A method for identifying the vortex-induced vibration modes of a flexible riser model based on the EMD method

Abstract

The present invention discloses a method for identifying the vortex-induced vibration modes of a flexible riser model based on the EMD method. According to the beam vibration theory under axial tension, the vibration displacement time history of each measuring point is deduced from the bending strain time history of each measuring point. Aiming at the multi-modal response characteristics of the flexible riser, the EMD method is used to decompose the vibration displacement time history to obtain the vibration displacement time histories of different frequencies. By adding harmonic signals based on the high-frequency decomposition results to the original data, the frequencies of the same-order decomposition results of each measuring point are ensured to be consistent. By comparing the phase differences of each measuring point with the same-frequency vibration, the vibration shape of the riser is obtained, thereby determining the vibration order corresponding to this frequency. The problem of identifying the vortex-induced vibration modes of the flexible riser is solved, effectively avoiding the modal mixing problem in the analysis results of the modal analysis method caused by the asymmetric vibration mode characteristics of the riser vortex-induced vibration, and having good adaptability. Therefore, the analysis results of the present invention are more accurate, and the expression of the vibration mode of the riser is more rapid and effective.

Description

Technical Field

[0001] The present invention relates to a method for identifying the vortex-induced vibration modes of a flexible riser model based on EMD, belonging to the field of modal identification of offshore engineering structures. Background Technique

[0002] Risers are essential key equipment for offshore oil and gas exploration and development, connecting the working platform and the seabed operation area. Generally, they are divided into drilling risers and production risers. The drilling riser is used to transport the liquid for drilling, while the production riser is used to transport the oil and gas from the seabed to the sea surface platform. Under the action of a certain oncoming flow, the riser will undergo lateral vibration. This is a special hydrodynamic phenomenon caused by the alternating shedding of vortices along the wake region of the riser, called the vortex-induced vibration of the riser. One of the main aspects of the analysis of the vortex-induced vibration response characteristics of a flexible riser is the analysis of the main vibration modes and their corresponding frequencies. At present, this analysis mainly uses the modal analysis method, that is, first assume the vibration modes of different vibration orders of the riser, usually simulated by sine functions, and then use the least squares method to estimate the modal weights corresponding to different modes. There are the following problems with this research method:

[0003] (1) The additional mass generated by the vibration of the riser will cause the non-uniform distribution of the mass of the riser, resulting in a certain difference between the sine function and the actual vibration mode.

[0004] (2) The traveling wave characteristics of the vortex-induced vibration of the flexible riser will enlarge the difference between the sine function and the actual vibration mode.

[0005] (3) Modal aliasing will affect the modal analysis, and the low-order modes will affect the analysis of the high-order modes during the high-order response. Summary of the Invention

[0006] The purpose of the present invention is to avoid the modal aliasing problem in the analysis results of the modal analysis method caused by the asymmetric vibration mode characteristics of the riser vortex-induced vibration.

[0007] In order to achieve the above object, the technical solution of the present invention is to provide a method for identifying the vortex-induced vibration modes of a flexible riser model based on the EMD method, which is characterized by including the following steps:

[0008] Step 1: Derive the vibration displacement time history of each measurement point based on the bending strain time history of each measurement point;

[0009] Step 2: For the multi-modal response characteristics of the flexible riser, use the EMD method to decompose the vibration displacement time history to obtain the vibration displacement time history of different frequencies. During this process, by adding a harmonic signal based on the high-frequency decomposition result to the original vibration displacement time history, to ensure that the frequencies of the same-order decomposition results of each measurement point are consistent;

[0010] Step 3: By comparing the phase differences of each measuring point with the same frequency vibration, obtain the vibration shape of the riser, thereby determining the vibration order corresponding to this frequency.

[0011] Preferably, step 1 includes basic data input and displacement calculation from strain.

[0012] Preferably, the basic data input includes the following steps:

[0013] Input the basic property parameters of the riser, axial force, and the strain time history data of each measuring point at a certain flow rate.

[0014] Preferably, the basic property parameters of the riser include length, radius, and elastic modulus.

[0015] Preferably, the riser strain time history data includes the position coordinates of each measuring point and the bending strain time history at each measuring point.

[0016] Preferably, the displacement calculation from strain includes the following steps:

[0017] According to the complex bending theory of beams, deduce the conversion matrix from strain response to displacement response, and based on the conversion matrix, convert the riser strain time history data of each measuring point into displacement time history data.

[0018] Preferably, in step 2, the harmonic signal based on the high-frequency decomposition result is obtained by the following method:

[0019] Perform EMD decomposition on the displacement time history data of each measuring point obtained in step 1, find the maximum frequency of the response as the frequency of the high-frequency harmonic, and determine the amplitude of the high-frequency harmonic according to the decomposition result. This high-frequency harmonic is the harmonic signal based on the high-frequency decomposition result.

[0020] Preferably, step 2 includes the following steps:

[0021] Step 201: Determine the high-frequency harmonic frequency and amplitude:

[0022] Perform EMD decomposition on the displacement time history data of each measuring point obtained in step 1, find the maximum frequency of the response as the frequency of the high-frequency harmonic, and determine the amplitude of the high-frequency harmonic according to the decomposition result;

[0023] Step 202: EMD decomposition;

[0024] Add the high-frequency harmonic obtained in step 201 to the displacement time history data of each measuring point obtained in step 1, and perform EMD decomposition. Subtract the high-frequency harmonic from the first decomposition result to obtain the true first-order decomposition result, that is, obtain the first-order vibration time history, and the other decomposition results are the vibration time histories of the remaining orders.

[0025] Preferably, step 3 includes the following steps:

[0026] Perform spectral analysis on the vibration time history of each order to obtain the response frequency;

[0027] Compare the phases of each measuring point in the same-order vibration to obtain the vibration mode of the riser, and then determine the vibration order.

[0028] Another technical solution of the present invention is to provide an application of the above-mentioned flexible riser model vortex-induced vibration mode identification method based on the EMD method, which is characterized in that it is used to identify the vibration mode of a beam-type structure.

[0029] According to the beam vibration theory under axial tension, the vibration displacement time history of the measuring points is deduced from the bending strain time history of each measuring point. For the multi-modal response characteristics of the flexible riser, the EMD method is used to decompose the vibration displacement time history to obtain the vibration displacement time history of different frequencies. By adding harmonic signals based on the high-frequency decomposition results to the original data, the frequencies of the same-order decomposition results of each measuring point are ensured to be consistent. By comparing the phase differences of each measuring point in the same-frequency vibration, the vibration shape of the riser is obtained, and thus the vibration order corresponding to this frequency is determined. The problem of vortex-induced vibration mode identification of flexible risers is solved. There is no need to assume the modal shape of the riser, effectively avoiding the modal confusion problem in the analysis results of the modal analysis method caused by the asymmetric modal shape characteristics of the vortex-induced vibration of the riser, and having good adaptability. Therefore, the analysis results of the present invention are more accurate, and the expression of the vibration mode of the riser is more rapid and effective. Description of the Drawings

[0030] Figure 1 Schematically shows the flow of the present invention;

[0031] FIG. 2(a) and FIG. 2(b) schematically show the mode identification results of the present invention. Among them, FIG. 2(a) is the 6th-order mode, and FIG. 2(b) is the 4th-order mode. Detailed Embodiments

[0032] The following further elaborates the present invention in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0033] A flexible riser model vortex-induced vibration mode identification method based on the EMD method disclosed in this embodiment is aimed at analyzing the vortex-induced vibration displacement response characteristics of the riser, can obtain the frequencies of each-order modal vibrations, obtain the actual deformation shape of the riser vibration, determine the orders of each-order modes of the riser, and then correspond the vibration frequencies and vibration modes without pre-supposing the vibration mode.

[0034] Specifically, the method disclosed in this embodiment includes the following steps:

[0035] Step 1, basic data input:

[0036] Input the basic property parameters of the riser, the axial force, and the riser strain time history data at each measuring point under a certain flow rate. In this embodiment, the basic property parameters of the riser include length, radius, and elastic modulus. The riser strain time history data includes the position coordinates of each measuring point and the bending strain time history at each measuring point.

[0037] Step 2, calculate displacement from strain:

[0038] According to the complex bending theory of beams, derive the conversion matrix from strain response to displacement response, and obtain the sub-function junbu_Dyn01 for converting strain time history to displacement time history based on the conversion matrix. After inputting the riser strain time history data in Step 1 into the sub-function junbu_Dyn01 for processing, the displacement time history data of each measuring point is obtained.

[0039] In this embodiment, the derivation of the conversion matrix includes the following steps A to D:

[0040] Step A, according to structural dynamics, there is the following relationship between the vibration displacement of the riser and the external load:

[0041]

[0042] where [M], [C], and [K] are the mass matrix, damping matrix, and stiffness matrix of the riser respectively, and [f] is the external load.

[0043] Step B, deform equation (1), and move the damping force, that is, the inertial force, to the right side of the equation, to obtain the relationship between the riser displacement and the generalized elastic force at any moment as:

[0044]

[0045] Step C, according to the beam bending theory, there is the following relationship between the displacement w of the riser bending deformation and the strain ε:

[0046]

[0047] where x is the coordinate of the riser along the axial direction, and R is the radius of the riser. According to equations (2) and (3), the relationship between the strain ε and the generalized elastic force can be obtained, that is:

[0048] [B]{ε} = {F} (4).

[0049] Step D, according to equations (2) and (4), the conversion relationship between the displacement w and the strain ε can be obtained, that is:

[0050] {w} = [K] -1[B]{ε} = [T]{ε}, where [T] is the transformation matrix.

[0051] Step 3: Determine the high-frequency harmonic frequency and amplitude:

[0052] Perform EMD decomposition on the displacement time history data of each measuring point obtained in Step 2, find the maximum frequency of the response as the frequency of the high-frequency harmonic, and determine the amplitude of the high-frequency harmonic according to the decomposition result.

[0053] Step 4: EMD decomposition;

[0054] Add the high-frequency harmonic obtained in Step 3 to the displacement time history data of each measuring point obtained in Step 2, and perform EMD decomposition. The EMD algorithm itself ensures that the decomposition results are arranged from high frequency to low frequency. Therefore, subtract the high-frequency harmonic from the first decomposition result to obtain the true first-order decomposition result, that is, obtain the first-order vibration time history, and the other decomposition results are the vibration time histories of the remaining orders.

[0055] In this embodiment, the purpose of adding the high-frequency harmonic is to enable the EMD decomposition results of each measuring point to be arranged in order of frequency from high to low. Otherwise, the frequencies of the decomposition results of the same order in the EMD decomposition results of each measuring point are not the same. For example, the first decomposition result of measuring point M may have the same frequency as the second decomposition result of measuring point N. Adding the high-frequency harmonic ensures that the decomposition results of the same order have the same frequency, and then the vibration mode and frequency of this order are obtained by comparing the decomposition results of the same order of different measuring points.

[0056] Step 5: Determine the vibration order and frequency:

[0057] Perform spectral analysis on the vibration time history of each order to obtain the response frequency;

[0058] Compare the phases of each measuring point in the vibration of the same order to obtain the vibration mode of the riser vibration, and then determine the vibration order.

Claims

1. A method for identifying the vortex-induced vibration modes of a flexible riser model based on the EMD method, characterized in that, It includes the following steps: Step 1: Derive the vibration displacement time history of each measuring point based on the bending strain time history of each measuring point; Step 2: Aiming at the multi-modal response characteristics of the flexible riser, use the EMD method to decompose the vibration displacement time history to obtain the vibration displacement time history of different frequencies. In this process, by adding a harmonic signal based on the high-frequency decomposition result to the original vibration displacement time history, to ensure that the frequencies of the same-order decomposition results of each measuring point are consistent; Step 3: By comparing the phase differences of each measuring point with the same-frequency vibration, obtain the vibration shape of the riser, so as to determine the vibration order corresponding to this frequency.

2. The method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method according to claim 1, wherein, Step 1 includes basic data input and displacement calculation from strain.

3. The method for identifying the vortex-induced vibration mode of the flexible riser model based on the EMD method according to claim 2, characterized in that, The basic data input includes the following steps: Input the basic property parameters of the riser, the axial force, and the riser strain time history data of each measuring point at a certain flow velocity.

4. The method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method according to claim 3, wherein The basic property parameters of the riser include length, radius, and elastic modulus.

5. The method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method according to claim 3, characterized in that The riser strain time history data includes the position coordinates of each measuring point and the bending strain time history at each measuring point.

6. The method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method according to claim 2, characterized in that, The displacement calculation from strain includes the following steps: According to the complex bending theory of the beam, derive the conversion matrix from strain response to displacement response, and based on the conversion matrix, convert the riser strain time history data of each measuring point into displacement time history data.

7. A method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method as claimed in claim 1, characterized in that, In Step 2, the harmonic signal based on the high-frequency decomposition result is obtained by the following method: Perform EMD decomposition on the displacement time history data of each measuring point obtained in Step 1, find the maximum frequency of the response as the frequency of the high-frequency harmonic, and determine the amplitude of the high-frequency harmonic according to the decomposition result. This high-frequency harmonic is the harmonic signal based on the high-frequency decomposition result.

8. The method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method according to claim 1, wherein The said Step 2 includes the following steps: Step 201: Determine the high-frequency harmonic frequency and amplitude: Perform EMD decomposition on the displacement time history data of each measuring point obtained in Step 1, find the maximum frequency of the response as the frequency of the high-frequency harmonic, and determine the amplitude of the high-frequency harmonic according to the decomposition result; Step 202: EMD decomposition; Add the high-frequency harmonic obtained in Step 201 to the displacement time history data of each measuring point obtained in Step 1, and perform EMD decomposition. Subtract the high-frequency harmonic from the first decomposition result to obtain the true first-order decomposition result, that is, obtain the first-order vibration time history, and the other decomposition results are the vibration time histories of the remaining orders.

9. The method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method according to claim 1, characterized in that The said Step 3 includes the following steps: Perform spectral analysis on each-order vibration time history to obtain the response frequency; Compare the phases of each measuring point in the same-order vibration to obtain the vibration mode of the riser vibration, and then determine the vibration order.

10. Application of a method for identifying the vortex-induced vibration mode of a flexible riser model based on the EMD method as described in claim 1, characterized in that, It is used to identify the vibration modes of beam-type structures.

Images