Method for fatigue analysis of offshore wind turbine support structures

Abstract

The present application relates to the technical field of fatigue analysis of offshore wind turbine support structure, and particularly provides a fatigue analysis method of offshore wind turbine support structure, aiming at solving the problem that the concept of stress concentration factor cannot obtain full-field cumulative damage distribution for the existing offshore wind turbine support structure. For this purpose, the fatigue analysis method comprises: establishing a dynamic model for the whole machine; calculating equivalent static loads according to the dynamic model; establishing a global-local finite element analysis model for the offshore wind turbine support structure; applying unit forces along the x-axis, y-axis and z-axis directions and unit moments around the x-axis, y-axis and z-axis to the beam element node at the top of the vertical support cylinder in the global-local finite element analysis model; performing finite element calculation to obtain a stress influence matrix; and calculating fatigue damage results of the welds between the inclined support cylinder and the vertical support cylinder according to the equivalent static loads and the stress influence matrix.

Description

Technical Field

[0001] This invention relates to the field of fatigue analysis technology for offshore wind turbine support structures, and specifically provides a fatigue analysis method for offshore wind turbine support structures. Background Technology

[0002] In recent years, frequent tower collapses of onshore wind turbines have brought increasing attention to their structural strength. Due to the higher design, installation, and maintenance costs of offshore wind turbines, the strength of support structures, especially fatigue strength, has become a research hotspot. The stress concentration factor (SCF) concept is widely used in support structure design due to its ease of application. Tripod support structures are a common type of support structure for offshore wind turbines. Typically, a tripod support structure consists of a vertical support cylinder, three vertical legs, three inclined support cylinders, three horizontal support cylinders, and three horizontal connecting cylinders. Three vertical pile legs, three inclined support tubes, three horizontal support tubes, and three horizontal connecting tubes are all evenly distributed in a circular array around the vertical support tube. One end of each of the three horizontal support tubes is welded to the lower end of the vertical support tube, and the other end of each of the three horizontal support tubes is welded to the three vertical pile legs. The upper ends of each of the three inclined support tubes are welded to the part of the vertical support tube that is higher than the horizontal support tube. The lower ends of each of the three inclined support tubes are welded to the three vertical pile legs. The two ends of each adjacent vertical pile leg are welded to the two ends of a horizontal connecting tube.

[0003] The intersection of the inclined and vertical support tubes in the tripod support structure has a complex shape and is subjected to multiple complex loads, including in-plane and out-of-plane bending moments and thrust. The concept of stress concentration factor is relatively mature in the application of offshore wind power support structures, but it still has the following shortcomings: it usually selects the weak point of ultimate strength or assumes fatigue failure point based on experience, which cannot obtain the cumulative damage distribution across the entire field and can easily lead to a more dangerous design structure.

[0004] Therefore, a new technical solution is needed in this field to solve the above problems. Summary of the Invention

[0005] The present invention aims to solve the above-mentioned technical problems, namely, the problem that the existing offshore wind power support structure cannot obtain the total field cumulative damage distribution by using the stress concentration factor concept.

[0006] This invention provides a fatigue analysis method for an offshore wind turbine support structure. The offshore wind turbine support structure includes a vertical support cylinder and at least three inclined support cylinders arranged in a circular array around the vertical support cylinder, along with corresponding vertical pile legs. The upper end of each inclined support cylinder is welded to the same height position of the vertical support cylinder, and the lower end of each inclined support cylinder is welded to the same height position of the corresponding pile leg. The fatigue analysis method includes the following steps: establishing a dynamic model of the entire offshore wind turbine and its support structure; calculating the equivalent static load based on the dynamic model; and performing fatigue analysis on the offshore wind turbine... A finite element analysis model is established for the unit's support structure. In this model, the connection between the inclined support cylinder and the vertical support cylinder is modeled as a solid model composed of discrete hexahedral solid elements, while the other parts of the model are modeled as one-dimensional beam elements. Unit forces along the x, y, and z axes and unit moments about the x, y, and z axes are applied to the beam element nodes at the top of the vertical support cylinder in the finite element model, and a stress influence matrix is ​​obtained through finite element calculation. The fatigue damage results of the weld between the inclined support cylinder and the vertical support cylinder are calculated based on the equivalent static load and the stress influence matrix.

[0007] In the preferred embodiment of the above fatigue analysis method, the dynamic model is: Where M is the overall mass matrix of the machine, C is the damping matrix of the machine, K is the overall stiffness matrix of the machine, and u(t) is the time-series displacement value of each node of the machine. The speed of each node in the entire machine. Let F(t) be the acceleration of each node of the machine, and F(t) be the load on the machine.

[0008] In the preferred technical solution of the above fatigue analysis method, the step of "calculating the equivalent static load according to the dynamic model" specifically includes: calculating the time-series displacement value of each node of the whole machine during each fatigue condition; calculating the time-series equivalent static load at the beam unit node at the top of the vertical support cylinder; wherein, under the action of the time-series equivalent static load, each node of the whole machine generates a displacement that is the same as the time-series displacement value.

[0009] In the preferred embodiment of the above fatigue analysis method, the step of "calculating the fatigue damage result of the weld between the inclined support cylinder and the vertical support cylinder based on the equivalent static load and the stress influence matrix" includes: calculating the time-series equivalent stress of the time-series equivalent static load at each node on the weld based on the stress influence matrix and the time-series equivalent static load; determining the SN curve of the weld; determining the number of occurrences of each stress cycle in each fatigue condition based on the time-series equivalent stress and the SN curve; determining the number of occurrences of each stress cycle in each fatigue condition during the design life; and calculating the cumulative fatigue damage value of each node on the weld based on Miner's rule based on the number of occurrences of each stress cycle in each fatigue condition and the number of occurrences of each stress cycle in each fatigue condition during the design life.

[0010] In the preferred embodiment of the above fatigue analysis method, the step of "determining the number of occurrences of each stress cycle in each fatigue condition based on the time-series equivalent stress and the SN curve" specifically includes: processing the time-series equivalent stress using the rainflow counting method to obtain the amplitude and number of multiple stress cycles during the occurrence of each fatigue condition; and determining the number of occurrences of each stress cycle based on the amplitude of each stress cycle and the SN curve.

[0011] In the preferred embodiment of the above fatigue analysis method, the specific steps for "determining the number of occurrences of each stress cycle under each fatigue condition during the design life" are as follows: the number of occurrences of each fatigue condition during the design life is calculated based on the design load condition and design life of the whole machine and the wind speed Weibull curve; the number of occurrences of each stress cycle during each fatigue condition during one occurrence and the number of occurrences of each fatigue condition during the design life are calculated.

[0012] By adopting the above technical solution, the equivalent static load is calculated based on the dynamic model of the whole machine. Then, the stress influence matrix is ​​calculated using a finite element analysis model (i.e., a global-local model) that uses a discrete hexahedral solid element model for the connection between the inclined and vertical support cylinders and a one-dimensional beam element model for other parts. Finally, the fatigue damage results of the weld between the inclined and vertical support cylinders are calculated based on the equivalent stress load and the stress influence matrix. This allows for the acquisition of the cumulative damage distribution across the entire field, resulting in more accurate fatigue analysis results. This avoids the problem that existing offshore wind power support structures cannot obtain the cumulative damage distribution across the entire field using the stress concentration factor concept. By using the established global-local model to solve the stress influence matrix and further calculating the fatigue damage results of the weld between the inclined and vertical support cylinders based on the equivalent stress load and the stress influence matrix, accurate stress values ​​for local details can be obtained. This avoids the huge computational burden required for fatigue analysis when using a discrete global finite element analysis model for the entire support structure. It also weakens the dependence of the boundary conditions applied to the local structure on the overall analysis results, making it easier to apply boundary conditions. Attached Figure Description

[0013] The preferred embodiments of the present invention are described below with reference to the accompanying drawings, in which:

[0014] Figure 1 This is a diagram showing the main steps of the fatigue analysis method for the support structure of offshore wind turbines in this invention;

[0015] Figure 2 This is a finite element analysis model of the support structure of an offshore wind turbine according to an embodiment of the present invention.

[0016] List of reference numerals in the attached diagram:

[0017] 1. Vertical support tube; 2. Inclined support tube; 3. Vertical pile leg; 4. Horizontal support tube. Detailed Implementation

[0018] First, those skilled in the art should understand that the embodiments described below are merely for explaining the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0019] Addressing the issue mentioned in the background section that existing offshore wind turbine support structures, employing the stress concentration factor concept, cannot obtain the total cumulative damage distribution across the entire field, this invention provides a fatigue analysis method for offshore wind turbine support structures. This method can obtain the total cumulative damage distribution across the entire field, yielding more accurate fatigue analysis results and avoiding the problem of existing offshore wind turbine support structures failing to obtain the total cumulative damage distribution across the entire field using the stress concentration factor concept. By utilizing an established global-local model to solve the stress influence matrix, and further calculating the fatigue damage results of the welds between the inclined and vertical support cylinders based on the equivalent stress load and the stress influence matrix, accurate stress values ​​for local details can be obtained. This avoids the enormous computational burden required for fatigue analysis using a discrete global finite element analysis model of the entire support structure. It also reduces the dependence of local structural boundary conditions on the overall analysis results, facilitating the application of boundary conditions.

[0020] The following reference Figure 1 and Figure 2 The present invention will now be described. Among other things, Figure 1 These are the main steps of the fatigue analysis method for the support structure of offshore wind turbines according to the present invention. Figure 2 This is a finite element analysis model of the support structure of an offshore wind turbine according to an embodiment of the present invention.

[0021] Reference Figure 2 In one embodiment of the present invention, the offshore wind turbine support structure comprises a vertical support cylinder 1, three inclined support cylinders 2, three vertical pile legs 3, and three horizontal support cylinders 4. The three inclined support cylinders 2, the three vertical pile legs 3, and the three horizontal support cylinders 4 are all evenly distributed in a circular array around the vertical support cylinder 1. The upper ends of the three inclined support cylinders 2 are welded to the same height position of the vertical support cylinder 1, and the lower ends of each inclined support cylinder 2 are welded to the same height position of the corresponding vertical pile leg 3.

[0022] like Figure 1 As shown, the main steps of the fatigue analysis method for the support structure of offshore wind turbines of the present invention include:

[0023] Step S100: Establish a dynamic model for the entire offshore wind turbine and its supporting structure.

[0024] Specifically, the dynamic model is as follows: Where M is the overall mass matrix of the machine, C is the damping matrix of the machine, K is the stiffness matrix of the machine, and u(t) is the time-series displacement value of each node of the machine. The speed of each node in the whole machine. Let F(t) represent the acceleration of each node in the entire machine, and F(t) represent the load on the entire machine. It should be noted that the dynamic model can be adjusted according to specific circumstances in practical applications. The entire machine structure is modeled using one-dimensional beam elements in the dynamic model.

[0025] Step S200: Calculate the equivalent static load based on the dynamic model.

[0026] Because the machine operates under various conditions in practical applications, its structure also experiences various fatigue conditions. The number of occurrences of each fatigue condition varies within the machine's design life. Step S200 specifically includes: calculating the temporal displacement values ​​of each node of the machine during each occurrence of a fatigue condition based on the dynamic model; and calculating the temporal equivalent static load at the beam element nodes at the top of the vertical support cylinder. Under the action of the temporal equivalent static load, each node of the machine generates a displacement identical to the temporal displacement value. It should be noted that the temporal equivalent static load can be directly calculated using existing finite element analysis software; the specific calculation process will not be elaborated here.

[0027] Step S300: Establish an overall-local finite element analysis model for the support structure of the offshore wind turbine.

[0028] In the overall-local finite element analysis model, the connection between the inclined support cylinder 2 and the vertical support cylinder 1 is modeled using discrete hexahedral solid elements, while other parts of the overall-local finite element analysis model are modeled using one-dimensional beam elements. Specifically, as shown... Figure 2 As shown, the structure consisting of a small section connecting the upper end of the inclined support cylinder 2 to the vertical support cylinder 1, and a small section connecting the vertical support cylinder 1 and the inclined support cylinder 2, is modeled using discrete hexahedral solid elements. The remaining parts of the inclined support cylinder 2, the remaining parts of the vertical support cylinder 1, as well as the vertical leg 3 and the horizontal support cylinder 4, are modeled using one-dimensional beam elements. For the weld structure in the solid model, it is not necessary to model the weld toe separately. The area surrounding the weld is meshed according to the standards recommended by the International Institute of Welding (specifically, the mesh is divided on the outer wall of the longitudinal support cylinder from the intersection of the outer walls of the inclined support cylinder and the longitudinal support cylinder, moving away from the inclined support cylinder at distances of 0.4, 0.4, and 0.6 times the wall thickness of the inclined support cylinder). For the equivalent stress at the weld toe, the equivalent stress at the weld toe is interpolated in subsequent calculations based on the equivalent stress at positions 0.8 times and 1.4 times the wall thickness of the inclined support cylinder.

[0029] Step S400: Apply unit forces along the x-axis, y-axis, and z-axis and unit moments around the x-axis, y-axis, and z-axis to the beam element nodes at the top of the vertical support cylinder 1 in the global-local finite element analysis model, and perform finite element calculations to obtain the stress influence matrix.

[0030] Specifically, the unit forces f1, f2, and f3 along the x-axis, y-axis, and z-axis, and the unit torques m1, m2, and m3 about the x-axis, y-axis, and z-axis are respectively referenced. Figure 2 The application orientations of Fx, Fy, Fz and Mx, My, Mz at the beam element nodes at the top of the vertical support cylinder 1.

[0031] In the finite element analysis software, the stress influence matrix of each node obtained after finite element calculation is as follows:

[0032] The first to sixth columns of the stress influence matrix represent the stress components caused by unit forces f1, f2, f3 and unit moments m1, m2, m3 at the same node.

[0033] For each node on the weld, calculate the stress influence matrix for each node.

[0034] Step S500: Calculate the fatigue damage results of the weld between the inclined support tube and the vertical support tube based on the equivalent static load and stress influence matrix.

[0035] Step S500 specifically includes the following steps:

[0036] The time-series equivalent stress at each node of the weld is calculated based on the stress influence matrix of each node and the time-series equivalent static load.

[0037] Determine the SN curve of the weld;

[0038] The number of possible stress cycles for each stress cycle in each fatigue condition is determined based on the time-series equivalent stress and SN curves.

[0039] Determine the number of occurrences of each stress cycle under each fatigue condition during the design life;

[0040] The cumulative fatigue damage value of each node on the weld is calculated based on Miner's rule, according to the number of possible occurrences of each stress cycle in each fatigue condition and the number of occurrences of each stress cycle in each fatigue condition during the design life.

[0041] The steps of "calculating the time-series equivalent stress at each node of the weld based on the stress influence matrix and time-series equivalent static load at each node of the weld" specifically include:

[0042] For each node on the weld, the stress component of each load in the time-series equivalent static load at that node is calculated according to the following formula (i.e., the stress component of the load at the node is quickly calculated using the principle of linear superposition):

[0043] Among them, F x F y F z M x M y M z Each load in the time-series equivalent static load is decomposed into Figure 2 Components of force in the x, y, and z axes and around Figure 2 The components of the torque along the x-axis, y-axis, and z-axis, σ x σ y σ z τ xy τ yz τ zx These are the stress components of the same load at this node in the time-series equivalent static load;

[0044] Calculate the equivalent stress value using the following formula:

[0045]

[0046] Calculate the signed equivalent stress value using the following formula:

[0047] σ SVM =sign(W T ·σ)·σ VM ,

[0048] Where W = [1 1 1 0 0 0] T σ represents the stress component of the same load at the same node in the time-series equivalent static load. SVM The value is the signed equivalent stress value.

[0049] The temporal equivalent stress (i.e., the signed equivalent stress values ​​arranged in temporal order) of each node on each weld seam is calculated using the above method.

[0050] The specific steps for "determining the SN curve of the weld" are as follows: Determine the SN curve at the weld according to the DNV-RP-C203 standard:

[0051]

[0052] Where N is the number of stress cycles that can occur; m is the negative reciprocal of the slope of the SN curve. Here, we select a cathodic protection seawater environment, and the m values ​​before and after 1×106 cycles are 3 and 5, respectively; lga is the intercept of the SN curve on the horizontal axis; t is the structural thickness at the expected crack initiation point, in mm; and k is the exponential parameter, which is taken as 0.2 here.

[0053] The steps of "determining the number of possible stress cycles for each fatigue condition based on the time-series equivalent stress and SN curves" specifically include:

[0054] After processing the time-series equivalent stress using the rainflow counting method, the amplitude and number of multiple stress cycles during each fatigue condition were obtained.

[0055] The number of occurrences of each stress cycle is determined based on the amplitude of each stress cycle and the SN curve (that is, the number of occurrences N of the stress cycle is calculated by substituting the amplitude of each stress cycle into the equation of the SN curve).

[0056] The steps for “determining the number of occurrences of each stress cycle under each fatigue condition during the design life” specifically include:

[0057] The number of occurrences of each fatigue condition during the design life is calculated based on the overall machine's design load conditions, design life, and wind speed Weibull curve.

[0058] The number of occurrences of each stress cycle in each fatigue condition during the design life is calculated based on the number of times each stress cycle occurs once during each fatigue condition and the number of times each fatigue condition occurs within the design life (for example, the number of occurrences of each stress cycle during each fatigue condition during the design life is multiplied by the number of times the corresponding fatigue condition occurs within the design life to obtain the number of occurrences of each stress cycle in each fatigue condition within the design life).

[0059] The steps of "calculating the cumulative fatigue damage value of each node on the weld based on Miner's rule, according to the number of possible occurrences of each stress cycle under each fatigue condition and the number of occurrences of each stress cycle under each fatigue condition during the design life" specifically include:

[0060] The fatigue damage value of a specific node is calculated using the following formula:

[0061]

[0062] Where D is the fatigue damage value, n i Let N be the number of times the i-th stress cycle occurs during the design life. i Let be the number of possible occurrences of the i-th stress cycle.

[0063] According to the above formula, the fatigue damage value of each node on the weld is calculated, and the fatigue damage result of the weld is obtained.

[0064] The fatigue analysis method described above allows for the acquisition of the total cumulative damage distribution across the entire field, resulting in more accurate fatigue analysis results. This avoids the problem of existing offshore wind power support structures, which, using the stress concentration factor concept, cannot obtain the total cumulative damage distribution. By utilizing the established global-local model to solve the stress influence matrix, and further calculating the fatigue damage results of the welds between the inclined and vertical support cylinders based on the equivalent stress load and the stress influence matrix, precise stress values ​​for local details can be obtained. This avoids the enormous computational burden required for fatigue analysis using a discrete global finite element analysis model of the entire support structure. It also reduces the dependence of local boundary conditions on the overall analysis results, facilitating the application of boundary conditions. Furthermore, the linear superposition principle, performed only after obtaining the stress influence matrix M, avoids large-scale finite element calculations and significantly improves the efficiency of fatigue calculations under time-series loads.

[0065] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after such changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A fatigue analysis method for the support structure of an offshore wind turbine, characterized in that, The offshore wind turbine support structure includes a vertical support cylinder and at least three inclined support cylinders arranged in a circular array around the vertical support cylinder, along with corresponding vertical pile legs. The upper end of each inclined support cylinder is welded to the same height position of the vertical support cylinder, and the lower end of each inclined support cylinder is welded to the same height position of the corresponding pile leg. The fatigue analysis method includes the following steps: A dynamic model is established for the entire offshore wind turbine and its supporting structure. The equivalent static load is calculated based on the dynamic model. A finite element analysis model is established for the support structure of the offshore wind turbine. In the finite element analysis model, the connection part between the inclined support cylinder and the vertical support cylinder is a solid model composed of discrete hexahedral solid elements, while the other parts of the finite element analysis model are one-dimensional beam element models. In the finite element analysis model, unit forces along the x-axis, y-axis, and z-axis and unit moments around the x-axis, y-axis, and z-axis are applied to the beam element nodes at the top of the vertical support tube, and the stress influence matrix is ​​obtained by finite element calculation. The fatigue damage results of the weld between the inclined support cylinder and the vertical support cylinder are calculated based on the equivalent static load and the stress influence matrix.

2. The fatigue analysis method according to claim 1, characterized in that, The dynamic model is as follows: Where M is the overall mass matrix of the machine, C is the damping matrix of the machine, K is the overall stiffness matrix of the machine, and u(t) is the time-series displacement value of each node of the machine. The speed of each node in the entire machine. Let F(t) be the acceleration of each node of the machine, and F(t) be the load on the machine.

3. The fatigue analysis method according to claim 2, characterized in that, The specific steps of "calculating the equivalent static load based on the dynamic model" include: Calculate the time-series displacement values ​​of each node of the entire machine during each fatigue condition occurrence. Calculate the time-series equivalent static load at the beam element node at the top of the vertical support tube; Specifically, under the action of the time-series equivalent static load, each node of the entire machine generates a displacement that is the same as the time-series displacement value.

4. The fatigue analysis method according to claim 3, characterized in that, The steps of "calculating the fatigue damage results of the weld between the inclined support cylinder and the vertical support cylinder based on the equivalent static load and the stress influence matrix" include: The time-series equivalent stress of the time-series equivalent static load at each node on the weld is calculated based on the stress influence matrix and the time-series equivalent static load. Determine the SN curve of the weld; The number of possible occurrences of each stress cycle in each fatigue condition is determined based on the time-series equivalent stress and the SN curve. Determine the number of occurrences of each stress cycle under each fatigue condition during the design life; The cumulative fatigue damage value of each node on the weld is calculated based on Miner's rule, according to the number of possible occurrences of each stress cycle in each fatigue condition and the number of occurrences of each stress cycle in each fatigue condition during the design life.

5. The fatigue analysis method according to claim 4, characterized in that, The step of "determining the number of possible occurrences of each stress cycle in each fatigue condition based on the time-series equivalent stress and the SN curve" specifically includes: After processing the time-series equivalent stress using the rainflow counting method, the amplitude and number of multiple stress cycles during each fatigue condition are obtained. The number of times each stress cycle can occur is determined based on the amplitude of each stress cycle and the SN curve.

6. The fatigue analysis method according to claim 5, characterized in that, The specific steps for "determining the number of occurrences of each stress cycle under each fatigue condition during the design life" are as follows: The number of occurrences of each fatigue condition during the design life period is calculated based on the design load conditions and design life of the whole machine and the wind speed Weibull curve. The number of occurrences of each stress cycle in each fatigue condition during the design life is calculated based on the number of times each stress cycle occurs once during each fatigue condition and the number of occurrences of each fatigue condition within the design life.

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