Dynamics of a Floating Platform Mounting a Hydrokinetic Turbine

Abstract

A two-dimensional mathematical model was developed to predict the dynamic response of a moored, floating platform mounting a tidal turbine in current and waves. The model calculates heave, pitch, and surge response to collinear waves and current. Waves may be single frequency or a random sea with a specified spectrum. The mooring consists of a fixed anchor, heavy chain (forming a catenary), a lightweight elastic line, and a mooring ball tethered to the platform. The equations of motion and mooring equations are solved using a marching solution approach implemented using MATLAB. The model was applied to a 10.7-m twin-hulled platform used to deploy a 0.86-m shrouded, in-line horizontal axis turbine. Added mass and damping coefficients were obtained empirically using a 1/9 scale physical model in tank experiments. Full-scale tests were used to specify drag coefficients for the turbine and platform. The computer model was then used to calculate full-scale mooring loads, turbine forces, and platform motion in preparation for a full-scale test of the tidal turbine in Muskeget Channel, Massachusetts, which runs north-south between Martha’s Vineyard and Nantucket Island. During the field experiments, wave, current, and platform motion were recorded. The field measurements were used to compute response amplitude operators (RAOs), essentially normalized amplitudes or frequency responses for heave, pitch, and surge. The measured RAOs were compared with those calculated using the model. The very good agreement indicates that the model can serve as a useful design tool for larger test and commercial platforms.