Optimal heave motion of some axisymmetric wave energy devices in sinusoidal waves

We present a mathematical model of the transient motion of a floating body wave energy device, including memory and an impulsive term. The memory is calculated from the damping coefficients in the frequency domain, and forms the kernel of an integral which is a component of the equation of motion to be solved numerically. The impulsive term is calculated from the added mass. Convergence of both memory and impulsive terms in water of finite depth is considered. It is shown that the (more general) transient memory model agrees with the usual added mass and damping model for the large time behaviour of a body with linear external constraints (power take-off), but that the full memory model is essential for studying either transient motion or motion with non-linear constraints. An examination of the equation of motion in the time domain suggests the possibility of implementing non-linear strategies such as latching to improve the efficiency of the system. Results are presented for a heaving hemisphere and two different truncated vertical circular cylinders (with geometries given by the Swedish and Danish heaving buoy devices). (C) 1997 Elsevier Science Ltd.