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This paper analyses the nonlinear forces on a moored point-absorbing wave energy converter (WEC) in resonance at prototype scale (1:1) and at model scale (1:16). Three simulation types were used: Reynolds Averaged Navier-Stokes (RANS), Euler and the linear radiation-diffraction method (linear). Results show that when the wave steepness is doubled, the response reduction is: (i) 3% due to the nonlinear mooring response and the Froude-Krylov force; (ii) 1-4% due to viscous forces; and (iii) 18-19% due to induced drag and non-linear added mass and radiation forces. The effect of the induced drag is shown to be largely scale-independent. It is caused by local pressure variations due to vortex generation below the body, which reduce the total pressure force on the hull. Euler simulations are shown to be scale-independent and the scale effects of the WEC are limited by the purely viscous contribution (1-4%) for the two waves studied. We recommend that experimental model scale test campaigns of WECs should be accompanied by RANS simulations, and the analysis complemented by scale-independent Euler simulations to quantify the scale-dependent part of the nonlinear effects.

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Newcastle University ePrints -eprint.ncl.ac.uk Yang S, Ringsberg JW, Johnson E, Hu ZQ. A comparison of coupled and decoupled simulation procedures for the fatigue analysis of wave energy converter mooring lines. Ocean Engineering 2016, 117, 332-345. AbstractMooring systems for floating wave energy converters (WECs) must be designed to survive the cyclical loads and motions to which they are subjected as a result of the wave load-WEC interaction and the motions of the WEC in the random elevation of the sea surface. The current study compares simulation procedures for the fatigue analysis of WEC moorings. The objective is to suggest a simulation procedure suitable for an extensive study of the behaviour of WEC systems and thus for the fatigue analysis of mooring lines. A cylindrical, floating WEC with four spread mooring lines is chosen as case study. The dynamics of the WEC system are simulated and analysed using both coupled and de-coupled models in the timedomain. Four types of simulation procedures are compared using commercial software, DNV DeepC, and an in-house solver, MooDy. A systematic fatigue response analysis based on variations of the numerical and physical parameters is conducted. The results are compared with respect to the fatigue damage calculated using the stress-based approach and the rainflow counting method. The simulation procedure using a coupled model is suggested as the preferred numerical method for capturing the interaction between the components of WEC system, which is proven to be crucial for the fatigue damage evaluation of mooring lines.

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