SPAR type offshore wind turbine fairlead structure fatigue analysis method

Abstract

The SPAR type offshore wind turbine fairlead structure fatigue analysis method provided by the invention specifically comprises the following steps: calculating an equivalent effect variable amplitude value of a fatigue dangerous point by adopting a critical plane method to obtain a fatigue damage parameter, and calculating the fatigue damage parameter according to a modified Manson-Conffin formula and a Miner linear cumulative damage theory. The fatigue damage of the fairlead of the offshore wind turbine under different working conditions of ice thickness or ice speed under the combined action of wind-wave and wind-ice is accurately evaluated; by establishing a numerical model of the fairlead and considering the geometric structure and stress characteristics of the fairlead, the fatigue life of the fairlead is accurately calculated, and the fatigue life and fatigue reliability of the fairlead are analyzed. According to the method, the fatigue life of the fairlead of the offshore wind turbine under multiple working conditions is analyzed, the service life of equipment is prolonged, the reliability is improved, and the blank of research on the fatigue influence of ice load parameters on the floating mooring fairlead structure is filled.

Description

Technical Field

[0001] The present invention relates to the field of offshore wind turbine structural engineering, and particularly to a fairlead structure for a SPAR-type floating wind turbine and a method for optimizing the fatigue life analysis thereof, especially for the fatigue reliability assessment and structural improvement of the fairlead under complex environmental loads (wind, wave, ice). Background Art

[0002] The fairlead of an offshore floating wind turbine is a key component connecting the mooring system and the wind turbine foundation, which is subjected to alternating loads for a long time and is prone to fatigue damage. The traditional fairlead structure has problems such as stress concentration and short fatigue life. Especially under the combined action of ice load and wind and wave, the risk of fatigue failure increases significantly. In the prior art, the design of the fairlead mostly relies on empirical formulas, lacking refined analysis for the complex environmental coupling effect, and the structural optimization method is insufficient, resulting in high maintenance costs and low reliability.

[0003] As a key part of the mooring system of an offshore wind turbine, the fairlead bears complex alternating loads, such as environmental loads like wind, wave, ice, and the tension change of the mooring cable. Fatigue analysis can help identify the stress concentration areas and fatigue weak links that may occur in the fairlead and its connection parts during long-term use, so as to take measures in advance for reinforcement or optimized design, thereby ensuring the structural safety of the offshore floating wind turbine.

[0004] The current fatigue assessment methods for offshore wind turbine mooring systems mainly focus on the fatigue damage assessment of mooring cables, and there is no systematic research and application on the fatigue analysis of fairleads. As an important connection part between the tower and the floating platform, the fairlead has a special structure, complex stress and is prone to failure, which is a weak link of the floating wind turbine. During the service period of the wind turbine, the fairlead is the most vulnerable part to fatigue damage. Once the fairlead undergoes plastic deformation or even fracture, it will accelerate the fatigue damage of the wind turbine structure, and even cause the instability and collapse of the wind turbine, resulting in serious impacts.

[0005] As a key part connecting the mooring cable and the floating wind turbine, the fairlead bears complex stress states and cyclic loads. Due to the geometric structure and stress characteristics of the fairlead, its fatigue damage risk is relatively high, and it has a great impact on the safety of the entire mooring system. Therefore, a fatigue analysis method for the fairlead of an offshore wind turbine is needed to accurately evaluate its fatigue life and ensure the safety and reliability of the floating wind turbine mooring system. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a fatigue analysis method for the fairlead structure of a SPAR-type offshore wind turbine to solve the technical problems of fatigue reliability assessment and structural improvement of the fairlead under complex environmental loads (wind, wave, ice).

[0007] The present invention adopts the following technical solutions to solve its technical problems:

[0008] The fatigue analysis method for the fairlead structure of a SPAR - type offshore wind turbine provided by the present invention is characterized in that the equivalent strain amplitude of the fatigue critical point is calculated by the critical plane method to obtain the fatigue damage parameter, and through the modified Manson - Conffin formula and the Miner linear cumulative damage theory, the fatigue damage of the fairlead of the offshore wind turbine under different working conditions including the combined action of wind - wave, wind - ice, ice thickness or ice speed is accurately evaluated; by establishing a numerical model of the fairlead, considering its geometric structure and stress characteristics, the fatigue life of the fairlead is accurately calculated, and the fatigue life and fatigue reliability analysis of the fairlead are carried out.

[0009] In the above - mentioned method, the following method can be adopted for the critical plane method calculation:

[0010] (1) Critical plane method:

[0011] Through Matlab programming, find out the critical failure plane where damage may occur, calculate the maximum shear strain plane at each moment, take the maximum shear strain plane with weighted average as the critical damage plane, obtain the corresponding shear strain time - history component and normal strain time - history component on the critical damage plane, use the rain - flow counting method to calculate the strain amplitude and cycle number of the positive - shear strain time - history component of the fairlead, and adopt the Von Mises criterion to synthesize the positive - shear strain amplitudes on the critical plane into an equivalent strain as the damage control parameter.

[0012] The present invention can adopt the following method to modify the Manson - Conffin formula:

[0013] (1) Combine high - cycle fatigue and low - cycle fatigue according to the Manson - Conffin formula. The total strain value is divided into elastic strain and plastic strain. Use the von Mises criterion to combine the two parameters γ max and on the critical plane into an equivalent strain, and use it as the damage control parameter on the critical plane, and derive the unified multiaxial fatigue life resistance equation:

[0014]

[0015] (2) Miner linear cumulative damage theory: The damage of a structural member subjected to n cycles is D = n / N, where N is the number of cycles required for material failure. When D = 1, the structure fails.

[0016] The present invention can adopt the following method to establish a numerical model of the fairlead:

[0017] The Spar floating wind turbine was modeled, the frame and floating foundation were simulated using the shell unit SHELL63, the motion response and cable tension of the floating wind turbine under environmental loads were analyzed, three symmetrically distributed anchor chains were simulated using catenary theory, and pre-tensioning was performed to ensure the static balance of the platform;

[0018] Based on the actual fairlead hole structure (such as Panamax or customized anti-ice cone connector), the hole shape, supporting structure and weld details are restored to adapt to the dynamic load transfer of the mooring cable. The fairlead hole is located at the connection between the floating platform and the tower. The displacement boundary conditions of the fairlead hole area are extracted from the overall model, and a local refined model is established. The hexahedral mesh is encrypted for stress concentration areas such as the hole and fillet.

[0019] In the above method, the numerical model of the fairlead hole can be: the geometric model takes the OC3-Hywind Spar floating wind turbine as the research object, including a 5MW NREL wind turbine, a Spar floating platform and a catenary mooring system, and adopts 3 anchor chains for mooring positioning with an angle of 120°; the fairlead hole adopts a segmented arc design (radius R1=200mm, R2=450mm), and the diameter of the anchor chain is 90mm.

[0020] Among the above methods, the following methods can be used to determine the geometric structure and force characteristics of the numerical model of the fairlead:

[0021] (1) Geometric structure judgment method:

[0022] Based on the existing standard models such as the Panama fairing hole, the geometric parameters are adjusted in combination with the structural characteristics of the floating wind turbine, and the fairing hole size (diameter, radius, thickness, etc.) is determined according to the design requirements of the mooring system. A parametric model is established, and matching optimization is performed in combination with the actual sizes of the floating platform and the tower by adjusting the parameters such as the fairing hole diameter, wall thickness, and cone angle:

[0023] (2) Analysis method of stress characteristics:

[0024] Dynamic load simulation, including mooring tension (time domain analysis), wind load (harmonic superposition method to simulate wind speed time history), wave load (JONSWAP spectrum / Stokes wave theory), ice load (secondary fracture theory to calculate ice force time history curve). Considering the combined effects of wind-wave and wind-ice, multi-condition time domain dynamic response simulation is performed through AQWA software to obtain the dynamic load input of the fairlead. Finite element transient analysis is performed to calculate the stress distribution (such as Von Mises stress) and strain time history curve of the key nodes of the fairlead, and to identify stress concentration areas (such as the edge of the fairlead and the connection of the anti-ice cone). Combined with the finite element results, the maximum equivalent stress point (such as the contact area between the fairlead and the anchor chain) is determined, and its stress amplitude, mean and cyclic characteristics are analyzed.

[0025] The present invention can accurately calculate the fatigue life of the fairlead. A dynamic fatigue assessment framework can be used to accurately calculate the fatigue life of the fairlead for fatigue life and fatigue reliability analysis of the fairlead. The specific steps are as follows:

[0026] (1) Obtaining dynamic loads and stress analysis. Establish a refined finite element model of the fairlead. Take the time history data of the anchor chain tension obtained by AQWA time domain analysis (such as the tension amplitude and direction change) as dynamic boundary conditions and apply them to the fairlead model for transient dynamic analysis. Extract the stress-strain time history curve of the critical points of the fairlead, identify the stress concentration areas (such as the edge of the fairlead and the anchor chain contact area) through the stress contour map, and determine the key positions for fatigue analysis.

[0027] (2) Stress cycle statistics and equivalent processing. Conduct rainflow counting on the stress time history data of the critical points, and statistically analyze the amplitude, mean value, and number of cycles of the variable amplitude stress cycle; correct the S-N curve through the Goodman equation to convert the variable amplitude stress cycle into an equivalent constant amplitude stress amplitude, which simplifies the life prediction process under complex load spectra and enhances engineering applicability. This variable amplitude stress cycle correction technology should consider the influence of the mean stress on the fatigue life.

[0028] (3) Multiaxial fatigue damage assessment. Extract the multiaxial stress-strain components of the critical points, determine the critical plane (the plane of maximum shear strain or normal strain), and calculate the equivalent uniaxial strain amplitude; correct the Manson-Conffin formula. Based on the strain-life theory, calculate the crack initiation life, and calculate the total damage in combination with the Miner linear cumulative damage theory. This method combines the critical plane method and the corrected Manson-Conffin formula to propose a fatigue damage parameter suitable for the multiaxial stress state of the fairlead, solving the problem of underestimating damage by the traditional uniaxial S-N curve.

[0029] (4) Probabilistic fatigue reliability analysis. Apply the Weibull method to calculate the fatigue reliability R and its reliability index β of the key nodes; quantify the influence of extreme ice thickness and ice speed conditions on the reliability of the fairlead. This innovation in the application of the probability density function uses the Weibull distribution to introduce the fatigue reliability assessment of the fairlead of the floating wind turbine, and quantifies the failure probability under different ice speed and ice thickness conditions, providing a probabilistic basis for risk decision-making.

[0030] The fatigue damage of the fairlead described in the present invention under different working conditions includes:

[0031] Condition 1: combined action of wind and wave. The damage mechanism is the wave dynamic response and wind load fluctuation. The damage manifestation is that the service life of the fairlead decreases with the increase of wind speed. The anchor chain tension is the largest under the action of the rated wind speed and the smallest under the action of the cut-out wind speed. The anchor chain tension under the action of the extreme wind speed is between the above two, but the significant wave height and spectral peak period change with the wind speed, and the change amplitude of the stress level under the cyclic load also increases, thus causing the fairlead to be damaged faster;

[0032] Condition 2: combined action of wind and ice. The damage mechanism is the periodic impact of ice force and the influence of ice speed. The damage manifestation is that the service life of the fairlead shows a trend of first increasing and then decreasing. With the increase of wind speed, the wind load first decreases and then increases, and the ice load continuously increases; under the combined action of the extreme wind speed and ice, the service life of the fairlead is 20 years;

[0033] Condition 3: influence of ice thickness and ice speed on damage. When the ice speed is between 0.2 m / s and 0.6 m / s, the fatigue reliability index of the fairlead increases significantly with the increase of ice speed; when the ice speed is greater than 0.6 m / s, the fatigue reliability index decreases slightly, indicating that the periodic impact of ice force dominates the fatigue damage mechanism in the low ice speed range. When the ice thickness is between 0.1 m and 0.25 m, the fatigue reliability index of the fairlead decreases slightly with the increase of ice thickness; when the ice thickness is greater than 0.25 m, the fatigue reliability index decreases significantly. When the ice thickness is 0.374 m, the fatigue reliability index reaches the minimum value of 4.87, and the impact of large ice blocks on offshore structures needs to be focused on.

[0034] The fatigue analysis method for the fairlead structure of the SPAR - type offshore wind turbine provided by the present invention is used to solve the technical problems of fatigue reliability assessment and structural improvement of the fairlead of the offshore wind turbine under complex environmental loads, and to realize the safety of the floating offshore wind turbine structure.

[0035] The present invention has the following main advantages compared with the prior art:

[0036] 1. Improve the hydrodynamic performance of the floating wind turbine foundation:

[0037] By using advanced simulation software (such as AQWA) and finite element analysis methods, the dynamic responses of the floating wind turbine foundation under different conditions are studied, including the actions under complex environments such as wind - wave and ice loads, so as to optimize the foundation design of the floating wind turbine, enabling it to meet higher safety requirements in practical applications. For example, it is found that the peak value of the pitch motion response of the Spar floating platform is the largest, and it is subjected to greater wave forces under the action of low - frequency waves, which provides an important reference for the design. In addition, through time - domain analysis, it is verified that the safety factors of the mooring anchor chains are all greater than 1.67, indicating their stability under extreme conditions.

[0038] 2. Extend the equipment life and improve the reliability:

[0039] The present invention establishes a finite element model of the fairlead, conducts a transient analysis on it, and combines a fatigue life prediction method to study the fatigue damage conditions under different working conditions, providing a theoretical basis for the fatigue life assessment of the fairlead. The present invention evaluates the fatigue life of key components such as the fairlead through a fatigue analysis model, providing a theoretical basis for the crack initiation path. In addition, the critical plane method based on the multiaxial fatigue damage criterion can effectively predict the material life, providing a reliable basis for practical engineering applications.

[0040] 3. The parameter sensitivity analysis provided by the present invention includes revealing laws such as the increase in ice speed leading to a shortening of the fracture period and the increase in ice thickness exacerbating the stress amplitude, thus filling the gap in the research on the influence of ice load parameters on the fatigue of floating mooring structures.

[0041] Description of the Drawings

[0042] Figure 1 It is a model diagram of the fan system, where: (a) Schematic diagram of the fan and loads (b) Schematic diagram of the mooring system

[0043] Figure 2 It is a time history diagram of the harmonic superposition method.

[0044] Figure 3 It is a schematic diagram of the action of wind load.

[0045] Figure 4 It is a schematic diagram of the AQWA hydrodynamic model, where: (a) Frequency domain analysis model, (b) Time domain analysis model.

[0046] Figure 5 It is a plan and elevation view of the fairlead, where: (a) Plan view of the fairlead, (b) Elevation view of the fairlead.

[0047] Figure 6 It is the finite element model and stress nephogram of the fairlead (the dangerous node is located at the part connected to the fan foundation), where: (a) Geometric model of the fairlead, (b) Finite element model of the fairlead, (c) Equivalent stress distribution diagram, (d) Node with the maximum stress.

[0048] Figure 7 It is a flowchart for calculating the fatigue life of the fairlead.

[0049] Figure 8 It is a flowchart for calculating the fatigue reliability of the fairlead.

[0050] Figure 9 It is a fatigue reliability index diagram of the fairlead, where, (a) Fatigue reliability index diagram of the fairlead under the action of different ice speeds, (b) Fatigue reliability index diagram of the fairlead under the action of different ice thicknesses. Detailed Implementation Manner

[0051] The present invention proposes a fatigue analysis method for offshore wind turbine fairing holes, uses the critical surface method to calculate the equivalent strain amplitude of the fatigue danger point to obtain the fatigue damage parameter, and accurately evaluates the fatigue damage of the offshore wind turbine fairing holes under different working conditions under the combined action of wind-waves, wind-ice, ice thickness or ice speed through the modified Manson-Conffin formula and Miner linear cumulative damage theory; by establishing a numerical model of the fairing hole, considering its geometric structure and stress characteristics, the fatigue life of the fairing hole is accurately calculated, and the fatigue life and fatigue reliability analysis of the fairing hole are carried out.

[0052] Embodiment 1:

[0053] The fatigue analysis method of the SPAR offshore wind turbine fairing hole structure provided in this embodiment includes the following steps:

[0054] 1. Fairlead hole structure:

[0055] This embodiment proposes a new type of fairlead hole structure, which has the following characteristics:

[0056] (1) Geometric dimensions:

[0057] The inner hole of the cable guide hole adopts a segmented arc design (radius R1=200mm, R2=450mm), and reinforcing ribs are added in the directions of 45°, 90°, 135°, and 270°. The thickness of the reinforcing ribs is increased at the places where the tensile force is applied to prevent damage caused by excessive tension. The thickness is 25mm to disperse stress concentration.

[0058] (2) Material strengthening:

[0059] Q345 steel is used and surface hardening treatment is performed in key contact areas to improve fatigue resistance.

[0060] (3) Anchor chain contact surface design:

[0061] The contact surface width is 60%-75% of the anchor chain diameter, and the pressure is evenly distributed, reducing the local stress peak.

[0062] 2. Fatigue life analysis method:

[0063] Combining AQWA and ANSYS software, a multi-axial fatigue life assessment method is proposed, which includes the following steps:

[0064] (1) Environmental load simulation:

[0065] The wind load time history is generated by the harmonic superposition method, the ice load time history is calculated based on the secondary fracture theory, and the wave load is simulated in combination with the wave spectrum.

[0066] (2) Dynamic response analysis:

[0067] Establish an overall model of the floating wind turbine, conduct frequency-domain and time-domain simulations, and obtain the stress-strain time-history data of the dangerous points of the fairlead.

[0068] (3) Critical damage surface determination:

[0069] Use the critical plane method to determine the maximum shear strain plane, and combine the rain-flow counting method to extract the variable-amplitude stress cycles.

[0070] (4) Fatigue life prediction:

[0071] Based on the modified Manson-Conffin formula and Miner's linear cumulative damage theory, calculate the crack initiation life of the fairlead.

[0072] 3. Reliability assessment method:

[0073] Introduce the Weibull distribution probability model, combine the Goodman criterion to modify the S-N curve, calculate the fatigue reliability indexes under different ice speed and ice thickness conditions, and quantify the influence of ice loads on the life.

[0074] Example 2:

[0075] The present invention takes the NREL 5MW wind turbine system with OC3-Hywind as the floating support structure as an application example, establishes a finite element model of the wind turbine, imports it into ANSYS / AQWA, conducts frequency-domain and time-domain calculations respectively, and obtains the motion responses of the degrees of freedom and the tensions of the anchor chains. Based on the Panama fairlead model, a new fairlead applicable to the wind turbine is built and a finite element model is established to analyze the fatigue reliability and evaluate the fatigue life of the wind turbine fairlead under different working conditions.

[0076] The following is described in conjunction with the accompanying drawings.

[0077] 1. The wind turbine system model is as Figure 1 shown, and the NREL_5MW Hywind single-column floating wind turbine system and the main parameters of each part (blade and nacelle parameters, tower parameters, spar floating body parameters, anchor chain parameters) are given in detail. Its rotor diameter is 126m, the swept area is 12463m 2 , the cut-in wind speed is 3m / s, the cut-out wind speed is 25m / s, the rated wind speed is 11.4m / s, and the height at the hub center is 90m. The wind power tower uses Q345 steel, with an elastic modulus of 2.1E11, a density of 8500 Kg / m 3 , a Poisson's ratio of 0.3, a yield strength of 345 MPa. The wind turbine foundation uses a Spar-type floating foundation, and the mooring system uses catenary positioning.

[0078] 2. Use the harmonic superposition method (the flow chart is as Figure 2The time history curve of the pulsating wind speed at each simulation point of the wind power tower is simulated (as shown), and the wind power tower is divided into 10 segments in total in the present invention. The wind load acts on the center of each segment, so the time history of the wind speed of 10 segments is simulated. The specific acting positions are as Figure 3 shown, and the magnitude of the instantaneous wind load at each simulation point of the tower is obtained through the conversion relationship between the wind speed and the wind pressure, and the aerodynamic load for the rotation of the wind turbine is obtained through the actuator disk theory.

[0079] 3. Considering the dynamic response of the structure under the action of normal ice thickness and the extreme ice thickness once in 50 years, the ice thickness once in 50 years is taken as 0.374 m and the normal ice thickness is 0.2 m according to the ice thickness probability density function. The setting of the ice condition should not only reflect the characteristics of the dangerous working condition but also reflect the movement response of the normal working condition. In the calculation working condition, the ice speed is divided into six cases: 0 - 0.2, 0.2 - 0.4, 0.4 - 0.6, 0.6 - 0.8, 0.8 - 1.0, 1.0 - 1.1 m / s, and the ice thickness is divided into six cases: 0.1 - 0.15, 0.15 - 0.2, 0.2 - 0.25, 0.25 - 0.3, 0.3 - 0.35, 0.35 - 0.374. The maximum value within the working condition range is selected for calculation. Considering the secondary fracture of the ice load, the magnitude of the ice load on the wind power tower under different working conditions is simulated through the generalized function of the dynamic ice force.

[0080] 4. The AQWA hydrodynamic model is as Figure 4 shown. The response amplitude operators (RAO) of the surge, pitch and heave of the structure of the wind turbine in the frequency domain are analyzed. Due to the structural symmetry, when the wave direction is the same as the surge and pitch directions, the influence is greater, and when it is perpendicular to the sway direction, the influence is smaller. Therefore, only these three degrees of freedom are analyzed. The wave period is 3 - 30 seconds, and the direction is -180° - 180°. One direction is divided every 60°, and there are 7 directions in total. The response amplitude operator of the system and each wave force are calculated, and the hydrodynamic performance of the floating foundation structure is relatively excellent and meets the use requirements.

[0081] 5. The time domain analysis of the wind turbine is carried out by using the two modules of AQWA - Drift and AQWA - Drift. It is assumed that the nacelle and the impeller are replaced by mass points, and the fixed wind thrust is respectively applied at the hub center and the tower to replace the aerodynamic load at the impeller and the wind load on the tower. The loads and responses of the wind turbine in three operating states are analyzed. It can be obtained from the analysis results that under the rated wind speed working condition, the wind thrust plays a major role in the movement responses of the surge, pitch and the anchor chain tension; under the extreme wind speed working condition, the influence of the ice load on the movement response is more significant. Since the anchor chain 1 is in the same direction as the load direction and the tension it receives is the largest, the safety factors of the mooring cables in this paper are all greater than 1.67 and meet the requirements.

[0082] 6. By statistically analyzing the motion responses of the wind turbine foundation under various working conditions, the maximum value, minimum value, average value, and variance of the responses are obtained. It is determined that the motion responses of the wind turbine foundation under the three working conditions are all within the safety values and meet the requirements.

[0083] 7. The plan and elevation views of the fairlead of the wind turbine are as follows Figure 5 shown. The inner hole of the fairlead adopts a segmented circular arc design (radii R1 = 200 mm, R2 = 450 mm), and stiffeners are added at 45°, 90°, 135°, and 270° directions with a thickness of 25 mm to disperse stress concentration. The finite element model is established in ANSYS as follows Figure 6 shown. The anchor chain tension is converted into pressure and applied on the surface of the fairlead. Assuming that the pressure is evenly distributed along the anchor chain direction on the contact surface and the pressure direction is perpendicular to the inner hole surface, the stress-strain time history curve of the most dangerous point of the fairlead is obtained through transient analysis calculation; according to the basic steps of multi-axial fatigue crack initiation life assessment by the critical plane method and the relevant theories of cumulative damage theory, the critical failure plane where damage may occur is found through Matlab programming; finally, the strain amplitude and cycle number are obtained by the rain flow counting method, and according to the Manson-Conffin formula and Miner's cumulative damage theory, the fatigue life of the fairlead under different load conditions is calculated. The flow chart of the fairlead fatigue life calculation is as follows Figure 7 shown.

[0084] 8. Considering the special structure, complex stress, and easy failure of the fairlead structure at the connection part between the wind turbine foundation and the mooring system, the fatigue reliability of the fairlead nodes under various working conditions is analyzed. The flow chart of the fairlead fatigue reliability calculation is as follows Figure 8 shown. First, the stress time history curve at the dangerous node of the fairlead obtained by AQWA calculation is used to calculate the cycle amplitude and number through the rain flow counting method, and then the variable amplitude stress cycle is converted into a constant amplitude stress cycle by using the equivalent stress amplitude formula; finally, based on the Weibull probability density function theory and combined with the Goodman criterion to correct the S-N curve, the fatigue reliability index of the fairlead under different load conditions is calculated, and the influence of ice force parameters (ice speed and ice thickness) on the fatigue reliability of the fairlead is discussed. When the ice thickness is constant, as the ice speed increases, the fatigue reliability of the fairlead decreases, and when the ice speed reaches a certain large value, the decreasing amplitude is large; when the ice speed is constant, as the ice thickness increases, the fatigue reliability of the fairlead increases, and when the ice thickness is very small, the increasing amplitude is particularly large. The existence of dynamic ice load has a non-negligible impact on the fatigue reliability of the wind turbine fairlead. The fatigue reliability indexes of the fairlead under different ice thicknesses and ice speeds are as Figure 9 shown.

[0085] The fatigue analysis method for the fairlead structure of a SPAR - type offshore wind turbine provided by the present invention adopts the integration of fairlead fatigue life assessment methods, including the calculation of multi - axial fatigue damage parameters and the variable - amplitude stress cycle correction technology; and the quantification of fatigue reliability based on the Weibull distribution, including the innovation of the application of the probability density function and the parameter sensitivity analysis, and has the following technical effects:

[0086] 1. Multidisciplinary analysis:

[0087] (1) Combined action of wind, wave and ice:

[0088] Most of the existing studies focus on the influence of a single load (such as wind or wave) on offshore wind turbines, while the present invention comprehensively considers the combined action of various environmental loads such as wind, wave and ice. By simulating the wind speed time - history through the harmonic superposition method, calculating the ice force time - history based on the secondary fracture theory, and combining with hydrodynamic analysis, it can more comprehensively reflect the actual stress situation of offshore wind turbines in complex marine environments.

[0089] (2) Multi - physical - field coupling:

[0090] It not only considers the static and dynamic responses of the structure, but also combines fatigue analysis, realizing a full - chain analysis from environmental loads to structural responses and then to fatigue life.

[0091] 2. High - precision numerical simulation:

[0092] (1) Combination of finite element and hydrodynamic analysis: An ANSYS software is used to establish a finite - element model of the floating wind - turbine tower and foundation, and it is imported into AQWA software for hydrodynamic performance analysis. This multi - software combined analysis method can more accurately simulate the motion response and stress distribution of the wind turbine in complex marine environments.

[0093] (2) Transient analysis and fatigue life assessment: The stress - strain time - history curve of the most dangerous point of the fairlead is obtained through transient analysis, and the fatigue life is evaluated by combining the critical - plane method and the Miner cumulative damage theory. This method can more accurately capture the fatigue behavior of the structure under dynamic loads.

[0094] 3. Fatigue reliability analysis:

[0095] (1) Probability analysis based on the Weibull distribution:

[0096] In existing studies, the assessment of fatigue life mostly adopts deterministic methods, while the present study introduces the Weibull distribution probability density function, which can more scientifically evaluate the fatigue reliability of the fairlead under different load conditions and provide a more reliable reference for engineering design.

[0097] (2) Analysis of the influence of ice load: Pay special attention to the influence of ice load on the fatigue life and reliability of fairleads, filling the gap in existing research in this field. Through the fatigue reliability analysis under different ice thicknesses and ice velocities, the significant influence of ice load on the fatigue performance of fairleads is revealed.

Claims

1. A fatigue analysis method for the fairlead structure of a SPAR-type offshore wind turbine, characterized in that, The equivalent strain amplitude at the fatigue critical point is calculated by the critical plane method to obtain the fatigue damage parameter. Through the modified Manson-Conffin formula and Miner's linear cumulative damage theory, the fatigue damage of the fairlead of the offshore wind turbine is accurately evaluated under different working conditions including the combined action of wind and wave, wind and ice, different ice thicknesses or ice velocities. By establishing a numerical model of the fairlead, considering its geometric structure and stress characteristics, the fatigue life of the fairlead is accurately calculated, and the fatigue life and fatigue reliability analysis of the fairlead are carried out.

2. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to claim 1, wherein, The following method is used to calculate the fatigue damage parameter by the critical plane method: Through Matlab programming, the critical failure plane where damage may occur is found, the maximum shear strain plane at each moment is obtained, and the maximum shear strain plane with weighted average is used as the critical damage plane. The corresponding shear strain time history component and normal strain time history component on the critical damage plane are obtained. The strain amplitude and cycle number of the positive shear strain time history component of the fairlead are calculated by the rain flow counting method, and the von Mises criterion is used to synthesize the positive shear strain amplitudes on the critical plane into an equivalent strain as the damage control parameter.

3. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to claim 1, characterized in that, The following method is used to modify the Manson-Conffin formula and Miner's linear cumulative damage theory: (1) Combine high-cycle fatigue and low-cycle fatigue according to the Manson-Conffin formula. The total strain value is divided into elastic strain and plastic strain. Use the von Mises criterion to combine the two parameters y max and into an equivalent strain, and use it as the damage control parameter on the critical plane. The derived unified multiaxial fatigue life resistance equation is: Wherein, is the strain range on the critical plane; Δε e is the elastic strain amplitude; Δε p is the plastic strain amplitude; σ′ f is the fatigue strength coefficient; ε′ f is the fatigue plasticity coefficient; b is the fatigue strength exponent; c is the fatigue plasticity exponent; E is the elastic modulus, γ max is the shear strain cyclic amplitude, is the normal strain cyclic amplitude N f is the fatigue life. (2) Miner's linear cumulative damage theory: The damage of a structural member subjected to n cycles is D = n / N, where N is the number of cycles required for material failure. When D = 1, the structure fails.

4. The fatigue analysis method for the fairlead structure of the SPAR-type offshore wind turbine according to claim 1, characterized in that, The following method is used to establish the numerical model of the fairlead: Model the Spar floating wind turbine. The shell element SHELL63 is used to simulate the frame and floating foundation. Analyze the motion response and cable tension of the floating wind turbine under environmental loads. The catenary theory is used to simulate 3 symmetrically distributed anchor chains, and the pre-tension is adjusted to ensure the static balance of the platform. Based on the actual fairlead structure, the shape of the orifice, the support structure and the weld details are mainly restored to adapt to the dynamic load transfer of the mooring cable. The fairlead is located at the connection between the floating platform and the tower. The displacement boundary conditions of the fairlead area are extracted from the overall model. The finite element software ANSYS is used to establish the fairlead model and divide the mesh. A locally refined model is established. The hexahedral mesh is encrypted in the stress concentration areas such as the orifice and fillet. The element type is the 3D solid element SOLID95. The materials of the fairlead and the foundation are both steel Q345.

5. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to claim 1, wherein, The numerical model of the fairlead is as follows: The geometric model takes the OC3-Hywind Spar type floating wind turbine as the research object, including a 5MW NREL wind turbine, a Spar floating platform and a catenary mooring system. It is moored and positioned by 3 anchor chains with an included angle of 120°. The fairlead adopts a segmented arc design (radius R1 = 200mm, R2 = 450mm), and the diameter of the anchor chain is 90mm.

6. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to claim 1, characterized in that, The following method is used to judge the geometric structure and stress characteristics of the numerical model of the fairlead: (1) Geometric structure judgment method: Based on existing standard models such as Panama fairleads, geometric parameters are adjusted in combination with the structural characteristics of floating wind turbines, and the fairlead size is determined according to the design requirements of the mooring system; a parametric model is established, and by adjusting parameters such as the diameter, wall thickness, and cone angle of the fairlead, and matching and optimizing in combination with the actual sizes of the floating platform and the tower: (2) Analysis method of force characteristics: Dynamic load simulation, including mooring tension, wind load, wave load, ice load; considering the combined action of wind-wave and wind-ice, multi-condition time-domain dynamic response simulation is carried out through AQWA software to obtain the dynamic load input of the fairlead; finite element transient analysis is used to calculate the stress distribution and strain time history curve of the key nodes of the fairlead, and identify the stress concentration area; Combined with the finite element results, determine the maximum equivalent stress point, and analyze its stress amplitude, mean value and cyclic characteristics.

7. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to claim 1, characterized in that The following method is used to accurately calculate the fatigue life of the fairlead, and then analyze the fatigue life and fatigue reliability of the fairlead. The dynamic fatigue assessment framework is used to accurately calculate the fatigue life of the fairlead. The specific steps are as follows: (1) Obtaining dynamic loads and stress analysis, establish a refined finite element model of the fairlead, use the time history data of the anchor chain tension obtained from the AQWA time-domain analysis as the dynamic boundary condition, apply it to the fairlead model, conduct transient dynamics analysis, extract the stress-strain time history curve of the dangerous point of the fairlead, identify the stress concentration area through the stress nephogram, and determine the key position for fatigue analysis; (2) Stress cycle statistics and equivalent processing, conduct rainflow counting on the stress time history data of the dangerous point, and statistically analyze the amplitude, mean value and cycle number of the variable amplitude stress cycle; convert the variable amplitude stress cycle into an equivalent constant amplitude stress amplitude through the Goodman equation, and correct the S-N curve to consider the influence of the mean stress on the fatigue life; (3) Multiaxial fatigue damage assessment, extract the multiaxial stress-strain components of the dangerous point, determine the critical plane, and calculate the equivalent uniaxial strain amplitude; Modify the Manson-Conffin formula, based on the strain-life theory, calculate the crack initiation life, and calculate the total damage in combination with the Miner linear cumulative damage theory. (4) Probabilistic fatigue reliability analysis, apply the Weibull method to calculate the fatigue reliability R and its reliability index β of the key node; quantify the influence of extreme conditions of ice thickness and ice speed on the reliability of the fairlead.

8. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to claim 1, wherein, The fatigue damage of the fairlead under different conditions includes: Condition 1, combined action of wind and wave, the damage mechanism is wave dynamic response and wind load fluctuation, and the damage manifestation is that the service life of the fairlead decreases with the increase of wind speed. The anchor chain tension is the largest under the rated wind speed, the smallest under the cut-out wind speed, and the anchor chain tension under the extreme wind speed is between the above two, but the significant wave height and spectral peak period change with the change of wind speed, and the change amplitude of the stress level under cyclic load also increases, thus causing the fairlead to fail faster; Condition 2, combined action of wind and ice, the damage mechanism is the periodic impact of ice force and the influence of ice speed, and the damage manifestation is that the service life of the fairlead shows a trend of first increasing and then decreasing. With the increase of wind speed, the wind load first decreases and then increases, and the ice load continuously increases; Condition 3: Influence of ice thickness and ice velocity on damage. When the ice velocity is between 0.2 m / s and 0.6 m / s, the fatigue reliability index of the fairlead increases significantly with the increase of ice velocity; when the ice velocity is greater than 0.6 m / s, the fatigue reliability index decreases slightly, indicating that the periodic impact of ice force dominates the fatigue damage mechanism in the low ice velocity range.

9. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to claim 8, characterized in that, In Condition 2, under the combined action of extreme wind speed and ice, the service life of the fairlead is 20 years. In Condition 3, when the ice thickness is between 0.1 m and 0.25 m, the fatigue reliability index of the fairlead decreases slightly with the increase of ice thickness; when the ice thickness is greater than 0.25 m, the fatigue reliability index decreases significantly; when the ice thickness is 0.374 m, the fatigue reliability index reaches the minimum value of 4.87, and the influence of large ice blocks on offshore structures needs to be focused on.

10. The fatigue analysis method for the fairlead structure of the SPAR type offshore wind turbine according to any one of claims 1 to 9, characterized in that, It is used to solve the problems of fatigue reliability assessment and structural improvement technology of the fairlead of offshore wind turbines under complex environmental loads, and to realize the safety of the structure of offshore floating wind turbines.

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