Otc18085 - Deepwater Oil Export System Pas Present Anf Future

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    Copyright 2006, Offshore Technology Conference

    This paper was prepared for presentation at the 2006 Offshore Technology Conference held inHouston, Texas, U.S.A., 1–4 May 2006.

    This paper was selected for presentation by an OTC Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Offshore Technology Conference and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Offshore Technology Conference, its officers, or members. Papers presented atOTC are subject to publication review by Sponsor Society Committees of the OffshoreTechnology Conference. Electronic reproduction, distribution, or storage of any part of thispaper for commercial purposes without the written consent of the Offshore TechnologyConference is prohibited. Permission to reproduce in print is restricted to an abstract of notmore than 300 words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. 

    Abstract

    To date, nine deepwater export systems have been installed by

    Oil Companies, or are under construction. The first deepwater

    export system, on the GIRASSOL field (Angola), was

    installed in 2001. Since then, a further three systems are now

    in place, one on the BONGA field (Nigeria) and two on the

    KIZOMBA field (Angola). A further five are under design orconstruction (ERHA, DALIA, GREATER PLUTONIO,

    AGBAMI and AKPO).

    All these export systems are based on the concept of a large

    surface buoy, shaped, in most cases, like a flat cylinder. These

     buoys are anchored to the sea bed by an array of semi-tautcomposite anchor lines and support several – generally two -

    mid-water export lines.

    Although these systems show clear differences in anchor line

    arrangement and composition, and also in export line diameter

    and configuration, they indicate that the design of deep waterexport systems has reached maturity.

    In its first part, the paper gives a general outline description of

    the nine export systems mentioned above. It explains what the

    key design drivers are and describes the design process,

    addressing successively the following issues:a.  derivation of mooring force and definition of the

    anchoring system;

     b.   buoyancy requirements and hydrostatic stability;c.  coupled motion response in waves;d.  fatigue in anchor lines and export lines.

    In the second part, the paper highlights the major limitations

    of the present systems. It describes the various concepts

    recently developed in the Industry and discusses their relative

    merits and drawbacks.

    In conclusion the paper will propose a way forward, facing the

    challenge of deeper water and harsher environments.

    Introduction

    At the beginning of 2006, there were four deepwater export

    systems installed and operating offshore West Africa. The first

    one, on the GIRASSOL field, offshore Angola, was installed

    in 2001. The GIRASSOL export system was followed by twoterminals on the KIZOMBA field, also offshore Angola

    (KIZOMBA A, installed in 2004 and KIZOMBA B, installed

    in 2005) and by one terminal on the BONGA field, offshore

     Nigeria, installed in 2004 and delivering oil to export tankers

    since December 2005.

    These terminals are connected to large new built FPSO’s, with

     production rates in excess of 200,000 bpd associated with

    storage capacity greater than 2 million barrels. All these

    FPSO’s are spread moored.

    The GIRASSOL terminal experienced severe problems after

    the successive failures of five of its anchor legs, due to a newfatigue phenomenon (see Ref. [1] and Ref. [2]). However, it

    was successfully repaired in May 2004 and has been operating

    normally since then. The three other terminals also operate to

    the satisfaction of their respective owners.

    There are five other deepwater export systems, presently under

    design or construction ERHA, AGBAMI and AKPO offshore

     Nigeria; DALIA and GREATER PLUTONIO, offshore

    Angola.

    Eight of the nine systems are or will be designed and supplied by SBM and one (DALIA) by APL.

    ERHA is hooked up to its anchor legs, but not yet connected

    to the export lines.

    Seen from some distance and from the surface, these

    deepwater systems, most of them at least, look very similar to

    the popular shallow water CALM buoys. One may recognize

    the familiar flat cylindrical buoy body, topped by a rotating

    turntable, to which the mooring hawsers and the floating hoses

    are connected. The buoy body diameter just appears a bitlarger than usual (see Figure 1).

    OTC 18085

    Deepwater Oil Export Systems: Past, Present, and FutureC. Blanc, J.-L. Isnard, and R. Smith, Single Buoy Moorings Inc.

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    2 OTC 18085

    Figure 1 – Typical deep water CALM buoy

    As a matter of fact, the first deepwater export buoy, on theGIRASSOL field, resulted from the extrapolation of the

    KUITO buoy, operating since 1999 offshore CABINDA in a

    water depth of 415 m, which, itself, was a clear enhancementof a conventional CALM buoy.

    The comparison with shallow water CALM terminals stops atthis point. Deep water terminals are indeed a different product,

    designed to meet fundamentally different requirements. The

     paper will address this issue. In the first section, it explains

    what the key design drivers are, and gives an overview of the

    design process. In the section which follows, the paper simplydescribes the main particulars of the existing terminals (with

    the exception of DALIA, for which the authors do not have

    access to design details). The paper then highlights the major

    limitations of the present systems. It briefly describes the

    solutions recently developed to overcome these limitations.Finally, in the conclusion, the paper will propose a wayforward, facing the challenge of deeper water and harsher

    environments.

    Key design drivers – Design process

    A deepwater export system is made of three main components,namely (see Figure 2):

    a.  one or several export lines; b.  an anchoring system;c.  a surface piercing buoy.

    Figure 2 – Deep water export system – Schematic view

    To follow the logical design sequence, the description should

    start with the export lines, which actually drive the design of

    the export system, and finish with the surface buoy.

    In practice, the design process is not so straightforward and

    the final design results from numerous iterations which aim at

    simultaneously optimizing those three components. For the

    sake of clarity, we shall follow a straight route.

    The export lines

    For the export lines, two main options have been adopted by

    the Industry: steel pipes and un-bonded flexible pipes. Steel

    export lines seem to be the most popular. They are used on allsystems except BONGA, which is designed to accommodate

    three rough bore flexible lines and DALIA, which will receive

    two lines of similar construction.

    The parameters which drive the selection of the line diameter

    are obviously the required throughput and the acceptable

     pressure losses in the lines.

    Irrespective of the line construction and of its diameter, the

    configurations adopted vary from the simple suspended “U”

    catenary shape (ERHA, GREATER PLUTONIO, AGBAMI)

    to the so-called “Lazy W” configuration (GIRASSOL,

    BONGA, DALIA, KIZOMBA A and B), with buoyancy

    elements distributed in a symmetrical pattern along the middle

    section of the lines. One must also mention the asymmetrical

    “Lazy W” adopted on AKPO. For this configuration, the

     buoyancy element section is moved toward the FPSO side (seeRef. [7], [8] and [9]).

    The export line configurations must accommodate the relativeoffsets of the FPSO on one end and of the surface buoy on the

    other end. This can represent a severe design constraint for the

    FPSO and terminal anchoring systems when un-bondedflexible lines are used. Steel lines appear to be much more

    tolerant on FPSO and buoy excursions, but their fatigue design

    is more critical. To achieve the specified design lives,

    limitations of the diameter and increase of the wall thickness

    associated with special treatments of the welded joints are

    generally required, at least on the buoy side.

    The export lines impose on the rest of the system the

    following design constraints:

    a.  a static horizontal pull, which has to be compensated by the anchor legs opposed to the export lines;

     b.  a static vertical pull, which drives, to a significantextent, the buoyancy requirement of the surface buoy;

    c.  limited wave induced first order motions at theirextremities. This is generally not an issue at the

    FPSO end, but it is definitively a serious one at the

     buoy end.

    xy

    z400 m

    Visual Orc aFlexat 13:07 on 14/01/2002: SparErha.dat (azimuth=270; elevation=0) Statics Complete

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    OTC 18085 3

    The anchoring system

    The design objectives to be met when designing the anchoring

    system of a deep water export system can be summarized as

    follows:

    a.  to resist the horizontal pull of the export lines; b.  to allow berthing and mooring of the largest tanker,

    up to the specified “operational” conditions, dueconsideration being given to relevant Ultimate Limit

    States (ULS e.g. extreme conditions) and Accidental

    Limit States (ALS e.g. loss of holding capacity of any

    one anchor leg);

    c.  to minimize the fatigue lives of the differentcomponents (anchor legs / anchor leg connection tothe buoy body / export lines / etc.);

    d.  to keep excursions within the specified envelope;e.  to minimize the vertical pull applied onto the buoy

    (this includes the ballast required to compensate the

    uneven distribution of the anchor leg vertical

    tension);

    f.  to minimize the loads imposed onto the variouscomponents of the anchor legs and consequently to

    minimize their size, while of course fully ensuring the

    compliance with the previous design objectives.

    The anchoring systems of the existing terminals have a

    number of common features. Firstly, the anchor legs are all of

    the semi-taut type. It means that the leg compositions adopt

    the following principles (from the top to the sea bed):

    a.  a short section of chain, which is present only to easeinstallation, to allow length adjustment at time ofhook-up and, if required, subsequent re-tensioning of

    the leg (especially if polyester is used, to

    accommodate its potential long-term creep); b.  a long section of rope, either made of steel

    (KIZOMBA A and B, ERHA, DALIA, GREATER

    PLUTONIO, AGBAMI, AKPO) or of polyester(GIRASSOL, BONGA). This long section, which

    transverses the water depth must be as light as

     possible with a view to minimizing the suspended

    weight of the anchoring system;

    c.  a relatively short section of chain, which connects thelower extremity of the rope to the anchor points. The

     purpose of this chain is to provide a minimum of

    catenary effect to the leg, which is of importance

    especially at low tensions. It also allows controlling

    the load and the up-lift angle at the anchor point.

    It is interesting to note that anchoring systems making use of

     polyester are generally stiffer than the all-steel systems. The

    restoring capacity of a polyester system is essentially provided

     by the axial elasticity of the polyester while a steel system

    combines catenary effects and material elasticity to achieve its

    restoring capacity. Although axially stiffer than the polyester,a steel wire rope exhibits higher catenary effects, which make,

    in the end, the anchor leg characteristics softer.

    The size of the chains is entirely controlled by fatigue

    considerations (including OPB - out-of-plane bending fatigue),

    while the size of the intermediate rope segment is determined

     by ULS and ALS consideration. It must be recalled also that

    chain fatigue is governed by the anchor leg pretension: the

    lower the pretension, the better the fatigue performances.

    Another common feature of these anchoring systems is the use

    of suction anchor piles. Other types of anchors could be

    envisaged, including vertically loaded anchors (VLA’s), or

    vertically loaded plate anchors (VELPA’s).

    On all systems the anchor legs are grouped in three bundles.

    The number of legs in each bundle varies from one project to

    the other. It is essentially driven by the horizontal pull applied

     by the export lines.

    To resist the horizontal pull of the export lines, it is necessary

    to have, opposite to the export lines, a bundle of legs. In order

    to minimize the tensions in these legs, it is necessary to have

    them as horizontal as possible, i.e. as long as possible.

    Increasing their length has also a beneficial effect: it decreases

    their axial stiffness (EA/L). However, at the same time, it

    increases their mass, which, obviously is having a detrimentaleffect on their dynamic response and, consequently, on their

    fatigue performances.

    In the case of ERHA, because of the unusually large

    horizontal pull of the export lines, it has been necessary to

    adopt a bundle of four legs in front of the export lines.

    Limiting to three legs would have imposed a chain diameter in

    excess of 6” to accommodate the fatigue requirements and

    excessively large anchor points, due to relatively poor soil

    conditions.

    To minimize the tensions in the “long legs”, it is required to

    reduce to a minimum the number of the “short legs”, whichare pulling in the same direction as the export lines, this

    minimum being obviously two legs per bundle. For these legs,

    the trade-off is between their horizontal pull, which must bekept to minimum for the reason explained above, and the

    transverse restoring capacity they have to ensure. This is why

    only two legs per bundle are preferred when the export lines

    impose a very large pull. This is why, also, these legs are

    made short, with a large top angle. This pattern is, for

    instance, the one adopted on KIZOMBA A and B and

    GREATER PLUTONIO.

    For systems with a limited pull (e.g. GIRASSOL, BONGA or

    AKPO), a conventional three-by-three pattern is perfectly

    suitable.

    As always, there are exceptions: AGBAMI shows a large

    horizontal pull and a conventional three-by-three pattern. This

    is simply a requirement from the Client.

    Another important consequence of the horizontal pull of theexport lines is the uneven distribution of the vertical load on

    the buoy, creating an overturning moment, which needs to be

    compensated by a ballast weight.

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    4 OTC 18085

    Figure 3 – Typical anchoring pattern

    The other important parameter which drives the design of theanchoring system is the mooring force applied by the export

    tanker under the worst permissible operational conditions.

    The extreme survival conditions (buoy alone in the 100-year

    return period storm) are not governing the design of any of the

    anchor leg components.

    In West African conditions, the largest load on the buoy isgenerated by the squall event when an export tanker is at berth

    on the buoy.

    The transient behavior of the tanker induces a quasi-static

    force on the buoy ranging between 180 and 250 tonnes,

    depending on the intensity of the squall (which is a function of

    the location and the specified return period) and, to a lesser

    extent, on the tanker size.

    These squall events are represented by time series of wind

    velocities and associated directions. The most severe ones

    show a rapid increase of the velocity (from nearly calm

    conditions up to 30 m/s in less than 500 seconds) associatedwith a 180 degree change of the wind direction during the

    same period of time. These squall events are generallyassociated with small waves. The current, at the time of the

    squall, can often be totally negligible. It means that one cannot

    rely on its stabilizing effect to minimize the mooring force.

    Squalls may come from almost any direction and, during the

    squall, the export tanker covers a broad angular sector. It istherefore unsafe to restrict the maximum mooring force within

    a limited sector. To derive design tensions and excursions, it is

    strongly recommended to apply the quasi-static mooring force

    in any direction.

    Some components of the anchor legs are then sized against

    this quasi-static design mooring force.

    a.  wire rope strength is governed by the one-leg brokencase (Class rules requirement),

     b.  wire rope length is optimized in order to give anoverall stiffness to the pattern that matches the

    allowable excursions,

    c.   bottom chain length is also optimized for stiffnessconsiderations and to avoid excessive uplift at the

    anchor point.

    At this point, it is important to note that, for a deep water

    export system, there is not a simple relationship between the

    mooring force experienced by the hawser and the tensions inthe legs. Clearly, only the quasi-static component of the

    hawser mooring force is transmitted to the anchor legs,

    through the turntable, the main bearing and the buoy body.

    The mooring hawsers and the anchor legs have totally

    “independent” dynamic responses. The dynamic amplification

    in the anchor legs is driven by the buoy first order motions.

    The anchor leg dynamic amplification factors (DAF’s)typically range between 1.1 and 1.3. The dynamic

    amplification in the hawsers is also due to the wave frequency

    motion of the buoy and, to a lesser extent, of the export tanker.

    However it is of a totally different nature. The DAF’s in the

    hawsers can be very high, up to a value of 2. This severe

    dynamic amplification is generally not associated with the

    largest quasi-static mooring force: wave dominated conditions

    do not govern the quasi-static design mooring force. On the

    contrary, during a squall, in the absence of significant waves,

    this DAF may fall to very low values, but it is likely to beassociated with a large quasi-static mooring force. Measuring

    the tension in the mooring hawsers may not give a relevant

    indication of the tensions seen by the anchor legs. Let usconsider, for instance, the following two scenarios, for which

    the measure THawser  of the hawser tensions is the same:

    a.  in the first scenario, the waves are insignificant andthe tension in the hawser is purely quasi-static. The

    tension in the most loaded leg TLeg  is also purely

    quasi-static and can be derived easily from THawser  

    from the static load excursion curves of the anchoring

     pattern: Hawser Leg TK T   ×= ;

     b.  in the second scenario, waves are present. The DAFin the hawsers is 2.0, while the DAF in the most

    loaded anchor leg is 1.3. In this case, the following

    applies:

    Hawser LegHawser Leg T

    0.2

    3.1K T

    0.2

    TK 

    3.1

    T××=⇔×= ;

    Hawser Leg TK 65.0T   ××= .

    In the first scenario, the tension is 54% larger than the tension

    in the second, for the same measured hawser tension. To

     properly estimate the level of tension in the anchor legs, it becomes necessary to low-pass filter the mooring force signal

    and to estimate its quasi-static and dynamic components

    separately.

    x

     y

    Hawser pull

    165º

    217.5º

    Swell direction

    0º-30º

    -50º

    30º50º

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    OTC 18085 5

    The surface piercing buoy

    The existing terminals make use of a surface piercing buoy,

    which, in most cases, is shaped like a flat cylinder. The main

    functions of this buoy are:

    a.  to keep the connection points of the mooringhawsers and of the floating hoses at the surface,

     b.  to ensure free weathervaning of the export tankerabout the buoy axis,

    c.  to support the fluid transfer line and to provideattachment to the anchor legs.

    To ensure these functions, the buoy body, which is earth-fixed,

    is topped by a rotating turntable. This turntable is made ofseveral platforms which are connected to the main

    weathervaning bearing. The platforms are either arranged in

    cantilever from the bearing (see Figure 4) or supported by

    wheels, running on a circumferential rail (see Figure 5). The

    continuity of the fluid transfer between the fixed and the

    rotating part is achieved by means of a swivel, generally of the

    axial type.

    Figure 4 – Turntable - cantilever-type

    Figure 5 – Turntable - bogie-type

    There are alternatives to this configuration. Some designers

     propose “reversed” or “turret” buoys, where the buoy body

    constitutes the rotating part, the anchor legs being connectedto a chaintable mounted on a bearing, following an

    arrangement very much similar to an FPSO internal turret.

    In any case, the buoy body is sized so that it has adequate

    floatation stability and buoyancy to support the suspended

    weight of the export lines, of the anchor legs and of all other

     pieces of equipment mounted on board.

    The hull subdivision generally consists of twelve

    compartments (six inner + six outer compartments) delimited

     by six radial and one circumferential bulkheads. Thiscircumferential bulkhead is positioned at least at 1.5 m from

    the outer shell.

    The damaged buoyancy requirements may differ significantly

    from one operator to the other. In general, it is asked to

    consider the flooding of any two adjacent compartments, butthis is not normally applied to two inner compartments.

    It is then required to ensure that the buoy remains afloat. The

    scenarios behind these requirements are the following:

    a.  collision with an export tanker resulting in the breachof the outer shell, in the vicinity of a radial bulkhead

    (two adjacent outer compartments flooded); b.  collision with an export tanker resulting in the breach

    of the outer shell and of the inner circumferential

     bulkhead in between two radial bulkheads (two

    adjacent inner and outer compartments flooded);

    c.  any one compartment adjacent to the sea can beflooded for any reason.

    In some cases, such as when the export lines are steel lines, a

    maximum allowable angle in damaged condition is specified

    (e.g. 15 deg). This requirement is set to “protect” the flexjointand to ensure that the maximum combined static and dynamic

     buoy angle remains within reasonable limits, which guarantees

    that the design of the flexjoint remains feasible.

    The damaged stability case is generally governed by the “one

    inner + one outer flooded compartment” case (two innerwould be worse but is not considered as a likely scenario). See

    Figure 6.

    Figure 6 – Damage stability – 1 inner + 1 outer cpts

    flooded

    The buoy static stability is also adequate to prevent the buoy

    capsizing should any single anchor leg fail. This criterion is

    generally easy to meet.

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    6 OTC 18085

    The buoy critical components, such as the main weathervaning

     bearing, are designed and constructed to resist occasional

    submersion, and therefore the buoy may accept appreciable

    heel inclinations.

    The buoy body is generally fitted with a skirt, which has two

    main functions:

    a.  to support the anchor leg chain-stoppers and to provide a path for the mooring loads between the

     buoy body and the anchor legs;

     b.  to act as a fender in case the export tanker “kisses”the terminal.

    This skirt plays an important role in the hydrodynamicresponse of the buoy.

    The buoy body has a center well, open to the sea, through

    which the fluid transfer piping is routed.

    In several instances, the buoy carries ballast, either solid or

    liquid, to ensure even keel conditions when the buoy isconnected to its export lines and its anchor legs. Adding

     ballast may also have a beneficial impact on the motion

    characteristics.

    Global analysis of deep water export systems

    The above description has not yet addressed the issue of the

    fatigue design of the anchor legs and of the export lines. This

    is in fact, the key one. Fatigue is essentially driven by the buoy

    first order motions. By definition, a buoy tends to follow thewaves, even when it is connected to heavy export lines and

    taut anchor legs.

    Fatigue can be critical for the anchor legs, and especially the

    top chain segments which are subject to combined tension-

    tension and out-of-plane bending (OPB) fatigue. It is howeverrather simple to overcome potential fatigue problems in the

    chains. A larger diameter and a longer connecting arm are the

    traditional answers to this issue.

    Fatigue problems in the export lines are somewhat more

    difficult and more expensive to resolve. These difficulties may

    impose a reduction in the diameter of the export lines, at the

    expense of the pressure losses, or to increase the wall

    thickness of the pipe, which has a direct cost impact. The

    quality of the welded joints needs also to be carefully

    controlled, should the export line designer want to benefit

    from more favorable design fatigue curves.

    Buoy motions in waves must not be considered as a design

     parameter. Buoy motions are the result of the dynamic

    response of the coupled system. However, it is possible to

     play, within a limited range, with the characteristics of the buoy and of the anchoring system to improve these motions.

    Some general guidance can be given:

    a.  a softer pattern (anchoring and export lines) willgenerally offer more favorable motions for fatigue-

    driven components, owing to its effect on surge

    natural period. However, this is to the detriment of

    the offsets;

     b.  a buoy with a larger displacement will tend to movemore favorably. This is also due to a shift in surge

    natural period. However, this could have an impact

    on heave resonance and should therefore be

    thoroughly thought through;

    c.  a way to influence heave resonance is to modify the buoy added mass by increasing the skirt diameter.

    However, this could have an impact on the heave

    resonance and the pitch/heave coupled motions.

    More generally, the final system performance is a trade-off

     between many of its characteristics and optimization should beconducted by checking, at all stages, that improvement of one

     parameter has not been detrimental to another one.

    Because of the criticality of the buoy motions for the fatigue

    design of the export lines, it is crucial to be capable of

     predicting them with a high accuracy and a high confidence.

    The buoy first order motions are clearly and strongly affected

     by the presence of:

    a.  the export lines: mass lumped to the buoy, with itseccentricity, induced damping and, to some extent,

    restoring stiffness;

     b.  the anchor legs: lumped mass, induced damping andrestoring stiffness;

    c.  the tanker through the mooring hawsers, whichessentially affect the natural periods of the system

    (especially in surge).

    (See Ref. [3])

    These coupling effects are well predicted by analysis software

    now commonly available in the Industry.

    Hydrodynamic loads on export lines and on anchor legs and

    their dynamic response are well documented in literature and

    are not seen as an issue. However, the hydrodynamic loads

    directly applied to the buoy are somewhat more delicate to

    assess. There are two main reasons for this:

    a.  the relatively small size of the buoy (compared to thetanker). In other words, a buoy is a hydrodynamic

    object intermediate between a so-called “small body”

    (the hydrodynamic loads can be derived using the

    Morison approach) and a so-called “large body” (the

    hydrodynamic loads can be derived using linear

    diffraction-radiation theory);

     b.  the presence of a skirt and to some extent of a centerwell open to the sea, which make the flow pattern

    rather complex to analyze.

    These two aspects have been extensively addressed during themodel test campaigns carried out for the BONGA, ERHA (see

    Ref. [4], [5] and [6]) and AGBAMI projects. The AKPO

     project will offer a new opportunity to refine the conclusions.

    However, it can already be said that:

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    OTC 18085 7

    a.   potential theory satisfactorily predicts the buoyhydrodynamic coefficients, provided due physical

    considerations are accounted for;

     b.  drag coefficient values are more difficult to calibratethan the diffraction-radiation coefficients. Sensitivity

    of the buoy motions with respect to the drag

    contribution is however relatively marginal. By

     performing a sensitivity analysis where the buoy dragcoefficients are varied to the conservative limits of

    their plausible range, the robustness of the design to

    uncertainty in the drag coefficients can be addressed;

    c.  the pitch drag modeling remains the most difficult parameter to calibrate. Computational fluid dynamic

    (CFD) analyses are a useful tool to get insight intothe flow pattern, especially around the edges of the

     buoy skirt. Model tests keep, of course, their interest,

     but the physical limitations of the basins impose a

    number of constraints on the designer. To keep a

    “reasonable” scale (typically between 1:20 and 1:60)

    it is necessary to truncate drastically both the anchor

    legs and the export lines.

    Figure 7 – Model testing at large scale

    Considerable developments have been made by SBM in recent

    years, in the dynamic analysis of deep water export systems.

    Performing a detailed fatigue analysis remains a heavy task,

    quite demanding in terms of computer resources, but it is now

     possible to rapidly arrive at an optimum combination of export

    lines, anchor leg pattern and surface buoy, for specific project

    conditions.

    The design process we just described is summarized in the

    flow-chart of Figure 8.

    Figure 8 – Design Process Flow Chart

    Main particulars of existing terminals

    Table 1 summarizes the main particulars of the terminals

    designed and supplied by SBM.

    One can see a large range of export line pulls, from 60 tonnes

    up to 280 tonnes. The impact on the anchor leg patterns and

    the levels of anchor leg pre-tension is obvious.

    As noted previously, the quasi-static mooring forces are

    spread in a much narrower range, from 180 tonnes to 255

    tonnes. This difference just reflects the difference between the

    intensity of the 1-year squall offshore Angola (1min. wind at

    10 m = 19.8 m/s) and the 10-year squall offshore Nigeria

    (1min. wind at 10 m = 29.7 m/s).

    Most of the buoys have been given a diameter of 23 m and a

    height of 8 m. These dimensions have been sufficient to

    accommodate the most demanding export line configuration

    met so far. In one case, the height of the 23 m diameter buoy

    had to be increased to 10 m. This is explained not only by the

    large vertical pull applied by the anchor legs and the export

    Flow ratePressure @ FPSO outlet

    Stabilized crude oil properties

    Field layout

    Distance FPSO - Terminal

    Export line number &

    diameterExport line configuration

    Static horizontal and vertical pulls

    Type of anchor leg pattern

    (3 * 3 or 1*3 + 2*2 or …)Orientation of the anchor leg

     pattern

    Anchor leg composition andstatic pre-tensions

    Export tanker sizeOperational conditions

    Quasi-static design mooringforce

    Allowablerelative offsets

    Static vertical pull of anchor

    legs / Eccentricity of the pull

    Required buoyancy and

     ballastHydrostatic stabilityrequirements (intact hull /

    flooded hull)

    Buoy body main dimensions(dia. / hull depth / center well) Number and size of internal

    compartments

    Flexjoints / bend-stiffenersangular limitations

    Buoy weight, inertia and CoG

    Global motion analysis

    (coupled dynamic)

    Fatigue analysis

    ChainsOK?

    YesExport

    lines OK?

    Yes

     No No

    Water de th

    Service life

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    8 OTC 18085

    lines, but also by the presence of a large amount of ballast

    needed to ensure even keel conditions and also to improve the

    motion response characteristics.

    The future deep water export terminals

    Is it possible to stretch the performances of the existing

    terminals to meet the new challenges that the Industry starts toface?

    Larger throughputs at lower pressure losses

    There is an obvious benefit in minimizing the pressure losses

    in the export lines. Even if a flow rate of 7500 m3/hr (one

     parcel of 1 million barrel offloaded in 24 hours) seems to be

    an acceptable level, adopting a 24” or even larger diameter

    would considerably reduce the power required for the

    offloading pumps. This would represent a significant CAPEX

    and OPEX saving. Today, the largest export line diameter is

    22” on ERHA. This diameter had even to be reduced to 20” at

    the buoy end, because of fatigue issues in the steel pipes. Toaccept larger diameters, the only viable options are:

    a.  to adopt a surface piercing buoy with better motioncharacteristics. Floaters of the spar type could be the

    answer. They could however be more expensive to

     build;

     b.  or to terminate the steel export lines on a submerged buoy, deep enough to stay away from the action of

    the waves. It obviously makes the design of the

    terminal more complex (the crude oil needs to be

    transferred to the surface, anyway). It is not easingthe operations and the maintenance of the installation

    (number of mechanical and structural components

     permanently submerged);c.  or to adopt, for the export lines, another type of

    construction, less prone to fatigue such as, for

    instance, bonded (rubber hoses) or unbonded flexible pipes. However, flexible pipes must be designed for

    smaller relative offsets, which make the task more

    difficult in deeper waters.

    Deeper waters

    Obviously, amongst the three main components of the system,

    the anchoring will be the most immediately affected by the

    deeper waters. It will be necessary to design anchoring

    systems:

    a.  sufficiently stiff to keep the relative excursions withinthe present limits (expressing offsets in terms of

     percentage of the water depth does not make a lot of

    sense for the export lines);

     b.  “light” enough to minimize the vertical pull appliedto the surface buoy;

    c.  with a small foot print to avoid interference with theanchoring pattern and the risers of the FPSO, so that

    the usual safe distance of one nautical mile between

    the FPSO and the export terminal does not need to be

    increased.

    Polyester and, more generally, fiber rope anchor legs offer an

    answer.

    Harsher environment

    The Industry has now gained substantial experience in the

    mild environment offshore West Africa. Designing for the

    ultra deep water provinces of the Far East and Brazil willrepresent a serious challenge, especially with regard to

    fatigue.

    The answers, in this case, will be essentially the same as for

    the larger export lines: “motion free” floaters and “fatigue

    free” export lines. The problem will become just morecomplex as fatigue will also become a burning issue for the

    anchoring system.

    To take-up these challenges SBM have developed, in parallel,

    new types of floaters such as the TSALM (Tendon Single

    Anchor Leg Mooring), the DDCALM (Deep Draft CALM),

    and a ballasted slender buoy, the later two being inspired fromthe spar. SBM have also developed a cost effective alternative

    to the steel or unbonded flexible pipes, TRELLINE.

    For the TRELLINE, the reader is referred to the 2006-OTC

    Paper 18065 (see Ref.[10]). The TSALM and the DDCALM

    are briefly described in the next sections.

    TSALM

    The system is configured along the lines of the long provenSingle Anchor Leg Mooring (SALM) concept with the

    following main features (see Figure 9):

    a.  a main buoyancy unit is located below the mainactive wave zone and safely below the keel of the

    deepest export tanker;

     b.  this main buoyancy is held in place by steel verticaltendons, assembled from standard drill string;

    c.  the export lines terminate at the main buoyancy;d.  separate tethers, on the reverse side, counter balance

    the horizontal pull of the export lines, maintaining the

    main buoyancy unit and tendons in a vertical

    orientation;

    e.  mounted atop the main buoyancy is a conventionalshallow water type SALM system;

    f.  the SALM buoy is used exclusively to moor theexport tanker;

    g.  the SAL swivel assembly is mounted on top of themain buoyancy unit and a subsea/floating hose carries

    the oil to the tanker manifold.

    The configuration promises to offer a cost effective mooring

    solution, extendable to deeper waters and with the following

     benefits:a.  it is based upon known and well proven SALM

    concept;

     b.  it utilizes in-house tendon technology;c.  the main buoyancy unit is not directly affected by

    surface wave action;

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    OTC 18085 9

    d.  the export lines terminate at the main buoyancy andare effectively decoupled from the surface mooring

     buoy;

    e.  the surface SALM is relatively small in diameter andtherefore more transparent to waves;

    f.  any tanker collision damage sustained at the surfaceSALM buoy can be remedied independently of the

    main buoyancy unit, tendons and oil export lines.

    Figure 9 – TSALM – Schematic view

    DDCALM

    The requirements which lead to the design of the DDCALM

    were:

    •  a stiff anchoring system to limit the offset at theexport line connection point;

    •  a large buoyancy to support the vertical pull induced by a minimum of two 24” export steel lines arranged

    in a simple “U” catenary shape;

    •  a virtually “quiet” attachment point of the anchor legsand of the export lines.

    The first two requirements can be met with a conventional

    deep water buoy. The third one, obviously, cannot.

    The solution consists of adopting a slender cylindrical floater,

    with the diameter limited to a minimum at the crossing of the

    free surface, making the buoy more transparent to the waves.The taut anchor legs, connected near the keel of the buoy,

    create a high degree of fixity. Contrary to conventional spars,

    it is not necessary to add ballast to stabilize the floater.

    Therefore the displacement and consequently the overall

    height (or length) of the buoy body can be reduced.

    The DDCALM consists of a column with a built-in chain table

    on the bottom for the hook-up of the anchor legs and a rotating

     part, with a built-in reserve of buoyancy, which is mounted on

    top of the column by means of a slewing bearing.

    The chain table incorporates also the receptacles for the export

    lines and is also designed, should it be required, to support the

     pigging loop and the associated remote controlled valves. The

    fluid transfer is then achieved via a large diameter hard pipe

    from the column bottom to the top.

    The rotating part incorporates a lattice structure for the

    mooring, a boarding ladder, outboard pipe supports for the

    floating hose lines to the shuttle tanker, the product swivel and

    associated piping and valving, navaids, safety andmaintenance equipment. Most of these items are situated on

    the turntable deck, six meters above water level.

    The column consists of an outer shell with bottom,

    intermediate and top decks forming a welded plate structure.

    (See Figure 10).

    Figure 10 – DDCALM – General arrangement

    The column is compartmented over its whole length by decks,

    in general every 5.0 meters and, when collision damage can be

    considered, every 2.5 meters. These decks are supported in the

    center by pipe columns.

    The turntable is constructed as a welded plate box structure. It

    is supported on the column by a slewing bearing, which is

    mounted on a circular machined support plate. Mooring points

    to which the mooring lattice structure is attached provide a

    direct load path for the mooring force to the bearing. The

    turntable top deck provides the structural support for the

    ancillary equipment which has to rotate and weathervane with

    FPSOSubmergedBuoy

    2 OOL’s

    2 Tendons

    1 CatenaryAnchor Leg

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    10 OTC 18085

    the moored vessel. The turntable provides the support for the

     pipe support platform and the boarding platform.

    This arrangement keeps the merits of conventional surface

     buoys:

    •  all the mechanical components remain above water,allowing easy access for maintenance, without the

    need for disconnection of the export lines;•  seen from the export tanker, it remains comparable to

    a CALM, and the tanker berthing operation remains

    identical.

    Figure 11 – DDCALM turntable

    The hydrodynamic performances of the DDCALM have been

    extensively model tested at MARIN and an accurate

    calibration of the analytical tools makes numerical simulationsa reliable means to optimize the design for specific project

    conditions.

    These performances clearly represent a major improvement

    compared with the conventional deep water buoys (see Figure

    12). The fatigue life in the export lines can be one order ofmagnitude higher than it would be with a conventional surface

     buoy.

    Figure 12 – First Order Motion Response RAO’s

    Ballasted slender buoy

    This concept uses the same basic principles as the DDCALM:

    reducing to a minimum the wave-piercing diameter of the

     buoy and moving the buoyancy below the water line.

    When the tanker pulls on the slender buoy, it tilts and the

    rotation angle is taken by the flexjoints, which connect the

    export lines to the buoy. To minimize this angle two options

    can be envisaged:

    a.  to locate the attachment point of the mooring hawsers

    at some depth below the surface. This is the optionadopted on the DDCALM;

     b.  to increase the restoring arm of the buoy, bymaximizing the separation between the center of

     buoyancy and the center of gravity. This is the option

    taken on the ballasted slender buoy.

    The ballasted slender buoy is made of a stack of three

    cylinders topped by a large diameter turntable section providing reserve buoyancy during damaged conditions:

    a.  the top cylinder crosses the free surface. As explainedabove, its diameter is kept minimal to ensure

    optimum motion characteristics;

     b.  the middle cylinder has an increased diameter and provides the required buoyancy;

    c.  finally the bottom cylinder, with a reduced diameteris used to accommodate variable water ballast and

    fixed solid ballast. The ballasts provide the vertical

    stability of the buoy.

    It is possible also to play with the elevation of the chaintable

    and of the export line attachment point to optimize the motions

    response and to obtain a “quiet” point at the export line

    attachment (see Figure 13).

    DDCALM Conventional buo

    Heave RAO

    0.00

    0.25

    0.50

    0.75

    1.00

    1.25

    1.50

    0.25 0.50 0.75 1.00 1.25

    Frequency rd/s

       A  m  p   l   i   t  u   d  e  m   /  m

    Surge RAO

    0.00

    0.25

    0.50

    0.75

    1.00

    1.25

    1.50

    0.25 0.50 0.75 1.00 1.25Frequency rd/s

       A  m  p   l   i   t  u   d  e  m   /  m

    Heave RAO

    0.00

    0.25

    0.50

    0.75

    1.00

    1.25

    1.50

    0.25 0.50 0.75 1.00 1.25

    Frequency rd/s

       A  m  p   l   i   t  u   d  e  m   /  m

    Surge RAO

    0.00

    0.50

    1.00

    1.50

    2.00

    2.50

    3.00

    3.50

    0.25 0.50 0.75 1.00 1.25Frequency rd/s

       A  m  p   l   i   t  u   d  e  m   /  m

      Buoy alone Buoy with tanker connnected

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    OTC 18085 11

    Figure 13 – Ballasted slender buoy

    The ballasted slender buoy maintains a static pitch angle less

    than 15 dg for a maximum export tanker pull of 250 tonnes in

    the intact condition and less than 20 dg in the one-line-broken

    case. This enables the use of a ±20 dg single acting flexjoint

    for the export line attachment to the buoy.

    The dynamic first order motions of the ballasted slender buoy

    allow for the use of 24” steel export lines with a 25-year

    service life, in a Brazilian environment.

    Conclusion

    The turn of the millennium witnessed further establishment of

    FPSO-based developments in deeper water and with this, the

    introduction of separate deep water oil export systems. Nine

    deep water oil export systems have been ordered so far with

    four already in service and operating to the satisfaction of their

    respective owners. The experience gained, together withfurther development of the analytical tools needed to evaluate

    the complex relationships between oil export line, anchor leg

     pattern and surface buoy behavior, provide confidence that the

    approach taken was not misplaced and that it can be repeated

    for future projects of a very similar nature.

    For coming projects based in still deeper waters, or in harsher

    environments, we can say that we now have a better

    appreciation of the challenges to be faced. Various mooring

    solutions to meet these challenges are on the drawing board

    and further work will be carried out to ensure that, when the

    time comes, we will be ready to meet the needs of the

    Industry.

    Abbreviations

    ALS Accidental Limit State

    CALM Catenary Anchor Leg Mooring

    CAPEX CApital EXpenditure

    DAF Dynamic Amplification Factor

    DDCALM Deep Draft Catenary Anchor Leg Mooring

    FPSO Floating Production Storage and Offloading

    OPB Out of Plane BendingOPEX OPerational EXpenditure

    SALM Single Anchor Leg Mooring

    TSALM Tendon Single Anchor Leg MooringULS Ultimate Limit State

    VELPA VErtically Loaded Plate Anchor

    VLA Vertically Loaded Anchor

    References

    [1]  Ph. Jean, K. Goessens & D. L’Hostis, OTC 17238,“Failure of Chains by Bending on Deepwater

    Mooring Systems”

    [2]  C. Melis, Ph. Jean, P. Vargas, OMAE 2005, “Out-Of-Plane Bending Testing of Chain Links”

    [3]  J.L. Cozijn, T.H.J. Bunnik, OMAE 2004,”CoupledMooring Analysis for a Deep Water CALM Buoy”

    [4]  C.Bauduin, C. Blanc, E. S. Elholm, G. de Roux, M.J.Santala, DOT 2004, “ERHA Deep Water Export

    System – Coupled Analysis and Model Tests

    Calibration”

    [5]  Z. Huang, M.J. Santala, H. Wang, T.-W. Yung, W.Kan, R Sandstrom, OMAE 2005, “ComponentApproach for Confident Predictions of Deepwater

    CALM Buoy Coupled Motions - Part 1: Philosophy”

    [6]  Z. Huang, M.J. Santala, H. Wang, T.-W. Yung, W.Kan, R Sandstrom, OMAE 2005, “Component

    Approach for Confident Predictions of Deepwater

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    12 OTC 18085

    CALM Buoy Coupled Motions - Part 2: Analytical

    Implementation”

    [7]  L. Lebon, J. Remery, DOT 2001, “A FatigueResistant Oil Off-Loading System using Flexible

    Pipe”

    [8]  G. Chaudhury, DOT 2002, “Cost Effective OilExport Flow Line for Fields in Deep Waters”

    [9]  S. Momtbarbon, S.H. Quintin, G. de Roux, OTC2005, “Experience with New Cost-Effective

    Solutions to Export Oil from Deepwater Floating

    Production Units Using Suspended Pipelines”

    [10] L. Rampi, Ph. Lavagna, OTC 2006, “TRELLINE – ACost Effective Alternative for OIl Offloading Lines

    (OOL)”

    Table 1 – Main particulars of existing deepwater export

    systems

     Notes: the figures given in the table refer to the buoy at rest

     position, with the export lines full of crude oil at its nominaldensity.

    Mean Maximum Minimum

    Water depth (m) 1200 1370 940

    Operational

    conditions

    10-year

    squall

    1-year

    squall

    Export tanker size VLCC

    Service life (years) 25 20

    Export lines -steel

    Steel –

    2*22”

    Lazy W

    Steel –

    2*16”

    Lazy W

    Export lines – 

    flexibles3*18.7” -ID

     Nominal flow rate

    (m3/h)6840 7500 6000

    Export line

    horizontal pull (t)190 280 60

    Export lines vertical pull (t)

    370 500 200

    Q.S. design mooring

    force (t)220 255 180

    Anchor leg vertical pull (t)

    700 850 540

    Pre-tension in most

    loaded leg (t)140 166 87

    Maximum excursion(intact – m)

    93 135 47

    Maximum excursion

    (one-leg-broken - m)108 145 58

    Total vertical pull (t) 1060 1350 750

    Buoy body diameter(m)

    23 19

    Buoy body height

    (m)10 8

    Buoy draft (m) 5.8 7.0 3.9