Evaluation of transient lift loads during a decommissioning operation with a crane vessel

Abstract

With the conversion of a heavy transport vessel into a crane vessel, Boskalis aims to strengthen its position in the offshore wind and decommissioning markets. The BOKALIFT 1, as the crane vessel will be named, is equipped with a 3,000 tonne crane and a class-2 Dynamic Positioning system. In order to utilise its exceptional large and strong deck area to its full extend, the crane is installed amidships at the starboard side of the vessel. Offshore lifts are consequently performed in transverse direction of the vessel, resulting in a large heeling moment during the lift operation. As the conversion is currently ongoing no operational experience has been gained yet. This thesis is therefore an assessment on the behaviour of this new asset and the crane loads during a lifting operation close to the maximum crane capacity. This is the case for a single-lift decommissioning operation, lifting a topside from a jacket support structure.

Two transient phenomena can occur throughout the decommissioning operation: snap loads and impact loads. Both phenomena cause large peaks in the crane loads. Snap loads occur in the rigging lines during the pre-tensioning. During this phase the rigging lines become taut and the motions of the crane hook are constrained. Impact loads can occur during the final lift phase of the topside. This phase is estimated to last several minutes and is driven by the capacity of the dedicated anti-heeling system. Its purpose is to compensate the large heeling moments. Since this operation will last multiple wave periods, the wave induced motion at the crane tip causes tension variations in the hoisting system. When this tension exceeds the required lift tension the topside is temporary lifted, with impact loads as a result.

An OrcaFlex model is built to perform time domain simulations of the pre-tensioning and lift phases of the decommissioning operation for several irregular sea states. For both phases, a crane load comparison is conducted for two different lifting schemes. In the first scheme the hoisting speed is dictated by the anti-heeling system in order to maintain even-keel conditions. This is favourable for the operation limits. For the second scheme this is disregarded and the maximum hoist speed is applied. The latter is expected to reduce the duration where impact loads are present.

In the pre-tensioning phase, the higher hoisting speed results in a larger amplitude of the load variation since the snap loads are dictated by the relative velocity of the crane hook. During the lift phase, the higher hoisting speed reduces the time during which impact loads occur and thereby also reduces the crane loads. However, the topside motions are significantly increased. The crane tip- and topside motions have shown to be the limiting criteria for the lift operation. Limiting the topside motions by bumpers, guides or extra tugger lines can increase the workability for sea states where the peak period is close to the vessels natural heave and pitch period. Conclusively this thesis shows that the BOKALIFT 1 is a very well-suited vessel for offshore decommissioning operations.