Chapter 3: Natural frequency analysis of offshore wind turbine monopiles

The UK has seen a significant expansion of offshore wind capacity in recent years with a 4 GW current installed capacity and 1·7 GW currently under construction (UKTI, 2015). Initial installations have been nearshore but as development continues the turbines are being located further offshore into deeper waters, ensuring that construction costs are kept to a minimum as water depths increase presents a significant design challenge.

Typically in shallow water, the installation comprises a foundation structure consisting of a steel monopile (MP) sunk into the seabed and a transition piece (TP), which together support the tower and the wind turbine generator (WTG), as illustrated in Figure 1. As the move into deeper water continues more substantial foundations are required and this may necessitate the use of costly jacket or gravity-based structures. However, the recent development of larger diameter MPs has increased the range of water depths that can be achieved with this type of foundation (Seidel, 2010) and this can be a cost-effective solution provided the design constraints can be satisfied.