Method for offshore wind turbine blade root bolt fatigue assessment

Abstract

This invention discloses a fatigue assessment method for offshore wind turbine blade root bolts, comprising the following steps: obtaining the load time mileage acting on the blade root based on environmental parameters, including sea state parameters of the wind turbine's operating sea area and the coupling effects of wind, waves, and currents; constructing a finite element model including bolts, nuts, transverse nuts, and bearings based on the geometric and material parameters of the blade root connection; numbering the bolts; and applying an initial preload F to the bolts in the model. p0 Different gradient magnitudes of external bending moments were applied to the blade root connection section. Multiple load steps were set for the finite element model, and preload and external bending moments were applied step by step. The relationship between the external bending moment and the maximum principal stress was obtained by linear fitting, and the bending moment time history was converted into the bolt maximum principal stress time history. The rainflow counting method was used to process the time domain stress data, and the stress amplitude and cycle number were statistically analyzed. Based on the extended S-N curve and Palmgern-Miner rule, bolt fatigue prediction was completed.

Description

A fatigue assessment method for offshore wind turbine blade root bolts Technical Field

[0001] This invention belongs to the field of offshore wind power generation technology, specifically relating to a fatigue assessment method for offshore wind turbine blade root bolts. Background Technology

[0002] Offshore wind turbines, subjected to the combined effects of wind, waves, and ocean currents, are susceptible to severe fatigue damage, particularly at critical components such as the blade root connection, potentially leading to catastrophic failures. The blade root connection is a core component of offshore wind turbines, responsible for transferring blade loads to the hub. As blade size increases, the requirements for fatigue strength in the blade root connection become increasingly stringent. Therefore, fatigue life assessment of the blade root connection during the design phase is crucial. However, bolt preload gradually diminishes during long-term service. Studies show that significant preload loss can occur in bolt connections within the first two years of operation. This preload loss alters the stress distribution at the blade root connection, affecting its fatigue strength. Traditional fatigue assessment methods are typically based on the SN curve and Palmgern-Miner's rule, but these methods have limitations when considering preload loss. Therefore, developing a fatigue assessment method that accurately considers the impact of preload loss is essential for ensuring the safe operation of offshore wind turbines.

[0003] In existing technologies, the changes in bolt stress response caused by preload loss are not fully considered, resulting in limited prediction accuracy. Typically, only the fatigue life of a single bolt is considered, neglecting the impact of preload loss on surrounding bolts. There is a lack of quantitative analysis on the influence of the degree, quantity, and location of preload loss on the fatigue life of the leaf root bolt. Summary of the Invention

[0004] This application provides a method for fatigue assessment of root bolts on offshore wind turbine blades, including the following steps:

[0005] Based on environmental parameters, including sea state parameters of the wind turbine's operating area and the coupling effects of wind, waves, and currents, the load time mileage acting on the blade root is obtained.

[0006] Based on the geometric and material parameters of the blade root connection, a finite element model containing bolts, nuts, transverse nuts, and bearings is constructed. The bolts are numbered, and an initial preload F is applied to the bolts in the model. p0 ;

[0007] Initial preload F p0 The formula is as follows:

[0008] F p0 =σ y A;

[0009] Where, σ yWhere A is the bolt yield strength, and A is the effective area of ​​the bolt.

[0010] External bending moments of different gradient magnitudes are applied to the blade root connection profile. The range of external bending moment values ​​covers the load magnitude of the offshore wind turbine under various sea conditions obtained from load analysis.

[0011] Multiple load steps are set for the finite element model, and the preload and external bending moment are applied step by step. The maximum principal stress σ on the bolt under different external loads is calculated. 1max The relationship between the applied bending moment and the maximum principal stress is obtained by linear fitting, and the bending moment time history is converted into the bolt maximum principal stress time history.

[0012] Rainflow counting: Use the rainflow counting method to process time-domain stress data, and statistically analyze stress amplitude and cycle number;

[0013] Based on the extended SN curve and the Palmgern-Miner rule, bolt fatigue prediction was performed.

[0014] The technical solution provided in this application also has the following technical features:

[0015] Preferably, in one embodiment of this application, the location of the dangerous bolt with the lowest fatigue life is found, and a preload loss ratio is set for the cover bolt and the bolts around it to study the influence of the amount of preload loss and the number of bolts with preload loss on the fatigue life of the blade root connection.

[0016] Preferably, in one embodiment of this application, the load time mileage acting on the blade root is obtained by obtaining the bending moment time history acting on the blade cross section under various sea states based on dynamic analysis.

[0017] Preferably, in one embodiment of this application, the obtained stress amplitudes are corrected by average stress based on the Goodman curve;

[0018] The Goodman curve formula is as follows:

[0019]

[0020] Where σ a This represents the actual stress amplitude, which is half of the stress range Δσ. e σ is the stress cycle amplitude under equivalent zero mean stress. m For the mean stress, σ u This represents the ultimate tensile strength of the steel.

[0021] Preferably, in one embodiment of this application, fatigue life is calculated using an extended SN curve, with the stress range Δσ and the number of stress cycles N input, according to BS EN 1993-1-9;

[0022] The extended SN curve expression is as follows:

[0023]

[0024] Where Δσ C The stress cycle number N is equal to 2 × 10 6 The corresponding stress range, Δσ D This represents the fatigue limit of constant amplitude.

[0025] Preferably, in one embodiment of this application, for bolts with a diameter greater than 30 mm, the SN curve is adjusted according to the bolt size effect to reflect the size influence;

[0026] The expression for the bolt size effect coefficient is as follows:

[0027]

[0028] Where d is the nominal diameter of the bolt.

[0029] Preferably, in one embodiment of this application, the Palmgren-Miner rule is applied to calculate the cumulative fatigue damage D of the bolt;

[0030]

[0031] Where n i N represents the number of stress cycles in the i-th group. i The constant amplitude stress amplitude σ i The number of loading cycles required for fatigue failure to occur under load; when D=1, the structure is considered to have experienced fatigue failure and is identified as the most dangerous bolt.

[0032] Preferably, in one embodiment of this application, the bolt number most prone to fatigue failure is determined based on fatigue analysis results, and the preload loss ratio k is used as the basis for determining the bolt number. pl Adjust the preload of this bolt while keeping the preload of other bolts constant to simulate different degrees of bolt preload loss;

[0033] Preload loss ratio k pl The expression is as follows:

[0034]

[0035] F pr k represents the remaining preload of the bolt. pl ≥0.8.

[0036] Preferably, in one embodiment of this application, a preload loss ratio is set for the dangerous bolt and several surrounding bolts; the overall impact of the number of bolts with preload loss on the fatigue life of the entire blade root connection system is analyzed.

[0037] Preferably, in one embodiment of this application, the distribution of the preload loss ratio for the critical bolt and several surrounding bolts is considered to be the same for all bolts, or the critical bolt is taken as the lowest value, and the ratio is considered to increase linearly or cosinely towards both bolts until k. pl =0.8.

[0038] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

[0039] 1. This application constructs a finite element model that includes details such as bolts and nuts, dynamically applies preload and external loads, and reasonably simulates the influence of preload loss on bolt stress distribution; by simulating the coupling effect of preload loss and external load through the finite element model, it solves the limitation of traditional methods that ignore preload changes;

[0040] 2. This application quantitatively evaluates the impact of different preload loss ratios on fatigue life by combining extended SN curves, Goodman mean stress correction, and bolt size effect coefficient; by setting preload loss ratios for multiple connecting bolts, it can effectively consider the overall impact of complex preload loss scenarios on the fatigue strength of bolt connections.

[0041] 3. Based on the cumulative results of fatigue damage, the most dangerous bolt is located, and by setting different preload loss ratios and quantities, its comprehensive impact on the stress distribution and fatigue life of surrounding bolts is analyzed; the high-risk bolt is accurately located and its failure is quantified to provide a scientific basis for the preventive maintenance of offshore wind turbines. Attached Figure Description

[0042] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0043] Figure 1 is a flowchart illustrating a method for fatigue assessment of offshore wind turbine blade root bolts according to the present invention.

[0044] Figure 2 shows the finite element model of the blade root connection structure of an offshore wind turbine.

[0045] Figure 3 is a schematic diagram of the leaf root coordinate system and bolt numbering;

[0046] Figure 4 shows different preload loss ratios k. pl Fatigue analysis results of the lower leaf root bolt;

[0047] Figure 5 shows the fatigue analysis results of the leaf root bolts under different numbers of failed bolts. Detailed Implementation

[0048] The specific embodiments of this application will be further described in detail below with reference to the accompanying drawings. These embodiments are only for illustrating this application and are not intended to limit the invention.

[0049] In the description of this invention, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0050] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0051] Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0052] A method for fatigue assessment and critical bolt location of offshore wind turbine blade root bolts considering preload loss, specifically including the following steps:

[0053] Determine the sea state parameters of the operating sea area of ​​the wind turbine to be analyzed, consider the coupling effect of wind, waves and current, and obtain the bending moment time history acting on the blade section under each sea state based on dynamic analysis;

[0054] Based on the geometry and material properties of the blade root connection, a detailed finite element model was constructed, including bolts, nuts, transverse nuts, and bearings. The bolts were numbered to facilitate subsequent location of critical bolts. An initial preload F was applied to the bolts in the model. p0 ;

[0055] The initial preload is calculated as follows:

[0056] F p0 =σ y A

[0057] Where, σ y Where A is the bolt yield strength, and A is the effective area of ​​the bolt.

[0058] Bending moments of different gradients are applied to the blade root connection profile, with the range of bending moment magnitudes covering the load magnitudes of the offshore wind turbine under various sea conditions obtained from load analysis.

[0059] Multiple load steps are set for the finite element model, and the preload and external bending moment are applied step by step. The maximum principal stress σ on the bolt under different external loads is calculated. 1max The relationship between the applied bending moment and the maximum principal stress is obtained based on linear fitting, thereby converting the bending moment time history into the bolt maximum principal stress time history.

[0060] Rainflow counting: The rainflow counting method is used to process time-domain stress data, and the stress amplitude and cycle number are statistically analyzed for fatigue analysis.

[0061] The average stress was corrected for each stress amplitude obtained using Goodman curves.

[0062] The Goodman curve formula is as follows:

[0063]

[0064] Where σ a This represents the actual stress amplitude, which is half of the stress range Δσ. e σ is the stress cycle amplitude under equivalent zero mean stress. m For the mean stress, σ u The ultimate tensile strength of steel;

[0065] According to BS EN 1993-1-9, fatigue life is calculated using the extended SN curve, with the stress range Δσ and the number of stress cycles N input.

[0066] The extended SN curve expression is as follows:

[0067]

[0068] Where Δσ C The stress cycle number N is equal to 2 × 10 6 The corresponding stress range, Δσ D This represents the fatigue limit of constant amplitude.

[0069] For bolts with a diameter greater than 30mm, the bolt size effect needs to be considered and the SN curve adjusted to reflect the size influence.

[0070] The expression for the bolt size effect coefficient is as follows:

[0071]

[0072] Where d is the nominal diameter of the bolt.

[0073] Apply the Palmgren-Miner rule to calculate the cumulative fatigue damage D of the bolt.

[0074]

[0075] Where n i N represents the number of stress cycles in the i-th group. i The constant amplitude stress amplitude σ i The number of loading cycles required for fatigue failure to occur under load; when D=1, the structure is considered to have experienced fatigue failure.

[0076] Based on the fatigue analysis results, the bolt numbers most prone to fatigue failure were determined, and the preload loss ratio k was used as a basis for further analysis. pl Adjust the preload of this bolt while keeping the preload of other bolts constant to simulate different degrees of bolt preload loss.

[0077] Preload loss ratio k pl The expression is as follows:

[0078]

[0079] F pr For the remaining preload of the bolts, engineering practice shows that the long-term preload loss of general bolted mechanical mechanisms is greater than k. pl Typically, it is between 0.9 and 0.8. Considering the complex working environment of offshore wind turbines, it is recommended that the preload loss ratio be no less than 0.8.

[0080] Centering on the critical bolt, preload loss ratios were set for it and several surrounding bolts. The overall impact of the number of bolts experiencing preload loss on the fatigue life of the entire blade root connection system was analyzed.

[0081] The distribution of the preload loss ratio for the critical bolt and several surrounding bolts is considered to be the same for all bolts, or the lowest value is set for the critical bolt, and the ratio is considered to increase linearly or cosinely towards both bolts until k. pl =0.8.

[0082] Specifically, in one embodiment of this application, as shown in Figure 1, the present invention provides a method for fatigue assessment and dangerous bolt location of offshore wind turbine blade root bolts considering preload loss. In this embodiment, based on the established fatigue assessment and location method, fatigue assessment and dangerous bolt location of a floating wind turbine blade root connecting bolt under the influence of preload loss are carried out.

[0083] The offshore wind turbine is a floating turbine combining a NERL 5MW turbine with an OC3-hywind Spar foundation. This turbine and platform foundation were developed by the National Renewable Energy Laboratory (NERL) to support various conceptual studies of offshore wind power technology; therefore, this turbine model is widely used as a benchmark for offshore wind turbine specifications and is highly representative. The turbine rotor is 126m long, and the blade roots are connected using T-bolts. The bolts are made of 10.9 grade high-strength steel, and the connecting flanges are made of stainless steel. The blade root connection geometry is shown in Table 1, and the parameters of the high-strength steel and stainless steel materials are shown in Table 2. The turbine uses a three-point mooring system, assuming that the wind, waves, and current are in the same direction and perpendicular to the turbine blade rotation direction. Load-time history curves under different sea states were obtained based on dynamic analysis.

[0084] As shown in Figure 2, the finite element model of the bolted connection structure applies fixed constraints to both the inner and outer surfaces of the pitch bearing to accurately simulate the blade root connection structure under operating conditions. All bolts are numbered from 0 to 119 as shown in Figure 3. An external bending moment M is applied to the blade root end face as shown in Figure 2, and a magnitude of 0.55σ is applied to the bolts. y The preload of A is determined. The maximum principal stress on the bolt is analyzed to obtain the relationship between the applied load and the maximum principal stress of each bolt, and the time history curve of the applied load is transformed into the time history curve of the maximum principal stress on each bolt.

[0085] The stress-time history of the blade root bolts was statistically processed using the rainflow counting method to convert it into usable stress cycles, and the mean stress was corrected using Goodman curves. The fatigue life of all blade root bolts was calculated based on Palmgren-Miner and SN curves, as shown in Figure 4 (k). pl As shown in the data at k=1.0, due to the order-of-magnitude difference in fatigue life between bolts in different locations, the calculation results are displayed as the reciprocal of the fatigue life. pl =1.0 represents the case of no bolt preload loss, and the analysis results are equivalent to the traditional bolt fatigue analysis results without considering bolt preload loss. The calculation results show that bolt #115 has the shortest fatigue life and is the most dangerous bolt. Therefore, different k values ​​are set for bolt #115. pl Meanwhile, the preload of other bolts was kept constant for analysis, and the results are plotted together in Figure 4. The analysis results show that when k pl When the preload loss is small (e.g., k = 0.8), the fatigue life of the bolt is significantly improved. This is because the reduction in preload, while maintaining tightness, alters the stress distribution on the 115# bolt and nearby bolts. However, only about 8 bolts are significantly affected by the preload loss of the 115# bolt. As the preload loss continues to increase, i.e., k... plWhen the stress level dropped to 0.6, the fatigue life of bolt #115 and its adjacent bolts decreased significantly. This indicates that the bolt joint loosened, leading to a significant increase in the stress range of the root bolt, which has a serious adverse effect on the bolt's fatigue life.

[0086] We selected bolt #115 and its surrounding bolts to set the preload loss ratio and performed fatigue life calculations to analyze the impact of the number of bolts with preload loss on fatigue life. The results are compared with those without considering preload loss, as shown in Figure 5. It can be seen that as the number of preloaded bolts increases, the fatigue life of the preloaded bolt and its surrounding bolts decreases. The more bolts with preload loss, the more bolts experience a decrease in fatigue life due to preload loss. When bolts #113-#117 experience preload loss, the fatigue life of nearly a quarter of the root bolts significantly decreases. It can be seen that preload loss has a significant impact on the fatigue life of bolted connections, but the traditional method, as shown in Figure 4 (k...),... pl =1.0 and the case without preload loss in Figure 5 cannot take into account the influence of this important factor; the method of this invention simulates the coupling effect of preload loss and external load, which breaks through the limitation of traditional methods that ignore preload changes.

[0087] It should be noted that although the above example only illustrates the fatigue analysis of bolted connections on a specific type of offshore wind turbine tower, in actual implementation, this step can also consider different bolt sizes, bolt quantities, bolt materials, and wind turbine foundation types. Furthermore, those skilled in the art will understand that the above equipment may only include the factors necessary to implement the embodiments of this specification, and not necessarily all the factors shown in the specification.

[0088] Table 1 Geometric Dimensions of Offshore Wind Turbine Blade Root Connection

[0089] Parameter values: Blade root outer diameter (m) 3.6; Blade root ply thickness (mm) 100; Number of T-bolts 120; Transverse nut hole diameter (mm) 66; Blade root length (m) 4.1; Bolt specification M36 surface

[0090] Table 2. Material Properties of Steel for Bolted Connection Structures

[0091] Main dimensions: density of high-strength steel and stainless steel ρ (kg / m³) 3 78507750 Young's modulus E (GPa) 207193 Poisson's ratio ν 0.29 0.30 Ultimate tensile strength σ u (MPa) 1040586 Yield Strength σ y (MPa) 940207 surface

[0092] The technical solution of this application can predict the remaining life of bolts using models based on fracture mechanics, etc., instead of the Palmgren-Miner rule, and can also achieve the fatigue life prediction in this invention.

[0093] The technical solution of this application can measure the change of bolt preload in real time through technical means such as fiber optic sensors or ultrasonic equipment, directly obtain data, and can also be used for subsequent modeling and fatigue analysis to achieve the purpose of the invention.

[0094] In summary, this invention aims to address the problem of insufficient consideration of the impact of preload loss on offshore wind turbine blade root bolts in existing fatigue analysis techniques, and proposes a fatigue assessment method that integrates finite element analysis, fatigue life prediction, and parameter research.

[0095] This application proposes a technical solution for locating and assessing the fatigue of dangerous bolts at the blade root, considering preload loss, by combining finite element analysis and fatigue life prediction. It comprehensively evaluates the stress state and fatigue strength of the entire blade root connection through holistic modeling, thereby quickly identifying the location of potentially failing dangerous bolts. Furthermore, by considering the impact of preload loss on the stress distribution and fatigue life of adjacent bolts, it analyzes the influence of different preload loss ratios, quantities, and locations on the fatigue life of blade root bolts, providing guidance for the design and maintenance of offshore wind turbine blade root connections. This invention innovatively combines finite element analysis and fatigue prediction methods, overcoming the shortcomings of existing technologies in preload loss assessment, and providing a comprehensive fatigue assessment and dangerous bolt location technology for offshore wind turbine blade root connections. This invention patent is of great significance in filling an industry gap and improving the safety and reliability of offshore wind turbine operation and maintenance.

[0096] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A method for fatigue assessment of blade root bolts in offshore wind turbines, characterized in that, The process includes the following steps: Based on environmental parameters, including sea state parameters of the wind turbine's operating area and the coupling effects of wind, waves, and currents, the load time mileage acting on the blade root is obtained; based on the geometric and material parameters of the blade root connection, a finite element model containing bolts, nuts, transverse nuts, and bearings is constructed, the bolts are numbered, and an initial preload is applied to the bolts in the model. Initial preload The formula is as follows: ;in, For the bolt yield strength, The effective area of ​​the bolt is defined. External bending moments of varying magnitudes are applied to the blade root connection section, with the range of values ​​covering the loads experienced by the offshore wind turbine under various sea conditions as determined by load analysis. Multiple load steps are set for the finite element model, applying preload and external bending moments step by step, and the maximum principal stress on the bolt is calculated under different external loads. The relationship between applied bending moment and maximum principal stress was obtained through linear fitting, and the bending moment time history was converted into the bolt maximum principal stress time history. Rainflow counting was used to process the time-domain stress data, and stress amplitude and cycle number were statistically analyzed. Based on the extended SN curve and Palmgern-Miner rule, bolt fatigue prediction was performed. Based on the fatigue analysis results, the bolt numbers most prone to fatigue failure were determined, and the preload loss ratio was used to predict the bolts. Adjust the preload of this bolt while keeping the preload of other bolts constant to simulate different degrees of bolt preload loss; the preload loss ratio... The expression is as follows: This represents the remaining preload of the bolt. 。 2. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 1, characterized in that, Locate the critical bolt position with the lowest fatigue life, and set a preload loss ratio for the cover bolt and its surrounding bolts to study the influence of preload loss and the number of bolts with preload loss on the fatigue life of the blade root connection.

3. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 1, characterized in that, The load time history acting on the blade root is obtained by obtaining the bending moment time history acting on the blade cross section under various sea states based on dynamic analysis.

4. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 1, characterized in that, The obtained stress amplitudes are corrected for average stress based on the Goodman curve; the Goodman curve formula is as follows: in This represents the actual stress amplitude, and its value is the stress range. Half of This represents the stress cycle amplitude under equivalent zero mean stress. For average stress, This represents the ultimate tensile strength of the steel.

5. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 1, characterized in that, According to BS EN 1993-1-9, using the extended SN curve, input the stress range. Fatigue life is calculated using the number of stress cycles N; the extended SN curve expression is as follows: ,in ,in in The number of stress cycles equal The corresponding stress range, This represents the fatigue limit of constant amplitude.

6. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 1, characterized in that, For bolts with a diameter greater than 30mm, the SN curve is adjusted according to the bolt size effect to reflect the influence of size; the expression for the bolt size effect coefficient is as follows: in This refers to the nominal diameter of the bolt.

7. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 1, characterized in that, Applying the Palmger-Miner rule: Calculating cumulative fatigue damage of bolts ； in For the first The number of stress cycles in a group. constant amplitude stress amplitude The number of loading cycles required for fatigue failure to occur under load; when If the bolt fails due to fatigue, it is considered to be the most dangerous bolt.

8. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 1, characterized in that, Centered on the critical bolt, preload loss ratios were set for it and several surrounding bolts; the overall impact of the number of bolts experiencing preload loss on the fatigue life of the entire blade root connection system was analyzed.

9. The fatigue assessment method for offshore wind turbine blade root bolts as described in claim 8, characterized in that, The distribution of the preload loss ratio for the critical bolt and several surrounding bolts is considered to be the same for all bolts, or the lowest value is set for the critical bolt, and the ratio is gradually increased towards both bolts using a linear or cosine function until it reaches the minimum value. 。

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