The majority of offshore wind farms utilize monopile substructures. As these wind farms are typically located in water depths less than 30 meters, the effect of breaking waves on these structures is of great concern to design engineers. This research investigation examines many of the practical considerations and alternative ways of estimating breaking wave forces. A survey of existing European wind farms is used to establish a realistic range of basic design parameters. Based upon this information a parametric study was pursued and a series of realistic design scenarios were evaluated. Comparisons include the sensitivity to the wave force model as well as to analytical and numerical wave theories used to evaluate the wave kinematics. In addition, the effect of different kinematics stretching techniques for linear waves is addressed. Establishing whether the bathymetry will induce spilling or plunging wave breaking is critical. Spilling wave breaking can be addressed using existing wave and wave force theories; however for plunging wave breaking an additional impact force must be introduced. Dimensionless design curves are used to display pertinent trends across the full range of design cases considered. This research study provides insight into the evaluation of the maximum breaking wave forces and overturning moment for both spilling and plunging breaking waves as a function of bottom slope.

Corrosion and fatigue have been dominant degradation mechanisms in offshore structures, with the combination of the two, known as corrosion fatigue, having amplified effects in structures in the harsh marine environments. Newer types of structure are now being developed for use in highly dynamic, harsh marine environments, particularly for renewable energy applications. However, they have significantly different structural details and design requirements compared to oil and gas structures, due to the magnitude and frequency of operational and environmental loadings acting on the support structures. Therefore, the extent of corrosion assisted fatigue crack growth in these structures needs to be better understood. In this research, fatigue crack growth in S355J2+N steel used for offshore wind monopile fabrications was investigated in air and free corrosion conditions. Tests were conducted on parent, HAZ and weld materials at cyclic load frequencies similar to what is experienced by offshore wind monopile support structures. The seawater used for testing was prepared according to ASTM D1141 specifications and was circulated past the specimens through a purpose designed and built corrosion rig at a rate of 3 l/min, at a temperature of 8-100C and at a pH of 7.78-8.1. A new crack propagation method accompanied by constant amplitude loading was used. Crack growth rates in parent, HAZ and weld materials were significantly accelerated under free corrosion conditions, at all the stress ratios used compared to in air environment. However, in free corrosion conditions, crack growth rates in the parent, HAZ and weld materials were similar, particularly at a lower stress ratio. The results are explained with respect to the interaction of the loading condition, environment and the rate of material removal by corrosion in the weldments. A new model was developed to account for mean stress effects on crack growth rates in air and in seawater, and was found to correlate well with experimental data as well as with the other mean stress models tested.