D6.4 Fatigue Assessment
This document constitutes Deliverable ‘D6.4 Fatigue assessment” of the LiftWEC project. LiftWEC is a collaborative research project funded by the European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement No 851885. It is the intention of the project consortium that the LiftWEC project culminates in the development of one or more promising configurations of a Wave Energy Converter operating through the use of one or more rotating hydrofoils that generate lift as the primary interaction with the incident waves.
In this report, a fatigue assessment is carried out on LiftWEC. The analysis is performed first through stress-life approaches in irregular sea-states. Firstly, damage is quantified through a probabilistic approach. It is assumed that LiftWEC operates at constant velocity which is equal to the peak frequency of a JONSWAP wave energy spectrum. Secondly, the damage quantified through the conventional time deterministic approach is compared to that computed through the probabilistic approach, yielding satisfactory results.
Critical areas of stress concentration are defined as “hot-spots”. In this report, we illustrate the damage computation methodology in one hot-spot. The hotspot is the fixed end of the foil for a rotor with the foil supported at both ends. We demonstrate the effect of passive structural dynamics in the damage that occurs in this hot spot. This is with the aim to develop methods that retard or avoid complete structural failure.
We compare the damage and power output in different sea states. Although, it is expected that more power translates into more damage, it is found that this is not the case in the selected hot spot. This is because power is a function of the tangential force, whilst the damage is a function of the radial force imposing the bending moments on the hydrofoils.
In terms of structural dynamics and their impact in the damage of the hot spot, it is found that passive compliance of the support structure and passive radial motion of a single foil do not alter significantly the power and damage footprints of the hot spot when the device is operated with constant speed in irregular seas. In contrast, it is found that a passively pitching foil can help in reducing the damage on the hot spot, although incurred at a penalty in power production.
It is the intention of this deliverable to provide guidance and a methodology to assess the fatigue life of a LiftWEC device, by utilising realistic sea-state simulations. This can help in designing strategies for coping with load variability and extending the fatigue life of certain components through passive or active actuation.