Reliability assessment of wave energy devices

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Wave Energy has a large potential for contributing to sustainable electricity production. Compared to other renewable energy sources like wind and solar power, the electricity production costs from wave energy are at present larger and less competitive. Therefore the cost of energy for electricity from wave energy converters needs to be decreased. One possibility to contribute to this problem is by finding an optimal and cost-optimised reliability level for wave energy devices (WEDs). Optimised reliability levels can be found using probabilistic design methods where explicit account for uncertainties connected to loads, strengths and calculation methods are considered. Sorensen et al. (2011) presented a framework for probabilistic design reliability analysis of wave energy devices which will be used as basis for this paper. At the moment there are many different WED concepts under development. For illustration in this paper the Wavestar power plant will be used. A prototype was installed in 2009 near Hanstholm at the west coast of Denmark and is feeding in electricity to the grid since January 2010. The strategy for the reliability assessment is divided into three major steps. In a first step the environmental conditions (wave and wind conditions) at Hanstholm are assessed. Based on the environmental conditions the loads onto the structure is calculated in a second step. Loads are calculated for different ultimate limit states (ULS) defined by a fault tree analysis (FTA). The fault tree analysis and the definition of ultimate limit states focuses on the following load cases for the structure:  Wave and wind loads during normal operation.  Wave and wind loads during operation simultaneous with failure of electrical components.  Wave and wind loads during operation simultaneous with failure of mechanical components.  Wave and wind loads during operation simultaneous with failure of control system. Other load cases are:  Wave and wind loads when the WED is in a ‘parked’ position.  Wave and wind loads during operation simultaneous with loss of grid. In a third step the reliability of the WED plant is assessed by using a probabilistic approach. The probabilistic reliability assessment accounts for the following uncertainties:  Uncertainties due to natural randomness of the parameters considered.  Uncertainties due to imperfect measurements (e.g. wave measurement device far away from location of WED).  Uncertainties due to imperfect knowledge of the mathematical model.  Uncertainties due to limited sample sizes.  Uncertainties about choice of probability distribution types. The probability of failure F P for a certain ultimate limit state is described by a limit state function and is derived by FORM/simulation methods. Based on the probability of failure the reliability index  is determined. The derived reliability index is then compared with reliability indices from related industries like offshore wind turbines. This enables to make suggestions and recommendations about the design of the WED. The different failure probabilities of the different failure modes are assessed by a cost-benefit consideration in order to assess the cost-optimal reliability level assuming that human safety issues are not important for the type of wave energy devices considered. In illustrative examples with the Wavestar device the importance of different choices in the stochastic modeling and different cost models are considered.