Brent spar disposal requires new approach

Brent Spar Layout [58269 bytes]

New structural analysis shows offshore dismantling of the derelict Brent spar loading buoy will be trickier than first expected and a new solution must be found.

So says Shell U.K. Exploration & Production, which disclosed those conclusions at a recent press conference (OGJ, July 15, p. 29).

Shell Expro, the operating joint venture of Shell U.K. Ltd. and Esso Exploration & Production U.K. Ltd., also unveiled a list of contractors working on outline proposals for disposal of the spar.

Earlier ideas infeasible

Shell said previous concepts for offshore dismantling of the spar for onshore disposal are not feasible because likely stresses in the structure during dismantling are higher than previously estimated.

Shell/Esso hired engineering consultant W.S. Atkins Consultants Ltd., London, to carry out an independent assessment of the spar structure using finite element analysis.

Eric Faulds, decommissioning manager at Shell Expro, said, "The analysis showed that, while the spar is floating it is strong, and could stay in the North Sea for another 20-30 years.

"The problem will be trying to get it out of the water. The outside pressure on the walls after its ballast water is pumped out would create problems for the contractors."

W.S. Atkins has showed, with techniques not available when the spar was installed in the 1970s, that it was highly stressed and close to the point of failure during the installation process.

This means the installation process cannot simply be reversed to dismantle the spar. The consultant says previous concepts for offshore dismantling are not feasible and that new solutions will have to be found.

Before the Atkins study results were known, Shell was considering dismantling the spar either by raising it higher in the water in its vertical position and removing it slice by slice from the top or rotating it into a horizontal position for loading onto a barge or for towing.

Shell said that during deballasting the spar's outer walls soon would reach their safety limit and could collapse. Even if its two damaged tanks were repaired first, deballasting would still be risky.

Buoyancy aids needed

Now the company believes buoyancy aids will be needed to reduce stresses during dismantling. It is considering use of a crane, air bags, compressed gas, or polystyrene balls.

"The new analysis shows that deballasting three tanks would need assisted buoyancy to give a weight loss of nearly 6,000 metric tons," said Shell, "raising it almost 16 m. Deballasting six tanks would need assisted buoyancy for a weight loss of nearly 4,100 metric tons, raising it 14 m.

"Removing the spar's topsides, which weigh 1,250 metric tons, would go some way towards this, but a lift of several thousand tons would still be needed, and suitably strong points in the structure would need to be identified."

The buoy is currently moored in Erfjord, Norway, while Shell/Esso prepares a new disposal plan for approval by U.K. government before disposal can go ahead.

The U.K. government had approved a plan to dump the buoy in deep water off Northwest Britain last summer, but international pro- tests forced Shell/Esso to abort the operation (OGJ, Nov. 27, 1995, p. 23).

Contractors listed

Shell/Esso subsequently invited suggestions for disposal of the spar and has asked 21 contractors to prepare outline disposal plans for Brent spar loading buoy, bearing in mind the new findings.

The 21 contractors are required to submit outline plans by July 31, after which Shell/Esso aims to select a short list of about six contractors to fully develop their disposal plans.

Heinz Rothermund, managing director of Shell Expro, said the aim of the first list is to utilize the ingenuity of contractors in the disposal task. The short list will feature several different disposal options for proposal to U.K. government.

"Whatever solution we propose needs government approval and must be at least as good or better than the original deepwater disposal plan," said Rothermund. "The emphasis is on getting it right and not rushing it.

"The Best Practicable Environmental Option (BPEO) will be determined on a balance of safety and the risks to human life, technical feasibility, economics, environmental impacts of all kinds, and widespread acceptability."

"We would not like to arrive at a short list of six very similar onshore scrapping options, for example, or six fish farm ideas," said Faulds.

"We are looking for a spread of realistic disposal options that can compete strongly with each other and with deepsea disposal to determine the BPEO."

Shell/Esso lists these contractors working up outline plans:

International groups Hollandia BV, Krimpen, Netherlands; Ramboll/Monberg & Thorsen Consortia, Esbjerg, Denmark; Rhodes Offshore Partners Inc., Toronto; ROS Holland BV, Ijmuiden, Netherlands; Thyssen Stahlunion GmbH, Dusseldorf; and Wood GMC, Aberdeen.
Recycling firm Mayer Perry Recycling, Erith, U.K.
Multidisciplinary offshore contractors Aker Offshore Partner AS, Stavanger; AMEC Process & Energy Ltd., Aberdeen; Brown & Root Energy Services Ltd., London; Kvaerner Installasjon AS of Hundvag, Norway; and McDermott Marine Construction Ltd., London.
Heavy lift contractors Heeremac VOF, Leiden, Netherlands; and Stolt Comex Seaway Ltd., Aberdeen.
Marine and civil engineering firms Hollandsche Staalbuow Maat-schappij BV, Gouda, Netherlands; Land & Marine Engineering Ltd., Wirral, U.K.; McAlpine Doris JV, Hemel Hempstead, U.K.; and Taylor Woodrow Civil Engineering Ltd., Southall, U.K.
Ship/rig repair yards NNC/ Cammell-Laird, Knutsford, U.K.; and UMOE Haugesund AS, Haugesund, Norway.
Dredging company Jan De Nul NV, Aalst, Belgium.

Faulds said Shell/Esso plans to disclose details of the plans submitted by contractors shortly after the end of July deadline. He said the company will not speculate on which type of disposal option is likely to be preferred, though the company's recent public statements on options have centered on offshore dismantling for onshore disposal.