Dynamic Responses of a Jacket-Type Offshore Wind Turbine Using Decoupled and Coupled Models

Abstract

This paper presents numerical studies of the dynamic responses of a jacket-type offshore wind turbine using both decoupled and coupled models. In the decoupled (hydroelastic) model, the wind load is included through time-dependent forces and moments at a single node on the top of the tower. The coupled model is a hydro-servo-aero-elastic representation of the system. The investigated structure is the OC4 (Offshore Code Comparison Collaboration Continuation) jacket foundation supporting the NREL 5-MW wind turbine in a water depth of 50m. Different operational wind and wave loadings at an offshore site with relatively high soil stiffness are investigated. The objective of this study is to evaluate the applicability of the computationally efficient linear decoupled model by comparing with the results obtained from the nonlinear coupled model. Good agreement was obtained in the eigen-frequency analysis, decay tests, and wave-only simulations. In order to obtain good results in the combined wind and wave simulations, two different strategies were applied in the decoupled model, which are 1) Wind loads obtained from the coupled model were applied directly as timedependent point loads in the decoupled model; and 2) The thrust and torque from an isolated rotor model were used as wind loads on the decoupled model together with a linear aerodynamic damper. It was found that, by applying the thrust force from an isolated rotor model in combination with linear damping, reasonable agreement could be obtained between the decoupled and coupled models in combined wind and wave simulations.

Decoupled simulations of offshore wind turbines with reduced rotor loads and aerodynamic damping

“…The rotor-nacelle-assembly in a decoupled analysis is usually simplified. Often an equivalent rotornacelle-assembly is modelled as a lumped mass on top of the tower with a mass moment of inertia (Ong et al, 2017;van der 10 Male and Lourens, 2015;Schløer et al, 2016). In this case, rotor loads must not include forces or moments from gravitational loads.…”

“…For this model a bending moment around the y-axis is applied on tower top in addition to the already applied thrust force (Ong et al, 2017;Schløer et al, 2016). The bending moment is supposed to represent the torque as explained in section 2.…”

“…frequencydomain calculations) is the application of a discrete dashpot at tower top for time-domain simulations (Schløer, et al, 2016;Ong et al, 2017) or to add an additional aerodynamic damping value to the damping in the transfer function for frequency -20 domain calculations (Salzmann and van der Tempel, 2005). Damping values for the transfer function or a damping coefficient for a discrete dashpot are derived in different ways.…”

Decoupled simulations of offshore wind turbines with reduced rotor loads and aerodynamic damping

“…These simulations are, therefore, often carried out with different analysis methods, such as frequency-domain calculations (van der Tempel and de Vries, 2005;Ziegler et al, 2015), substructuring techniques (van der Valk and Rixen, 2012), and/or simplified or reduced models . For most of the reduced models the aerodynamic loading is simplified by removing the rotor nacelle assembly from the support structure and replacing the aeroelastic computation with precomputed or stochastic generated rotor loads acting as a point force or moment at tower top (Dong et al, 2011;Abhinav and Saha, 2015;Kim and Lee, 2015;van der Male and Lourens, 2015;Schløer, et al, 2016;Ong et al, 2017) The main advantages are the faster simulation time, since the aeroelastic computation rather is a time-consuming task, and the possibility to use standard finite-element or multi-body software . It has been shown that the use of rotor load time series combined with an efficient substructuring technique (van der Valk and Rixen, 2012) can speed up the dynamic analysis for a commercial support structure design by a factor of 375.…”

“…frequencydomain calculations) is the application of a discrete dashpot at tower top for time-domain simulations (Schløer, et al, 2016;Ong et al, 2017) or to add an additional aerodynamic damping value to the damping in the transfer function for frequency-domain calculations (Salzmann and van der Tempel, 2005). Damping values for the transfer function or a damping coefficient for a discrete dashpot are derived in different ways.…”