Floating wave energy converters (WECs) operating in the resonance region are strongly affected by non-linearities arising from the interaction between the waves, the WEC motion and the mooring restraints. To compute the restrained WEC motion thus requires a method which readily accounts for these effects. This paper presents a method for coupled mooring analysis using a two-phase Navier–Stokes (VOF–RANS) model and a high-order finite element model of mooring cables. The method is validated against experimental measurements of a cylindrical buoy in regular waves, slack-moored with three catenary mooring cables. There is overall a good agreement between experimental and computational results with respect to buoy motions and mooring forces. Most importantly, the coupled numerical model accurately recreates the strong wave height dependence of the response amplitude operators seen in the experiments

A numerical method that coupled a Reynolds-averaged Navier-Stokes solver with a quasi-static and a dynamic mooring model was employed and compared. Decay tests of the freely floating and the moored buoy in calm water determined the system's linear and quadratic damping characteristics. In regular waves, predicted heave and pitch amplitudes and high-frequency surge amplitudes based on the quasi-static mooring model were similar to those based on the dynamic mooring model and similar to comparable measurements. However, the low-frequency surge and pitch motions based on the quasi-static model deviated significantly from the corresponding motions based on the dynamic model and from measurements. In irregular waves, buoy motion amplitudes, especially in surge, based on the quasi-static model were overpredicted. The accurate evaluation of the moored buoy in irregular waves, especially when the moored buoy underwent large translations, was possible only by accounting also for the dynamic loads acting on the mooring lines.

Mooring systems exhibit high failure rates. This is especially problematic for offshore renewable energy systems, like wave and floating wind, where the mooring system can be an active component and the redundancy in the design must be kept low. Here we investigate how uncertainty in input parameters propagates through the mooring system and affects the design and dynamic response of mooring and floaters. The method used is a nonintrusive surrogate based uncertainty quantification (UQ) approach based on generalized Polynomial Chaos (gPC). We investigate the importance of the added mass, tangential drag, and normal drag coefficient of a catenary mooring cable on the peak tension in the cable. It is found that the normal drag coefficient has the greatest influence. However, the uncertainty in the coefficients plays a minor role for snap loads. Using the same methodology we analyze how deviations in anchor placement impact the dynamics of a floating axi-symmetric point-absorber. It is shown that heave and pitch are largely unaffected but surge and cable tension can be significantly altered. Our results are important towards streamlining the analysis and design of floating structures. Improving the analysis to take into account uncertainties is especially relevant for offshore renewable energy systems where the mooring system is a considerable portion of the investment.

Industrial realization of both floating offshore wind and wave energy technologies requires reductions in the current high levelized cost of energy. Reducing mooring fatigue loads could decrease levelized cost of energy, as mooring is expected to be a major cost of these systems. Previous work on improving mooring reliability and costs has focused on material and design. In this exploratory study, we quantify how placing WECs in front of a FOWT could reduce fatigue damage incurred by FOWT mooring cables in long-crested wave conditions. We use SWAN to quantify the WEC-induced sea state modifications and obtain wave spectra at the FOWT location. The spectra are then input into WEC-Sim and MooDy to model the mooring cable behavior. Fatigue analysis with Rainflow Counting is used to quantify the fatigue loading on the mooring cables. Results from this study show a 8% reduction in fatigue damage to mooring cables over the lifetime of the structure. These results indicate that co-location could have a beneficial effect on FOWT mooring cable fatigue. In future work, these results will be leveraged to perform optimal O&M planning and reliability-based design optimization of floating offshore wind turbines.