Abstract
Structural design of the floater is an important aspect in developing cost efficient and reliable floating wind turbines. It is difficult to well account for the effect of strong non-linear dynamic characteristics and transient loading events, e.g. wind turbine faults, of floating wind turbines in a frequency-domain finite element analysis. The time-domain approach which implements the Morison's formula cannot accurately account for the hydrodynamic loads on the hull of floating wind turbines. While, the conventional hybrid frequency-time domain approach (based on the potential flow theory) fails to capture structural responses of the hulls since a rigid-body global model rather than a finite element model of the hull is employed. The present paper deals with the development and verification of a time-domain approach that can be easily implemented in various state-of-the-art computer codes for wind turbine analysis, e.g. Simo/Riflex/Aerodyn, OrcaFlex and FAST + CHARM3D, to extend their capabilities to analyze global forces and moments in structural components of a generic floater subject environmental loads from e.g. wind and waves. The global forces and moments in the structural components might be used as inputs of design formulas for structural strength design checks and/or used as boundary conditions in a sub-model finite element analysis to determine structural responses such as stresses. The proposed approach focuses on modeling of the inertia and external loads on the hull and mapping of the loads in the finite element model of the hull. In the proposed approach, floating wind turbines are considered as a system of several structural components, e.g. blades, rotational shaft, nacelle, tower, mooring lines, columns, pontoons and braces, rather than one rigid-body, while a finite element model for the hull is developed to represent the global stiffness of the structural components. The external and inertial loads on the hull are modeled as distributed loads rather than the integrated forces and moments. The conventional hybrid frequency-time domain approach, which is available in the state-of-the-art computer codes, is implemented to model the hydrodynamic loads on each structural component with essential modifications with respect to the corresponding hydrodynamic coefficients, e.g. added mass and potential damping coefficients and wave excitation forces. Approaches for modeling the hydrostatic pressure forces, gravity loads, drag forces and inertial loads on each structural component are also illustrated. Second order and higher order terms of the hydrostatic and hydrodynamic loads and the hydroelasticity effects are not accounted for in the present paper but can be further included. So far, the proposed approach has been implemented in the computer code Simo/Riflex/Aerodyn to analyze global forces and moments in the hull of a semi-submersible wind turbine. Good agreement between the reference values and the simulation results has been observed and indicates that the developed time-domain numerical models are reliable. The simulation results show that the low-frequency aerodynamic loads and fluctuations of hydrostatic pressure forces on and gravity of the floating wind turbine are important contributions to the structural responses, in particular, in the low-frequency range.
Highlights
Onshore wind energy has been well developed while the potential of offshore wind energy is substantial, in relatively deep water
In the low frequency range, the global forces and moments in the structural components of the hull are sensitive to fluctuations of the hydrostatic pressure forces on the structural components. This is because: 1) the aerodynamic loads on the Rotor Nacelle Assembly (RNA) and tower can excite significant rotational motions in particular in the low frequency range comparing to the motions excited by the wave excitation loads on the hull, while the first order terms of the fluctuations of the hydrostatic pressure forces are proportional to the motions; 2) wave excitation loads are expected to be small since wave energy is expected to be very limited in the low-frequency range; and 3) inertia and radiation loads are proportional to the first order derivative and second order derivative of the motions and are expected to be small in the low-frequency range
The global forces and moments in the structural components might be used as inputs of design formulas for structural strength design checks and/or used as boundary conditions in a sub-model finite element analysis to determine structural responses such as stresses, etc
Summary
Onshore wind energy has been well developed while the potential of offshore wind energy is substantial, in relatively deep water (e.g. deeper than 80 m). The Morison's formula is implemented in some cost effective computer codes [22] to model the hydrodynamic loads on the hull of floating wind turbines. Wind loads on blades and tower are typically considered as distributed loads This approach has been implemented in some computer codes [22] to analyze structural responses of the RNA, tower and mooring lines and rigid-body motions of the hull of floating wind turbines [21,40e45]. The proposed approach can be implemented in various state-of-the-art computer codes, e.g. Simo/Riflex/ Aerodyn, OrcaFlex [22] and FAST þ CHARM3D [22], to extend their capabilities to analyze global forces and moments in structural components of a generic floater subject to linear and non-linear environmental loads, e.g. wind and waves. An application of the proposed approach for ULS design check for the structural design of the hull of a semi-submersible wind turbine is available in Ref. [15]
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