Tower Top Motion of a Bottom-Fixed Offshore Wind Turbine in the Hammerhead Configuration: Root Causes and System Identification Tests

Offshore wind turbine (OWT) installation is a challenging task and there are significant gaps in understanding their unique dynamics. The partially installed state of the OWT where the monopile, tower and the nacelle have been installed is called the hammerhead configuration. In this configuration, the nacelle exhibits complex tower top motions with rapidly changing elliptical orbits, which poses a major challenge to the safe deployment of OWTs. The aim of this study is to investigate the root causes behind the complex orbital motion of OWTs during installation. A numerical model of the hammerhead configuration of an OWT has been established. Various scenarios, including different sea states, wave headings, and alterations to the hammerhead configuration (i.e., varying eccentric distances and varying masses) were tested to evaluate the effect of each parameter individually. Additionally, a Froude-scaled experimental model of the OWT system was designed and constructed. System identification tests were carried out on the model to verify the natural frequencies and mode shapes of the as-built system. A novel methodology for designing a fully flexible scaled model of an OWT in hammerhead configurations apt for wave tank testing is presented.