Ship-cargo interaction for large and stiff cargo

Abstract

An important trend exhibited by the offshore wind market is the increase in the size of wind turbines, leading to longer and stiffer monopiles with larger diameter-to-thickness ratios. Current transport analysis is focused on loads resulting from hydrodynamic accelerations, without taking loads into account resulting from differences in bending deflection between the vessel and cargo. This thesis covers a study carried out on the structural response of a monopile and sea-fastening system subjected to displacement-based loads. The loadcase follows from a vessel excited using a regular wave leading to bending deflections and rigid body accelerations. The saddles used to support the monopile are modeled as unilateral springs, emphasizing the need to use a non-linear solution method to obtain structural responses. The harmonic nature of hydrodynamic-based loads led to the selection of the harmonic balance method (HBM) used as solution method in the structural model. A novel understanding how cargo properties, seafastening properties and seafastening arrangements influence the structural response of the coupled cargo-seafastening system. Various parametric studies are performed to identify behaviors related to the total structural response. Based on this study, the conclusion can be drawn that a large number of saddles in combination with a low stiffness is desired to minimize the structural response of the cargo and sea-fastening system. Furthermore, the influence of lashing stiffness and pretension is limited with respect to the total response. At last, a parametric analysis on the properties of the cargo showed that certain sea-fastening arrangements can be identified which comply with design criteria.