Torsionally coupled dynamic performance analysis of asymmetric offshore platforms subjected to wave and earthquake loadings

The dynamic equations of motion of asymmetric offshore platforms under three different environmental conditions: seismic action, wave action and their combination are established in this paper. In establishing these motion equations, three typical eccentricity types including mass eccentricity, rigidity eccentricity and their combination were considered, as are eccentricities that occur uni-directionally and bi-directionally. The effects of the eccentricity type, the dynamic characteristics and the environmental conditions on the torsional coupling response of platforms are investigated and compared. An effort has also been made to analyze the influence of accidental eccentricity on asymmetric platforms with different eccentricity in two horizontally orthogonal directions. The results are given in terms of non-dimensional parameters, accounting for the uncoupled torsional to lateral frequency ratio. Numerical results reveal that the eccentricity type has a great influence on the torsionally coupled response under different environmental conditions. Therefore, it is necessary to consider the combination of earthquake and wave action in the seismic response analysis of some offshore platforms.