Before Pieter Schelte's arrives on location, the legs of the topside support structure are cut, and further preparations performed as required, from the platform deck or using a support vessel. Once complete, the topsides are raised using hydraulic clamps mounted on eight horizontal lifting beams located at the bow of Pieter Schelte. These clamps will 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 Pieter Schelte's active motion compensation system. Friction clamps close carefully around the platform legs, the natural strong points of the platform; in the case of a gravity-based platform, special purpose strong points are mounted to the underside of the topsides to facilitate the lift-off operation.
Once the clamps are 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 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 the 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 minimal.
Due to the large motion compensation capacity (both vertical and horizontal) of Pieter Schelte’s topsides lift units, the dynamic forces transferred to topsides during engaging and pre-tensioning are very low, even when she is working in less favourable sea states.
Since the lift-off procedure is displacement driven and ample lift capacity is provided, the lift system does not require accurate knowledge of the platform weight or where the centre of gravity is located.
After Pieter Schelte has safely transported the structures to sheltered water, they are transferred to a cargo barge for final loading onto the quayside. A 200 m long cargo barge has been specifically designed to facilitate this operation. Alternatively, if the water depth is sufficient, the structures can be skidded directly from Pieter Schelte onto the quayside.