Spread-moored barge with surface trees in wellbay confirmed in model tests

All calm in the moonpool

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Th 6005f401
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Since 1995, Bouygues Offshore, in asso ciation with Sedco Forex, has been devel oping new concept spread-moored barges housing surface trees in a central wellbay. The program started with the multi function barge (MFB), a dry wellhead floating production, storage, and offloading (FPSO) vessel with integrated drilling facilities, then expanded to include the wellhead barge - similar to the MFB, but without storage. Riser system design and analysis has been provided by the Institut Francais du Petrole and 2H Offshore Engineering in the UK.

Both the new concepts are particularly attuned to conditions in the Gulf of Guinea. What makes them so cost-effective is the possibility they offer of sharing a drilling unit with surface trees and processing plant on a high-capacity floating production unit (FPU).

Development has been undertaken with the technical and financial support of BP Amoco, Conoco, Elf, ExxonMobil, Shell, and Statoil, with numerous different site-specific studies. Recently, basin model tests were arranged to confirm maturity of the concepts. Since late 1999, intensive tank trials have been performed in the South of France, focusing on integration of the self-tensioned risers into the central wellbay. The tests, on 1/60th scale models, simulated wave and current loading impact on the hull, moorings, and risers in a basin with a 10-meter-deep central pit.

The tests were designed to shed light on the interaction, under different wave and current conditions, of the barge, the volume of water in the wellbay (moonpool) and the risers. Altogether, 155 wave and current tests have been performed. Current in the direction of wave propagation or in the opposite direction is generated by 12 helical pumps coupled with two large water circulation frames outside the basin, allowing for a laminar flow within the measurement area. Tests have covered a wide range of wave periods, using regular and irregular waves generated by 12 oscillating, duck-type wave makers. Two different barge sizes were tested, corresponding to the minimum processing WHB and MFB.

The fully instrumented test bench placed within the model's moonpool includes a fluid particle image laser velocimeter, used to qualify and quantify wave and current-induced water circulation in the moonpool and also to measure precisely the local particle velocity field. This is the first such application anywhere of instrumentation within the model itself, and for such a purpose. In addition, 10 different sets of instrumentation were arranged around the barge models, including the following:

  • Real-time measurement of the small motions of the vessel and the very limited motions of the riser wellheads, using the Rodym 6D camera tracking system
  • Capacitive water gauges to determine moonpool-free surface elevation
  • Three video cameras directed within the moonpool, and installed in each hull model behind plexiglass walls, to confirm the uniform and gentle local/global water motions, and also to visualize riser behavior
  • Measurement of horizontal accelerations at the lower parts of the large aircans to validate with the motion measurements the riser/ riser and riser/barge clearance issues.

Results, conclusions

Overall performance of the barge design (regarding operational requirements in the tested configuration) was excellent, confirming the ability of the barges to house aircan top-tensioned, pivot-guided risers. Also, the theoretical or numerical behavior prediction tools applied all gave very accurate information, so that interaction, barge global motion behavior, water mass "enclosed" in the wellbay, and riser effects are all fully manageable.

Theoretical results have been obtained with an appropriate numerical model of the barge only. Model test and theoretical results, in terms of barge response, correlate well. A few hydrodynamic coefficients have been forced to fit analytical models of the moonpool behavior and overall barge mode eigen damping source prediction.

The results highlighted the limited coupling effects between the barge and the moonpool eigen modes and also revealed slightly more damping than anticipated. The conclusion to be drawn is that the barges are typical of large FPSOs which move very little in mild environments. The motions can be predicted using simple tools.

Concerning moonpool hydrodynamics, the analytical theory employed by Bouygues Offshore to evaluate both the deformation at each eigen mode and the water elevations in the moonpool appears to be valid for a moonpool without risers, but is marginally impacted by the presence of risers or the super-position of waves and current.

The most important conclusion, however, is that the wellbay fits its protective purpose entirely. Limited modal amplifications due to the moonpool were as expected for narrow wave periods. As a consequence, the relative water elevation at any point of the moonpool is always very limited and far from interacting with design or operational requirements. Above all, local water flow analysis shows that for such barges, the flow of the water mass within the moonpool is nearly laminar. The free surface is uniform during its global elevation, and moreover, fluid motions are weaker within the moonpool than outside the hull. In fact, the wellbay is really a calm place compared with the open sea.

Potentially, the biggest success of the test campaign was the riser/barge behavior. The cameras, motion sensors, and the eyes of the human observers all suggested that:

  • The risers do not move apart from following the horizontal motions of the barge.
  • No single riser is acting differently with respect to its neighbor, and the barge is slowly heaving, pitching, and rolling around the 24 risers modeled, without disturbing their vertical high stability.
  • Riser absolute motions are very low (almost immeasurable).
  • The dynamics of the moonpool water mass are too gentle to interfere with the risers' top sections.

Investment group keeps faith with entrepreneurs, despite downturn

Oil and gas service contractors in France have experienced mergers or restructuring programs over the past year, in common with their chief clients, Elf and TotalFina. Their motives for doing so are identical - to boost value to shareholders. - The CSO Constructor is one of two Coflexip Stena Offshore vessels that have been upgraded recently for all-around deepwater duties.

One of the key investors in the French sector is the ISIS Group, founded in 1975 by the Institut Francais du Petrole. ISIS, which was listed on the Paris Stock Exchange in 1997, maintains interests in 16 advanced technology/service specialists, active both upstream and downstream. Most of these companies are French, although of late, ISIS has diversified with investments outside France, as part of its strategy to become more international.

IFP, in turn, remains ISIS' principal shareholder, with a 53% interest. Most of the outstanding shares are publicly owned. ISIS' main mission is to promote industrial application of promising research and development by supporting or helping to establish new companies. It divides its portfolio into four divisions, the most offshore-biased being exploration and production. Within this category are the geophysical contractor CGG, Coflexip Stena Offshore, Geoservices, and consultants DV Offshore, in which ISIS holds respective stakes of 15.5, 18, 35 and 25%.

CGG suffered last year from the worldwide exploration slump, and was therefore forced to seek new ways to raise capital. One success last December was the merger of its seismic equipment subsidiary Sercel with one of its main competitors in the US - Geoscience Syntron. The enlarged operation, headquartered in Nantes, and with a subsidiary in the US, claims 45% of the world market for marine and land geophysical equipment, according to ISIS' Executive Vice-President Jacques Burger. Among its products, Sercel will produce the Syntrak system developed by Syntron.

The acquisition was engineered via the Beacon Group, which made a public offering for Syntron's parent company and brought these shares to CGG. Beacon Group acquiesced to the move because it was impressed by CGG's management, says Burger. "ISIS is effectively diluted in this operation to 15.5% of a bigger group. However, we were entirely pleased with the outcome of this exercise. It will lead to decreased costs through a strong restructuring program."

Coflexip Stena Offshore may have raised its stock through buying Houston-based pipeline engineering company RJ Brown from Kvaerner. RJ Brown is known for its innovation in deepwater bundles and risers. One of its recent projects was the world's first multiple flowline steel catenary riser and bundle for Amerada Hess' Penn State project in the Gulf of Mexico. Coflexip also raised its stake in India offshore contractor, Peerless Shipping & Oilfield Services, to 58%, thereby strengthening its Asia-Pacific capability.

Geoservices also suffered from the continuing depression in exploration drilling. Its 1999 sales fell by 35%, which in turn led to 25% staff cuts. However, the company is maintaining its research and development impetus, Burger insists, having joined a new European Union project on seismic-while-drilling. It also continues to work with IFP on drillstring vibration monitoring, and the two may also collaborate on multiphase flow metering.

The SMB Division includes Toulon-based Principia (ISIS 90%). "Thirty percent of their activity is offshore," he says, "and they want to increase that further. One of their most promising products is the Deeplines mooring and riser analysis software. They have recently produced a new version for vortex-induced vibration. They have also undertaken studies for the Girassol riser towers."

In 1998, ISIS branched out of France by acquiring 15% of Dutch-owned platform maintenance group, Dietsmann. This January, it also invested in a new company in Charleroi, Belgium called Diamant Drilling Services. Its founder, Robert Delwiche, helped develop the world's first commercially successful PDC bit in 1976.

Delwiche worked for several major service contractors, but decided to go independent to pursue his own advanced drill bit designs, says Burger. "Based on his knowledge of the market, he wants to adapt his designs to specific rocks that have to be drilled through. He believes it's possible to have more custom-made products and an improved rate of penetration and overall efficiency." Most of the assembly work will be managed by specialist offshoots from the local mining and steel sectors.

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