The green energy industry is more and more concerned due to global warming effect and oil shortage. Besides wind, tidal and solar power, the ocean wave energy is also an important renewable green energy resource. Based on the floating object motion theory in waves, many floating wave energy converting devices are invented. One concept developed by Pelamis, which is now been commercially applied, is tohinge several cylinders with universal joints as a floating bodies system. The joint forces between the floating bodies can be converted into electricity power using generators inside. The hydrodynamic motion behavior and joint force of the system needsto be predicted to analyze the best power conversion efficiency and design the safe structure for the machinery.

In this paper, a frequency domain analysis method based on potential theory is applied in calculating the wave frequency motion of the floating wave energy convertor system. The incoming wave is simplified using linear sinusoidal wave. The system motion is assumed to be linear. The hydrodynamic forces of radiation and diffraction are solved using three dimensional pulsing source method. The joint forces between floating bodies are solved together with the motions.

In order to analyze the mooring line force of this device when in survival condition, two ways are used. One way is using frequency domain method to solve the higher order hydrodynamics and the mooring forces. The other is the time domain analysis method, in which the impulse response method is applied. The hydrodynamic coefficients of frequency domain are applied in time-domain analysis. The second order slowly varying force is not taken into consideration in the time domain analysis. The mooring line is assumed to be purely elastic.

Two small scale model tests are performed. One model, with flexible joints, is used to test the frequency response of the motions. The other model is equipped with fixed joints is used for testing the mooring line force in heavy irregular waves. The experiment results are compared with the numerical results.

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