Abstract
Marine renewable energy is poised to advance the Blue Economy through providing power to several industries and specialized markets. With the Biden administration vision of 30 GW by 2030 from offshore wind, the deployment of marine hydrokinetic (MHK) energy devices and offshore wind turbines (OWT) on a shared anchoring configuration provides several advantages. The Department of Energy estimates that the Kitty Hawk offshore wind lease area in North Carolina has a potential power generating capacity of 2.8 GW. In the shallow waters of these wind lease areas, a monopile foundation is the most likely to be used for the fixed bottom OWT. The research herein aims to address the concept of shared anchoring systems utilizing monopiles. Using critical loading scenarios, a systematic assessment of the excess capacity of OWT monopiles as a function of the embedded length to diameter (Le/D) ratios in the range of 5-10 is conducted in view of the Ultimate Limit State (ULS) and Serviceability Limit State (SLS) per IEC standards. In addition, the effect of various monopile wall thickness to diameter (tp/D) ratios on the natural frequencies and foundation damping for the same Le/D ratios has been investigated.
To investigate the concept of shared anchoring, the Sandia reference model “RM3” device, which is a two-body floating point absorber (FPA), is considered the MHK device to be moored to the monopile in a shared anchoring configuration. The Siemens SWT-3.6-107 3.6 MW turbine is used as the structure with rotor operational frequency (1P) ranging from 0.083 Hz to 0.216 Hz. The effect of wave and wind loading on the mooring system of the FPA is simulated using ANSYS AQWA. Then, coupled numerical analyses of the monopile are conducted using PLAXIS 3D while applying the tension loading from the FPA mooring lines on the OWT. Analyses confirm the viability of monopile capacity for supporting FPA arrays in a co-location configuration. It is concluded that OWT monopiles with D=5m, lengths (Le) = 28-48m and wall-thicknesses (tp) > 40mm meet ULS and SLS specifications with and without the shared anchoring approach. By increasing tp from 40mm to 60mm, natural frequency (fn) increased from 0.3 Hz to 0.325 Hz, and foundation damping decreased from 0.44% to 0.32%. For these OWT monopiles, fn is well inside the frequency range of the dynamic load caused by passing blades (3P), which is from 0.25 Hz to 0.65 Hz. The addition of loading from FPA as a co-located configuration slightly changed the natural frequency and foundation damping. The effect of reducing tp on Dynamic Amplification Factor (DAF) was found to be slight.