TENSION RAFT JACKET EYED FOR USE IN DEEP WATER

Tension Raft Jacket

A group of companies led by a unit of Norway's Aker AS has formed a joint industry project (JIP) to study deepwater production using tension raft jackets (TRJs).

In Phase 1 of the JIP, the international group by yearend aims to assess the technical and economic feasibility of using TRJs capable of supporting heavy deck weights to produce oil and gas in 3,000 and 5,000 ft of water in mild offshore regions like the Gulf of Mexico. Depending on Phase 1 results, Phase 2 of the program in 1996 could include construction and testing of a model TRJ.

Later JIP studies are to focus on requirements of medium deck weight TRJs working in mild and harsh environments in 3,000 ft of water and mild regions in 5,000 ft of water.

For the study, a TRJ with heavy deck weight is considered capable of producing as much as 100,000 b/d of oil equivalent (BOE/d). A TRJ with medium deck weight capability would be able to produce as much as 40,000 BOE/d.

As examples of harsh environments, JIP participants cite the Norwegian Sea or Atlantic Ocean west of the Shetland Islands.

JIP leader Aker Omega Inc. (AOI), Houston, began preliminary work on the TRJ concept early last month.

Seven production companies and three Aker units have agreed to support the JIP by contributing $25,000 apiece in services or money. Definitive agreements were to be signed soon with all participants.

Oil company participants are expected to include units of Amoco Corp., British Petroleum Co. plc, Exxon Corp., BHP Petroleum Pty. Ltd., Norsk Hydro AS, Shell Oil Co., and Texaco Inc. Aker units taking part are Norwegian Contractors and Aker Engineering, both of Oslo, and Aker Gulf Marine, Corpus Christi, Tex.

AOI is a unit of Aker Oil & Gas Technology Inc., itself a part of Aker's oil and gas technology division.

TENTATIVE SCHEDULE

At an Apr. 18 meeting in Houston, JIP participants reviewed drafts of the study's proposed design basis and operating philosophy agreements and assigned group members to develop more information defining the scope of the design basis.

"Starting in early May, we're going to take that final design basis and start engineering," said Phil Abbott, AOI's JIP manager.

At the group's next meeting, tentatively due about the end of June, participants are to begin presenting results of base case studies focusing on use of a heavy deck weight TRJ in 3,000 ft of water.

HYBRID STRUCTURE

In a journal of Petroleum Technology article, AOI described the TRJ as a hybrid structure that uses a concrete raft installed below the current of a 100 year storm to support a steel jacketed platform topped by a steel deck. The raft, which provides almost all the system's net buoyancy, is moored vertically with steel tendons in a manner similar to a tension leg platform (TLP).

In a typical installation, a TRJ jacket and raft would be joined onshore in a vertical position and towed as a unit to the wellsite, where it would be set in place by a derrick barge.

Installed in deep water, AOI said, the system behaves somewhat like a TLP but with much less response to low period waves. The TRJ handles changes in deck, riser, and tendon loads more easily than a TLP, it can be adapted more easily than a TLP for use in ultradeep water, it should require as little as 3 years from start of engineering to first oil, and it could be built by most Gulf of Mexico fabrication yards.

However, the TRJ concept has not been studied much and never has been model tested, so many issues or possible disadvantages remain unresolved.

For example, design of a TRJ could include storage, but with more difficulty than some competing deepwater production systems. During installation, cost could exceed that of hooking up a TLP if the TRJ's deck is too large to be integrated and handled by a derrick barge.