Session: 08-15-01: Wind and Water Power

Paper Number: 94502

94502 - An Efficient Time-Domain Model to Simulate Parametric Resonances in a Floating Body Free to Move in Six Degrees of Freedom 

Many wave energy converters oscillate in modes other than their power absorbing mode, making them prone to parametric resonances. When these happen, motions in the non-absorbing modes are amplified, while motions in the absorbing modes are usually diminished. Parametric resonance can thus dramatically impair the performance of wave energy converters, both in terms of power production and loads associated with amplified motions. It is therefore important to be able to predict and quantify the effect of parametric resonance on wave energy converters with sufficient accuracy in order to make the necessary design adjustments to rectify or avoid it as much as possible. 

In this paper, we present a computationally efficient time-domain model of a floating body under wave action. The body can be thought of as a representation of a generic point absorber. The time-domain model assumes all wave forces to be linear but considers inertia and restoring forces up to second order in the body motions. This introduces couplings between modes of motion which are not normally present in a completely linear model. The wave excitation and radiation forces are otherwise linear, and the excitation coefficients, added mass and radiation damping coefficients can be evaluated using standard linear diffraction and radiation solvers, without any additional post-processing compared to a linear time-domain model. This results in a very efficient model. The simulation speed on a standard computer is approximately 40 times faster than real time.

The model is applied to a floating axisymmetric body which absorbs energy through heave, but otherwise is free to move in six degrees of freedom. Horizontal springs with low stiffness represent the mooring lines and provide restoring stiffness in surge, sway, and yaw. Under certain conditions, we show that the floating body responds parametrically with large amplitudes in modes other than heave, including in the out-of-plane modes such as sway and yaw, provided it is given some small initial displacement in one of these modes. These motions have oscillation periods different from the wave periods, which is typical for parametrically excited motions. Long-period and large-amplitude yaw motions are also observed, which may be the first time such motion is reported for a floating axisymmetric body like this. Although the predictions have not been validated against physical measurements in this paper, they have been confirmed by simulations using a state-of-the-art computational fluid dynamics model. Validations through physical model testing is planned as a future work. 

​​​

Presenting Author: Thanh Toan Tran NREL

Presenting Author Biography: Thanh Toan Tran received a Ph.D. in Aerospace Engineering from Gyeongsang National University, South Korea, in 2016. Before that, he received his B.Sc. from the Ho Chi Minh City University of Technology in 2008. His graduate research has focused on high-fidelity fluid-structure interaction simulation of floating offshore wind turbines considering the influence of aero-hydrodynamic coupling.<br/><br/>Prior to NREL, he was a Postdoctoral Researcher at UC Berkeley and has developed a high-fidelity simulation of wave energy converters. At NREL, he is working on a combination of marine energy (ME) and offshore wind (OSW) projects. On the ME side, he is involved in the development of numerical tools for wave energy converter devices. On the OSW side, he is working on verification and validation of offshore wind platforms using computational fluid dynamics simulation. Specifically, he focuses on better understanding the physical behaviors of the system at the low-frequency regime.

Authors:

Adi Kurniawan The University of Western Australia
Thanh Toan Tran NREL
Yi-Hsiang Yu National Yang Ming Chiao Tung University