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With the Nansen Spar leading the way, large trussSpars are becoming an increasingly common sightin the Gulf of Mexico. This new technology oftenproves a more cost-effective version of the classicSpar. The truss Spar design replaces the lowercylindrical hull of the original Spar with an openstructure to improve stability, reduce hull weightand to increase performance in high current situations.The main purpose of the model tests carried outfor the two firms was to verify their computations,increase their knowledge database and captureunexpected phenomena.

World’s largest truss Spar

Last year, CSO -Aker Maritime, a Technip-CoflexipGroup affiliate, ordered on behalf of BP and theirHolstein and Mad Dog partners the most extensivemodel test programme ever for a truss Spar. Thesetests concern the world’s largest truss Spar,scheduled to come into action in the BP-operatedHolstein field in mid-2004, in a 1,324 m waterdepth. The test programme involved an extensiveVIV test programme, horizontal tow tests, upendingtests and vertical tow tests, using four differentmodel test basins.

Radboud van Dijk & Arjan VoogtA.J.Voogt@marin.nl

Two leading truss Spar players choose MARIN model testing

MARIN was very proud when the two main

players in the truss Spar field, CSO -Aker and

J. Ray McDermott, both decided to use MARIN’s

facilities and expertise for model tests.

During the VIV tests the strake design was optimised and also VIV tests with the bare Sparhull were performed. The horizontal tow testsfocused on heave plate loads, bending momentsand motions. Both tow by hard tank and soft tankwere tested to determine the most favourable towset-up. For the upending tests the actual ballastingsequence was tested, using nine individual ballasttanks. The vertical tow tests provided an answer torequired tow loads in upended position and freedrift speed in hurricane conditions. All of themodel test results were used for comparison withits own simulation codes.

Medusa

The tests for J. Ray McDermott concern a Sparscheduled to come into action in the Murphy-operated Medusa field by late 2002, in a 677 mwater depth. For this in-place Spar, model tests were carried outin MARIN’s Offshore Basin. The size of thisbasin enables accurate modelling of all individualmooring lines, top tensioned risers and steel catenary risers without truncation.

The tests made clear that it is important to generate the actual current flow with realistic turbulence levels, because the Spar motion behaviour is based on the mean current loads andthe current velocities at the hard tank. To guaran-tee these sheared currents with constant velocityand low turbulence intensity the current genera-tion system recirculates water with pumps outsidethe basin.

In-place Medusa Spar.

Hans Cozijn & Tim BunnikJ.L.Cozijn@marin.nl

* References Bluewater Energy Services

T.H.J. Bunnik, G. de Boer, J.L. Cozijn, J. van der Cammen,

E. van Haaften, E. ter Brake.“Coupled mooring analysis

in large scale model tests on a deepwater CALM buoy

in mild wave conditions”, proceedings OMAE, 2002

(to be published).

Bonga SPM buoy tests

For SBM of Monaco model tests were carried outon the Shell Bonga Field SPM system and its asso-ciated asymmetric spread mooring system at a scaleof 1:60. Since at this model scale it is not possibleto model the actual local water depth of 1,000 mdue to available maximum model basin depth of10.5 m, an equivalent mooring system was used inthe available scaled water depth of 600 m.This equivalent mooring system was designed sothe load-displacement characteristics and dynamicresponse characteristics were identical to the char-acteristics of the full length mooring system.Furthermore, three 19”Bonga Oil OffloadingRisers (BOORs) in double wave configurationwere modelled in the tests.

Initially, model tests with the buoy alone were carriedout in order to confirm the adequacy of the designof the buoy itself, the mooring system and theBOORs. The buoy motions were measured, as wellas the tensions in the mooring lines and the risers.Underwater video recordings were made to ensurethat the line spacing was sufficient to avoid impactbetween the mooring legs and the BOORs. Inaddition, model tests with the shuttle tankermoored to the SPM buoy were carried out toinvestigate the shuttle tanker motions and bowhawser loads. For these tests various operationaland survival conditions were used.

Large scale model tests

For Bluewater Energy Services, MARIN performeda research project to investigate the coupled

As the deepwater CALM buoy becomes a common solution for

offloading from spread moored West of Africa FPSOs, MARIN is

carrying out an increasing number of model tests in a bid to

improve buoy designs. Report outlines two recent model test

projects carried out in its Offshore Basin.

motions of a CALM buoy and its mooring system(risers and anchor lines). Numerical simulations ofthe buoy motions were carried out with a newly-developed tool taking into account the couplingbetween the buoy motions and the dynamics ofthe mooring system. Until now, the interactionbetween mooring line dynamics and first-ordermotions of floaters was often neglected in nume-rical simulations. For a CALM buoy however, thiscannot be done since the mass of the buoy and themass of the mooring system are of the same orderof magnitude.

The aim of the project was to investigate the firstorder buoy motions. Results of model tests thenserved as input to the numerical model (dampingvalues and wave force measurements) and to validatethe numerical model (buoy motions and toptensions in the risers and mooring lines in waves).A comparison between the model tests and thesimulations showed that the surge and heavemotions of the buoy can be predicted well with thenumerical model.*

MARIN helps improve CALM buoy design 01-397-35

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Horizontal tow test in breaking waves.

Bonga SPM buoy.

report 76 26-03-2002 10:18 Pagina 7