Seismic Response of Offshore Wind Farm Jacket Structure Supported on the Pile Foundation

Abstract

The main objective of this research is to investigate the response of the pre-piled offshore wind farm jacket structure when the structure undergoes operational cyclic (wind+wave) loads combined with earthquake loads. Hence, a two-layered soil condition was chosen, with the shallow-loose layer overlying a deep-denser layer. The scaled model of an exact jacket configuration was designed and tested in a centrifuge along with the wind tower and nacelle arrangement, which matches the dynamic properties of an equivalent 9MW turbine. Two cases were considered, one with lateral load (wind load) and another with no lateral load during the earthquake. The lateral load was applied at the nacelle level using an in-flight mass on a pneumatic piston arrangement to generate the equivalent pile head moment due to the wave and wind loads. The key findings are that the generation of excess pore pressures causes the soil to liquefy under strong earthquakes, eventually leading the structure to settle and rotate more than its allowable serviceability limit state (SLS). While it is evident that the lateral loads combined with seismic load cause excessive settlement and rotation, the structure also shows considerable settlement and rotation without any lateral load during the seismic event. Also, the effect of including operational wind and wave loads in combination with the seismic loads recommended by DNV-ST-0437 (2016) and DNV-RP-0585 (2021) are also discussed.