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 will begin with the 23,500-t Brent Delta topside in 2017.
Pioneering Spirit successfully executed her maiden heavy lift project, removal of the 13,500 t Yme mobile offshore production unit (MOPU) in the North Sea, for Repsol Norge AS on 22 August 2016. The Yme MOPU is a jack-up type platform standing on three steel legs of 3.5 m diameter, which are inserted approximately 10 m inside the subsea storage tank columns at 93 m water depth.
With this platform removal, Allseas was able to demonstrate the unique single-lift capabilities of Pioneering Spirit.
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 legs of the topsides support structure are cut and additional preparations are performed as required. When Pioneering Spirit arrives on location, the topsides are lifted using a series of hydraulic clamps, which have been previously adjusted to the exact dimensions of the platform legs (in some cases specially constructed support yokes are positioned at predetermined strong points at 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 (or the support yokes are positioned underneath the topsides). Once the clamps (or yokes) are connected, tension in the lift system is gradually increased to transfer the weight of the topsides from the jacket to the vessel. Finally, a 2-m rapid lift eliminates the risk of impact between topsides and jacket.
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. After cuts have been made in the foundation piles of the jacket at seabed level (or above the pile clusters as required), the tilting lift beams raise the jacket at the main legs. The beams rotate the jacket onto the deck and it is then skidded inboard. Jacket removal can be performed upon completion of the topsides removal, without Pioneering Spirit first having to offload the topsides.
Transfer of jackets and topsides
For the transfer of jackets and topsides to/from Pioneering Spirit, a 200 m long, 57 m wide cargo barge has been built. The barge, Iron Lady, with its relatively shallow draught, will enable the loading or offloading of structures to/from the dismantling yard in case the water depth at the quayside is insufficient for Pioneering Spirit. The transfer of structures between Pioneering Spirit and the barge takes place in sheltered water close to the yard.