Point absorber wave energy converters (WEC) are subjected to random wave loads. In addition, the power production of a WEC and the motions are considerably influenced by the applied control mechanism. For small waves, with a wave period close to the natural period of the oscillating system, the power output may be controlled passively by means of a constant damping coefficient. The energy is extracted proportionally to the square of the body’s velocity. If the wave period is away from the natural period, reactive power may be applied in order to enlarge the resonance bandwidth. Recent studies on a point absorber have shown that the stresses at a particular section of the structure depend on the control parameters. The power is increased by choosing a more advanced control mechanism. The consequences are that the stress amplitudes are higher than for the more conservative control case. In this study, the focus is given on the fatigue damage calculation of a structural detail by taking into account the control parameters of the power take-off system. The predicted fatigue damage is calculated based on the spectral approach. Finally the question will be answered which control strategy is more favorable regarding the trade off between the fatigue damage and power production of the wave energy device.

In this paper the fatigue behavior of laser welded T-joints of stainless steel AISI304 is investigated experimentally. In the fatigue experiments 36 specimens with a sheet thickness of 1 mm are exposed to one-dimensional cyclic loading. Three different types of specimens are adopted. Three groups of specimens are used, two of these are non-welded and the third is welded with a transverse welding (T-Joint). The 13 laser welded specimens are cut out with a milling cutter. The non-welded specimens are divided in 13 specimens cut out with a milling cutter and 10 specimens cut out by a plasma cutter. The non-welded specimens are used to study the influence of heat and surface effects on the fatigue life. The fatigue life from the experiments is compared to fatigue life calculated from the guidelines in the standards DNV-RP-C203 and EUROCODE 3 EN-1993-1-9. Insignificant differences in fatigue life of the welded and non-welded specimens are observed in the experiments and the largest difference is found in the High Cycle Fatigue (HCF) area. The specimens show a lower fatigue life compared to DNV-RP-C203 and EUROCODE 3 EN-1993-1-9 when the specimens are exposed to less than 4.0 1E06 cycles. Therefore, we conclude that the fatigue life assessment according to the mentioned standards is not satisfactory and reliable.

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This paper investigates the effect of a passive and reactive control mechanism on the accumulated fatigue damage of a wave energy converter (WEC). Interest is focused on four structural details of the Wavestar arm, which is used as a case study here. The fatigue model is set up as an independent and generic toolbox, which can be applied to any other global response model of a WEC device combined with a control system. The stress responses due to the stochastic wave loads are computed by a finite element method (FEM) model using the frequency-domain approach. The fatigue damage is calculated based on the spectral-based fatigue analysis in which the fatigue is described by the given spectral moments of the stress response. The question will be discussed, which control case is more favorable regarding the tradeoff between fatigue damage reduction and increased power production.