Interaction Between Mooring Line Damping and Response Frequency as a Result of Stiffness Alteration in Surge

The design and operation of a chain mooring for a wave energy converter (WEC) is considered. Experimental measurements of a mooring line were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The laboratory procedures were designed to resemble tests undertaken earlier in the year at ‘full’ scale in 24 m water depth. This paper describes and compares these measurements and relates the results to earlier work on mooring lines by Webster [1]. Measurements of both the damping and response frequencies of the mooring are described. Although the present results support partly the conclusions of the earlier work, care must be taken in how these are applied when one is considering mooring line design for WECs. It is concluded that there are significant differences for a WEC for both operational and limit state design in comparison with a more conventional offshore system such as an FPSO or CALM. Although the primary requirement is still one of station-keeping two further considerations may be of great importance. Firstly if a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere with one another and this will have great significance for the loading experienced by the line. This can be exacerbated by variations in tidal range which will have a larger effect in comparison with a conventional deepwater mooring. A second factor may arise if the mooring system response is critical to the WEC energy extraction process. If the mooring becomes part of the ‘tuned’ system then changes in the mooring properties of damping and natural frequency could seriously affect energy conversion efficiencies.Copyright © 2006 by ASME