A Method for the Frequency Domain Seakeeping Analysis of Offshore Structures in the Early Design Stage

The motion analysis of floating offshore structures is a major design aspect which has to be considered in the early design stage. The existing design environment E4 is an open software framework, which is being developed by the Institute of Ship Design and Ship Safety, comprising various methods for design and analysis of mainly ship-type structures. In context of the development to enhance the design environment E4 for offshore applications this paper presents a method to calculate the response motions of semi-submersibles in regular waves. The linearised equations of motion are set up in frequency domain in six degrees of freedom and the seakeeping behaviour is calculated in terms of the amplitudes of the harmonic responses. The hydrodynamic forces onto the slender elements of the semi-submersible are accounted for by a Morison approach. As the drag and damping forces depend quadratically on the amplitudes, these forces are linearised by an energy-equivalence principle. The resulting response amplitude operators of the semi-submersible are validated by comparison with model tests. The method represents a fast computational tool for the analysis of the seakeeping behaviour of floating offshore structures consisting of slender elements with circular cross sections in the early design stage.