Dragged surge motion of a tension leg structure

Abstract

The coupling problem of a two-dimensional tension leg structure interacting with a monochromatic linear wave train has been solved analytically. Fluid-induced drags, including form drag and inertia drag, on linearly elastic tension legs have been considered in the present study. A Morison type equation of relative motion is applied to quantify the drags. The nonlinear form drag is then replaced by a linear drag according to Lorentz's hypothesis of equivalent work. Potential theory is utilized to facilitate the coupling problem of the interaction among waves, the “large” floating structure, and the “small” tension legs. The boundary value problem is then incorporated into a scattering and a radiation problem. They are solved separately and combined to resolve all unknowns. The coupled flow field and the dragged surge motion of the structure are then described in terms of the incident wave properties. Analytical solutions show that the inertia drag on tension legs is negligible compared to that due to the evanescent waves caused by the wave-structure interaction. However, the form drag on the legs does alter the structure motion and, consequently, the wave field, especially when wave periods are close to structure's resonant frequency.