Spectral-Based Fatigue Analysis of a Semi-submersible Platform

Offshore structures are exposed to wide range of cyclic loadings which causes fatigue damage and hence it is an important design consideration which determines the durability of the structure. Due to the complexity of the random loading conditions, determination of fatigue life is a tedious task. Therefore, it is important to develop a reasonable loading spectrum for fatigue assessment using cumulative fatigue damage (CFD) theory. There are few methods by which we can predict fatigue life of a structure which include fatigue crack propagation (FCP) model, by using conventional empirical relations and by developing fatigue stress response spectrum. In this paper, fatigue life of a semi-submersible platform is determined by analyzing critical joints which are exposed to cyclic loading and also by considering random numbers of short-term wave conditions which follow Rayleigh distribution. Structure considered for the study is a sixth-generation semi-submersible platform known as COSL Prospector made in CIMC Yantai Raffles shipyard in China designed for North Sea. Firstly, a hydrodynamic diffraction analysis is carried out in order to understand the response of the structure under various wave directions and frequencies. Wave-induced pressure on the column-bracing connections are determined and these joints are considered as critical locations for fatigue analysis. Suitable wave spectrum is then developed for each sea state conditions. In order to determine the stress concentration factor of the column-bracing joint, a local geometrical model of the connection is created followed by generating a fatigue stress energy spectrum and complex fatigue stress transfer function which is later described to the model. The minimum fatigue life of the local model is determined, and the results are compared with the specification required.