NPD: Decommissioning of concrete platforms in the North Sea feasible through planning

Source: Norwegian Petroleum Directorate

The North Sea has a unique challenge in decommissioning its offshore facilities, especially when considering removing the massive concrete installations of the past. Nonetheless, it is technically feasible to remove concrete installations resting on the seabed -- but this is comprehensive decommissioning work that requires thorough planning and risk management. 

This is the conclusion reached by the consultancy firm Dr. techn. Olav Olsen, which has prepared the report “Disposal of concrete installations” on assignment for the Norwegian Petroleum Directorate. The report describes the technical challenges associated with raising the concrete substructures from the seabed and making them float again. In addition, it discusses issues related to transport of the installations from the field to land. 

Dr. techn. Olav Olsen is one of the inventors of Condeep - a concrete gravity base platform that was produced between 1975 and 1995. 

There are 14 such platforms in the North Sea, of which 11 are on the Norwegian shelf. Statfjord A from 1977 is the oldest that is still in operation, and Troll A from 1995 is the newest. The concrete platforms built before 1978 were not designed to be removed, and this concerns three of the ones currently on the Norwegian shelf. Other concrete structures in addition to the Condeeps include the storage tank on Ekofisk and the floating process and living quarter platform Troll B. 

According to Kolbjørn Høyland, head of offshore and marine structures in Dr. techn. Olav Olsen, no concrete platform resting on the seabed has been removed from the Norwegian shelf so far. With the exception of Frigg TCP2 and the Ekofisk Tank, the reason for this is that they are still in operation. The report therefore builds on knowledge from the planning and construction of the Condeeps, and experience from removal of a steel platform resting on the UK continental shelf. 

”It is technically feasible to remove the platforms. But this is comprehensive work that requires careful planning and a thorough review of risk,” says Høyland. 

In the worst case, an accident during the preparation operation, refloating, towing or dismantling could have serious consequences such as loss of human lives and damage to the environment. 

None of the installations are the same, so the challenges will vary, Høyland explains. The greatest general challenge is mapping the platforms' condition. Solid knowledge regarding the individual platform is therefore the most important premise for a successful refloating operation. 

Several are more than 30 years old, and an overview must be prepared of what has happened over the years: What remodelling has been done since the platforms were new, and whether they have been damaged along the way. 

”We have not identified much corrosion in these platforms, but we must however check whether the reinforcement steel has been exposed to corrosion. The strength of the structure must be checked, to verify that it can withstand the new strains in connection with removal,” says Høyland. 

A mapping of the strains concrete substructures were exposed to when they were joined with the decks and installed on the fields will be included in the calculation of how much strain the substructures can withstand.
 
Natural buoyancy 

The most important method for raising the substructures is to drain the ballast water in the storage tanks, thus giving the structure natural buoyancy. According to Høyland, this is most likely sufficient for most facilities, except for platforms such as Draugen, Gullfaks C and Troll A, where the skirts at the bottom of the substructure are especially deep due to the soft seabed. On these the platforms, it is essential to be able to increase the pressure from the bottom, in addition to deballasting. 

To raise them in a controlled and safe manner, all conductor openings at the bottom of the platforms must be plugged, so that no water leaks in during the operation. If that happens, the pressure cannot build up the way it should, Høyland explains. 

Another risk factor that must be included in the calculation, is the cement mortar that was filled up under the platforms when they were installed. This was done to equalise the pressure and weight on the seabed. According to Høyland, it is unsure what will happen with the mortar when the platform is raised: Whether it will remain on the seabed, or whether parts of it will remain, or possibly fall off during or after the raising.
”It can cause instability, and the platform could start to tilt. But this is manageable, and we can take precautions,” he says. 

The Norwegian Petroleum Directorate is continuing the work on disposal of concrete facilities by announcing a new project. The project will study the risk in connection with transport to land, and technical challenges associated with dismantling of the platforms at onshore facilities.



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