Pile installation fatigue damage from impact driving records – A case study of offshore monopiles

Currently, impact pile driving is the preferred method of installation for offshore wind monopile foundations, thanks to the good stability of the foundation achieved after driving and fairly reliable driveability predictions possible for this installation method. Despite these benefits, a possible drawback of impact pile driving can be the high forces exerted on the foundation during installation. Since roughly 90% of steel structure failures during service life is due to fatigue or fracture, in order to promote circularity in the renewable energy sector and to allow a safe re-use of the wind turbine foundations, a correct estimation of the fatigue damage cumulated during the installation process is needed.
The purpose of this paper is to address the topic of cumulative fatigue damage of offshore monopiles installed by impact driving based on driveability prediction results and actual driving records. The paper shows that the monopile fatigue damage predicted in the design stage, often based on numerical models considering constant maximum impact energy, can be overly conservative when compared to the fatigue damage calculation performed based on the actual driving records. This conclusion demonstrates the need for appropriate piling strategies to be able to reduce the actual fatigue damage in the foundation.