[LECTURE] Design of Fixed Offshore Structures

8/10/2019 [LECTURE] Design of Fixed Offshore Structures

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Offshore Structures: General Introduction

OBJECTIVE/SCOPE

To identify the basic vocabulary, to introduce the major concepts for offshore platform

structures, and to explain where the basic structural requirements for design are generated.

PREREQUISITES

None.

SUMMARY

The lecture starts with a presentation of the importance of offshore hydro-carbon exploitation,the basic steps in the development process (from seismic exploration to platform removal)

and the introduction of the major structural concepts (jacket-based, GBS-based, TLP,

floating). The major codes are identified.

For the fixed platform concepts (jacket and GBS), the different execution phases are briefly

explained: design, fabrication and installation. Special attention is given to some principles of

topside design.

A basic introduction to cost aspects is presented.

Finally terms are introduced through a glossary.

1. INTRODUCTION

Offshore platforms are constructed to produce the hydrocarbons oil and gas. The contribution

of offshore oil production in the year 1988 to the world energy consumption was 9% and isestimated to be 24% in 2000.

The investment (CAPEX) required at present to produce one barrel of oil per day ($/B/D) and

the production costs (OPEX) per barrel are depicted in the table below.


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Condition CAPEX $/B/D OPEX $/B

Conventional

Average 4000 - 8000 5

Middle East 500 - 3000 1

Non-Opec 3000 - 12000 8

Offshore

North Sea 10000 - 25000 5 - 10

Deepwater 15000 - 35000 10 - 15

World oil production in 1988 was 63 million barrel/day. These figures clearly indicate the

challenge for the offshore designer: a growing contribution is required from offshore

exploitation, a very capital intensive activity.

Figure 1 shows the distribution of the oil and gas fields in the North Sea, a major contribution

to the world offshore hydrocarbons. It also indicates the onshore fields in England, theNetherlands and Germany.


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2. OFFSHORE PLATFORMS

2.1 Introduction of Basic Types

The overwhelming majority of platforms are piled-jacket with deck structures, all built in steel(see Slides 1 and 2).

Slide 1: Jacket based platform - Southern sector North Sea

Slide 2: Jacket based platform - Northern sector North Sea


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Slide 4 shows an integrated deck (though excluding the living quarters and helideck) being

moved from its assembly building.

Slide 4 : Integrated topside during load out

5.2.2 Structural Design for Integrated Topsides

For the smaller decks, up to approximately 100 MN weight, the support structure consists of

trusses or portal frames with deletion of diagonals.

The moderate vertical load and shear per column allows the topside to be supported by

vertical columns (deck legs) only, down to the top of the piles (situated at approximately +4 m

to +6 m L.A.T. (Low Astronomic Tide).

5.2.3 Structural Design for Modularized Jacket-based Topsides

A major modularized topside weighs 200 to 400 MN. In this case the MSF is a heavy tubular

structure (Figure 4), with lateral bracing down to the top of jacket.


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5.3 Structural Design for Modularized Gravity-based Topsides

The topsides to be supported by a gravity-based substructure (see Figure 2) are in a weight

range of 200 MN up to 500 MN.

The backbone of the structure is a system of heavy box-girders with a height of approximately

10 m and a width of approximately 12 - 15 m (see Figure 5).

The substructure of the deck is rigidly connected to the concrete column and acts as a beam

supporting the deck modules. This connection introduces wave-induced fatigue in the deck

structure. A recent development, foreseen for the Norwegian Troll platform, is to provide a

flexible connection between the deck and concrete column, thus eliminating fatigue in the

deck [10].

6. EQUIPMENT AND LIVING QUARTER MODULES

Equipment modules (20-75 MN) have the form of rectangular boxes with one or two

intermediate floors.

The floors are steel plate (6, 8 or 10 mm thick) for roof and lower floor, and grating forintermediate floors.


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In living quarter modules (5-25 MN) all sleeping rooms require windows and several doors

must be provided in the outer walls. This requirement can interfere seriously with truss

arrangements. Floors are flat or stiffened plate.

Three types of structural concepts, all avoiding interior columns, can be distinguished:

conventional trusses in the walls.

stiffened plate walls (so called stressed skin or deck house type).

heavy base frame (with wind bracings in the walls).

7. CONSTRUCTION

7.1 Introduction

The design of offshore structures has to consider various requirements of construction

relating to:

1. fabrication.

2. weight.

3. load-out.

4. sea transport.

5. offshore installation.

6.

module installation.7. hook-up.

8. commissioning.

A documented construction strategy should be available during all phases of the design and

the actual design development should be monitored against the construction strategy.

Construction is illustrated below by four examples.

7.2 Construction of Jackets and Topsides

7.2.1 Lift Installed Jackets

The jacket is built in the vertical (smaller jackets) or horizontal position (bigger jackets) on a

quay of a fabrication site.

The jacket is loaded-out and seafastened aboard a barge. At the offshore location the barge

is moored alongside an offshore crane vessel.


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The jacket is lifted off the barge, upended from the horizontal, and carefully set down onto the

seabed.

After setting down the jacket, the piles are installed into the sleeves and, driven into the

seabed. Fixing the piles to the jacket completes the installation.

7.2.2 Launch Installed Jackets

The jacket is built in horizontal position.

For load-out to the transport barge, the jacket is put on skids sliding on a straight track of steel

beams, and pulled onto the barge (Slide 5).

Slide 5 : Jacket being loaded onto barge by skidding

At the offshore location the jacket is slid off the barge. It immerses deeply into the water and

assumes a floating position afterwards (see Figure 6).


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Two parallel heavy vertical trusses in the jacket structure are required, capable of taking the

support reactions during launching. To reduce forces and moments in the jacket, rocker arms

are attached to the stern of the barge.


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The next phase is to upright the jacket by means of controlled flooding of the buoyancy tanks

and then set down onto the seabed. Self-upending jackets obtain a vertical position after the

launch on their own. Piling and pile/jacket fixing completes the installation.

7.2.3 Topsides for a Gravity-Based Structure GBS)

The topside is assembled above the sea on a temporary support near a yard. It is then taken

by a barge of such dimensions as to fit between the columns of the temporary support and

between the columns of the GBS. The GBS is brought in a deep floating condition in a

sheltered site, e.g. a Norwegian fjord. The barge is positioned between the columns and the

GBS is then deballasted to mate with and to take over the deck from the barge. The floating

GBS with deck is then towed to the offshore site and set down onto the seabed.

7.2.4 Jacket Topsides

For topsides up to approximately 120 MN, the topside may be installed in one lift. Slide 6

shows a 60 MN topside being installed by floating cranes.

Slide 6 : Installation of 60MN K12-BP topside by floating crane

For the modularized topside, first the MSF will be installed, immediately followed by the

modules.


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7.3 Offshore Lifting

Lifting of heavy loads from barges (Slide 6) is one of the very important and spectacular

construction activities requiring a focus on the problem when concepts are developed.

Weather windows, i.e. periods of suitable weather conditions, are required for these

operations.

7.3.1 Crane Vessel

Lifting of heavy loads offshore requires use of specialized crane vessels. Figure 7 provides

information on a typical big, dual crane vessel. Table 1 (page 16) lists some of the major

offshore crane vessels.


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7.3.2 Sling-arrangement, Slings and Shackles

For lifting, steel wire ropes in a four-sling arrangement are used which directly rest in the four-

point hook of the crane vessel, (see Figure 8). The heaviest sling available now has a

diameter of approximately 350 mm, a breaking load of approximately 48 MN, and a safe

working load (SWL) of 16 MN. Shackles are available up to 10 MN SWL to connect the

padeyes installed at the module's columns. Due to the space required, connecting more than

one shackle to the same column is not very attractive. So when the sling load exceeds 10

MN, padears become an option.


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Table 1 Major Offshore Crane Vessels

Operator Name Mode Type Lifting capacity (Tonnes)

Fix 2720

Thor MonohullRev 1820

Fix 2720Odin Monohull

Rev 2450

Fix 4536 + 3628 = 8164Hermod Semisub

Rev 3630 + 2720 = 6350

Fix 3630 + 2720 = 6350

Heerema

Balder SemisubRev 3000 + 2000 = 5000

Fix 4000DB50 Monohull

Rev 3800

Fix 1820DB100 Semisub

Rev 1450

Fix 3360

DB101 SemisubRev 2450

McDermott

DB102 Semisub Rev 6000 + 6000 = 12000

Micoperi M7000 Semisub Rev 7000 + 7000 = 14000

ETPM DLB1601 Monohull Rev. 1600

Notes:

1. Rated lifting capacity in metric tonnes.

2. When the crane vessels are provided with two cranes, these cranes are situated at

the vessels stern or bow at approximately 60 m distance c.t.c.

1. 3. Rev = Load capability with fully revolving crane.

Fix = Load capability with crane fixed.

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7.4 Sea Transport and Sea Fastening

Transportation is performed aboard a flat-top barge or, if possible, on the deck of the crane

vessel.

The module requires fixing to the barge (see Figure 9) to withstand barge motions in rough

seas. The sea fastening concept is determined by the positions of the framing in the module

as well as of the "hard points" in the barge.

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7.5 Load-out

7.5.1 Introduction

For load-out three basic methods are applied:

skidding

platform trailers

shearlegs.

7.5.2 Skidding

Skidding is a method feasible for items of any weight. The system consists of a series of steel

beams, acting as track, on which a group of skids with each approximately 6 MN load

capacity is arranged. Each skid is provided with a hydraulic jack to control the reaction.

7.5.3 Platform Trailers

Specialized trailer units (see Figure 10) can be combined to act as one unit for loads up to 60

- 75 MN. The wheels are individually suspended and integrated jacks allow adjustment up to

300 mm.

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The load capacity over the projected ground area varies from approximately 55 to 85

kN/sq.m.

The units can drive in all directions and negotiate curves.

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7.5.4 Shearlegs

Load-out by shearlegs is attractive for small jackets built on the quay. Smaller decks (up to 10

- 12 MN) can be loaded out on the decklegs pre-positioned on the barge, thus allowing deck

and deckleg to be installed in one lift offshore.

7.6 Platform Removal

In recent years platform removal has become common. The mode of removal depends

strongly on the regulations of the local authorities. Provision for removal should be considered

in the design phase.

8. STRUCTURAL ANALYSIS

8.1 Introduction

The majority of structural analyses are based on the linear theory of elasticity for total system

behaviour. Dynamic analysis is performed for the system behaviour under wave-attack if the

natural period exceeds 3 seconds. Many elements can exhibit local dynamic behaviour, e.g.

compressor foundations, flare-stacks, crane-pedestals, slender jacket members, conductors.

8.2 In-place Phase

Three types of analysis are performed:

Survival state, under wave/current/wind attack with 50 or 100 years recurrence

period.

Operational state, under wave/current/wind attack with 1 or 5 years recurrence

period, under full operation.

Fatigue assessment.

Accidental.

All these analyses are performed on the complete and intact structure. Assessments at

damaged structures, e.g. with one member deleted, and assessments of collision situations

are occasionally performed.

8.3 Construction Phase

The major phases of construction when structural integrity may be endangered are:

Load-out

Sea transport


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11. CONCLUDING SUMMARY

The lecture starts with the presentation of the importance of offshore hydro-carbon

exploitation, the basic steps in the development process (from seismic exploration to

platform removal) and the introduction of the major structural concepts (jacket-based,

GBS-based, TLP, floating). The major codes are identified.

For the fixed platform concepts (jacket and GBS), the different execution phases are

briefly explained: design, fabrication and installation. Special attention is given to the

principles of topside design.

A basic introduction to cost aspects is presented.

Finally terms are introduced within a glossary.

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12. GLOSSARY OF TERMS

AIR GAP Clearance between the top of maximum wave and underside of the topside.

CAISSONS See SUMPS

CONDUCTORS The tubular protecting and guiding the drill string from the topside down to 40

to 100m under the sea bottom. After drilling it protects the well casing.

G.B.S. Gravity based structure, sitting flatly on the sea bottom, stable through its weight.

HOOK-UP Connecting components or systems, after installation offshore.

JACKET Tubular sub-structure under a topside, standing in the water and pile founded.

LOAD-OUT The operation of bringing the object (module, jacket, deck) from the quay onto the

transportation barge.

PADEARS (TRUNNIONS) Thick-walled tubular stubs, directly receiving slings and

transversely welded to the main structure.

PADEYES Thick-walled plate with hole, receiving the pin of the shackle, welded to the main

structure.

PIPELINE RISER The piping section which rises from the sea bed to topside level.

SEA-FASTENING The structure to keep the object rigidly connected to the barge during

transport.

SHACKLES Connecting element (bow + pin) between slings and padeyes.

SLINGS Cables with spliced eyed at both ends, for offshore lifting, the upper end resting in

the crane hook.

SPREADER Tubular frame, used in lifting operation.

SUBSEA TEMPLATE Structure at seabottom, to guide conductors prior to jacket installation.

SUMPS Vertical pipes from topside down to 5-10 m below water level for intake or discharge.

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TOPSIDE Topside, the compact offshore process plant, with all auxiliaries, positioned above

the waves.

UP ENDING Bringing the jacket in vertical position, prior to set down on the sea bottom.

WEATHER WINDOW

A period of calm weather, defined on basis of operational limits for the offshore marine

operation.

WELLHEAD AREA Area in topside where the wellheads are positioned including the valves

mounted on its top.

13. REFERENCES

[1] API-RP2A: Recommended practice for planning, designing and constructing fixed offshore

platforms.

American Petroleum Institute 18th ed. 1989.

The structural offshore code, governs the majority of platforms.

[2] LRS Code for offshore platforms.

Lloyds Register of Shipping.

London (UK) 1988.

Regulations of a major certifying authority.

[3] DnV: Rules for the classification of fixed offshore installations.

Det Norske Veritas 1989.

Important set of rules.

[4] AISC: Specification for the design, fabrication and erection of structural steel for buildings.

American Institute of Steel Construction 1989.

Widely used structural code for topsides.

[5] AWS D1.1-90: Structural Welding Code - Steel.

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American Welding Society 1990.

The structural offshore welding code.

[6] DnV/Marine Operations: Standard for insurance warranty surveys in marine operations.

Det norske Veritas June 1985.


[7] ABS: Rules for building and classing offshore installations, Part 1 Structures.

American Bureau of Shipping 1983.


[8] BV: Rules and regulations for the construction and classification of offshore platforms.

Bureau Veritas, Paris 1975.


[9] ANON: A primer of offshore operations.

Petex Publ. Austin U.S.A 2nd ed. 1985.

Fundamental information about offshore oil and gas operations.

[10] AGJ Berkelder et al: Flexible deck joints.

ASME/OMAE-conference The Hague 1989 Vol.II pp. 753-760.

Presents interesting new concept in GBS design.

14. ADDITIONAL READING

1. BS 6235: Code of practice for fixed offshore structures.

British Standards Institution 1982.

Important code, mainly for the British offshore sector.

2. DoE Offshore installations: Guidance on design and construction, U.K. Department of

Energy 1990.

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Governmental regulations for British offshore sector only.

3. UEG: Design of tubular joints (3 volumes).

UEG Offshore Research Publ. U.R.33 1985.

Important theoretical and practical book.

4. J. Wardenier: Hollow section joints.

Delft University Press 1981.

Theoretical publication on tubular design including practical design formulae.

5.

ARSEM: Design guides for offshore structures welded tubular joints.

Edition Technip, Paris (France), 1987.

Important theoretical and practical book.

6. D. Johnston: Field development options.

Oil & Gas Journal, May 5 1986, pp 132 - 142.

Good presentation on development options.

7. G. I. Claum et al: Offshore Structures: Vol 1: Conceptual Design and Hydri-

mechanics; Vol 2 - Strength and Safety for Structural design.

Springer Verlag, London 1992.

Fundamental publication on structural behaviour.

8.

W.J. Graff: Introduction to offshore structures.

Gulf Publishing Company, Houston 1981.

Good general introduction to offshore structures.

9. B.C. Gerwick: Construction of offshore structures.

John Wiley & Sons, New York 1986.

Up to date presentation of offshore design and construction.

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10.T.A. Doody et al: Important considerations for successful fabrication of offshore

structures.

OTC paper 5348, Houston 1986, pp 531-539.

Valuable paper on fabrication aspects.

11.D.I. Karsan et al: An economic study on parameters influencing the cost of fixed

platforms.

OTC paper 5301, Houston 1986, pp 79-93.

Good presentation on offshore CAPEX assessment.


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depth and deep water conditions. Corresponding particle paths are illustrated in Figures 3 and

4. Note the strong influence of the water depth on the wave kinematics. Results from high-

order wave theories can be found in the literature, e.g. see [9].

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2.2.2 Wave Statistics

In reality waves do not occur as regular waves, but as irregular sea states. The irregular

appearance results from the linear superposition of an infinite number of regular waves with

varying frequency (Figure 5). The best means to describe a random sea state is using the

wave energy density spectrum S(f), usually called the wave spectrum for simplicity. It is

formulated as a function of the wave frequency f using the parameters: significant wave

height Hs(i.e. the mean of the highest third of all waves present in a wave train) and mean

wave period (zero-upcrossing period) To. As an additional parameter the spectral width can

be taken into account.


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Designs for ductility level earthquakes will normally require inelastic analyses for which the

seismic input must be specified by sets of 3-component accelerograms, real or artificial,

representative of the extreme ground motions that could shake the platform site. The

characteristics of such motions, however, may still be prescribed by means of design spectra,

Lecture 15A.2

which are usually the result of a site specific seismotectonic study. More detail of the analysis


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of earthquakes is given in the Lectures 17: Seismic Design.

2.5 Ice and Snow Loads

Ice is a primary problem for marine structures in the arctic and sub-arctic zones. Ice formationand expansion can generate large pressures that give rise to horizontal as well as vertical

forces. In addition, large blocks of ice driven by current, winds and waves with speeds that

can approach 0,5 to 1,0 m/s, may hit the structure and produce impact loads.

As a first approximation, statically applied, horizontal ice forces may be estimated as follows:

Fi= CifcA ......................................... (7)

Where,

A is the exposed area of structure,

fcis the compressive strength of ice,

Ciis the coefficient accounting for shape, rate of load application and other factors, with usual

values between 0,3 and 0,7.

Generally, detailed studies based on field measurements, laboratory tests and analytical work

are required to develop reliable design ice forces for a given geographical location.

In addition to these forces, ice formation and snow accumulations increase gravity and wind

loads, the latter by increasing areas exposed to the action of wind. More detailed information

on snow loads may be found in Eurocode 1 [8].

2.6 Loads due to Temperature Variations

Offshore structures can be subjected to temperature gradients which produce thermal

stresses. To take account of such stresses, extreme values of sea and air temperatures

which are likely to occur during the life of the structure must be estimated. Relevant data for

the North Sea are given in BS6235 [6]. In addition to the environmental sources, human

factors can also generate thermal loads, e.g. through accidental release of cryogenic material,

which must be taken into account in design as accidental loads. The temperature of the oil

and gas produced must also be considered.


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[8] Eurocode 1: "Basis of Design and Actions on Structures", CEN (in preparation).


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[9] Clauss, G. T. et al: "Offshore Structures, Vol 1 - Conceptual Design and Hydromechanics",

Springer, London 1992.

[10] Anagnostopoulos, S.A., "Dynamic Response of Offshore Structures to Extreme Wavesincluding Fluid - Structure Interaction", Engr. Structures, Vol. 4, pp.179-185, 1982.

[11] Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co., Houston, 1981.

[12] Graff, W.J., "Introduction to Offshore Structures", Gulf Publishing Co., Houston, 1981.

[13] Gerwick, B.C. Jr., "Construction of Offshore Structures", John Wiley, New York, 1986.

Table 1 Results of Linear Airy Theory [11]

Phase = kx - t

Relative water depth d/L

Deep water

d/L 0,5

Finite water depth

d/L < 0,5

Velocity potential

Surface elevation z

Dynamic pressure pdyn=

acos

gaekzcos

acos

Water particle velocities

horizontal u =

vertical w =

aekzcos

aekzsin

Water particle accelerations

horizontal u' =

vertical w' =

a2ekzsin

-a2ekzcos

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Wave celerity c =

Group velocity cgr=

Circular frequency =

Wave length L =

Wave number k =

co=

cgr=

=

Lo=

ko=

c =

cgr=

=

L =

kd tanh kd =

Water particle displacements

horizontal

vertical

Particle trajectories

-aekzsin

aekzcos

Circular orbits

Elliptical orbits

Where a=


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Lecture 15A.3

conditions can be taken as static. Typical values of friction coefficients for calculation of

skidding forces are the following:


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steel on steel without lubrication..................................... 0,25

steel on steel with lubrication...........................................0,15

steel on teflon.................................................................. 0,10 teflon on teflon................................................................. 0,08

3.3 Transportation Forces

These forces are generated when platform components (jacket, deck) are transported

offshore on barges or self-floating. They depend upon the weight, geometry and support

Lecture 15A.3

conditions of the structure (by barge or by buoyancy) and also on the environmental

conditions (waves, winds and currents) that are encountered during transportation. The types


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of motion that a floating structure may experience are shown schematically in Figure 3.

In order to minimize the associated risks and secure safe transport from the fabrication yard

to the platform site, it is important to plan the operation carefully by considering, according to

API-RP2A [3], the following:

1.

Previous experience along the tow route

2. Exposure time and reliability of predicted "weather windows"

3. Accessibility of safe havens

4. Seasonal weather system

5. Appropriate return period for determining design wind, wave and current conditions,

taking into account characteristics of the tow such as size, structure, sensitivity and

cost.


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Lecture 15A.3

Table Characteristic Lo


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LIMIT STATES FOR TEMPORARY

PHASES

Progressive Collapse

Serviceability

LOAD

TYPE

Service

ability

Fatigue

Ultimate

Abnormal

effects

Damage

condition

DEAD EXP

LIVE SPE

DEFORM

ATIONEXPECTE

ENVIRON

MENTAL

Depen

dent onoperati

onal

require

ments

Expect

ed load

history

Value

dependent on

measures taken

Dependent on operational requirements

ACCIDEN

TAL NOT APPLICABLE

Dependent on

operational

requirements

NOT APPLIC

ads according to NPD [4]


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LIMIT STATES FOR NORMAL OPERATIONS

Progressive Collapse

Fatigue Ultimate

Abnormal

effectsDamage condition

ECTED VALUE

CIFIED VALUE

D EXTREME VALUE

Expect

ed load

history

Annual

exceedance

probability 10-2

Annual

exceedanc

e

probability

10-4

Annual exceedance probability 10-2

ABLE

Annual

exceedanc

eprobability

10-4

NOT APPLICABLE


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P are permanent loads (structural weight, dry equipments, ballast, hydrostatic pressure).

L are live loads (storage, personnel, liquids).


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D are deformations (out-of-level supports, subsidence).

E are environmental loads (wave, current, wind, earthquake).

A are accidental loads (dropped object, ship impact, blast, fire).

3.3.2 Material factors

The material partial factors for steel is normally taken equal to 1,15 for ULS and 1,00 for PLS

and SLS design.

3.3.3 Classification of Design Conditions

Guidance for classifying typical conditions into typical limit states is given in the following

table:

Loadingsondition

P/L E D A

Design

Criterion

Construction P ULS,SLS

Load-Out P reduced wind support

disp

ULS

Transport P transport wind

and wave

ULS

Tow-out

(accidental)

P flooded compart PLS

Launch P ULS

Lifting P ULS

In-Place

(normal)

P + L wind, wave &

snow

actual ULS,SLS

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In-Place

(extreme)

P + L wind & 100

year wave

actual ULS

SLS


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In-Place

(exceptional)

P + L wind & 10000

year wave

actual PLS

Earthquake P + L 10-2quake ULS

Rare

Earthquake

P + L 10-4quake PLS

Explosion P + L blast PLS

Fire P + L fire PLS

Dropped

Object

P + L drill collar PLS

Boat

Collision

P + L boat impact PLS

Damaged

Structure

P + reduced L reduced wave

& wind

PLS

4. PRELIMINARY MEMBER SIZING

The analysis of a structure is an iterative process which requires progressive adjustment of

the member sizes with respect to the forces they transmit, until a safe and economical design

is achieved.

It is therefore of the utmost importance to start the main analysis from a model which is close

to the final optimized one.

The simple rules given below provide an easy way of selecting realistic sizes for the main

elements of offshore structures in moderate water depth (up to 80m) where dynamic effects

are negligible.


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Lecture 15A.4

p(t) =

The plot of the amplitudes pjversus the circular frequencies jis called the amplitude power

spectra of the loading Usually significant values of p j only occur within a narrow range of


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spectra of the loading. Usually, significant values of pjonly occur within a narrow range of

frequencies and the analysis can be restricted to it.

The relationship between response and force vectors is expressed by the transfer matrix H,

such as:

H = [-M 2+ i x C + K]

the elements of which represent:

Hj,k=

The spectral density of response in freedom j versus force is then:

The fast Fourier transform (FFT) is the most efficient algorithm associated with this kind of

analysis.

6.4.2 Time Domain Analysis

The response of the i-th mode may alternatively be determined by resorting to Duhamel's

integral:

Xj(t) =

The overall response is then obtained by summing at each time step the individual responsesover all significant modes.

6.5 Direct Integration Methods

Direct step-by-step integration of the equations of motion is the most general method and is

applicable to:

non-linear problems involving special forms of damping and response-dependent

loadings.

Lecture 15A.4

responses involving many vibration modes to be determined over a short time

interval.

The dynamic equilibrium at an instant is governed by the same type of equations, where all

matrices (mass, damping, stiffness, load) are simultaneously dependent on the time and


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Page 14 of 14

structural response as well.

All available integration techniques are characterized by their stability (i.e. the tendency for

uncontrolled divergence of amplitude to occur with increasing time steps). Unconditionally

stable methods are always to be preferred (for instance Newmark-beta with = 1/4 or Wilson-

theta with = 1,4).


The analysis of offshore structures is an extensive task.

The analytical models used in offshore engineering are in some respects similar to

those used for other types of steel structures. The same model is used throughout the

analysis process.

The verification of an element consists of comparing its characteristic resistance(s) to

a design force or stress. Several methods are available.

Simple rules are available for preliminary member sizing.

Static in-plane analysis is always carried out at the early stage of a project to size the

main elements of the structure. A dynamic analysis is normally mandatory for every

offshore structure.


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Lecture 15A.5

[9] UEG, Node Flexibility and its Effect on Jacket Structures/CIRIA Report UR22, 1984.

[10] Hallam M.G., Heaf N.J. & Wootton L.R., Dynamics of Marine Structures/ CIRIA Report

UR8 (2nd edition), October 1978.

[11] Wilson J.F., Dynamics of Offshore Structures/Wiley Interscience, 1984.

[12] Clough R.W. & Penzien J., Dynamics of Structures/McGraw-Hill, New York, 1975.


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Page 15 of 16

[ ] g , y , ,

[13] Newland D.E., Random Vibrations and Spectral Analysis/Longman Scientific (2nd

edition), 1984.

[14] Zienkiewicz O.C., Lewis R.W. & Stagg K.G., Numerical Methods in Offshore

Engineering/Wiley Interscience, 1978.

[15] Davenport A.G., The Response of Slender Line-Like Structures to a Gusty Wind/ICE

Vol.23, 1962.

[16] Williams A.K. & Rhinne J.E., Fatigue Analysis of Steel Offshore Structures/ICE Vol.60,

November 1976.

[17] Anagnostopoulos S.A., Wave and Earthquake Response of Offshore Structures:

Evaluation of Modal Solutions/ASCE J. of the Structural Div., vol. 108, No ST10, October

1982.

[18] Chianis J.W. & Mangiavacchi A., A Critical Review of Transportation Analysis

Procedures/OTC paper 4617, May1983.

[19] Kaplan P. Jiang C.W. & Bentson J, Hydrodynamic Analysis of Barge-Platform Systems in

Waves/Royal Inst. of Naval Architects, London, April 1982.

[20] Hambro L., Jacket Launching Simulation by Differentiation of Constraints/ Applied Ocean

Research, Vol.4 No.3, 1982.

[21] Bunce J.W. & Wyatt T.A., Development of Unified Design Criteria for Heavy Lift

Operations Offshore/OTC paper 4192, May 1982.

[22] Walker A.C. & Davies P., A Design Basis for the J-Tube Method of Riser Installation/J. of

Energy Resources Technology, pp. 263-270, September 1983.

Lecture 15A.5

[23] Stahl B. & Baur M.P., Design Methodology for Offshore Platform Conductors/J. of

Petroleum Technology, November 1983.

[24] DnV - Rules for the Classification of Steel Ships, January 1989.


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Page 16 of 16


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Lecture 15A.6


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Page 3 of 25

2.1 Design Loads

These loads are those transferred from the jacket to the foundation. They are calculated at

the mudline.

2.1.1 Gravity loads

Gravity loads (platform dead load and live loads) are distributed as axial compression forces

on the piles depending upon their respective eccentricity.

Lecture 15A.6

2.1.2 Environmental loads

Environmental loads due to waves, current, wind, earthquake, etc. are basically horizontal.

Their resultant at mudline consists of:

shear distributed as horizontal forces on the piles.

overturning moment on the jacket, equilibrated by axial tension/ compression insymmetrically disposed piles (upstream/downstream).


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y y p p ( p )

2.1.3 Load combinations

The basic gravity and environmental loads multiplied by relevant load factors are combined in

order to produce the most severe effect(s) at mudline, resulting in:

vertical compression or pullout force, and

lateral shear force plus bending.

2.2 Static Axial Pile Resistance

The overall resistance of the pile against axial force is the sum of shaft friction and end

bearing.

2.2.1 Lateral friction along the shaft shaft friction)

Skin friction is mobilized along the shaft of the tubular pile (and possibly also along the inner

wall when the soil plug is not removed).

The unit shaft friction:

for sands: is proportional to the overburden pressure,

for clays: is calculated by the "alpha" or "lambda" method and is a constant equal to

the shear strength Cuat great depth.

Lateral friction is integrated along the whole penetration of the pile.

2.2.2 End bearing

End bearing is the resultant of bearing pressure over the gross end area of the pile, i.e. with

or without the area of plug if relevant.

The bearing pressure:

for clays: is equal to 9 Cu.


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Lecture 15A.6

The energy of the ram hitting the top of the pile generates a stress wave in the pile, which

dissipates progressively by friction between the pile and the soil and by reflection at the

extremities of the pile.

The plastic displacement of the tip relative to the soil is the set achieved by the blow. Curves

can be drawn to represent the number of blows per unit length required to drive the pile at

different penetrations.


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The wave equation, though representing the most rigorous assessment to date of the driving

process, still suffers a lack of accuracy, mostly caused by the inaccuracies in the soil model.

3. DIFFERENT KINDS OF PILES

Driven piles are the most popular and cost-efficient type of foundation for offshore structures.

As shown in Figure 2, the following alternatives may be chosen when driving proves

impractical:

insert piles.

drilled and grouted piles.

belled piles.


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Lecture 15A.6

account the changes in load direction during lifting). Padeyes are then carefully cut before

lowering the next pile section.


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Lecture 15A.6

Sketch E shows the different steps for the positioning of pile sections:

pile or add-on lifted from the barge deck.

rotation of the crane to position add-on.

installing and lowering of the pile add-on.

4.4.2 Pile connections

Different solutions for connecting pile segments back-to-back are used:


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either by welding, Shielded Metal Arc Welding (SMAW) or flux-cored, segments held

temporarily by internal or external stabbing guides as shown in Figure 4. Welding time

depends upon:

- pile wall thickness: 3 hours for 1in. thick (25,4mm); 16 hours for 3in. thick, (76,2mm)

(typical).

- number and qualification of the welders.

- environmental conditions.

or by mechanical connectors (as shown in Figure 4):

- breech block (twisting method).

- lug type (hydraulic method).

4.4.3 Hammer placement

Figure 5 shows the different steps of this routine operation:

Lecture 15A.6


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lifting from the barge deck.

positioning over pile by booming out or in (the bell of the hammer acts as a stabling

guide... very helpful in rough weather).

alignment of the pile cap.

lowering leads after hammer positioning.

Each add-on should be designed to prevent bending or buckling failure during installation and

in-place conditions.

Lecture 15A.6

4.4.4 Driving

Some penetration under the self weight of the pile is normal. For soft soil conditions, particular

measures are taken to avoid an uncontrolled run.

Piles are then driven or drilled until pile refusal.

Pile refusal is defined as the minimum rate of penetration beyond which further advancement

of the pile is no longer achievable because of the time required and the possible damage to

the pile or to the hammer A widely accepted rate for defining refusal is 300 blows/feet (980


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Page 16 of 25

the pile or to the hammer. A widely accepted rate for defining refusal is 300 blows/feet (980

blows/meters).

4.5 Pile-to-Jacket Connections

4.5.1 Welded shims

The shims are inserted at the top of the pile within the annulus between the pile and jacket leg

(see Figure 6) and welded afterwards.

Lecture 15A.6

4.5.2 Mechanical locking system

This metal-to-metal connection is achieved by a hydraulic swaging tool lowered inside the pile

and expanding it into machined grooves provided in the sleeves at two or three elevations as

shown on Figure 7.


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Page 17 of 25

Lecture 15A.6

This type of connection is most popular for subsea templates. It offers immediate strength and

the possibility to re-enter the connection should swaging prove incomplete.

4.5.3 Grouting

This hybrid connection is the most commonly used for connecting piles to the main structure

(in the mudline area). Forces are transmitted by shear through the grout.

Figure 8 shows the two types of packers commonly used. The expansive, non-shrinking grout

must fill completely the annulus between the pile and leg (or sleeve).


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must fill completely the annulus between the pile and leg (or sleeve).

Lecture 15A.6

Bonding should be excellent; it is improved by shear connectors (shear keys, strips or weld

beads disposed on the surface of the sleeve and pile in contact with the grout).

The width of the annulus between pile and sleeve should be maintained constant by use of

centralizers and be limited to:

1,5in. minimum, (38,1mm)

about 4in. (101,6mm) maximum (to avoid destruction of the tensile strength of the

grout by internal microcracking).


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Packers are used to confine the grout and prevent it from escaping at the base of the sleeve.

Packers are often damaged during piling and are therefore:

installed in a double set.

attached to the base of the sleeve to protect them during pile entry and driving.

Thorough filling should be checked by suitable devices, e.g. electrical resistance gauges,

radioactive tracers, well-logging devices or overflow pipes checked by divers.

4.6 Quality Control

Quality control shall:

confirm the adequacy of the foundation with respect to the design.

provide a record of pile installation for reference to subsequent driving of nearby piles

and future modifications to the platform.

The installation report shall mention:

pile identification (diameter and thickness).

measured lengths of add-ons and cut-offs.

self penetration of pile (under its own weight and under static weight of the hammer).

blowcount throughout driving with identification of hammer used and energy, as

shown in Figure 9.

record of incidents and abnormalities:

- unexpected behaviour of the pile and/or hammer.

- interruptions of driving (with set-up time and blowcount subsequently required to break the

pile loose).

- pile damage if any.

elevations of soil plug and internal water surface after driving.

Lecture 15A.6

information about the pile/structure connection:

- equipment and procedure employed.

- overall volume of grout and quality.

- record of interruptions and delays.


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Page 20 of 25


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Lecture 15A.6

TABLE 2 Large pile driving hammers

A. Air/Steam Hammers

Make Model

Rated

Energy

(ft lb )

Ram

Weight

(ki )

Max.

Stroke

( )

Std.

Pilecap

Weight

Typical

Hammer

Weight

( /l d )

Rated

Operating

Pressure

Steam

Consumption

(lb ht)

Air

Consumption

(lb ht)

Hose

ST/F

Rated

BPM


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Page 23 of 25

(ft-lbs) (kips) (m)(kips)

(w/leads)

(kips)(psi)

(lbs ht) (lbs ht) .....

Conmaco

6850

5650

5300

300

200

510.000

325.000

150.000

90.000

60.000

85

65

30

30

20

72

60

60

36

36

57,5

59,0

12,7

12,7

12,7

312

262

92

86

74

180

160

160

150

120

31.500

8.064

6.944

5.563

7.500

1.711

1.471

1.195

2 @

4

3 @

4

4

3

3

40

45

46

54

59

Menck

(MRBS)

12500

8800

8000

7000

5000

4600

3000

1800

850

1.582.220

954.750

867.960

632.885

542.470

499.070

325.480

189.850

93.340

275,58

194,01

176,37

154

110,23

101,41

66,14

38,58

18,96

69

59

59

49

59

59

59

59

50

154,32

103,62

85,98

92,4

66,14

52,91

33,07

22,05

11,5

853

600

564

583

335

313

205

125

64

171

150

142

156

150

142

142

142

142

53.910

32.400

30.860

30.800

20.940

19.840

12.130

7.060

3.530

26.500

16.700

15.900

14.830

10.400

9.900

6.000

3.700

1.950

2 @

6

8

8

4 @

4

6

6

5

4

3

36

36

38

35

40

42

42

44

45

MKT

OS-60

OS-40

OS-20

18.000

120.000

60.000

60

40

20

36

36

36 38,65 150 3 60

Lecture 15A.6

C. Hydraulic Hammers

Make Model

Rated Energy

(ft-lb)

Ram Weight

(kips)

Standard

Pilecap

Weight

(kips)

Hammer

Weight

(kips)

Typical

Operating

Pressure

(psi)

Rated

Oil Flow

(gal. min)

Rated

BPM

4000

3000A

1.200.000

800.000

205

152

490

414


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Page 24 of 25

Note 1: With the heavier hammers in the range given, the wall thicknesses must be near the

upper range of those listed in order to prevent overstress (yielding) in the pile under hard

driving.

HMB

3000

1500

900

500

725.000

290.000

170.000

72.000

139

55

30,8

9,5

33

17,6

1,1

172

88

27,5

40-70

Menck

MRBU

MHU

1700

MHU

900

MH

195

MH

165

MH

145

MH

120

MH 96

MH 80

760.000

1.230.000

650.000

141.000

119.000

105.000

87.000

69.000

58.000

132

207

110

22,0

19,0

16,5

13,9

11,0

9,3

84

77

6,0

6,0

6,0

6,0

1,9

1,9

415

617

386

59

51

46

40

27

24

3400

3400

3100

3550

3190

2755

2320

2830

2465

845

845

580

98

103

102

103

75

75

50-80

32-65

48-65

38

42

42

44

48

48

Lecture 15A.6

Note 2: With diesel hammers, the effective hammer energy is from one-half to two-thirds the

values generally listed by the manufacturers and the above table must be adjusted

accordingly. Diesel hammers would normally only be used on 36-in. or less diameter piles.

Note 3: Hydraulic hammers have a more sustained blow, and hence the above table can be

modified to fit the stress wave pattern.

TABLE 3 Typical values of pile sizes, wall thickness and hammer energies

Pile Outer Diameter Wall Thickness Hammer Energy


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Page 25 of 25

(in.) (mm) (in.) (mm) (ft-lb) (kN-m)

24

30

36

42

48

60

72

84

96

108

120

600

750

900

1.050

1.200

1.500

1.800

2.100

2.400

2.700

3.000

5/8 - 7/8

7/8 - 1

1 - 1

17- 1

17 - 1

1 - 2

1 - 2

1 - 2

1 - 2

1 - 2

15-21

19

21-25

25-32

28-44

28-44

32-50

32-50

32-50

37-62

37-62

50.000 - 120.000

50.000 - 120.000

50.000 - 180.000

60.000 - 300.000

90.000 - 500.000

90.000 - 500.000

120.000 - 700.000

180.000 - 1.000.000

180.000 - 1.000.000

300.000 - 1.000.000

300.000 - 1.000.000

70 - 168

70 - 168

70 - 252

84 - 120

126 - 700

126 - 700

168 - 980

252 - 1.400

252 - 1.400

420 - 1.400

420 - 1.400


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Lecture 15A.7

Tubular Joints in Offshore Structures

OBJECTIVE/SCOPE

To present methods for the design of large tubular joints typically found on offshore

structures.

PREREQUISITES

Lecture 15A.1: Offshore Structures: General Introduction


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RELATED LECTURES

Lecture 15A.8: Fabrication

Lecture 15A.12: Connections in Offshore Deck Structures

SUMMARY

The lecture defines the principle terms and ratios used in tubular joint design. It presents the

classifications for T, Y, X, N, K and KT joints and discusses the significance of gaps, overlaps,

multiplanar joints and the details of joint arrangements. It describes design methods for static

and fatigue strength, presenting some detailed information on stress concentration factors.

1. INTRODUCTION

The main structure of topside consists of either an integrated deck or a module support frame

and modules. Commonly tubular lattice frames are present, however a significant amount of

rolled and built up sections are also used.

This lecture refers to the design of tubular joints. These are used extensively offshore,

particularly for jacket structures. Connections of I-shape sections or boxed beams whether

rolled or built up, are basically similar to those used for onshore structures. Refer to the

corresponding lectures for appropriate design guidance.

Two main calculations need to be performed in order to adequately design a tubular joint.

These are:

1. Static strength considerations

2. Fatigue behaviour considerations

Lecture 15A.7

The question of fatigue behaviour always has to be addressed, even where simple

assessment of fatigue behaviour shows this will not be a problem. The joint designer must

therefore always be "fatigue minded".

2. DEFINITIONS

The following definitions are universally acknowledged [1]: (refer to Figure 1 for clarification):


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The CHORD is the main member, receiving the other components. It is necessarily a through

member. The other tubulars are welded to it, without piercing through the chord at the

intersection.

Other tubulars belonging to the joint assembly may be as large as the chord, but they can

never be larger.


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Lecture 15A.7

2.2 Geometrical ratios

= Can slenderness ratio

= Brace to chord diameter ratio (always 1)

= Chord slenderness ratio

= Brace to chord thickness ratio


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Page 4 of 23

= Relative gap

These are non-dimensional variables for use in parametrical equations.

3. CLASSIFICATION

Load paths within a joint are very different, according to the joint geometry. The following

classification is used, see Figure 2.

Lecture 15A.7

3.1 T and Y Joints

These are joints made up of a single brace, perpendicular to the chord (T joint) or inclined to it

(Y joints).

In a T joint, the axial force acting in the brace is reacted by bending in the chord.

In a Y joint, the axial force is reacted by bending and axial force in the chord.

3.2 X Joints

X joints include two coaxial braces on either side of the chord


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X joints include two coaxial braces on either side of the chord.

Axial forces are balanced in the braces, which in an ideal X joint have the same diameter and

thickness. In fact, other considerations such as brace length, which can be very different on

each side of the chord, may lead to two slightly different braces. Angles may be slightly

different as well.

The important point to note is the balance of forces in the braces. If the axial force in one

brace is far higher than the one in the other brace, the joint may be classified as a Y (or a T)

joint rather than an X joint.

3.3 N and K Joints

These joints include two braces. One of them may be perpendicular to the chord (N joint) or

both inclined (K joint).

The ideal load pattern of these joints is reached when axial forces are balanced in the braces,

i.e. net force into chord member is low.

3.4 KT Joints

These joints include three braces.

The load pattern for these joints is more complex. Ideally axial forces should be balanced

within the braces, i.e. net force into chord member is low.

3.5 Limitations

For a joint to be able to be fabricated and to be effective, the geometrical ratios given in

Section 2.2 have limitations. Table 3.1 shows these limits and their typical ranges.

Lecture 15A.7

Table 3.1 Geometrical Limits and Typical Ranges

LimitationsParameter Typical range

min max

0,4 - 0,8 0,2 1

12 - 20 10 30


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Page 6 of 23

0,3 - 0,7 0,2 1 (2)

40- 90 30(3) 90(1)

(1) Physical limitation

(2) Brace shall be less or equal to chord thickness (see punching shear)

(3) Angle limitation to get a correct workmanship of welds.

3.6 How to classify a joint

This classification deals only with braces located in one plane.

It must always be remembered that this classification is based on load pattern as well as the

geometry. Engineering judgement must therefore be used to classify a joint. For example a

geometrical K joint may be classified as.

a K joint when forces are balanced within braces.

a Y joint when the force in one brace is reacted predominantly by the chord, rather

than by the second brace.

Lecture 15A.7

4. GAP AND OVERLAP

4.1 Definitions

The GAP is the distance along the chord between the weld toes of the braces (Figure 3).


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The theoretical gap is the shortest distance between the outer surfaces of two braces,

measured on the line where they cross the chord outer surface. The real gap is the onemeasured at the corresponding location, between actual weld toes.

A brace OVERLAPS another brace when one brace is welded to the other brace.

The overlapping brace is always the thinner brace.

The overlapped brace is always completely welded to the chord.

Lecture 15A.7

4.2 Limitations

The minimum gap allowed is 50mm. This limitation is set to avoid two welds clashing. This is

important because the gap is a highly stressed zone.

4.3 Multiplanar Joints

The same definitions and limitations apply to multiplanar joints.

5. JOINT ARRANGEMENT

As a rule, welds in a joint have to be kept away from zones of high stress concentration.


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The following practice, see Figure 4, should be followed:

1. The chord circumferential welds are to be located at either 300mm or a quarter of the

chord diameter, whichever is the greater, from the nearest point of a brace-chord

connection.

2. The brace circumferential welds are to be located at either 600mm or a brace

diameter, whichever is the greatest, from the nearest point of the brace-chord

connection.

3. The actual gap shall not be less than 50mm. To achieve this, most designers use a

70 or 75mm theoretical gap.

4. Eccentricity and offset are to be kept within a quarter of the chord diameter. When

higher values can not be avoided, secondary moments have to be introduced in the

structural analysis by introducing extra nodes.

5. Thickness transitions are smoothed to a 1 in 4 slope, by tapering the thicker wall.

Lecture 15A.7


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Page 9 of 23

Lecture 15A.7

6. STATIC STRENGTH

6.1 Loads taken into account

The loads considered in a joint static strength design are the axial force, the in-plane bending

moment and the out-of-plane bending moment for each brace.

The other components (transverse shear and brace torsion moment) are usually neglectedsince unlike the preceding loads, these loads do not induce bending in the chord wall.

Nevertheless, their presence must never be forgotten and in some specific cases, their effects

must be assessed. The axial load, in-plane and out-of-plane bending moments are normally

the dimensioning criterion for tubular joints.


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6.2 Punching shear

6.2.1 Acting punching shear

The acting punching shear is the shear stress developed in the chord by the brace load.

The acting punching stress vpis written as:

vp= f sin

where f is the nominal axial, in-plane bending or out-of-plane bending stress in the brace

(punching shear for each kept separate), see Figure 5.

Lecture 15A.7

6.2.2 Allowable punching shear

Allowable punching shear values in the chord wall are determined from test results carried out

on full scale or on reduced scale models.

Tests are performed on experimental rigs such as the one shown in Figure 6. They are

performed for a single load-case (axial force, in-plane bending, or out-of-plane bending).


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The ultimate static strength obtained through these tests can then be expressed in terms of

punching shear, as defined above.

Statistical treatments of results allow formulae to be defined for the allowable punching shear

stress.

Lecture 15A.7

6.2.3 The API method

Several offshore design regulations are based on the punching shear concept [1,2]. The

following method is presented in API RP2A [2]:

A. Principle

This method applies to a single brace without overlap, for a non-stiffened joint. Whenthe joint includes several braces, each brace connection is checked independently.

Punching shear for each load component (axial force, in-plane bending, and out of

plane bending) is calculated and compared to the allowable punching shear stress for

the appropriate load and geometry.


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Interaction formulae are given for combined loading, combining the three punching

shear ratio calculated for each component.

B. Allowable punching shear stress

The allowable punching shear stress for each load component is:

Vpa= QqQf

where: Fycis the yield strength of the chord member

Qqis to account for the effects of type of loading and geometry, see Table 6.1.

Qfis a factor to account for the nominal longitudinal stress in the chord

Qf= 1 -

fAX, fIPB, fOPBare the nominal axial, in-plane bending and out of plane bending stresses in the

chord

Lecture 15A.7

Value for and Qqare given in Table 6.1

Table 6.1 Values of Q

q

for allowable punching shear stress f rom APIRP2A

Load component Axial load In-plane bending Out of plane bending

Stress in brace fax fby fbz

Acting punching shear Vpx= faxsin Vp= fbysin Vp= fbzsin

K joints

T & Y Joints


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Page 13 of 23

T & Y Joints

Qqw/o diaphragm

X

w diaphragm

Tension Compression

0,030 0,045 0,021

Qg= 1,8 - 0,1 for 20

Qg= 1,4 - 4 g/D for > 20

but Qgmust be 1,0

Q= for > 0,6

QB= 1,0 for 0,6

Lecture 15A.7

C. Loading Combination

For combined loadings involving more than one load component, the following equations shall

be satisfied:

where: IPB refers to in-plane bending component

OPB refers to out-of-plane bending component

AX f t i l f t


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AX refers to axial force component

and

ax

where: arc sin term is in radians.

6.3 Overlapping joints

The parametric formulae discussed in Section 6.2 were specifically established for non-

overlapping joints with no internal reinforcement. These formulae cannot be used foroverlapping joints.

In an overlapping joint, part of the load is transferred directly from one brace to the other

through the overlapping section, without that part of the load transferring through the chord.

The static strength of an overlapping joint is higher than a similar joint without an overlap.

API RP2A, [2] allows the static shear strength of the overlapping weld section to be added to

the punching shear capacity of the brace-chord connection, see Figure 7.

Lecture 15A.7


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The allowable axial load component perpendicular to the chord, P(in Newtons) should be

taken to be:

P= (vpaT l1) + (2vwatwl2)

where:

vpais the allowable punching shear stress (MPa) for axial stress.

l1is the circumference for that portion of the brace which contacts the chord (mm), see Figure

7.

vwais the allowable shear stress for weld between braces (MPa).

twis the lesser of the weld throat thickness or the thickness t of the inner brace (mm).

l2is the projected chord length (one side) of the overlapping weld, measured perpendicular to

the chord (mm), see Figure 7.

6.4 Reinforced joints

6.4.1 Definition

Large chord wall thickness may be reduced by stiffening the chord. The most usual

reinforcement consists of ring stiffening inside the chord.


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Lecture 15A.7

In-plane bending- as for K joint

Validity range

The above equation for T/Y, K and KT joints are generally valid for joint parameters within the

following limits:

8,333 33,3

0,20 0,8

0,3 0,8 unless stated otherwise

6 667 40 unless stated otherwise


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Page 19 of 23

6,667 40 unless stated otherwise

0 90unless stated otherwise.

8. FATIGUE ANALYSIS

A fatigue analysis of a joint consists of the following steps:

1. Calculation of nominal stress ranges in the brace and the chords

2. Calculation of hot-spot stress range

3. Calculation of joint fatigue lives using S-N curves for tubular members at joints.

8.1 Nominal stress range

Nominal stress ranges in braces and chords are calculated by a global stress analyses.

Lecture 15A.7

8.1.1 Wave histogram

A wave histogram has to be obtained for each direction around the platform. A simple form of

a wave histogram is as follows:

Wave height metres) Average number per year

0-1,5

1,5-3

3-4,5

3 100 000

410 000

730 000


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Page 20 of 23

4,5-6

6-8

8-10

5 000

800

20

8.2.2 Nominal stress ranges

Nominal stress ranges can be calculated by following the steps below:

1. Wave heights are grouped in "blocks", for which just one stress range will be

calculated. Different wave directions need to be considered with a minimum of three

"blocks" per wave direction.

2. For each block one representative wave is chosen, whose action is supposed to

represent the action of the whole block. The highest wave of the block is normally

chosen.

3. Nominal stresses for each joint component are then calculated for different phase

angles of the chosen wave, for one complete cycle (360 ). The nominal stress range

for the joint component is defined as the difference between the highest and the

lowest stress obtained for a full wave cycle. Four to twelve phase angles per wave

are usually considered.

8.2 Hot spot stress ranges

Hot spot stress ranges are then evaluated for each chosen joint location by applying

parametric formulae [4] (or by applying the SCF calculated from a detailed analysis).


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Lecture 15A.7

8.4 Cumulative Fatigue Damage Ratio

The stress responses should be combined into the long term stress distribution, which should

then be used to calculate the cumulative fatigue damage ratio, D, given by:

D =

Where,

n is the number of cycles applied at a given stress range

N is the number of cycles to cause failure for the given stress range (obtained from

appropriate S N curve)


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Page 22 of 23

appropriate S-N curve).

In general the design fatigue life of each joint and member should be at least twice the

intended service life of the structure, i.e. a safety factor of 2,0.

For critical elements whose sole failure would be catastrophic, use of a larger safety factor

should be considered.


Terminology, geometric ratios and joint classifications are now standardised for

tubular joints.

The presence of gaps and overlaps significantly influence joint behaviour.

Determination of static strength is generally based on the concept of punching shear,

with the allowance of overlapping joints.

Special analysis are required for reinforced joints.

Stress concentration factors (SCF) are defined for most commonly occurring joints.

Determination of fatigue strength is based on nominal stress range multiplied by

appropriate SCF.

Lecture 15A.7

10. REFERENCES

[1] Offshore Installations: Guidance on Design, Construction and Certification. Fourth Edition,

HMSO, 1990.

[2] Recommended Practice for Planning, Designing and Constructing Fixed Offshore

Platforms, API RP2A Nineteenth Edition.

[3] Young, Warren C, Roark's Formulae for Stress and Strain. Sixth Edition, McGraw-Hill.

[4] Stress Concentration Factors for Simple Tubular Joints, 1989, Volumes 1 to 5, Lloyds

Register of Shipping-Offshore Division.


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Lecture 15A.8


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Lecture 15A.9


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Lecture 15A.9


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Jackets destined for deeper water are heavier and are usually erected on their side and

launched from a barge (Figure 2). This method of construction is currently applicable for

jackets up to 25000 tonnes. A launched jacket usually requires additional buoyancy tanks with

extensive pipework and valving to enable the legs and tanks to be flooded in order to ballast

the jacket into the vertical position on site. For instance, in the case of the Brae 'B' jacket (alarge 19000 tonne jacket installed in 100m water depth in the North Sea) it was necessary to

provide 11000 tonnes of additional buoyancy. This buoyancy was primarily to limit the jacket

trajectory through launch (i.e. to stop it hitting the sea bed) but was also essential for

maintaining bottom clearance during up-ending. The additional buoyancy took the form of two

'saddle' tanks, two pairs of twin 'piggy-bank tanks' and twelve 'cigar' tubes installed down the

pile guides (Figure 3). Altogether the auxiliary buoyancy added about 3,300 tonnes additional

weight to the jacket.

Lecture 15A.9


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Lecture 15A.9

Very large jackets, in excess of launch capacity, have been constructed as self-floaters in a

graving dock, towed offshore subsequent to flooding the dock, and installed on location by

means of controlled flooding of the legs (see Figure 4).


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1.3 Installation Planning

The installation of a jacket consists of loading out, seafastening and transporting the structure

to the installation site, positioning the jacket on the site and achieving a stable structure in

accordance with the design drawings and specifications, in anticipation of installation of the

platform topsides.

An important aspect is the avoidance of unacceptable risk during offshore activities from

loadout through to platform completion. It is recognised that the potential cost to the projectassociated with failure to successfully execute marine activities is particularly high. Normally

therefore the contractor is obliged to produce procedures for these activities which

demonstrate that the risk of failure has been reduced to acceptable levels. He is also required

to demonstrate that, prior to the commencement of an activity, all the necessary preparations

have been completed.


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Lecture 15A.9

5. Lloyds Register of Shipping, Rules and Regulations for the Classification of Fixed

Offshore Installations, 1989. Based on Lloyd's experience from certification of over

500 platforms world-wide.

Table 1 Major Offshore Crane Vessels

Operator Name Type Mode Lifting Capacity

Thor Monohull

Fix

Rev

2720

1820

Odin Monohull

Fix

Rev

2720

2450

Hermod Semisub

Fix

Rev

4536 + 3628 = 8164

3630 + 2720 = 6350

Heerema

Fix 3630 + 2720 = 6350


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Balder Semisub

Fix

Rev

3630 + 2720 = 6350

3000 + 2000 = 5000

DB50 Monohull

Fix

Rev

4000

3800

DB100 Semisub

Fix

Rev

1820

1450

DB101 Semisub

Fix

Rev

3360

2450

McDermott

DB102 Semisub Rev 6000 + 6000 = 12000

Micoperi M7000 Semisub Rev 7000 + 7000 = 14000

Notes:

1. Rated lifting capacity in metric tonnes

2. When the crane vessels are provided with two cranes, these are situated at the

vessels stern at approximately 60m distance etc.


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Lecture 15A.10

Superstructures

I

OBJECTIVE/SCOPE

To introduce the functional requirements; to identify major interfaces with the process,

equipment, logistics, and safety; to introduce the structural concepts for jacket and gravity

based structure (GBS) topsides; to elaborate on structural design for deck floors.

PREREQUISITES

Lectures 1A& 1B: Steel Construction

Lecture 2.4: Steel Grades and Qualities

Lecture 2.5: Selection of Steel Quality

Lectures 3.1: General Fabrication of Steel Structures

Lecture 6.3: Elastic Instability Modes


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Page 1 of 21

Lecture 7.6: Built-up Columns

Lectures 8.4: Plate Girder Behaviour & Design

Lectures 11.2: Welded Connections

Lecture 12.2: Advanced Introduction to Fatigue

Lectures 15A: Structural Systems - Offshore

SUMMARY

The topside lay-out is discussed, referring to API-RP2G [1], and to general aspects of

interface control and weight control.

The different types of topside structures (relevant to the type of substructure, jacket or GBS)

are introduced and described. These types are:

1. integrated deck.

2. module support frame.

3. modules.

Floor concepts are presented and several aspects of the plate floor design are addressed.

Lecture 15A.10

1. INTRODUCTION

This lecture deals with the overall aspects of the design of offshore topsides.

The topside of an offshore structure accommodates the equipment and supports modules and

accessories such as living quarters, helideck, flare stack or flare boom, microwave tower, and

crane pedestals.

The structural concept for the deck is influenced greatly by the type of substructure (jacket or

GBS) and the method of construction, see Figures 1 and 2.


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Lecture 15A.10

Heavy decks, over 10,000 tons, are provided with a module support frame onto which a

number of modules are placed. Smaller decks, such as those locat

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[LECTURE] Design of Fixed Offshore Structures

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