A Simulation Approach for Large Relative Motions of Multi-body Offshore Operations in Waves

The traditional approach to simulate offshore multi-body operations in waves is to carry out a potential-flow based linear diffraction analysis in the frequency domain and to apply the results in a time-domain simulation through impulse response functions and time-traces of first and second-order wave forces. A linear multi-body diffraction analysis is valid for small oscillatory motions of the offshore structures around a mean position. In some cases however relative motions may become large. Examples are float over operations, the approach/berth of an LNG carrier towards an LNG FPSO or a tandem offloading operation. In these cases the traditional approach is questionable and, as will be shown in this paper, insufficient in certain cases. A simulation approach was therefore developed for operations with large relative motions. When the (low-frequency) horizontal relative motion in the time-domain simulation exceeds a certain threshold level a new linear diffraction analysis is automatically carried out at the present relative positions of the bodies. The linear diffraction analysis is parallelized using the nodes of a desktop computer cluster, running each frequency on a different node, which enables a relatively quick analysis. The time-domain simulation is then restarted at the time level where the relative motions have become too large, but with the results of the new diffraction analysis (updated impulse response functions and wave force time traces). The simulation approach is applied to the approach of an LNG carrier to an LNG FPSO. In the paper it will be shown that the wave drift forces are varying considerably depending on the relative position and wave direction, thereby possibly affecting the approach manoeuvre, which justifies the use of this more complicated technique. In the present work the large relative motions are restricted to the horizontal plane. in future work vertical motions will also be included and validation with model tests will be carried out.