dry tree fdpso unit for brazillian waters

7/22/2019 Dry Tree FDPSO Unit for Brazillian Waters

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Copyright 2002, Offshore Technology Conference

This paper was prepared for presentation at the 2002 Offshore Technology Conference held in

Houston, Texas U.S.A., 69 May 2002.

This paper was selected for presentation by the 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 or its officers. Electronic reproduction,distribution, or storage of any part of this paper for commercial purposes without the writtenconsent of the Offshore Technology Conference is prohibited. Permission to reproduce in printis restricted to an abstract of not more than 300 words; illustrations may not be copied. The

abstract must contain conspicuous acknowledgment of where and by whom the paper waspresented.

AbstractIn the last decade Brazilian offshore oil fields have become oneof the most interesting areas containing prospects in water depthsranging from 500 - 3000 m. Many of these field developmentsrequire a stand-alone production and storage facility with tankerexport. An FPSO is the natural solution and should ideally be

installed during the drilling phase to obtain early recovery. Tofurther improve the FPSO concept a dry production tree deck hasbeen included in the moon pool of a spread moored FPDSO.This concept presents to the industry a cost effective solution

where drilling, production and work over activities can becarried out from one single floater. The paper describes this new

concept with its unique feature that the rigid production risers aresuspended from, and tensioned by, a Tension Leg Deck (TLD)located above the water level in the middle of the FPDSO moonpool. The BOP stack and the surface completed production treesare located on the TLD and therefore easy accessible. Thetension or uplift force of the deck is provided by weights. The

weights are located in dedicated hull compartments where theirvertical motions are guided by hinged lever arms. These lever

arms are located transverse in the longitudinal direction of theFPDSO vessel. The FPDSO-TLD facility has the flexibility tosupport all required drilling and production equipment for a wide

range of field development scenarios. The FPDSO-TLD conceptpresented in this paper was studied for a Brazilian deepwaterfield development scenario where an FPSO and a nearby Dry

Completion Unit is the base case development scenario. TheFPDSO-TLD concept has been model tested.

IntroductionThe current drive to produce oil in deeper and deeper watershas pushed oil companies to review their field development

strategies. Deepwater fields generally differ from their shallowwater counterparts in some or all the following aspects:

Absence of or limitations in existing infrastructures,

Reservoir size,

Reservoir horizontal extent and thickness of oil pockets,

Oil properties and well maintenance,

Faster return on large capital expenditure.

In the new and promising deep water regions (West ofAfrica and Brazil) infrastructures are scarcely available on theseabed to export the oil and gas production. This has paved theway for floating production systems which have flourished inrecent years in various forms (FPSOs, TLPs etc..).

The size of the proven reserves in these deepwater fieldscalls for large storage facilities that can only be provided by

ship-shaped floating production systems (new-built barge orconverted tanker).

The new deepwater fields often differ from shallow waterfields by their geology. While the latter consisted ofhorizontally localized oil pockets with a substantially thick oillayer, the deepwater fields come in shallow and thin oil

pockets scattered over a large area. This new situation requireswellheads to be placed at large distances from each other.

Optimum production rates and oil recovery often requiresa comprehensive work-over and well maintenance program.Given the high day rates of drilling/work-over rigs and thelimited fleet able to operate in deep water, availability of on-

board work-over if not full drilling facilities brings about adefinite economic advantage.

Large capital expenditures are involved in these deepwaterfield developments. Ways of reducing the lead-time to first oilmust be investigated. An earlier return on investment can beachieved by starting production from a few pre-drilled wells

soon after FPSO hook-up and long before the drilling phase is

over. The remaining wells are drilled directly from theFP(D)SO if full drilling capability is incorporated.

In summary, the typical deepwater field in West of Africaor Brazil would call for a hub, preferably a large FPDSO, andsatellite units located above secondary drilling centers within a

few miles of the hub. Well maintenance issues would call forcompletion/work-over facilities to be included.

Dry well heads as on SPARs and TLPs facilitate work-over and well maintenance activities. Direct and continuousaccess to the wellhead is possible without ROV assistance.Furthermore, these concepts allow the use of casing strings for

OTC 14256

A Dry Tree FPDSO Unit for Brazilian WatersL. Poldervaart, J. Pollack, Single Buoy Moorings, Inc.


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2 L. POLDERVAART, J. POLLACK OTC 14256

risers with potential cost savings as compared to flexible risersolutions. The above reasons account for the popularity ofdry trees.

The subject of this paper is to present a dry tree FPDSOunit that could be used in deepwater field developments

offshore Brazil. The dry tree design is based on the TensionLeg Deck (TLD) concept developed at SBM. This concept,

unlike the spar and TLP systems, is rather insensitive topayload and does not rely on buoyancy to pretension the

risers. Instead, the riser deck is pre-tensioned by weightsconnected to the deck by means of cables going over sheaves.

These weights are either located in a dedicated hullcompartment or underwater well below the sea surface toavoid wave action. This tensioning system allows for dynamicvessel motions and draft variations associated with storage.

The concept was first designed as a Dry Completion Unit (DCU) for W. A. waters. Therefore the paper will follow the

path from the DCU to the FPDSO-TLD for Brazilian waters.Modeltest results will be presented from modeltest performedat MARIN the Netherlands, during the months of February

and March on this concept.The design basis of the DCU is discussed in Section 2.

Section 3 covers model test experiments performed atMARINTEK and the verification of the numerical tools usedfor detailed analysis and engineering. Applicability of theconcept to a Brazil environment is discussed in Section 4.

DCU design premisesIn this section, the design premise for the DCU is outlined. Asthe weight-based tensioning system is new, it is worthwhile todiscuss first the physical principle of the TLD

TLD physics. The TLD concept illustrated in Figure 1 is anovel means of tensioning hard risers. This concept has thefollowing main components (see Figure 1):

a tendon stabilized deck to which production risers areattached. This deck with trees is located above the meansea level;

cables running over a vessel fixed sheave system hold thedeck in place;

weights hanging from the cables well below the mean sea

level keep the required pretension in the risersand tendons.

This concept can be adapted to a large variety of surfacefloaters provided the motion characteristics are suitable.

Figure 1 shows the TLD concept in the moon-pool of abarge or tanker. The barge shown in Figure 1 is merely

supporting the sheaves. When the barge heaves up by onemeter, the deck remains in place while the suspended weights

move up by two meters. Likewise, when the barge heavesdown by one meter, the suspended weights move down by twometers with the deck remaining in its position. The TLDconcept effectively de-couples the wave frequency motions of

the floater from those of the deck thus allowing the barge withits TLD to ride a storm with all risers/tethers connected.

In Figure 1, the weights used to pretension the risers arelocated well below the mean sea level. In Figure 2 analternative design is illustrated whereby the weights are

located inside the hull and are guided in their vertical motionby hinged lever arms.

More details about the TLD can be found in references [1]and [2].

Design Basis for DCU. The basis of design for the satelliteDCU is as follows:

West of Africa (Angola) environment.

Small overall horizontal dimensions for flexibility inchoice of yard,

Small draught for easy quay side outfitting,

Limited storage capacity,

12 risers (9 5/8 in OD casing strings and wall thickness0.395 in),

4 tendons to support the Tension Leg Deck (again 9 5/8in OD),

4m spacing between the trees on the deck, Large enough moon-pool to insure deck/hull clearance,

Environment. The environment is typical of Block 17in Angola.

Two wave spectra have been assumed correspondingrespectively to the long south Atlantic swells and toshorter seas:

JONSWAP spectrum =2,Hs=4.2m, Tp=15.4s,


A third spectrum has been assumed to explore larger

environments:


The water depth was originally set to 1300m. This water

depth could not however be modelled at a reasonable scale ina 10m deep wave basin. A smaller depth of 680m was choseninstead to also allow current generation in the model basin.

Preliminary design of tensioning system. Deck size andweight: The TLD deck dimensions are dictated by the numberof risers and by the 4m riser spacing. Assuming a circular

pattern, the radius is about 7.6m. Space must be allotted forthe tendon attachments at the four corners of the deck. Asquare deck of 18m by 18m is adequate. A deck weight of 100tons has been estimated.

Riser/tendon weight: The weight in water of 16 (12 risersand 4 tendons) 9 5/8 in risers with wall thickness 0.395 in is

approximately: 5440 kN i.e. about 555 tons.Pretension weights: When the barge heaves the suspended

weights also heave by twice as much. These accelerationsinduce fluctuations in the tension member connecting theweights to the deck. A good measure of these tensionfluctuations is the so-called Dynamic Amplification Factor

(DAF) defined as the ratio of the tension standard deviation tothe mean tension. An approximate formula for this DAF is

simply gaDAF Z /3/1= where 3/1Za refers to the

significant single amplitude total heave acceleration and g to

the gravitational acceleration. With this approximate formula,


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OTC 14256 A DRY TREE FPDSO UNIT FOR BRAZILIAN WATERS 3

DAFs are shown to be independent of the weights and dependonly upon the first order acceleration RAOs of the floater andthe wave environment

DCU dimensions. The DCU is a square barge with asquare moon-pool in the middle. Its main dimensions are

summarized in Table 1.The moon-pool walls are vertical. The moon-pool

dimensions are dictated by riser clearance at keel level (bargehorizontal offset and roll/pitch motion). A clearance of 3.5m is

provided all around the deck in calm water.TLD system layout. The TLD system is illustrated in

Figure 3. Eight weights each with a 300-ton mass are used topretension tendons and risers. Each weight is connected to thedeck via a cable running over two sheaves. The position offirst/outer sheave is such that weights do not cause

interference with the barge hull or risers. The position of theinner sheave allows the connection to the deck located inside

the moon-pool.For simplicity the 12 risers and 4 tendons were lumped

into 4 equivalent risers (in mass and stiffness).

Mooring layout. The DCU is held on station by an eight-line mooring system (4 bundles of 2 lines). The line

composition consists of a 150m-long bottom chain segment, a900m long wire segment and a 50m top chain segment.

Model tests and numerical calibrationModel test.

General description. The model test campaign wasperformed in the ocean basin at MARINTEK in July 1998.The scale used is 1:90 [3]. The scaled model is shownin Figure 3.A comprehensive series of tests was performed including loadexcursion verifications, decay tests in surge, heave and pitch

in calm water and finally irregular wave tests. These testswere performed each time with and without the TLD with aview to assessing its influence on the DCU motion response.

Table 2 summarizes the irregular wave tests performed.The first three tests were performed with the design waveheight. Test 5230 was performed to extrapolate the behaviourto more severe environments. As the first order motions were

expected to be larger in their test, the elevation of the sheaveaxis above main deck was increased by 2.5m to prevent any

contact between the deck and the sheave. Test 5250 wasperformed to investigate the influence of the pretension weighton DAF.

Result highlights.

Natural periods. Decay tests with and without TLD systemgave the natural periods in surge, heave and pitch as shown in

table 3. Note that the surge natural period is decreased by 25%when the TLD system is connected. When the barge movesthe tendons and risers incline from the vertical thus providingan additional restoring force proportional to the pretension

weights. The decrease in period is therefore primarily due to astiffness increase.

Changes in heave and pitch natural periods are small/marginalowing to the mass and added mass ratios involved.

First order motions. Standard deviations of surge, heaveand pitch motions are shown in Table 4 for all irregularwave tests.

The following comments are in order:

With the TLD system mounted and a 4.2m significantwave height, the most probable maximum single

amplitude heave and pitch motions assuming a Rayleigh

distribution are respectively 3.9m and 7.1; Increasing the wave height from 4.2m to 5.5m causes a

proportionally smaller increase in the heave and pitchresponses. This suggests that viscous effects play an

important role;

Changing the pretension weights from 300 tons (5240) to200 tons (5250) has only a minor effect on the standard

deviations of the surge, heave and pitch motions. Thisconfirms an earlier conclusion that the motion of thebarge is decoupled to a large extent from the suspendedweights.

Tensions in TLD system. Tensions in the TLD cable

systems have been measured at 6 locations referred to as T1,T2, TLD1, TLD2, TLD3 and TLD4 as indicated in Figure 4.Tension T1 and T2 are top tensions for the bow and starboardrisers. TLD1 to TLD4 are tensions measured along the cablesconnecting suspended weights to deck. Tensions TLD2 and

TLD4 are measured just above the suspended weights.Sheaves were modelled with roller bearing thus implying 1%to 2% friction.

A DAF is calculated as the ratio of the standard deviationof TLD2 and TLD4 tensions to the weight in water used forpretension. These DAF values are listed in Table 5.

The following remarks can be made:

DAFs are higher for TLD4 which is located up-wave

(see Figure 4), The largest DAF for Hs=4.2m (all tests except 5230)is 8.2%,

Increasing the wave height from 4.2m to 5.5m inducesroughly a 25% increase in DAF,

The DAF values for Hs=4.2m and test # 5240 and 5250 areunaffected by the decrease of the pretension weights from 300tons to 200 tons.

Most Probable Minimum top riser tensions (T1 and T2) arelisted in Table 6.The following can be pointed out:

These MPM values must be greater than the riser weightin water. In the model tests the risers were flooded and

their weight in water was 1360 kN. The above tableshows that the risers remain in tension throughout thewater column.

It is expected that the minimum tension be larger in thepresence of current (test # 5210) and lower with the largerwave height (Test # 5230) or the smaller weights

(Test 5250).

Numerical calibration against model tests.

General.A numerical model of the DCU is built with theAQWA suite of programs. This numerical model is tuned to


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match the experimental results. A similar exercise wasperformed with a TLD system mounted on a 300,000 dwttanker [4] also tested at MARINTEK in July 1998.

A view of the AQWA model is shown in Figure 5.Result highlights. Figure 6 shows a comparison of the

measured and calculated load-excursion curves. A goodagreement is found.

The details of the calibration of decay tests and irregularwave tests will be reported elsewhere

Conclusion. Model tests have been performed that confirmed

the validity of the TLD concept for West of Africa. DynamicAmplification Factors of about 8.5% have been measured.Minimum tensions at mud-line are such that the risers are welltensioned throughout the water depth.

Successful calibration of numerical tools againstexperimental results has already been reported for a TLD

system on a 300,000 dwt tanker [4].Our numerical tools are well suited for the detailed

analysis and engineering of the TLD systems.

New FPDSOTLD layout/design forBrazil EnvironmentThe TLD concept has been experimentally validated for a 65mx 65m square DCU in West African environment. When thewave height was increased to Hs=5.5m, the measuredDynamic Amplification Factors were about 10%. For a typicalBrazil environment, this figure would be even higher thus

making the small DCU floater unsuitable for this oil region.Large spread moored FPDSO-TLD designs for West of

Africa have been designed for relatively small beam wavesprovided the floater bow is oriented towards SSW [1]. Theassociated roll motion is small enough to insure riser/hull

clearance. Should this floater be spread moored bow to theSouth or to SSW in Brazil, the roll response in beam waveswould surely be prohibitive from both the riser/hull clearanceand tension fluctuations points of view.

A new hull shape must therefore be developed to be spreadmoored offshore Brazil. This section will highlight thepreliminary stage of the design process of this new hull shape

and of the TLD system mounted on it.

Design requirement.

Brazilian environment,

Storage capacity: 1.5MBBLs,

Wells: 12 productions/water injection wells (outer casing13 3/8 x 68 lb/ft, tubing string 7 5/8 x 29.7 lb/ft, powercontrol coil 2 3/8 x 5 lb/ft),

Topside capacity: 100,000 Bopd.

Design environment. The water depth is assumed tobe 1200m.

The following 100-year Brazilian design waves are listedin table 7 for the sectors from East to South-West. Tidal

variations are as follows:

Tidal Astron. Max. +Tidal Atm. Max = 1.5m,

Tidal Astron. Min. +Tidal Atm. Min = -0.4m,The total tidal range is thus 1.9m.

Hull design. The stated requirements include a 1.5 MBBLstorage capacity and a 100,000 bopd process capacity. Thedesign philosophy adapted for this hull design is as follows:

Rectangular barge shape,

Presence of horizontal skirts to increase the roll and heave

natural periods,

Limitation of the overall beam (including skirts) to around80m for flexibility in the choice of the yard.

Several hull shapes have been considered as shown in Table8. The models with a 16m draft result from a detailedinvestigation regarding the ballast and storage capacity requiredachieving a near zero-draft variation during operation. Theseexercises lead to the 16m draft, instead of the earlier assumed22m draft. Figure 7 illustrates the Tex 11 barge mesh.

The radii of gyration are respectively 0.35*Bpp and0.25*Lpp for roll and pitch. Table 9 summarizes the naturalperiods and radiation damping (expressed as percentages ofcritical damping) for the heave, roll and pitch dominatedcoupled modes. The heave natural period is higher with widerskirts. Decreasing the height of the skirts from 5m to 2.5m

results in a small reduction of the heave and roll periods.Having skirts at the bow and stern increases the heave, rolland pitch natural periods by 4, 7 and 15% respectively. Again,the wider the skirts are, the larger the roll natural period is.Radiation damping is negligible in roll.

First order response. First order heave motion RAOs forbeam waves are shown for all configurations in Figure 8. Oneobserves that the implementation of the moon-pool appears to

be beneficial in terms of maximum response at the peak. Notethat the above motion responses assume no other sources ofdamping other than radiation. Viscous damping is to beexpected due to the sharp edges of the skirt

The barge is spread moored with the bow pointing to SSW.The environments of Table 10 have been assumed. The wavesare respectively from the port beam, bow quartering andhead directions.

Table 11 summarizes the largest significant singleamplitude heave motion and acceleration response at COG

and the environment responsible for these values. Theseresults have been obtained by a frequency domain approachwith radiation damping only for heave.

Table 12 summarized the largest roll and pitch angles andthe environment these are associated with. These results areobtained with a frequency domain approach (LF ignored). A

2% of critical has been added in roll to account for viscouseffects at the keel. No additional damping has been taken inpitch. We observe that for the 16m draft hulls (Tex5, Tex7,Tex8 and Tex11), the wave frequency roll motion is quitesmall while the wave frequency pitch motion is larger. For theabove hulls, the roll natural periods are much longer than the

wave periods associated with the spectra of Table 10 whilst, incontrast, the pitch natural periods are close to the wave


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spectrum peak periods. The largest roll motions occur for thebeam waves coming from the ESE and the largest pitchmotions for the head waves from the SSW.

It is shown that a low frequency (LF) roll motion alsoexists and must be considered in the design.

Heave responses are important at sheave locations. Thesewill govern the riser dynamic tensions. Responses have

therefore also been computed for Tex11 at each corner of themoon-pool: bow port (x=20m, y=12m,z=42m), bow starboard

(x=20m, y=-12m,z=42m), stern port (x=-20m, y=12m,z=42m)and stern starboard (x=-20m, y=-12m,z=42m).

Table 13 summarizes the heave motion and accelerationresponses at these locations for hull Tex11. This hull has beenchosen for the design, as it satisfies the construction criteria ofhaving a beam less than 80m.

The SSE environment is therefore the designing one forheave response. It is found that the heave response at the

sheave locations can be larger (up to 10%) than at COG owingto the contributions of pitch and roll. This increase is relativelysmall due to the short distances to the principal axes and

would be higher if the moon-pool was located away fromamidships. The maximum heave acceleration is found to

be 0.40 m/sec. The associated DAF is therefore0.40m/sec/g = 4.1%.

For the Tex11 barge, a maximum double amplitude heavemotion of 8.5m is found at the stern corner of the moon-poolon the starboard side for the bow-quartering SSE swells.

Low frequency roll motion.LF roll motions exist due to thehulls large roll natural periods (roll period for Tex11 is 21.5si.e. above the wave period range) and to the non-trivial secondorder roll drift moment. This has recently been confirmedduring a model test campaign performed in October 2000 in

MARIN new offshore basin [4],[5]. Second order rollmoments, even if much smaller than first order moments, cangenerate a large roll response at the natural period dependingon the amount of damping.

Assuming an approach similar to that used to evaluate thestandard deviation of the low frequency surge motion of amoored structure based on surge force spectral density,

mooring stiffness and damping, we can estimate the lowfrequency roll motion standard deviation once the second

order drift moment in roll is computed. The roll stiffness isprimarily hydrostatic and the roll damping is conservativelyassumed to be 2%. With these assumptions, the significantsingle amplitude of the LF roll motion is found to be 1.1 for

the beam waves with Hs=6.7m and 2.8 for the bow quarteringwaves with Hs=7.0m. Assuming a Rayleigh distribution, the

3-hour maximum LF roll motion is then 5.2.As the phasing between the LF and wave frequency (WF)

is not clear, we adopt the method often used for the LF andWF surge motion i.e. we combine the LF significant with the

WF maximum and vice versa. The most severe combinationyields a 6.3 maximum single amplitude low+wave frequency

roll motion. This value will be used in the TLD weight andclearance calculations.

Having most of the roll response at low frequencies meansthat the roll acceleration will come primarily from the smallwave frequency component. This is beneficial in terms of total

heave acceleration, as only wave frequency motions willcontribute. Figure 9 shows the roll motion and acceleration

response for the Tex11 hull at 90.Estimation of riser weight. The requirements call for 12

production/water injection risers. The risers are assumed tohave the following characteristics:

13 3/8 OD outer shell, 68 lb/ft dry weight,

7 5/8 OD tubing string, 29.7 lb/ft dry weight,

2 3/8 OD power control, 5 lb/ft dry weight,

The annulus space is assumed filled with seawater and thetubing string with 8lb/gal oil.The tendons are 10 3/4 OD with 15.1mm wall thickness

and are filled with air.The weights in water and axial stiffness are listed below:

Tendon: 44 tons, 2.1 MN/m,

Production/water injection riser: 161 tons, 2.2 MN/m,

The axial stiffness above is that of the outer shell as thetubing strings and power control are assumed fitted with slipjoints at mud-line.

TLD system.One rectangular 32m-long and 12m-wide deckwill support the 12 risers. This deck will be sheltered from

wave action in the 40m long and 24 m wide moon-pool. Thetotal suspended weight (including a 400 ton deck) is 2500tons. With a conservative 5% DAF, eight suspended weightseach of 385 tons are required.The weights keeping the risers under tension are located indedicated hull compartment and are each guided by a hinged

arm (see Figures 10 and 11). The length of the arm is

dictated by: Range of barge vertical motions,

Maximum sheave fleet angle allowed.The range of vertical motions is broken down according toTable 14.

The wave frequency total heave corresponds to thecontribution of all three vertical motions at sheave locationsand comes form Table 13 (2.3 * 1.86 * 2 = 8.5m). The lowfrequency roll motion must also be accounted for as it willinduce a heave motion at the sheave location. The maximumsingle amplitude LF roll motion has been estimated to be5.2deg. A distance of the sheave axis to barge longitudinal

axis to of 11m is assumed. This results in the LF motion range

of 2m = 2 * 11m * tan(5.2).The total range of heave motion is 17.3m. It is important

to point out that this range results from simple superpositionof various contributions without accounting for the probabilityof occurrence of joint events. This approach is therefore

quite conservative.A typical arm length able to cope with the above range of

vertical motion and admissible fleet angle from the sheavesis 15 m.


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Riser/hull clearance. When the barge rolls, the moon-poolwalls near the keel come closer to the risers and tendons. Areliable estimate of the maximum single amplitude roll motion

(low and wave frequency) is therefore important for clearanceanalysis. The value estimated is 6.3.

Owing to the barge horizontal excursion (limited to 7% ofthe water depth), the risers will have a static inclination from

the vertical of atan (0.07) = 4.0. In total, the relative rollangle is 6.3+4 = 10.3. Note that this value is conservative

since the largest roll motions occur for bow-quarteringwaves that are unlikely to cause a 7% excursion in the

beam direction.Based on the above assumptions, the lateral displacement

of a riser/tendon from its initial position in calm water ish*tan(10.3) where h is the height of the sheave axis above

keel. Assuming a height h = 42m, the lateral displacement isfound to be 7.6m. In calm water, the distance of a production

riser from the moon-pool wall is w/2-e = 24/2 -3.2 = 8.8mwhere w is the moon-pool width and e the riser distance to thedeck longitudinal axis. In this configuration, a clearance of

1.2m is provided with the vertical moon-pool wall. Additionalclearance can be provided if the moon-pool walls are slightly

flared near the keel.A similar calculation made for a tendon shows it could be

4.4m from the longitudinal axis before interference occurs.Should the tendon be further out, the moon-pool would beflared out.

Conclusions. A large number of hull shapes with various sizesof horizontal skirts near the keel, with and without moon-pools have been investigated for a motion and accelerationresponses using a frequency domain analysis. A 320m long,64m beam, rectangular barge fitted all around with 7.5m-wide

skirts has been shown capable of supporting dry trees in aBrazilian environment.

A 40m-long, 24m-wide moon-pool located amidships isprovided for the Tension Leg Deck supporting 12 steel risersin a 1,200m water depth. Preliminary design calculations callfor eight 385-ton weights to keep the risers under tension at alltime. These weights are located in dedicated hull

compartments and guided in their vertical travel by 15m longarms pivotally fixed to reinforced hull compartment walls.

Clearances between risers/tendons and the moon-pool wallsnear the keel are adequate.

General conclusionsA small, spread moored, Dry Completion Unit (DCU)supporting dry trees on a TLD has been validated

experimentally as being well suited for West Africanfield developments where multiple drilling centers couldbe required.In a proposed field layout, these DCUs are connected to an

FPSO hub. This floater could also feature dry trees and a TLDsystem to support steel production/water injection risers [1].

As the small DCU is not suitable for a Brazilian environment,a new hull shape has been designed which is able to cope with

the waves a spread-moored floater would be subjected to. Apreliminary design of the TLD system shows that there are nofeasibility hurdles that cannot be overcome for a spread-

moored FPDSO-TLD offshore Brazil.In this paper, various floater shapes have been investigated for

supporting risers using the TLD system. It has been shownhow the performance of the TLD system (riser tension

fluctuations) is closely linked to the motion and accelerationcharacteristics of the floater. This illustrates that an integrated

hull/TLD design can properly support hard riser systems in themore severe environments offshore Brazil.

References1. An FPSO with dry well heads located on a novel Tension Leg

Deck as Stand Alone Deepwater Field Development by L.

Poldervaart, J. Pollack.Proceedings of Deep Offshore TechnologyConference, 19-21 October 1999. Stavanger, Norway.

2. A Surface Tree Riser Tensioning Systems for FPSOs by J.

Pollack, L. Poldervaart and M. Naciri, Proceedings of 2000Offshore Technology Conference. Paper OTC 11902.

3. Tension Leg Deck (TLD) SBM test campaign. Main report,MARINTEK, November 1998.

4. The Tension Leg Deck, From Drawing Board to NumericalDesign Tools by J. Pollack, M. Naciri and L. Poldervaart.

Proceeding of the 2000 Offshore Mechanics and ArticEngineering Conference. Paper OMAE-00-4001 - February 14-

17. 2000, New Orleans - Louisiana.5. Model tests on a rectangular barge with different skirt

configurations MARIN report. December 2000.6. Non-linear low-frequency roll excitation of a rectangular

barge by M. Naciri and N. Lledo. Proceeding of the 2001Offshore Mechanics and Artic Engineering Conference. PaperOMAE-01-1247 - June 4th8th, 2001. Rio de Janeiro Brazil


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Length [m] 65

Beam [m] 65

Depth [m] 19

Length moon-pool [m] 25

Beam moon-pool [m] 25

Draught [m] 7

Displacement [m3] 25200

Table 1 DCU main particulars

Test # Hs(m) Tp(s) Vc(m/s) Mooring TLD weight

5011 4.2 15.4 - Horizontal N/A

5110 4.2 15.4 - Catenary N/A

5210 4.2 15.4 1.1 Catenary 8 x 300 tons

5230 (*) 5.5 15.4 - Catenary 8 x 300 tons

5240 (*) 4.2 15.4 - Catenary 8 x 300 tons

5250 (*) 4.2 15.4 - Catenary 8 x 200 tons(*)These tests are performed with sheaves lifted up by 2.5m.

Table 2 Irregular wave tests performed on DCU

Degree

of freedom

Nat. period w/o TLD

[s]

Nat. period with TLD [s]

Surge 240 180

Heave 10.2 10.5

Pitch 9.0 9.0

Table 3 Natural periods with and without TLD ( 8 x 300 tons)

Table 4 Standard deviation of barge motions

Test # TLD2 TLD4

5210 6.9 7.1

5230 9.1 10.15240 7.1 8.2

5250 7.1 8.1

Table 5 DAF values (%) Table 6 Most Probable Minimumtop riser tension

Sector Hmax[m] THmax[s] Hs[m] Tp[s] Tz[s]

E 8.7 11.7 4.7 9.2 6.9 2.19

SE 12.4 12.1 6.7 11.4 8.5 1.58

S 13.0 12.1 7.0 14.7 11.1 1.62

SW 14.6 12.2 7.8 15.4 11.5 1.70

Table 7 100 year wave conditions in Campos Basin

Table 8 Hull shapes considered for diffraction/radiation

Test # surge [m] heave [m] pitch []

5011 3.45 1.02 1.85

5110 2.40 0.96 1.70

5210 2.66 0.90 1.50

5230 (*) 3.04 1.19 2.21

5240 (*) 2.12 1.00 1.89

5250 (*) 2.05 0.98 1.83

Test # T1 (kN) T2 (kN)

5210 3550 3689

5230 2848 32985240 3317 3480

5250 2120 2270

Hull Id Lpp[m] Bpp[m] D [m] Draft [m] Skirts w & h

[m]

Skirt

Location

Moonpool

Dim. [m]Tex1 320 60 32 22 12x5 All around N/A

Tex2 320 60 32 22 12x5 On the side N/A

Tex3 320 70 32 22 5x5 On the side N/A

Tex4 320 70 32 22 5x2.5 On the side N/A

Tex5 320 64 32 16 10x3 All around N/A

Tex7 320 64 32 16 10x3 All around 80x24

Tex8 320 64 32 16 10x3 All around 40x24

Tex11 320 64 32 16 7.5x5 All around 40x24


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Hull Id Skirts width

& height [m]

Skirt location Moonpool

Dim. [m]

Heave Roll Pitch

Tex1 12x5 All around N/A 17.2s/5.7% 35.1s/0% 17.4s/2.3%

Tex2 12x5 On the side N/A 16.6s/5.5% 32.8s/0% 15.2s/3.1%

Tex3 5x5 On the side N/A 14.9s/9.8% 19.9s/0.1% 13.5s/6.8%

Tex4 5x2.5 On the side N/A 14.7s/10.8% 19.4s/0.2% 13.5s/7.6%

Tex5 10x3 All around N/A 15.3s/8.2% 23.4s/0% 15.2s/4.8%

Tex7 10x3 All around 80x24 15.2s/8.3% 22.3s/0% 15.0s/4.7%

Tex8 10x3 All around 40x24 15.3s/8.3% 23.1s/0% 15.3s/4.8%Tex11 7.5x5 All around 40x24 14.8s/8.5% 21.5s/0% 14.6s/5.1%

Table 9 Periods of natural modes and radiation damping

Table 10 100 year wave conditions for response calculations

Hull Id Skirts width &height [m]

SkirtLocation

Motion (m) Env. Acceleration (m/s2) Env

Tex1 12x5 All around 2.1 SSE 0.28 SSE

Tex2 12x5 On the side 2.0 SSE 0.27 SSE

Tex3 5x5 On the side 1.9 ESE 0.39 ESETex4 5x2.5 On the side 1.9 ESE 0.41 ESE




Tex11 7.5x5 All around 2.1 SSE 0.37 ESE

Table 11 Wave frequency significant single-amplitude COG heave motion and acceleration

Table 12 Significant single-amplitude wave frequency roll and pitch motions

Table 13 Significant single-amplitude total heave motion and

acceleration at various locations for hull Tex11

(1) To allow for some draft variations.(2) Assuming a 7% excursion in the damage case.

Table 14- Range of heave motions

Sector Hs[m] Tp[s] Tz[s] Dir of propagation []

ESE 6.7 11.4 8.5 1.6 -90 (beam)

SSE 7.0 14.7 11.1 1.6 -135 (quartering)

SSW 7.8 15.4 11.5 1.7 -180 (head)

Hull Id Skirts dim.

[m]

Skirt

Location

Roll

[]

Env. Pitch [] Env.

Tex1 12x5 All around 1.3 ESE 2.6 SSW

Tex2 12x5 On the side 1.2 ESE 2.2 SSE

Tex3 5x5 On the side 1.1 ESE 1.8 SSE

Tex4 5x2.5 On the side 1.2 SSE 1.8 SSE




Tex11 7.5x5 All around 1.2 ESE 2.3 SSW

Location Motion (m) Env. Acceleration (m/s2) Env

CoG 2.1 SSE 0.37 SSE

Bow/P 2.2 SSE 0.37 SSE

Bow/SB 2.2 SSE 0.40 SSE

Stern/P 2.3 SSE 0.38 SSE

Stern/SB 2.3 SSE 0.40 SSE

Origin Associated range

Draft variations (1)[m] 2.0

Set down (2)[m] 2.9

Tide variations [m] 1.9

Wave frequency total heave at sheave location [m] 8.5

LF roll induced heave at sheave location [m] 2.0

Total [m] 17.3


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Figure 1 Schematic of TLD system with weights in water

Figure 2 Schematic of TLD system with weights in a dedicated hull compartment.


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Figure 3 DCU in MARINTEK ocean basin

Figure 4 Layout of cable and sheave system and location of load cells

65m25m

18m

TLD

T


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Figure 8 Heave motion RAOs in beam waves

Figure 9 Roll motion and acceleration RAOs at 90 for hull Tex11

Figure 10 Isometric view of tensioning system Figure 11 Isometric view of tensioning system andsupport superstructure

dry tree fdpso unit for brazillian waters

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