QUAD Lift Workability Analysis

Abstract

A QUAD lift is a new lifting method in which dual crane vessels combine their vessel capability to increase their offshore lift performance. The use of the Jumbo J-Class vessels in a QUAD lift creates the opportunity to increase the offshore lift capacity and to install structures with larger dimensions. When floating vessels are close to each other in an offshore environment, their motion will be different than in the freely-floating situation because of hydrodynamic coupling and wave diffraction forces. The main objective of this thesis is to create a model of the QUAD Lift method which predicts the vessel and load motions and evaluate the workability such a lift. Both potential solvers AQWA and OrcaWave are used to assess the hydrodynamic parameters of the interacting vessels. The gap between the vessels is 40 m and the vessel configuration is such that the cranes are parallel to each other. In between the vessels, transversal wave resonance induces peaks in the frequency dependent radiation forces of the vessels. An additional damping lid in between the vessels effectively reduces the resonance behaviour, which is overestimated by potential solvers. The damping lid has an negligible effect on the final workability of the QUAD lift. A 18-DoF linear Matlab model is created which includes the mechanical connection between the vessels and the load. The cranes and the cables are modelled as linear springs. The natural frequencies and eigenvectors show large coupling between the vessel roll and the load sway motion. Tugger lines between the vessel and load are added to shift the natural frequencies of the system and to decrease the large horizontal responses of the load. A parametric study is done on the effect of the load mass, cable lengths and wave directions on the system motion in the most probable wave condition in the Central North Sea. An increase of the mass of the load leads to larger vessel and load motions. The shorter the cable length, the larger the vessel and load motions. The motions are most severe in beam and quarter waves. Depending on the stiffness of the tugger lines the workability can be improved up to 85, 55 and 24 % in respectively head, quarter and beam waves. The limiting factor for the workability is the off-lead angle of the cranes. Broadening of the off-lead angle limit of the crane shows great potential to further increase the workability.