Hydrodynamics of the multi-float wave energy converter M4 with slack moorings: Time domain linear diffraction-radiation modelling with mean force and experimental comparison

•Multi-float multi-mode linear diffraction modelling extended to include mean forces.•Mean forces due to energy absorption by mechanical and radiation damping included.•Model results compared with basin experiments for 6-float wave energy converter M4.•Device response and power approximately predicted in all waves including extreme.•Mooring forces predicted reasonably in moderate waves but not in large waves. The response and power capture of the multi-float wave energy converter M4 has been shown to be approximately predicted by linear diffraction-radiation modelling, giving relatively high capture widths. In this study we model a 6-float configuration which has been investigated experimentally in irregular waves and take account of the mooring system. Second-order mean forces are included due to excitation forces as is standard, but also due to the rate of energy extraction from mechanical damping and radiation damping resulting from body motion. There are also second order effects due to coupling between heave and surge. Operational conditions with mechanical damping and extreme conditions without mechanical damping (giving worst case response) are investigated. The motion is almost independent of mooring forces from a single point mooring, known as slack-moored. In operational conditions mean mooring forces are in approximate agreement with experiment except for larger periods and peak forces are similar. For extreme waves large rotational response is predicted reasonably by linear diffraction-radiation modelling with a small representative drag coefficient, as is horizontal acceleration, although peak values may be underestimated for the largest wave heights. However mean mooring forces are markedly underestimated and peak values generally overestimated. The overall conclusion is that linear modelling gives reasonable prediction of response in all wave conditions, and power capture when operational, but resulting second order mean forces only give approximate predictions for smaller wave heights.