For the safe operation of floating wave energy converters (WEC's) it is required to obtain deeper understanding of the moored device as a coupled system. The motion of a WEC is dependent on the stiffness, mass and the damping characteristics of the body and the mooring system, as well as influenced by the power take of system. In order to understand the effect of the individual contributions to the coupled system, the influence of the individual parameters needs to be understood. Typical offshore oil and gas installations, in moderate water depths, are often moored using catenary configurations within linear load-extension characteristics. In many applications the damping contribution due to the mooring system is within a linear regime and hence can be applied in a linear fashion. Earlier investigation relating to mooring damping and stiffness can be found in [1-6]
Due to their physical size and the wish to install the WEC in an energetic environment, there is an increased risk that the moor will be operating in a load-extension range with non-linear characteristics. Furthermore, it is likely that for the station keeping of WECs compliant configurations are used with fibre rope sections. These would have an effect on the stiffness and damping characteristic of the mooring system [7, 8], which could importantly affect the coupled motion of a moored WEC device. The effect towards the power conversion and/or ultimate peak loads needs to be understood to allow for a safe station keeping that does not adversely affect the energy conversion.
A detailed investigation of mooring line stiffness and damping properties has been conducted at large scale tests in the Scapa Flow at Orkney. Three different mooring arrangements namely, compliant chain, compliant hybrid and nylon rope s-shape were investigated. Preliminary analysis was presented to indentify the stiffness characteristics for these large scale mooring tests in open sea environment . This publication presents the continuation of the study by the second author analysing outstanding data from the Scapa Flow test series [9,10,11]. .Within the analysis presented here the main focus will be given to the energy dissipation (damping), comparing the different arrangements using the 'indicator-diagram method'. The intention of this study is to identify the damping properties of the individual mooring arrangements in order to provide information towards the analysis of a coupled system.
The outcome will present the dissipated energy (damping) results for the tests and will relate them to the preliminary stiffness analysis. This will then be used to discuss the suitability of these mooring arrangements with respect to the requirements of different WEC devices. It is also hoped to provide further information on the variability of the damping as a result of a change in frequency characteristics.