Expanding on existing methodologies for the design optimization of mooring systems for offshore renewable energy devices, this paper explores the optimization of the axial stiffness of a mooring system, a key property that governs the motion of the moored renewable energy device. The optimization using a covariance matrix adaptation evolutionary strategy is executed with respect to both the motion response of the moored device, and the cumulative fatigue damage in the mooring lines. Previous mooring system optimization work has focused on geometry optimization of the system considering known material properties, whilst this paper explores the optimization of the axial stiffness properties of the mooring system identifying ways in which the response of the system can be optimized through changes in the material properties. Considering the case of a moored heaving buoy similar to an offshore renewable energy device, the present case study optimizes the mooring system simultaneously minimizing the cumulative fatigue damage in the mooring lines while also maximizing the heave response which is responsible for the energy generated by the device.


As society moves to reduce the greenhouse gas emissions associated with electricity generation, renewable energy and in particular offshore renewable energy devices are seen as playing an increasing role in achieving the global emissions targets. Though offshore renewable energy continues to be deployed at increasing rates, it still remains a relatively nascent field which requires innovative engineering solutions in order to be cost competitive with more traditional energy sources. With this aim in mind, optimization techniques are increasingly being applied to the design and operation of offshore renewable energy devices in order to maximize the relative efficiency of these developments and identify novel solutions to the unique challenges faced by this sector (Baños et al., 2011, Iqbal et al., 2014).

In recent years as the number of offshore wind farms developed has rapidly increased, the closer to shore shallow water sites are approaching capacity. As a result of this, floating offshore wind turbines, which can operate further from shore and in deeper waters have become a major research focus. A vital component of these floating offshore renewable energy devices are the mooring systems which at the same time keep the device on station for the desired operational life while at the same time allowing the required motion from which energy can be generated. The mooring system therefore represents a key component which must be carefully designed to suit both the device and the site at which the device is being deployed (Harris et al., 2004, Johanning et al., 2006b).

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