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Chapter 1
Introduction
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Content
General considerations
Platform functions and types
Historical background
Design process
Standards and regulations
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General considerationsOffshore engineering is a branch of civil engineering involved with the construction of sea structures, far form the coast.Such structures are normally called platforms. They are usually employed in oil industry, but there are also platforms for broadcasting, navigational lighting, radar surveillance, space operations, oceanographic research and so on.The general design requirements for an offshore platform are similar to any industrial structure; the first step in the design is to develop a concept of the structure based on its functional requirements, environmental restraints and construction method. The function of an offshore platform is to provide a secure working support, so the platform must be structurally adequate to withstand both operational and environmental loading; in addition, it must be practical to construct and economically feasible.Nevertheless, due to their characteristics, to environmental andgeographic aspects, to the construction procedures, offshore platforms are very peculiar structures, and several unusual aspects must be taken into account in its design phase: environmental loadings, construction phases, economic aspects, linked to the type, construction procedure and date and to the installation site.
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Platform functions and typesDuring the last five decades, several types of offshore platforms haves been designed and used; this variety of platform types is due to different factors: technological and scientific progress, economical factors, need to exploit deeper natural reservoirs, ecological constraints.A possible, of course incomplete, classification is the following one.
jack upsemi-submersibledrilling ship
Mobile offshore platforms
TLP
free standing towerguyed towerspar towercompliant
concrete gravitysteel gravity
steel jacketsteel tower
rigid
Fixed offshore platforms
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Platform functions and typesFixed offshore platforms are normally used for production, whilemobile platforms are almost exclusively used for exploration anddrilling phases.The difference between fixed and compliant platforms is in the way they face environmental (namely wind and wave) lateral actions.As its name clearly indicates, a fixed platform is a traditionalstructure, in the sense that its deformation under lateral loads is small, but it is located into the sea water.Unlike fixed, compliant platforms are designed to move under lateral forces, so that the effects of these forces are mitigated. The trade-off in a compliant platform is between excursion amplitude and restraining force. Compliant platforms are used in deep water, where the stiffness of a fixed platform decreases while its cost increases, and they are the only technical solution in very deep waters (> 500 m).The fluid-structure interaction is a capital aspect in platform design, but it assumes a biggest role in the case of a compliant platform.
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Platform functions and typesGeneral scheme of offshore platforms in relation with water depth.
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jacketjack-up
semi-submersibledrilling ship
TLP
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Platform functions and typesSteel jacket: it is the most classical and widely used offshore platform. A typical jacket is shown in the figure.It is composed of three principal parts: the deck, carrying the topsides (living quarters, drilling derrick, consumables, facilities, helideck, flare etc.), the jacket itself and the foundation piles. Steel jackets are normally used in shallow to moderate deep waters (from 20 to 100 m), but they have been used up to 500 m of water.
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Platform functions and typesIf not too big, jackets are built in a dock and then charged onto a barge by a crane. Large jackets are built on one side, directly on a barge or on rails to be the skidded onto a barge. An important characteristic of a jacket is a small floatability: in fact, legs are not plugged, as they are the templates for the piles, and the braces are normally too small to ensure the necessary buoyancy. So, a barge is needed to carry them to the field, where the jacket is put into water by a crane, for small jackets, or directly skidded or rolled off the barge (launching operation) for large jackets. w
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Platform functions and typesIn the first case, the design phase must take into consideration the lifting phase, while in the second cases a complete analysis of the launching phase must be done, in order to asses the transient stress distribution and to control the actual behaviour of the jacket during the transportation and the launching phases, namely if an additional buoyancy is needed and if the jacket touch the sea bottom during launching.Numerical investigations are normally used to simulate these phases, which considerably condition the design of a steel jacket. In the next pages, these phases are outlined. w
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Platform functions and types
Launching phases of a steel jacket.ht
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Platform functions and typesTwo pictures of a jacket installed by a crane and of the mating of a deck and a jacket.
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Platform functions and typesThe construction phases of a jacket. 1: construction in a coast yard;
2: transportation by barge on the oil field;
3: launching of the jacket.
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Platform functions and typesAnother characteristic of steel jackets is the foundation system: as previously said, piles are directly inserted into the legs and they directly support the deck. Sometimes, especially for larger platforms, additional skirt piles are necessary. Once the piles driven, they are grouted into the legs to join them to the platform. Once the piles installed and grouted, the deck is placed at the top of the jacket by a crane. Normally, all the installments and facilities are already installed onto the deck before its mating with the deck.
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Platform functions and typesSteel tower: it is a large jacket where the piles cannot be inserted in the legs mainly for economical reasons.In fact, too long piles are too expensive. So, when the platform is located in deep waters, the jacket becomes very heavy and the piles cannot be as long as the legs. They become skirt piles inserted in sleeves around the outside of the legs. In this way, the legs are plugged and normally sufficient to ensure the buoyancy: the jacket does not need a barge to be carried on the site, as it floats (eventually with auxiliary buoyancy) and can be towed. This is very convenient both for economical and construction aspects.They exist tower structures installed in more than 400 m water depth.
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Platform functions and typesSome relevant achievements of jacketed structures.
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Platform functions and typesSteel gravity platforms: this type of structure, rarely used, uses its own weight to counter the lateral actions due to wind and waves that tend to overturn the platform: the weight is used as a stabilizing force. Nevertheless, the real reason for using gravity platforms is thenature of the soil: when it is of solid rock, it is impossible to drive piles into it, so the gravity solution is the only possible one.Normally, gravity platforms are concrete platforms, but in some cases a steel solution can be adopted, in relation with several factors, mainly economic considerations. Normally, the structure has a certain number of large tanks, flooded by water or by crude oil, to ballast the platform and provide the necessary weight to counter overturning lateral forces.These tanks, in the transportation phase, provide the necessary buoyancy.An important feature of all the gravity platforms is that they can be removed for demobilization or re-use.
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Platform functions and typesThe Maureen Alpha platform is a steel gravity platform with a weight of 112000 t, height of 241 m. It has been installed in 1983 in the North Sea; in 2001 it has been removed and replaced on another oil field.
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Platform functions and typesLoanga platform(Nigeria): it is a steel gravity platform, with inclined risers to optimize the exploitation of the field.
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Platform functions and typesConcrete gravity platforms: they are the hugest and most impressive structures ever built. In this platforms, the steel structure supporting the deck is totally or partially replaced by a concrete structure of large dimensions.
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Platform functions and typesConcrete gravity platforms are used when some particular circumstances are present:
economical factors: in some cases, the construction of a very large concrete structure can be cheaper than the construction of a steel structure;ecological factors: a concrete platform can be very huge, so as to concentrate onboard some industrial treatments of the crude and to allow a great stocking capacity in the ballast cells;construction conditions: the pile driving operation for a steel jacket needs usually 5 to 10 days; in the North Sea it is rare to have such a period of fine weather; the installation in the oil field of a concrete gravity platform, complete with its deck, requires a shorter period (1 to 2 days); decommissioning aspects: concrete gravity platforms can be decommissioned and eventually re-used;soil conditions: when the soil is made of rock it is impossible to drive piles into it: the gravity solution is then the only one possible; geographical conditions: the presence of calm and deep waters not far from the oil field is an important factor for the construction phases.
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Platform functions and typesThe above factors have often been determinant in the choice of this kind of platforms in the North Sea.Nevertheless, rather recently concrete gravity platforms have been commissioned in other parts of the world (East Russia, Philippines and so on).These structures can reach a height of 400 m and weigh more than800000 t.
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Platform functions and types
Source: www.ogp.org.uk
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Platform functions and types
Source: www.ogp.org.uk
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Platform functions and typesIt is important to understand the construction phases of this kind of platforms: the fundamental aspect is the Archimedes' force.In fact, the concrete substructure is built onshore, in a dock under the sea level.Once the base is ready, the dock is flooded and the base floats; it is then towed in deeper but calm waters, where the construction of the substructure continues on the floating base.
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Platform functions and typesOnce the substructure ready, it is towed in sufficiently deep and calm waters (a fiord gives the optimal conditions) for the mating operation: the deck, carried by two barges with all the topsides, is mated to the concrete substructure just by an operation of ballasting and deballasting the concrete substructure with water.All these operations are outlined in the following scheme.
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Platform functions and typesMalampaya concrete gravity platform (Philippines; the first concrete gravity platform in Asia): all the construction phases.
1. construction in a dock under the sea level
2. construction of the towers
3. flooding of the dock: the platforms is ready to be towed
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Platform functions and types
Source: www.arup.com
4. towing of the concrete substructure
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6. mating
5. ballasting of the concrete substructure7. the final platform
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Platform functions and typesThe disaster of Sleipner A: this platform produces oil and gas in the North Sea in a water depth of 82 m. The first concrete base structure for Sleipner A sprang a leak and sank under a controlled ballasting operation during preparation for deck mating in Gandsfjorden outside Stavanger, Norway on 23 August 1991. The loss was caused by a failure in a cell wall, resulting in a serious crack and a leakage that the pumps were not able to cope with. The wall failed as a result of a combination of a serious error in the finite element analysis and insufficient anchorage of the reinforcement in a critical zone. A better idea of what was involved can be obtained from the photos in the following page. The top deck weighs 57,000 t, and provides accommodation for about 200 people and support for drilling equipment weighing about 40,000 t. When the first model sank in August 1991, the crash caused a seismic event registering 3° on the Richter scale, and left nothing but a pile of debris at 220 m of depth. The failure involved a total economic loss of about $700 million.
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Platform functions and typesThe post accident investigation traced the error to inaccurate finite element approximation of the linear elastic model of the cells (using the popular finite element program NASTRAN). The shear stresses were underestimated by 47%, leading to insufficient design. In particular, certain concrete walls were not thick enough. More careful finite element analysis, made after the accident, predicted that failure would occur with this design at a depth of 62 m, which matches well with the actual occurrence at 65 m.
Source: http://www.ima.umn.edu/~arnold/disasters/sleipner.html
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Platform functions and typesFree standing towers: they are classical towers but so slender that their structural behavior is that of a compliant structure: large sway displacements and high oscillating period. Baldpate: the highest, freestanding compliant structure in the world.Characteristics:
water depth: 501 m;sway response cycle: 30 s;lateral displacement: 3 m;cross section: 42.6 x 42.6 m (bottom),
27.4 x 27.4 m (top);weight of the tower: 28900 t;weight of deck and topsides: 2700 t;foundation: 12 piles driven for 130 m.
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Platform functions and typesGuyed towers: these compliant platforms are composed by a slender jacket, normally pin-joined at its base, whose vertical stable position is ensured by the buoyancy of the structure itself and by a series of mooring catenary lines.The structure can oscillate under the lateral actions, the restoring force being provided by the buoyancy and the mooring lines. The clump weights provide additional restraining forces in case of storm, when they are lifted off the seafloor. These platforms are used for water depth in the range 200-600 m, and they can be re-used.
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Platform functions and typesSPAR towers: these platforms are composed by a large steel tube as substructure directly supporting the deck and topsides.The tube is ballasted so as its floating stable equilibrium position is vertical (including topsides), and moored by tensioned risers and by mooring lines (catenaries). On the lateral surface of the large vertical cylinder there are helicoids, installed to counter vortex-shedding.
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Platform functions and typesTLP (Tension Leg Platforms): they are floating structures anchored to the seafloor by a series of vertical tendons (tethers) pre-tensioned by extra-buoyancy. The tethers are made by steel pipes.A TLP is composed by 4 principal parts: the foundation template, the tethers, the hull and the deck.Some TLPs (e. g. Heidrun) have a concrete hull.
TLP are very large structures, able to host great payloads. So, they are used for great fields and can host some refining processes and have a good storage capacity.TLP can be used from 150 m of water depth on, and theoretically there is no limit of water depth for their use.The restoring force is given by extra buoyancy; this is obtained deballasting the TLP hull once the tethers installed. TLPs can be re-used.
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Platform functions and typesThe constructing phases of a TLP are similar to those of a concrete gravity platform, and are sketched in the figure.
a: construction of the hull in a dock;b: towing the hull to the mating site;c: towing the deck to the mating site;d: mating;e: tethers positioning;f: deballasting for tensioning the tethers.
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Platform functions and typesTLPs have known a wide diffusion all over the world in the last decade. The water depth, in 20 years, has been multiplied by a factor of ten.
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884502906106622510600016602displacement (t)
2601835380300004170topsides weight (t)
2032121772437001950hull's weight (t)
110 x 11087,6 x 103,1100 x 10055 x55dimension (m)
54column height (m)
262422,624,412,2diam. col. (m)
1158345872395536water depth (m)
UrsaHeidrunAugerSnorreJolliet
884502906106622510600016602displacement (t)
2601835380300004170topsides weight (t)
2032121772437001950hull's weight (t)
110 x 11087,6 x 103,1100 x 10055 x55dimension (m)
54column height (m)
262422,624,412,2diam. col. (m)
1158345872395536water depth (m)
UrsaHeidrunAugerSnorreJolliet
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Platform functions and typesSome pictures of TLPs (source: Atlantia Offshore LTD).
Snorre
Magnolia
Marlin
Heidrun
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Platform functions and typesRecently, a new TLP concept has been developed: it is that knownas mini TLP.In this solution, only one column is present, just as in the case of spare towers.The column is anchored to the seafloor by pretensioned tethers that are fixed at the end of three pontoons at the bottom of the cylinder.These TLP have a less payload capacity, and are normally used for deep water small fields.
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Platform functions and typesDrilling ships: like all the mobile systems, drilling ships are used mostly for the drilling phase, but they can be used, at least temporarily, also as FPS (Floating Production System).A drilling ship is, as its name indicates, a common ship equipped with a drilling system (a derrick tower).It is maintained in its position by a system of mooring catenaries, eventually assisted by servo-motors and GPS positioning.
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Platform functions and typesSemi-submersibles: as their name indicates, these are special ships, normally composed of two pontoons, some columns and a deck. The deck is equipped for all the drilling operations.A semi-submersible is a complete platform, that can navigate as it is furnished of motors. Once in place, its positioning is provided by a system of catenaries normally controlled by a GPS system.Recently a concrete semi-submersible has been constructed.
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Platform functions and typesJack-ups: these are special mobile platforms, normally used for the drilling operations.They are triangular barges, completely equipped for the drilling operations and disposing of three truss legs.
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These legs can be lifted or lowered by motors. When the legs are lifted, the jack-up can navigate just as a common ship. Once arrived on the field, the jack-up lowers the legs so as to be fixed in the drilling place and it lifts itself at the right height above the sea level.
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Platform functions and typesIn this course, only fixed platforms will be considered, as the case of mobile platforms concerns much more naval engineering.Nevertheless, several considerations which will be made in the following, such as dynamic response, wind and wave force analysis and so on, concern as well mobile platforms, namely jack ups.
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Historical background1896. Summerland, California: the first oil and gas operations over water, with wells drilled from piers extending from shore. 1909 or 1910. Ferry Lake, Louisiana: wells drilled using a wood deck erected on a platform supported by cypress trunks driven aspiling.1924 and after. Lake Maracaibo, Venezuela: wells drilled from wooden platforms supported on timber pilings. 1930s: the oil industry moves into the marsh and swamplands of South Louisiana, and then, as a natural extension, into shallow waters of the Gulf of Mexico, using existing technology for timber structures. 1937. Gulf of Mexico: the first platform close to shore; it is a traditional timber-pile structure.1945. Gulf of Mexico: Magnolia Petroleum Company builds a wooden structure in about 6 m of water and drills the first offshore well remote from shore.
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Historical background1947. Gulf of Mexico: the first steel platform is installed out of sight of land, in 6 m of water by Superior Oil Company, and another in 5.4 m of water by the Kerr McGee-Phillips-Stanolind group. These early steel platforms were fabricated entirely offshore. They were supported by a large number of small steel pilings (40-60) driven in varying directions and to varying depths. After the pilings weredriven, horizontal pipe braces were laid out on the constructionbarge and cut to fit.1948. The first prefabricated substructure sections assembled on the site.
The first steel platform (1947).
1950. The first onshore fabrication of unitized substructures, referred to as templates or jackets. The steel jacket was placed on the ocean floor, where it acted as a template for the steel piles that were driven through its tubular legs.
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Historical background1955 to 1978. The water depth for steel jacket platform construction passes from about 30 to 311.8 m: a progression of ten times in 20 years.1973. North sea: the first concrete gravity platform, Ekofisk, is installed in 70 m of water.1983. Gulf of Mexico: Exxon installs the first guyed tower, Lena, in 304 m of water. 1984. North Sea: Conoco installs Hutton, the first TLP, in 147 m of water, used as a drilling and production platform.1991. Bullwinkle, the highest rigid steel jacket is installed in 412 m of water.1995. The highest concrete platform is installed in 330 m of water.1998. Baldpate, the highest steel jacket, is installed in 501 m of water: it is a compliant free standing platform. 1999. A TLP is installed for the first time in more than 1000 m of water: Ursa, in 1158 m of water, in the Gulf of Mexico.2005. The TLP Magnolia is installed in 1425 m of water.
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Historical background
1998; 501
1984; 147
1991; 4121978; 311,8
1965; 68,51955; 301947; 6
1976; 258,6
1975; 144
1997; 980
1994; 872
1989; 536
1996; 894
1999; 1158
2004; 1311
2005; 1425
1977; 151
1993; 2501989; 217
1973; 70
1995; 330
0
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1940 1950 1960 1970 1980 1990 2000 2010
Year
Wat
er d
epth
(m)
JacketsTLPConcrete
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Design processThe design of an offshore platform is a very complex process, asseveral different aspects must be taken into account.Normally, in a preliminary phase data concerning the industrial activity, the construction facilities, the environmental conditions must be collected.These data concern the payload, that is the total weight, surface and distribution of the topsides facilities, installments and plants; all this, of course, depends on the kind of industrial activity of the platform (see hereafter).Then, the environmental and geographic conditions must be known;the design storm must be selected and the characteristic wave determined, which is fundamental to assess the wave lateral forces on the platform.The wind characteristics must also be determined, in order to evaluate the maximum horizontal force acting on the superstructure.The soil stratigraphy must also be known, which is capital for foundation design.
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Design processIn fact, the number and length of piles, which are in connection with soil properties and stratigraphy but also with the platform dimensions and with the amount of horizontal actions, can affectgreatly the design of the platform. Moreover, the existence of bad soil layers or of solid rock can suggest, and eventually impose, the choice of a gravity platform.The construction phases must always be taken into account duringthe design process. In fact, small jackets and decks are lifted and installed by cranes, and so these phases must be considered and submitted to structural calculation, as very different from the normal conditions the jacket and the deck are designed for.Greater jackets are skidded onto barges and then launched into the sea, eventually with the aid of a crane or of floating units. Also these phases must be carefully considered and analyzed, as very different from the usual situation of the jacket.
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Design processFinally, the transitory phase of pile driving must be studied: during this phase, the jacket must not overturn under the action of moderate waves. In the design phase, the corrosion aspects and the marine growthmust be considered: for the first problem, an additional thickness of the steel members is used (normally 5 mm), and also the use of sacrificial anodes, while for the latter an extra thickness (about 50 mm) is taken into account to calculate the wave action and the total immersed weight.Another important factor is time: it is always important to construct the platform in as less time as possible, for evident economicalreasons, and this can affect considerably the structural choices and the construction phases. For rigid structures installed in water depths to about 100 m, static analyses are normally adequate, because these structures are sufficiently stiff, so that dynamic effect can be safely ignored.
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Design processUnlike this case, that is in deeper water and anyway for any situation in which the fundamental period of the structure exceeds 3 sec (compliant structures), the effect of cyclic platform motion caused by wave action becomes an important factor and must be carefully analyzed: the fluid-structure interaction becomes an important problem and must be properly taken into account in the design phase.Usually, a preliminary design is done, taken into consideration a cardinal rule in offshore engineering: the onshore work must be maximized and the offshore work minimized, and this for economical reasons. This preliminary design is useful to obtain the overall dimensions of the platform, the number of piles and so on.Once this preliminary design done and approved, a detailed design is done, in which all the structural parts are studied and calculated in detail.
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Design processIn this phase, also the exact construction sequence is attentively considered and taken into consideration in the structural calculations.The knowledge of the climatic and geographic data allows to determine some fundamental dimensions of the structure.In fact, once the LAT (Lowest Astronomical Tide) and the design wave known, the height hof the jacket can be determined:
Here, a is the wave amplitude, tmax is the maximum tide and ag is the so-called air-gap, a safety height, normally 0.5 m.Actually, no point of the deck must be positioned under the height h, and this to put all the installments off the wave action.
agatLATh +++= max
ag
a
LAT tmax
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Design processThe most important parameter, however, in the design phase is the economical aspect: this point conditions very much the choice of the structure type.In the figure, it is shown a comparison of the relative cost trends for different platform types, for mild-sea (e.g. the Gulf of Mexico) and for extreme-sea conditions (e.g. the North Sea).It is apparent that the cost of bottom-supported platforms increases rather quickly with the water depth: beyond a certain water depth, these are no more cost-effective.
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Standards and regulationsThe offshore installments must be safe to protect the human, marine, and coastal environments, and to preserve the very considerable investments of time and resources involved. It is the purpose of this section to identify the methods by which responsible parties seek assurance of the structural integrity of offshore platforms.Until the late 1960s, structural integrity in the U.S. offshore industry was largely the responsibility of the designers, who worked to avariety of standards drawn from coastal and onshore engineering experience. The first published design standard for fixed offshore platforms was issued in 1969 by the American Petroleum Institute (API).Actually, the structural integrity can be checked through one of three similar, but different, procedures known as verification, certification, and classification, having the characteristic that each is carried out by specialist organizations independent of both the owner and the designer.
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Standards and regulationsThe object of quality assurance procedures is to verify that the best available technical and environmental knowledge has been appliedduring the phases of design, construction, installation, and operation of an offshore platform. Classification is a quality assurance service provided by one of the private organizations known as classification societies, and begins during the design phase of a structure. Criteria for classification are the society’s published standards, known as rules or guides. The procedure begins with submittal of engineering calculations, specifications and fabrication drawings so that the society can verify compliance of the design with the rules. During the subsequent phases of construction, installation, and operation, surveys are conducted as necessary to ensure completeand continuing adherence to standards. The classification procedure includes periodic inspections and special damage surveys to ensure that integrity and serviceability are maintained throughout the life of the installation.
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Standards and regulationsWell-known classification societies include the American Bureau of Shipping, Det Norske Veritas (Norway) and Lloyd’s Register of Shipping (U.K.). These and other societies have expanded their scope of services beyond the traditional classification of vessels of commerce to encompass many other types of marine structures, including offshore structures. Certification is a quality assurance procedure under which the owner or a government mandates adherence to specified standards or rules for design or construction and requires verification ofcompliance by one of a limited number of authorized certification agents. While an authorized certification agent may apply the standards of its own organization in the process of evaluation, the standardsspecified by the owner or government having jurisdiction prevail in cases of conflict.
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Standards and regulationsExamples of assuring structural integrity by certification are the procedures adopted by the U. K. and by Norway.In the U.K. regulations stipulate that a fixed offshore structure must be certified by American Bureau of Shipping, Det Norske Veritas(Norway), Lloyd’s Register of Shipping (U.K.), Bureau Veritas(France), Germanischer Lloyd (Germany) or Halcrow Ewbank and Associates Certification Group (U. K.).The U.K. regulations make it clear that the owner has the responsibility for sound design and construction as well as adequate maintenance of the offshore installation. Norwegian regulations specify only the Norwegian Petroleum Directorate (NPD) as the approval agency, but NPD customarily contracts with independent review agencies to confirm compliancewith its rules. The owner carries responsibility for assuring structural integrity.
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Standards and regulationsVerification is the procedure used in the U.S. to assure structural integrity of offshore platforms. It is administered by the government under the Platform Verification Program. In the USA, verification is mandatory for a new platform and formajor modifications to an existing platform if any of the following conditions will exist:
it is installed in water deeper than 120 m; it has a natural period greater than 3 s; it is installed in an area having unstable bottom conditions; it is installed in a frontier area; it uses an unusual design concept in comparison to typical installations in the region.
In 1979, a study conducted in the USA revealed that in the Gulf of Mexico, over 32000 platform years of service had been accumulated with 37 platforms lost.
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Standards and regulationsOf these, 34 losses were due to overloading from hurricane waves, and 3 losses occurred because of hurricane wave loading combinedwith wave-induced motion of unstable sea-floor deposits. This represents a failure rate of 0.1 percent annually. By 1984, five years after the period covered by the above statistics, approximately 14000 platform years of service had been added without further losses. Present offshore technology, standards, and regulations provide adequate safety of fixed offshore structures against general collapse.The general knowledge about this topic has spread rapidly all over the world and at present all the countries have similar and equivalent rules.
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Nevertheless, there are some rules that are considered as reference norms:
API RP 2A - Recommended Practice for planning, designing, constructing fixed offshore platforms. In the following of this course, if not specified differently, API norms are intended to be used (abridged in API);Det Norske Veritas – Rules for the design, construction and inspection of off-shore structures (abridged in the following with DNV);British Standard 6235 – Code of practice for fixed offshore structures (abridged in the following with BS);DOE – Offshore installations: guidance on design and construction; German norms.NTS: Norsok Standard – Actions and action effects (abridged in the following with NTS – Norwegian Technology Standards, or NorsokStandard).
Standards and regulations
