Safety level checking method for design formula of offshore wind turbine monopile foundation under extreme environmental conditions

Abstract

The present application relates to offshore wind power engineering and structural reliability analysis technical field, disclose a kind of based on reliability analysis's offshore wind turbine single pile foundation design formula safety level checking method under extreme environmental conditions.The method is based on the historical information of marine environmental data, establish the probability distribution model of environmental random variable and related structural parameters acting on offshore wind turbine, and under the condition of preset recurrence period, combined with soil condition characteristics, the key structural response of single pile foundation is obtained by finite element simulation simulation.Further based on the statistical results of structural response, the probability distribution of the annual maximum bending moment of single pile foundation is determined, the limit state function corresponding to the design formula is constructed, and the reliability analysis method is used to calculate the reliability index and the corresponding safety level of the design formula.The present application can provide a theoretical basis for the trade-off and decision-making of economic cost and safety level in offshore wind turbine foundation design under the premise of ensuring structural safety.

Description

Technical Field

[0001] This disclosure belongs to the field of offshore wind power technology, specifically involving a method for verifying the safety level of offshore wind turbine monopile foundation design formulas under extreme environmental conditions based on reliability analysis. Background Technology

[0002] Offshore wind turbine foundations can be categorized into several structural types, including monopile foundations, gravity foundations, multipile foundations, suction foundations, and floating foundations. Among these, monopile foundations, being the simplest structural type, are currently the dominant foundation type for offshore wind power. Statistics show that over 75% of offshore wind turbine foundations worldwide utilize monopile foundations.

[0003] Due to the complex seabed conditions, rough seas, ocean currents, and floating ice, the construction cost of offshore wind turbine foundations accounts for approximately 20% to 30% of the total cost. Therefore, controlling the cost of the foundation structure is crucial in the design of offshore wind turbines. The cost of the offshore wind turbine foundation structure is closely related to its safety and reliability; the two are mutually restrictive. Improving the safety of the foundation structure and even the entire wind turbine requires increasing the cost of the foundation structure, while reducing the economic efficiency of the foundation structure means reducing the safety and reliability of the entire wind turbine. Therefore, studying the safety and reliability of offshore wind turbine foundation structures is of great significance. The safety and reliability of offshore wind turbine foundation structures are directly related to the design formulas in standards or specifications, so it is necessary to verify these formulas. Furthermore, for monopile foundations, extreme environmental conditions are often the controlling conditions; therefore, understanding the safety level of the design formulas under such conditions is of great importance. Summary of the Invention

[0004] To address the aforementioned technical issues, this disclosure proposes a method for verifying the design formula of offshore wind turbine monopile foundations under extreme environmental conditions based on reliability analysis. This method combines finite element model simulation, statistics, and reliability analysis to accurately evaluate the safety level of the design formula in the standard when applied to offshore wind turbine monopiles under extreme environmental conditions.

[0005] To achieve the above-mentioned technical objectives, the specific technical solution adopted in this disclosure is as follows:

[0006] A method for verifying the safety level of offshore wind turbine monopile foundation design formulas under extreme environmental conditions based on reliability analysis includes the following steps:

[0007] Step 1: Determine the design formula to be checked Select a model fan that matches the applicable conditions of the design formula to be checked;

[0008] Step 2: Based on historical information of marine environmental data, statistical analysis is conducted to establish the marginal probability distribution and joint probability distribution of the multidimensional environmental random variables acting on offshore wind turbines for a preset statistical time interval, and the corresponding marine environmental condition parameters under the preset return period conditions are calculated accordingly.

[0009] Step 3: Based on the marine environmental parameters obtained in Step 2 under the preset return period conditions, and determining the soil condition characteristics of the offshore wind turbine site, the maximum bending moment within the single pile foundation of the wind turbine is calculated using finite element simulation, combining the marine environmental parameters and soil condition characteristics. and the axial force at the corresponding position ;

[0010] Step 4: Based on the maximum bending moment and axial force obtained in Step 3, Using the formula to be verified, the steel pipe pile is designed, and the standard values ​​of the geometric design parameters of the steel pipe pile are calculated.

[0011] Step 5: Establish the distribution of random variables involved in the design formula, whereby the random variables are used to characterize the uncertainties of structural parameters, load parameters, model parameters, and environmental parameters;

[0012] Step 6: Based on the probability distribution of marine environmental conditions and soil properties obtained in Step 2, generate multiple sets of extreme environmental conditions and soil parameters samples, and use finite element simulation based on the samples to obtain the maximum bending moment sample of a single pile foundation under short-term working conditions, thereby determining the probability distribution of the bending moment response of the single pile foundation.

[0013] Step 7: Based on the probability distribution of wind speed at the hub established in Step 2, determine the annual duration characteristics of the offshore wind turbine when the wind speed exceeds the operating conditions, and derive the distribution function of the probability distribution of the annual maximum bending moment of the single pile foundation.

[0014] Step 8: Introduce the axial force obtained in Step 3, the design standard value determined in Step 4, the probability distribution of each random variable established in Step 5, and the probability distribution of the annual maximum bending moment obtained in Step 7, to construct the design formula described in Step 1. Corresponding limit state function The reliability index and corresponding safety level of the design formula under extreme environmental conditions were calculated using the first second moment method or the Monte Carlo method.

[0015] Furthermore, in step 1, the design formula takes the following form:

[0016] (1)

[0017] In the formula, express a design parameter indicating a function indicating the safety factor introduced by the design.

[0018] Further, in step 2, the multi-dimensional environmental random variables include: the wind speed at the hub of the offshore wind turbine , the significant wave height , the spectral peak period , and the tidal subsurface flow velocity . The preset statistical time interval is 1 hour, and the preset return period is 50 years.

[0019] Further, in step 3, the soil condition characteristics include the undrained shear strength of clay , the internal friction angle of sand . If the soil is stratified, the standard values of the soil conditions of multiple soil layers need to be obtained.

[0020] Further, in step 3, if there is a lack of statistical data, for clay, the undrained shear strength is taken as 20, 40, 60, 80 kPa, and for sand, the internal friction angle is taken as 20°, 25°, 30°, 35°, 40° for calculation respectively.

[0021] Further, in step 4, the geometric design parameters include the yield strength , the diameter , the wall thickness . The design standard value includes the minimum wall thickness . If there are no determined fixed design parameters, the yield strength can be taken as 235 MPa or 345 MPa, the diameter , unit: m, approximately taken as , where is the rated power of the model wind turbine, unit: MW.

[0022] Further, in step 5, the random variables include the uncertainty of the permanent load , the uncertainty of the calculation model , the uncertainty of the yield strength , the uncertainty of the dynamic model , the uncertainty of the wave model and the probability distribution of the soil characteristics obtained based on step 4. If there is a lack of statistical data, then:

[0023] (1) The uncertainty of the permanent load obeys a normal distribution with a mean of 1.05 and a coefficient of variation of 0.05;

[0024] (2) Uncertainty of yield strength It follows a normal distribution with a mean of 1.15 and a coefficient of variation of 0.08;

[0025] (3) Uncertainty of dynamic model It follows a log-normal distribution with a mean of 1.0 and a coefficient of variation of 0.10;

[0026] (4) Uncertainty of wave model It follows a log-normal distribution with a mean of 1.0 and a coefficient of variation of 0.10;

[0027] (5) Undrained shear strength of clay Unit: kPa, follows a normal distribution with mean undrained shear strength standard value and coefficient of variation of 0.15;

[0028] (6) Angle of internal friction of sand The unit is rad, and it follows a modified normal distribution with the following probability density function:

[0029]

[0030] In the formula: , for The upper and lower limits, For scale parameters; , s and average and standard deviation The approximate relation is expressed by the following formula:

[0031]

[0032] The mean is the standard value of the internal friction angle of sand, the coefficient of variation is 0.15, the lower limit is a=10°=0.1745 rad, the upper limit is b=60°=1.0472 rad, and the scale parameter is s=1.4371 rad.

[0033] Furthermore, step 6 specifically includes the following steps:

[0034] Step 6.1: Using the probability distributions of marine environmental conditions and soil properties obtained in Step 2, randomly generate sample values ​​for the variables, and use the probability distribution of soil properties determined in Step 5 to generate sample values ​​for soil properties; a total of all generated sample values ​​are... Group;

[0035] Step 6.2: Use the data generated in Step 6.1 Sample values ​​of marine environmental conditions and soil were analyzed. The 10-minute finite element simulation yielded the following results. By analyzing the sample values ​​of the maximum bending moment of each pile, the probability distribution of the maximum bending moment of the pile over 10 minutes is obtained. ;

[0036] Furthermore, in step 6, the wind speed at the hub needs to be greater than the cut-out wind speed. The interval is generated.

[0037] Furthermore, in step 7, the annual duration characteristic is the cumulative annual duration during which the wind speed at the hub exceeds the cut-out wind speed. The expected probability distribution of the annual maximum bending moment is determined by the annual extreme value statistical method.

[0038] The beneficial effects of this disclosure are: Based on reliability analysis, this invention provides reliability indicators and corresponding safety levels for the design formula of offshore wind turbine foundations, providing important theoretical support for the trade-off and decision-making between the economic cost and safety level of offshore wind turbine foundations. Attached Figure Description

[0039] Figure 1 This is a publicly available flowchart. Detailed Implementation

[0040] The following specific embodiment will illustrate the application of the technical solution disclosed herein. Unless otherwise specified in the embodiment, conditions are determined according to conventional conditions or the user's experience.

[0041] Step 1: Determine the design formula to be checked Select a model fan that matches the applicable conditions of the design formula to be checked.

[0042] Typically, design formulas in standards take the following form:

[0043] (1)

[0044] In the formula, express Design parameters, such as the outer diameter and thickness of the pile section; express These functions include bending moment and axial force. Indicates the design introduction A safety factor. This depends on the physical meaning and specific design formulas; the value of the safety factor may be related to the design parameters.

[0045] This embodiment uses a simplified design formula: consider a system subjected to axial force. and bending moment The round steel pipe has a diameter of , with a wall thickness of , and a yield strength of , and the design requirements satisfy the formula

[0046] (2)

[0047] Where is the safety factor.

[0048] The model wind turbine in this step can be created by the applicant according to specific requirements or an existing general model wind turbine can be used.

[0049] Step 2: According to the historical information of ocean environmental data, perform fitting analysis to obtain the marginal probability distribution and joint probability distribution of the wind speed at the hub of the offshore wind turbine, the significant wave height , the spectral peak period , and the probability distribution of the tidal subsurface flow velocity at an hourly time interval, and calculate the wind speed at the hub, the significant wave height, and the tidal subsurface flow velocity with a return period of 50 years, that is, once in 50 years.

[0050] Generally speaking, in the obtained ocean environmental data, the wind speed is not the wind speed at the hub, but the wind speed at a specific height, such as 10 m or 100 m above sea level. At this time, the power law or logarithmic wind profile can be used to approximately convert the wind speed at a specific height to the wind speed at the hub of the wind turbine. For example, the power law wind profile: <-->

[0051] (3)

[0052] In the formula: is the wind speed at 100 m, is the hub height, unit: m, is the power law exponent.

[0053] Generally, the wind speed follows a Weibull distribution, and the significant wave height conditioned on the wind speed follows a normal or lognormal distribution. If the data shows other distribution characteristics, the data shall prevail.

[0054] The environmental conditions with a return period of 50 years are calculated as follows:

[0055] (4)

[0056] In the formula: is the wind speed at the hub , the significant wave height or the tidal subsurface flow velocity , Given its 50-year return period; for The inverse function of the probability distribution function; for for or In this case, parameters Take 8766, for for In this case, parameters The value is 2922.

[0057] The reason for choosing a 50-year return period is that the offshore wind turbine industry typically considers a 50-year return period as an extreme environmental condition. Parameters The values ​​are derived from commonly used time intervals for marine environmental conditions during offshore wind turbine design. For wind speed and ocean currents, a 1-hour interval is commonly used, with 8766 1-hour intervals per year; for wave height, a 3-hour interval is commonly used, with 2922 3-hour intervals per year. If the user has other requirements, the 50-year and [other time intervals] values ​​here can be adjusted. It can also be adjusted according to needs.

[0058] Step 3: Based on the marine environmental conditions with a 50-year return period obtained in Step 2, and the soil condition characteristics of the offshore wind turbine site are determined, the maximum bending moment within the single pile foundation of the wind turbine is calculated using finite element simulation based on the marine environmental conditions and soil condition characteristics. and the axial force at the corresponding position ;

[0059] The soil condition characteristics include clay having undrained shear strength. Sand has an internal friction angle If the soil is layered, then the soil condition standard values ​​for the multiple layers of soil need to be obtained.

[0060] The maximum bending moment within the steel pipe pile during this step It usually appears in the soil portion of the pile.

[0061] Step 4: Based on the maximum bending moment inside the steel pipe pile and the axial force at the corresponding location obtained in Step 3. The steel pipe piles are designed using the formula to be verified. The basic design geometric parameter of the steel pipe pile is the yield strength. ,diameter Wall thickness Fixed yield strength ,diameter The minimum wall thickness that satisfies the design formula can be calculated. That is, the design standard value of steel pipe piles.

[0062] Use the maximum bending moment obtained in step 3 and a fixed yield strength ,diameter The minimum wall thickness can be obtained by solving equation (1). If no specific conditions are given, then the yield strength... The pressure is 235MPa or 345MPa because Q235 or Q345 steel is commonly used in the offshore wind turbine industry; diameter Unit: m, approximated as ,in The rated power of the model wind turbine, in MW, is the preferred value.

[0063] Step 5: Establish the distribution of random variables involved in the design formula, including the uncertainty of permanent loads. Uncertainty in computational models Uncertainty of yield strength Uncertainty in dynamic models Wave model uncertainty And uncertainties in soil properties, including: clay having undrained shear strength Distribution, sandy soil has an internal friction angle If the soil is stratified, then the probability distribution of the soil properties of each layer needs to be obtained.

[0064] In this step, if there is no statistical data, it is preferred to...

[0065] (1) Uncertainty of permanent load It follows a normal distribution with a mean of 1.05 and a coefficient of variation of 0.05;

[0066] (2) Uncertainty of yield strength It follows a normal distribution with a mean of 1.15 and a coefficient of variation of 0.08;

[0067] (3) Uncertainty of dynamic model It follows a log-normal distribution with a mean of 1.0 and a coefficient of variation of 0.10;

[0068] (4) Uncertainty of wave model It follows a log-normal distribution with a mean of 1.0 and a coefficient of variation of 0.10;

[0069] (5) Undrained shear strength of clay Unit: kPa, follows a normal distribution with mean undrained shear strength standard value and coefficient of variation of 0.15;

[0070] (6) Angle of internal friction of sand The unit is rad, and it follows a modified normal distribution with the following probability density function:

[0071] (5)

[0072] In the formula: , for The upper and lower limits, For scale parameters; , s and average and standard deviation The approximate relation is expressed by the following formula:

[0073] (6)

[0074] The mean is the standard value of the internal friction angle of sand, the coefficient of variation is 0.15, the lower limit is a=10°=0.1745 rad, the upper limit is b=60°=1.0472 rad, and the scale parameter is s=1.4371 rad.

[0075] Step 6.1: Using the joint probability distribution of wind speed at the hub, significant wave height, and spectral peak period obtained in Step 2, randomly generate sample values ​​for these random variables, where the wind speed at the hub needs to be greater than the cut-out wind speed. Interval generation; using the probability distribution of soil properties from step 5 to generate sample values ​​for soil properties. A total of all generated sample values... Group.

[0076] The specific method for generating the wind speed, meaningful wave height, and spectral peak period at the hub in this step is as follows: The wind speed at the hub is generated from the truncated random variable corresponding to the conditional distribution function (7). Sample values, in Under the condition of generating meaningful wave height Sample values, in and The wave spectrum peak period is generated under the condition of The sample values.

[0077] (7)

[0078] To strike a balance between the good fit to random variables and computational cost, the following was determined: The preferred value.

[0079] Step 6.2: Use the steps from step 6.1 Sample values ​​of marine environmental conditions and soil were analyzed. A 10-minute finite element simulation was performed to obtain... By analyzing the sample values ​​of the maximum bending moment of each pile, the probability distribution of the maximum bending moment of the pile over 10 minutes was obtained. .

[0080] Step 7: Using the wind speed probability distribution at the wheel hub obtained in Step 2, calculate the total duration each year during which the wind speed at the wheel hub exceeds the cut-out wind speed. The expected number of times the wind speed at the hub exceeds the cut-out duration by 10 minutes per year is, in units of minutes. The distribution function of the probability distribution of the annual maximum bending moment is:

[0081] (8)

[0082] Equation (8) assumes that the bending moments of the steel pipe piles are independent for all 10-minute periods within a year. The expected number of 10-minute periods each year in which the wind speed at the hub exceeds the cut-out duration is... for .

[0083] Step 8: Introduce the axial force obtained in Step 3, the design standard value determined in Step 4, the probability distribution of each random variable established in Step 5, and the probability distribution of the annual maximum bending moment obtained in Step 7, to construct the design formula described in Step 1. Corresponding limit state function The reliability index and corresponding safety level of the design formula under extreme environmental conditions were calculated using the first second moment method or the Monte Carlo method.

[0084] This step involves obtaining the design formula. Corresponding limit state function The specific method is as follows: Design formula The design parameters were changed to standard values, and statistically obtained random variables were introduced. Removing the safety factor introduced by the design, we can obtain the limit state function of the form (9):

[0085] (9)

[0086] Taking equation (2) as an example, its corresponding limit state function is:

[0087] (10)

[0088] Then, by applying the first second moment method or the Monte Carlo method to equation (10), the reliability index and the safety level of the design formula can be obtained.

[0089] The above is merely one embodiment of the present invention. Those skilled in the art can modify the embodiments of the present invention without departing from the technical spirit of the invention. All embodiments without the inventive step of those skilled in the art are within the protection scope of the present invention.

Claims

1. A method for verifying the safety level of offshore wind turbine monopile foundation design formulas under extreme environmental conditions based on reliability analysis, characterized in that, Includes the following steps: Step 1: Determine the design formula to be checked Select a model fan that matches the applicable conditions of the design formula to be checked; Step 2: Based on historical information of marine environmental data, statistical analysis is conducted to establish the marginal probability distribution and joint probability distribution of the multidimensional environmental random variables acting on offshore wind turbines for a preset statistical time interval, and the corresponding marine environmental condition parameters under the preset return period conditions are calculated accordingly. Step 3: Based on the marine environmental parameters obtained in Step 2 under the preset return period conditions, and determining the soil condition characteristics of the offshore wind turbine site, the maximum bending moment within the single pile foundation of the wind turbine is calculated using finite element simulation, combining the marine environmental parameters and soil condition characteristics. and the axial force at the corresponding position ; Step 4: Based on the maximum bending moment and axial force obtained in Step 3, Using the formula to be verified, the steel pipe pile is designed, and the standard values ​​of the geometric design parameters of the steel pipe pile are calculated. Step 5: Establish the distribution of random variables involved in the design formula, whereby the random variables are used to characterize the uncertainties of structural parameters, load parameters, model parameters, and environmental parameters; Step 6: Based on the probability distribution of marine environmental conditions and soil properties obtained in Step 2, generate multiple sets of extreme environmental conditions and soil parameters samples, and use finite element simulation based on the samples to obtain the maximum bending moment sample of a single pile foundation under short-term working conditions, thereby determining the probability distribution of the bending moment response of the single pile foundation. Step 7: Based on the probability distribution of wind speed at the hub established in Step 2, determine the annual duration characteristics of the offshore wind turbine when the wind speed exceeds the operating conditions, and derive the distribution function of the probability distribution of the annual maximum bending moment of the single pile foundation. Step 8: Introduce the axial force obtained in Step 3, the design standard value determined in Step 4, the probability distribution of each random variable established in Step 5, and the probability distribution of the annual maximum bending moment obtained in Step 7, to construct the design formula described in Step 1. Corresponding limit state function The reliability index and corresponding safety level of the design formula under extreme environmental conditions were calculated using the first second moment method or the Monte Carlo method.

2. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis as described in claim 1, characterized in that: In step 1, the design formula takes the following form: (1) In the formula, express One design parameter, express One function, Indicates the design introduction A safety factor.

3. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis as described in claim 1, characterized in that: In step 2, the multi-dimensional environmental random variables include: the wind speed at the hub of an offshore wind turbine , significant wave height , spectral peak period , and the velocity of the tidal subsurface current . The preset statistical time interval is 1 hour, and the preset return period is 50 years.

4. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis as described in claim 1, characterized in that: In step 3, the soil condition characteristics include the clay having an undrained shear strength. The internal friction angle of the sand is... If the soil is layered, then the soil condition standard values ​​for the multiple layers of soil need to be obtained.

5. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis as described in claim 4, characterized in that: In step 3, if statistical data is lacking, the undrained shear strength for clay is taken as 20, 40, 60, and 80 kPa, and the internal friction angle for sand is taken as 20°, 25°, 30°, 35°, and 40°, and calculations are performed accordingly.

6. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis as described in claim 1, characterized in that: In step 4, the geometric design parameters include yield strength. ,diameter Wall thickness The design standard values ​​include minimum wall thickness. If there are no fixed design parameters, the yield strength can be used. 235MPa or 345MPa, diameter Unit: m, approximated as ,in Rated power of the model wind turbine, unit: MW.

7. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis as described in claim 1, characterized in that: In step 5, the random variables include the uncertainty of the permanent load. Uncertainty in computational models Uncertainty of yield strength Uncertainty in dynamic models Wave model uncertainty And based on the probability distribution of soil properties obtained in step 4, if statistical data is lacking, then: (1) Uncertainty of permanent load It follows a normal distribution with a mean of 1.05 and a coefficient of variation of 0.05; (2) Uncertainty of yield strength It follows a normal distribution with a mean of 1.15 and a coefficient of variation of 0.08; (3) Uncertainty of dynamic model It follows a log-normal distribution with a mean of 1.0 and a coefficient of variation of 0.10; (4) Uncertainty of wave model It follows a log-normal distribution with a mean of 1.0 and a coefficient of variation of 0.10; (5) Undrained shear strength of clay Unit: kPa, follows a normal distribution with mean undrained shear strength standard value and coefficient of variation of 0.15; (6) Angle of internal friction of sand The unit is rad, and it follows a modified normal distribution with the following probability density function: In the formula: , for The upper and lower limits, For scale parameters; , s and average and standard deviation The approximate relation is expressed by the following formula: The mean is the standard value of the internal friction angle of sand, the coefficient of variation is 0.15, the lower limit is a=10°=0.1745 rad, the upper limit is b=60°=1.0472 rad, and the scale parameter is s=1.4371 rad.

8. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis according to claim 1, characterized in that: Step 6 specifically includes the following steps: Step 6.1: Using the probability distributions of marine environmental conditions and soil properties obtained in Step 2, randomly generate sample values ​​for the variables, and use the probability distribution of soil properties determined in Step 5 to generate sample values ​​for soil properties; a total of all generated sample values ​​are... Group; Step 6.2: Use the data generated in Step 6.1 Sample values ​​of marine environmental conditions and soil were analyzed. A 10-minute finite element simulation was performed to obtain... By analyzing the sample values ​​of the maximum bending moment of each pile, the probability distribution of the maximum bending moment of the pile over 10 minutes is obtained. .

9. A method for verifying the design formula of a single pile foundation for an offshore wind turbine under extreme environmental conditions based on reliability analysis, as described in claim 8, characterized in that: In step 6, the wind speed at the hub needs to be greater than the cut-out wind speed. The interval is generated.

10. The method for verifying the design formula of offshore wind turbine monopile foundation under extreme environmental conditions based on reliability analysis as described in claim 7, characterized in that: In step 7, the annual duration characteristic is the cumulative annual duration during which the wind speed at the hub exceeds the cut-out wind speed. The expected probability distribution of the annual maximum bending moment is determined by the annual extreme value statistical method.

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