A Study of Slowly Varying Drift Forces on Multi-Body Floating System
The mean and slowly varying drift forces play a vital role in the study of the behavior of moored offshore structures, because their mean periods are close to the natural period of oscillation of the system that causes large motion amplification. The present study is based on far-field method where the velocity potential is calculated by 3D sink-source technique. The numerical calculations have been carried out for a parallel arrangement of FPSO and an LNG carrier. The numerical accuracy in the determination of the wave exciting forces and the hydrodynamic reacting forces influences the motion response that eventually affects the drift forces and moments of each body. The computations of motion responses and drift forces are carried out for a number of different wave heading angles and for different separation distances between the FPSO and LNG carrier. The numerical results are compared with the experimental ones to justify the validity of the improved numerical accuracy of the present computations. It has been observed that due to the lack of accuracy in the numerical scheme adopted for the computer code, the computed results of drift forces and moments sometimes shows completely opposite trend than that of the experimental ones.