FUNDAMENTALS OF FPSOs

MODULE 3

FPSO Design

AUTHOR

John Preedy PhD

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CONTENTS

Page No.

WELCOME TO MODULE 3 4

MODULE 3 SUMMARY 5

LEARNING OUTCOMES 6

1. FPSO HULL DESIGN AND CONSTRUCTION 7

1.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.1.1 Location and Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.1.2 New Build vs Tanker Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.1.3 FPSO Function and Field Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.1.4 Weight and Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.1.5 Vessel Motions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.2 New Cylindrical Shaped FPSO Vessel � Sevan Marine SPS . . . . . . . . . . . . . . 16

1.3 Hull/Vessel Design Methods and Requirements. . . . . . . . . . . . . . . . . . . . . . 17

1.3.1 FPSO Hull/Vessel Design Through Full Hydrodynamics Studies . . . . . 18

1.3.2 FPSO Hull/Vessel Design Through Testing with Models . . . . . . . . . . . 21

1.3.3 Computer Simulation and Model Testing . . . . . . . . . . . . . . . . . . . . . 22

1.3.4 Hydrodynamic Model Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1.3.5 Wind Tunnel Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2. ACCOMMODATION (AND CONTROL ROOM) AND HELIDECK 26

2.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.2 Accommodation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.3 Helideck . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.4 Escape and Evacuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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3. FPSO TOPSIDES LAYOUT 30

3.1 Layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2 General Arrangement Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.2.1 Converted Tanker FPSO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.2.2 New Build FPSO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4. MOORING SYSTEM 35

4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2 Mooring Systems Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.2.1 Restoring Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.2.2 Environmental Loading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.3 Mooring System Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.3.1 Spread Mooring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.3.2 Single-point Mooring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4.3.3 Catenary and Taut Mooring System . . . . . . . . . . . . . . . . . . . . . . . . 41

4.4 Mooring System Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.4.1 Vessel Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.4.2 Mooring Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.4.3 Analysis Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.5 Mooring System Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.5.1 Winches and Fairleads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.5.2 Mooring Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.5.3 Man-made Fibre Ropes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.5.4 Anchor Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5. TURRETS 49

5.1 Types of Turret-mooring Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

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5.1.2 External Turrets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.1.3 Internal Turrets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.2 Turret Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.3 Bearing Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.4 Mooring Lines and Risers Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.5 Fluids and Electric Transfers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6. RISERS 56

6.1 Introduction to Flexible Dynamic Risers. . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.1.1 Types of Pipe Structures and Main Manufacturers . . . . . . . . . . . . . . 57

6.1.2 Devices Connected or Attached to Flexible Risers . . . . . . . . . . . . . . 58

6.1.3 New Flexible Risers for Deepwater Developments . . . . . . . . . . . . . . 59

6.2 Hybrid Riser Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.3 Steel Catenary Risers for Deepwater Developments . . . . . . . . . . . . . . . . . . 60

© Copyright IIR Limited 2010. All rights reserved. These materials are protected by international copyright laws. This manual is only for the use of course participants

undertaking this course. Unauthorised use, distribution, reproduction or copying of these materials either in whole or in part, in any shape or form or by any means electronically, mechanically, by photocopying, recording or otherwise, including, without limitation, using the manual for any commercial purpose whatsoever is strictly forbidden without prior written consent of IIR Limited.

This manual shall not affect the legal relationship or liability of IIR Limited with, or to, any third-party and neither shall such third-party be entitled to rely upon it. All information and content in this manual is provided on an �as is� basis and you assume total responsibility and risk for your use of such information and content. IIR Limited shall have no liability for technical errors, editorial errors or omissions in this manual; nor any damage including but not limited to direct, punitive, incidental or consequential damages resulting from or arising out of its use.

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WELCOME TO MODULE 3

In Module 2 we looked at the processes and preparation involved in planning and setting up an FPSO project, from licence approvals and Þ eld development plans to project development strategies, contracts and considerations in choosing the appropriate vessel.

In Module 3 we will examine the actual physical design and technology of FPSOs. How are hulls designed, tested and built? How is the topside layout planned and organised? What options are available for mooring the vessel, or for turrets and risers? This is where we start looking at the �nuts and bolts� of FPSOs.

You will now be familiar with the procedure, but let me encourage you again to make full use of the online course forum and to follow the directed learning suggestions raised at certain points during the course material. If everyone makes use of these resources, we will all have a much richer learning experience.

John Preedy

Course Director

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MODULE 3 SUMMARY

FPSO Design

Hull � hydrodynamics and model testing�

Hull � new construction or tanker conversion�

Accommodation and helideck�

Topsides plant layout�

Mooring systems�

Type: spread or turret moored !

Components !

Design !

Installation !

Turrets�

External turrets !

Internal turrets !

Fluid swivel !

Risers�

Flexible dynamic risers !

Hybrid riser towers !

Steel catenary risers !

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LEARNING OUTCOMES

By the end of this module you will understand:

the factors that determine the design of an FPSO�

the location and environmental factors�

the crude oil storage requirements�

the principal issues affecting the topsides layout of an FPSO�

the principles involved in the design of the mooring system�

the types and components of the mooring systems�

how the turret and ß uids transfer swivel works�

the principal issues involved in the selection of a riser system�

the comparison between the design requirements for new build and tanker conversion � FPSOs

You will also have an insight into:

the importance of hydrodynamic studies and other computer simulations and model � testing in the design of the FPSO

safety issues in the siting of the accommodation and helideck�

You will also be able to:

list and describe the factors that drive the design of an FPSO�

Recognise and describe the different riser systems�

Have experience of interpreting technical diagrams and ß ow charts�

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1. FPSO HULL DESIGN AND CONSTRUCTION

1.1 Introduction

FPSOs generally are an amalgam of marine and petroleum functions, and therefore, present many specialised challenges for those involved in their creation.

An FPSO system is an offshore production facility that is typically ship-shaped. Well ß uids from the reservoir are processed by the deck based topsides processing equipment to produce a dead dry crude oil, separating co-produced gas and water. The vessel stores crude oil in tanks located in the hull of the vessel. The crude oil is periodically ofß oaded to shuttle or trading tankers for transport to shore.

FPSOs may be used as production facilities to develop marginal oil Þ elds or Þ elds in deepwater areas remote from the existing pipeline infrastructure. FPSOs have been used to develop offshore Þ elds around the world since the late 1970s. They have been used predominately in the North Sea, Brazil, Southeast Asian/South China Seas, the Mediterranean Sea, Australia, and off the West Coast of Africa. Currently there are more than 200 FPSOs in operation or under construction worldwide.

The FPSO is moored in the Þ eld. In very calm weather areas this may be by spread mooring. In areas with heavier sea states the vessels are single-point moored meaning they can weathervane (i.e. take up a heading of least resistance to the weather�s loading forces).

1.1.1 Location and Environment

The design of the FPSO monohull vessel is dependent on the environmental conditions at the Þ eld location. Typically in North Sea locations the Þ eld sizes are small to medium (40,000 bbls to 150,000 bbls production per day) and the environment is severe with waves 12 to 18m. Alternatively in West Africa some Þ elds are much larger (up to 250,000 bbls production per day) and the environmental conditions are very mild (waves up to 4m).

This inß uences:

Requirement for a turret mooring to allow the FPSO to weathervane and minimise � environmental loads on the mooring system in harsh environments or spread moored in the mild environments.

Selection of suitable hull size and form with good motion characteristics.�

Freeboard�

Production facilities designed to minimise motion downtime.�

Hull size to provide adequate buffer storage to minimise tanker or shuttle tanker ofß oading � downtime.

Hull structure design (strength and fatigue)�

Environmental performance�

Marine installation design and procedures�

New builds or tanker conversions�

An additional important requirement is the desirability for the FPSO to remain on station for the duration of Þ eld life, without dry-docking for inspection, maintenance or repair. This is because of potential difÞ culties in riser and mooring disconnection and reconnection in the harsh environment and the economic penalty of lost or deferred production. This is especially important for long life FPSOs with high production throughput and complex riser systems.

1.1.2 New Build vs Tanker Conversion

Service in the NW Atlantic region places even higher demands on FPSO design. The more onerous wave climate imposes higher hull loads and requires extensive upgrading of existing hulls to achieve strength and

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fatigue performance. The fatigue requirement, in particular, coupled with the problem of in situ repair, strongly encourages the use of new build custom-designed vessels for this area.

Examples of new build FPSOs are:

Figure 1.1 � West of Shetlands FPSO Schiehallion � new build FPSO with turret forward and accommodation aft.

Figure 1.1

New Build FPSO � Schiehallion FPSO

New build FPSO with conÞ guration of turret at bow and accommodation aft.

Source � SBM and BP plc.

Figure 1.2 � Northern North Sea FPSO Asgard � new build FPSO with accommodation at the bow and the turret one-third from the bow.

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Figure 1.2

New Build FPSO � Asgard FPSO

New build FPSO with conÞ guration of accommodation at bow and turret one-third from bow.

Source � Statoil.

Figure 1.3 � FPSO for sheltered area � Bohai Bay in China � new build FPSO.

Figure 1.3

New Build � FPSO for Bohai Bay in China

The FPSO is deployed in the Bohai Bay. The Bay has very shallow water depth of some 20 to 30m. The mooring system is a �tower� system from APL (designated SYS).

Source � APL.

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For tanker conversion to an FPSO, the strength and fatigue assessments must consider the tanker operations prior to conversion. After conversion, in addition to the tank loadings, account must also be taken of the topside loads and mooring system loads. These conditions, prior to and after conversion, together with required assessments are addressed.

Conversion requirements include:

A determination of the environmental severity factors (ESFs) based on the vessel�s past � trading history, expected transit route and projected onsite location

A reassessment of the scantling determination based on the onsite, transit, inspection and � repair conditions

For FPSOs with internal turrets a detailed Þ nite element analysis study must be carried out � to assess the strength requirement around the turret position.

Examples of tanker conversions are:

Figure 1.4 � Tanker being converted to a FPSO (Curlew).

Figure 1.4

Tanker Conversion � Hole for Turret

In a conversion the turret is always set at the bow (a naturally strong area). However after removing some of the original vessels steel it is necessary to conduct Þ nite element analyses and weld back new steel plates. The

amount of steel is not big (a few 100 tonnes), but is lots of individual plates welded at speciÞ c locations.

Source � SBM.

Figure 1.5 � Tanker Conversion FPSO Uisge Gorm, initially operating in the Fife Field.

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Figure 1.5

Tanker Conversion � Bluewater Uisgi Gorm Operating on Fife Field

Bluewater is one of the companies which owns and rents FPSOs. This is a tanker conversion.

Source � Bluewater.

Figure 1.6 � ABO FPSO � A tanker conversion for a West African Þ eld deployed on a spread-mooring system.

Figure 1.6

Tanker Conversion � ABO FPSO

A tanker conversion at Keppel Shipyard in Singapore. The original tanker was a single-sided vessel. Before conversion in Singapore it went to a yard in S Korea where a second skin was added internally to make it

equivalent to a double-sided vessel. This upgraded it to the modern standard.

Source � Keppel Shipyard.

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Figure 1.7 � VLCC tanker conversion for FPSO in the Mediterranean � with an external turret.

Figure 1.7

Tanker Conversion � Firenze FPSO

Firenze FPSO is operating on the Aquila Field in the Adriatic Sea by Agip. The FPSO was converted from a VLCC tanker (which had 2 million barrels storage). Aquila is a relatively small Þ eld (as can be seen from the relatively small amount of process plant for 50,000 barrels oil per day). So why such a large vessel? It is because of the

problem with a tanker conversion. Here Agip preferred to use a vessel from its own tanker ß eet (although too large) rather than purchase an unknown tanker from outside.

Source � SBM.

Directed Learning: Some half of the world�s tanker conversions to FPSOs have taken place in the KEPPEL SHIPYARD in Singapore. Visit their website at www.keppelshipyard.com Enter PUBLICATIONS then BROCHURES then SHIPPING & CONVERSIONS. There are two sets of information: (1) A general brochure on FPSO/FSO Conversions and (2) Track Record examples of a number of their conversions. Download any vessels of interest � select some with internal and external turrets.

1.1.3 FPSO Function and Field Life

Introduction

The function of an FPSO is determined by the type and quantities of the ß uids that it has to process and export, and by the storage requirements for its crude oil product. The size of the reservoir, in terms of recoverable reserves and its producibility, will inß uence both the size of production facilities and the time the FPSO will be in the Þ eld. The export route will generally determine the storage capacity of the FPSO.

However, when an existing vessel as opposed to a new, purpose-built vessel is considered for a speciÞ c development application, it may well be that the storage capacity, the available deck space and load bearing capabilities along with the remaining fatigue life of the hull will determine the function and the Þ eld life. The conversion of the tanker will involve considerable structural modiÞ cations, especially if it is to be an FPSO with an internal turret.

The two following sections on the sizing of the hull and on the sizing of the production facilities identify the principal issues for consideration by FPSO owners, operators and designers in selecting a suitable unit to develop a Þ eld.

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FPSO Hull Sizing

The main drivers of hull sizing are:

Crude oil storage. For new-build custom designed FPSOs, hull size is driven by the crude oil storage requirement. This is sized on the peak oil production rate and is sufÞ cient to cover the shuttle tanker cycle time between each ofß oading. The cycle time comprises:

Loading time at Þ eld�

Sailing time to/from port (including weather delay en-route)�

Port discharge duration�

Connect/disconnect time to FPSO�

Waiting on weather time to cope with a speciÞ ed winter storm. This is dependent on the � wave height and wind speed thresholds for connection and disconnection.

Other practical factors may inß uence the crude storage volume. For instance, if the shuttle tanker is part of a pool arrangement with a common size of tanker, the FPSO storage can be the same volume, to avoid the shuttle tanker sailing with part-Þ lled tanks or leaving part of the parcel behind.

For high throughput Þ elds, it may be more economical to provide additional shuttle tanker(s) to give more frequent ofß oading during peak production, instead of enlarging crude oil storage on the FPSO. Towards the end of Þ eld life, when production rates have declined, the frequency of shuttle tanker ofß oading can be reduced and it may be economic to have a pool arrangement with other Þ elds for sharing shuttle tanker utilisation and operating costs.

Hull sizing should also provide sufÞ cient segregated ballast capacity to ensure adequate ballast draught to avoid bottom slamming forward and provide required sea-keeping performance and stability.

On converted tankers, it is usually not possible to obtain a tanker that matches the optimum crude storage requirements outlined above. If the selected tanker has less than the required storage volume, the shuttle tanker ofß oading frequency can be increased during peak production, and vice-versa where storage is greater than required. Tankers that are greatly oversized for their FPSO role will have a cost penalty in heavier moorings.

Where crude oil is exported via pipeline or there is a separate FSU, there is no need for crude oil storage unless some buffer storage is speciÞ ed to cover outage of the pipeline or FSU. The hull size can be reduced commensurate with a reduction in crude oil storage, provided there is sufÞ cient deck space for production facilities and adequate sea-keeping performance.

See Figure 1.8.

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Figure 1.8

FPSO Hull Storage and Other Tanks

In a large new build FPSO the tanks conÞ guration allows many compartments. This example is a 2 million barrel storage capacity, three tanks across and six tanks long. The double-sided construction allows ballast storage in the wing sides. Storage is also provided for: trimming water; slops and offspec water; drinking water; diesel;

methanol; and other chemical storage.

Source � J E & P Associates.

Deck space. Hulls sized for the crude oil storage requirement provide adequate deck space for deck mounted production facilities for small/medium-sized Þ elds. On large throughput FPSOs where production facilities may be complicated with water injection and gas processing, the hull may not be large enough to provide enough deck space for a workable single-level layout with adequate lay-down area and future tie-in space. For new builds, it may be economically attractive to increase hull size rather than constrain topsides deck area.

Sea-keeping performance. Good sea-keeping performance is important in the environment to ensure:

Crew safety and operability�

High production uptime�

Reliable helicopter operation�

Riser top-end motion is within limits�

Sea-keeping is a function of size and shape of vessel.

Generally, conventional hull shapes can be expected, from experience, to give good motion performance with minimal production facility downtime provided attention is paid to motion sensitive facilities (separator design, gas turbines etc.).

Small or novel hull forms require detailed examination of their notion characteristics at an early design stage.

Sizing of Production Facilities

The key factors which inß uence the main deck load which the FPSO has to carry in the form of production facilities are:

The number of major systems�

Reservoir characteristics�

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Choice of export route�

Single or twin production trains�

Possible redeployment on another, different Þ eld�

Major systems. The main systems which will have a signiÞ cant bearing on the size and conÞ guration of the FPSO�s production facilities are:

Crude oil separation�

Gas processing (dehydration and compression) and possible export�

Produced water treatment and handling including re-injection�

Seawater treatment and injection�

Crude oil export via shuttle tanker or pipeline�

Main power generation, i.e. size and numbers of units.�

Export system. The choice of crude oil export system will also inß uence facility sizing in that not only will ß ow rates to a pipeline and a shuttle tanker be different but also the levels of separation in the oil processing train to achieve the different crude oil speciÞ cations for each route. In the case of heavy, viscous or waxy crudes, low ß uid arrival temperatures may require extensive facilities involving, among other items, waste heat recovery units on gas turbine drivers.

1.1.4 Weight and Space

Weight Control

FPSO weight is dominated by the weight of the crude storage and hull steel weight. Production equipment represents only a small proportion, typically 5 � 7% of the total displacement, and therefore, in contrast to other types of ß oating structure, weight of production equipment is usually not a critical feature of FPSO operations.

Production equipment weight control is however important in design of the production module structure and deck reinforcement and from a lifting viewpoint during fabrication.

The Þ rst objective is to arrive at the �lightship� weight and centres of gravity, i.e. vessel with empty tanks and production equipment on-board but no ß uids in production/utility systems. The accuracy of this lightship weight/centre of gravity estimate can be checked once the FPSO is reasonably complete and aß oat by the �inclining experiment� which uses draught measurements and stability checks to produce FPSO as-built weight/c of g.

The �lightship� data is used as the basis for producing the various �ship conditions� with different tank contents and ß uids in production equipment, mooring loads, etc. which check draught and stability information against design and regulatory constraints.

Space Control

The large deck area on FPSOs has facilitated the use of single-level modules from cost and safety points of view, and has led to the view that FPSOs are not space-limited. This is generally the case on simple, lower throughput topsides but space control has become a problem on complex, high capacity topsides. This is manifested in poor lay-down areas and access for maintenance. Utility equipment, e.g. switchgear, may be relegated to within the hull and this can cause problems during construction and hook-up. Alternative locations, e.g. within the forecastle space, may put critical equipment in a location which may be more vulnerable to wave damage.

1.1.5 Vessel Motions

Human Response

The FPSO is constantly in motion, even in the most benign weather conditions. It is therefore important that crew selection should take account of the susceptibility of personnel to motion sickness, not just in moderate to severe sea states but also in calmer conditions.

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A crew member arriving on the FPSO from the helicopter has no time to acclimatise to the moving environment and so the individual is expected to function as near normally as possible in terms of decision- making and performing routine tasks. Even though an individual may not be susceptible to motion sickness, he or she still has to maintain balance while moving around to avoid bumping into equipment and Þ ttings. As FPSO motions become more pronounced the need to preserve balance becomes greater and so the individual�s attention becomes more focused on self-protection and injury avoidance.

Effect on Equipment

On a Þ xed offshore platform, the production and utilities equipment is effectively static in terms of the motions which it might experience from, say, wave slam on the jacket structure. On an FPSO, the equipment is moving all the time, following the roll, pitch, yaw, heave, surge and sway of the hull.

It is therefore essential that before the speciÞ cations are drawn up for each item of equipment, the specifying engineer has a comprehensive understanding of the subtleties of the motions and their implications. The speciÞ cations should then fully identify the motions and the accelerations which the equipment can be subject to. If possible, speciÞ c local conditions should be taken into account. All sea states which the FPSO can experience must be addressed, and while in severe weather conditions equipment may be shut down, some items such as pressure vessels and towers may still contain liquid inventory.

1.2 New Cylindrical Shaped FPSO Vessel � Sevan Marine SPS

Some years ago Tentech (part of Aker Maritime) proposed a new FPSO structure called the BUOYFORM. The hull was a simple compact structure consisting of a vertical cylindrical section breaking the sea level and a circular conical shaped underwater hull. Deck structure on top to be supported by the conical hull part. Ballast and crude oil tanks were arranged radially in the lower part of the main hull.

The Sevan Company has, since its origin, focused its business on the development of a new cylinder shaped platform type for storage and production of hydrocarbons in deep and shallow waters (FPSO). The platform is designed to operate in all types of offshore conditions and with a range of storage capabilities (from 300,000 to 2 million barrels).

The main competitive advantage of the Sevan platform is that it combines internal oil storage capacity and ability to carry high topside weights with a low construction cost compared to other FPSOs. The design of the cylindrical shape and the below waterline �skirt� conÞ guration gives very favourable motions in all sea states. Roll and pitch are more or less eliminated and converted to a moderate heave motion.

The Sevan FPSO is used as a ß oating production and storage for hydrocarbons. The hydrocarbons are stored in tanks in the hull of the platform and are transported by shuttle tankers or by pipeline to onshore reÞ neries.

The hull is Þ tted with machinery, power generators, transformers, electric boards, Þ re control systems, ballast pumps and cargo pumps.

The main components on deck are living quarters with control rooms, workshop, life vessels, helipad, cranes, on- and off-loading system for oil, and anchor winches.

The FPSO also has a processing plant for hydrocarbons installed on deck. Depending on the characteristics of the Þ eld, the processing plant has different modules for processing oil, gas, and water.

There is also a power generating module that delivers power to the platform�s additional machinery. The processing module has an oil production capacity of 10,000�200,000 bbls.

It has been used to develop the Piranema Field in the Sergipe-Alagoas basin of Brazilian waters and a number of North Sea Þ elds. See Figure 1.9.

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Figure 1.9

New Cylindrical FPSO from Sevan Marine

This new cylindrical shaped vessel (a full FPSO) Þ rst appeared a few years ago. There are up to six operating around the world.

Source � Sevan Marine.

Directed Learning: To learn more about this FPSO go to the SEVAN MARINE website at www.sevanmarine.com Click TECHNOLOGY and select THE SEVAN TECHNOLOGY. This contains a detailed presentation. There are some six FPSOs of this type either in use or proposed. What do you think of this new approach? Do you think it will become more widespread in the world? Share you Þ ndings with others on the course.

1.3 Hull/Vessel Design Methods and Requirements

FPSOs sometimes appear deceptively simple, being based on conventional ship-shaped hulls, with which there should be plenty of experience in design and construction. However, the reality is that the duty of the FPSO is completely different from that of the trading VLCC, and although the hull shape may be similar, an FPSO is in fact a very complex system.

Some of the features that make it complicated are:

The vessel is permanently installed at a Þ xed location, and so must survive the worst � weather at that location (though some systems, mostly in typhoon or iceberg areas, have been designed to disconnect in the worst weather conditions)

Process equipment on deck is vulnerable to green water damage, with potentially � dangerous consequences

Being ship-shaped, environmental forces and motions vary greatly depending on relative � heading to the weather

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The vessel will often change heading (weathervane) in order to face in a favourable � direction to the weather (though some FPSOs are spread-moored in benign areas, and therefore cannot change heading)

Weathervaning is often facilitated by a �turret� mooring system, and the turret, which � must resist all the mooring loads, becomes a key mechanical component in design, and a major cost driver

The FPSO�s motions and excursions are the controlling design parameters for the associated � riser system

Natural weathervaning properties can be uncertain in complex non-co-linear metocean � environments

There may often be a compromise required in turret location between the requirement to � provide reliable natural weathervaning, but at the same time minimise dynamic mooring loads and inertial loads at the turret

Thruster-assisted mooring systems are sometimes included to permit active heading � control and to reduce mooring loads.

All the issues listed above are related to the way in which the FPSO interacts with the metocean environment. Calculation and computer simulation are used but many of the above are not necessarily reliably estimated by these techniques. Physical model testing is therefore still an alternative important aspect of FPSO design, and is called upon to deal with design concept issues, such as verifying the effectiveness of the hull form or the selected turret location, and detailed design issues, such as the anticipated bearing loads on the turret caused by the FPSO motions and the dynamic mooring loads.

1.3.1 FPSO Hull/Vessel Design Through Full Hydrodynamics Studies

Most new (and some tanker conversions) will have had detailed hydrodynamic studies carried out to determine the proposed FPSO vessel motions in the various sea states in its area of operation. This will enable the vessel�s design to be matched to its operational functions of processing the well ß uids and storing/exporting the dead crude. In addition the information will be input for the umbilicals, riser tower and mooring system design. These will normally be carried out by detailed computer modelling and hydrodynamic studies.

An example of this approach is given for a large West African FPSO. The FPSO�s dimensions are:

Length 315m

Breadth 60m

Depth 50m

Design Draught 23m

Water Ballast Capacity 150,000t

Oil Storage Capacity 260,000t

There are a number of hydrodynamics software programs available. One such is MOSES software. See Figure 1.10.

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Figure 1.10

Hydrodynamic Studies for FPSO Vessels

Hydrodynamics studies are carried out by computer software (represented here by MOSES). This calculates various parameters which mathematically describe the vessel shape and how it will behave in various sea states.

Source � J E & P Associates.

The aim of a hydrodynamics study is to describe the hydrodynamic behaviour of the FPSO on its Þ eld location. This sea-keeping analysis will provide motion Response Amplitude Operators (RAOs) and motion statistics at various locations for both extreme and fatigue sea states. The mean drift data will also be calculated and given through the Quadratic Transfer Functions (QTFs).

See Figure 1.11.

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Figure 1.11

Response Amplitude Operators (RAOs)

RAOs are part of the mathematical output from hydrodynamic studies.

Source � J E & P Associates

Three representative conÞ gurations have been investigated among the vast number of loading conditions which the FPSO will meet over the course of its life:

Loading Condition Mean Draught

Ballast, 14.2m 1.

Mid-Load, 17.5m2.

Full Load. 23m3.

Example of Typical Results from a Hydrodynamics Study

The FPSO natural periods for the three considered loading conditions are given in Figure 1.12.

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Figure 1.12

Sample Hydrodynamics Study Results

Source � J E & P Associates.

The most severe FPSO motions are due to swell coming from the dominant weather direction (in this case SSW).

Such data will be used to design the mooring and riser systems. Additionally it will assist in the correct location for topsides process plant which are sensitive to vessel motions.

1.3.2 FPSO Hull/Vessel Design Through Testing with Models

Physical model testing is an alternative important aspect of FPSO design, and is called upon to deal with design concept issues, such as verifying the effectiveness of the hull form or the selected turret location, and detailed design issues, such as the anticipated bearing loads on the turret caused by the FPSO motions and the dynamic mooring loads.

It should be recognised that the role of model-testing in relation to numerical simulation has changed over the years. Twenty years ago mooring systems were optimised mainly by model testing, whereas now much of the optimisation of both the vessel and mooring system is performed on the computer. Model tests are now mainly used for validation and conÞ rmation of the design calculations. With FPSOs this has become essential owing to the very large number of different metocean conditions that may have to be analysed in order to obtain design data in extreme loading conditions.

Model testing and computer simulation have always been complementary techniques. It is often necessary to complement model tests with analysis in order to gain understanding of the system performance. Numerical models usually provide much greater scope for parameter variations than is possible in a model test programme.

There are a number of aspects of offshore hydrodynamics which cannot yet be solved by numerical simulation alone. These include viscous effects, such as roll damping of ships and vortex-induced vibrations of risers. Major advances have been made in computational ß uid dynamics (CFD), but as yet these techniques can only be regarded as a partial solution to the problem. Further examples of difÞ cult areas for numerical simulation are wave drift force and relative motion prediction in extreme waves, interactions between FPSOs and shuttle tankers, and DP thruster interactions.

In the context of deepwater moored FPSOs the integrated use of model tests, numerical simulations and full-scale measurements are all clearly essential components in the determination of system performance. It may be important to take account of the effects of mooring and riser dynamics on low frequency motion damping of the FPSO, line and riser dynamics on the quasi-static line and riser loads, and also direct current

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and wave loads on the riser and moorings. These effects can become very signiÞ cant in very deepwater, and it is not always possible to represent them even in the most up-to-date model basin. For ultra deepwater (>1,000m) passive equivalent mooring systems or active equivalent mooring systems are preferred to ultra-small-scale testing (1:>100), and these require numerical simulation to facilitate the determination of the characteristics of the hybrid or equivalent system.

Directed Learning: The UK HSE commissioned a study REVIEW OF MODEL TESTING REQUIREMENTS FOR FPSOs by BTM Fluid Mechanics Ltd. The report is available under www.hse.gov.uk/research/otopdf/2000/oto00123.pdf (or use title in Google). Review the report and assess the various methods therein.

1.3.3 Computer Simulation and Model Testing

COMPUTATIONAL FLUID DYNAMICS (CFD) Computational � having to do with mathematics, computation Fluid Dynamics � the dynamics of things that ß ow

CFD is a computational technology that enables the study of dynamics of things that ß ow. CFD can build a computational model that represents a system or device. The ß uid ß ow physics is applied to this virtual prototype, and the software outputs a prediction of the ß uid dynamics. CFD is a sophisticated analysis technique. It not only predicts ß uid ß ow behaviour, but also the transfer of heat, mass (such as in perspiration or dissolution), phase change (such as in freezing or boiling), chemical reaction (such as combustion), mechanical movement (such as an impeller turning), and stress or deformation of related solid structures (such as a mast bending in the wind).

See Figure 1.13.

Figure 1.13

Computational Fluid Dynamics (CFD) Study on Troll Field Floater

Wind ß ow results. The arrows give the speed and direction of the wind around the structure.

Source � BTM Fluid Mechanics Ltd.

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1.3.4 Hydrodynamic Model Testing

Wave Basin Tests

The design of a new FPSO often involves a programme of hydrodynamic model tests in a wave basin. See Figure 1.14.

Figure 1.14

Wave Tank Model Testing

Model FPSO in wave tank. Studies on �green� water coming over FPSO bow under test conditions.

Source � APL/Axis.

These tests may have several distinct purposes, such as:

To evaluate mooring, riser and vessel installation procedures, and to identify possible � difÞ culties during the installation process;

To assist with training of personnel involved with the installation; �

To verify the vessel�s predicted motions in extreme and operational sea states, for use in � assessments of mooring and thruster capability, human habitability, vessel operability and weather downtime;

To verify extreme mooring line loads predicted using standard mooring design and analysis � procedures;

To verify or investigate the dynamic behaviour of the mooring and riser systems; �

To verify the predicted capability of the thruster system to maintain the heading of a � turret-moored FPSO in extreme and operational sea conditions;

To assess the occurrence and severity of green water on deck; �

To assess the severity of wave slamming on the vessel�s bow; and�

To verify the vessel�s predicted accelerations, which are used as input to structural loading � and fatigue analyses of the hull structure, and the loading of items such as seafastenings, cranes and production equipment;

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To verify predicted forces on the mooring turret, and to identify any unexpected features � of the loading;

To evaluate the combined behaviour of the vessel and shuttle tanker in various operating � scenarios, and the possibility of �Þ shtailing� or other undesirable motions.

The list of possible reasons for carrying out wave basin model testing is long. The multi-purpose nature of such tests, the costs of hiring large test facilities, and the technical limitations of all facilities sometimes leads to compromises being made in selecting the model scale and test conditions.

Other Hydrodynamic Tests

Towing tank tests (either with or without waves) are sometimes undertaken in order to evaluate towing and installation procedures, especially in situations where strong currents are possible. Vortex-induced motions and unstable behaviour sometimes develop in these circumstances. Towing tests allow tow loads and possible motion suppression strategies to be evaluated.

Uniform currents are sometimes simulated by towing the installation in still water. The ß ow quality is far more uniform and steady than can be achieved in a wave and current basin, but towing the model limits the duration of the test run, and wave conditions may vary between the beginning and end of the test.

FPSO models are sometimes towed at various headings through still water in order to measure current force coefÞ cients, for input to mooring and thruster analyses. These coefÞ cients may alternatively be obtained by wind tunnel tests on the underwater hull form.

1.3.5 Wind Tunnel Testing

Wind Loading

Wind heeling moments on the FPSO�s hull and superstructure are necessary input to a stability analysis. Wind forces and moments are also necessary inputs to analyses of the mooring and thruster systems. These forces and moments are sometimes estimated using standard calculation procedures, such as those deÞ ned by the certifying authorities and the International Maritime Organisation. These calculation procedures are of uncertain accuracy, however, and may not take adequate account of interactions between the many different structures on the deck of an FPSO. Wind tunnel tests provide more reliable estimates.

See Figure 1.15.

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Figure1.15

Wind Tunnel Model Testing and Measurement Probes

Source � BTM Fluid Mechanics Ltd.

Ventilation, Environmental and Safety Studies

Wind tunnel tests may also be used, in conjunction with computational ß uid dynamics (CFDs) modelling, to assess ventilation problems on-board an FPSO, and as an input to various environmental and safety case assessments. Tests of this type have been used to assess and optimise regions over the helideck which are affected by disturbed ß ow and by temperature rises due to turbine exhaust emissions. These tests have also been used to model gas releases, Þ re scenarios and to identify regions of poor ventilation. CFD and wind tunnel techniques are now regarded as complementary capabilities.

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2. ACCOMMODATION (AND CONTROL ROOM) AND HELIDECK

2.1 Introduction

The accommodation/control room and helideck are located as far as possible from the most hazardous processes (gas compression equipment). They will be separated from the processing plant by thick blast walls (often 6cm thick steel). Detailed safety studies will be conducted on these facilities, see Figure 2.1.

Figure 2.1

Finite Element Studies on Accommodation and Helideck

Finite element studies are used to study possible Þ re and blast damage to the accommodation and helideck. Studies of credible incidents to the helideck will also be analysed.

Source � J E & P Associates.

The living quarters will very likely contain:

the temporary refuge along with Þ re Þ ghting equipment provisions�

emergency control and response room(s)�

control rooms, radio room, ofÞ ces and meeting rooms�

dining, recreation and leisure facilities and locker rooms�

galley, laundry facilities and stores�

HVAC plant rooms and battery rooms�

Sick bay and medical rooms�

Helicopter reception facilities�

The owner will have to decide on whether to provide single or two-person cabins and the possible temporary conversion of two-person cabins into three-person cabins for short, clearly deÞ ned periods such as planned shutdown. Provision may also have to be made for male and female crew members.

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Normally moveable Þ xtures such as tables and chairs may have to be secured in position because of the movement of the FPSO in heavy weather.

Basis for Crew Sizing

It is the general objective of FPSO owners and operators to have a minimum crew size that is consistent with maintaining the highest standards of safety, environmental performance, Þ eld uptime and the preservation of asset integrity.

Crew size will be driven by several factors, not the least of which are the size and complexity of the production facilities. The maintenance strategy, e.g. batch maintenance, will also inß uence crew numbers.

2.2 Accommodation

On converted vessels, the existing accommodation is usually retained in its aft position. On a new-build vessel, the accommodation position is largely determined by the position of the turret.

For forward turrets, the accommodation is placed at the stern to maximise the separation between the accommodation and the major hazards of the swivel and the production facilities.

For amidships turrets, the accommodation is placed at the bow upwind of the major hazards. The acceptability of motions for crew and helicopter operability should be checked at an early stage in the design because of the high combined heave and pitch vertical motions at the bow.

The advantages and disadvantages of each position are as follows:

Position of Accommodation

Advantages Disadvantages

Forward Accommodation/� helideck is upwind of major hazard and Þ re or smoke in an emergency

High vertical motion at bow may affect � crew comfort and helicopter operations.

Natural weathervaning of FPSO is difÞ cult � to achieve without placing HP swivel close to accommodation

An amidships turret achieves separation � between accommodation and HP swivel but requires substantial thrusters power to weathervane the FPSO

Aft Reduced vertical � motion (compared to bow) for crew comfort and helicopter operations

Natural weathervaning � of FPSO can be achieved without thrusters

Large separation of � accommodation from HP swivel

Accommodation/helideck is downwind of � major hazards and Þ re or smoke in an emergency. However thrusters could be used to rotate FPSO clear of Þ re/smoke and create a lee side for lifeboats

2.3 Helideck

See Figure 2.2.

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Figure 2.2

Helideck and Free Fall Lifeboats

These are of importance because they provide the emergency exit routes in any emergency situation.

Source - Azur Offshore Ltd.

Helideck design should meet the location requirements. In the UK North Sea this is CAA CAP 437 Offshore Helicopter Landing Areas: A Guide to Criteria, Recommended Minimum Standards and Best Practice. The siting of helidecks on FPSOs will generally be above the living quarters for logistics and safety reasons, and will therefore be at the forward or aft end of the FPSO. The positioning of the helideck should take account of the wind environment around the FPSO, particularly turbulence, vertical component of wind velocity and hot gas plumes from ß ares or turbine exhausts, both at the helideck and on the approaches. The height of the helideck and the air gap between it and the living quarters below should ensure as clean a ß ow of air as possible and should be determined using wind tunnel testing.

Helideck layout should allow for either parking of a broken-down helicopter or platform craneage should have helideck coverage and capacity to permit helicopter removal.

Helicopter Operations

Helicopter operations may be affected by vessel motion, especially those at wave frequency. The limiting factors for a Puma/Tiger are, for example:

Roll Max 3 degrees (half amplitude)

Pitch Max 3 degrees (half amplitude)

Vertical motion Max 5 metres (combined heave and pitch)

Wind speed Max 35 knots cross wind

2.4 Escape and Evacuation

Escape. During layout development studies of the FPSO main and production decks the location of the main or primary escape routes from these areas to the living quarters and the Temporary Refuge (TR) can have a major inß uence on the overall layout.

In the case of a large FPSO with substantial production facilities covering the main deck, the �free� open deck area may be considerably smaller than on a large vessel with small production capacity. It is therefore

29

important to assess the escape route requirements at a very early stage to ensure that rapid escape from the more congested areas is not compromised.

One or more escape tunnels can be used to provide direct, protected access from the process and utility areas on the main deck into the TR, usually located in the living quarters. The escape tunnel has to be sized to take account of the range of credible scenarios of major incidents and the personnel likely to be involved. The tunnel should be capable of withstanding credible explosions and Þ res to permit personnel to escape within a deÞ ned time and to allow Þ re-Þ ghting and rescue crews to gain access if and when it is deemed prudent to undertake these activities.

The position of the tunnel will be determined by the Þ re and explosion scenarios but it is most likely to be on the outside edge of the main deck running the length of the production facilities, as a minimum, and possibly the entire length of the main deck. It is unlikely to be located within the production areas unless safety studies indicate otherwise. The tunnel may be totally enclosed and positively pressurised or it may be open on the seaward side.

There will be intermediate access points from production and utilities areas into the tunnel and layout studies must ensure that the access into these access points is as direct as possible and clear of obstructions.

The principles of direct, unobstructed escape access apply equally to enclosed areas in the hull (i.e. the bow section and in the aft machinery spaces or in any superstructure), to the production areas and to the main decks where the congestion factors due to cargo tank pipe-work will be greater.

Attention should be given to secondary escape routes which may have to be used if primary routes are not available. Both primary and secondary routes should be provided with clear route markings.

Evacuation. The main evacuation methods from the FPSO will be by helicopter, subject to proximity and availability of helicopter services and to weather conditions, or via lifeboat. In the UK North Sea locations the access requirements to the helideck are set out in CAA guidelines CAP 437. Whether a freefall or a davit-launched TEMPSC is chosen, lifeboat access will be close to the TR. Covered access may or may not be provided depending on the outcome of Þ re scenario studies with input from the wind tunnel tests.

Consideration has also to be given to secondary means of evacuation such as liferafts or other proprietary methods in situations where it may not be possible for personnel to gain access to or be able to use the escape tunnel. The position of and access to these secondary evacuation points must be included in the overall layout development studies.

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3. FPSO TOPSIDES LAYOUT

3.1 Layout

See Figure 3.1.

Figure 3.1

Topsides Process Plant Layout � General

Note the safety layout with the accommodation set as far away as possible from the most hazardous process � gas compression. Also the oil separation is at the midship � the position on a moving vessel with the least

motions.

Source � J E & P Associates.

Introduction

The layout of an FPSO will probably depend on whether the vessel is a new, purpose-built vessel or a new intercept vessel or an existing unit. With a new, purpose-built vessel the designer has two main decisions to make which will inß uence the overall layout, namely the position of the living quarters and the position of the turret. In the case of an existing vessel, there is probably only one decision to be made namely the location of the turret as the living quarters will already have been Þ xed.

The following sections consider the principal issues which affect layout. First and foremost, the main considerations are those of the safety of the crew and of reducing their exposure to the hazards encountered on an offshore installation storing and processing hydrocarbons. Whatever conÞ guration is used, the scale of the hazards reduces towards the living quarters, the temporary refuge and the principal points of evacuation.

Turret

All turret/swivel positions in the North Sea, and other harsh environments are internal to the hull to minimise accelerations on ß exible risers, for structural support of the turret, protection of the swivel and accessibility for maintenance. In less harsh environments the turrets may be located externally. In the locations of very mild conditions the FPSOs will be spread moored, with no need for weathervaning.

There is a choice of internal turret position (see Figure 3.2).

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Figure 3.2

Position of Turret

Source � J E & P Associates.

Forward(a)

A forward location provides natural weathervaning of the FPSO with use of minimal or no thrust power. This is the more common system in the North Sea and is used on vessels with a high-pressure swivel which permits unrestricted rotation.

A forward turret is structurally preferable as it is located away from the central highly stressed part of the hull and less longitudinal reinforcement is needed. This is especially important on converted tankers.

The acceptability of riser top-end motions and accelerations need to be checked at an early stage for forward turrets because of the high combined heave and pitch vertical motions close to the bow.

Just forward of amidships(b)

This location requires vessels with substantial thruster power to control weathervaning. Where this type of turret is used, the vessel has a wind-on system of high pressure transfer hoses which permit rotation of approximately 270 degrees before the system must be �unwound�.

The turret opening close to amidships introduces major structural reinforcement problems on a converted vessel but is more easily incorporated at the design stage on a new-build vessel.

Deck space. Hulls sized for the crude oil storage requirement provide adequate deck space for deck mounted production facilities for small/medium-sized Þ elds. On large throughput FPSOs where production facilities may be complicated with water injection and gas processing, the hull may not be large enough to provide enough deck space for a workable single-level layout with adequate lay-down area and future tie-in space. For new builds, it may be economically attractive to increase hull size rather than constrain topsides deck area.

Flares, Exhausts and Vents

Flares. The position of the ß are structure will be determined largely by the position of the living quarters. A bow-mounted living quarters will give rise to a stern mounted ß are and vice versa. Once a location has

32

been selected, account will have to be taken of the ß are stack in relation to the main ß are headers and knockout drums to achieve satisfactory pressure drops in the ß are system.

The height of the stack will be chosen after careful consideration of ß are radiation levels and the recommended human exposure guidelines. Thermal studies are conducted to meet any regulatory requirements. The radiation levels on adjacent structures and equipment will also have to be taken into account. The potential for liquid carry over and ignited droplets should also be considered to avoid the occurrence of �ß aming rain� and its fall out on production areas.

The ß are structure will probably also carry a low pressure hydrocarbon atmospheric vent, discharging at an intermediate point up the structure. The interaction of the plume from the vent outlet with surrounding structures and work places has to be considered in any layout assessment. The risk of ignition of the plume by the ß are in still air conditions has also to be checked. The outputs from wind tunnel tests should also be used.

Exhausts. The exhausts which dominate the main deck skyline are those from the main turbo-generators and from any gas turbine drivers used on compressor trains. Although Þ xing the location of the turbines themselves may be straightforward, the routing of ducting and selection of the Þ nal discharge location may not be as simple. Turbines providing main electrical power will be located in the least hazardous area of the main deck; the hot exhausts could have an adverse interaction with the helideck. Wind tunnel tests will assist in the selection of the optimal location of the exhaust outlet.

In the case of a vessel conversion, the existing in-deck generators may be retained to provide main power. Again the routing and the discharge points of these exhausts have to be carefully considered to avoid interactions with adjacent structures and work areas.

Vents. In addition to the atmospheric vent discussed above, the other large vent discharge point to be considered is that of the cargo tank venting system. This can be incorporated into the main ß are structure but may discharge at some intermediate elevated location along the main deck. The wind tunnel test programme should include a check on the main tanks� vent outlet. Still air conditions should also be taken into account.

Noise. Noise levels from the operating equipment will need to be evaluated, and minimised by good acoustic insulation. Areas requiring ear defenders will have to be deÞ ned.

3.2 General Arrangement Considerations

See Figure 3.3.

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Figure 3.3

Topsides Process Plant Layout � Detailed

Source � SBM

The primary factors governing the deck layout of an FPSO are:

The process equipment must be located as close to amidship as possible to minimise the � effects of vessel motions on the separators in particular;

The accommodation block with the helicopter platform must be located clear of hazardous � zones and ideally upwind of the ß are stack; and

In the North Sea, export is always likely to be by tandem-moored tanker and so the � ofß oading facilities as well as the metering arrangements will be at the stern if the turret is near the bow (or at the bow if the turret is at the stern).

The application of these objectives makes a distinction between a converted tanker FPSO and a new-built FPSO.

3.2.1 Converted Tanker FPSO

In a conversion, it is desirable to make as much use as possible of the existing facilities (tanks, accommodation, pumping, power generation). Existing pumping and power generation facilities can often be integrated into the new production facilities. However, in practice it is frequently necessary to rebuild the accommodation block to cater for a larger number of crew, for more stringent safety requirements and also to provide large areas for control and monitoring features.

With the accommodation block either retained or replaced at the stern of the converted vessel, it is attractive to Þ t the turret aft (i.e. toward the stern) since it will place the accommodation upwind of the process and storage areas. However this is difÞ cult, mostly for structural reasons since the normal stern end of a tanker is not designed to carry high shear loads and is also an inconvenient shape to accommodate a large diameter turret moonpool.

Generally the turret must be located at the forward end, where it can be Þ tted externally in a new specially constructed extension of the bow, or it can be Þ tted internally just aft of the collision bulkhead or in No.1 centre tank. For North Sea applications the second option � internal turret � is more suitable because of the

34

severe motions experienced at the bow. In either case the accommodation block is located downwind of the turret and the process equipment.

3.2.2 New Build FPSO

In a new-built FPSO the objectives above are most easily satisÞ ed by placing the accommodation and helideck block at the extreme forward end with the turret aft of this position. This ensures that the accommodation and control areas are upwind of all hazardous functions: turret, process equipment and ß are.

However, more recently, the reverse arrangement with the accommodation at the stern end, has also been considered and following thorough assessments of the risks involved, has been accepted.

The International Regulations regarding FPSOs are presented in Figure 3.4.

Figure 3.4

International Standards for Design, Construction and Operation of FPSOs

Source � Wikipedia.

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4. MOORING SYSTEM

4.1 Introduction

Mooring systems for FPSOs have evolved initially from the components and technology already available in the marine industry, i.e. the conventional chains, winches and anchors. However, the technology has evolved very rapidly to satisfy the speciÞ c requirements of offshore applications: more severe environments, larger vessels, longer design lives, deeper waters.

Production vessels are either moored by a spread-moored arrangement or single-point mooring. The latter allows the vessel to weathervane (take up the heading of least resistance to the environmental conditions of the waves, tides or wind). These options are represented in Figure 4.1 and illustrated in Figure 4.2.

Figure 4.1

Mooring Types

Source � J E & P Associates.

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Figure 4.2

Types of FPSO Mooring

Source � J E & P Associates.

4.2 Mooring Systems Basics

4.2.1 Restoring Forces

The primary purpose of a mooring system is to maintain a ß oating structure on station within a speciÞ ed tolerance, typically based on an offset limit determined from the conÞ guration of the risers. The mooring system provides a restoring force that acts against the environmental forces which want to push the unit off station. In the following diagrams the main components of a mooring system restoring force are explained.

The connection between the mooring system and the body of the vessel is where the restoring force of the mooring system acts. At this connection point there are two force components present; horizontal and vertical. The horizontal component of the mooring line�s tension acts as a restoring force. The vertical component acts as a vertical weight on the vessel. In deepwater the vertical force can be quite considerable.

The tensions in a mooring line are split into two components: the restoring force that opposes the environmental loading, and the lateral force, which may be balanced by another mooring line.

4.2.2 Environmental Loading

See Figure 4.3.

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Figure 4.3

Environmental Weather Data

General wave data from around the world and detailed data from West Africa. SigniÞ cant wave height = two-thirds of the distance between the bottom and crest of each wave. Note how calm the conditions are in

W Africa compared with the North Sea. In the North Sea internal turrets are a must. In W Africa spread mooring is the norm.

Source � J E & P Associates

When there is no external loading on the system the vessel will not move from its static equilibrium position. When environmental loading does occur an imbalance in the system will occur. To restore equilibrium the mooring system restoring force must become equal to that of the environmental load. This is achieved through the vessel offsetting from its original position. As this occurs the �windward� lines will pick up tension and the �leeward� lines will shed tension.

The vessel will offset until the �windward� lines have generated a restoring force that balances the environmental loading. This means that the distance between the anchor and fairlead will increase, and thus the tension at the fairlead will also increase.

4.3 Mooring System Types

There are two main types of mooring systems: spread mooring and single-point mooring (SPM).

4.3.1 Spread Mooring

See Figure 4.4.

38

Figure 4.4

Spread Mooring

Spread mooring is possible in very calm areas where weathervaning is not required. It is common in W Africa.

Source � Keppel Shipyard and J E & P Associates.

This conventional mooring approach is used for non-weathervaning FPSOs. Spread system can be applied to ship-shaped vessels as long as the environmental conditions are relatively benign and the weather direction is fairly uniform without strong cross currents. In a location such as the North Sea, the forces which can be generated on the beam of a spread-moored FPSO, plus the motions in such conditions, effectively prohibit such a mooring arrangement.

The mooring lines can be chain, wire rope, Þ bre rope or a combination of the three. Either conventional drag anchors or anchor piles can be used to terminate the mooring lines.

4.3.2 Single-point Mooring

Single-point moorings (SPMs), such as internal or external turrets, are used primarily for ship-shaped units. They allow the vessel to weathervane, which is necessary to minimise environmental loads on the vessel by heading into the prevailing weather. There is a wide variety in the design of SPMs, but they all perform essentially the same function.

The main types in increasing order of environment severity are:

� Fixed tower � Figure 4.5

A Þ xed tower is suitable for shallow water depths (20�50m) and small wave heights (about 5m signiÞ cant). It can be connected to the ß oating vessel by a simple hawser. However, to avoid the risk of extensive damage in the event of a minor collision between the tower and the FPSO, the hawser is usually replaced by a yoke and pendulum system. Fixed tower mooring systems are common in the shallow water locations of a number of Chinese FPSO developments, for example the Bohai Bay area with the new ALP SYS system as described below.

39

Figure 4.5

Single-point Mooring � APL�s SYS Tower Mooring System in Bohai Bay (China)

Bohai Bay is sheltered and very shallow � 20m to 30m water depth. This allows the cost effective use of a tower mooring system.

Source � APL.

� CALM buoy (Catenary Anchor Leg Mooring) � Figure 4.6

A CALM buoy is suitable over a wider range of water depths (30�50m) and larger wave heights (up to 8m signiÞ cant). It can be connected to the ß oating vessel by a yoke and pendulum system similar to that for the Þ xed tower, or by a rigid arm that is hinged or rigidly connected to the buoy and hinged to the vessel.

40

Figure 4.6

Single-point Mooring � FPSO Moored to Catenary Anchor Leg Moored (CALM) Buoy and External Turret Moored FPSOs

Source � SBM.

� External turret � Figure 4.7

An external turret eliminates the CALM buoy and allows the turntable and swivels to be directly attached to the vessel bow or stern. It is suitable for deepwaters and large wave heights. It can be used up to the point where the combined heave and pitch motions may cause slamming on the bottom of the turret (depending on vessel size and length, up to approximately 12m signiÞ cant wave height).

� Internal turret � Figure 4.8

An internal turret is convenient when a large number of risers are to be installed, and therefore a large turret and swivel assembly are required. An internal position also reduces the risk of slamming due to the reduction of the effect of pitch. Consequently internal turrets can be used in deepwaters and the most severe environments (up to 18m signiÞ cant wave height).

41

Figure 4.7

Single-point Mooring � Internal Turret Moored FPSO

Source � SBM.

4.3.3 Catenary and Taut Mooring System

Two main types of mooring system can be used for either the spread or single-point system: taut-leg and catenary. Both methods allow the system to withstand the applied forces, but through different mechanisms.

A �catenary� system generates restoring force through the lifting and lowering of the line onto the seabed, plus a limited amount of line stretch.

A �taut-leg� system makes use of the material properties of the mooring line, namely its elasticity.

4.4 Mooring System Design

4.4.1 Vessel Dynamics

Waves will cause a vessel to move in all six degrees of freedom: surge, sway, heave, roll, pitch and yaw.

The motion of the vessel to individual waves is called its wave frequency or Þ rst-order response. As a mooring line moves through the water it will be subject to dynamic line drag and inertia loading and sometimes a whipping effect. It is possible to take this into account by undertaking a dynamic mooring analysis, but this does increase computing time signiÞ cantly.

The compliance of a mooring system is such that conventionally the presence of the mooring system is not considered to affect the wave frequency response. The overall mooring system stiffness and associated natural frequency will inß uence its second order or low frequency slow drift response.

4.4.2 Mooring Design

The development of the mooring system will require a number of inputs listed in Figure 4.8.

42

Figure 4.8

Mooring Design � Input Information

Source � J E & P Associates.

Mooring Layout

The mooring layout should be designed to distribute the loads in the individual lines as equally as possible and also to give sufÞ cient redundancy to the overall system. The important factors are:

The strength of each line;�

Seabed topography and soil friction;�

Prevailing directions of wind, waves and current;�

Proximity of other Þ xed structures on the seabed such as templates and pipelines or in the � water column, such as risers and riser mid-water arches, etc.;

Other storage or drilling vessels moored in the vicinity;�

Future operational activities in the Þ eld (e.g. well workover).�

Note that mooring systems are often symmetrical but they don�t have to be. For speciÞ c environmental conditions asymmetric systems may be more effective.

Environmental Data

Mooring systems are normally designed for the 100-year storm conditions, i.e. for the combination(s) of wave height, wind and current velocities which are likely to occur once in a 100 years. These conditions are established by extreme value analysis and extrapolation based on environmental data measured over a sufÞ cient length of time.

Typical values of waves in 100-year storms are given in Figure 4.3.

4.4.3 Analysis Methods

See Figure 4.9.

43

Figure 4.9

Mooring Line Analysis

The analysis is carried out using specialist software. The approach illustrated is an older method of a �quasi static� model. In this there are three components of loading (the vessel�s mooring system is

designed to resist such loadings).

The mooring system is a �soft� system allowing the vessel to move around about the mooring point. The level of excursion is called the �offset�.

Source J E & P Associates.

The essence of mooring design is to optimise the behaviour of the mooring system such that the excursions of the surface vessel do not exceed the allowable ß exible riser offsets, while at the same time ensuring that the line tensions are within their allowable values. Thus the mooring system load offset curve should not be too hard or too soft. Hence, considerable iteration work may be required to optimise a system for a particular location.

The results obtained are the tensions in the mooring lines and the excursions of the FPSO vessel.

4.5 Mooring System Components

The main components of mooring systems, from the points of attachment of the mooring lines on the FPSO to the seabed, are:

Winches�

Fairleads�

Mooring lines (chains, wire ropes or hybrid materials)�

Surface or submerged buoys�

Clump weights�

Anchor system (drag, embedment, gravity, pile or suction anchors).�

4.5.1 Winches And Fairleads

When reaching the FPSO, the mooring lines are guided through fairleads, which can be either sheaves (pulleys) or bending shoes. The sheaves can handle both chain and wire rope whilst the bending shoe is

44

designed for wire rope only and is coated with a special high density nylon bearing material to reduce friction.

The payout, haul-in and tensioning of a mooring chain are normally accomplished with a rotary winch (windlass) or a traction winch.

4.5.2 Mooring Lines

See Figure 4.10.

Figure 4.10

Mooring Chain and Wire Rope

Source � J E & P Associates and SBM.

In ß oating production systems, chain and wire are the most commonly used mooring line materials; chain and wire are also often used in combination.

Chain provides weight and therefore stiffness through the catenary effect whilst the wire rope provides greater elasticity and therefore compliance at high tension levels.

Thus the combination of chain and wire rope provides optimal performance in a wide range of water depths. In shallow waters (less than 100m), the use of heavy chain through the water column provides the initial high catenary stiffness and the use of wire rope (sheathed in this case) on the seabed provides the compliance at high tensions. In deeper waters, the use of wire rope through the water column helps to reduce the vertical loads and the use of chain at the touchdown point provides the stiffness required. At the fairleads on the FPSO, chains are often preferred to avoid bending loads and also for easier handling.

As offshore applications go into deeper waters (below 1,000m), man-made Þ bres (e.g. Kevlar, polyester and polyethylene) become beneÞ cial because of their superior strength to weight ratios.

Hybrid Systems (Combinations of Chain and Wire Rope) � See Figure 4.11.

45

Figure 4.11

Typical Shallow Water Catenary Mooring System (North Sea)

Note � for a water depth of 120m the mooring lines step out more than 600m � this takes up a lot of seabed space. The hybrid mooring lines are made up from sections of chain and sections of wire rope.

Chain connects the Þ rst section to the turret. The section on open water is wire rope � cheaper and more efÞ cient. The section at touchdown on the seabed is chain � here there is high erosion of the chain by the seabed materials. The section along the seabed continues with chain � this has a good friction contact with the seabed.

Lastly the line is Þ nished with the attachment anchor or pile.

Source � J E & P Associates.

In support of cost efÞ ciency, it is common to use a combination of chain and wire rope. A typical arrangement for a shallow water FPSO (i.e. 120m water depth) might be:

1st section from turret = 27.5m chain�

Portion through water column = 100m wire rope�

Touch down section = 300m heavy weight chain�

Final touch down section = 270m light weight chain�

Pile end.�

A semi-taut hybrid mooring line from a spread-moored FPSO is represented in Figure 4.12.

46

Figure 4.12

Typical Spread Semi-Taut Moored System (West Africa)

In this spread-moored system the chain is attached to the chain stopper porch external to the hull at main deck level. The chain runs down the side of the vessel and through the underwater fairlead at the bottom of the hull.

It then angles out to the seabed. It is a hybrid system of chain and wire rope.

The mooring is a semi-taut system which is like the catenary mooring (Figure 4.11) but with very little line on the seabed. The attachment to the seabed is a suction pile. Above the stopper porch there are deck

sheaves. When the mooring lines are attached to the pulling lines they run through the sheaves and are pulled by a linear winch set on the main deck.

Source � J E & P Associates.

4.5.3 Man-made Fibre Ropes

See Figure 4.13.

47

Figure 4.13

Deepwater Mooring Systems

Use of polyester rope. The taut mooring system design uses less seabed space (line at 45 degrees).

Source � SBM and J E & P Associates.

The technology of man-made Þ bre ropes is at the development stage, although it is advancing rapidly.

The materials mostly used for Þ bre ropes are polyesters, aramids, high modulus polyethylene (HMPE) and polyesters.

The densities of these materials are close to unity (0.98�0.99 for HMPE, 1.38�1.40 for polyester and aramid) and therefore Þ bre ropes are almost neutrally buoyant.

The axial stiffness of Þ bre ropes is a more critical parameter than that of either chain or wire rope, because the stiffness is mostly contributed by axial stretch. The stiffness is not constant and is inß uenced by load level, range, frequency and history. For example there is a factor of two or more between stiffness at installation and stiffness of a worked rope.

The fatigue life of Þ bre ropes is inß uenced by creep, abrasion and external wear. Under constant load conditions the Þ bres will creep (i.e. load backs off). This means the lines must be re-tensioned periodically.

4.5.4 Anchor Systems

See Figure 4.14.

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Figure 4.14

Mooring Line Attachment to the Sea Bed

The attachment anchor or pile types are: drag embedment anchors, hammered piles or suction piles. The type is selected by seabed conditions and pulling loads.

Source � J E & P Associates.

Detailed site information is required for anchor system design. This should include geophysical data (bathymetry) and geotechnical borehole data (soil conditions).

Soil conditions at the installation site can greatly affect the selection of an anchor system and soil investigations should include both the nature and the depth of the seabed material.

A large variety of anchor systems is available; they include:

Gravity anchors�

Conventional drag anchors�

Drag embedment anchors�

Suction anchors�

Pile anchors�

Directed Learning: A company that makes anchor systems is Vryhof Anchors. Visit their website www.vryhof.com Go to DOWNLOADS. There you will Þ nd a detailed anchor manual (more than 100 pages). Use it to learn more about anchor systems.

49

5. TURRETS

5.1 Types of Turret-mooring Systems

5.1.1 Introduction

The purpose of a turret-mooring system in a mono-hull FPSO vessel is twofold:

It moors the vessel to the seabed; �

It links the vessel to the subsea wells via the ß exible risers and control umbilicals.�

Different types of turret-mooring systems exist which satisfy these two requirements and it is convenient to split them into two groups:

External turret-mooring systems (with 5% of disconnectable one).�

Internal turret-mooring systems (with 5% of disconnectable one).�

5.1.2 External Turrets

See Figure 5.1.

Figure 5.1

Disconnectable External Turret � Cossack Pioneer FPSO

A disconnection capability is required because the FPSO is located in a region of the world that experiences cyclones. As the cyclone approaches the Þ eld must be shut down, the FPSO then disconnects and moves out of

the path of the storm. Re-connection takes place once the cyclone has passed by.

Source � Woodside Energy Ltd.

The turret is mounted externally to the hull of the vessel either at the bow or at the stern. In its simpler version the turret is secured to a cantilever beam extending from the main deck of the vessel. Consequently the bearing arrangement is located above water and both the mooring lines and the risers are also attached above water. The advantages of this turret are its simplicity and minimum requirement for integration into

50

the hull of the vessel, hence its lower cost. Its main limitation is linked to the requirement to provide sufÞ cient cantilever extension length and height in order to avoid interference between the mooring lines and the bow or stern of the vessel. This in turn limits its application to shallow water depths and relatively mild environments.

A variation is the disconnectable riser turret mooring which is speciÞ cally designed for environments prone to cyclones. The turret consists of a large buoyant riser column which is suspended from a cantilever beam extending from the vessel deck. The mooring lines and risers are attached to this column and remain attached to it in all conditions. Disconnection takes place between the top of the column and the cantilever beam above a certain threshold in cyclone conditions.

5.1.3 Internal Turrets

See Figure 5.2.

Figure 5.2

Large Internal Turret � Schiehallion FPSO

FPSO in west of Shetlands location with 100-year waves up to 18m signiÞ cant wave height.

Source � SBM

The turret is mounted internally inside the hull of the vessel, normally in the forward half (i.e. between midship and bow). It consists of a large cylindrical structure rotating inside a cylindrical moon pool in the hull. The bearing arrangement can be mounted at vessel deck level above water, or inside the moon pool below water, or it can be a combination of both. The mooring lines and the risers are attached to the base of the turret, below water.

The turret is integrated inside the hull of the vessel and is therefore protected from direct wave loads and the risk of collision. Other advantages are its capacity for handling a large number of risers and its ability to withstand severe environments (northern North Sea, west of Shetlands) in deepwater.

These advantages are gained at the expense of a more complex turret structure and turret integration inside the hull of the vessel, and consequently a higher cost.

There are several designs of internal turret mooring systems which may be distinguished by features such as:

Turret size as deÞ ned by the diameters of the turret and the moon pool; �

Type and position of bearing arrangements;�

51

Requirement for thruster assistance or not;�

Ability to accommodate any type of ß uid and electric transfer system. �

Disconnectable versions of the internal turret-mooring system also exists. This design is characterised by a large buoy located beneath the keel of the vessel. When disconnected, the buoy submerges to a pre-determined depth approximately 35-40 metres below the surface where it stabilises whilst still supporting the mooring lines and the risers. As before, this design is for applications in cyclone prone areas. See Figure 5.3.

Figure 5.3

Submerged Internal Turret Production (STP) Disconnectable Turret

Turret system on Pierce Field FPSO, North Sea.

Source � APL.

Finally a more signiÞ cant variation of the internal turret is the STP (Submerged Turret Production). This consists of a buoy located beneath the keel of the vessel, but supporting only the ß exible risers. There are no mooring lines since the vessel is held on station by its thrusters controlled by a dynamic positioning system. When disconnected, the buoy submerges to a pre-determined depth and the vessel may sail away.

5.2 Turret Position

The turret is usually mounted in the forward half of the vessel and the accommodation can � be either forward, in front of the turret, or it can be aft (i.e. stern end of the vessel);

If the vessel is passive (i.e. no thrusters) or has a minimum thruster capacity, the turret � can be external at the bow in mild environments, and internal within the Þ rst 25% of the vessel length from the bow in severe environments; and

If the vessel is active (i.e. signiÞ cant thruster capacity) the turret is internal and is � positioned just forward of midship.

5.3 Bearing Arrangement

See Figures 5.4 and 5.5.

52

Figure 5.4

Espadarte FPSO Turret System (Brazil)

The Petrobras Field in Brazil are developed with many risers (connecting to seabed ß owlines from each well). Some FPSOs there have up to 75 risers. This large number of risers leads to very large turrets, with many activity decks in the turret. Note � because there is no subsea manifolding of the ß owlines,

this function is performed in the turret decks. A smaller number of lines are then required through the swivel system above.

Source � SBM

Figure 5.5

Turret Design Showing Chain Locking Table and the Bearing System

The mooring lines are pulled into the correct tension by a deck mounted winch, and dropped into the chain stopper receptacle on the chain table at the bottom of the turret. In this internal turret the main bearing system is towards

the bottom (it is a three-roller bearing system). It is below the waterline and is sealed from the seawater. The risers enter the turret bottom via guides and continue to their connection at the turret main deck level.

Source � Azur Offshore Ltd and SBM.

53

Bearings may be:

Rigid bearings such as ball bearings and roller bearings.�

And compliant bearings such as bogey wheels and friction pads. �

Rigid bearings can withstand very large loads around a comparatively small diameter. They can be made to the size required and therefore do not inß uence the size of the turret. The maximum size is of the order of 10m in one solid ring and 17m in segmented form. These bearing systems may be at the main deck level or (more usually) below the waterline. Often, both these types have secondary support systems.

Compliant bearings can also withstand very large loads, but the larger the load the larger the bearing. The reason is that as the load increases, the number of wheels or pads to support it also increases, and these can only be accommodated around larger diameter tracks. Typical track diameters are generally in excess of 20m. These bearings are at the main deck level on the FPSO.

5.4 Mooring Lines and Riser Connections

See Figure 5.6.

Figure 5.6

Turret Underside

Source lower turret chain table where mooring lock to FPSO and entry guides for the risers to come up through turret to main deck level.

Source � Azur Offshore Ltd.

The turret ends of the mooring lines always consist of chains since these are easier to handle than steel cables. The chains can be connected either at the bottom or at the top of the turret and are generally handled from the main deck.

These two types of mooring lines connections reß ect the types of turrets:

Connection at the bottom of the turret is used for external turrets and for small diameter 1. internal turrets equipped with rigid bearings; and

Connection at the top of the turret is used for large diameter internal turrets equipped 2. with compliant bearings.

If connected at the bottom of the turret, the chains normally terminate at articulated, ratchet action chain stoppers mounted on the chain table. The chains are tensioned by cables which are run through one or more fairleads and up to the deck.

54

The chains are tensioned by winches which can be mounted either on the turret or on the vessel deck; additional fairleads are necessary in the latter case. The extra lengths of chains which are needed for installation are stored in chain lockers on board the turret. Alternatively the chains may be cut above the chain stoppers, once tensioned, and the surplus lengths of chains may be stored in lockers aboard the vessel, thereby saving space on the turret.

The risers are generally connected up to rigid piping in a dry environment, inside the turret.

Each riser penetrates the turret bottom through its chain table or ring pontoon and is guided up by individual hawser tubes or funnels. At the entrance point each riser is protected by a bend restrictor which may be inclined from vertical to suit the catenary suspension angle and minimise the strains in the riser.

The risers are generally terminated close to main deck level in an area which is protected, accessible and well ventilated for reason of potential gas leakage. They are connected to rigid piping by standard couplings, but may be welded for further reduction of the risk of leakage upstream of the ESD valves.

5.5 Fluids and Electric Transfers

See Figure 5.7.

Figure 5.7

Toroidal Swivel Design and Swivel Stack

The swivel allows the pathway of the risers (Þ xed to the seabed) to connect to the line attached to the FPSO (which can freely rotate � weathervane). Each upward ß ow line goes into the toroidal section. The inner

part (attached to the turret) is separate from the outer part (attached to the vessel). The separate section has high pressure seals. A swivel may have many ß uid paths. Each pass sits on

top of another and the arrangement is known as a swivel stack.

In addition to the ß ow pathways for the ß uids from the reservoir of water injected into the reservoir the swivel must transfer electrical signals and control ß uids and chemicals required at the subsea equipment

level. The swivel components for these are towards the top section. Lastly at the very top there is an inline swivel which is usually reserved for transfer of high pressure gas.

Source � SBM.

Since the turret and the FPSO vessel rotate with respect to each other, there is a requirement for the transfer of ß uids and electric lines between the turret and the FPSO. The technology presently available for ß uid transfer consists of:

The toroidal multi-path ß uid swivel.�

55

The toroidal multi-path swivel is a mechanical assembly with bearings and active seals. Only few manufacturers in Europe and the US have experience with this critical technology. In addition, for maintenance purposes, a large gantry is installed above the turret.

The toroidal swivel has a number of advantages:

It offers unlimited rotation in either direction;�

It is compact, relatively light and can be installed on top of any turret;�

It can operate in severe environments;�

It requires little maintenance; and�

It can accept multi-phase unprocessed ß uids from the subsea production system.�

Its technical limitations are:

The maximum size of seal diameter, presently of the order of 2.0m, which controls the � number of ß ow paths which can be accommodated; consequently manifolding upstream of the swivel may be required for large applications;

The pressure of the ß uids, particularly for gas applications (present maximum of the order � of 6,250 psi working pressure).

The size of a toroidal swivel is dependent upon the number and size of the ß ow paths it needs to accommodate. A recent example is the swivel for the Norne FPSO:

Four toroidal paths, 10-inch at 230 bars. �

Seal diameter,�

One in-line gas path, 8-inch at 335 bars, �

Total stack height 8.9m,�

Total stack weight 93 tonnes.�

The transfer of electricity, hydraulic ß uids for power or control and chemicals on-board the turret or subsea systems are accommodated by conventional electro-hydraulic swivels and swivel ring systems.

Examples of multi-path swivel stacks are shown in Figure 5.7.

5.6 Conclusions

It must be remembered that turrets are part of a weathervaning FPSO. Spread-moored FPSOs do not require turrets. Additionally, with the spread-moored FPSO the risers simply drape over the vessel�s sides rather than being grouped through the turret.

Directed Learning: To Þ nd out more about turrets and swivel systems visit the SBM Offshore website. Go to www.sbmoffshore.com Go to MEDIA INFORMATION and the DOWNLOAD CENTRE. Download the brochures on TURRET SOLUTIONS and SWIVEL STACKS. Review the information.

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6. RISERS

6.1 Introduction to Flexible Dynamic Risers

See Figure 6.1.

Figure 6.1

Flexible Dynamic Risers

Flexible dynamic risers permit the connection from the seabed to a vessel which can have signiÞ cant surface movements. The continuous risers have a low part which is in tension. Above an arch is formed by laying

it over a rigid support arch or by distributing blocks of foam along its length. The section connecting to the production vessel is in a catenary shape. This catenary section provides the reservoir

of length to accommodate the vessel�s motions (as shown in Figure 6.2).

Source � J E & P Associates.

The production riser system carries well ß uids up from the seabed to the host for production. Some risers will carry ß uids (water and gas for injection into the Þ eld) down from the host to the reservoir. Other risers will carry the dead crude and gas for export to shore.

Early ß oating production vessels deployed steel riser pipes, but these had to be disconnected (after Þ eld shut down) during storm periods. Flexible dynamic risers allow the FPSO to remain fully operational during storm conditions. Such risers provide allowance for the movement of the vessel.

The basis of the ß exible dynamic riser is that it can cope with the FPSO�s movements, even in extreme conditions, by means of its conÞ guration. The riser comes up from the seabed (a �tension� section). It then has an arch conÞ guration � this is either by means of draping it over a mid-water arch or by supporting it with foam blocks distributed along this section. The Þ nal part is the catenary hang section from the arch up to the connection into the FPSO. This catenary portion provides the reserve of length to accommodate the vessel�s movement. See Figure 6.2.

57

Figure 6.2

Basic Movement Requirements of a Flexible Dynamic Riser between Seabed and FPSO

Source - J E & P Associates.

The dynamic ß exible riser concept has been used successfully for several years in various parts of the world in relatively temperate weather conditions and was used by the industry for applications in the most harsh environments, North Sea included, since 1985. See Figure 6.3.

Figure 6.3

FPSO with Mooring Lines and Risers

Source � SBM.

6.1.1 Types of Pipe Structures and Main Manufacturers

In the 1980s two types of structure have been on offer, the BONDED rubber based one and the UNBONDED thermoplastic/steel one.

58

Today only UNBONDED structures are manufactured for ß exible dynamic risers. Bonded structures are only used for short lengths of ß exible pipe jumpers at the top of drilling risers and for ß oating ofß oading hoses, where they can easily be replaced.

A typical unbonded structure with all the various layers and their functions is illustrated in Figure 6.4.

Figure 6.4

Technip Flexible Dynamic Riser Construction and Latest Integrated Production Riser Bundle

The ß exible pipe is a complex fabrication of steel layers and plastic layers. The steel provides the core and the tensile and hoop strength. The plastic provides the ß uid containment requirements. The layers are

not bonded together, but can move with respect to each other.

The latest risers may have insulation and electrical trace heating elements.

Source � TECHNIP Ltd.

Manufacturers have made very large investments to build large and complex factories. The three main manufacturers are:

TECHNIP with factories in France, Brazil, Angola and Malaysia.1.

WELLSTREAM with factories in the USA and UK.2.

NTK with one factory in Denmark.3.

Other factories are planned.

6.1.2 Devices Connected or Attached to Flexible Risers

From the ß oater to the seabed a signiÞ cant number of devices are used. These include:

End Þ ttings�

Buyoncy modules�

Mid-water arch�

59

Bend stiffeners and bend restrictors�

Fairing vanes and other guiding tubes and cones�

PLEM and riser base.�

6.1.3 New Flexible Risers for Deepwater Developments

The problem of FLOW ASSURANCE does impose a means of maintaining heat in risers, in particular during shut downs.

TECHNIP pioneered the IPB system in 1998 for deepwater projects in Angola (�1,500m). There is extensive thermal insulation, but also an active electrical trace heating wires built into the structure.

Directed Learning: To learn more about ß exible dynamic risers visit the TECHNIP website www.technip.com. Search under SUBSEA. Click Publications. Click Brochures. Look at FLEXIBLE PIPE and FLEXI FRANCE. These give a lot of information on their products � review the information and collect for your Þ le.

6.2 Hybrid Riser Systems

In the early 1990s Mobil proposed a fully welded riser tower concept for 1,400m of water. The same design concept was considered by the ALTO MAR GIRASSOL Group (ETPM, BOUYGUES OFFSHORE, STOLT COMEX) and further designed for the Girassol Field development in 1998. This design was selected against a ß exible riser system with hot water heating.

This new system was locally fabricated and was installed during the summer 2001. It is represented in Figure 6.5.

60

Figure 6.5

Riser Tower System Details

The riser tower was used in the Girassol Field � Angola. Each of the three riser towers has six 8� pathways for ß ow and two 2� pathways for service ß uids, all surrounded by insulated foam blocks. The towers are some

1.3 km in length. At the bottom they attach to the pile mount with a RotoLock stab-in connection. At the top there is a buoyancy can which tensions the riser and provides the connection

of the ß exible jumper connections to the FPSO riser porch.

Source � Azur Offshore Ltd.

6.3 Steel Catenary Risers for Deepwater Developments

The use of free-hanging steel catenary risers is ideal for benign environments and low vessel motions. A good example of innovation, is the export riser for the Gulf of Mexico Shell Auger TLP. See Figure 6.6.

61

Figure 6.6

Steel Catenary Risers � Auger TPL (GoM)

With a relatively stable TLP platform, and in some 800m water the steel riser can be draped from the platform to the seabed in a catenary shape.

Source � Azur Offshore Ltd.

For more severe loading conditions, limitations come into play, relating to touch-down dynamics, maximum vessel offset and increased top-end response.

The predominant pipe material considered for the riser is �standard� X 65 grade steel. A small amount of X 80 grade is also used, a light stress-joint (or possibly a short length of titanium pipe) at the top-end ensures that combined stresses are below 50% of yield throughout. Low dynamic response means that a long fatigue life is predicted.

Maximum dynamic rotation over riser end-sections is about ten degrees, and seabed and vessel ends are conÞ gured at a small angle to the vertical, corresponding to a static rotation in the mean position. Uplift on the riser base and vessel weight loading are 100 tonnes and 160 tonnes respectively. Installation would be by controlled depth tow of the entire riser or of several sections joined on-site using mechanical connectors.

The status of SCRs to date is:

The Shell AUGER TLP platform saw the installation of two SCRs 12-inch export risers in 1994. They are in API 5L X 52 pipeline steel, coated with three layers of polyethylene and Vortex Induced Vibration (VIV) suppression strakes in the top 150m. The water depth is 858m, they have a length of 1500m, an inclination angle of 11 degrees +/� 2 degrees from vertical and are connected at pontoon level hang-off points with an elastomeric ß exjoint.

Since the AUGER installation from a TLP, a lot more work has been done, in particular:

Petrobras XVIII semi-sub production platform. Marlim Field. 1998. 10-inch riser in 910m � of water. Experiment for mooring.

STRIDE JIP has performed, in 1998 in the UK, test initiatives into several areas for the � design of SCR. Tests have been carried out offshore with six-inch and 10-inch risers.

Morpeth Field Mini-TLP GoM. 1998. Two risers, 12-inch and 8-inch for oil and gas export, � have been installed in 470m.

SCRs have been installed on the Bonga FPSO operating in Nigeria (see� Figure 6.7).

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Figure 6.7

First Steel Catenary Riser on FPSO (Bonga FPSO in Nigeria)

The Bonga Field in Nigeria is the Þ rst use of steel catenary risers deployed from an FPSO. It is highly instrumented and should provide useful information to the industry about further potential use with FPSOs.

Source � Shell plc.

All the developed conÞ gurations fulÞ l both the Ultimate Limit State (ULS) conditions and fatigue due to Þ rst order wave action and due to vortex induced vibrations. Also, the Fatigue Limit State (FLS) governs the global conÞ guration of the SCR concept.

In order to achieve a conÞ dent design, several design aspects must be studied in detail:

First order wave loading�

Vortex Induced Vibration�

Differential effects (from the large volume structure)�

Riser/soil interaction�

Fatigue capacity.�