Cutting of the support legs of the topsides and further preparations as required, will be done from the platform deck or using a support vessel, prior to arrival of Pieter Schelte.
Topsides are lifted by using the hydraulically operated lifting clamps, mounted on eight beams at the bow of Pieter Schelte. These clamps have been previously adjusted to the exact dimensions of the platform legs.
Whilst the vessel itself is in slight motion due to wave action, all motion of the clamps relative to the platform is eliminated by engaging the active motion compensation system. This enables careful closing of the friction clamps around the platform legs, the natural strong points of the platform (or, in case of a gravity platform, strong points mounted to the underside of the topsides specifically for the lift-off operation).
With the clamps 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
2 m rapid lift-off eliminates the risk of re-impact between topside and jacket.
Analysis has shown that in hostile environments such as the North Sea, a motion compensation system is essential on a single-lift vessel to eliminate impact forces on large topsides, due to the giant masses involved. In the absence of such a system, local damage will occur even when the wave height is small and vessel motions are very limited. Due to the large motion compensation capacity (both vertical and horizontal) of Pieter Schelte’s topsides lift units, the dynamic forces introduced in the topsides during engaging and pre-tensioning are very low, even when working in less favourable sea states.
The lift system does not require accurate knowledge of the platform weight or the position of the centre of gravity, as the lift-off procedure is displacement driven and ample lift capacity is provided.
After Pieter Schelte has transported the structures to sheltered water, they are transferred to a cargo barge for final loading onto a quayside. A 200 m long cargo barge has been designed for this operation. Alternatively, if the water depth is sufficient, they can be skidded directly from Pieter Schelte onto the quayside.
