Breaking wave loads on monopiles for offshore wind turbines and estimation of extreme overturning moment

Abstract

Measurements of breaking wave forces on vertical circular columns have been re-analysed, where substantial magnification over forces because of non-breaking non-linear waves may occur, by a factor of up to 2.8. The analysis shows that this factor increases as the depth parameter kd decreases, being close to unity for kd > 1.5 (k is the wave number and d is depth). Non-breaking wave forces are predicted reasonably by non-linear stream function wave theory using Morison's equation with empirical drag and inertia coefficients. A study was then made of the magnitude of wave overturning moment in relation to hub moment due to wind on standard 2 and 5 MW turbines with a 6 m diameter column. This showed that the wave moment in extreme conditions is greater than wind ‘hub’ moment for depths greater than about 7 and 13 m for the 2 and 5 MW turbines, respectively. Monopiles are normally used in depths below about 30 m and extreme moments due to waves occur when waves are depth-limited and defined by the Miche criterion, well below the limiting value of kd for the onset of depth-induced breaking. The maximum moment for these depths is expected to be predicted reasonably.