Grounding Resistance Characteristics of Gravity Foundations of Offshore Wind Turbines in the Intertidal Zone

Compared with onshore wind turbines, offshore wind turbine develops very quickly in recent years due to less land cover, higher wind speed and steady wind direction. At present, gravity foundations of offshore wind turbines are widely applied in Rudong intertidal wind farm in China. However, up to now, few attentionsare paid to grounding of gravity foundations. In this paper, grounding resistance characteristics of gravity foundations of intertidal wind turbines were calculated and analyzed systematically. Various impact factors, including depth of seawater, resistivity and thickness of clay and gravel layers, and size and depth of pile pumped into ground, were considered. Analytical results show that grounding resistance is affected significantly by depth of seawater. If seawater depth is less than 0.3 m, power-frequency and impulse grounding resistances in intertidal zone of gravity foundation offshore turbine will, in some conditions, exceed limited value specified in standards. As seawater depth increases, both power-frequency and impulse grounding resistances decrease rapidly. Moreover, the two types of grounding resistances increase with increase of soil resistivity of clay and gravel layers, and decrease with increase of clay layerthickness, although these effects become weaker as depth of seawater increases. Structure size of gravity foundation, including its diameter and driven depth, has limited effect on resistances. These results are of great significancefor grounding design of gravity foundations of offshore wind turbines.