Natural Frequency and Foundation Damping of Colocated and Hybrid Systems Sharing Wind Turbine Monopiles under Operational Conditions

Abstract

This paper explores the utilization of the excess capacity of monopiles supporting offshore wind turbines (OWTs) by deploying hybrid and colocated wave and current marine energy concepts. However, there is a lack of guidance in literature and standards relating to the evaluation of natural frequency and foundation damping for monopile-supported wind turbines in shared anchoring configurations. The work presented here represents one of the first studies quantifying the dynamic characteristics of simulated colocated and hybrid energy systems sharing the anchoring capacity of OWT monopiles. A 3D finite-elements numerical model is developed and used to perform dynamic simulations of the OWT monopile. The hybrid system is modeled as a marine hydrokinetic device represented by an annulus plate installed around the monopile perimeter, providing additional mass participating in system damping and vibration while inducing additional loading due to drag forces on its perimeter. For a colocated system, tension forces from the mooring lines of a wave hydrokinetic device are first computed by ANSYS-AQWA then these forces are applied to the monopile in the 3D numerical model. The dynamic analyses are first performed to calibrate the numerical model parameters using data from three existing OWT projects (i.e., Burbo Bank, Walney 1, and Gunfleet Sands). Results from a parametric study varying diameter and length of the monopile, demonstrated the effect of the added mass in the hybrid configuration, as well as the influence of the tension forces of the mooring lines in the colocated configuration on the natural frequency and foundation damping. Results indicate that the simulated hybrid and colocated systems do not alter the natural frequency of the system; however, the foundation damping of the hybrid system is approximately 1.5 to 2 times higher than that of the monopile supporting only the wind turbine tower. Results also demonstrated the minimum obtained natural frequency (f1,dmin) for a combination of diameter, length, and relative densities (i.e., DP = 5 m, LP = 38 m, and DR = 25%–100%) is higher than the minimum frequency of the dynamic loading caused by the passing blades (f3P,min = 0.25 Hz) while the maximum obtained natural frequency (f1,dmax) is lower than the maximum frequency of the 3P dynamic load (f3P,max = 0.65 Hz). As such, for the cases analyzed herein, f1,d falls within the frequency range of the 3P dynamic load, and the system operates in-between the two frequency zones of soft–stiff and stiff–stiff. To ensure operational natural frequency within the soft–stiff zone in these cases, reducing the dimensions of the tower would be necessary.