One-way coupled dynamic analysis of floating platform with wave energy converters

Abstract

In this paper, a six degree-of-freedom motion of a very large floating platform which is equipped with a number of wave energy converters (WECs) was carried out. Six cylindrical-shaped WECs designed to slide along the vertical shaft are located at each side of the rectangular platform, so a total of 24 WECs existed on the platform. Since WECs extract electrical energy from the relative heave motion between WECs and the platform, and the relative heave motion is dynamically coupled by the power take-off (PTO) mechanism, the platform responses are affected by the WECs. To examine the effect of the WECs on the platform, an analysis of one-way coupling, which only considered the PTO damping of the static WECs on the platform, was carried out. The one-way coupling was simulated under the following conditions: All the WECs were statically fixed at a mean water level (MWL) without heaving, but sliding motions were still allowed along the shaft. As a result, the relative heave motion between the floating platform and the WECs was partially implemented. The equation of motion of the floating platform in the time domain was established, and the responses of the one-way coupled platform were then compared with the case of the platform without any coupling effects from the WECs. The hydrodynamic coefficients and wave exciting forces were obtained from WAMIT, the 3D diffraction/radiation pre-processor code based on the boundary element method. The analysis of the dynamic responses of the floating platform with or without one-way coupling in the time domain was carried out using a hull-mooring coupled dynamic analysis tool, CHARM3D. The analysis showed that WECs may induce significant damping of platform motions in heave, roll, and pitch. It also showed that the mooring line top tensions are also influenced by the damping effect of WECs. Copyright ？ 2015 by the International Society of Offshore and Polar Engineers (ISOPE).