Inspection and maintenance costs are significant contributors to the cost of energy for wave energy converters. Maintenance can be performed after failure (corrective) or before a breakdown (preventive) occurs. Furthermore, helicopter and boat can be used to transport equipment and personnel to the device for operation and maintenance actions. This article focusses on a risk-based inspection and maintenance planning approach involving minimization of the overall repair costs including costs due to lost electricity production. The study includes real weather data and damage accumulation as well as uncertainties related with imperfect weather forecasts, costs, structural damage accumulation, inspection accuracy and the applied maintenance strategies. This article contains a case study where the risk-based maintenance strategy is applied for the Wavestar device.
Wave energy converters (WECs) use the wave energy as a resource in order to produce electricity. There are working principles that exist on prototype level and are expected to reach commercialized level soon. When moving from prototype level to commercialization, the operation and maintenance (O&M) costs become important. Based on studies available in Carbon Trust (2006), O&M costs reach 57 % of the operational costs for a specific WEC. Maintenance actions depend on the weather conditions as they define the accessibility by boat/helicopter. Not only high failure rates of the installed components on a WEC but also the limited accessibility drive O&M costs for WECs. The total resulting O&M costs can be minimized when considering risk-based O&M strategies (Sørensen, 2009).
For WECs cost optimal O&M strategies are important as WECs are placed in harsh environmental locations and the device might not be accessible for a couple of weeks (during winter months). A risk-based O&M approach considers repair costs as well as the lost income from no electricity production in order to minimize the overall expected maintenance costs.
There are studies on risk-based maintenance and operation actions available for offshore wind turbines (see e.g. Nielsen and Sørensen, 2011; Nielsen, 2013) and for WECs (Ambühl et al., 2015). But these studies assume perfect weather forecast, which is not the case for real applications. In order to better estimate the total expected maintenance costs, the weather forecast should be assumed to be an uncertain parameter. Uncertain weather forecast may lead to longer waiting times as well as cancellation and postponing of maintenance actions due to stronger environmental conditions than forecasted.
This paper accounts for risk-based maintenance and operation actions taking weather forecast uncertainties into account. The paper is based on the methodology presented in Ambühl et al. (2015) and presents the applicability of the methodology with an example that focusses on the Wavestar WEC that is placed at Danish North Sea Westcoast.