5.2 Development of offshore fields - Treccani

5.2.1 Introduction

Over the years different types of offshore structureshave been designed for a multitude of purposes.There is no doubt, however, that the development ofhydrocarbon exploration and production provided avital boost to their spread. The need to transferdrilling rigs and production plants offshore, and theresulting need to design and build support structures,the difficulties of building platforms capable ofwithstanding increasingly extreme environmentalconditions have led to offshore research andengineering becoming one of the most interestingand innovative branches of technologicaldevelopment.

The first offshore drilling activities took placeduring the late 1930s in the Gulf of Mexico. The firstoffshore production platforms of modern design wereinstalled from the 1950s onwards, but it was in theearly 1970s that a genuine boom took place in theoffshore industry. The 1980s saw the consolidation ofdevelopment technologies for moderately deep waters,following the exploitation of almost all easilyaccessible large fields. In the 1990s attention moved tothe potential for developing small fields, which couldonly guarantee small economic returns. Hydrocarbonexploration focused mainly on very deep waters, thustriggering the development of innovative technologieswith the aim of constructing production platforms atever increasing depth.

Generally speaking, an offshore platform has theprimary function of allowing the production ofhydrocarbons from the subsurface, with minimumtreatment and maximum respect for safety andenvironmental protection. These are then transportedto plants on the coast for definitive treatment prior tocommercialization. Due to the high costs of offshore

construction, facilities in open waters must besimplified and reduced as far as possible. The requisitefacilities are thus kept to a bare minimum, and, duringthe construction phase, offshore construction work areas limited as possible by maximizing onshoreprefabrication.

Over the years, technological advances and theconstant quest for innovative solutions have led tothe development of different types of structures tosupport production facilities. The main factorinfluencing development typologies is the depth ofwater in which the platforms must be installed. Wehave already dealt with offshore drilling rigs (seeChapter 3.4); this chapter will provide a detaileddescription of offshore production platforms. Wewill first examine the most common developmenttypologies for moderately deep waters; we will thenbriefly outline development strategies for smallreservoirs and the technical expedients adopted foreconomically profitable exploitation; finally, we willdiscuss development typologies for very deepwaters, describing the structures which have beenused in recent years, and those which are provingthemselves to be most promising for the future.

5.2.2 Development in shallow waters

Generals

An offshore unit in shallow waters generallyconsists of a structural platform, able to supportdrilling rigs, wells and wellheads, processing plants forthe primary treatment of hydrocarbons, support andsafety facilities, and accommodation for workers. Inrelatively shallow waters, no more than 300-400 m

609VOLUME I / EXPLORATION, PRODUCTION AND TRANSPORT

5.2

Development of offshore fields

deep, platforms are generally fixed structures, whichstand on the seafloor, to which they are rigidlyanchored. However, it is impossible to use rigidsolutions at greater depths; under these conditions theplatforms must be free to oscillate in response toenvironmental loads.

Below we will describe in detail the facilitiespresent on an offshore platform, and the structuraltypologies most frequently used to build fixedplatforms. It should be remembered that the discoveryof large new fields in moderately deep waters hasbecome a genuine rarity in recent years. The onlypotential for development in shallow waters isrepresented by the ability to exploit small,geographically dispersed, hydrocarbon reserves whosedevelopment would not be economically viable, orwould involve excessive risk, were suitabletechnological solutions not adopted.

Fixed platforms

Fixed platforms generally consist of a supportstructure which rests rigidly on the seabed and

supports the production plants (topside), keeping themat a sufficient distance from the surface of the water toavoid them being hit by waves (Fig. 1).

Given the relatively shallow waters in which theseplatforms are installed (a maximum of 300-400 m butnormally less than 200 m), fixed platforms cangenerally be easily linked via subsea pipelines toterminals along the coast, where the hydrocarbons arecollected. As a result, these platforms do not usuallyneed storage facilities. The hydrocarbons produced bythe wells flow through the primary treatment plants onthe platform, and then pass continuously throughsubsea pipelines to onshore gathering stations. Herethey undergo further processing, bringing them to therequired conditions for the final refining andcommercialization phases.

Surface production facilities (topside)The development of an offshore field requires

the following topside facilities: a) drilling rigsand/or workover rigs; b) wellheads, placed at thetops of the wells to control the flow of hydrocarbonsfrom the seabed to the surface; c) processing plantsfor the hydrocarbons produced and, where necessary,facilities for the injection of gas or water into thereservoir; d ) utility systems to support primaryprocessing; e) safety and emergency facilities; f ) pumping or compression systems to propel the oiland gas to gathering stations along the coast; g) control systems for operations and relevantcontrol rooms; h) technical rooms and laboratories;i) accommodation and common rooms for workers;j) flares to allow gas to be burned in case ofemergency; k) equipment for moving materials;l) transport systems for workers.

These different components may be unified andintegrated on a single platform (Fig. 2), or hosted onindependent structures, generally linked by bridgesallowing the passage of workers, piping, andelectrical and instrumentation wiring. Which of thesetwo alternatives is selected depends on the depth ofthe seabed and the size of the plants. Severalplatforms of modest size allow for greater flexibilityand are inherently safer; they also simplify thedesign, construction and installation process ascompared to that required for a large integratedplatform. From the point of view of safety and theflexibility of construction the optimal solution is tohost worker accommodation, processing and supportplants, drilling rigs and flare on separate platforms.However, as depth increases, so does the impact ofcosts on support structures: at depths over 100 m,these structures are so expensive that facilities mustbe concentrated as far as possible on a singleplatform.

610 ENCYCLOPAEDIA OF HYDROCARBONS

DEVELOPMENT PHASE OF HYDROCARBON FIELDS

drilling rig

flare

living quarters and helideck

production deck

jacket supportingstructure

Fig. 1. Fixed platform.

In the case of integrated platforms, the generallayout is governed by safety issues: processing plants,drilling rigs, utilities, accommodation and controlrooms are therefore kept as far as possible in separateareas. Accommodation and control rooms are placedas far away as possible from the potentially high-riskprocessing and drilling zone. Utilities, which are muchless dangerous, are concentrated in the area betweenthe processing and drilling zone and theaccommodation area.

Environmental conditions also influence theconfiguration of topside facilities: the flare is locatedas far away as possible from the accommodation area,and to the lee side of it with respect to the prevailingwinds. The orientation of the platform itself isdesigned to expose the smallest possible area of thesupport structures to waves and currents.

All the facilities are supported by steel structureson several levels, connected by columns and diagonalgirders. For reasons of expenditure, both structuresand facilities must be prefabricated onshore as far aspossible. This entails the need to build completemodules, which are as large as possible at onshoreconstruction yards; these can then be transported andinstalled offshore. The size of these prefabricated unitstherefore depends on the availability and liftingcapabilities of the crane barges used for offshoreinstallation. Until the second half of the 1980s, evenlarge platforms were built by prefabricating modulesof mass no greater than 1,500-2,000 t onshore; thesewere installed singly and connected together offshore,thus wasting a considerable amount of time andmoney. From the late 1980s, the development of a new

generation of crane barges for installation has allowedfor the construction of integrated modules ofenormous size and weight (up to around 12,000 t).This minimizes the number of offshore liftingoperations, and thus drastically reduces the timerequired to complete the platform and start up itsfacilities.

A modern topside structure includes a mainmodule (deck), which is prefabricated onshore andinstalled offshore directly onto the support structure,which keeps it at a sufficient distance from the surfaceof the water (see below). Depending on the size of theplatform, the deck may have four, six or eight maincolumns, which transfer the load to the structure uponwhich they rest. If not all the facilities can becontained within the deck due to the weightrestrictions mentioned above, we need to constructother modules which are installed on the main module.

Accommodation for workers (up to 100-150people on large platforms) and common rooms aregenerally built as an independent module, given theirdifferent typology, more architectural thanengineering. The accommodation module is alsoconstructed in steel, with external cladding in load-bearing corrugated metal sheets. The helideck used forthe transport of workers is usually built on top of theaccommodation module, of which it forms an integralpart.

The flare, needed to burn gas in emergencies orwhen the production is started up, is also usuallybuilt as an independent module, with a metalframework structure of triangular or square section.This framework is so long that the flare has to betransported to the installation site restinghorizontally on a barge, and then installed verticallyon the platform. It is therefore impossible toprefabricate the flare as an integral part of theproduction module, even were the total weight toallow this.

The drilling and, if necessary, workover rig isalso an independent module, which can beremoved after operations end and reused todevelop other fields.

Drilling and workover rigs Production wells can be drilled using rigs hosted

directly on the production platform or, in shallowwaters, using special jack-up drilling vessels, whichoperate directly above the platform. The jack-up is avessel consisting of a hull and framework legs. It istowed to the drilling location, where its legs arelowered until they rest on the seabed; the hull is thenlifted until it reaches operating height.

To reduce the amount of time required fordevelopment, it is often preferable to carry out


DEVELOPMENT OF OFFSHORE FIELDS

Fig. 2. Topside of a fixed platform (Eni-Saipem).

pre-drilling before the platform is installed. The wellsare drilled through a structure previously installed onthe seabed (template), which is used as a guideduring the drilling phase. At depths of less than 100m, pre-drilling is carried out using a jack-up rig. Atgreater depths, we must use floating rigs ofsemisubmersible type. These consist of a metal deckhosting the drilling rigs; the deck is supported byfour, six or eight columns of large diameter, which inturn rest on submerged pontoons. While sailing,semisubmersible units have a limited draft (6-8 m),whereas in operating mode they can be submerged,reaching drafts in the order of 20 m, thus notablyincreasing stability. Where the seabed is suitable forfixed platforms, semisubmersible drilling rigs aremoored using a system of anchors, chains and steelcables. In deep waters, however, they are kept inposition using computer-controlled thrusters, whichrefer to satellite positioning systems.

When pre-drilling ends, a pair of docking piles areinstalled next to the wells drilled through the pre-installed template structure. These piles act as guidesduring the subsequent installation of the productionplatform above the pre-drilled wells, to ensure that thestructure is positioned within extremely tightdimensional tolerance limits.

After the installation of the platform has beencompleted, the pre-drilled wells are completed withlight facilities, also used for workovers. These areusually modularized, allowing them to be easilyremoved from the platform when operations end, andwhere possible reused to develop other fields. Whenthe wells have been completed, we install thewellheads, and then connect these to the processingplants.

The wells from the seabed to the surface and the wellheads

The hydrocarbons from the reservoir arechannelled from the seabed to surface plants throughconductor pipes, supported by the same structurewhich supports the topside.

Once the well has been completed, a wellhead orchristmas tree is installed at the top of each conductor;these allow us to alter the direction of flow fromvertical to horizontal, to channel the hydrocarbonstowards treatment plants, to close the well and regulatethe flow. The wellhead consists of a series of valves,operated manually or using appropriate devices. Thewing valves, usually gate valves, are used to open orclose the well, whilst the flow is regulated using valves

with adjustable calibrated orifices (choke valves)which are far more resistant to abrasion. Afterinstallation, each wellhead is connected to a manifoldinto which the reservoir fluids from the wells arechannelled before processing.

Processing plants for the hydrocarbons producedThe high cost of offshore units means that

treatment carried out on the platform must be kept to aminimum, reducing the corrosiveness of thehydrocarbons to allow their transport to gatheringstations along the coast. Any further treatmentrequired will be carried out here; on the platform weonly carry out treatment involving the separation,dehydration, and heating or cooling of reservoir fluids.

Separation allows us to separate gases fromliquids (crude oil, condensates and water). Duringthis process, any sand dragged along with thefluids, which may cause erosion can also beremoved.

Dehydration allows us to remove the watercontained in crude oil or natural gas, in order to avoidthe formation of hydrates during transport through thepipeline; these solidify when the fluid cools and riskcausing obstructions. The most frequently usedmethod involves bringing the hydrocarbons intocontact with a pure glycol solution. Facilities for theseparation of water from crude oil also remove otherimpurities present and collect the petroleum vapourscontained within it separately.

The fluids produced are heated and cooled inaccordance with varying processing requirementsand the properties of the products to be treated.

At times, after a few years of production, we mayneed to inject water or gas into the reservoir to maintainreservoir pressure at an acceptable level. In these cases,the platform also hosts facilities for re-injection throughpurpose-drilled wells or depleted production wells (seeChapters 5.3 and 5.4).

Utility systems for primary processingIn order for the hydrocarbon treatment plants

to function, and for the platform to operate withthe requisite safety and reliability, it must alsoinclude a series of utility systems. Thesefacilities are:• Power generated unit to power all the electrical

equipment on the platform. This usually consists ofseveral turbines running on both gas and diesel oil(normally natural gas produced from the reservoir;gas oil during start-up or if production is halted).



• Treatment plant for the gas used to power theturbines.

• Plants for the injection of chemicals, under theform of corrosion inhibitors, into the exportpipeline (e.g. methanol, usually injected every timethe platform is started up).

• Glycol regeneration unit for the glycol used todehydrate gas; as it exits the dehydration column,the regeneration unit separates the glycol from thewater, and recovers it.

• Diesel oil distribution system: the diesel oil is storedin tanks and used to power turbines, emergencygenerators, fire pumps and other motors.

• Unit which supplies compressed air to all the fieldequipment and other utilities on the platform.

• Refrigeration units: the need to cool processes andsupport facilities is met by using refrigeratingwater which circulates in a closed loop, and iscooled in seawater exchangers.

• Seawater collection and distribution plant: seawateris pumped to the platform by submerged pumpsinstalled inside tubular caissons at a depth of a fewtens of metres. Seawater is used as a coolant inexchangers, to feed desalination and purificationplants, and during drilling operations.

• Desalination and purification plant: this water isthen distributed to worker accommodation,utilities, laboratories, the drilling rig and theemergency showers needed for instantdecontamination of personnel.

• Plants to collect discharges from equipment andwaste water.

• Treatment plant for the water separated fromreservoir fluids during processing: this water istreated to recover the hydrocarbons remaining in itafter the primary separation process; the recoveredhydrocarbons are fed into the production system,whereas the water is discharged into the sea aftertreatment to limit the pollutants it contains as far aspossible.

• Sewage treatment plant to treat sewage fromworker accommodation and utilities.

• Nitrogen generation plant to power some specificutilities.

• Biocide liquid distribution plant, used to preventorganic growth inside the pipes of fire-fightingsystems.

Safety and emergency systemsSafety systems significantly condition offshore

units; those normally used on platforms are as follows:

• Emergency generator system: consisting of one ormore generators powered by diesel oil, whichbecome operative if the primary generator systemsfail.

• UPS (Uninterruptible Power Supply) system:consisting of a series of batteries to power vitalplatform systems which become operative if boththe primary and emergency generators fail.

• Shut-down system: which shuts down productionin case of accident.

• Detection system, which uses a series of sensorsplaced throughout the platform to detect thebeginnings of a fire, smoke or gas leaks, and thusactivate alarm and protection systems.

• Active fire-fighting systems: these use water,foam, carbon dioxide and inert gas, and protect theentire platform; the water is pumped directly fromthe sea, whereas the other substances are stored intanks.

• Passive fire-fighting systems, consisting in theapplication of appropriate materials resistant tohigh temperatures on all those parts of structuresand facilities at risk of prolonged exposure to firein case of accident, and whose collapse couldprejudice the safety of the entire platform.Additionally, the well and processing zone isgenerally isolated from other areas of the platformwith explosion-proof walls.

• Personnel evacuation systems: generally life boatsand life rafts, suitably distributed around theplatform.

• Security and protection systems for workers: theseare located at strategic points around the platform,and include life-jackets, gas masks, showers foruse in case of contact with dangerous substances,etc.

• Alarm systems: these consist of acoustic and visualdevices which are switched on automatically incase of emergency.

• Telecommunications systems: these allow workerson the platform to communicate internally andwith the outside world to request help in case ofemergency.

Oil and gas pumping or compression systemsOften, after treatment carried out on the platform,

the pressure of the hydrocarbons produced isinsufficient to propel them onshore through subseapipelines, and we need to increase it. For gasespecially, pressure is usually initially sufficient toavoid the installation of compressors to propel it



onshore; however, over the years, pressure tends todecrease as a result of continued production, and itlater becomes necessary to install compressionsystems.

The platform also hosts the devices (pigs) whichare propelled through the entire pipeline by the fluidpressure, allowing it to be cleaned, and theconditions of the pipeline to be inspected. The pig isinserted through sidelines in the main pipeline (pigtrap), with trap doors for the insertion or recovery ofthe pig.

Control system and control roomsThe production and processing plants, and support

and safety systems are constantly monitored by a datacapture and processing system run from a controlroom which represents the heart of the platform. Fromthe control room, operators can work on the entireplatform, using control panels which show thefunctioning of the platform in a schematic way usinggraphic displays; these also allow operators tointervene remotely.

The functioning of the platform is monitoreduninterruptedly 24 hours a day, usually by two groupsof operators who work on different panels: one groupworks on the processing plants and utilities systems,whilst the other monitors the electrical generator anddistribution systems.

The monitoring and data capture system constantlyrecords operational data from all the appliances. Theirworking history can thus also be used to plan andrecord maintenance work on the platform.

Technical rooms and laboratoriesAlongside the control room, platforms usually host

other technical rooms: one or more electrical rooms,where the electrical distribution switchboard, batteriesand transformers are installed; a room containing therefrigeration units for air-conditioning plants; a workshop for minor repairs or maintenance work, andlaboratories to carry out chemical and physicalanalyses of production fluids.

Accommodation and living quarters for workersPlatforms are generally manned. Workers (up to

100-150 people on large platforms) are housed in aspecific area of the platform, which for safetyreasons is as far as possible from wells andprocessing plants. Accommodation and commonrooms are generally grouped together in a specialmodule on several floors. Alongside the cabins for

personnel, this also hosts other common areas suchas: offices and meeting rooms, infirmary, radio andtelecommunications room, kitchens, storeroom,laundry, canteen, recreation rooms, TV rooms, gym,etc. The accommodation and common rooms, servedby an air-conditioning and ventilation unit, areslightly pressurized to prevent the entrance of anytoxic gases, which may leak from facilities in case ofaccident.

FlaresA unit is needed to collect the discharges from the

various processing plants (hydrocarbon and natural gasvapours), and dispose of these. The gas to beeliminated is sent to a burner placed at the far end of ametal framework, known as the flare; its length,depending on the maximum amount of gas which canbe burned, may easily reach a hundred metres. Theflare is oriented so as to be downwind of the prevailingwinds.

Equipment for moving materialsMaterials are moved onto the platform using

cranes, placed so as to serve the entire upper decksurface of the topside. Access for the cranes to thelower decks is ensured by the presence of suitablypositioned cantilevered loading bays. The cranes,which can lift several tens of tonnes, are used to loadand unload materials onto or from the transport vesselswhich supply the platform. Materials are movedaround the platform on monorails serving all criticalareas; less heavy materials can be moved usingtransport trolleys.

Personnel transfer systemsPlatform workers are usually transported by air

using helicopters. More rarely, and only ingeographical locations where weather conditions allowthis, workers may also be transported by sea; in thelatter case, the platform is equipped with a jetty tomoor the transport vessels and the equipment neededfor mooring. In any case, all platforms have ahelicopter pad placed on the roof of theaccommodation module, in order to guarantee fastevacuation in case of medical emergencies oraccidents.

Support structuresThe modules hosting surface facilities must be

supported by adequate structures resting on the seabed,which serve the following main purposes:



• To maintain the topside at a sufficient distancefrom the surface of the water to avoid waves hittingthe facilities.

• To transfer their own weight and that of thefacilities above to the seabed.

• To transfer the loads caused by environmentalfactors such as waves, currents, winds andearthquakes to the seabed, resisting rigidly.

• To resist and transfer to the seabed the loadscaused by potential collisions with ships.

• To support the conductors which link the wellheadsat the surface to the wells and thence the reservoir.

• To support vertical sections of any subsea pipelines(risers) which rise from the seafloor to theproduction plant at the surface.

• To support those parts of surface facilities whichinteract with seawater, typically water intakes anddischarges.

• To support the conductors inside which subseatubes rise to the surface; these may be power orfibre optic cables, or other flexible tubes of smalldiameter which carry hydraulic control fluids tothe subsea pipeline shutoff valves, allowing theseto be controlled from the platform.

• To support berthing and mooring facilities forships used to transport workers.Two typologies are widely used to construct the

rigid support structures for surface facilities in depthsof water up to 200-300 m (and in some extremely rarecases up to 400 m). The most common is certainly asteel lattice structure ( jacket), designed to be asnarrow and light as possible; this structure transfersloads to a system of foundation piles. The secondtypology, by contrast, is massive and heavy, and isgenerally made of reinforced concrete; this structuresupports the topside by resisting environmental loadsthrough gravity alone.

Steel lattice support structures (jacket)The jacket is a steel lattice structure constructed

from tubular members. This structure usually consistsof four or eight legs of large diameter, generallydesigned to diverge from the vertical by a few degreesso that the base of the structure is larger than its top,thus enabling it to transfer loads to the seabed moreeffectively (Fig. 3).

The legs are connected to one another by a seriesof tubes welded both vertically and horizontally toform a three-dimensional lattice structure. Thediameters of the legs depend on the size and weight ofsurface facilities, and the depth of the seabed, and is

generally in the order of a few metres. The othertubular members forming the framework are smaller,but still about a metre in diameter.

The structure’s mass also depends on the size ofthe facilities, and above all on the depth of the seabedupon which it is to be installed. It may range from afew thousand tonnes in depths of water under 100 m,to 20,000-30,000 t for depths in the order of 200 m, upto as much as 40,000-50,000 t (giant jackets) in depthsover 300 m.

All the vertical members descending from thetopside which must be supported by the jacket (seeabove) are inserted inside guides or welded to fixedsupports held up by suitably positioned elements of theframework.

Jackets are built onshore in construction yards.Except in those rare cases when they are of verylimited size they are constructed in a horizontalposition and, once finished, are loaded onto a barge fortransport to the offshore installation site. Usually theweight of the structure is such that it cannot be liftedinto place; the jacket is therefore launched in ahorizontal position, up-ended in the water, and thensunk and positioned definitively with the help of acrane barge. All these intermediate phases of



Fig. 3. Fixed platform jacket.

buoyancytanks

jacketsupportingstructure

mud mat

pilesleeves

installation require the jacket to be equipped with aseries of structural features with a merely temporaryfunction; to allow it to be loaded onto the barge andsubsequently launched in open waters the jacket musthave two parallel skids running almost its wholelength. These allow the structure, when in a horizontalposition, to slide along parallel tracks placed both onthe wharf at the onshore construction site and on thetransport barge.

Once launched in open waters near the finalinstallation site, the jacket must be able to float. Somelarge buoyancy tanks, attached to the main structureand made of steel plate suitably reinforced withinternal rings, must therefore be included. A controlledstaged flooding of the buoyancy tanks during thephases of up-ending and positioning on the seabed isthen carried out. After installation has been completed,the buoyancy tanks are removed to reduce the surfacearea exposed to waves and currents. To reduce the sizeof the buoyancy tanks, the tubular members formingthe structural framework are also made watertight; thelegs of the jacket are divided into compartmentsequipped with valves, and these too are floodedsequentially during the phases of up-ending andapproach to the seabed.

Once it has come to rest on the seabed, thejacket is supported by a temporary foundation,designed to support the weight of the structurebefore the foundation piles are installed. This temporary foundation, or mud mat, consistsof a reinforced metal plate of appropriate size,welded to the elements of the framework formingthe lower floor plate of the jacket, at the seabedlevel. Since the marine environment exposes thesteel structure to significant corrosion, the upper part of the jacket, subject to the actionof waves, is given an appropriate number of coatsof paint. The remaining submerged part of thestructure is shielded by a cathodic protectionsystem using sacrificial anodes in aluminiumalloy (which has a lower electrochemical potentialthan steel), suitably placed along all structuralelements.

As already mentioned, the definitive foundation ofthe jacket consists of piles, which must transfer to theseabed all environmental loads and those derivingfrom the facilities above. The piles are steel tubes oflarge diameter, and are driven into the seabed with thehelp of hydraulically operated subsea pile hammersbefore the topside modules are installed. Where theseabed is extremely hard, we may need recourse to

drilling. The depth to which the piles are driven intothe seabed depends on the loads they must support,and the properties of the ground, and may easilyexceed a hundred metres.

For small jackets with modest surface facilitiesin shallow waters, the piles may be driven throughthe main legs of the structure. When the number ofpiles required is larger than the number of legs, andthe depths of water greater, the piles are driventhrough sleeves placed at the base of the jacket andjoined to the legs in such a way as to transfer theloads of the structure to the foundations. Structuralcontinuity between piles, sleeves and jacket isensured by a ring of concrete poured at the end ofpiling operations, to fill the hollow space betweeneach pile and its sleeve.

The jacket is usually also equipped with a numberof utility systems allowing the staged flooding ofwatertight compartments during the installationphases, and the concrete to be poured to connect thepiles and the structure. This staged flooding is carriedout by opening the valves installed in each watertightcompartment; these can be operated manually or usingpneumatic systems.

The concrete is generally pumped from thesurface to the various sleeves through anothersystem of tubes and valves. In order to prevent theconcrete escaping as it is poured, the space betweenthe pile and the sleeve must be sealed at the base ofthe pile sleeve using rubber rings (grout packers);these are inflated with nitrogen once pile driving hasterminated.

Gravity support structuresUnlike jackets, gravity support structures do not

need foundation piles to transfer the various loads tothe seabed, but simply rest on the seafloor using theforce of gravity to maintain stability. Whereas jacketsare built to be as narrow and light and insensitive tothe loading of waves and currents as possible, gravitystructures depend on their mass and base dimensionsfor resistance and stability.

To guarantee the required stability whilst notexceeding the load-bearing capacities of the seabed onwhich the structure rests, we need to construct anextremely massive base with large dimensions.Consequently, gravity structures are usually made ofreinforced concrete. In this case, the topside issupported by cylindrical columns of large diameter,usually four in number. These columns, below the areasubject to wave loading, are set into a large base



consisting of cells created with concrete walls (Fig. 4).Since the size of the structure allows it, manyinstallations of this type are also used to store thecrude oil produced, by virtue of tanks constructedinside the cells of the base. There is thus no need totransport the crude oil onshore through subseapipelines, since it can be collected by tankers directlyfrom the offshore production site.

A gravity support structure is built in a yard with alarge dry dock. When construction work has finished,the dock is flooded so that the structure, whose base iswatertight, can float. It is then towed out of the dock toa sheltered area of sea near the construction yard. Herea crane barge can install the topside modules on thesubstructure. Once the modules have been installed,these can be connected together, and test thefunctioning of facilities. The ability to complete theinstallation of facilities in a sheltered area near thecoast, thus protected from adverse weather conditions,allows for an optimal use of time, and represents oneof the advantages of this type of solution. The entire

platform is then floated to the offshore installationsite, where it is flooded and sunk in a slow andcontrolled way. In order to increase its mass, and thusthe stability of the structure, some of the cells in thebase are filled with inert materials once installation iscomplete.

The gravity solution in reinforced concrete hasbeen used for some large fields in the North Sea, suchas those along the Norwegian coast, with the platformbeing constructed inside the deep and very shelteredfjords. It has been extremely difficult to use thistypology in other geographical contexts. In a fewisolated cases, steel gravity structures have been built.Attempts have also been made to build hybridstructures, with a concrete base and upper steelframework, but none of these solutions have beenparticularly successful.

One possible alternative application for thistypology is represented by the need to exploitreservoirs in arctic waters: only a gravity structure inextremely massive reinforced concrete can withstandcollision with an iceberg, or the compressional loadsresulting from the formation of pack ice. For thesepurposes, the reinforced concrete base has acylindrical or truncated conical shape. In any case,gravity platforms represent a possible specializedtypology, with advantages only in special contexts.

FoundationsPile foundations represent the most commonly

used typology for the construction of rigid platforms,and are generally coupled with framework structures(see above).

A recently adopted alternative to pilefoundations involves constructing steel cylindersabout ten metres in diameter, open at the base andclosed at the top. These are placed at the fourbottom corners of the jacket and integrated intothe structural framework so as to protrude a givennumber of metres beneath the lowest level of thestructure (Fig. 5): when the jacket is positioned onthe seabed, the cylinders penetrate the upperlayers of the seafloor due to the weight of thestructure. After this initial penetration iscomplete, a pump system depressurizes the spaceinside the cylinders by sucking out the water, sothat the cylinders can slowly sink into the seabed,driven by the pressure difference between interiorand exterior.

As far as gravity platforms are concerned, on theother hand, as we noted earlier no special foundation



concretegravityplatform

topside

Fig. 4. Gravity platform.

systems are required. The vertical compression loadsare distributed over such a large load-bearing area thatlocal pressures are limited, and the weight of thestructure is so high as to balance any overturningmoments.

Design of the facilities

Engineering the development of an offshore field,and in particular of a rigid platform, takes place indistinct stages, each with a well-defined aim.

Preliminary design and feasibility studiesOnce we have discovered the field, evaluated its

production capabilities and analysed the properties ofthe hydrocarbons, the first engineering phase begins.This involves drawing up a preliminary design andfeasibility study, with the aim of defining the mostsuitable solution. During this first stage, cost estimatesare made in order to evaluate the economic return oninvestments. More specifically, this first stage involvesthe following activities:• Preparation of functional specifications defining

the geographical location of the platform, theprocessing typology with the necessary systemsand subsystems, the typology of the main supportstructures, and supplying a rough estimate of themass and size of facilities and structures.

• Definition of surface facilities (primary processingsystems, support and safety systems, the interfacewith gathering systems for the fluids produced bythe wells and the interface with systems to exportthe fluids after treatment), and preliminary sizingof major plants.

• Definition of safety and environmental protectionrequirements (identification of safe temporary

refuge areas and escape routes, and active andpassive protection systems), and evaluation of theacceptability of environmental working conditionsin terms of noise, vibrations and the presence oftoxic substances.

• Definition of drilling package, well completionand workover systems.

• Definition of platform support structures and estimate of the mass of the topside structures, sup-port structure, foundations, flare, accommodationand helideck.It should be stressed that designing an offshore

installation is heavily influenced by the prefabricationmethods used in onshore yards, and by themethodologies used for offshore transportation,installation, completion and start-up. The designprocess should therefore take into account all possibleproblems and constraints, attempting to identify theoptimum compromise.

Once the preliminary design stage has beencompleted, a preliminary estimate of investmentexpenditure is prepared with operating costs and,where necessary, decommissioning costs, thusobtaining the cash flow. This allows us to carry outan economic analysis with the aim of evaluating theeconomic profitability of the project over time. Ifthe results of the economic analysis are positive, thenecessary go-ahead, on the basis of the developmentplan chosen, might be obtained.

Collection of environmental dataThe second stage involves collecting all necessary

environmental data.We therefore carry out in-depth

geomorphological studies to define the exactconformation and depth of the seabed on which theplatform is to be installed, and geotechnicalinvestigations to assess the mechanical properties ofthe ground. Geotechnical studies require coresamples to be taken in the subsurface to considerabledepths if we intend to use a pile foundation, withtests aimed at determining the mechanical propertiesof the various layers. We also measure some of theproperties of the seawater, such as the temperatureand salinity at various depths, if these are not alreadyavailable from databases. The meteorologicalconditions which the platform will face (winds,waves and currents), are obtained from statisticaldata for the area already available in the literature, orextrapolated using simulations with mathematicalmodels.



Fig. 5. Jacket with a foundation alternative to piles (Eni-Saipem).

Basic designAfter gathering all the basic data needed for

design, both with reference to the properties of thehydrocarbons to be produced and environmental andgeotechnical parameters, the third phase begins. Thisinvolves defining all aspects of the drilling rig andprocessing plants, support facilities, safety systems,structures, etc.

Diagrams of energetic flow and of the materials areprepared, and the necessary process analyses toestablish the characteristics of the hydrocarbontreatment plant are carried out, while the size of theplants and the materials used to build them isdetermined. In the same way, the characteristics andthe size of utility systems are determined.

The well drilling and completion systems is thendefined by preparing specifications for these, anddrawing up general criteria for health and safety, theenvironment and acceptable risk; specifications foractive and passive safety systems are prepared, andplans for the classification of hazardous areas, safetyand rescue systems, and for escape routes aredefined.

General layouts of the offshore complex, floor plotand elevations of facilities, appliances and mainpipelines, plans of maintenance and distributionsystems and utilities, plans of fire barriers andexplosion-proof walls, are then prepared.Specifications for next detailed planning and theclassification of the piping required are developed;specifications for piping materials, paints, claddingand insulation are drawn so as to be ready forsubsequent requests for quotes from suppliers. Afterdetermining the specifications for the design ofmachinery and equipment, and carrying out thenecessary studies, all the appliances required aredefined, identifying their dimensions, capacity andmass, and drawing up specifications for subsequentrequests for quotes from suppliers; specifications forthe containment of noise and vibrations are alsodrawn up.

This is followed by the preparations of basicspecifications for the electrical system, defining theelectrical layout of the platform, carrying out basiccalculations and drawing up a preliminary list ofelectrical loads; layout for switchboard rooms,substations and cabins are prepared and theproperties of the relevant equipment are defined,drawing up specifications and data sheets needed forthe subsequent request for quotes from suppliers.Layout for the routes taken by the main cables are

then studied and optimized. Design specificationsfor instruments, automation and telecommunicationsare drawn up, and the characteristics of theequipment needed (control, safety, alarm andtelecommunications systems, control and safetyvalves, measuring and control apparatus, etc.) aredefined and used for subsequent quotes fromsuppliers; layout for instrumentation systems,control rooms and cabins, telecommunicationssystems and the main routes taken by cables are alsoprepared.

After establishing the project criteria andpreparing design specifications, all the analysesrequired for the sizing of the main structures arecarried out. These analyses must take intoconsideration all the loads, including temporary ones,to which the structures will be subjected duringconstruction (loading onto barges for transport,transportation, offshore installation) and later duringthe operational life of the platform. Specifically, fordeck structures and topside modules account must betaken of: a) static behaviour during the life of theplatform, considering its various configurations(addition or removal of appliances, presence oftemporary well workover equipment, etc.); b) behaviour during earthquakes; c) behaviour oftransport vessels during loading and transportationphases; d) the phases of loading, transportation andoffshore installation by lifting; e) fire resistance, todetermine the extent of protective coating systems onthe main structures; f ) response to accidents such asexplosions or the dropping of suspended loads; g) vibrations caused by rotating machinery.

Similarly, for support structures such as the jacket,exposed to waves and currents, we must take accountof: a) the static behaviour of the structure subject to allthe loads deriving from both the topside and theenvironment; b) the response to seismic loads; c) structural fatigue, extremely important due to thecyclical nature of hydrodynamic forces; d) thestructural behaviour of the foundations; e) thebehaviour of vessels during loading and transportationoperations; f ) behaviour during launching, for jacketslaunched from barges; g) behaviour during lifting, forjackets installed by crane barges; h) behaviour duringup-ending and sinking and temporary stability afterthe structure has been placed on the seabed, before thecompletion of piling; i) the installation of foundationpiles; l) accidents such as collision with ships andsuspended loads dropped from the topside; m) cathodic protection system.



Once the design of the main structures iscompleted, the size of the most important secondarystructures (such as staircases, walkways, grids, themain supports for appliances and pipes, supports forrisers, J-tubes, caissons, etc.) must be determined andspecifications to be used for the purchase of materialsare to be prepared. Finally, layout for the buildings andthe relevant structural and architectural drawings aredeveloped, while specifications for air-conditioningand ventilation systems, and for electrical systems andutilities are drawn up.

Detailed engineeringAt this point the final detailed design phase begins.

This represents the definitive refinement of theprevious phase, and aims to engineer all the facilitiesrequired, up to the issue of purchase orders formaterials and equipment, and the preparation of thetechnical drawings required by yards for construction.During this phase, all the definitive calculation reportsrequired for the project to be approved and certified,procedures for the construction, transportation,installation, commissioning and start-up of theplatform and write the operating manuals are drawnup. Finally, all the drawings made during the detailedengineering phase are handed over to the constructionyards.

Construction

For reasons of expenditure, the building of offshorestructures and facilities must be completed as far aspossible onshore, before they are transported tolocation in open waters and installed in their finalconfiguration. The size and mass of the prefabricatedunits that can be finished onshore depend on theavailability and capacity of the barges used fortransportation and later for installation. This must betaken into consideration when modularizing a platform.

The structures to be prefabricated onshore are ofconsiderable size and mass. Construction yards musttherefore stretch over a large area, and the groundmust be consolidated to guarantee high load-bearingcapacities. They must also have suitable liftingequipment and a wharf able to support large loads, andto berth and moor the large barges onto which thestructures are loaded.

Topside constructionGenerally, the topside structure consists of a main

module (deck), which is installed offshore directlyonto the support structure. The facilities are builtinside a steel structure of several storeys, connected toone another by columns and diagonal girders. If all thefacilities cannot be contained within the deck, due to

the limitations dictated by the transportation andinstallation phases, we need to build other modules,which are prefabricated separately and then installedon the main module.

Regardless of the specific construction sequencesadopted by individual yards, the following two rulesshould be followed to optimize time and expenditure:work as much as possible inside covered sheds, inorder to reduce the risk of inactivity due to adverseweather conditions; carry out as many activities aspossible at ground level and on several fronts to reducecosts and allow us to proceed with several activitiessimultaneously. Consequently, it is essential to identifythe largest possible number of subelements which canbe prefabricated under cover, and then devise aninstallation sequence allowing the prefabricatedcomponents to be assembled as far as possible atground level.

In accordance with these guidelines, theconstruction of a deck, or a typical module containingfacilities, usually follows a sequence resembling thatdescribed below:• Preparation of the areas of the yard needed for the

various phases of construction.• Installation of slipways (concrete blocks on which

steel guides are installed), on which the deck willfirst be assembled and then skidded during loadingonto the transport barge.

• Assembly of the loose structural elements receivedfrom the steelworks, prefabricating completestructural subunits of such a size that they can beconstructed under cover (structural frameworksforming the complete floor plates of all theelements, vertical structural components such ascolumns and diagonal girders).

• Transfer of structural subunits to another shed



Fig. 6. Constructing the deck: rotating a prefabricatedstructural framework (Eni-Saipem).

where sandblast and painting of the surfaces iscarried out.

• Assembly and painting of loose piping elements(pipes, bends, flanges, etc.), to form theprefabricated piping spools to be installedsubsequently, whilst simultaneously prefabricationis undertaken.

• Prefabrication in the workshop of cable trays andtheir supports.

• Assembly of structural subunits, to create theframework for the various floor plates of the deck:each structural level can be assembled at groundlevel upside-down, thus allowing the installation ofstructural supports for piping and cable trays,mainly suspended beneath the framework (verticalcolumns and diagonal girders between one storeyand another can also be welded to the frameworkwhile it is upside-down on the ground).

• 180° rotation of the first floor plate andsubsequent installation in its final position on theslipways (Fig. 6).

• Installation of facilities on the first levelframework; once this operation has beencompleted, the appliances are lifted and installed intheir final positions on the floor plate.

• Mounting of prefabricated piping elements,installation of valves and connection of pipingspools to appliances (Fig. 7).

• Installation of the second floor plate and itsappliances, and so forth for subsequent levels (Fig. 8).



Fig. 7. Constructing the deck: mounting the prefabricated tubes (Eni-Saipem).

Fig. 8. Constructing the deck: installing the structural floor plates (Eni-Saipem).

• Laying of electrical and instrumentation cables onthe cable trays when welding work is almostcomplete, so as to minimize the risk of them beingdamaged.

• Systematic check that all components of theplatform have been completed, and initial testing ofthe functioning of facilities (commissioning); thiswill be completed offshore, after installation hasbeen finished and before the platform is started up.

• Mounting of temporary equipment needed foroffshore installation, such as lifting slings.Once finished, the deck (or module) must be

loaded onto a barge for transportation to the openwater installation site. The loading operation is usuallycarried out by sliding the structure along the slipways,using a system of hydraulic jacks. The barge is mooredinto position, and must be ballasted following acarefully designed sequence, in order to constantlycompensate for the transferral onboard of the load, andchanging tides. Suitable steel box girders are installedto create a bridge between the slipways on the wharfand those on the barge.

Construction of the support structure: jacketThe structural framework of the jacket is built in

onshore construction yards following the sameprinciples adopted for the topside: in other words,attempting to maximize prefabrication under cover andground level assembly. The jacket consists of a three-dimensional frame made of tubular steel memberswelded to one another.

As already mentioned, depending on thedimensions of the topside and the depths of water inwhich it is to be installed, this structure generallyconsists of four or eight legs of large diameter linkedby a series of tubular members, welded together toform both vertical walls and horizontal floor plates.

Small jackets, for installation on undemandingseabeds and in geographical areas whereenvironmental conditions are not particularly severe,may be constructed by welding the tubular membersdirectly to one another and to the main legs, thuswelding only from the outside.

For large jackets, the joints between the tubularmembers are subjected to considerable static anddynamic loads and fatigue, and the connecting weldsmust therefore be continued inside the tubularmembers. The joints are therefore prefabricated units,made up of an inner tube of larger diameter, to whichthe tubular members which end in the node arewelded (Fig. 9). This allows us to weld both along theexternal circumference and inside each member.Given their complex geometry, a result of the largenumber of elements which meet in each node, thesejoints are subjected to thermal treatment at hightemperatures after prefabrication, to release thestresses caused by weld shrinkage. The tubularmembers forming the framework and which linktogether the prefabricated joints are created by thecircumferential welding of several base elements(ferrules); each ferrule in turn is made in a workshopby rolling steel sheets of suitable thickness followedby longitudinal welding.

Other elements which can be prefabricated undercover are: a) the structural units containing conductorguides; b) the box girder sections forming the slipwaysalong which the jacket is slid for loading onto thetransport barge and later for launching in open waters;c) the mud mat structures forming the temporaryfoundations for the jacket before piling; d) thebuoyancy tanks which ensure that the jacket can floatduring its installation; e) the sleeves which act asguides for the piles; f ) the walkways and other smallstructures.

The walls of the jacket are generally built in ahorizontal position, in the open and in a very largearea of the yard; this area must be suitably prepared byinstalling slipways on which the jacket is assembled,and along which it will slide during loading onto thetransport barge. The nodes and prefabricated tubularmembers are welded together on the ground to createthe vertical walls of the framework.

For four-legged jackets, two opposite walls areconstructed contemporaneously at ground levelfacing one another, so that when rotated through90°, they stand parallel to one another. Most of thetubular members forming the floor plates which linkthe walls of the framework are welded to one of thetwo walls while it is still at ground level. Afterassembling these two opposite walls, they are themthrough 90° using a series of crawler cranes. The twowalls are then connected by welding the members



Fig. 9. Constructing the jacket: prefabricated structural node (Eni-Saipem).

forming the floor plates, already connected to one ofthe two walls. The remaining diagonal girderslinking the two walls are installed and welded ataltitude. The anodes, risers, tanks, tubes forcementing and for the ballasting of watertightcompartments during installation, and all the otherstructures supported by the jacket are welded to thewalls of the framework as far as possible while theseare still on the ground.

For eight-legged jackets the central block isconstructed first; the external walls are then built atground level, facing the central block. Aftercompleting ground level assembly, the two externalwalls are rotated through 90°, and joined to the centralblock. After completing the framework, the buoyancytanks, assembled beforehand, are lifted and installed ataltitude. The successful use of this buildingmethodology depends on the ability of theprefabricated units to meet extremely stringent controland dimensional tolerance criteria. When the jacket isfinished, it is loaded onto the barge (Fig. 10). Theloading operation, as for the topside, involves slidingthe structure along the slipways, whilst the barge,moored perpendicular to the wharf, is graduallyballasted following a carefully devised sequence.

Construction of support structures:gravity platforms

For gravity structures, especially those inreinforced concrete, the procedure is completelydifferent. These structures are built in their finalvertical configuration in large dry docks. They arebuilt from the bottom up, with the progressiveinstallation of the reinforcement and slip forms andramp bridge deck, followed by concrete casting.Once construction is complete, the dry dock isflooded, allowing the structure to float. The platformis then towed out of the dock by tugs and floated to a

sheltered area near the construction yard where thetopside modules are lifted into place by a cranebarge.

Transport and installation

Onshore prefabrication introduces a series ofissues linked to the need to transport structures oflarge size and weight from yards on the coast to theinstallation site, and then install these in openwaters.

Transport and installation of the supportstructure: jackets

The vessel generally used to transport jackets fromconstruction yards to the offshore installation site is asteel lighter, or barge, generally designed andconstructed for this purpose. Given their considerablesize, jackets are usually built and then transported in ahorizontal position on the barge; only in those rarecases where the structures are designed for waters onlya few tens of metres deep can they be built andtransported in a vertical position.

Once placed in transport configuration after theloading operations, the jacket is rigidly connected tothe structure of the barge, which is then towed by oneor more tugs to the offshore installation site. Thecrane barge needed for installation operations isbrought here simultaneously. As soon as weatherforecasts are favourable, the installation operationbegins by removing the elements connecting thebarge and the jacket. Usually the structure must beplaced in the water with an operation known as‘launching’, since the jacket can be lifted directly offthe barge only in those rare cases when its weight iscompatible with the maximum lifting capacity of theinstallation vessel.

The launching operation is extremely delicate: thebarge is first ballasted so that it leans a few degreestowards the stern. The jacket is then pushed byappropriate hydraulic jacks to overcome the frictionbetween the slipways and the guide girders on thebarge. At this point the structure begins to slide freelydue to gravity, thanks to the inclination of the vessel.The jacket then continues to slide until it definitivelyleaves the barge. In some cases the jacket’s buoyancytanks are designed and positioned so that the jacketcan rotate autonomously until it is in a verticalposition after entering the water. Usually, however, thejacket floats in the water in a horizontal position afterlaunching. It is therefore needed to up-end it bydifferentially flooding the buoyancy tanks and otherwatertight compartments built into the structure’s legs.The launching and up-ending operations are carefullyplanned in minute detail, so as to ensure that the



Fig. 10. Loading the jacket onto the transport barge (Eni-Saipem).

structure is always stable, and does not exceed theminimum distance from the seabed.

After launching and up-ending, the jacket floats ina vertical position, with its base about ten metresabove the seabed. The final positioning manoeuvrefollows: the crane grabs the jacket using suitableslings, the flooding of watertight compartments iscompleted, and the crane gently lowers its hook,allowing the jacket to sink until it rests on the seabedin its final position. In order to guarantee that thejacket is correctly positioned, it is constantlymonitored during the final phases of installation usingan acoustic system which continuously identifies thestructure’s position with respect to a reference systempreviously installed on the seabed.

When final positioning is complete, the jacket issupported by its temporary foundation. The crane isunhooked, and the crane ship is prepared to install thefoundation piles. The piles, too, are prefabricatedonshore, and transported offshore lying horizontallyon a barge. The barge is brought alongside the craneship, and each pile is up-ended and installed by thecrane inside the sleeves on the jacket. Each pile is thendriven by a pile hammer until it reaches the depthspecified by the project. After pile-driving, the pilesneed to be cemented in place: the cement mortar ispumped from the crane barge into the gaps betweenpile and sleeve, through tubes previously placed alongthe jacket structure. Once the cement has set, thestructural link between the foundation piles and thestructure is complete.

All the phases of installation require underwaterobservation of the various manoeuvres which occurbeneath the surface of the sea. For this purpose we usesubmarine robots (ROV, Remote Operated Vehicle),manoeuvred from the ship’s bridge with remotecontrol systems. The ROVs are placed in the waterwhen operations begin, and allow us to see, usingunderwater television cameras, the touchdown of thejacket, installation and pile-driving. In some cases, theROVs are equipped with manipulators, and can also beused to manoeuvre the valves for cementing.

The installation sequence ends with the removal ofthe temporary structures used only for the offshoreinstallation operation, such as: buoyancy tanks,temporary platforms to support the lifting slings andthe hydraulic control systems used to operate thesubsea valves required for flooding and cementingoperations. Until a few years ago, these removaloperations required teams of divers, who had to workunder saturation conditions due to the depths, thus

forced to spend days afterwards in a hyperbaricchamber built on board the installation vessel.Technological developments, and the need to work inincreasingly deep waters, has allowed the design ofspecial ROVs. As well as allowing for the observationof operations, these can also work underwater, thanksto mechanical arms and manipulators. Today, diversare used only for work in shallow waters, whereasoperations at depth are mainly carried out byunderwater robots.

Transport and installation of support structures:gravity structures

Given their enormous size and weight, gravitysupport structures cannot be transported by barge,but must be floated. After leaving the dry dock, thestructure is attached to a sufficient number of tugs,which transport it, floating in a vertical position, to asheltered area not far from the construction yard.Sheltered from the danger of adverse sea conditions,the topside modules are then installed, lifted by acrane barge from their transport barges andpositioned on top of the support structure. Once theinstallation of the modules is complete, these areconnected together, and the functioning of facilitiesis tested. The complete platform is then towed to theoffshore installation site.

After reaching its destination, the structure isslowly sunk by the gradual and controlled flooding ofthe watertight compartments built into its base. Thetugs, arranged around it in a circle, keep it undercontrol until touchdown has occurred. Its positionand orientation are monitored as described forjackets.

Once the structure has come to rest on the seabed,all the watertight compartments are flooded toincrease stability. To further increase its weight, somepurpose-built compartments at the base of thestructure are filled with inert materials.

Transport and installation of the topsideThe deck and the modules containing topside

facilities are transported from the construction yard tothe offshore installation site on special barges, as wehave seen for jackets transportation. The considerablemasses to be transported require the construction ofgrid structures to spread the loads over the deck of thetransport vessel, at the points where the topside rests.After completing loading operations, elements areinstalled to ensure that the structures remain integralwith the barge during transport. The barge is then



towed to the offshore site, where the vessel used forinstallation is already in place. When weather forecastsare favourable for the coming hours, the barge ismoored to the crane barge.

Over the past decades, significant developmentshave taken place in the construction of installationvessels. New vessels have appeared on the marketequipped with cranes of high capacity and highstability hulls, which can also operate under conditionswhen the sea is not perfectly calm. The most powerfulvessels have a pair of cranes which work in tandem,and semisubmersible hulls which considerablyincrease their draft during operations, thus allowingthem to remain extremely stable and relativelyunaffected by wave motion.

The lifting slings, pre-installed at the constructionsite, are linked to the crane’s hook (or hooks, with twocranes lifting in tandem), and the connecting elementsare removed and the crane can lift the load. When thestructure is completely free, the barge is unmoored,and towed away from the operation zone. The cranebarge, with the load suspended from its hook,subsequently approaches the substructure (jacket orgravity platform), onto which the deck or module is tobe installed (Fig. 11). When the vessel is in position,operations to lower the deck onto the substructurebegin; to ease correct positioning, the design includespurpose-built conical sleeves or guides which allowthe installation of the structures to be completedwithin the requisite dimensional tolerance.

The installation vessel generally works whileanchored to its mooring system, consisting of anchorsand steel cables. Modern ships of higher capacity alsohave a dynamic positioning system which allows the

vessel to maintain a predetermined position withrespect to a satellite control system autonomously, byactivating and orientating propeller thrusters. Thisspeeds up installation operations, since the vessel canoperate without being moored.

After completing the installation of the deck, andany further modules forming the topside, it isnecessary to link the substructure and the deck, andthe deck and the other modules. Between the deck andthe substructure it is necessary to weld the legs andjoin together corresponding pipes. Connecting thedeck to the other modules is more difficult, especiallywhen the processing plant is divided between severalmodules. As well as the welds, required to guaranteestructural continuity, it is needed join up all the pipesand install all the electrical and instrumentation cableswhich pass between the modules. This phase is themost critical, since it is extremely expensive and time-consuming. The primary objective of a good project isthus to reduce these post-installation operations to aminimum, with an optimal subdivision of platformsystems between the various modules. The availabilityof crane barges of ever greater capacity has alsoallowed us to design and construct increasinglyintegrated topsides over the years. Once all the workneeded to connect the modules to one another hasbeen finished, the topside facilities are complete andready for the subsequent commissioning and start-upactivities.

An alternative installation methodology to liftingwhich allows us to overcome the limitations resultingfrom the maximum capacity of crane barges is thefloat over. This procedure involves transporting thedeck on a barge which is narrower than the spacebetween the structure’s legs, so that the legs areoutside the external sides of the vessel. The barge mustbe towed and positioned inside the previously installedsupport structure, so that the legs of the deck are in



Fig. 11. Installing the deck: lifting with a crane barge (Eni-Saipem).

Fig. 12. Installing the deck: positioning on the supportstructure by float over (courtesy of ARUP Energy).

line with the legs of the structure on which they willcome to rest (Fig. 12). At this point the topsidestructure is lowered until it comes to rest on thesupport one. The structure is lowered using sand jacks,and by ballasting the barge. After completing themating operation, the barge continues to be ballasteduntil it is completely free of the structure above, andcan thus be towed away from the platform. Thisapparently simple operation is in fact extremelydelicate, and heavily affected by weather conditions,which must be absolutely favourable. However, it doesallow us to construct extremely large integratedtopsides.

5.2.3 Development of marginal fields

Characteristics

Most large offshore hydrocarbons basins have nowentered the phase of maturity, having been inproduction for several decades; at the same time, newdiscoveries of large fields in shallow waters havebecome an absolute rarity. The technological solutionswhich will be discussed briefly in this section aim toexploit small geographically dispersed hydrocarbonreserves in shallow waters, whose development wouldnot be economically viable, or would entail excessiverisk, were some specific expedients – which havebecome available only in recent years – not used.Since reservoirs of this type are characterized by beingon the borderline of development viability, they aredescribed as marginal, dependent on the levels of oiland gas prices, development costs linked to theproperties of the field to be produced, and the level ofeconomic risk which the operator is able to sustain.Generally speaking, marginality is linked to two basicfactors: the small amount of hydrocarbon reserves inthe reservoir concerned, and its distance from otherexisting installations. Development potential dependson our ability to contain expenditure as much aspossible, and the need to reduce to a minimum thetime required for the transition from the engineeringphase to the operational phase.

The time factor is crucial for the development ofa marginal field: each year of delay in the start ofproduction reduces the field’s economic value.Consequently, once it is decided to develop amarginal field, this must be done as quickly aspossible, and the production period must be as short

as possible in order to allow a rapid return of theinitial financial exposure. Another factorcharacterizing marginal fields is the greateruncertainty of our knowledge of reservoir properties,given the economic impossibility of undertakingexpensive data collection campaigns.

The technological solutions used to develop amarginal field must therefore allow us to: minimizethe expenditure and time required for development byidentifying simple solutions, as standardized aspossible, which reduce the need to treat thehydrocarbons offshore to the minimum; reuseproduction facilities for another field once thereservoir is depleted, since investment expenditureoften cannot be repaid by their use on a singleplatform.

Development typologies

There are three basic typologies for thedevelopment of marginal fields, which meet theseconditions: small fixed platforms, which are light andunmanned, with a minimum of topside facilities;subsea production systems linked to floating treatmentplants; subsea production systems linked to existingplatforms.

Fixed platformsOn small fixed platforms, clearly advantageous

for very shallow waters (maximum 50 m), thetopside must be as light as possible. Consequently,treatment and support facilities must be reduced to aminimum.

Thanks to the simplification of facilities,operations can be controlled remotely from anotherplatform or from the shore. There is thus no need for ahuman presence, and therefore for accommodation, onthe platform. Some of the technological expedientsallowing us to minimize the number of facilities to beinstalled are: the use of multiphase fluid pumpingtechnologies, allowing water-oil emulsions to betransported in pipelines, and thus avoiding the need toseparate these on the platform; the reduction to aminimum of electricity consumption, allowing us touse alternative energy sources such as photovoltaiccells or wind generators; the simplification of the fire-fighting system, made possible by the lack of a humanpresence on the platform. Simplifying facilities alsoallows for considerable structural reduction, and thus afurther decrease in the total mass of the topside, easingthe construction, transportation and installation phases



and thus significantly reducing the time andexpenditure required.

Supporting substructures also benefit from thelightness of the topside module. These are constructedin steel, in the shape of either a four-legged frameworkor a tripod with only three legs. In very shallowwaters, the substructures may consist of only a singlesteel column of large diameter (3-5 m), on which weinstall the production topside, and inside which thewell conductors and the export pipelines are placed(Fig. 13).

To guarantee an economic return on investments,we must be able to reuse the structures on other fields.Their design must therefore take into account possiblefuture use, and allow them to be easily removed andreinstalled with minimal modifications.

Floating platformsIn deeper waters it becomes preferable to use

floating treatment plants linked to subsea

production systems; this is also the solution mostfrequently adopted for the development of largefields in deep waters (see below). For reasons ofcost, production facilities are generally housed onconverted oil tankers, which also allow the storageof the crude oil produced. The crude oil isperiodically transferred to tankers moored near thefloating platform, and then transported to onshoretreatment plants.

The subsea production system consists of a steelstructure (template) installed on the seabed andanchored with foundation piles. The structure isinitially used as a guide for the drilling of wells.When drilling has been completed, the productionmodule is installed on the template; this contains thesubsea wellheads, a manifold to collect the fluidsproduced, and the control system for the wellheads.The production module is then protected by aframework structure which prevents potential damageto facilities caused by fishing or objects droppedfrom the surface.

The tanker converted into a production facility isanchored near the subsea template. The risers are theninstalled; these are the pipelines which allow thehydrocarbons to rise to the surface facilities from thewellheads (see below). Given the limited depth of theseabed, the risers must be made of flexible materials,so as to work with modest radii of curvature and thedynamic motion of the vessel to which they are linkedwithout being damaged.



Fig. 13. Development of marginal fields:simplified support structure (Eni-Saipem).

subsea pipelines

existing platform

control umbilical

subsea productionsystem

Fig. 14. Development of marginal fields: subsea production system connected to an existing platform.

Other flexible cables of small diameter(umbilicals) link the surface vessel to the wellheads onthe seabed; inside these are the electrical controlcables and the small tubes which transport the fluidsneeded to manoeuvre the valves on the wellhead andany chemicals needed during the start-up of the wells.

Existing platformsWhen the marginal field is near an already

producing platform, we can avoid using a floatingtreatment plant by linking the subsea productionsystems to the existing platform using one or morepipelines to transport the reservoir fluids and anumbilical to control the subsea systems (Fig. 14). Inthis case, we usually need to modify the existingfacilities on the platform to increase their treatmentcapacity. Since the reservoir fluid is transportedwithout prior treatment and is highly corrosive, thepipeline cannot be made of simple carbon steel, butmust be made of special corrosion resistant steel, orcarbon steel plated inside with suitable alloys.Furthermore, to avoid the pipeline being blocked bythe formation of waxes or hydrates on its walls whenthe untreated reservoir fluid cools to below giventemperatures, we need to provide it with externalthermal insulation. For all these reasons, the pipelinebecomes extremely expensive, and it is economicallyviable to develop the marginal field in this way only ifthe subsea wells are installed no more than a fewkilometres from the existing platform.

5.2.4 Development in very deep waters

Generals

The drastic decrease in discoveries of new fieldson the continental shelf, and thus in shallow ormoderately deep waters, has led offshore petroleumexploration towards very deep waters. The difficultiesfaced in designing and installing production platformsat depths ranging from 300-400 m up to over 2,000 mare easily understood. From a structural point of viewit is impossible, and economically untenable, to extendthe typological solutions adopted for shallow waters tothese great depths. We must abandon the supportstructure which rigidly resists the loading of waves andcurrents; whilst the platform must remain anchored tothe seabed, less rigid structures must be used, allowingfor considerable displacements, and consequentlyadaptation to wave motion.

If we have a surface production platform which isnot rigidly connected to the seabed, however, thisintroduces the problem of bringing the fluids to the

surface inside pipelines which are sufficiently flexibleto withstand the stresses caused by large displacementsand the direct action of waves and currents.

Great depths also mean high hydrostatic pressuresand low temperatures. To obtain a high degree ofthermal insulation insulating materials with lowdensity and thermal conductivity must be installed onthe pipelines. However, low density materials are alsopoorly resistant to high pressures, thus making itnecessary to find new technological solutionscombining thermal insulation with resistance to highpressures. From an operational point of view, floatingdrilling rigs, able to maintain their position andoperate without needing to be anchored had to bedesigned. As far as the construction of productionfacilities is concerned special vessels for theinstallation of structures and pipelines, and to carryout subsea operations at great depth needed to be built.

Here we will first describe some typologicalsolutions suited to very deep waters. These, whilstguaranteeing considerable elasticity, low oscillationfrequencies and thus adaptability to the dynamics ofwave motion, nevertheless keep the displacements ofsurface facilities within values allowing for theinstallation of surface wellheads and the use oftraditional completion and workover technologies,similar to those used on rigid platforms. We will thensee how, in some cases, the flexibility of this system andthus its displacements are such that this solution cannotbe adopted; in this case the surface treatment plantsmust be coupled with production systems with subseawellheads and non-conventional well completionsystems. Finally, we will briefly describe the differenttypes of risers, in other words the pipelines which allowthe transfer of reservoir fluids from the seabed tosurface facilities in very deep waters.

Platforms in deep waters with surface wellheads

Compliant towerConventional platforms in moderately deep waters

are so rigid that the structure’s natural oscillationperiod is lower than that of the most powerful waves,thus avoiding phenomena of resonance. To apply thesame principle at great depths (with equally highrigidity), we would need structures of such enormoussize and weight that they would be impossible to build,as well as economically unviable.

The compliant tower was designed to resolve thisproblem. This is a platform similar to a conventionalplatform, with a metal framework support structure ofjacket type, but very narrow and with far lowerrigidity, and thus with natural oscillation periods muchhigher than those of conventional rigid platforms, even



higher than those of the most powerful waves. In thisway we avoid dangerous phenomena of resonancethanks to oscillation periods higher than those ofwaves, rather than lower as is the case for conventionalrigid platforms.

The structures and topside facilities are identical tothose on a conventional platform, including drillingtechnologies and access to wells from the surface. Thesupport consists of a steel framework structure ofsquare section, similar in type to a jacket (see above).Its low rigidity is obtained by keeping the section ofthe framework very narrow in relation to the depth ofwater, and above all by creating a joint in the structurewhich forms a hinge whose rotations are controlled byresistant members that act as springs.

This structural hinge may, for example, be built ata certain distance from the base of the structure. Inthis case, the framework is constructed in twosections: the lower base section is a rigid structureidentical to a conventional jacket, whose foundationsare created using piles in an equally conventionalway; the upper section, much taller, has a squarecross-section of constant size along its entire height,and rests on the lower section (Fig. 15). The four legsof the upper section end in pins, which, duringoffshore installation, are inserted into the legs of thelower section; the two sections are linked by concretecasting to seal the gap between pins and legs. Thestructural hinge is built into the lower part of theupper structure (tower). The rotations of the upperpart of the structure around the hinge are controlledby eight steel tubes (two per corner) which run mostof the way along its length. When the tower oscillatesin one direction, the tubes on the opposite side comeinto traction, and act as springs which return thetower to a position of equilibrium. The greatadvantage of this solution lies in the fact that thesame technologies and structural typologies are usedas for conventional platforms. The structures are builtand installed in the same way as rigid platforms (seeabove).

As far as transportation is concerned, both sectionsof the structure are transported by barges to theinstallation site. The lower structure is launched, up-ended and installed on the seabed with the help of acrane barge suited to operating at great depths. Thefoundation piles are then driven in and cemented.After completing the installation of the lowerstructure, the upper structure is in turn launched andup-ended by sequentially flooding the watertightcompartments inside the legs, and then positionedabove the lower structure. After hooking up the twostructures, concrete is cast to join the two sections. Theseveral day period required for the concrete to set ishighly critical: since the upper structure is in acondition of partial stability, this phase of installationmust be undertaken in good weather conditions, andwith favourable forecasts for the entire duration of theoperation. After completing the installation of thetower, we proceed to install the deck and any othertopside modules, lifting them with a crane barge as wehave seen for platforms in shallow waters. During thedesign phase, we should take into considerationproblems linked to the large displacements (especiallyvery high fatigue, and the wear caused by constantrelative movements and friction between the steelpipes and their guides). Platforms of this type havebeen built on seabeds about 500-600 m deep. Atgreater depths, however, the solutions described beloware used.



topside

tower upper section

tower lower section

Fig. 15. Compliant tower.

Tension Leg Platform (TLP)In the platform known as a Tension Leg Platform

(TLP), the hydrocarbon treatment plants are housed ona special floating hull with excellent stability, which iskept in position by a system of vertical tendonsanchored to the seabed (Fig. 16).

The hull is made of steel, and consists of fourvertical columns of large diameter (about 20 m),

stiffened by internal longitudinal and circumferentialribs. The columns, placed at the corners of a squarewhose sides may be up to 80 m long, are connected atthe top to a structural platform, designed to supporttreatment facilities and occasionally drillingequipment. Facilities and living quarters are builtinside steel modules to be installed above the hull, asfor rigid platforms (Fig. 17). The wells are preferablypre-drilled using a drilling rig suitable for greatdepths, and then completed from the top of the TLP.

In this case we are dealing with asemisubmersible hull; once it has been ballasted inoperating conditions it has a very high draft, allowingit to remain extremely stable despite the loading ofwave motion. Since only the columns cut through thesurface of the water, this considerably reduces theloading exerted by waves on the structure. Protectionfrom corrosion is ensured by the presence ofsacrificial aluminium anodes on underwater parts,and by painting the areas exposed to the atmospherenear the waterline and the internal ballastingcompartments. The hull is anchored to the seabedwith 12-16 tubular steel tendons whose diameter islower than a metre, attached to the structure near thebase of the columns (3-4 tendons per column). Thesetendons are fixed to the seabed with foundation piles,which are driven into the ground to a depth sufficientto guarantee resistance to the high tractions releasedby the tendons. Thanks to the action of the tendons,maximum movements are contained within limitsenabling us to use well completion technologies andwellheads similar to those used on conventionalplatforms in shallow waters.

The difficulties in designing a TLP are mainlylinked to the correct simulation of the loading ofwind, waves and currents on the hull, the accurateevaluation of the system’s dynamic response, andthus the motion induced, the tensions in the tendons,and the minimal distance of the deck from the wavecrests. To refine the design and optimize its size, weshould always carry out trials on models in a tank. Inparticular it is essential to achieve optimization of thepretension in the tendons, obtained by selecting asuitable value for the hydrostatic lift on the hull. Thepretension must be high enough to ensure that thetendons always remain taut regardless of operatingconditions, but at the same time must not beexcessive, to avoid the hull becoming oversized.Once we have optimized the pretensions in thetendons and the height of the hull, these determinethe choice of the diameter of the columns and the



floating hull

risers

tendons

drilling templatewith tendonreceptacles

Fig. 16. Tension Leg Platform (TLP).

size of the pontoons linking these at the base. Itshould be remembered that the tendons and allelements subject to cyclical loads must be carefullydesigned against fatigue. During the design processwe should also optimize the horizontal dimensions ofthe hull, taking into consideration the layout andsafety requirements for hosting surface facilities.

The hull of the TLP and the modules containingfacilities are constructed at onshore yards equipped forbuilding conventional rigid platforms. Transportationis then effected: for this purpose a special vessel isused which, using a ballasting system, can immerse itsdeck so as to allow the hull of the TLP to float freelyon its base pontoons when it is to be launched. Thehull must then be transported near the site where it isto be installed and the topside modules integrated.Transport may occur on the vessel onto which the hullhas been loaded, or the hull may be allowed to floatfreely immediately after loading, and then be towed.Transport using a barge is more expensive, but muchfaster.

The construction, transportation and installation ofthe topside modules are carried out as for rigidplatforms, in particular gravity platforms. Themodules containing facilities may also be installedwhile the hull is temporarily anchored near the yard ina sheltered area of sea. The tendons are prefabricatedonshore in sections of length under a hundred metres,and then transported offshore where they areassembled using mechanical mating systems as they

are lowered into the sea in a vertical position. Thefoundation piles are prefabricated onshore, andtransported offshore as for the foundation piles of ajacket.

Offshore installation requires the presence of acrane barge. The first stage of this process involvesinstalling and driving the piles using a subsea pilehammer suited to great depths. After the piles havebeen installed, we begin to assemble the tendons inparallel on the two long sides of the crane barge. In themeantime, the TLP is towed to the installation site.The completely assembled tendons are left hangingfrom the sides of the crane barge, awaiting the arrivalof the TLP. The TLP is then moored to the crane barge,and the tendons transferred one by one and hookedinto place at the base of the columns of the hull. Thetendons are then lowered from the TLP until they canbe inserted into the relevant slots on the tops of thepiles or on the foundation template. The tendons arehooked to the piles with the help of submarine workvehicles controlled from the surface (ROVs). Once thetendons have been hooked into place, the ballast wateris pumped out of the TLP’s hull to bring the tendons



quarters module

power moduleprocess module

wellbay module

drilling module

Fig. 17. Tension Leg Platform (TLP):hull and topside.

Fig. 18. Mini-TLP.

into tension in accordance with project specifications.The installation of the TLP can now be consideredcomplete.

This type of platform represents a viable solution,and has already found a practical application inoffshore fields at depths ranging from 500 m to about1,200 m. However, it does not allow the crude oilproduced to be stored, so export pipelines are needed.Alternatively, the TLP can be used alongside a storagevessel (FSO, Floating Storage Offloading vessel) and aloading buoy to which the tankers connect to load theproduct. Both these units must be permanently moorednear the TLP, and linked to one another and with theTLP using pipelines, which are generally flexible, forthe transfer of products. Recently a type of TLP hasbeen built with a hull of completely different designfrom that described above, and which represents agood alternative especially for topside facilities ofmodest size and weight (Fig. 18). This type ofplatform, known as a mini-TLP, has a hull consistingof a single central column supporting the deck and itsfacilities. Three pontoons radiate out from the base ofthe central column. The tendons (six in total) areconnected to the ends of the pontoons.

Spar platformA different floating production system for deep

waters is the Spar (Fig. 19). The hull of the Sparconsists of a cylindrical tower structure about 25 min diameter and 200-250 m high, which floats in avertical position thanks to a special arrangement ofwatertight compartments. The structure is ofconventional naval type, in steel reinforced bycircumferential ribs and transverse and radialbulkheads. The bulkheads serve to subdivide the hullinto various watertight or floodable compartments,allowing it to float as desired. The inner central partof the tower is hollow, enabling it to hold theproduction risers which carry the hydrocarbons fromthe wells to the wellheads at the surface, and thenceto treatment plants. Each riser is kept taut by abuoyancy tank which is also installed inside thehollow centre of the tower. Once the tower has beenpositioned and ballasted in operating conditions, itfloats vertically with a draft equivalent to over 90%of its length.

The watertight compartments which provide thenecessary hydrostatic lift are concentrated in the upperpart of the hull. At the lower end, other watertightcompartments are built to keep the tower floatinghorizontally during the transportation and installation

phases. These are later flooded, allowing the hull toadopt its vertical operational position andcompensating for any offset in the centre of gravity ofthe structure and its topside facilities. Protection fromcorrosion is ensured by the presence of aluminiumsacrificial anodes on the lower parts, and by paintingareas exposed to the atmosphere near the water line,and the internal ballasting compartments. To preventthe vortex shedding generated by the passage ofcurrents along the cylindrical surface of the towercausing unwanted phenomena of dynamic oscillation,spiral strakes are installed on the outer surface alongits entire length, similar to those which can be seen onfactory chimneys.

The production plants and living quarters arecontained in one or more topside modules installed on



fairleadsstudless anchor chain

sheathed spiral strandwire rope

Fig. 19. Spar platform.

top of the tower, and built inside steel frames, as forthe topsides of rigid platforms. The wells are pre-drilled using a drilling rig suited to deep waters, andthen completed from the top of the tower.

The Spar is anchored with a mooring systemconsisting of a series of cables arranged in a circlearound the tower and tethered to the seabed usingpiles, or suction anchors. A suction anchor consists ofa cylinder about 5 m in diameter and ten metres tall,open at the base and closed at the top, to which thechain at the end of the mooring cable is attached. Thecylinder, once resting on the seabed, is driven into theground by creating a pressure inside which is lowerthan external pressure. This is done by extracting thewater using a system of subsea pumps placed on thetop of the cylinder itself. The mooring lines are steelcables, ending with sections of chain at both ends, andstretched so as to increase their rigidity and thusreduce the tower’s movements.

The design of the hull depends on modes oftransportation and installation, and on operatingconditions. Appropriate trials on models in tanks arecarried out to test the results of theoretical simulations.In order to define the optimal pretension to be appliedto the mooring cables, the mooring methodologies andan evaluation of the dynamic response of the systemare important. Fatigue checks are essential todetermine the size of the local structures to which themoorings are connected. The mooring system isdesigned to keep the tower in position even under theworst possible weather conditions.

Construction involves subdividing the tower into agiven number of sections. The sections are thenaligned and welded together in open air. Due to itsexcessive length, the structure is usually built andtransported in two sections whose size and weight issuch that they can be prefabricated inside coveredworkshops. These can be finally assembled in thewater, by joining together the two sections of the towerfloating horizontally at the quay of a yard near thefinal installation site, or on land in a dry dock.Construction procedures for topside modules areidentical to those seen earlier for rigid platforms.

Offshore installation takes place in two distinctphases, both requiring the assistance of a crane bargeand remote-controlled underwater work vehicles(ROVs). During the first phase, the moorings areinstalled. Each mooring pile is lowered to the seabedwith its mooring line already attached, and is driveninto the ground by a subsea pile hammer suited toworking at great depth. If suction anchors are used

instead of piles we proceed in the same way. Thesecond phase of installation begins by transporting thetower from the assembly yard to the offshoreinstallation site. The tower is towed by tugs, floatinghorizontally. After reaching the installation site, thewatertight compartments allowing the tower to rotatethrough 90o are progressively flooded until it reachesits definitive vertical position. The end of eachmooring line is then recovered from the seabed to thedeck of the crane ship, and transferred to the Spar.Once all the lines have been recovered, they are tightenuntil they reach the tension values specified during theengineering phase.

At this point the module or modules containingproduction facilities are installed, using methodsidentical to those seen earlier for rigid platforms. Thefinal operation is to lift the buoyancy tanks from thetransport barge and install these in the central part ofthe tower; the production risers are later attached tothese (see below). This type of platform represents avalid solution for deep waters, and has already foundpractical applications in a large number of cases indepths up to 1,700 m.

Recently, a platform named Truss Spar wasdesigned and built; this represents a development ofthe Spar (Fig. 20). The Truss Spar differs from theSpar in the lower part of the tower, which consists ofa steel framework instead of a cylindrical hull. Thewatertight compartment at the base of the tower,



topside

mean water level

hard tankstrake

mooring line

truss

riser

Fig. 20. Truss Spar platform.

required during horizontal transportation and toup-end the tower, is now obtained by installing aparallelepiped at the base of the frame. At the variouslevels of the framework are floor plates, which allowan improved damping of vertical movements. Theadvantages of the Truss Spar over the classic solutionare as follows: a) a saving of structural steel and thusreduction of construction costs; b) reduced totalheight of the tower; c) vertical movements reducedthanks to the presence of floor plates acting asdampers; d) lower loading of currents thanks to theframework which replaces a substantial portion of thecylindrical section; e) fewer vibrations caused byvortex shedding.

Floating production units coupled with subsea production systems

There is another class of floating productionsystems which represents an alternative or addition tothe production systems discussed earlier. Unlike thoseexamined previously, these do not allow thecompletion of the wells from the surface and theinstallation of surface wellheads; these units musttherefore be linked to subsea production systems andwellheads. These units are vessels which are eithersemisubmersible or have conventional hulls, on boardwhich the production facilities are installed. In thiscase, the wells, either individual or grouped on atemplate installed on the seabed, are pre-drilled beforethe installation of the production unit, and thencompleted with subsea wellheads. The subseaproduction system is created as for the development ofmarginal fields in shallow waters, to which we referfor a detailed discussion (see above).

The floating production unit is moored in a centralposition with respect to the subsea wells, to which it isthen joined by rigid or flexible pipelines which carrythe reservoir fluids from the wellheads to surfacefacilities (see below). Other flexible tubes of smalldiameter (umbilicals) link the subsea productionsystems to the surface units; these contain theelectrical cables and hydraulic fluids required tocontrol the wellheads from the surface and anychemicals needed during the start-up of the wells.

Semisubmersible unitsA production unit of semisubmersible type (Fig. 21)

is identical in structure to the semisubmersible drillingrigs (see above). Under operating conditions, the unitis ballasted until it reaches a considerable draft, and

therefore high stability and reduced motion due towave loading. The disadvantage of this type of hull isthat it has no storage capacity; pipelines for the exportof products are therefore needed, which are difficult tomake and very expensive for deep waters.Alternatively, the production unit may be used incombination with a storage vessel (FSO) and a loadingbuoy, moored near the semisubmersible productionunit.

Floating hullThe advantage of using a vessel with a

conventional hull is precisely that it has large holds inwhich to build storage tanks. In this case, theproduction unit is also used for storage (FPSO,Floating Production Storage Offloading vessel), and itis sufficient to hook it up to a loading buoy to ensurethe export of the fluids produced (Fig. 22). In this casethe hull may come from a converted tanker or be



Fig. 21. Floating production plants:semisubmersible hull (Eni-Saipem).

Fig. 22. Floating production plants:conventional FPSO hull (Equatorial Guinea, Ministry of Mines,Industry and Energy).

purpose-built. Purpose-built hulls are generally madeof steel, although some are made of reinforcedconcrete. In any case, the treatment plants, support andsafety facilities and living quarters are installed on themain deck.

An FPSO is shaped like a conventional ship, andthus has a preferential orientation with respect towaves, currents and winds. The most effective way ofmooring an FPSO is to use a rotating turret, to whichthe mooring cables are attached, and around which thevessel can rotate through 360o, thus keeping the bowinto the wind and significantly reducing the load onthe moorings (Fig. 23). Only where the production unitmust be installed in an area where intense wind andwaves come from a single quadrant can the vessel bemoored in a fixed position, with mooring cables at thebow and stern, and with the bow facing in thedirection of the prevailing winds. If a rotating turret isused, it can be installed outside the FPSO’s prow bymeans of a cantilevered structure or located inside thevessel. The external solution is more convenient if theturret is being installed on a converted tanker, whilstinternal installation is preferable if the vessel ispurpose-built: since the turret can be installed in anarea closer to the centre of rotation of the ship’smotion, the turret is subjected to lower loads. In anycase, the turret is a highly critical and expensivecomponent of this system, because in addition to themooring cables, the production risers must also bejoined to the same rotating system, and the transfer offluids to the manifold on the vessel must beguaranteed without the danger of leaks due toruptures.

The mooring system is an equally importantcomponent of a floating production system; there arevarious alternative solutions. A conventional mooringwith a catenary cable system loses rigidity in deepwaters and thus causes the ship to move considerably,as well as requiring extremely long cables with highweight, which are therefore very expensive. In order toincrease rigidity whilst simultaneously reducing thelength of the cables, it is preferable to use a mooringsystem with shorter cables that must therefore beextremely taut. Traditionally, a mooring line consistsof a steel cable with sections of chain at both ends, andwith the lower end joined to the anchor. If shortercables are used, the relatively lower length of themooring lines and the high loads to which they aresubjected exert significant vertical loads on the seabedanchoring system. In this case, traditional anchorsmust be replaced with piles or suction anchors, as we

have described above for the mooring of the Spar (see above).

As far as the mooring cables are concerned, cableshave recently been manufactured in synthetic fibres,with the production of increasingly light and resistantprototypes. Polyester fibre in particular has revealeditself to be a good alternative to steel cables for use invery deep waters: however, the disadvantage of thesecables is that they are less resistant to abrasion, andpotential surface cuts; in this case, too, the ends mustbe made of chains.

Designing the hull presents the problems typical ofship design, whilst that of the treatment plants followsthe same criteria adopted for rigid platforms. Indeed,the procedure is often less complex, since the deckareas available for installation are larger, and there arethus fewer constraints on facilities. In this case, too,for safety reasons the layout of facilities is governed



Fig. 23. FPSO: mooring system with turret and risers (A) and the detail of the turret (B).

buoyancymodules

riseranchor pile

B

A

by the attempt to segregate the dangerous processingareas and flare in an area as far away as possible fromliving quarters, generally with utility systems inbetween. This is easier for facilities on FPSO’s, thanksto the length of the vessel: accommodation is locatedat one end (bow or stern), with the processing area andflare at the other end. If the vessel is purpose-built, theaccommodation is placed at the bow (in the moresecure windward position), whereas in convertedtankers accommodation is kept in its original positionat the stern. Due to rolling and pitching motions,special attention must be devoted to the design ofprocessing plants, and to fatigue-assessment; if thevessel is converted, the fatigue check must also takeearlier use into account.

As far as construction is concerned, building anFPSO does not present particular problems. The hull,both for converted tankers and newly constructedvessels, is built in a traditional shipyard. The treatmentplants, on the other hand, are built as the typicalmodules of offshore platforms, and thus in a differentyard. Once finished, the hull is sailed to the quay ofthe yard where the facilities modules have beenconstructed, and moored. Here the modules areinstalled and integrated on the deck. Once integrationis complete, and we have checked that the facilities arefully functional, the vessel can leave the yard and betowed to the offshore installation site. The mooringsystem is installed and rested on the seabed before thevessel arrives. As soon as the production unit reachesthe installation site, the moorings are recovered, andattached one by one to the vessel, as seen for the Sparplatform (see above).

We then install the production risers and umbilicalswith the devices to control the wellheads, which arelayed from an appropriate vessel (see below), andlinked to the ship and the subsea production system.

The problem of exporting the crude oil producedand stored in the holds of the FPSO is resolved byplacing a loading buoy next to the production vessel.The buoy is moored near the FPSO and linked to it byflexible or rigid steel pipelines; the buoy also has amooring line and a flexible tube, to the end of whichthe tankers which come to collect the crude oil areconnected.

The flexible pipelines which transfer crude oilfrom the FPSO to the buoy are highly complex andexpensive since, to resist the high pressures and loads,they are made by superimposing numerous layers ofspecial plastic materials, alternating with metallicspiral armour. The solution using rigid steel tubes ismuch cheaper, but presents considerable problems dueto the high dynamic loads and fatigue caused by therelative motion between FPSO and buoy. To reducethese effects, the rigid tubes cannot be installed with a

natural catenary configuration, but must follow aspecial w-shaped configuration, obtained by installingbuoyancy modules along the middle section.

Risers

In deep water developments, the substantialdistance between the seabed and production facilities,and the fact that these facilities are themselves hostedon non-rigid structures and are thus subject tosignificant displacements, cause considerableproblems in the construction of the pipelines (risers)which transport the reservoir fluids from the wells tothe surface.

As already mentioned, the displacements of TLPand Spar platforms as a result of waves and currentscan be kept within limits allowing the installation ofsurface wellheads and the use of vertical steel risers.These limited displacements can be absorbed by thelength of the pipeline and the flexibility of the steelwithout the need for special materials or geometricalconfigurations, required for floating productionsystems.

During production, the vertical risers act as guidesfor the succession of tubing elements, which carry thehydrocarbons from the reservoir to surface facilities;they also function as containment facilities if leaksoccur within the tubing. During well workover andreservoir stimulation activities, the riser also functionsas a guide for the equipment which must be loweredinto the well from the surface.

Each riser links a well to its wellhead at thesurface, and consists of a steel pipe about 25 cm indiameter, thick enough to resist the pressure andstresses induced by the static and dynamic loads towhich it is subjected. The riser is installed directlyfrom the platform, using a traditional drilling derrick,and joining together prefabricated sections ofpipeline. The first tube is lowered into the water, andthis is then joined to the second using a specialthreaded joint, the pair of tubes is lowered, and thethird tube joined to the top, and so forth until theentire pipeline is complete. When it reaches theseabed, the pipeline is joined to the head of the pre-drilled well using a special connector. Due to highbending stresses and fatigue the lower part of theriser, near the connector, is made of special materialssuch as titanium alloy special flexible joints can alsobe used.

Each riser must be kept suitably taut both tosupport its own weight and to control its dynamicbehaviour; tension is exerted from the surface usingdevices attached to the platform hull or steel buoyancytanks surrounding the top of the riser. The weight ofthe riser in water may nevertheless be reduced by



installing buoyancy modules made of specialpolyurethane foam around some sections of thepipeline. To reduce the loads due to the movements ofthe platform, the tops of the risers are not rigidlyattached to the structure but using purpose-builtdevices that allow relative movements. The wellheadsmust then be joined to the manifold which channelsthe fluids produced to the treatment plants throughflexible pipes.

The solution using vertical steel risers, however,cannot be adopted for floating production systems, inother words semisubmersible platforms or FPSOs; inthese cases the displacements of the hull due to wavesand currents cannot be absorbed by the flexibility ofthe vertical steel risers, the wellheads cannot beinstalled on the surface, and we must therefore usesubsea production systems. The risers must havegeometries and be made of materials which ensure thatthey are sufficiently flexible to absorb without damagethe considerable displacements at the top, and, at thesame time, resist high fatigue. The main solutions areas follows: a) flexible risers; b) rigid steel risers; c) vertical steel risers grouped inside a tower notsupported by the production hull (riser tower); d) independent vertical steel risers not supported bythe production hull.

In all cases, thermal insulation is needed tomaintain the temperature of the reservoir fluids abovegiven minimum values, to prevent hydrates or waxesforming and being deposited on the walls of thepipeline.

Flexible risersThe first floating production systems were

constructed to exploit marginal fields in shallowwaters. Here, given the limited length of the pipelines,flexible risers were needed. With the increase in deepwater applications, this solution was also extended togreat depths. It is necessary to develop a pipelinewhich, though resistant to high internal and externalpressures, can adopt modest radii of curvature withoutbeing damaged. To this end, the wall of the tube isformed of a series of concentric layers: a first internallayer, in contact with the fluid, is a thin sheet ofstainless steel; outside this are alternating layers inspecial polymers and spiral steel armour (Fig. 24).Given the complexities of manufacturing, productioncosts are extremely high.

Thanks to its flexibility, this type of riser can beinstalled with a catenary configuration, with the upperend suspended from the production facilities and thelower end resting on the seabed until it is joined to thesubsea production system. The greatest problems arelinked to the difficulties of guaranteeing that theproduct lasts for the entire life of the platform.

Especially critical from this point of view are fatigueand the movements which generate abrasion betweenthe different layers of the tubing.

The tube is manufactured in special yards on thecoast. Risers of limited length and small diameter canbe wound onto purpose-made reels, which are thentransferred onto the installation vessel. For very deepwaters, and therefore considerable lengths, the flexiblepipeline is loaded directly on board the vessel, woundaround large diameter drums of vertical axis. Theinstallation vessel carrying all the flexible pipelinescan then sail to the offshore site, where the risers arelayed extremely quickly, by unwinding the reel or thedrum around which they are wrapped.

Despite high production costs, this type of riser hasgiven good practical results. The benefits of rapidinstallation and the consequent savings in terms of timeand money are countered by the fact that theinstallation vessel in most cases has to load the productdirectly at the production site, which is often at aconsiderable distance from the offshore installationsite. Although flexible pipelines have lower thermalconductivity than non-insulated metal pipelines, theyare not particularly suited to applications where a highdegree of thermal insulation is required.

Rigid risersTo reduce the cost of flexible pipelines,

technological solutions have been developed over thepast decade for the construction of rigid steel risers.Designing and installing a rigid riser is far morecomplex than a flexible riser, particularly ifthe installation site is in a geographical area whereenvironmental conditions are especially severe. In particular, we need to take into consideration the



tensil armour layer

spiral armour layer

pressure armour

carcass layer

external layer

anti-wear layer

anti-collapse layer

pressure sheat (polyamide)

Fig. 24. Flexible riser: section showing different layers.

dynamic response of the pipeline, subjected to themotion of the vessel to which it is attached and to theloading of waves and currents. The least expensiveconfiguration is a simple catenary, which can be usedin very deep waters and in sites where environmentalconditions are fairly favourable. In this case theweight of the pipeline is entirely supported by thehull to which it is connected with a special flexiblejoint. At the lower end, the riser is placed on theseabed for a section sufficient to ensure that thepipeline remains attached to it thanks to the frictionalforce between pipe and ground. The contact pointbetween the pipe and the ground moves freelyaccording to systerm dynamics. The bottom end ofthe riser is then linked to the subsea productiontemplate using a section of pipeline, which is usuallyflexible, flanged at both ends. Where it is notpossible to use a simple catenary geometry, due toparticularly severe weather conditions or aninsufficiently deep seabed, we need to use a ‘wave’configuration. In this case, the natural catenary of theriser is modified at a given depth, by installingbuoyancy modules along the pipeline which force theriser to adopt a wave configuration, and thencontinue with a catenary until it reaches the seabed.The configuration thus obtained allows the pipelineto have a dynamic response, so that it is subjected tolower loads (Fig. 25).

Resistance to fatigue is another highly criticalaspect of the design of rigid risers. Fatigue is causedby three factors: wave motion, the motion of thevessel, and the vibrations induced by the vortexshedding created by the passage of currents. As far as

vortices are concerned, we need to install helicalstrakes on the upper part of the riser (most exposed tocurrents), to prevent vortex shedding. The most criticalparts of the riser in the context of dynamic loads andfatigue are those near the link to the hull and thosenear the point of contact with the seabed; at thesepoints, very thick steel and particularly careful weldsshould be used. Using titanium alloy for the entireriser, thanks to its properties of resistance and highelasticity, would allow us to reduce the thickness of thepipeline considerably, and a corresponding saving ofmaterials. However, this solution has not yet beenadopted given the high costs of titanium, and thecomplexities of welding techniques.

As far as installation is concerned, three differentmethodologies are used, each of which in turnconditions onshore manufacturing techniques: layingfrom an installation vessel, using the method known asJ lay; laying from an installation vessel, using themethodology known as reel lay; floating transport andup-ending at the installation site.

The J lay methodology is considered the mostsecure, although it is more time-consuming: thepipeline is constructed offshore by welding togethersections of tube previously assembled in sectionswhose maximum length is about 50 m. The firstsection of pipeline is inserted into a tower on theinstallation vessel, and is then lowered into the sea in avertical position, keeping the top above water, to allowa second section of pipeline to be inserted into thetower. At this point, the top of the first section and thebottom of the second section are welded together; thetwo joined sections of pipeline are in turn lowered intothe water, allowing us to install the third section, andso on until the riser is complete. Once the layingoperation is finished, the head of the riser with itselastic joint is placed in the water by lowering a cablelinked to a winch. A second cable linked to a winch onthe production vessel recovers the head, which is theninserted into a slot in the hull.

In the reel lay methodology, the pipeline is entirelyprefabricated onshore, and then wrapped around awide reel on the installation vessel. Although the reelhas a large diameter, the pipeline suffers considerableplastic deformation during the reeling operation. Whenthe vessel reaches the installation site, the reel rotates,releasing the pipeline, which is lowered into the water.The pipeline, deformed by loading, must then bestraightened by imposing an opposite deformation.The need to introduce these plastic deformationsmakes this procedure quite risky for the installation ofrisers, since their effect on fatigue resistance has notyet been fully evaluated.

In floating transportation, the riser is completelyprefabricated at an onshore yard, as near as possible to



FPSO

steel riser

floatingbuoys

riser base

Fig. 25. Steel riser: ‘wave’ configuration.

the offshore installation site. The complete pipeline isthen launched directly from the yard, by sliding italong purpose-built roller systems and exertingtraction from the sea using tugs. Once in the water, theriser can be transported using a series of tugs. Uponreaching the installation site, the pipeline is sunkcompletely, and the head is then recovered by apowerful winch on the production vessel. Thismethodology is very risky since the behaviour of theriser during transportation is difficult to predict. Highcosts are also incurred by the need to create a specialarea of the yard for prefabrication and to construct alarge number of temporary buoyancy modules able toresist high hydrostatic pressures, and which must bedisconnected from the pipeline before its finalinstallation.

As far as thermal insulation is concerned, rigidrisers can be treated on the surface with coatingsresistant to high pressures; however, it is necessary forthe low thermal conductivity requirements not to beparticularly restrictive. In the case of catenary rigidrisers, the possibility of using pipe in pipe, the mosteffective technological solution currently available toguarantee high thermal insulation, is as yet unproven.The pipe in pipe consists of a double concentricpipeline: the inner pipe has the task of resisting thepressure of the reservoir fluid; the outer pipe mustresist the extremely high hydrostatic pressures causedby great depths. Insulation is guaranteed by insertingmaterial with excellent thermal insulation properties inthe gap between the two pipes.

Riser towerThe motion of the production vessel has an

enormous influence on the dynamic behaviour of therisers connected to it. The riser tower provides asolution allowing us to decouple as far as possible theproduction vessel’s motion from that of the riser. Inthis case, the steel risers which carry the reservoirfluids from the subsea wells to the surface arecollected inside a cylindrical tower, also made of steel,hinged to a base foundation (Fig. 26). The tower is keptin a stable vertical position by the hydrostatic lift of alarge watertight cylindrical tank connected to its top; ittherefore does not need to be supported by theproduction vessel. The foundation consists of acylinder of large diameter which acts as a suctionanchor (see above). The height of the riser tower isslightly lower than the depth of the seabed on which itis installed, so that the top of the buoyancy tankremains submerged at a sufficient depth from thesurface to avoid the effects of wave motion. Theconnection between the risers and the productionvessel is guaranteed by installing a bundle of flexibletubes which, in a catenary configuration, link the

flanges on the vessel’s side to the correspondingflanges at the tops of the risers, at the head of thetower. Thanks to this configuration, the motion of thetower and the vessel are relatively independent, sincethe two systems are linked only by highly flexibleelements. The fact that a considerable number of risersare grouped inside a single tower also allows us toobtain a high degree of thermal insulation. We caninstall special insulating foam with very low thermalconductivity values inside the tower, in the gapsbetween the tubes.

Due to the dimensions of this structure (whichmay be considerably above 1,000 m in length), thetower, including all its internal risers, must beconstructed onshore, near the coast and perpendicularto it. Once complete, the tower is launched by slidingit along special rollers and exerting traction from thesea using tugs. It is then floated a few metres belowthe surface of the water; after reaching the installationsite it is slowly up-ended by flooding the tubes inside,



FPSO

suction anchor

flexible lines

risers tower

buoyancy can

Fig. 26. Riser tower.

disconnecting the temporary buoyancy modules andkeeping the upper end connected to a tug. After up-ending is complete, the base of the tower isattached to its foundation, installed previously. Wethen install the buoyancy tank, which is placed in thewater by a crane barge, allowing its watertightcompartments to flood freely. Finally, the flexibletubes connecting the risers and the production vesselare installed. Pipelines, rigid or flexible, are placedon the seabed to connect the subsea wellheads to theflanges of the corresponding risers at the base of thetower and thus allow the hydrocarbons to flow intothe risers.

The riser tower, as well as resolving the problem ofrigid risers attached directly to the vessel, also has theadvantage that it can be constructed and installedindependently from the production vessel, whoseconstruction is particularly critical in terms of time.The installation of the riser tower, subsea productionsystems and the pipelines connecting these to thetower can occur before the production vessel reachesthe installation site. The only activity needing to beundertaken after mooring the vessel is the installationof the flexible tubes connecting it to the tops of therisers.

The negative aspects of this solution are thedifficulties of onshore prefabrication and limitedflexibility with respect to possible evolutions in thedevelopment of the field. Grouping the risers inside asingle structure introduces considerable inflexibilitywith respect to any modifications required during laterphases of development, or potential maintenance work.

Independent vertical risers not supported by the production vessel

A development of the riser tower concept involvesinstalling the vertical steel risers individually, ratherthan grouped inside a tower. Each riser is hinged to itsown base foundation, is kept taut by its own buoyancytank and is connected to the production vessel by itsown flexible tube (Fig. 27). This solution has all theadvantages of the riser tower, and at the same timeeliminates its negative aspects. The fact that the risersare installed independently offers maximum flexibility,since the risers associated with the drilling of futuresubsea wells can be installed when they becomenecessary, without requiring any advance operations.The paths followed by the connecting tubes to thesubsea production systems are also simplified, sincethese do not all have to converge towards the base ofthe riser tower, but simply reach the base of their ownrisers. Nor do we need a large onshore prefabricationarea, since individual risers can be constructedoffshore, using the J lay method for laying pipelines indeep waters (see above).

The solution using independent vertical risers isalso suitable for cases where a high degree of thermalinsulation is required since the pipe in pipe conceptcan be used for the riser tubes (see above).

The only potentially critical aspect is represented bythe risk of interferences and collisions between therisers, caused by their independent motion. A largenumber of studies, simulations and trials using modelshave nevertheless led to the conclusion that the riserstend to move in synch with each other, thus avoiding therisk of interference. The first application of this solutionhas been installed for a seabed about 1,300 m deep.

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Alliot V., Carrè O. (2002) Riser tower installation, in:Proceedings of the annual Offshore Technology Conference,Houston (TX), 6-9 May, OTC 14211.

API (American Petroleum Institute) (1993) Recommendedpractice for planning, designing and constructing fixedoffshore platforms, Washington (D.C.), API.

Dorgant P.L. et al. (2001) System selection for deepwaterproduction installations, in: Proceedings of the annualOffshore Technology Conference, Houston (TX), 30 April-3 May, OTC 12966.

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Edel J.C. et al. (1999) Fabrication of the Baldplate compliant



independent risers

Fig. 27. Independent risers not supported by the production vessel.

tower, in: Proceedings of the annual Offshore TechnologyConference, Houston (TX), 3-6 May, OTC 10918.

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Fabio PallaviciniEni - Saipem

San Giuliano Milanese, Milano, Italy



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5.2 Development of offshore fields - Treccani

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