15a.1 offshore structures

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15A.1: Offshore Structures:

General Introduction

OBJECTIVE/SCOPE

To identify the basic vocabulary, to introduce the major concepts for offshore platformstructures, 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-carbonexploitation, 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. Thecontribution of offshore oil production in the year 1988 to the world energy consumptionwas 9% and is estimated 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.

Condition CAPEX $/B/D OPEX $/B

Conventional

Average 4000 - 8000 5


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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 thechallenge for the offshore designer: a growing contribution is required from offshoreexploitation, 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 inEngland, the Netherlands 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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A second major type is the gravity concrete structure (see Figure 2), which is employedin the North Sea in the Norwegian and British sectors.

A third type is the floating production unit.

2.2 Environment

The offshore environment can be characterized by:

• water depth at location• soil, at seabottom and in-depth• wind speed, air temperature• waves, tide and storm surge, current• ice (fixed, floes, icebergs)

• earthquakes (if necessary)

The topside structure also must be kept clear of the wave crest. The clearance (air gap)usually is taken at approximately 1.50 m, but should be increased if reservoir depletionwill create significant subsidence.

2.3 Construction

The environments as well as financial aspects require that a high degree of prefabricationmust be performed onshore. It is necessary to design to limit offshore work to a minimum.The overall cost of a man-hour offshore is approximately five times that of an onshore

man-hour. The cost of construction equipment required to handle loads, and the cost forlogistics are also a magnitude higher offshore.

These factors combined with the size and weight of the items, require that a designermust carefully consider all construction activities between shop fabrication and offshoreinstallation.

2.4 Codes

Structural design has to comply with specific offshore structural codes. The worldwideleading structural code is the API-RP2A [1]. The recently issued Lloyds rules [2] and the

DnV rules [3] are also important.

Specific government requirements have to be complied with, e.g. in the rules ofDepartment of Energy (DoE), Norwegian Petroleum Direktorate (NPD). For the detaildesign of the topside structure the AISC-code [4] is frequently used, and the AWS-code[5] is used for welding.


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In the UK the Piper alpha diaster has led to a completely new approach to regulationoffshore. The responsibility for regulatory control has been moved to the Health andSafety Executive (HSE) and the operator has to produce a formal safety assessment (FSA)himself instead of complying with detailed regulations.

2.5 Certification and Warranty Survey

Government authorities require that recognized bodies appraise the aspects of structuralintegrity and issue a certificate to that purpose.

The major certification bodies are:

• Det norske Veritas (DnV)• Lloyds Register of Shipping (LRS)• American Bureau of Shipping (ABS)• Bureau Veritas (BV)

• Germanischer Lloyd (GL)

Their requirements are available to the designer [2, 3, 6, 7, 8].

Insurance companies covering transport and installation require the structures to bereviewed by warranty surveyors before acceptance. The warranty surveyors applystandards, if available, on a confidential basis.

3. OFFSHORE DEVELOPMENT OF AN OIL/GAS

FIELD

3.1 Introduction

The different requirements of an offshore platform and the typical phases of an offshoredevelopment are summarized in [9]. After several initial phases which include seismicfield surveying, one or more exploration wells are drilled. Jack-up drilling rigs are usedfor this purpose for water depths up to 100 - 120 m; for deeper water floating rigs areused. The results are studied and the economics and risks of different development plansare evaluated. Factors involved in the evaluation may include number of wells required,fixed or floated production facilities, number of such facilities, and pipeline or tanker off-loading.

As soon as exploitation is decided and approved, there are four main technical activities, prior to production:

• engineering and design• fabrication and installation of the production facility• drilling of production wells, taking 2 - 3 months/well• Providing the off loading system (pipelines, tankers, etc.).


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The drilling and construction interaction is described below for two typical fixed platformconcepts.

3.2 Jacket Based Platform for Shallow Water

First the jacket is installed. The wells are then drilled by a jack-up drilling unit standingclose by with a cantilever rig extending over the jacket. Slide 3 shows a jack-up drillingunit with a cantilever rig. (In this instance it is engaged in exploratory drilling and istherefore working in isolation.)

Slide 3: Cantilevered drilling rig: Self-elevating (jack-up) exploration drilling platform.

Design and construction of the topside are progressed parallel to the drilling, allowing production to start soon after deck installation. For further wells, the jack-up drilling unitwill be called once again and will reach over the well area of the production deck.

As an alternative to this concept the wells are often accommodated in a separate wellhead platform, linked by a bridge to the production platform (see Slide 1).

3.3 Jacket and Gravity Based Platform for Deep Water

The wells are drilled from a drilling rig on the permanent platform (see Slide 2). Drillingstarts after the platform is built and completely installed. Consequently production starts

between one and two years after platform installation.

In recent years pre-drilled wells have been used to allow an earlier start of the production.In this case the platform has to be installed exactly above the pre-drilled wells.

4. JACKETS AND PILE FOUNDATION


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4.1 Introduction

Jackets, the tower-like braced tubular structures, generally perform two functions:

• They provide the substructure for the production facility (topside), keeping it

stable above the waves.• They support laterally and protect the 26-30 inch well conductors and the pipeline

riser.

The installation methods for the jacket and the piles have a profound impact on thedesign.

4.2 Pile Foundation

The jacket foundation is provided by open-ended tubular steel piles, with diameters up to2m. The piles are driven approximately 40 - 80 m, and in some cases 120 m deep into the

seabed.

There are basically three types of pile/jacket arrangement (see Figure 3):

Pile-through-leg concept, where the pile is installed in the corner legs of the jacket.

Skirt piles through pile sleeves at the jacket-base, where the pile is installed in guidesattached to the jacket leg. Skirt piles can be grouped in clusters around each of the jacketlegs.


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Vertical skirt piles are directly installed in the pile sleeve at the jacket base; all otherguides are deleted. This arrangement results in reduced structural weight and easier piledriving. In contrast inclined piles enlarge the foundation at the bottom, thus providing astiffer structure.

4.3 Pile Bearing Resistance

Axial load resistance is required for bearing as well as for tension. The pile accumulates both skin friction as well as end bearing resistance.

Lateral load resistance of the pile is required for restraint of the horizontal forces. Theseforces lead to significant bending of the pile near to the seabed.

Number, arrangement, diameter and penetration of the piles depend on the environmental

loads and the soil conditions at the location.

4.4 Corrosion Protection

The most usual form of corrosion protection of the bare underwater part of the jacket aswell as the upper part of the piles in soil is by cathodic protection using sacrificial anodes.A sacrificial anode (approximate 3 KN each) consists of a zinc/aluminium bar cast abouta steel tube and welded on to the structures. Typically approximately 5% of the jacketweight is applied as anodes.

The steelwork in the splash zone is usually protected by a sacrificial wall thickness of 12

mm to the members.

5. TOPSIDES

5.1 Introduction

The major functions on the deck of an offshore platform are:

• well control• support for well work-over equipment

• separation of gas, oil and non-transportable components in the raw product, e.g.water, parafines/waxes and sand• support for pumps/compressors required to transport the product ashore• power generation• Accommodation for operating and maintenance staff.

There are basically two structural types of topside, the integrated and modularizedtopside which are positioned either on a jacket or on a concrete gravity substructure.


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5.2 Jacket-based Topsides

5.2.1 Concepts

There are four structural concepts in practice. They result from the lifting capacity of

crane vessels and the load-out capacity at the yards:

• the single integrated deck (up to approx 100 MN)• the split deck in two four-leg units• the integrated deck with living quarter module• The modularized topside consisting of module support frame (MSF) carrying a

series of modules.

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 consistsof trusses or portal frames with deletion of diagonals.

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


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The topsides to be supported by a gravity-based substructure (see Figure 2) are in aweight range of 200 MN up to 500 MN.

The backbone of the structure is a system of heavy box-girders with a height ofapproximately 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 inthe deck structure. A recent development, foreseen for the Norwegian Troll platform, isto provide a flexible connection between the deck and concrete column, thus eliminatingfatigue in the deck [10].

6. EQUIPMENT AND LIVING QUARTER

MODULES

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


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

In living quarter modules (5-25 MN) all sleeping rooms require windows and severaldoors 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 constructionrelating 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 designand 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.


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The jacket is loaded-out and seafastened aboard a barge. At the offshore location the barge is moored alongside an offshore crane vessel.

The jacket is lifted off the barge, upended from the horizontal, and carefully set downonto the seabed.

After setting down the jacket, the piles are installed into the sleeves and, driven into theseabed. 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 ofsteel 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 waterand 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 takingthe 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 buoyancytanks and then set down onto the seabed. Self-upending jackets obtain a vertical positionafter 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 thentaken by a barge of such dimensions as to fit between the columns of the temporarysupport and between the columns of the GBS. The GBS is brought in a deep floatingcondition in a sheltered site, e.g. a Norwegian fjord. The barge is positioned between thecolumns 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 ontothe seabed.

7.2.4 Jacket Topsides

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

7.3 Offshore Lifting

Lifting of heavy loads from barges (Slide 6) is one of the very important and spectacularconstruction activities requiring a focus on the problem when concepts are developed.Weather windows, i.e. periods of suitable weather conditions, are required for theseoperations.


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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 ofthe 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 thefour-point hook of the crane vessel, (see Figure 8). The heaviest sling available now has adiameter 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 morethan one shackle to the same column is not very attractive. So when the sling loadexceeds 10 MN, padears become an option.

Table 1 Major Offshore Crane Vessels


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Operator Name Mode Type Lifting capacity (Tonnes)

Fix 2720Thor Monohull

Rev 1820

Fix 2720Odin MonohullRev 2450

Fix 4536 + 3628 = 8164Hermod Semisub

Rev 3630 + 2720 = 6350

Fix 3630 + 2720 = 6350

Heerema

Balder Semisub

Rev 3000 + 2000 = 5000

Fix 4000DB50 Monohull

Rev 3800

Fix 1820DB100 Semisub

Rev 1450

Fix 3360DB101 Semisub

Rev 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.

7.4 Sea Transport and Sea Fastening

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


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The module requires fixing to the barge (see Figure 9) to withstand barge motions inrough seas. The sea fastening concept is determined by the positions of the framing in themodule as well as of the "hard points" in the barge.

7.5 Load-out


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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 ofsteel beams, acting as track, on which a group of skids with each approximately 6 MNload capacity is arranged. Each skid is provided with a hydraulic jack to control thereaction.

7.5.3 Platform Trailers

Specialized trailer units (see Figure 10) can be combined to act as one unit for loads up to60 - 75 MN. The wheels are individually suspended and integrated jacks allowadjustment up to 300 mm.


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The load capacity over the projected ground area varies from approximately 55 to 85kN/sq.m.


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The units can drive in all directions and negotiate curves.

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, thusallowing 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 dependsstrongly on the regulations of the local authorities. Provision for removal should beconsidered in the design phase.

8. STRUCTURAL ANALYSIS

8.1 Introduction

The majority of structural analyses are based on the linear theory of elasticity for totalsystem 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 jacketmembers, 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 atdamaged structures, e.g. with one member deleted, and assessments of collision situationsare 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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• Upending of jackets• Lifting.

9. COST ASPECTS

9.1 Introduction

The economic feasibility of an offshore project depends on many aspects: capitalexpenditure (CAPEX), tax, royalties, operational expenditure (OPEX).

In a typical offshore field development, one third of the CAPEX is spent on the platform,one third on the drilling of wells and one third on the pipelines.

Cost estimates are usually prepared in a deterministic approach. Recently cost-estimatingusing a probabilistic approach has been developed and adopted in major offshore projects.

The CAPEX of an installed offshore platform topside amounts to approximately 20ECU/kg.

9.2 Capital Expenditure (CAPEX)

The major elements in the CAPEX for an offshore platform are:

• project management and design• material and equipment procurement• fabrication• transport and installation• hook-up and commissioning.

9.3 Operational Expenditure (OPEX)

In the North Sea approximately 20 percent of OPEX are required for offshore inspection,maintenance and repair (IMR).

The amount to be spent on IMR over the project life can add up to approximately half theoriginal investment.

IMR is the area in which the structural engineer makes a contribution by effort in design,selection of material, improved corrosion protection, accessibility, basic provisions forscaffolding, avoiding jacket attachments dangerous to divers, etc.

10. DEEP WATER DEVELOPMENTS

Deep water introduces a wide range of extra difficulties for the operator, the designer andconstructor of offshore platforms.


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Fixed platforms have recently been installed in water of 410 m. depth, i.e. "Bullwinkle"developed by Shell Oil for a Gulf of Mexico location. The jacket weighed nearly 500 MN.

The maximum depth of water at platform sites in the North Sea is approximately 220 mat present. The development of the Troll field situated in approximately 305 m deep

water is planned for 1993.

In the Gulf of Mexico and offshore California several fixed platforms in water depths of250 - 350 m are in operation (Cerveza, Cognac). Exxon has a guyed tower platform(Lena) in operation in 300 m deep water.

An option for deeper locations is to use subsea wells with flowlines to a nearby(approximately maximum 10 km) fixed platform at a smaller water depth. Alternativelysubsea wells may be used with flexible risers to a floating production unit. Subsea wellsare now feasible for 300 - 900 m deep water. The deepest wells have been developed offBrasil in moderate weather conditions.

The tension leg platform (TLP) seems to be the most promising deepwater productionunit (Figure 11). It consists of a semi-submersible pontoon, tied to the seabed by vertical prestressed tethers. The first TLP was Hutton in the North Sea and recently TLP-Jollietwas installed at a 530 m deep location in the Gulf of Mexico. Norwegian Snorre andHeidrun fields have been developed with TLPs as well.


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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 seismicexploration 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 isgiven to the principles of topside design.

• A basic introduction to cost aspects is presented.• Finally terms are introduced within a glossary.

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 topsidedown 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 quayonto the transportation barge.

PADEARS (TRUNNIONS) Thick-walled tubular stubs, directly receiving slings andtransversely welded to the main structure.

PADEYES Thick-walled plate with hole, receiving the pin of the shackle, welded to themain 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 duringtransport.

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


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SLINGS Cables with spliced eyed at both ends, for offshore lifting, the upper end restingin the crane hook.

SPREADER Tubular frame, used in lifting operation.

SUBSEA TEMPLATE Structure at seabottom, to guide conductors prior to jacketinstallation.

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

TOPSIDE Topside, the compact offshore process plant, with all auxiliaries, positionedabove 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 marineoperation.

WELLHEAD AREA Area in topside where the wellheads are positioned including thevalves 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.


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[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.

American Welding Society 1990.

The structural offshore welding code.

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

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.


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15A.2: Loads (I) : Introduction

and Environmental Loads

OBJECTIVE/SCOPE

To introduce the types of loads for which a fixed steel offshore structure must bedesigned. To present briefly the loads generated by environmental factors.

PREREQUISITES

A basic knowledge of structural analysis for static and dynamic loadings.

SUMMARY

The categories of load for which a pile supported steel offshore platform must bedesigned are introduced and then the different types of environmental loads are presented.The loads include: wind, wave, current, earthquake, ice and snow, temperature, sea bedmovement, marine growth and tide generated loads. Loads due to wind, waves and

earthquake are discussed in more detail together with their idealizations for the varioustypes of analyses. Frequent references are made to the codes of practice recommended bythe American Petroleum Institute, Det Norske Veritas, the British Standards Institutionand the British Department of Energy, as well as to the relevant regulations of the Norwegian Petroleum Directorate.

1. INTRODUCTION

The loads for which an offshore structure must be designed can be classified into thefollowing categories:

1. Permanent (dead) loads.2. Operating (live) loads.3. Environmental loads including earthquakes.4. Construction - installation loads.5. Accidental loads.

Whilst the design of buildings onshore is usually influenced mainly by the permanent andoperating loads, the design of offshore structures is dominated by environmental loads,


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where:

ρ is the wind density (ρ ≈ 1.225 Kg/m3)

Cs is the shape coefficient (Cs = 1,5 for beams and sides of buildings, Cs = 0,5 for

cylindrical sections and Cs = 1,0 for total projected area of platform).

Shielding and solidity effects can be accounted for, in the judgement of the designer,using appropriate coefficients.

For combination with wave loads, the DNV [4] and DOE-OG [7] rules recommend themost unfavourable of the following two loadings:

a. 1-minute sustained wind speeds combined with extreme waves.

b. 3-second gusts.

API-RP2A [2] distinguishes between global and local wind load effects. For the first caseit gives guideline values of mean 1-hour average wind speeds to be combined withextreme waves and current. For the second case it gives values of extreme wind speeds to be used without regard to waves.

Wind loads are generally taken as static. When, however, the ratio of height to the leasthorizontal dimension of the wind exposed object (or structure) is greater than 5, then thisobject (or structure) could be wind sensitive. API-RP2A requires the dynamic effects ofthe wind to be taken into account in this case and the flow induced cyclic wind loads dueto vortex shedding must be investigated.

2.2 Wave Loads

The wave loading of an offshore structure is usually the most important of allenvironmental loadings for which the structure must be designed. The forces on thestructure are caused by the motion of the water due to the waves which are generated bythe action of the wind on the surface of the sea. Determination of these forces requires thesolution of two separate, though interrelated problems. The first is the sea state computedusing an idealisation of the wave surface profile and the wave kinematics given by anappropriate wave theory. The second is the computation of the wave forces on individualmembers and on the total structure, from the fluid motion.

Two different analysis concepts are used:

• The design wave concept, where a regular wave of given height and period isdefined and the forces due to this wave are calculated using a high-order wavetheory. Usually the 100-year wave, i.e. the maximum wave with a return period of100 years, is chosen. No dynamic behaviour of the structure is considered. Thisstatic analysis is appropriate when the dominant wave periods are well above the


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period of the structure. This is the case of extreme storm waves acting on shallowwater structures.

• Statistical analysis on the basis of a wave scatter diagram for the location of thestructure. Appropriate wave spectra are defined to perform the analysis in thefrequency domain and to generate random waves, if dynamic analyses for extreme

wave loadings are required for deepwater structures. With statistical methods, themost probable maximum force during the lifetime of the structure is calculatedusing linear wave theory. The statistical approach has to be chosen to analyze thefatigue strength and the dynamic behaviour of the structure.

2.2.1 Wave theories

Wave theories describe the kinematics of waves of water on the basis of potential theory.In particular, they serve to calculate the particle velocities and accelerations and thedynamic pressure as functions of the surface elevation of the waves. The waves areassumed to be long-crested, i.e. they can be described by a two-dimensional flow field,

and are characterized by the parameters: wave height (H), period (T) and water depth (d)as shown in Figure 1.

Different wave theories of varying complexity, developed on the basis of simplifyingassumptions, are appropriate for different ranges of the wave parameters. Among themost common theories are: the linear Airy theory, the Stokes fifth-order theory, thesolitary wave theory, the cnoidal theory, Dean's stream function theory and the numerical


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theory by Chappelear. For the selection of the most appropriate theory, the graph shownin Figure 2 may be consulted. As an example, Table 1 presents results of the linear wavetheory for finite depth and deep water conditions. Corresponding particle paths areillustrated in Figures 3 and 4. Note the strong influence of the water depth on the wavekinematics. 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 irregularappearance results from the linear superposition of an infinite number of regular waveswith varying frequency (Figure 5). The best means to describe a random sea state is usingthe wave energy density spectrum S(f), usually called the wave spectrum for simplicity. Itis formulated as a function of the wave frequency f using the parameters: significantwave height Hs (i.e. the mean of the highest third of all waves present in a wave train)


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and mean wave period (zero-upcrossing period) To. As an additional parameter thespectral width can be taken into account.

Wave directionality can be introduced by means of a directional spreading function

D(f,σ), where σ is the angle of the wave approach direction (Figure 6). A directionalwave spectrum S (f,σ) can then be defined as:

S (f,σ ) = S(f).D (f,σ ) (3)


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The response of the structure, i.e. forces, motions, is calculated by multiplication of thewave energy spectrum with the square of a linear transfer function. From the resultingresponse spectrum the significant and the maximum expected response in a given timeinterval can be easily deduced.

For long-term statistics, a wave scatter diagram for the location of the structure is needed.It can be obtained from measurements over a long period or be deduced from weatherobservations in the region (the so-called hindcast method). The scatter diagram containsthe joint probability of occurrence of pairs of significant wave height and mean wave period. For every pair of parameters the wave spectrum is calculated by a standardformula, e.g. Pierson-Moskowitz (Figure 6), yielding finally the desired response


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spectrum. For fatigue analysis the total number and amplitude of load cycles during thelife-time of the structure can be derived in this way. For structures with substantialdynamic response to the wave excitation, the maximum forces and motions have to becalculated by statistical methods or a time-domain analysis.

2.2.3 Wave forces on structural members

Structures exposed to waves experience substantial forces much higher than windloadings. The forces result from the dynamic pressure and the water particle motions.Two different cases can be distinguished:

• Large volume bodies, termed hydrodynamic compact structures, influence thewave field by diffraction and reflection. The forces on these bodies have to bedetermined by costly numerical calculations based on diffraction theory.

• Slender, hydrodynamically transparent structures have no significant influence onthe wave field. The forces can be calculated in a straight-forward manner with

Morison's equation. As a rule, Morison's equation may be applied when D/L ≤ 0.2,where D is the member diameter and L is the wave length.

The steel jackets of offshore structures can usually be regarded as hydrodynamicallytransparent. The wave forces on the submerged members can therefore be calculated byMorison's equation, which expresses the wave force as the sum of an inertia force proportional to the particle acceleration and a non-linear drag force proportional to the

square of the particle velocity: (4)

where

F is the wave force per unit length on a circular cylinder (N)

v, |v| are water particle velocity normal to the cylinder, calculated with the selected wavetheory at the cylinder axis (m/s)

are water particle acceleration normal to the cylinder, calculated with the selected wavetheory at the cylinder axis (m/s2)

ρ is the water density (kg/m3)

D is the member diameter, including marine growth (m)

CD, CM are drag and inertia coefficients, respectively.

In this form the equation is valid for fixed tubular cylinders. For the analysis of themotion response of a structure it has to be modified to account for the motion of thecylinder [10]. The values of CD and CM depend on the wave theory used, surface


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roughness and the flow parameters. According to API-RP2A, CD ≈ 0,6 to 1,2 and CM ≈ 1,3 to 2,0. Additional information can be found in the DNV rules [4].

The total wave force on each member is obtained by numerical integration over thelength of the member. The fluid velocities and accelerations at the integration points are

found by direct application of the selected wave theory.

According to Morison's equation the drag force is non-linear. This non-linear formulationis used in the design wave concept. However, for the determination of a transfer functionneeded for frequency domain calculations, the drag force has to be linearized in a suitableway [9]. Thus, frequency domain solutions are appropriate for fatigue life calculations,for which the forces due to the operational level waves are dominated by the linear inertiaterm. The nonlinear formulation and hence time domain solutions are required fordynamic analyses of deepwater structures under extreme, storm waves, for which thedrag portion of the force is the dominant part [10].

In addition to the forces given by Morison's equation, the lift forces FD and the slammingforces FS, typically neglected in global response computations, can be important for localmember design. For a member section of unit length, these forces can be estimated asfollows:

FL = (1/2) ρ CL Dv2 (5)

FS = (1/2) ρ Cs Dv2 (6)

where CL, CS are the lift and slamming coefficients respectively, and the rest of thesymbols are as defined in Morison's equation. Lift forces are perpendicular to the

member axis and the fluid velocity v and are related to the vortex shedding frequency.Slamming forces acting on the underside of horizontal members near the mean water

level are impulsive and nearly vertical. Lift forces can be estimated by taking CL ≈ 1,3 CD.For tubular members Cs ≈ π.

2.3 Current Loads

There are tidal, circulation and storm generated currents. Figure 7 shows a wind and tidalcurrent profile typical of the Gulf of Mexico. When insufficient field measurements areavailable, current velocities may be obtained from various sources, e.g. Appendix A ofDNV [4]. In platform design, the effects of current superimposed on waves are taken into

account by adding the corresponding fluid velocities vectorially. Since the drag forcevaries with the square of the velocity, this addition can greatly increase the forces on a platform. For slender members, cyclic loads induced by vortex shedding may also beimportant and should be examined.


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2.4 Earthquake Loads

Offshore structures in seismic regions are typically designed for two levels of earthquakeintensity: the strength level and the ductility level earthquake. For the strength levelearthquake, defined as having a "reasonable likelihood of not being exceeded during the platform's life" (mean recurrence interval ~ 200 - 500 years), the structure is designed torespond elastically. For the ductility level earthquake, defined as close to the "maximumcredible earthquake" at the site, the structure is designed for inelastic response and to

have adequate reserve strength to avoid collapse.

For strength level design, the seismic loading may be specified either by sets ofaccelerograms (Figure 8) or by means of design response spectra (Figure 9). Use ofdesign spectra has a number of advantages over time history solutions (base accelerationinput). For this reason design response spectra are the preferable approach for strengthlevel designs. If the design spectral intensity, characteristic of the seismic hazard at thesite, is denoted by amax, then API-RP2A recommends using amax for the two principalhorizontal directions and 0,5amax for the vertical direction. The DNV rules, on the otherhand, recommend amax and 0,7 amax for the two horizontal directions (two differentcombinations) and 0,5 amax for the vertical. The value of amax and often the spectral

shapes are determined by site specific seismological studies.


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Designs for ductility level earthquakes will normally require inelastic analyses for whichthe seismic input must be specified by sets of 3-component accelerograms, real orartificial, 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 designspectra, which are usually the result of a site specific seismotectonic study. More detail ofthe analysis of earthquakes is given in the Lectures 17: Seismic Design.


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2.5 Ice and Snow Loads

Ice is a primary problem for marine structures in the arctic and sub-arctic zones. Iceformation and expansion can generate large pressures that give rise to horizontal as wellas 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 asfollows:

Fi = Cif cA (7)

where:

A is the exposed area of structure,

f c is the compressive strength of ice,

Ci is the coefficient accounting for shape, rate of load application and other factors, withusual values between 0,3 and 0,7.

Generally, detailed studies based on field measurements, laboratory tests and analyticalwork 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 andwind loads, the latter by increasing areas exposed to the action of wind. More detailedinformation 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 thermalstresses. To take account of such stresses, extreme values of sea and air temperatureswhich are likely to occur during the life of the structure must be estimated. Relevant datafor 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 ofcryogenic material, which must be taken into account in design as accidental loads. Thetemperature of the oil and gas produced must also be considered.

2.7 Marine Growth

Marine growth is accumulated on submerged members. Its main effect is to increase thewave forces on the members by increasing not only exposed areas and volumes, but alsothe drag coefficient due to higher surface roughness. In addition, it increases the unitmass of the member, resulting in higher gravity loads and in lower member frequencies.Depending upon geographic location, the thickness of marine growth can reach 0,3m or


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more. It is accounted for in design through appropriate increases in the diameters andmasses of the submerged members.

2.8 Tides

Tides affect the wave and current loads indirectly, i.e. through the variation of the level ofthe sea surface. The tides are classified as: (a) astronomical tides - caused essentiallyfrom the gravitational pull of the moon and the sun and (b) storm surges - caused by thecombined action of wind and barometric pressure differentials during a storm. Thecombined effect of the two types of tide is called the storm tide. Tide dependent waterlevels and the associated definitions, as used in platform design, are shown in Figure 10.The astronomical tide range depends on the geographic location and the phase of themoon. Its maximum, the spring tide, occurs at new moon. The range varies fromcentimetres to several metres and may be obtained from special maps. Storm surgesdepend upon the return period considered and their range is on the order of 1,0 to 3,0m.When designing a platform, extreme storm waves are superimposed on the still water

level (see Figure 10), while for design considerations such as levels for boat landing places, barge fenders, upper limits of marine growth, etc., the daily variations of theastronomical tide are used.

2.9 Sea Floor Movements

Movement of the sea floor can occur as a result of active geologic processes, storm wave pressures, earthquakes, pressure reduction in the producing reservoir, etc. The loadsgenerated by such movements affect, not only the design of the piles, but the jacket aswell. Such forces are determined by special geotechnical studies and investigations.


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• Environmental loads form a major category of loads which control many aspectsof platform design.

• The main environmental loads are due to wind, waves, current, earthquakes, ice

and snow, temperature variations, marine growth, tides and seafloor movements.• Widely accepted rules of practice, listed as [1] - [13], provide guideline values for

most environmental loads.• For major structures, specification of environmental design loads requires specific

studies.• Some environmental loads can be highly uncertain.• The definition of certain environmental loads depends upon the type of analysis

used in the design.

4. REFERENCES

[1] Eurocode 8: "Structures in Seismic Regions - Design", CEN (in preparation).

[2] API-RP2A, "Recommended Practice for Planning, Designing and Constructing FixedOffshore Platforms", American Petroleum Institute, Washington, D.C., 18th ed., 1989.

[3] OCS, "Requirements for Verifying the Structural Integrity of OCS Platforms".,United States Geologic Survey, National Centre, Reston, Virginia, 1980.

[4] DNV, "Rules for the Design, Construction and Inspection of Offshore Structures",Det Norske Veritas, Oslo, 1977 (with corrections 1982).

[5] NPD, "Regulation for Structural Design of Load-bearing Structures Intended forExploitation of Petroleum Resources", Norwegian Petroleum Directorate, 1985.

[6] BS6235, "Code of Practice for Fixed Offshore Structures", British StandardsInstitution, London, 1982.

[7] DOE-OG, "Offshore Installation: Guidance on Design and Construction", U.K., Dept.of Energy, London 1985.

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

[9] Clauss, G. T. et al: "Offshore Structures, Vol 1 - Conceptual Design andHydromechanics", Springer, London 1992.

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


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[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 θ = kx - ω t

Relative water depth d/L

Deep water

d/L ≥ 0,5

Finite water depth

d/L < 0,5

Velocity potential

θ

Surface elevation z

Dynamic pressure

pdyn =

ζa cos θ

ρ gζa ekz cos θ

ζa cos θ

Water particle velocities

horizontal u =

vertical w =

ζa ω ekz cos θ

ζa ω ekz sin θ

Water particleaccelerations

horizontal u' =

vertical w' =

ζa ω2 ekz sin θ

-ζa ω2 ekz cos θ


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Wave celerity c =

Group velocity cgr

=

Circular frequency

ω =

Wave length L =

Wave number k =

co =

cgr =

ω =

Lo =

ko =

c =

cgr =

ω =

L =

kd tanh kd =

Water particledisplacements

horizontal ξ

vertical ζ

Particle trajectories

-ζa ekz sin θ

ζa ekz cos θ

Circular orbits Elliptical orbits

Where ζ a =


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Sealed tubular members must be designed for the worst condition when flooded or non-flooded.

2. OPERATING (LIVE) LOADS

Operating loads arise from the operations on the platform and include the weight of allnon-permanent equipment or material, as well as forces generated during operation ofequipment. More specifically, operating loads include the following:

a. The weight of all non-permanent equipment (e.g. drilling, production), facilities (e.g.living quarters, furniture, life support systems, heliport, etc), consumable supplies, liquids,etc.

b. Forces generated during operations, e.g. drilling, vessel mooring, helicopter landing,crane operations, etc.

The necessary data for computation of all operating loads are provided by the operatorand the equipment manufacturers. The data need to be critically evaluated by the designer.An example of detailed live load specification is given in Table 1 where the values in thefirst and second columns are for design of the portions of the structure directly affected by the loads and the reduced values in the last column are for the structure as a whole. Inthe absence of such data, the following values are recommended in BS6235 [1]:

a. crew quarters and passageways: 3,2 KN/m2

b. working areas: 8,5 KN/m2

c. storage areas: γH KN/m2

where

γ is the specific weight of stored materials, not to be taken less than 6,87KN/m3,

H is the storage height (m).

Forces generated during operations are often dynamic or impulsive in nature and must betreated as such. For example, according to the BS6235 rules, two types of helicopterlanding should be considered, heavy and emergency landing. The impact load in the first

case is to be taken as 1,5 times the maximum take-off weight, while in the second casethis factor becomes 2,5. In addition, a horizontal load applied at the points of impact andtaken equal to half the maximum take-off weight must be considered. Loads fromrotating machinery, drilling equipment, etc. may normally be treated as harmonic forces.For vessel mooring, design forces are computed for the largest ship likely to approach atoperational speeds. According to BS6235, the minimum impact to be considered is of avessel of 2500 tonnes at 0,5 m/s.


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3. FABRICATION AND INSTALLATION LOADS

These loads are temporary and arise during fabrication and installation of the platform orits components. During fabrication, erection lifts of various structural componentsgenerate lifting forces, while in the installation phase forces are generated during

platform loadout, transportation to the site, launching and upending, as well as duringlifts related to installation.

According to the DNV rules [2], the return period for computing design environmentalconditions for installation as well as fabrication should normally be three times theduration of the corresponding phase. API-RP2A, on the other hand [3], leaves this designreturn period up to the owner, while the BS6235 rules [1] recommend a minimumrecurrence interval of 10 years for the design environmental loads associated withtransportation of the structure to the offshore site.

3.1 Lifting Forces

Lifting forces are functions of the weight of the structural component being lifted, thenumber and location of lifting eyes used for the lift, the angle between each sling and thevertical axis and the conditions under which the lift is performed (Figure 1). All membersand connections of a lifted component must be designed for the forces resulting fromstatic equilibrium of the lifted weight and the sling tensions. Moreover, API-RP2Arecommends that in order to compensate for any side movements, lifting eyes and theconnections to the supporting structural members should be designed for the combinedaction of the static sling load and a horizontal force equal to 5% this load, applied perpendicular to the padeye at the centre of the pin hole. All these design forces areapplied as static loads if the lifts are performed in the fabrication yard. If, however, the

lifting derrick or the structure to be lifted is on a floating vessel, then dynamic loadfactors should be applied to the static lifting forces. In particular, for lifts made offshoreAPI-RP2A recommends two minimum values of dynamic load factors: 2,0 and 1,35. Thefirst is for designing the padeyes as well as all members and their end connectionsframing the joint where the padeye is attached, while the second is for all other memberstransmitting lifting forces. For loadout at sheltered locations, the corresponding minimumload factors for the two groups of structural components become, according to API-RP2A,1,5 and 1,15, respectively.


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3.2 Loadout Forces

These are forces generated when the jacket is loaded from the fabrication yard onto the barge. If the loadout is carried out by direct lift, then, unless the lifting arrangement isdifferent from that to be used for installation, lifting forces need not be computed,


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because lifting in the open sea creates a more severe loading condition which requireshigher dynamic load factors. If loadout is done by skidding the structure onto the barge, anumber of static loading conditions must be considered, with the jacket supported on itsside. Such loading conditions arise from the different positions of the jacket during theloadout phases, (as shown in Figure 2), from movement of the barge due to tidal

fluctuations, marine traffic or change of draft, and from possible support settlements.Since movement of the jacket is slow, all loading conditions can be taken as static.Typical values of friction coefficients for calculation of skidding forces are the following:

• steel on steel without lubrication............................................ 0,25• steel on steel with lubrication............................................... 0,15• steel on teflon.................................................................. 0,10• teflon on teflon................................................................. 0,08


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


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2. Exposure time and reliability of predicted "weather windows"3. Accessibility of safe havens4. Seasonal weather system5. 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.

Transportation forces are generated by the motion of the tow, i.e. the structure andsupporting barge. They are determined from the design winds, waves and currents. If thestructure is self-floating, the loads can be calculated directly. According to API-RP2A [3],towing analyses must be based on the results of model basin tests or appropriateanalytical methods and must consider wind and wave directions parallel, perpendicular

and at 45° to the tow axis. Inertial loads may be computed from a rigid body analysis ofthe tow by combining roll and pitch with heave motions, when the size of the tow,magnitude of the sea state and experience make such assumptions reasonable. For opensea conditions, the following may be considered as typical design values:

Single - amplitude roll: 20°

Single - amplitude pitch: 10°

Period of roll or pitch: 10 second

Heave acceleration: 0,2 g

When transporting a large jacket by barge, stability against capsizing is a primary designconsideration because of the high centre of gravity of the jacket. Moreover, the relative

stiffness of jacket and barge may need to be taken into account together with the waveslamming forces that could result during a heavy roll motion of the tow (Figure 4) whenstructural analyses are carried out for designing the tie-down braces and the jacketmembers affected by the induced loads. Special computer programs are available tocompute the transportation loads in the structure-barge system and the resulting stressesfor any specified environmental condition.


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3.4 Launching and Upending Forces

These forces are generated during the launch of a jacket from the barge into the sea andduring the subsequent upending into its proper vertical position to rest on the seabed. Aschematic view of these operations can be seen in Figure 5.


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There are five stages in a launch-upending operation:

a. Jacket slides along the skid beams

b. Jacket rotates on the rocker arms

c. Jacket rotates and slides simultaneously

d. Jacket detaches completely and comes to its floating equilibrium position

e. Jacket is upended by a combination of controlled flooding and simultaneous lifting bya derrick barge.

The loads, static as well as dynamic, induced during each of these stages and the forcerequired to set the jacket into motion can be evaluated by appropriate analyses, whichalso consider the action of wind, waves and currents expected during the operation.

To start the launch, the barge must be ballasted to an appropriate draft and trim angle andsubsequently the jacket must be pulled towards the stern by a winch. Sliding of the jacketstarts as soon as the downward force (gravity component and winch pull) exceeds thefriction force. As the jacket slides, its weight is supported on the two legs that are part of

the launch trusses. The support length keeps decreasing and reaches a minimum, equal tothe length of the rocker beams, when rotation starts. It is generally at this instant that themost severe launching forces develop as reactions to the weight of the jacket. Duringstages (d) and (e), variable hydrostatic forces arise which have to be considered at allmembers affected. Buoyancy calculations are required for every stage of the operation toensure fully controlled, stable motion. Computer programs are available to perform thestress analyses required for launching and upending and also to portray the wholeoperation graphically.


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4. ACCIDENTAL LOADS

According to the DNV rules [2], accidental loads are loads, ill-defined with respect tointensity and frequency, which may occur as a result of accident or exceptionalcircumstances. Accidental loads are also specified as a separate category in the NPD

regulations [4], but not in API-RP2A [3], BS6235 [1] or the DOE-OG rules [5].Examples of accidental loads are loads due to collision with vessels, fire or explosion,dropped objects, and unintended flooding of bouyancy tanks. Special measures arenormally taken to reduce the risk from accidental loads. For example, protection ofwellheads or other critical equipment from a dropped object can be provided by speciallydesigned, impact resistant covers. According to the NPD regulations [4], an accidentalload can be disregarded if its annual probability of occurrence is less than 10

-4. This

number is meant as an order of magnitude estimate and is extremely difficult to compute.Earthquakes are treated as an environmental load in offshore structure design.

5. LOAD COMBINATIONSThe load combinations used for designing fixed offshore structures depend upon thedesign method used, i.e. whether limit state or allowable stress design is employed. Theload combinations recommended for use with allowable stress procedures are:

a. Dead loads plus operating environmental loads plus maximum live loads, appropriateto normal operations of the platform.

b. Dead loads plus operating environmental loads plus minimum live loads, appropriateto normal operations of the platform.

c. Dead loads plus extreme (design) environmental loads plus maximum live loads,appropriate for combining with extreme conditions.

d. Dead loads plus extreme (design) environmental loads plus minimum live loads,appropriate for combining with extreme conditions.

Moreover, environmental loads, with the exception of earthquake loads, should becombined in a manner consistent with their joint probability of occurrence during theloading condition considered. Earthquake loads, if applicable, are to be imposed as aseparate environmental load, i.e., not to be combined with waves, wind, etc. Operating

environmental conditions are defined as representative of severe but not necessarilylimiting conditions that, if exceeded, would require cessation of platform operations.

The DNV rules [2] permit allowable stress design but recommend the semi-probabilisticlimit state design method, which the NPD rules also require [4]. BS6235 permits bothmethods but the design equations it gives are for the allowable stress method [1]. API-RP2A is very specific in recommending not to apply limit state methods. According tothe DNV and the NPD rules for limit state design, four limit states must be checked:


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1. Ultimate limit state

For this limit state the following two loading combinations must be used:

Ordinary: 1,3 P + 1,3 L + 1,0 D + 0,7 E, and

Extreme : 1,0 P + 1,0 L + 1,0 D + 1,3 E

where P, L, D and E stand for Permanent (dead), Operating (live), Deformation(e.g., temperature, differential settlement) and Environmental loads respectively.For well controlled dead and live loads during fabrication and installation, theload factor 1,3 may be reduced to 1,2. Furthermore, for structures that areunmanned during storm conditions and which are not used for storage of oil andgas, the 1,3 load factor for environmental loads - except earthquakes - may bereduced to 1,15.

2. Fatigue limit state

All load factors are to be taken as 1,0.

3. Progressive Collapse limit state


4. Serviceability limit state


The so-called characteristic values of the loads used in the above combinations and limitstates are summarized in Table 2, taken from the NPD rules.


• In addition to environmental loads, an offshore structure must be designed fordead and live loads, fabrication and installation loads as well as accidental loads.

• Widely accepted rules of practice, listed in the references, are usually followed forspecifying such loads.

• The type and magnitude of fabrication, transportation and installation loads

depend upon the methods and sequences used for the corresponding phases.• Dynamic and impact effects are normally taken into account by means of

appropriate dynamic load factors.• Accidental loads are not well defined with respect to intensity and probability of

occurrence. They will typically require special protective measures.• Load combinations and load factors depend upon the design method to be used.

API-RP2A is based on allowable stress design and recommends against limit state


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design, BSI favours allowable stress design, while DNV and NPD recommendlimit state design.

7. REFERENCES

[1] BS6235, "Code of Practice for Fixed Offshore Structures", British StandardsInstitution, London, 1982.

[2] "Rules for the Design, Construction and Inspection of Offshore Structures", Det Norske Veritas (DNV), Oslo, 1977 (with corrections 1982).

[3] API-RP2A, "Recommended Practice for Planning, Designing and Constructing FixedOffshore Platforms", American Petroleum Institute, Washington, D.C., 18th ed., 1989.

[4] "Regulation for Structural Design of Load-bearing Structures Intended forExploitation of Petroleum Resources", Norwegian Petroleum Directorate (NPD), 1985.

[5] DOE-OG, "Offshore Installation: Guidance on Design and Construction", U.K.Department of Energy, London 1985.

8. ADDITIONAL READING

1. OCS, "Requirements for Verifying the Structural Integrity of OCS Platforms".,United States Geologic Survey, National Centre, Reston, Virginia, 1980.

2. Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co.,Houston, 1981.

3. Graff, W.G., "Introduction to Offshore Structures", Gulf Publishing Co., Houston,1981.4. Gerwick, B.C. Jr., "Construction of Offshore Structures", John Wiley, New York,

1986.

Table 1 Minimum design live load specification

Loads to be taken into account(kN/m2)

For portions of the structure For the structure as awhole

Zone considered Flooring and

joists

Other

components

(3)

Process zone (around wells andlarge-scale machines)

5 (1) 5 (1) 2.5

Drilling zone 5 (1) 5 (1) 2.5


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Catwalks and walking platforms(except emergency exits)

3 2.5 1

Stairways (except emergencyexits)

4 3 0

Module roofing 2 1.5 1

Emergency exits 5 5 0

STORAGE

Storage floors - heavy

Storage floors - light

18

9

12

6

8 (2)

4 (2)

Delivery zone 10 10 5

Non-attributed area 6 4 3

(1) Accumulated with a point load equal to the weight of the heaviest part likely to beremoved, with a minimum value of 5 kN. Point loads are assumed as being applied to a

0,3m × 0,3m surface.

(2) Applied on the entirety of the flooring surface (including traffic).

(3) This column gives the loads to be taken into account for the structure's overallcalculation. These values are the input for the computer runs.

Table 2 Characteristic Loads according to NPD [4]

LIMIT STATES FOR TEMPORARY PHASES LIMIT STATES FO

Progressive Collapse

LOAD

TYPE

Serv

iceability

Fatig

ue

Ultimate Abnormal effects Damage condition

Serviceabili

ty

Fatig

ue

DEAD

EXPECTED VALUE

LIVE SPECIFIED VALUE


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DEFORMATION

EXPECTED EXTREME VALUE

ENVIRONMENTAL

Dependentonoperationalrequirements

Expected loadhistory

Valuedependent on

measures taken

Dependentonoperationalrequirements

Expected load history

ACC

IDENTAL

NOT APPLICABLE Dependent on

operationalrequirements

NOT APPLICABLE

Lecture 15A.4 - Analysis I

OBJECTIVE/SCOPE

To present the main analysis procedures for offshore structures.

PREREQUISITES

Lecture 15A.1: Offshore Structures: General Introduction

Lecture 15A.2: Loads I: Introduction and Environmental Loads


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Lecture 15A.3: Loads II: Other Loads

RELATED LECTURES

Lecture 15A.5: Analysis II

SUMMARY

Analytical models used in offshore engineering are briefly described. Acceptance criteriafor the verification of offshore structures are presented.

Simple rules for preliminary member sizing are given and procedures for static in-placeand dynamic analysis are described.

1. ANALYTICAL MODEL

The analysis of an offshore structure is an extensive task, embracing consideration of thedifferent stages, i.e. execution, installation, and in-service stages, during its life. Manydisciplines, e.g. structural, geotechnical, naval architecture, metallurgy are involved.

This lecture and Lecture 15A.5 are purposely limited to presenting an overview ofavailable analysis procedures and providing benchmarks for the reader to appreciate thevalidity of his assumptions and results. They primarily address jackets, which are moreunusual structures compared to decks and modules, and which more closely resembleonshore petro-chemical plants.

2. ANALYTICAL MODELThe analytical models used in offshore engineering are in some respects similar to thoseadopted for other types of steel structures. Only the salient features of offshore modelsare presented here.

The same model is used throughout the analysis process with only minor adjustments being made to suit the specific conditions, e.g. at supports in particular, relating to eachanalysis.

2.1 Stick Models

Stick models (beam elements assembled in frames) are used extensively for tubularstructures (jackets, bridges, flare booms) and lattice trusses (modules, decks).


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2.1.1 Joints

Each member is normally rigidly fixed at its ends to other elements in the model.

If more accuracy is required, particularly for the assessment of natural vibration modes,

local flexibility of the connections may be represented by a joint stiffness matrix.

2.1.2 Members

In addition to its geometrical and material properties, each member is characterised byhydrodynamic coefficients, e.g. relating to drag, inertia, and marine growth, to allowwave forces to be automatically generated.

2.2 Plate Models

Integrated decks and hulls of floating platforms involving large bulkheads are described

by plate elements. The characteristics assumed for the plate elements depend on the principal state of stress which they are subjected to. Membrane stresses are taken whenthe element is subjected merely to axial load and shear. Plate stresses are adopted when bending and lateral pressure are to be taken into account.

3. ACCEPTANCE CRITERIA

3.1 Code Checks

The verification of an element consists of comparing its characteristic resistance(s) to adesign force or stress. It includes:

• a strength check, where the characteristic resistance is related to the yield strengthof the element,

• a stability check for elements in compression where the characteristic resistancerelates to the buckling limit of the element.

An element (member or plate) is checked at typical sections (at least both ends andmidspan) against resistance and buckling. This verification also includes the effect ofwater pressure for deepwater structures.

Tubular joints are checked against punching under various load patterns. These checksmay indicate the need for local reinforcement of the chord using overthickness or internalring-stiffeners.

Elements should also be verified against fatigue, corrosion, temperature or durabilitywherever relevant.

3.2 Allowable Stress Method


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This method is presently specified by American codes (API, AISC).

The loads remain unfactored and a unique coefficient is applied to the characteristicresistance to obtain an allowable stress as follows:

Condition Axial Strong axis bending

Weak axis bending

Normal 0,60 0,66 0,75

Extreme 0,80 0,88 1,00

"Normal" and "Extreme" respectively represent the most severe conditions:

• under which the plant is to operate without shut-down.

• the platform is to endure over its lifetime.

3.3 Limit State Method

This method is enforced by European and Norwegian Authorities and has now beenadopted by API as it offers a more uniform reliability.

Partial factors are applied to the loads and to the characteristic resistance of the element,reflecting the amount of confidence placed in the design value of each parameter and thedegree of risk accepted under a limit state, i.e:

• Ultimate Limit State (ULS):

corresponds to an ultimate event considering the structural resistance with appropriatereserve.

• Fatigue Limit State (FLS):

relates to the possibility of failure under cyclic loading.

• Progressive Collapse Limit State (PLS):

reflects the ability of the structure to resist collapse under accidental or abnormalconditions.

• Service Limit State (SLS):

corresponds to criteria for normal use or durability (often specified by the plant operator).

3.3.1 Load factors


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Norwegian Authorities (2, 4) specify the following sets of load factors:

Load CategoriesLimit State

P L D E A

ULS (normal) 1,3 1,3 1,0 0,7 0,0

ULS (extreme) 1,0 1,0 1,0 1,3 0,0

FLS 0,0 0,0 0,0 1,0 0,0

PLS (accidental) 1,0 1,0 1,0 1,0 1,0

PLS (post-damage) 1,0 1,0 1,0 1,0 0,0

SLS 1,0 1,0 1,0 1,0 0,0

where the respective load categories are:

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

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

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 forPLS and SLS design.

3.3.3 Classification of Design Conditions

Guidance for classifying typical conditions into typical limit states is given in thefollowing table:

Loadings Condition

P/L E D A

Design

Criterion


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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

In-Place (extreme) P + L wind & 100 year wave actual ULS

SLS

In-Place (exceptional) P + L wind & 10000 yearwave

actual PLS

Earthquake P + L 10-2 quake ULS

Rare Earthquake P + L 10-4 quake 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 adjustmentof the member sizes with respect to the forces they transmit, until a safe and economicaldesign is achieved.

It is therefore of the utmost importance to start the main analysis from a model which isclose to the final optimized one.


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The simple rules given below provide an easy way of selecting realistic sizes for the mainelements of offshore structures in moderate water depth (up to 80m) where dynamiceffects are negligible.

4.1 Jacket Pile Sizes

• calculate the vertical resultant (dead weight, live loads, buoyancy), the overallshear and the overturning moment (environmental forces) at the mudline.

• assuming that the jacket behaves as a rigid body, derive the maximum axial andshear force at the top of the pile.

• select a pile diameter in accordance with the expected leg diameter and thecapacity of pile driving equipment.

• derive the penetration from the shaft friction and tip bearing diagrams.• assuming an equivalent soil subgrade modulus and full fixity at the base of the

jacket, calculate the maximum moment in the pile and derive its wall thickness.

4.2 Deck Leg Sizes

• adapt the diameter of the leg to that of the pile.• determine the effective length from the degree of fixity of the leg into the deck

(depending upon the height of the cellar deck).• calculate the moment caused by wind loads on topsides and derive the appropriate

thickness.

4.3 Jacket Bracings

• select the diameter in order to obtain a span/diameter ratio between 30 and 40.

• calculate the axial force in the brace from the overall shear and the local bendingcaused by the wave assuming partial or total end restraint.

• derive the thickness such that the diameter/thickness ratio lies between 20 and 70

and eliminate any hydrostatic buckle tendency by imposing D/t


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The static in-place analysis is the basic and generally the simplest of all analyses. Thestructure is modelled as it stands during its operational life, and subjected to pseudo-staticloads.

This analysis is always carried at the very early stage of the project, often from a

simplified model, to size the main elements of the structure.

5.1 Structural Model

5.1.1 Main Model

The main model should account for eccentricities and local reinforcements at the joints.

Typical models for North Sea jackets may feature over 800 nodes and 4000 members.

5.1.2 Appurtenances

The contribution of appurtenances (risers, J-tubes, caissons, conductors, boat-fenders, etc.)to the overall stiffness of the structure is normally neglected.

They are therefore analysed separately and their reactions applied as loads at theinterfaces with the main structure.

5.1.3 Foundation Model

Since their behaviour is non-linear, foundations are often analysed separately from thestructural model.

They are represented by an equivalent load-dependent secant stiffness matrix;coefficients are determined by an iterative process where the forces and displacements atthe common boundaries of structural and foundation models are equated.

This matrix may need to be adjusted to the mean reaction corresponding to each loadingcondition.

5.2 Loadings

This Section is a reminder of the main types of loads, which are described in more detailin Lectures 15A.2 and 15A.3.


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5.2.1 Gravity Loads

Gravity loads consist of:

• dead weight of structure and equipments.

• live loads (equipments, fluids, personnel).

Depending on the area of structure under scrutiny, live loads must be positioned to produce the most severe configuration (compression or tension); this may occur forinstance when positioning the drilling rig.

5.2.2 Environmental Loads

Environmental loads consist of wave, current and wind loads assumed to actsimultaneously in the same direction.

In general eight wave incidences are selected; for each the position of the crest relative tothe platform must be established such that the maximum overturning moment and/orshear are produced at the mudline.

5.3 Loading Combinations

The static in-place analysis is performed under different conditions where the loads areapproximated by their pseudo-static equivalent.

The basic loads relevant to a given condition are multiplied by the appropriate loadfactors and combined to produce the most severe effect in each individual element of the

structure.

6. DYNAMIC ANALYSIS

A dynamic analysis is normally mandatory for every offshore structure, but can berestricted to the main modes in the case of stiff structures.

6.1 Dynamic Model

The dynamic model of the structure is derived from the main static model.

Some simplifications may however take place:

• local joint reinforcements and eccentricities may be disregarded.• masses are lumped at the member ends.• the foundation model may be derived from cyclic soil behaviour.


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6.2 Equations of Motion

The governing dynamic equations of multi-degrees-of-freedom systems can be expressed

in the matrix form:

MX'' + CX' + KX = P(t)

where

M is the mass matrix

C is the damping matrix

K is the stiffness matrix

X, X', X'' are the displacement, velocity and acceleration vectors (function

of time).

P(t) is the time dependent force vector; in the most general case it may depend on thedisplacements of the structure also (i.e. relative motion of the structure with respect to thewave velocity in Morison equation).

6.2.1 Mass

The mass matrix represents the distribution of masses over the structure.

Masses include that of the structure itself, the appurtenances, liquids trapped in legs ortanks, the added mass of water (mass of water displaced by the member and determinedfrom potential flow theory) and the mass of marine growth.

Masses are generally lumped at discrete points of the model. The mass matrixconsequently becomes diagonal but local modes of vibration of single members areignored (these modes may be important for certain members subjected to an earthquake).The selection of lumping points may significantly affect the ensuing solution.

As a further simplification to larger models involving considerable degrees-o

15a.1 offshore structures

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