There are many different working principles for wave energy converters (WECs) which are used to produce electricity from waves. In order for WECs tobecome 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 andmaintenance (O&M) costs as well as the initial investment costs.Furthermore, there is a control system for WEC applications which defines theharvested energy but also the loads onto the structure. Therefore, extremeloads but also fatigue loads are important to the structural designs of WECdevices. The extreme loads on WEC structures during extreme environmentalconditions can be limited by moving the device in storm protection/idle modewhere no electricity is produced.
Structural reliability assessments use a probabilistic approach that includes uncertaintiesrelated to the limited amount of data and the considered models used tocalculate 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 structuresare mainly based on experiences from offshore wind turbines as well asstructures used in oil and gas industry. But nevertheless, WEC-specificuncertainties need to be quantified, and the required structural reliabilitylevels, which are important to e.g. the calibration of safety factors, need tobe defined for WEC applications.
O&M operations can be performed preventively or correctively. Access to thedevice can be by boat, which is cheaper, but takes longer and is limited by thewave characteristics or by helicopter, which is more expensive but faster than byboat, and is also constricted to small cargo and its operation is limited by thewind speed.
Many WECs have a storm protection mode/idle mode where the extreme loads duringextreme environmental conditions are minimized. Therefore, extreme loads may occurduring operation where electricity is produced. Hence, loads during operationcan be extrapolated to extreme loads during operation. The control systemmainly drives fatigue loads during operational mode. Since WEC devices areunmanned and have limited access (mainly during winter months) due to strongenvironmental conditions, failure modes where electrical/mechanical componentsas well as the control system fail and lead to abnormal loads on to thestructure 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 thesisquantifies the uncertainties related to the wave and wind condition assessmentsas well as the uncertainties linked to long-term and extreme environmentalmodeling. Different ultimate limit states as well as fatigue limit states areconsidered in various papers. The study will estimate the influence of systemfailure (failure of electrical/mechanical components or the control system) instructural design considering the most critical system failure modes of theWavestar device. Furthermore, extreme mooring loads are extrapolated frommeasurements of the lab-scaled floating WEC WEPTOS. Calibration of safetyfactors are performed for welded structures at the Wavestar device includingdifferent control systems for harvesting energy from waves. In addition, a casestudy of different O&M strategies for WECs is discussed, and an example ofreliability-based structural optimization of the Wavestar foundation ispresented.
The work performed in this thesis focuses on the Wavestar and WEPTOS WEC deviceswhich are only two working principles out of a large diversity. Therefore, inorder to gain general statements and give advice for standards for structuralWEC designs, more working principles should be investigated using themethodologies presented in this thesis.