Application of the Most Likely Extreme Response Method for Wave Energy Converters

Abstract

Extreme loads are often a key cost driver for wave energy converters (WECs). As an alternative to exhaustive Monte Carlo or long-term simulations, the most likely extreme response (MLER) method allows mid- and high-fidelity simulations to be used more efficiently in evaluating WEC response to events at the edges of the design envelope, and is therefore applicable to system design analysis. The study discussed in this paper applies the MLER method to investigate the maximum heave, pitch, and surge force of a point absorber WEC. Most likely extreme waves were obtained from a set of wave statistics data based on spectral analysis and the response amplitude operators (RAOs) of the floating body; the RAOs were computed from a simple radiation-and-diffraction-theory-based numerical model. A weakly nonlinear numerical method and a computational fluid dynamics (CFD) method were then applied to compute the short-term response to the MLER wave. Effects of nonlinear wave and floating body interaction on the WEC under the anticipated 100-year waves were examined by comparing the results from the linearly superimposed RAOs, the weakly nonlinear model, and CFD simulations. Overall, the MLER method was successfully applied. In particular, when coupled to a high-fidelity CFD analysis, the nonlinear fluid dynamics can be readily captured.

A Wave Energy Converter Design Load Case Study

“…The grid shown in Figure 5 was attained with grid resolution and convergence studies, including those reported in [2], an example of which is provided in Figure 6. The refined grid regions were established based on guidance from STAR-CCM+ [24], previously reported extreme sea state CFD studies [6,9,[25][26][27][28], accurately modeling the wave propagation, minimizing wave reflections, minimizing y+ on the RM3 model surface, and accurately resolving the velocity gradients around the model while keeping the total number of cells at a minimum. The final grid resolutions at the water surface and total number of cells for each case are reported in Table 3.…”

“…There was an average y + value of 3.18 on the RM3 surface. Of the total number of cells reported in Table 3 for each case, ∼3.3 × 10 6 were for the FEA model, where a large number of elements were required to model the slender support structures and accurately map the fluid pressure and shear forces. An example of the coupled FEA model results, corresponding to the same simulation and point in time as Figure 5, is given in Figure 7.…”

“…Another approach to reducing the simulation timeframe is the use of an equivalent design, or focused wave. Using this approach, a series of waves are identified that will generate the most-likely extreme wave or WEC response for a specified sea state [6].…”

A Survey of WEC Reliability, Survival and Design Practices

“…A number of studies have applied the New Wave approach in wave tank testing of WECs [53][54][55][56][57]. The most-likely-extreme-response (MLER) method was used for a WEC by Quon et al [58]. This approach utilizes both the desired spectrum and an estimate for the linear response of the system to define a design-wave.…”

“…Thies et al found large uncertainties in the spread mooring system's line fatigue loads. A number of studies have also considered fatigue in WEC design in conjunction with the assessment of control strategies [58,109].…”