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ABSTRACT: In a number of experiments and field tests of point absorbers, snap loads have been identified to cause damage on the mooring cables. Snap loads are basically propagating shock waves, which require special care in the numerical modeling of the mooring cable dynamics. In this paper we present a mooring cable model based on a conservative formulation, discretized using the Runge-Kutta discontinuous Galerkin method. The numerical model is thus well suited for correctly capturing snap loads. The numerical model is verified and validated using analytic and experimental data and the computed results are satisfactory.
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1 INTRODUCTION
Floating Wave Energy Converters (WECs) of Point Absorber (PA) type need to be designed to survive in highly energetic seas, often in relatively shallow water. PAs are small devices in relation to the wave length of the prominent waves and are typically designed to move with large amplitudes near resonance in one or more modes of motion. The combination of large amplitude motion, a small device displacement and limited water depth makes it difficult to design the mooring system so that snap loads can be avoided. Snap loads have been seen to do large damage in several experiments and field tests on WECs (Thies et al. 2012, Harnois 2014, Savin et al. 2012). However, measurements are sometimes difficult to interpret and clear conclusions regarding the exact nature of the loads are still lacking (Harnois 2014). It is therefore important that the numerical methods used for PAs are able to accurately handle snap loads events. The correct prediction of snap loads is vital in order to be able to take suitable measures to avoid or mitigate the effect of snap loads in mooring lines.
Snap loads often occur when a mooring line is retightened after a period of slack. The load is essentially a discontinuous shock wave whose amplitude can be very high depending on the line local strain rate and the line material stiffness (Hennessey et al. 2005). Mooring line materials generally have negligible bending stiffness when compared to axial tension, which makes their dynamics governed by a hyperbolic equation, see e.g. Montano et al. (2007). The shock wave problem is a well studied mathematical field governed by well-known theorems (Lax &...