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.

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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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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


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


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


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

Otc18085 - Deepwater Oil Export System Pas Present Anf Future

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