For the design of FPSOs the vertical bending moment is a key parameter to ensure safe operation. If analyzed at water line level, however, the unknown influence of longitudinal forces may distort the results. Hence, a segmented FPSO model with midship force transducers at two levels is investigated in various deterministic wave sequences to identify the vertical bending moment and its associated neutral axis as well as the superimposing longitudinal forces. It is shown that the neutral axis is far below the water line level, with the consequence, that extreme cyclic loads at deck level would be expected. However, as the associated longitudinal forces — even if significant — generate a counteracting moment, this effect is largely compensated. Both, frequency- and time-domain results are presented. With frequency-domain analysis the profound data for the standard assessment of structures, concerning seakeeping behaviour, operational limitations and fatigue are obtained. In addition, time-domain analysis in real rogue waves gives indispensable data on extremes, i.e. motions and structural forces.

A growing amount of reports on heavy lift operations involving huge crane vessels prove that investigations on the motion behavior of multi-body systems are vital regarding the combined aspects of safety and economics. In this paper a method of transforming frequency-domain into time-domain results is presented. With the panel program WAMIT (WAMIT Inc.) the Response Amplitude Operators (RAO) of the motions in six degrees of freedom of the structures involved in a lift operation are calculated. The multi-bodies RAOs differ significantly from those of the single structures (without interaction effects). The consideration of hydrodynamic coupling is therefore essential for the prediction of accurate relative motions between the structures. Frequency-domain results are still important when determining operational limitations. But only with simulations in time-domain the relation between cause and reaction can be studied in detail. Results from simulations provide for example decision support for finding uncritical starting points of the lift off operation. By Fouriertransforming the RAOs the impulse-response functions are obtained. Having the impulse-response function the time-dependent system responses in arbitrary deterministic wave registrations are determined by convolution. This method allows fast and effective time-domain simulations of multi-body systems. Results are presented for a crane semisubmersible and a conventional transport barge. The influence, particularly the sensitivity of wave height and wave length on the response is shown in wave packets.

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