15A11 Superstructures II

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ESDEP WG 15A

STRUCTURAL SYSTEMS: OFFSHORE

Lecture 15A.11 - Superstructures IIOBJECTIVE/SCOPE

To elaborate on structural steel concepts for integrated decks, module support frames, and modules. Toshow principles and methods of construction (from yard to offshore site).

PRE-REQUISITES

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

Lecture 7.6: Built-up Columns

Lectures 8.4: Plate Girder Behaviour and Design

Lecture 11.2: Welded Connections

Lecture 12.2: Advanced Introduction to Fatigue

Lecture 15A: Offshore Structures

SUMMARY

Structural systems for each type of topside structure are introduced, i.e. truss, portal frame, box girder,and stressed skin.

Some special topics of design are addressed and the different construction phases are presented in moredetail, i.e.:

1. fabrication2. weighing3. load out4. sea transport5. offshore installation especially deckmating

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6. module installation7. hook-up8. commissioning.

A brief discussion on inspection and repair and on platform removal concludes this lecture.

1. INTRODUCTIONThis lecture deals with the structural design of jacket-based offshore deck structures, following theintroduction in Lecture 15A.10.

Heavy decks, over 10.000 tons, are provided with a module support frame onto which a number ofmodules are placed, see Lecture 15A.1, Figs. 4 and 5. Smaller decks, such as those located in thesouthern North Sea, are nowadays installed complete with all equipment in one lift to minimize offshorehook-up. Most of this lecture refers to this type of integrated deck as described in Lecture 15A.10.

The selection of the concept for the structural deck is made in close cooperation with the otherdisciplines.

For the design of the deck structure, the in-place condition has to be considered, together with thevarious previous stages such as fabrication, load-out, transport and installation.

A structural system for a deck structure comprises several of the following elements:

2. MAIN STRUCTURE DESIGN

2.1 Introduction

Some major topics in topside structural design are reviewed below.

2.2 Main Structure-Portal Frame Design

A portal frame design has been used in recent major projects in the Dutch sector such as Amoco P15,Placid K12 [5] and Penzoil L8.

The main girder/column joint, as shown in Figure 1, is very important in determining the height. It ismost practical to position the longitudinal and transverse main girder flanges at the same elevation.

Floors (steel plate or grating) }

Deck stringer (H beams, bulbs or troughs) } Discussed in

Horizontal bracing } Lecture 15A.10

Deck beams }

Primary girders }

Vertical trusses or bracing } Discussed in

Deck legs } this lecture




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Haunching of the transverse main girder , which is more lightly loaded-in-plane, however is not anoption as these girders become highly loaded during transport.

The severe restraint of welding a tubular in a diaphragm requires the selection of TTP steel for thecolumn section.

Due to the high importance of the diaphragm plates in the overall integrity of the structure and the

welding constraints on the web plates in between, TTP-steel is chosen also for the diaphragm.

Another option is to weld the girders directly onto the unstiffened can section of the column. Theassessment of ultimate resistance as well as fatigue strength has been the subject of recent research (seeLecture 15A.12).

Further improvement of the theoretical and experimental background is required. For lighter loaded trussstructures, this non-stiffened type of joint has been used successfully.

A third solution is to weld the girders directly to the can section of the column, which is internallystiffened by rings. Its most severe disadvantage is the difficulty of inspecting the column interior.

The disadvantage of both direct girder-column joints is that the girder sizing is governed by the veryhigh moments at the column/beam transition point.

Cast steel nodes form an alternative to the welded designs.

Member selection for portal frame structures with increasing section moduli usually includes:

300 mm wide rolled beam. 400 mm wide rolled beam. 450 mm /460 mm wide rolled beam.

castellated beams fabricated from rolled beams, giving a height 1,5 times the original beamheight. built-up girders fabricated from rolled beam T-sections with a web plate welded-in-between. plate girder.

The plate girder of course provides the greatest flexibility for design, material selection andprocurement, though its cost per tonne is approximately twice that of a rolled beam.

2.3 Main Structure-Truss Design

Most offshore structures of moderate size have been provided with a truss-type structure. Typically such

trusses consist of rolled beams as chords and tubulars as diagonals.

Truss design requires several choices which affect the structural efficiency and have impact on otherdisciplines:

number and configuration of braces falling or rising braces intermediate load carrying of chords presence of external moments on joints braces: tubulars or H-rolled sections




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chords: rolled section or plate girders truss joints: locally reinforced chord or prefabricated node section.

Figure 2 shows different arrangements of braces (basically N or W-type) obtained by variation of thenumber of nodes. It should be kept in mind that all diagonals and verticals form obstructions for pipingand cable routings of all kinds.

For the transverse trusses, transparency is even more important, especially near the well area. Thenumber of members required should therefore be reduced to a minimum.

Providing a W-truss with light verticals should be evaluated against choosing a heavier chord section.

If a joint, e.g. at the top deck, is subject to severe moments due to lifting, ventstack, or crane pedestal forexample, much of the bracing stress would result from unintended bending. Generally the deck legrestraint creates a similar problem in the lower deck. An evaluation should yield a preferred locationtherefore for the node of the end brace.

The truss deflects under its vertical load which leads to restraint of the chord in the column and tobending of the chord. Both effects can quite severely effect the efficiency. The chord section should be




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kept compact therefore and not given too much height.

Tubulars (circular, square or rectangular) or rolled sections can be chosen for the braces.

The choice depends primarily on the loads and the chord width. A chord width of 300mm canaccommodate a 10 in. brace only. Thus a wider chord flange is preferred.

2.4 Main Structure-Stressed Skin Design

A third major structural option is the stressed skin concept, where full height plate walls take thefunction of the truss or the frame.

Modules for living quarters are frequently built to this concept. Other types of modules have not beenbuilt with stressed skin since the obstruction they cause during construction is severe.

For smaller stressed skin modules, trapezoid corrugated plate can be used to provide a wall in a frame ofsquare hollow sections.

For bigger modules, flat plate stiffened with through-stiffeners is used for the walls.

The detailed design can only be made with a clear plan for assembling the module which shows thepanels that must be prefabricated.

2.5 Non-Load Bearing Walls

Blast or fire walls are provided in offshore platforms. Due to their function full welding to the mainstructure is often unavoidable, see Figure 3a.




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Special attention is required concerning:




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the capability of the walls to comply with the deformation of the main structure during load-out,sea transport, lifting and in-service.

the strength of welds to the main structure being stronger than the plate to avoid rupture andpotential crack initiation of the main structure.

One solution is to provide a flexible detail, see Figure 3b and 3c, with stiffeners falling short.

2.6 Crane Pedestals

Crane pedestal, are discussed briefly below.

It is structurally economical to put the crane pedestal on top of a main column. For a truss type the mainstructure will be close to the platform periphery so a moderate length of crane boom is sufficient.

For a portal frame type with columns closer to the outer periphery, the pedestal requires a specialcolumn in order to avoid using a crane with large boom length. Figure 4 depicts such a solution.




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The functions of the main structure with respect to the crane pedestal are:

to provide torsional support preferable at top deck level to provide lateral restraint at top deck level to provide lateral restraint at the lower end of the pedestal to provide vertical support, preferably at the lower end of the pedestal.

Bending restraint by deck beams and/or main structure girders is not required and should be reducedwhere possible. Torsion caused by slewing of the crane should preferably be resisted by the floor plate,the stiffest element.

It has become practice to include the tapered top section of the pedestal in the supply package of thecrane. The top section contains the large flange for the slewing bearing.

Fatigue due to crane operations is a design criterion and requires careful detailing of the pedestal and theadjoining structure.

3. ANALYSIS OF DECK STRUCTURES

3.1 Introduction

Although the analysis of deck structures is a standard task, several aspects require special attention:

Plate girder design Strength of joints Strength of the floor plate Lifting points Modelling of floor plates Support of modules.

3.2 Plate Girder Design

Design of plate girders requires selection of many dimensional variables and of approaches for assessingload-carrying resistance. Lectures 8.4deal in more detail with plate girder design.

Web buckling due to bending, normal force and shear restricts the slenderness of the web which isexpressed as the height of the web (h) divided by the web thickness (t).

API-RP2A [2] refers to the AISC manual [3] which gives the figures below for material with yield-stressof 355 MPa:

Allowable bending stress 0,66 Fy 0,60 Fy

Ratio web height h to thickness t 90 138

Ratio flange width b to thickness t 18 27

Instead of the above approach, more recent research, [3] and [6], allows use of the post-bucklingstrength. The depth/thickness limits given above do not then apply.




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3.3 Strength of Joints

The most important joints in a topside steel structure are:

the ring stiffened joint between rolled beams or plate girders with a circular column. the non-stiffened joint between rolled beams or plate girders with a circular column. the tubular brace joint to single web beams. the non-overlapped tubular joint.

These joints are discussed in Lecture 15A.12.

3.4 Lifting Points

The effect of lifting points on deck design is considerable. For example the local forces that act on thelifting points (Figure 5) have to be transmitted safely through to the deck structure.




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There are two types of lifting points, trunnions and padeye, Figure 6.

Trunnions, though favourable from other points of view, see Section 4, can generate considerable offset




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of the sling force with respect to the topdeck system points. Significant bending is generated which istransferred to the topdeck girders to the extent that they contribute to joint stiffness. It is most efficientto leave these bending moments in the column, by providing stiff columns.

Padeyes generally provide a good opportunity to minimize or eliminate offset, as far as they can besituated on top of the column. The requirement of recessed padeyes (recessed padeyes are those whichare positioned between the top and bottom flange elevation) or the presence of other structures on the

top deck can lead to very eccentric positioning and resulting heavy moments. For this reason the liftingconcept must be developed in the concept phase of the structural development.

API-RP2A [1] requires larger load factors to be used for members direct-loaded by padeyes ortrunnions.

3.5 Modelling of Floor Plates

There are two points of major interest:

representation of the floorplate in the structural model true elevation

There are several ways to model the plate. The most direct is to choose a computer-program whichallows selection of plate elements. A second option is to define representative members which model theplate stiffness by diagonals.

The deck plate is often positioned in the model at the elevation of the centre line, i.e. the mid height ofthe main structure girders, in order to save nodes in the model. It should however be recognised that this"error" of elevation, amounting to 0,5 - 1m, can affect the results. A separate evaluation should then beperformed on the effect to this deliberate "error" at least at some critical points.

3.6 Support of Modules

Modules and deck structures interact structurally. API-RP2A [1] requires that modules are modelled aselastic structures for the analysis of the supporting deck. In the 1970's major difficulties arose in thedecks for concrete gravity structures, because modules were represented as a set of loads for thedifferent load cases, acting at the support points, and neglecting structural interaction. The basicphenomenon of this interaction is that the distribution of the support reactions of the module is quiteunequal and varies with the load case. Dimensional control of the module as well as the support, withcorrective measures, further provide control over the module - deck interaction. Some modules, such asliving quarter modules, gas compressor and injection modules, are often placed on anti-vibration pads inorder to isolate them from vibrations.

4. CONSTRUCTION

4.1 Introduction

In Lecture 15A.1the principal aspects of construction of offshore structures and their major equipmentwas introduced.

For topsides more specific aspects are discussed below.




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4.2 Fabrication

4.2.1 Operations

The design should allow efficient prefabrication of major sections. Prefabrication will avoid congestionin one working area and it speeds up the whole construction process.

Prefabrication and assembly shall properly incorporate the aspects of installation of major and smallermechanical equipment, as well as outfitting with piping, electrical and instrument cables and lines. Itshould be recognized that major mechanical and electrical equipment is often not available at the start ofassembly and must be brought in during fabrication.

4.2.2 Design aspects

Since the overhead space is well covered by extensive piping routes as well as cable trays duringconstruction, "late" structural work should preferably not be positioned overhead in that underfloor area.

Fabrication of offshore steel structures is principally assembly by welding.

The prefabrication concept and joint detailing should maximize welding productivity with manyhorizontal welds preferably made using SMAW technology.

Support to the topside during construction should be well controlled to avoid settlement and to keepwithin construction tolerances.

Special consideration should be given to the selection of materials suitable for the fabrication. Wherethick-walled elements are involved requiring Post Weld Heat Treatment (PWHT), the design shouldposition such welding and the PWHT in the prefabrication phase.

4.3 Weight Engineering

The topside must be kept under strict weight control, as explained in Lecture 15A.10. To that end thetopside is usually weighed prior to load out. The basic design of a weighing system usually consists of aset of hydraulic jacks with electrical load cells on top, installed between the topside and the shop floor.The accuracy of such systems is typically 0,5-1%.

Accuracy is necessary in order to check the actual position of the centre of gravity. Knowledge of theposition is vital for the installation.

The system for support of the topside should be similar to the anticipated method of load out.

4.4 Load Out

4.4.1 Operations

The load out usually combines two operations:

moving the topside from the fabrication hall to the nearby quay. moving the topside from the quay onto the barge.




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The short journey on land can be complicated when the track is not flat or curves have to be taken.

The most preferred option for load out is therefore to use a platform trailer with individual suspendedwheels, see Figure 7 and Slide 1.




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Slide 1 : General arrangement of a load out through skidding

The trailer drives from the quay over a rocker flap resting on the quay and the barge and then slowlyonto the barge. The barge is kept in right trim by ballast pumping.

When it reached the right position, the topside is set down on the beam grid of the sea fastening.

4.4.2 Design aspects load out

When using platform trailers the lower deck should be designed to meet three basic load-outrequirements:

the bottom flange plates of the transverse beams should all be in one plane.

the distance of transverse beams should not exceed approximately 7 m. the lower deck should be able to take an upward reaction typically in the range of 50-60 kN/m2of

ground area.

A uniform distribution of loads is assumed for platform trailers. Skid systems which are not providedwith a proper load sharing system will lead to a non-uniform load distribution.

Design for load-out requires coordination with sea fastening design.

4.5 Sea Transport and Sea Fastening

4.5.1 Operations

Sea transport is a very critical operation, especially for topsides (see Slide 2).




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Slide 2 : Seafastening of 105MN Brent C topside

After completion of the load out and full fastening to the barge, the barge is ballasted to its target draftand cleared for the transport.

The barge is towed by one or two tugs to the offshore location. There the barge is positioned closealongside the crane vessel.

Prior to lifting, the sea fastening is cut free.

Planning the sea transport contains several steps:

identification of critical clearances, e.g. (harbour depth, width of bridges or locks, etc inshore) barge selection (a.o. stability, dynamic behaviour, location of bulkends). evaluation of sea route (weather, length of tow). assessment of barge motions in sea state. development of a sea fastening concept. assessment of deck/module integrity. assessment of barge integrity.

There is also the option with some crane vessels to transport the top side on board. Usually an extra takeover is required as the draft of the crane vessel exceeds the depth at the fabricator's quay. The advantagehowever is that sea fastening requires less effort. Furthermore, the offshore operation is simpler andquicker, as the most critical and weather sensitive operation - lift off the barge- is avoided.

4.5.2 Design aspects of sea transport and sea fastening

Several elements of the structure are dominated by the load condition during transport, see Lecture15A.1.




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All equipment in or on the topside is also subject to heavy loads, e.g. control panels, generator skids,platform crane, during transport.

Internal bracing of a topside for transport is not favoured since it creates obstacles and risk of damage orfire to cables, instruments, piping and equipment during subsequent removal. External bracing is alsonot without problems. The width of the topside requires an extra wide barge. It is difficult to find"strong" points on the topside exterior. The basic concept is therefore to fix the topside to the barge by

its columns only.

The designer should be aware that the bending stiffness of the topside often exceeds that of the barge.Considerable "composite" action can result when the barge deflects in heavy head-on seas.

It is very important for any sea fastening concept to consider aspects of de-seafastening, i.e. cutting free,prior to lift off, and the need to remain safe in a moderate sea state.

De-seafastening should not require any handling by cranes. Braces cut loose at one end should thereforeremain stable and safe while fixed at one end only.

Design of the sea fastening should not require any welding in the column joint, since the topside wouldnot then be ready for immediate set down onto the jacket.

When the tow is more than one or two days long, fatigue may have to be considered on critical nodes.

4.6 Installation

4.6.1 Operations

Installation on the substructure can be:

deck mating with a deep submerged floating GBS (Slide 3) lifting onto an already installed jacket (Slide 4).

Slide 3 : Deckmating of the 500MN Gullfaks-C topside




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Slide 4 : Installation of 60MN K12-BP topside by floating crane

Deck mating is a floating operation in a sheltered location, e.g. a Norwegian fjord or Scottish loch. Deckmating requires that the deck is temporarily supported with the final supports free. This requirementcreates a very awkward load situation for the deck structure.

Lifting is the usual installation method for jacket-based topsides. During development of a platformconcept, the lift strategy should be defined as part of the overall construction strategy. The liftingcapacity of crane vessels is defined by hook-load and reach.

The required reach is determined mainly by the width of the topside and/or the transport barge.

The major steps are:

review of the weight report. assessment of "critical" elevations. assessment of feasible crane vessels. development of a lift concept. preliminary sizing of slings, shackles, trunnions, etc. concept design of guides and bumpers. analysis of deck or module structure for lift condition.

4.6.2 Design aspects of installation by lifting

The lift concept consists of several elements:

the single or dual crane lift the sling configuration choice of topside pick-up points the necessity (or not) for spreader bars or even spreader frames the single, double or paired slings the choice of padeyes, or trunnions.




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Crane vessels were listed in Lecture 15A.1. Slings are available up to over 400mm nominal diameterwith safe working loads of 20-25 MN.

A basic element in all elevations is the inevitable tolerance in sling length which leads to an unequaldistribution of sling forces (typically 25%-75%) in a four sling lift. The unequal sling forces lead tosignificant stresses in the module (see Figure 8).

The use of spreader bars leads to a fully balanced lift without distorting the module. However thespreader bar is quite expensive and usually leads to a requirement for a higher hook elevation.

The use of a spreader frame should only be considered in exceptional cases and does not prevent moduledistortion. The padeye/shackle option is limited by the safe working load (maximum 10MN) of thebiggest shackle. The trunnion can accommodate higher loads.




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4.7 Hook up

Hook up is the completion of all joints and connections after installation.

For economic reasons, the overall construction strategy should keep hook up work to a minimum.Critical hook up work is the work required immediately to secure the object in order to survive the nextstorm.

4.8 Commissioning

Commissioning is not relevant to the structural design.

4.9 Inspection Maintenance and Repair (IMR)

These activities are a major source of operational expenditure, OPEX, as introduced in Lecture 15A.1.

Some requirements are:

inspection of the primary structure is a statutory, fully planned activity. inspection is only possible when proper access to the area or joint is provided. gaining access is costly and requires space to be left behind equipment. minimum provisions, e.g. small clamps under the deck, greatly speeds up scaffolding. crack growth through fatigue is slow. A crack is usually detectable before one quarter of its life is

passed. dirt accumulation promotes corrosion damage. maximum use should be made of the results of inspection. Evaluation should lead to modification

of the inspection programme where appropriate.

4.10 Removal

Removal requirements are different from country to country. In some depths of water full removal isrequired in some countries from the mudline upward. Elsewhere only the structure 75 m or more abovethe mudline must be removed.

Extensive engineering of removal is required to achieve a safe and effective operation. In the Gulf ofMexico removed structures are dumped in the form of reefs. It is very difficult and inefficient at presentto include conceptual removal engineering in the design phase. When re-use of the facility is planned,then removal engineering should be developed early in the design.

5. CONCLUDING SUMMARY

Structural systems for each type of topside structure were introduced, i.e. truss, portal, box girder,and stressed skin systems.

In the section on design some topics were addressed in more detail. In the section on construction the different phases were presented in more detail, i.e.

i. fabricationii. weighingiii. load out




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iv. sea transportv. offshore installation especially deckmatingvi. module installationvii. hook-upviii. commissioning

A brief discussion on inspection and repair and on platform removal concluded the lecture.

6. REFERENCES

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

American Petroleum Institute, 18th ed., 1989.

The structural offshore code, governs the majority of platforms.

[2] AISC: Allowable stress design manual (ASD).

9th ed., American Institute of Steel Construction, 1989.

Widely used structural code for topsides.

[3] API-Bulletin 2V: Bulletin on design of flat plate structures.

American Petroleum Institute, 1st ed., 1987.

Valuable specialist addendum to API-RP2A.

[4] API-Bulletin 2U: Bulletin on stability design of cylindrical shells.

American Petroleum Institute, 1st ed., 1987.

Valuable specialist addendum to API-RP2A.

[5] D.v.d. Zee & A.G.J. Berkelder: Placid K12BP biggest Dutch production platform.

IRO Journal, nr. 38, 1987, pp 3-9.

Presents a recent example for a portal framed topside.

[6] R. Narayanan: Plated structures/Stability and Strength.

Applied Science Publishers, London, 1983.

Good designers guide to plated structures design.

[7] ANON: Gullfaks C platform deckmating.

Ocean Industry, April 1989, pp 24.




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Good description of the actual mating of deck to GBS.

[8] A.G.J. Berkelder: Seafastening 105 MN Brent C deck.

Bouwen met Staal, nr.24 1979.

Presentation of seafastening design for GBS topside.

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15A11 Superstructures II