Abstract
Mooring systems are among the most critical sub-systems for floating marine energy converters (MEC). In particular, the occurrence of peak mooring loads on MEC mooring systems must be carefully evaluated in order to ensure a robust and efficient mooring design. This understanding can be gained through long-term field test measurement campaigns, providing mooring and environmental data for a wide range of conditions. This paper draws on mooring tensions and environmental conditions that have been recorded (1) for several months during the demonstration of an MEC device and (2) over a period of 18 months at a mooring test facility. Both systems were installed in a shallow water depth (45 m and 30 m, respectively) using compliant multi-leg catenary mooring systems. A methodology has been developed to detect peak mooring loads and to relate them to the associated sea states for further investigation. Results indicate that peak mooring loads did not occur for the sea states on the external contour line of the measured sea states, but for the sea states inside the scatter diagram. This result is attributed to the short-term variability associated with the maximum mooring load for the given sea state parameters. During the identified sea states, MEC devices may not be in survival mode, and thus, the power take-off (PTO) and ancillary systems may be prone to damage. In addition, repeated high peak loads will significantly contribute to mooring line fatigue. Consequently, considering sea states inside the scatter diagram during the MEC mooring design potentially yields a more cost-effective mooring system. As such, the presented methodology contributes to the continuous development of specific MEC mooring systems.
Highlights
The use of renewable energy has been increasing over the last 40 years and will keep increasing because of climate change concerns, the rise of the price of fossil resources, the need for energy independence and growing energy demand
The key results of the assessment of environmental conditions associated with peak mooring load conditions are presented for the South West Mooring Test Facility (SWMTF) and for the Bolt-2 Lifesaver installation
The general environmental conditions are assessed at both sites, by plotting the contours of the joint percentage of occurrences of the measured general environmental conditions
Summary
The use of renewable energy has been increasing over the last 40 years and will keep increasing because of climate change concerns, the rise of the price of fossil resources, the need for energy independence and growing energy demand. The main challenge for the development of marine renewable energy is to be cost effective. The predicted electricity generating costs of marine energy converters (MEC) have been significantly decreasing in the last twenty years [1]. Floating MECs require mooring systems, which are essential for the MEC’s survival. The cost of the mooring systems is much more critical for an MEC project than for an oil and gas project. In the example given by Fitzgerald [2], for an oil and gas project, the mooring costs represent less than 2% of the lifetime costs, while the mooring costs are equal to 18% of the wave energy project lifetime costs
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