THE SIMULATION OF SLOW-DRIFT MOTIONS OF OFFSHORE STRUCTURES

The large amplitude surge-sway-yaw 'slow-drift' motions of a floating body constrained by weak restoring forces in random waves are consider ed. A multiple time scales approximation is employed to separate the f ast time scale associated with the linear motions from the slowly vary ing motions. The ideal fluid free surface flow is approximated by a pe rturbation series expansion for small slow-drift velocities and wave-s teepness, and is solved around the instantaneous position of the body. The linear zero-speed and forward-speed velocity potentials are solve d for arrays of vertical cylinders, using exact interaction theory. Th e horizontal mean drift forces and the wave-drift damping are obtained , and results for realistic configurations are compared with well-esta blished methods. The surge-sway-yaw equations of the slow-drift motion s are solved numerically in the time domain under the influence of sho rt-crested, random waves, including viscous forces. The random wave-si gnal is generated by the filtering of white Gaussian noise. The slowly -varying forces are obtained using the Newman approximation and effici ent summations of time series. The results are compared with full QTF- matrix (Quadratic Transfer Function-matrix) computations of the exciti ng force. The use of a robust random number generator and the Fast Fou rier Transform allows for efficient simulations of long records of the slow-drift motions, and the study of its statistical parameters. The sensitivity upon the simulation length, transients, drag-coefficient a nd directional spreading are demonstrated. Copyright (C) 1996 Elsevier Science Limited