The authors of this paper developed a non-linear time domain analysis procedure and calculation tools based on these procedures to predict the motion response of moored semisubmersibles under wave, wind and current loading. These tools have been validated with published results. In the first part of the paper a brief summary of the nonlinear analysis procedures is given, together with some results obtained from predictions and published experimental and theoretical results. In the second part of the paper the results of parametric studies investigating the effects of variations in external wave, wind and current force magnitudes and directions on the dynamic motion response of semisubmersible platforms are illustrated. These results show the sensitivity of certain types of response values as the external forces vary in magnitude and in direction. Such results provide essential information for probability based design procedures. In the last part of the paper, time domain models are used to simulate the incidents occurred in the North Sea in 1990, in which mooring lines of some semi-submersibles were failed.


Motion response prediction of offshore structures may be carried out using time domain or frequency domain models or model tests. Frequency domain analysis uses the simplified, linearised form of the motion equations and it is very economical. Time domain analysis, unlike frequency domain models, is adequate to deal with non-linearities such as viscous damping and mooring forces, but most of the time it requires sophisticated solution techniques and it is expensive to be employed. If the non-linearities in the system is significant or the external forces acting on the structure is time dependent, then time domain methods must be employed. Calculations of wave induced motions of semisubmersibles usually have been carried out using the Morison equation formulation.

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