Safe and efficient oil-platform removal

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Safe and efficient oil-platform removal

Aker Marine Contractors (AMC) has developed a new method of removing offshore platforms to meet international decommissioning guidelines. Trelleborg Viking supplied rubber elements for the AMC buoyancy tank assemblies.

Safe and efficient oil-platform removal
What happens to an offshore oil platform when all the oil has been extracted? It must be removed of course! This is a huge and costly task, but Aker Marine Contractors (AMC) has developed a new method for removing offshore platforms that should facilitate decommissioning operations for the oil industry. Trelleborg technology makes an important contribution to the new method.

AMC’s decommissioning technology is focused on the removal of jackets, the steel substructures that stand in the sea and support the platform topsides containing the production facilities. In the North Sea alone, there are approximately 500 jackets that will eventually have to be retrieved to shore.

The method involves attaching buoyancy tanks to the four corner legs of the jacket, so that when the legs are cut, the jacket will float in the water. It can then be towed to land. It sounds simple, but in practice it is not, according to AMC purchaser Bjørn Andre Jarli.

AMC has designed and constructed four buoyancy tank assemblies – BTAs – each consisting of two buoyancy tanks joined by a mid-section. The BTAs are large structures, standing 67 meters high and weighing over 1,200 tons. Rubber elements delivered by Trelleborg Viking, more than 750 of them in different shapes and sizes with a total weight of more than 38 tons, are fitted to the BTAs where they will be in contact with the jacket legs. Within the clamps mounted at the top and bottom of the assemblies and at places along the length of the assemblies, they act as deflection elements absorbing any impacts.

“We selected Trelleborg Viking for several reasons,” says Jarli. “Its offer was cost effective and the company was able to meet our very short delivery schedule. They also proposed the best solution. It is essential that the rubber elements provide good grip between the clamp and the jacket to ensure there is no slippage while the clamp is holding the leg. In addition, the elements could not be too heavy, because it was important to keep the overall weight of the BTAs down.”
The rubber elements in the jaws of the clamps also have to be able to tolerate the very high forces generated during refloating – jackets are heavy structures, some weighing more than 20,000 tons.

The big test for AMC’s jacket removal technology will come in the summer of 2008, when it will be used to refloat the jacket of a redundant North Sea platform. If the method proves successful, AMC will be well positioned to compete for more such assignments with its new technology.

The fact that Trelleborg Viking, an operating unit of Trelleborg Engineered Systems, a business area of Trelleborg Group, has the ability to be flexible in the face of changing specifications was an important factor in helping it secure this job, according to Sales Manager Stein Hovde. “We were able to convince AMC that we could meet the required delivery time and remain flexible up to the day of delivery,” he says.

Trelleborg Viking staff worked two, and at times, even three shifts to ensure the schedule was met. Some of the work was sub-contracted to Trelleborg Hercules in Singapore. “This is another of our strengths,” says Hovde. “We can call on other Trelleborg companies to cooperate.”

Decommissioning by refloating

The decommissioning market is forecast to grow substantially within the next few years, as International guidelines require oil companies to remove almost all the oil and gas platforms they have installed in the North Sea. A similar approach is being adopted elsewhere in the world.

Floating offshore platforms can be towed back to shore, but platforms fixed to the seabed present a special challenge. To date, floating cranes have been mainly used to remove the production equipment on the deck and then lift the substructure or jacket.

A lot of work and expense is involved – the weights of both topsides and jackets can be anything from a few hundred tons to more than 30,000.

In the North Sea, the overall “decommissioning liability” – the bill faced by the industry – is estimated at some USD 25 billion. Most of this will be spent on removing fixed platforms, of which there are more than 500.
Accordingly, the industry is keen to see the development of new techniques, such as AMC’s refloating method, which offers an economic, efficient and safe alternative to floating cranes.

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