Study of Mooring Systems for Offshore Wave Energy Converters

Wave energy converters need to be secured to prevent them form drifting away in the sea or getting damaged. In offshore regions, with intermediate to deep waters, the use of mooring systems to keep the converters on station is an attractive solution because of its reduced cost. Due to the specificities of offshore wave energy converters (for example, the need to oscillate in resonance), the development of an appropriate mooring solution requires the study of mooring configurations different from those used in typical offshore structures. To provide some assistance in this task, three different outcomes are presented: a numerical model for mooring cables; a technique to track mooring cables in small scale models; and results of experimental tests of small scale models of different mooring configurations. The numerical model uses the spectral/hp-element discontinuous Galerkin method to solve the partial differential equation of perfectly flexible cables. It has exponential convergence to the solution and exhibits shock capturing features. The tracking technique records underwater videos of mooring cables in small scale models. These videos are processed using an especially developed algorithm that determines and outputs the geometry of the cable, represented by its medial axis, as a function of time. The cable is detected in its full length, and not only at selected points, so there is no need for interpolation to estimate its true shape. The mooring configurations studied in small scale physical models are the chain catenary, and two compact configuration using taut cables, floaters and clumpweights. The physical model is composed of a cylindrical buoy representing a generic wave energy converter that is kept on station by the different mooring configurations studied. Data is provided about the maximum and minimum tension in the cables, the displacements of the buoy, the response amplitude operators for the three configurations, among other parameters, when the model is subjected to regular and irregular waves. The tools and results mentioned allow researchers and developers to get a deeper insight into the dynamic behaviour of mooring cables, and to have more information in the selection of an appropriate mooring configuration. The numerical model makes it possible to carry out simulations of moored devices in highly dynamic conditions using coarse discretisations. The technique to track mooring cables can be used to validate numerical simulations, as well as to get a better understanding of the most active regions of mooring cables, accurate estimates of damping, etc. The results from the experiments pin-point the advantages and disadvantages of the three configurations studied, and make it easier to select the best one for a specific device and situation.