Reliability of Wave Energy Converters: revised version

There are many different working principles for wave energy converters (WECs) which are used to produce electricity from waves. In order for WECs to become successful and more competitive to other renewable electricity sources, the consideration of the structural reliability of WECs is essential. Structural reliability considerations and optimizations impact operation and maintenance (O&M) costs as well as the initial investment costs. Furthermore, there is a control system for WEC applications which defines the harvested energy but also the loads onto the structure. Therefore, extreme loads but also fatigue loads are important to the structural designs of WEC devices. The extreme loads on WEC structures during extreme environmental conditions can be limited by moving the device in storm protection/idle mode where no electricity is produced. Structural reliability assessments use a probabilistic approach that includes uncertainties related to the limited amount of data and the considered models used to calculate the loads/stresses as well as uncertainties given by Mother Nature (e.g. inter-annual variation of extreme values) and measurement uncertainties. Due to limited amount of knowledge, reliability considerations for WEC structures are mainly based on experiences from offshore wind turbines as well as structures used in oil and gas industry. But nevertheless, WEC-specific uncertainties need to be quantified, and the required structural reliability levels, which are important to e.g. the calibration of safety factors, need to be defined for WEC applications. O&M operations can be performed preventively or correctively. Access to the device can be by boat, which is cheaper, but takes longer and is limited by the wave characteristics or by helicopter, which is more expensive but faster than by boat, and is also constricted to small cargo and its operation is limited by the wind speed. Many WECs have a storm protection mode/idle mode where the extreme loads during extreme environmental conditions are minimized. Therefore, extreme loads may occur during operation where electricity is produced. Hence, loads during operation can be extrapolated to extreme loads during operation. The control system mainly drives fatigue loads during operational mode. Since WEC devices are unmanned and have limited access (mainly during winter months) due to strong environmental conditions, failure modes where electrical/mechanical components as well as the control system fail and lead to abnormal loads onto the structure should also be accounted for in the structural design. Before using a probabilistic reliability approach to start optimizing WEC structures, WEC-specific uncertainties need to be found and quantified. This thesis quantifies the uncertainties related to the wave and wind condition assessments as well as the uncertainties linked to long-term and extreme environmental modeling. Different ultimate limit states as well as fatigue limit states are considered in various papers. The study will estimate the influence of system failure (failure of electrical/mechanical components or the control system) in structural design considering the most critical system failure modes of the Wavestar device. Furthermore, extreme mooring loads are extrapolated from measurements of the lab-scaled floating WEC WEPTOS. Calibration of safety factors are performed for welded structures at theWavestar device including different control systems for harvesting energy from waves. In addition, a case study of different O&M strategies for WECs is discussed, and an example of reliability-based structural optimization of the Wavestar foundation is presented. The work performed in this thesis focuses on the Wavestar and WEPTOS WEC devices which are only two working principles out of a large diversity. Therefore, in order to gain general statements and give advice for standards for structural WEC designs, more working principles should be investigated using the methodologies presented in this thesis.