Vibration-feature-based monitoring method for pile slippage of offshore wind power monopile foundation

Abstract

In the present invention, a modal parameter identification method is used to analyze collected acceleration data, so as to identify the vibration frequency of a monopile foundation. On the basis of a physical model and data analysis, this method can more accurately reflect the actual state of a pile body, thus improving the accuracy and reliability of monitoring. Compared with a conventional monitoring method based on pile tip resistance and a pile driving speed, a vibration-feature-based monitoring method has a lower cost. In addition, the present invention does not require complex devices and high maintenance costs, and thus has higher economic benefits.

Description

A monitoring method for pile slippage in offshore wind turbine monopile foundations based on vibration characteristics Technical Field

[0001] This invention belongs to the field of quality and safety management at offshore wind power construction sites, specifically a method for monitoring pile slippage in offshore wind power monopile foundations based on vibration characteristics. Background Technology

[0002] To ensure the quality and safety of offshore wind turbine monopile foundation construction, sensors are currently installed on the steel pipe piles and connected to a data acquisition and analysis device via cables to monitor the pile side friction and dynamic penetration resistance in real time during pile driving. By using sensors mounted on the pile to monitor these parameters in real time, the theoretical values ​​of pile end resistance obtained from numerical simulations are compared with the data measured on-site to assess the stability and safety of the pile foundation. However, the pile side friction and dynamic penetration resistance indicators are significantly affected by seabed geological conditions, leading to substantial deviations between actual measurements and numerical simulations. This can easily result in misjudgments of pile slippage, thus impacting construction progress.

[0003] Therefore, a new monitoring method for pile slippage in offshore wind turbine monopile foundations is urgently needed. Summary of the Invention

[0004] Purpose of the invention: To address the problems existing in the prior art, this invention proposes a method for monitoring pile slippage in offshore wind power monopile foundations based on vibration characteristics. The aim is to improve the accuracy of pile slippage monitoring by analyzing the vibration of the pile body to determine whether pile slippage has occurred in the monopile foundation.

[0005] Technical solution:

[0006] This invention proposes a method for monitoring pile slippage in offshore wind turbine monopile foundations based on vibration characteristics, comprising:

[0007] A numerical model of a single pile foundation for offshore wind power is constructed. Geological exploration data is collected to construct a soil numerical model. The pile driving process is simulated according to actual construction requirements by combining the single pile foundation numerical model and the soil numerical model to obtain the depth variation function of the single pile foundation.

[0008] One-dimensional finite element calculations are performed based on the depth variation function to establish the motion equations of the damped system and obtain the simulated natural frequency of the single pile foundation.

[0009] An acceleration sensor is installed on the actual monopile foundation to collect measured data of the monopile foundation during actual pile driving operations, and the actual natural frequency of the monopile foundation is calculated based on the measured data.

[0010] The actual natural frequency is compared with the numerically simulated natural frequency to determine whether the actual natural frequency matches the simulated natural frequency. If the matching condition is not met, it is considered that a pile slip has occurred, and an early warning is issued.

[0011] Furthermore, the equation of motion for the damped system is expressed as:

[0012] In the formula, M is the mass matrix, C is the damping matrix, K is the stiffness matrix, and u is the displacement vector in the x-direction. Further, the stiffness matrix K is expressed as: k x =1.2E s

[0013] In the formula, n represents the number of degrees of freedom, E is the elastic modulus of the steel, I is the moment of inertia of the section, and H is the height of the divided element.

[0014] Furthermore, the mass matrix M is represented as:

[0015] In the formula, m n Let be the concentrated mass of the nth unit of a single pile foundation.

[0016] Furthermore, the damping matrix C is expressed as: c n =2ξωm n

[0017] In the formula, c n Let ξ be the damping of the nth element of the monopile foundation, and ξ be the damping ratio of the monopile foundation. s Let c be the soil damping ratio. x For the soil damping caused by the interaction between the foundation soil and the single pile foundation, k x ρ is the unit value of stiffness of the interaction between the foundation soil and the single pile foundation, ω is the simulated natural frequency of the single pile foundation, and ρ is the unit value of stiffness. s V is the density of the soil. s Let d be the soil shear wave velocity, d be the pile diameter, and a0 = ωd / V.

[0018] Furthermore, the simulated natural frequency ω of the monopile foundation is calculated using the following formula:

[0019] In the formula, k represents the stiffness of a single pile foundation, and m represents the mass of a single pile foundation.

[0020] Furthermore, the measured data of the monopile foundation during actual pile driving operations are processed using a low-pass filter method after collection.

[0021] Furthermore, the method for calculating the actual natural frequency of a single pile foundation is a random subspace method.

[0022] Furthermore, the determination of whether the actual natural frequency matches the numerically simulated natural frequency is evaluated using the correlation of the frequency response function shape, expressed as:

[0023] In the formula, H pq,e (ω) represents the measured value of the frequency response function at input at point q and output at point p, H pq,u (ω) represents the calculated value of the frequency response function with input at point q and output at point p, T represents the matrix transpose operation, SAC pq This indicates the degree of correlation between the shape curves of the two;

[0024] Frequency response function H pq Represented as:

[0025] In the formula, j is an imaginary number, under the r-th order mode shape, For the displacement of the measuring point p, Let k be the displacement of the measuring point q. r For modal stiffness, m r For quality, c r For damping.

[0026] Furthermore, the matching conditions include:

[0027] SAC pq The value range is 0 to 1, when SAC pq When SAC = 1, it indicates that the actual natural frequency perfectly matches the numerically simulated natural frequency. pq When =0, it means that the actual natural frequency is completely unrelated to the natural frequency in the numerical simulation;

[0028] Set a matching threshold based on actual needs, when SAC pq If the value is less than the matching threshold, it is considered that the matching condition is not met. Beneficial effects:

[0029] This invention utilizes modal parameter identification to analyze collected acceleration data to identify the vibration frequency of a single pile foundation. Based on physical models and data analysis, this method more accurately reflects the actual state of the pile compared to existing technologies, improving monitoring accuracy and reliability. Compared to traditional methods for monitoring pile end resistance and driving velocity, the vibration-based monitoring method is less expensive. Furthermore, this invention does not require complex equipment or high maintenance costs, resulting in high economic benefits. Attached Figure Description

[0030] Figure 1 is a flowchart of the method of the present invention;

[0031] Figure 2 is a schematic diagram of the numerical model of the soil.

[0032] Figure 3 shows a single pile foundation and a numerical model of simulated pile driving.

[0033] Figure 4 is a schematic diagram of the depth variation function curve;

[0034] Figure 5 is a schematic diagram of the frequency variation curve with the depth of pile penetration.

[0035] Figure 6 is a schematic diagram of the accelerometer sensor arrangement. Detailed Implementation

[0036] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments. The method flow of the present invention is shown in Figure 1, and includes the following steps:

[0037] Step 1, Construction of soil numerical model

[0038] Establishing a soil numerical model based on geological exploration results is a crucial step in geotechnical engineering analysis. First, detailed geological exploration data needs to be collected, including soil layer distribution and physical and mechanical parameters. This data is then used to construct a three-dimensional soil numerical model to simulate the distribution and properties of actual soil layers. A schematic diagram of the soil numerical model constructed in this embodiment is shown in Figure 2.

[0039] Step 2, Numerical simulation of pile driving process

[0040] Based on the design drawings of the offshore wind turbine monopile foundation, a numerical model was constructed. During the construction process, the structural form, dimensions, and material properties of the monopile foundation were carefully considered to ensure the model's accuracy and reliability. The pile driving process was simulated according to the pile driving speed specified in the construction organization design. Figure 3 shows a schematic diagram of the simulated pile driving using the monopile foundation numerical model and the soil numerical model, where the left side represents the monopile foundation numerical model and the right side represents the simulated pile driving process. Through simulation, key indicators such as vibration frequency, deformation trend, and stability of the monopile foundation during the pile driving process can be predicted, providing important reference data for subsequent construction.

[0041] The depth variation function (DVF) of the single pile foundation in each typical soil layer is obtained. The typical soil layer refers to soil layer parameter data obtained from geological exploration. A schematic diagram of the DVF curve obtained in this embodiment is shown in Figure 4. After determining the depth variation function of the single pile foundation, one-dimensional finite element analysis is performed based on the actual soil layer distribution to obtain the vibration frequency variation curve with the pile penetration depth. The motion equations of the damped system are then established.

[0042] In the formula, M is the mass matrix, C is the damping matrix, K is the stiffness matrix, and u is the displacement vector in the x-direction. Specifically, the above matrices are established as follows:

[0043] (1) The stiffness matrix K is established based on the stiffness equation of the members according to the displacement method of structural mechanics:

[0044] In the formula, n represents the number of degrees of freedom, and the matrix subform is as follows:

[0045] In the above formula, E is the elastic modulus of steel, I is the moment of inertia of the section, and H is the height of the divided unit.

[0046] This invention considers the interaction between piles and soil, E s The elastic modulus of the soil is represented by k, which is the unit value of the interaction stiffness between the foundation soil and the pile. x For k x =1.2E s .

[0047] (2) Mass matrix M:

[0048] In the formula, m n Let be the concentrated mass of the nth unit of a single pile foundation.

[0049] (3) The damping matrix C is constructed as a diagonal matrix according to classical damping, as follows:

[0050] Where c n According to Rayleigh damping construction, we have c n =2ξωm n In the formula, ω is the natural frequency of the damped motion system of a single pile foundation, and ξ is the damping ratio of the single pile foundation. In this embodiment, ξ = 0.02.

[0051] Adding soil damping to the foundation term of the damping matrix, the soil damping for pile-soil interaction is taken as:

[0052] In the formula ξ s Let k be the soil damping ratio. x ρ is the unit value of stiffness obtained in (1). s V is the density of the soil. s Let be the soil shear wave velocity, d be the pile diameter of the single pile foundation, and a0 = ωd / V.

[0053] By performing eigenvalue analysis on the stiffness, mass, and damping matrices of the single pile foundation calculated using the above formulas, the formula for calculating the simulated natural frequency ω is obtained:

[0054] In the formula, k represents the stiffness of a single pile foundation, and m represents the mass of a single pile foundation.

[0055] Since the vibration frequency of a single pile foundation is related to the foundation depth and pile diameter, the single pile foundation used in this embodiment has a variable diameter section. Define a dimensionless l0, l0 = h / 2a, where a is the pile diameter of the single pile foundation and h is the foundation depth. The curve of the vibration frequency of the single pile foundation as a function of the pile depth is shown in Figure 5.

[0056] Step 3: Accelerometer placement and vibration signal acquisition

[0057] During pile driving, accelerometers are installed around the pile to monitor the vibration of the area below the top flange of the foundation in real time. In this embodiment, to verify the accuracy of the data measured by different sensors, wireless accelerometers are installed at four different locations, as shown in Figure 6. By using complementary data from different locations to correct the blind spots of a single sensor, the overall measurement accuracy is improved, ensuring real-time synchronization of key parameters and early warning of anomalies. The accelerometers selected in this embodiment have built-in power supplies and Bluetooth data transmission capabilities. These sensors will measure and record the acceleration changes of the single pile foundation during pile driving, providing important data for subsequent analysis.

[0058] Step 4, Modal parameter identification and early warning analysis

[0059] After acquiring the acceleration response data of a single pile foundation during pile driving, the data first needs to be filtered. The purpose of filtering is to remove noise and interference from the data, making the subsequent calculation results more accurate and reliable. This invention monitors the first-order natural frequency of the structure, therefore a low-pass filtering method is used.

[0060] The natural frequencies of a structure are calculated using the stochastic subspace method. The stochastic subspace method is a time-series analysis-based approach that establishes the dynamic equations of the structure and uses observed acceleration response data to solve for its natural frequencies. This method has advantages such as simple calculation and wide applicability, and is therefore widely used in engineering practice. For a detailed explanation of the stochastic subspace method, please refer to the paper "Research on Real-time Extraction Method of Modal Characteristics of Offshore Platforms Based on Stochastic Subspace Method" (Huang Yan, Chen Tao, Zhu Benrui. [J]. Vibration and Shock, 2021, 40(03):147-155. DOI:10.13465 / j.cnki.jvs.2021.03.020.), which will not be elaborated here.

[0061] Considering the complexity and uncertainty of actual structures, initial soil numerical models often fail to accurately and reliably reflect the true state of the structure. Therefore, the core objective of soil numerical model correction is to use modal test data from experimental models to adjust and optimize the original soil numerical model, ensuring that its simulation results match experimental observations. To ensure the matching degree between the soil numerical model and measured data, the correlation of the frequency response function shape is used for evaluation, expressed as:

[0062] In the formula, H pq,e (ω) and H pq,u (ω) represents the measured and calculated values ​​of the frequency response function at input point q and output point p, respectively, and T represents the matrix transpose operation. SAC pq This indicates the degree of correlation between the shape curves of the two, with a value ranging from 0 to 1. When SAC pq When SAC = 1, it indicates a perfect linear correlation between the two, meaning the natural frequency simulated by the soil numerical model perfectly matches the actual natural frequency of the measured structure. pq When = 0, it indicates that the two are completely uncorrelated. In the formula, the frequency response function H... pq It can be expressed as the following formula:

[0063] In the formula, j is an imaginary number, j 2 =-1, k represents the displacements of the measuring points and under the r-th mode shape. r m r and c r Let represent the modal stiffness, mass, and damping of the r-th mode, respectively.

[0064] After matching the finite element model with the measured data, the calculated natural frequencies are compared with the numerical simulation results to determine whether they are within the allowable range. Numerical simulation uses computer simulation technology to predict the dynamic performance of a structure under different working conditions. By comparing the actual measured values ​​and the numerical simulation results, it is possible to assess whether the safety performance of the structure meets the design requirements.

[0065] If the natural frequency is found to exceed the allowable range, the early warning device is triggered. In this embodiment, the early warning device is used to monitor the structural safety status in real time. When a potential safety hazard is detected, it will issue an alarm signal to remind relevant personnel to take emergency measures to prevent pile slippage.

Claims

1. A method for monitoring pile slippage in offshore wind turbine monopile foundations based on vibration characteristics, characterized in that, include: A numerical model of a single pile foundation for offshore wind power is constructed. Geological exploration data is collected to construct a soil numerical model. The pile driving process is simulated according to actual construction requirements by combining the single pile foundation numerical model and the soil numerical model to obtain the depth variation function of the single pile foundation. One-dimensional finite element calculations are performed based on the depth variation function to establish the motion equations of the damped system and obtain the simulated natural frequency of the single pile foundation. An acceleration sensor is installed on the actual monopile foundation to collect measured data of the monopile foundation during actual pile driving operations, and the actual natural frequency of the monopile foundation is calculated based on the measured data. The actual natural frequency is compared with the simulated natural frequency to determine whether the actual natural frequency matches the simulated natural frequency. If the matching condition is not met, it is considered that the pile has slipped and an early warning is issued.

2. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 1, characterized in that, The equation of motion for the damped system is expressed as: In the formula, M is the mass matrix, C is the damping matrix, K is the stiffness matrix, and u is the displacement vector in the x-direction.

3. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 2, characterized in that, The stiffness matrix K is represented as: In the formula, n represents the number of degrees of freedom, E is the elastic modulus of the steel, I is the moment of inertia of the section, and H is the height of the divided element.

4. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 3, characterized in that, The mass matrix M is represented as: In the formula, m n Let be the concentrated mass of the nth unit of a single pile foundation.

5. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 4, characterized in that, The damping matrix C is represented as follows: c n =2ξωm n In the formula, c n Let ξ be the damping of the nth element of the monopile foundation, and ξ be the damping ratio of the monopile foundation. s Let c be the soil damping ratio. x For the soil damping caused by the interaction between the foundation soil and the single pile foundation, k x ρ is the unit value of stiffness of the interaction between the foundation soil and the single pile foundation, ω is the simulated natural frequency of the single pile foundation, and ρ is the unit value of stiffness. s V is the density of the soil. s Let d be the soil shear wave velocity, d be the pile diameter, and a0 = ωd / V.

6. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 5, characterized in that, The simulated natural frequency ω of the single pile foundation is calculated using the following formula: In the formula, k represents the stiffness of a single pile foundation, and m represents the mass of a single pile foundation.

7. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 6, characterized in that, The measured data of the monopile foundation during actual pile driving operations were processed using a low-pass filter method after collection.

8. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 7, characterized in that, The method for calculating the actual natural frequency of a single pile foundation is a random subspace method.

9. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 8, characterized in that, The determination of whether the actual natural frequency matches the numerically simulated natural frequency is evaluated using the correlation of the frequency response function shape, expressed as: In the formula, H pq,e (ω) represents the measured value of the frequency response function at input at point q and output at point p, H pq,u (ω) represents the calculated value of the frequency response function with input at point q and output at point p, T represents the matrix transpose operation, SAC pq This indicates the degree of correlation between the shape curves of the two; Frequency response function H pq Represented as: In the formula, j is an imaginary number, under the r-th order mode shape, For the displacement of the measuring point p, Let k be the displacement of the measuring point q. r For modal stiffness, m r For quality, c r For damping.

10. The method for monitoring pile slippage in offshore wind turbine monopile foundations according to claim 9, characterized in that, The matching conditions include: SAC pq The value range is 0 to 1, when SAC pq When SAC = 1, it indicates that the actual natural frequency perfectly matches the numerically simulated natural frequency. pq When =0, it means that the actual natural frequency is completely unrelated to the natural frequency in the numerical simulation; Set a matching threshold based on actual needs, when SAC pq If the value is less than the matching threshold, it is considered that the matching condition is not met.

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