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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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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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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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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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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
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