Motion based cable integrity limits for quadrant assisted pull-in operations on submarine inter-array cables

Abstract

The installation of subsea cables connecting offshore wind turbines to the grid is a delicate process. This is especially the case for the operation of connecting the second end of the cable to the turbine. The applied method of using a quadrant means that in the workability analyses, multibody dynamics, line dynamics and sea state dynamics need to be combined, resulting in lengthy simulation requirements. The objective of this thesis is to determine vessel motion based limits to cable integrity in order to simplify workability analyses. This method allows the problem to be analyzed in the frequency domain, resulting in computational efficiency gains. In order to arrive at the desired result a literature study is performed regarding cable loading and cable failure modes encountered during quadrant assisted cable pull-ins. On that basis a detailed investigation into the relations between vessel motion and mechanical cable responses is carried out. To achieve this, a representative cable and a set of high but realistically encountered sea states are simulated. The obtained relations are then compared to the cable integrity limits for curvature, tension and compression to acquire limits expressed in terms of motion parameters such as acceleration, velocity and displacement. The results from these simulations show that: 1) maximum cable tension is closely correlated to upward heave velocity of the crane tip, 2) maximum cable compression is closely correlated to downward heave velocity, 3) maximum curvature is most closely correlated to downward heave velocity. It is concluded from the results that the cable response can be determined from the crane tip heave motion, which in turn is known from the vessel motions. This means that analysis of such a problem is possible in the frequency domain. As the results show that heave motion is governing in cable failure, heave compensation in the crane is recommended for the operations considered. In addition, an enhancement of the analysis process is proposed by extracting the linear relations and vessel motion limits from a small set of time domain simulations and assessing the situation thoroughly in a frequency domain analysis. The configurations considered exclude any effects added by cable protection systems or interaction effects with rigid bodies in the vicinity of the cable. Even though the analysis process is generally applicable and constitutes a significant improvement in computational load, the obtained relations may be generalized further by implementing a closed formula relating vessel motion to cable failure, or application of the cable protection system to the assessed configuration. Additional research in these directions is encouraged.