When oil and gas fields end production, their facilities need to be removed and disposed of (or recycled), a process known as “decommissioning”. Decommissioning of offshore structures is a highly complex and technical exercise that poses significant health and safety challenges. The process encompasses the planning, approval, implementation, removal and disposal (or re-use) of an offshore structure.
In the North Sea alone, around 470 installations will require decommissioning over the next 30 to 40 years. Royal Dutch Shell is leading the way; in February 2015 the company set out plans to decommission the iconic Brent platforms in the oilfield north-east of Shetland – using Allseas’ heavy-lift vessel Pioneering Spirit. Removals began with the 24,000-t Brent Delta topside in 2017.
Pioneering Spirit set a new world lifting record with the safe and successful removal of Brent Delta on 28 April 2017. The lift operation, comprising move in around the platform, yoke connection, de-ballasting and “fast lift”, took just 12 hours. Brent Delta was Allseas' second decommissioning job, following the removal of the Repsol Norge-operated 13,500 t Yme mobile offshore production unit (MOPU) from the Norwegian sector of North Sea in August 2016.
Pioneering Spirit is able to remove entire topsides, up to 48,000 t, in a single lift. Platform removal is executed by lifting the topsides facilities at their natural strong points and bringing them to shore in one piece. Topsides removal in a single lift will significantly reduce the man-hours spent on offshore cleaning, partitioning, installing lifting points and rigging platform modules. This will considerably reduce health and safety risks. Avoiding offshore cleaning and purging of process facilities will also reduce risk to the environment.
Prior to Pioneering Spirit’s arrival, the lift preparations are performed as required and the topsides leg are cut from the platform deck. When Pioneering Spirit arrives on location the topsides are lifted, for example by using clamps or yokes mounted on the topsides lift system beams. These clamps or yokes have been previously adjusted to the exact dimensions of the platform legs or the underside of the topsides.
Waves will rock the vessel during the lifting operation, but thanks to the motion compensation system the clamps remain motionless relative to the platform. The clamps are carefully closed around the platform legs (in the case of support yokes, these are positioned at predetermined strong points at the underside of the topsides). With the clamps or the yokes connected, pre-tension in the lift system is gradually increased in order to transfer the weight of the topsides from the jacket to the vessel. In the final stage, a rapid lift-off, using hydraulic lift cylinders with a maximum stroke of 4 m, ensures sufficient clearance and eliminates the risk of re-impact between topside and jacket during and after the lift.
Once the topsides have been removed, the platform jacket may be lifted and brought ashore. When removing large jackets, conventionally they need to be cut up into small sections suitable for crane lifts, requiring lengthy and complex subsea cutting operations. Pioneering Spirit will be able to remove jackets weighing up to 25,000 t in a single lift, thereby limiting subsea work and associated risks.
Jackets are lifted over the stern of Pioneering Spirit, for which the vessel is repositioned. After cuts have been made in the foundation piles of the jacket at seabed level, or above the pile clusters as required, the jacket lift beams raise the jacket at the main legs. The jacket is subsequently reclined onto the vessel deck and skidded further inboard. Jacket removal can be performed upon completion of a topsides removal, without first requiring the vessel to offload the topsides.
Transfer of jackets and topsides
For the transfer of topsides to/from Pioneering Spirit, the 57 m wide, 200 m long cargo barge Iron Lady has been built. In case the water depth at the quayside is too shallow for Pioneering Spirit, structures are first loaded onto the barge for transfer to/from the yard. The transfer of structures between Pioneering Spirit and Iron Lady takes place in sheltered waters close to the yard.