Hybrid nonlinear floating fan hydrodynamic load coupling calculation method

Abstract

The invention discloses a hybrid nonlinear floating fan hydrodynamic load coupling calculation method. The method comprises the steps of obtaining a velocity potential of a wave field in a computational domain through OceanWave3D numerical simulation (a wave field calculation model), and performing post-processing to obtain wave field data; dividing an instantaneous wet surface according to the six-degree-of-freedom motion, calculated based on an OpenFAST (floating fan calculation model), of the floating fan in each time step, and performing pressure integration to obtain a Front-Krylov force; updating the hydrostatic stiffness matrix according to the real-time position change of the buoyancy center of the fan, and calculating hydrostatic restoring force; linear radiation force and diffraction force are calculated through time domain convolution of frequency domain data, nonlinearity of the radiation force and the diffraction force is considered based on an instantaneous wetting volume change coefficient, and hydrodynamic load and structural motion response calculation in the current time step is completed after the OpenFAST is coupled. According to the method, coupling calculation between the open source code OpenFAST and OceanWave3D is achieved, and compared with commercial software at the present stage, the method is more accurate in wave load calculation under the extreme wave sea condition.

Description

Technical Field

[0001] The present invention relates to a hybrid nonlinear floating wind turbine hydrodynamic load coupling calculation method, which is mainly used for time-domain nonlinear wave working condition calculation of floating wind turbines. Background Art

[0002] Floating wind turbines are widely used in offshore wind energy development. In complex deep-sea environments, wave loads can cause platform motion and stress on mooring systems. Extreme waves can even cause platforms to capsize and anchor chains to break. To calculate the wave loads on floating wind turbines, the main research methods currently used include: 1. CFD integrated numerical simulation; 2. Potential flow theory.

[0003] Both the fully nonlinear potential flow theory and the CFD method can be applied to the calculation of wave loads under strongly nonlinear sea conditions. The calculations are accurate, but both require solving the nonlinear equations of all nodes in the computational domain at each time step, which results in low computational efficiency and consumes a lot of computing resources.

[0004] For small-amplitude waves and platform motion, linear potential flow theory is the most widely used. Based on the Cummins method, the wave-excited forces, added mass, and radiation damping matrices of the structure are calculated in the frequency domain and then convolved into the time domain to solve the structural dynamics. This method has been extended to the calculation of second-order wave forces (nonlinearities) such as sum-frequency forces, difference-frequency forces, and average drift forces through quadratic transfer functions (QTFs). Commercial software such as AQWA, Orcaflex, SIMA, and WAMIT, as well as academic codes such as WEC-Sim and Charm3D, all use this method to calculate hydrodynamic loads. It is computationally efficient and provides high fidelity results for small-amplitude motions. However, the accuracy of linear potential flow theory decreases in sea conditions with strong wave nonlinearities, steep waves, and significant heave motion of the floating body.

[0005] Based on the above considerations, there is an urgent need for a calculation method for the hydrodynamic loads of floating wind turbines with high computational efficiency and that can take into account the strong nonlinearity of waves, so as to be used for the calculation of the structural motion response of floating wind turbines under complex sea conditions. Summary of the Invention

[0006] To address the challenges of low computational efficiency of CFD and fully nonlinear potential flow solutions, and the low accuracy of linear potential flow theory in calculating wave loads under strong nonlinear wave conditions, this paper provides a hybrid nonlinear method for coupling hydrodynamic load calculations for floating wind turbines. This method provides more accurate and efficient wave load calculations for extreme wave conditions.

[0007] In order to achieve the above objectives, the technical solution adopted by the present invention is:

[0008] A hybrid nonlinear method for coupling hydrodynamic load calculations for floating wind turbines uses parameters from a full-scale floating wind turbine model. A wave field model and a floating wind turbine model are established in the nonlinear potential flow solvers OceanWave3D and OpenFAST, respectively. Numerical simulations are performed in OceanWave3D to obtain the velocity potential of the wave field, which is then post-processed to obtain wave field data, including dynamic pressure and wave surface elevation data. The initial hydrostatic stiffness matrix is calculated based on the mass distribution and waterplane area at the initial equilibrium position.

[0009] The six-degree-of-freedom motion of the floating wind turbine in the previous time step is obtained in OpenFAST. Combined with the wave field data of OceanWave3D, the instantaneous wet surface division and the pressure integration of the instantaneous wet surface are performed to obtain the time-domain Froude-Krylov force. The hydrostatic stiffness matrix is updated according to the real-time buoyancy center position change of the floating wind turbine, and the hydrostatic restoring force is calculated based on the hydrostatic stiffness matrix. The linear radiation force and linear diffraction force are calculated through time-domain convolution of the frequency domain data. The nonlinearity of the radiation force and diffraction force is considered by the instantaneous wet volume change coefficient. Specifically, the instantaneous wet volume change coefficient is multiplied by the linear radiation force and the linear diffraction force respectively to obtain the nonlinear radiation force and the nonlinear diffraction force.

[0010] Based on the Froude-Krylov force, hydrostatic restoring force, nonlinear radiation force, and nonlinear diffraction force, the hydrodynamic load of the floating wind turbine is finally obtained, which can be used to calculate the structural motion response of the floating wind turbine.

[0011] In the above technical solution, further, the coupling calculation method is implemented based on an OceanWave3D-OpenFAST coupling program, which includes OceanWave3D, a hydrodynamic load calculation module and OpenFAST; the coupling calculation method is specifically:

[0012] Based on the parameters of the full-scale floating wind turbine model, the wave field calculation model and floating wind turbine calculation model were established in OceanWave3D and OpenFAST, respectively.

[0013] According to the wave field calculation model, the velocity potential of all grid nodes in the OceanWave3D calculation domain is calculated based on the calculation domain range, grid accuracy, incident wave spectrum, significant wave height and period, wave-making area range and wave-breaking area range. Based on the velocity potential of all grid nodes in the calculation domain, the dynamic pressure and wave surface elevation data of all grid nodes in the wave field are calculated.

[0014] According to the floating wind turbine calculation model, the six-degree-of-freedom motion of the floating wind turbine is calculated based on relevant parameters such as blades, tower, floating platform, and mooring system.

[0015] Obtain the six-degree-of-freedom motion of the floating wind turbine in OpenFAST at the previous time step, specifically the six-degree-of-freedom displacement, velocity, and acceleration data of the wind turbine float;

[0016] According to the six-degree-of-freedom motion of the floating wind turbine in the previous time step, the grid element position and element normal vector of the wind turbine are updated in real time. The instantaneous wet surface is divided according to the wave surface elevation data of the wave field, and the pressure of the instantaneous wet surface is integrated according to the dynamic pressure of the wave field to obtain the Froude-Krylov force. According to the change of the wetted surface, the buoyancy center position of the floating wind turbine in the current time step is updated, and the hydrostatic stiffness matrix is updated based on the buoyancy center position of the floating wind turbine to calculate the hydrostatic restoring force.

[0017] The linear radiation force and linear diffraction force are calculated by time domain convolution of frequency domain data, and the nonlinearity of the radiation force and diffraction force is considered by the instantaneous wetting volume change coefficient to obtain the nonlinear radiation force and nonlinear diffraction force.

[0018] At each time step, the hydrodynamic load calculation module receives the dynamic pressure and wave surface elevation data from OceanWave3D and the six-degree-of-freedom motion data of the wind turbine in OpenFAST, calculates the Froude-Krylov force, hydrostatic restoring force, nonlinear radiation force, and nonlinear diffraction force, thereby obtaining the hydrodynamic load of the floating wind turbine and transmitting the hydrodynamic load of the floating wind turbine to OpenFAST.

[0019] Furthermore, based on the obtained hydrodynamic loads, the structural motion response of the floating wind turbine can be solved through the structural motion equation.

[0020] Furthermore, the calculation equation for the hydrodynamic load is:

[0021] F=F FK +αF DIFF +αF add +αF damping +F hys

[0022] Among them, F FK is the Froude-Krylov force, which is solved directly in the time domain by integrating the wave field pressure on the wet surface; F DIFF is the linear diffraction force, which is obtained by multiplying the diffraction force matrix in the frequency domain normalized by the wavefront elevation and then performing time domain convolution; F add With F damping is the additional mass force and radiation damping force in the linear radiation force. The additional mass force is calculated by the additional mass matrix and the six-degree-of-freedom acceleration in the frequency domain, and the radiation damping force is calculated by the radiation damping matrix and the velocity; F hysis the hydrostatic restoring force. The change of the fan buoyancy center is considered when dividing the instantaneous wetted surface at each time step. The hydrostatic stiffness matrix is updated in real time based on the fan buoyancy center, and the hydrostatic restoring force is calculated based on the hydrostatic stiffness matrix. α is the instantaneous wetted volume change coefficient, specifically the ratio of the instantaneous wetted volume to the initial wetted volume.

[0023] The present invention is beneficial in that:

[0024] This hybrid nonlinear floating wind turbine hydrodynamic load coupling calculation method can be used to calculate the hydrodynamic loads of floating wind turbines under nonlinear wave conditions. Compared to linear potential flow theory-based software such as AQWA / Orcaflex / WAMIT, this method considers the fully nonlinear Froude-Krylov force and hydrostatic restoring force, and uses the instantaneous volume change coefficient to correct the linear diffraction force and linear radiation force to obtain nonlinear radiation force and nonlinear diffraction force. Therefore, the results are more accurate when calculating wave loads in extreme wave conditions. Compared with full-scale CFD simulation and fully nonlinear potential flow solution methods, this method significantly improves the calculation efficiency of hydrodynamic loads and structural motion responses of floating wind turbines under complex sea conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic flow chart of the coupling method of the present invention.

[0026] Figure 2 Illustration of the computational domain in OceanWave3D (above) and the mesh element of the structure (below).

[0027] Figure 3 、 4 5 are the platform motions of the three main degrees of freedom of the floating wind turbine obtained by the coupling calculation method (Hybrid Method) of the present invention under the condition of irregular wave effective wave height of 2m and period of 15s, compared with the CFD verification, and the motions of the main degrees of freedom of the platform are basically consistent. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further described below with reference to the accompanying drawings and specific embodiments.

[0029] like Figure 1This is a flow chart of a hybrid nonlinear floating wind turbine hydrodynamic load coupling calculation method of the present invention. The method is implemented based on an OceanWave3D-OpenFAST coupling program, which includes OceanWave3D, a hydrodynamic load calculation module, and OpenFAST. A wave field calculation model is constructed in OceanWave3D. The input of the wave field calculation model includes wave input conditions (such as the calculation domain range, grid accuracy, incident wave spectrum, water depth, effective wave height and period, wave-making area range and wave-breaking area range, etc.), and the output is the velocity potential of the grid node; a floating wind turbine calculation model is constructed in OpenFAST. The input of the floating wind turbine calculation model is the relevant parameters of the wind turbine blades, tower, nacelle, hub, float, and mooring system, and the output is the six-degree-of-freedom motion of the floating wind turbine. At the same time, OpenFAST and OceanWave3D are set to use the same wave input conditions to calculate the hydrodynamic load and structural dynamic response of the wind turbine. Run the OceanWave3D-OpenFAST coupling program. In each time step of the operation, OpenFAST calls the hydrodynamic load calculation module. During the calling process, the hydrodynamic load calculation module receives the dynamic pressure and wave surface elevation data of OceanWave3D and the six-degree-of-freedom motion data of the wind turbine in OpenFAST, calculates the Froude-Krylov force, hydrostatic restoring force, nonlinear radiation force and nonlinear diffraction force, thereby obtaining the hydrodynamic load of the floating wind turbine, and transmits the hydrodynamic load of the floating wind turbine to OpenFAST. During the interaction, it is necessary to consider using Euler angle transformation to transform the six-degree-of-freedom motion data and load data. After the interaction is completed, the hydrodynamic load can be integrated and transmitted to OpenFAST. OpenFAST then integrates the wind turbine aerodynamic load, hydrodynamic load and mooring load internally, and then calculates the motion response of structures such as the float, tower, nacelle, wind rotor, and transmission chain. The time step is advanced until the calculation is completed.

[0030] Figure 2Illustrations of the OceanWave3D computational domain and the structure's mesh bins are shown. For 0° incident waves, the OceanWave3D computational domain (above) is 750m × 400m × 200m. The wave inlet boundary is designated "Inlet," the outlet is "Outlet," the bottom seabed is "Bottom," and the top boundary, also serving as the wave's free surface, is designated "Top." Wave generation is achieved using a relaxation zone (Inlet relaxation zone) and wave cancellation using a damping zone (Outlet relaxation zone). The relaxation zone covers an area of X = 0-100m, while the damping zone covers an area of X = 600-750m. The figure below shows the structure's mesh bins. The structure's surface is decomposed into 50,000 bins. During the coupled calculation, the structure's mesh bins are partitioned into wetted surfaces based on the wave elevation curve. Pressure integration is then performed on the wetted surfaces to calculate the Froude-Krylov force. A hybrid nonlinear coupled calculation method for hydrodynamic loads of floating wind turbines is implemented based on the OceanWave3D-OpenFAST coupling program. The method includes the following steps:

[0031] Based on the parameters of the full-scale floating wind turbine model, a wave field calculation model and a floating wind turbine calculation model were established in OceanWave3D and OpenFAST, respectively.

[0032] According to the wave field calculation model, the velocity potential of all grid nodes in the OceanWave3D calculation domain is calculated based on the calculation domain range, grid accuracy, incident wave spectrum, significant wave height and period, and the range of the wave-making and wave-breaking areas. Based on the velocity potential of all grid nodes in the calculation domain, the dynamic pressure and wave surface elevation data of all grid nodes in the wave field are calculated.

[0033] According to the floating wind turbine calculation model, the six-degree-of-freedom motion of the floating wind turbine in the previous time step is obtained;

[0034] Based on the six-degree-of-freedom motion of the floating wind turbine in the previous time step, the wind turbine's grid element position and element normal vector are updated in real time. The instantaneous wet surface is divided based on the wave field's wave surface elevation data, and the instantaneous wet surface pressure is integrated according to the dynamic pressure of the wave field to obtain the Froude-Krylov force. Based on the wetted surface changes, the buoyancy center position of the floating wind turbine in the current time step is updated. Based on the buoyancy center position of the floating wind turbine, the hydrostatic stiffness matrix is updated to calculate the hydrostatic restoring force.

[0035] The linear radiation force and the linear diffraction force are calculated by time-domain convolution of the frequency-domain data, and the linear radiation force and the linear diffraction force are multiplied by the instantaneous wet volume change coefficient respectively to obtain the nonlinear radiation force and the nonlinear diffraction force;

[0036] At each time step, the hydrodynamic load calculation module receives dynamic pressure and wave surface elevation data from OceanWave3D and the turbine's six-degree-of-freedom motion data from OpenFAST. It calculates the Froude-Krylov force, hydrostatic restoring force, nonlinear radiation force, and nonlinear diffraction force to derive the hydrodynamic loads on the floating turbine. This load is then transmitted to OpenFAST. Furthermore, the structural motion response of the floating turbine can be solved using the structural equations of motion.

[0037] The calculation equation for the hydrodynamic load in the coupling program is:

[0038] F=F FK +αF DIFF +αF add +αF damping +F hys

[0039] Among them, F FK is the Froude-Krylov force, which is solved directly in the time domain by integrating the wave field pressure on the wet surface; F DIFF is the diffraction force, which is obtained by multiplying the diffraction force matrix in the frequency domain normalized by the wavefront elevation and then performing time domain convolution; F add With F damp is the additional mass force and radiation damping force in the linear radiation force. The additional mass force is calculated by the additional mass matrix and the six-degree-of-freedom acceleration in the frequency domain, and the radiation damping force is calculated by the radiation damping matrix and the velocity; F hys is the hydrostatic restoring force. The change of the center of buoyancy is considered when dividing the instantaneous wet surface at each time step. The hydrostatic stiffness matrix is updated in real time, and the hydrostatic restoring force is calculated based on the hydrostatic stiffness matrix. α is the ratio of the instantaneous wetted volume to the initial wetted volume.

[0040] Depend on Figure 3 、 4 As shown in Figures 5 and 6, the phase and amplitude of the floating body's motion response after coupling the proposed method to OpenFAST are essentially consistent with the CFD results, confirming the reliability of the results. Furthermore, computational efficiency is significantly improved. For a 3-hour sea state, the proposed method takes only 8-12 hours to calculate, far less than the 3-4 weeks required for CFD calculations.

[0041] Of course, the above are only specific application examples of the present invention. The present invention has other implementation methods. Any technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A hybrid nonlinear floating wind turbine hydrodynamic load coupling calculation method, characterized by: Based on the parameters of the full-scale floating wind turbine model, a wave field calculation model and a floating wind turbine calculation model are established respectively. The wave field calculation model is used to calculate the velocity potential of the wave field, and the velocity potential of the wave field is post-processed to obtain wave field data, specifically including the dynamic pressure intensity and wave surface elevation data of the wave field. Based on the mass distribution and waterplane area at the initial equilibrium position, the initial hydrostatic stiffness matrix is calculated. The floating wind turbine calculation model is used to calculate the six-degree-of-freedom motion of the floating wind turbine in the previous time step. Combined with the wave field data, the instantaneous wetted surface division and the instantaneous wetted surface pressure integration are performed to obtain the Froude-Krylov force. The hydrostatic stiffness matrix is updated according to the real-time buoyancy center position change of the floating wind turbine, and the hydrostatic restoring force is calculated based on the hydrostatic stiffness matrix. The linear radiation force and the linear diffraction force are calculated through time-domain convolution of the frequency domain data, and the instantaneous wetted volume change coefficient is multiplied by the linear radiation force and the linear diffraction force, respectively, to obtain the nonlinear radiation force and the nonlinear diffraction force. Based on the Froude-Krylov force, hydrostatic restoring force, nonlinear radiation force and nonlinear diffraction force, the hydrodynamic load of the floating wind turbine is finally obtained.

2. The hybrid nonlinear floating wind turbine hydrodynamic load coupling calculation method according to claim 1 is characterized by: The coupling calculation method is implemented based on an OceanWave3D-OpenFAST coupling program, which includes OceanWave3D, a hydrodynamic load calculation module, and OpenFAST. The specific steps of the coupling calculation method are as follows: Based on the parameters of the full-scale floating wind turbine model, a wave field calculation model and a floating wind turbine calculation model were established in OceanWave3D and OpenFAST, respectively. According to the wave field calculation model, the velocity potential of all grid nodes in the OceanWave3D calculation domain is calculated based on the calculation domain range, grid accuracy, incident wave spectrum, significant wave height and period, and the range of the wave-making and wave-breaking areas. Based on the velocity potential of all grid nodes in the calculation domain, the dynamic pressure and wave surface elevation data of all grid nodes in the wave field are calculated. According to the floating wind turbine calculation model, the six-degree-of-freedom motion of the floating wind turbine in the previous time step is obtained; Based on the six-degree-of-freedom motion of the floating wind turbine in the previous time step, the wind turbine's grid element position and element normal vector are updated in real time. The instantaneous wet surface is divided based on the wave field's wave surface elevation data, and the instantaneous wet surface pressure is integrated according to the dynamic pressure of the wave field to obtain the Froude-Krylov force. Based on the wetted surface changes, the buoyancy center position of the floating wind turbine in the current time step is updated. Based on the buoyancy center position of the floating wind turbine, the hydrostatic stiffness matrix is updated to calculate the hydrostatic restoring force. The linear radiation force and the linear diffraction force are calculated by time-domain convolution of the frequency-domain data, and the linear radiation force and the linear diffraction force are multiplied by the instantaneous wet volume change coefficient respectively to obtain the nonlinear radiation force and the nonlinear diffraction force; At each time step, the hydrodynamic load calculation module receives the dynamic pressure and wave surface elevation data from OceanWave3D and the six-degree-of-freedom motion data of the wind turbine in OpenFAST, calculates the Froude-Krylov force, hydrostatic restoring force, nonlinear radiation force, and nonlinear diffraction force, thereby obtaining the hydrodynamic load of the floating wind turbine and transmitting the hydrodynamic load of the floating wind turbine to OpenFAST.

3. The hybrid nonlinear floating wind turbine hydrodynamic load coupling calculation method according to claim 1 is characterized by: The calculation equation for hydrodynamic load is: F=F FK +αF DIFF +αF add +αF damping +F hys Among them, F FK is the Froude-Krylov force, which is solved directly in the time domain by integrating the wave field pressure on the wet surface; F DIFF is the linear diffraction force, which is obtained by multiplying the diffraction force matrix in the frequency domain normalized by the wavefront elevation and then performing time domain convolution; F add With F damping is the additional mass force and radiation damping force in the linear radiation force. The additional mass force is calculated by the additional mass matrix and the six-degree-of-freedom acceleration in the frequency domain, and the radiation damping force is calculated by the radiation damping matrix and the velocity; F hys is the hydrostatic restoring force. The change of the fan buoyancy center is considered when dividing the instantaneous wetted surface at each time step. The hydrostatic stiffness matrix is updated in real time based on the fan buoyancy center, and the hydrostatic restoring force is calculated based on the hydrostatic stiffness matrix. α is the instantaneous wetted volume change coefficient, specifically the ratio of the instantaneous wetted volume to the initial wetted volume.

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