Hydrodynamic coefficients of a monolithic circular offshore structure

Expressions for both the rectilinear and rotational inertial and damping coefficients for a circular monolithic tower of uniform radius are derived. The analysis matches the fluid velocity, derived from potential theory, with the structural velocity in sway. That is, the motions of the tower are assumed to be in a vertical plane. The analysis is then applied to a tower composed of (lumped-mass) elements, where the expressions for the added-mass and damping coefficients are shown to be functions of wave number. The added-mass is shown to be a product of two wave systems: a travelling wave system, which is responsible for the radiation damping, and a standing wave system, called the evanescent system, which is attached to the structure. The added-mass of the evanescent system is negative for small wave numbers, while that of the travelling waves is positive. The negative sign simply means that the inertial force of the evanescent waves is 180° out of phase with that of the travelling system. Furthermore, it is shown that the contributions of the two wave systems to the total added-mass of the structure counteract each other, resulting in a total added-mass which varies gradually with the wave number. Finally, the analysis is applied to an experiment, and results of the analysis and the experiment are found to agree rather well.