A modal frequency-based multi-span pipeline discrimination method

Abstract

The application provides a modal frequency-based multi-span pipeline discrimination method, which comprises the following steps: S1, calculating a single-span modal frequency f0 of a suspended pipeline A section; S2, calculating a multi-span modal frequency f of the suspended pipeline A section i ; S3, calculating a difference between the single-span modal frequency f0 and the initial multi-span modal frequency f i , and comparing a ratio of the difference to the single-span modal frequency f0 with a predetermined threshold value to determine an initial critical span length ratio; S4, gradually increasing an initial span interval ratio α1 by a predetermined span interval ratio interval γ to obtain a plurality of equally-spaced span interval ratios and critical span length ratios corresponding to each span interval ratio; S5, drawing a multi-span discrimination curve, and discriminating a suspended pipeline section under a working condition of the suspended pipeline A section based on the multi-span discrimination curve. The method judges the influence degree between spans according to the change of the pipeline modal frequency under different span interval ratios and span length ratios, and improves the discrimination accuracy.

Description

Technical Field

[0001] This invention relates to the field of marine pipeline engineering technology, and more specifically to a method for identifying multi-span pipelines based on modal frequencies. Background Technology

[0002] Subsea pipelines are continuous, and vortex-induced vibrations in adjacent suspended spans can have mutual effects. When a subsea pipeline consists of multiple suspended spans, if adjacent spans influence each other's vibration characteristics and responses, these mutually influencing suspended spans are called multi-spans. If adjacent suspended spans do not influence each other, their vibration response can be calculated as an isolated single span; otherwise, the multi-span form must be considered. The mutual influence between multi-spans may lead to an increase in amplitude and even change the range of vortex-induced vibration responsiveness of the suspended spans. The interaction between multi-spans cannot be ignored in the calculation of vibration response and fatigue.

[0003] Modal characteristics are the inherent vibrational properties of a structural system. Each mode has a specific natural frequency, damping ratio, and mode shape. During the typical development stage of the span length of subsea pipelines, the modal frequencies of the structure are highly sensitive to changes in boundary conditions, added mass, axial force, and span length; any change in these conditions will lead to a change in the modal frequencies of the suspended pipeline. However, different state changes can also result in the same modal frequencies, making it impossible to accurately distinguish between multiple pipelines simply based on a large number of natural frequency samples.

[0004] Therefore, establishing a highly accurate multi-span pipeline identification method based on modal frequencies has become an urgent technical problem to be solved in the field of marine pipeline engineering. Summary of the Invention

[0005] The purpose of this invention is to provide a method for identifying multi-span subsea pipelines based on modal frequencies to address the aforementioned problems in the prior art. This method considers the influence of the interaction between the subsea soil and the pipeline, and determines the degree of influence between spans based on the changes in the pipeline's modal frequencies under different span ratios and span-length ratios, thereby establishing a new method for identifying multi-span subsea pipelines.

[0006] According to the present invention, a method for identifying multi-span pipelines based on modal frequencies is provided. The subsea pipeline includes a suspended pipeline segment A and a suspended pipeline segment B separated by inter-span soil. The method includes the following steps:

[0007] S1, calculate the single-span modal frequency f0 of the suspended pipe section A based at least in part on the soil spring stiffness of the interspan soil;

[0008] S2, set the initial value of the span ratio as the initial span ratio α1 and the initial value of the span length ratio as the initial span length ratio α2, and calculate the multi-span modal frequency f of the suspended pipe section A based at least in part on the soil spring stiffness of the span soil, the initial span ratio α1 and the initial span length ratio α2. i Where the span ratio is the length L of the soil in the span. s The length L of the suspended pipe section A A The ratio, the span-to-length ratio is the length L of section B of the suspended pipe. B The length L of the suspended pipe section A A The ratio;

[0009] S3, Calculate the single-span modal frequency f0 and the initial multi-span modal frequency f i The difference is calculated and compared with the ratio of the single-span modal frequency f0 and a predetermined threshold to determine the initial critical span ratio.

[0010] S4, gradually increase the initial span ratio α1 at predetermined span ratio intervals γ, and repeat steps S2-S3 to obtain multiple equally spaced span ratios and the critical span length ratio corresponding to each span ratio;

[0011] S5. Using the multiple span ratios obtained in step S4 and the critical span length ratio corresponding to each span ratio, a multi-span discrimination curve is plotted, and multi-span discrimination is performed on the suspended pipeline section with the same working conditions as section A of the suspended pipeline based on the multi-span discrimination curve.

[0012] According to one embodiment of the present invention, in step S3, determining the initial critical span ratio includes:

[0013] If the ratio is not less than the predetermined threshold, then the initial span ratio α2 is taken as the critical span ratio;

[0014] If the ratio is less than a predetermined threshold, the initial span ratio α2 is gradually increased at predetermined span ratio intervals β, and steps S2-S3 are repeated until the critical span ratio is obtained.

[0015] According to one embodiment of the present invention, step S3 includes: determining a predetermined threshold based at least in part on operating conditions.

[0016] According to one embodiment of the present invention, a predetermined threshold is determined based on the span length ratio and modal frequency corresponding to the inflection point of the curve of modal frequency changing with span length ratio when the span ratio is fixed, and the single span modal frequency f0.

[0017] According to one embodiment of the present invention, the operating conditions include soil texture and pipeline parameters.

[0018] According to one embodiment of the present invention, the pipe parameters include at least one of the following: pipe outer diameter, pipe inner diameter, pipe elastic modulus, pipe density, fluid density inside the pipe, and pipe tension.

[0019] According to one embodiment of the present invention, step S5 includes plotting a multi-span discrimination curve with the span ratio as the abscissa and the critical span-length ratio as the ordinate, and plotting the span ratio and span-length ratio of the suspended pipeline section with the same working conditions as section A of the suspended pipeline onto the multi-span discrimination curve:

[0020] If the point of the suspended pipeline section is located in the area below the multi-span discrimination curve, then the suspended pipeline section has no influence on the adjacent suspended pipeline sections and constitutes an independent single-span pipeline.

[0021] If the suspended pipeline section is projected to the area above the multi-span discrimination curve, then the suspended pipeline section and the adjacent suspended pipeline section will influence each other and together form a whole multi-span pipeline.

[0022] According to one embodiment of the present invention, the subsea pipeline further includes a suspended pipeline section C separated from the suspended pipeline section B by a second span of soil, and the method further includes:

[0023] S6, use multi-span discrimination curves to distinguish between suspended pipe section A, suspended pipe section B and suspended pipe section C respectively;

[0024] S7. Based on the discrimination results of each of the suspended pipe sections A, B, and C, perform multi-span discrimination on the suspended pipe sections A, B, and C.

[0025] According to one embodiment of the present invention, step S7 includes:

[0026] If suspended pipe section A and suspended pipe section B affect each other, and suspended pipe section B and suspended pipe section C affect each other, then suspended pipe section A, suspended pipe section B and suspended pipe section C together constitute a whole multi-span pipe.

[0027] If suspended pipe section A and suspended pipe section B affect each other, and suspended pipe section B and suspended pipe section C do not affect each other, then suspended pipe section A and suspended pipe section B together constitute a multi-span pipe as a whole, and suspended pipe section C constitutes an independent single-span pipe.

[0028] If there is no influence between suspended pipe section A and suspended pipe section B, and suspended pipe section B and suspended pipe section C influence each other, then suspended pipe section A constitutes an independent single-span pipe, and suspended pipe section B and suspended pipe section C together constitute a whole multi-span pipe.

[0029] If there is no influence between suspended pipe section A and suspended pipe section B, and no influence between suspended pipe section B and suspended pipe section C, then suspended pipe section A, suspended pipe section B, and suspended pipe section C each constitute an independent single-span pipe.

[0030] By adopting the above technical solution, the modal frequency-based multi-span pipeline discrimination method of this invention considers the significant influence of the mid-span soil length between multiple spans on the natural frequency of the pipeline, and analyzes the pipeline accordingly. As the span length increases, the mutual influence between adjacent spans decreases, and the change in pipeline modal frequency gradually diminishes. When the pipeline modal frequency no longer changes, it can be considered a single span, and the influence between the two spans is ignored. Multi-span discrimination lays a theoretical foundation for the reasonable analysis of the vibration response and fatigue of subsea pipelines, and optimizes and supplements the prediction and analysis methods and processes for subsea multi-span pipelines. Attached Figure Description

[0031] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings:

[0032] Figure 1 This is a flowchart of a multi-span pipe discrimination method based on modal frequency according to an embodiment of the present invention;

[0033] Figure 2 This is a schematic diagram of a two-span pipeline model according to an embodiment of the present invention;

[0034] Figure 3 This is a flowchart of determining the critical span ratio according to an embodiment of the present invention;

[0035] Figure 4 A modal frequency-span ratio curve according to an embodiment of the present invention is shown;

[0036] Figure 5 A multi-span discrimination curve diagram according to an embodiment of the present invention is shown;

[0037] Figure 6 This is a schematic diagram of a three-span pipe model according to an embodiment of the present invention;

[0038] Figure 7 This is a flowchart of a multi-span pipe discrimination method based on modal frequency according to another embodiment of the present invention. Detailed Implementation

[0039] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0040] Figure 1 A flowchart of a multi-span pipeline discrimination method based on modal frequency according to an embodiment of the present invention is shown. This embodiment uses adjacent sections in a subsea pipeline—such as… Figure 2The example shown is a subsea pipeline with two spans, A and B, illustrating the method for identifying two-span pipelines. Span A has a length L. A The suspended pipeline section B has a length of L B The two are connected by a length of L s The soil sections are separated by spans. The method generally includes the following steps:

[0041] S1, the single-span modal frequency f0 of the suspended pipe section A is calculated at least in part based on the soil spring stiffness of the soil between the spans.

[0042] S2, set the initial value of the span ratio as the initial span ratio α1 and the initial value of the span length ratio as the initial span length ratio α2, and calculate the multi-span modal frequency f of the suspended pipe section A based at least in part on the soil spring stiffness of the span soil, the initial span ratio α1 and the initial span length ratio α2. i Where the span ratio is the length L of the soil in the span. s The length L of the suspended pipe section A A The ratio, the span-to-length ratio is the length L of section B of the suspended pipe. B The length L of the suspended pipe section A A The ratios are as follows. Among them, the initial span ratio α1 and the initial span length ratio α2 can be determined or adjusted according to the actual working conditions. For example, they can be set to the span ratio and span length ratio common between pipelines in the sea area where the pipeline section to be judged is located.

[0043] S3, Calculate the single-span modal frequency f0 and the initial multi-span modal frequency f i The difference is calculated and compared with the ratio of the single-span modal frequency f0 and a predetermined threshold to determine the initial critical span ratio.

[0044] S4, gradually increase the initial span ratio α1 at predetermined span ratio intervals γ, and repeat steps S2-S3 to obtain multiple equally spaced span ratios and the critical span length ratio corresponding to each span ratio.

[0045] S5. Using the multiple span ratios obtained in step S4 and the corresponding critical span ratios for each span ratio, a multi-span discrimination curve is plotted. Based on the multi-span discrimination curve, multi-span discrimination is performed on the suspended pipeline section with the same operating conditions as section A of the suspended pipeline. The operating conditions mainly include soil texture and pipeline parameters, such as soil viscosity, soil spring stiffness, pipeline outer diameter, pipeline inner diameter, pipeline elastic modulus, pipeline density, fluid density inside the pipeline, and pipeline tension. Besides the above main factors, environmental factors such as seawater density and seawater velocity can also affect specific operating conditions. However, since submarine suspended pipelines are usually in a low-velocity environment, the impact of environmental factors is minimal and can generally be ignored.

[0046] Step S1 aims to calculate the single-span modal frequency of section A of the suspended pipe in a single-span mode. This single-span modal frequency will then be used as a standard value for comparison with the single-span modal frequency calculated in step S2 in a multi-span mode. The specific calculation methods for the modal frequencies of suspended pipes in single-span or multi-span modes are well known to those skilled in the art and will not be elaborated upon here.

[0047] Figure 3 A flowchart for determining the critical span ratio according to an embodiment of the present invention is shown in detail. When determining the initial critical span ratio, if the ratio is not less than a predetermined threshold, the initial span ratio α2 is used as the critical span ratio; if the ratio is less than the predetermined threshold, the initial span ratio α2 is gradually increased at predetermined span ratio intervals β, and steps S2-S3 are repeated until the critical span ratio is obtained. The predetermined threshold can be determined at least partially based on operating conditions, for example, based on the span ratio and modal frequency corresponding to the inflection point of the curve showing the change of modal frequency with span ratio at a fixed span ratio, and the single-span modal frequency f0.

[0048] In the finite difference calculation process of the present invention, the magnitudes of the predetermined span ratio interval γ and the predetermined span length ratio interval β determine the precision of the difference calculation, and those skilled in the art can set them according to the actual situation.

[0049] In an embodiment of the present invention, the specific parameters of section A of the suspended pipeline are shown in Table 1:

[0050] Table 1 Parameters of Section A of the Suspended Pipeline

[0051]

[0052]

[0053] In Table 1, "Main Span" refers to section A of the suspended pipeline, and "Adjacent Span" refers to section B of the suspended pipeline.

[0054] Figure 4 The figure shows the modal frequency-span ratio curves of section A of the suspended pipe under different span ratios. As shown in the figure, when the span ratio is constant, the modal frequency initially decreases slowly with the increase of the span ratio, and then drops sharply. There is an inflection point between the slow decrease region and the sharp drop region. Table 2 shows the modal frequencies when the span ratio is fixed at 0.5, and the calculation results of different modal frequencies compared with the standard value (i.e., the single-span modal frequencies of section A of the suspended pipe calculated in single-span mode). Specifically, the single-span modal frequency f0 and the initial multi-span modal frequency f0 are calculated. i The difference is calculated, and this difference is compared with the single-span modal frequency f0 to obtain the ratio between the two.

[0055] Table 2. Modal frequency calculation results when the span ratio is 0.5

[0056]

[0057] Combination Figure 4 The modal frequency-span ratio curve for a mid-span ratio of 0.5 shows that the inflection point is approximately located between a span ratio of 0.9 and 1. Further refined calculations of the modal frequencies in this range yielded the results shown in Table 3.

[0058] Table 3. Calculation results of refined modal frequencies when the span ratio is 0.5

[0059]

[0060]

[0061] As shown in Table 3, the modal frequencies in the inflection point region are approximately 5% lower than the standard values. Therefore, 5% can be set as a predetermined threshold. Alternatively, those skilled in the art can also set different values ​​as predetermined thresholds based on the above method, depending on the different pipeline and soil parameters.

[0062] Figure 5 The diagram shows the multi-span discrimination curve obtained when the predetermined threshold is 5%. The multi-span discrimination curve is plotted with the span ratio on the x-axis and the span length ratio on the y-axis. When the span ratio and span length ratio of a suspended pipeline section with the same operating conditions as section A of the suspended pipeline are plotted onto the multi-span discrimination curve:

[0063] If the point of the suspended pipeline section is located in the area below the multi-span discrimination curve, then the suspended pipeline section has no influence on the adjacent suspended pipeline sections and constitutes an independent single-span pipeline.

[0064] If the suspended pipeline section is projected to the area above the multi-span discrimination curve, then the suspended pipeline section and the adjacent suspended pipeline section will influence each other and together form a whole multi-span pipeline.

[0065] Preferably, the present invention also relates to a method for distinguishing between two or more spans. Figure 6 Taking the three-span model shown as an example, the subsea pipeline further includes a suspended pipeline segment C adjacent to the suspended pipeline segment B, with the two separated by the inter-span soil. Figure 7 As shown, the discrimination method for two or more spans is based on the two-span discrimination method, and it specifically includes:

[0066] S6. Use multi-span discrimination curves to distinguish between suspended pipe sections A, B, and C respectively. Specifically, each of suspended pipe sections A, B, and C is distinguished using multi-span discrimination curves under its respective operating conditions.

[0067] S7, based on the individual discrimination results of suspended pipe sections A, B, and C, performs multi-span discrimination on suspended pipe sections A, B, and C. Specifically:

[0068] If suspended pipe section A and suspended pipe section B affect each other, and suspended pipe section B and suspended pipe section C affect each other, then suspended pipe section A, suspended pipe section B and suspended pipe section C together constitute a whole multi-span pipe.

[0069] If suspended pipe section A and suspended pipe section B affect each other, and suspended pipe section B and suspended pipe section C do not affect each other, then suspended pipe section A and suspended pipe section B together constitute a multi-span pipe as a whole, and suspended pipe section C constitutes an independent single-span pipe.

[0070] If there is no influence between suspended pipe section A and suspended pipe section B, and suspended pipe section B and suspended pipe section C influence each other, then suspended pipe section A constitutes an independent single-span pipe, and suspended pipe section B and suspended pipe section C together constitute a whole multi-span pipe.

[0071] If there is no influence between suspended pipe section A and suspended pipe section B, and no influence between suspended pipe section B and suspended pipe section C, then suspended pipe section A, suspended pipe section B, and suspended pipe section C each constitute an independent single-span pipe.

[0072] The above describes the method for identifying three spans. The identification of spans exceeding three spans can be performed sequentially based on the principles of the three-span identification method. Therefore, the suspension method of each suspended segment of the submarine pipeline can be identified one by one.

[0073] The preferred embodiments of this disclosure have been described in detail above with reference to the accompanying drawings. However, this disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of this disclosure, various simple modifications can be made to the technical solutions of this disclosure, and these simple modifications all fall within the protection scope of this disclosure.

[0074] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately. Furthermore, various different embodiments of this disclosure can also be combined arbitrarily, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A method for identifying multi-span pipes based on modal frequencies, characterized in that, The subsea pipeline comprises a suspended pipeline section A and a suspended pipeline section B separated by spanning soil. The method includes the following steps: S1, at least in part based on the soil spring stiffness of the span soil, calculate the single-span modal frequency of the suspended pipe section A. f 0; S2, set the initial value of the span ratio as the initial span ratio α1 and the initial value of the span length ratio as the initial span length ratio α2, and calculate the multi-span modal frequency of the suspended pipe section A based at least in part on the soil spring stiffness of the soil in the span, the initial span ratio α1 and the initial span length ratio α2. f i Wherein, the span ratio is the length L of the soil mass in the span. s The length L of the suspended pipe section A A The ratio, wherein the span ratio is the length L of the suspended pipe section B. B The length L of the suspended pipe section A A The ratio; S3, Calculate the single-span modal frequency. f 0 and the multimodal frequency f i The difference, and the difference is compared with the single-span modal frequency. f The ratio of 0 to a predetermined threshold is compared to determine the initial critical span ratio; S4, gradually increase the initial span ratio α1 at predetermined span ratio intervals γ, and repeat steps S2-S3 to obtain multiple equally spaced span ratios and a critical span length ratio corresponding to each span ratio; S5. Using the multiple span ratios obtained in step S4 and the critical span ratio corresponding to each span ratio, a multi-span discrimination curve is plotted, and multi-span discrimination is performed on the suspended pipeline section with the same working conditions as the suspended pipeline section A based on the multi-span discrimination curve.

2. The method according to claim 1, characterized in that, In step S3, determining the initial critical span ratio includes: If the ratio is not less than the predetermined threshold, then the initial span ratio α2 is taken as the critical span ratio; If the ratio is less than the predetermined threshold, the initial span ratio α2 is gradually increased at predetermined span ratio intervals β, and steps S2-S3 are repeated until the critical span ratio is obtained.

3. The method according to claim 2, characterized in that, Step S3 includes: determining the predetermined threshold based at least in part on the operating conditions.

4. The method according to claim 3, characterized in that, Based on the inflection point of the curve showing the modal frequency changing with the span ratio under a fixed span ratio, and the single-span modal frequency, the span ratio and modal frequency are shown. f 0. Determine the predetermined threshold.

5. The method according to claim 3, characterized in that, The operating conditions include soil texture and pipeline parameters.

6. The method according to claim 5, characterized in that, The pipeline parameters include at least one of the following: pipeline outer diameter, pipeline inner diameter, pipeline elastic modulus, pipeline density, fluid density inside the pipeline, and pipeline tension.

7. The method according to claim 1, characterized in that, Step S5 includes plotting the multi-span discrimination curve with the span ratio as the abscissa and the critical span-length ratio as the ordinate, and plotting the span ratio and span-length ratio of the suspended pipe section that is in the same working condition as the suspended pipe section A onto the multi-span discrimination curve: If the point of the suspended pipeline section is located in the area below the multi-span discrimination curve, then the suspended pipeline section has no influence on the adjacent suspended pipeline section and constitutes an independent single-span pipeline. If the projection point of the suspended pipeline section falls above the multi-span discrimination curve, then the suspended pipeline section and the adjacent suspended pipeline section influence each other and together form a whole multi-span pipeline.

8. The method according to claim 1, characterized in that, The subsea pipeline further includes a suspended pipeline section C separated from the suspended pipeline section B by a second span of soil, and the method further includes: S6, use multi-span discrimination curves to distinguish between the suspended pipe segment A, the suspended pipe segment B, and the suspended pipe segment C respectively; S7, based on the discrimination results of each of the suspended pipe segments A, B, and C, perform multi-span discrimination on the suspended pipe segments A, B, and C.

9. The method according to claim 8, characterized in that, Step S7 includes: If the suspended pipe section A and the suspended pipe section B affect each other, and the suspended pipe section B and the suspended pipe section C affect each other, then the suspended pipe section A, the suspended pipe section B, and the suspended pipe section C together constitute a whole multi-span pipe. If the suspended pipe section A and the suspended pipe section B affect each other, and the suspended pipe section B and the suspended pipe section C do not affect each other, then the suspended pipe section A and the suspended pipe section B together constitute a multi-span pipe as a whole, and the suspended pipe section C constitutes an independent single-span pipe. If there is no influence between the suspended pipe section A and the suspended pipe section B, and the suspended pipe section B and the suspended pipe section C influence each other, then the suspended pipe section A constitutes an independent single-span pipe, and the suspended pipe section B and the suspended pipe section C together constitute a whole multi-span pipe. If there is no influence between suspended pipe section A and suspended pipe section B, and no influence between suspended pipe section B and suspended pipe section C, then suspended pipe section A, suspended pipe section B, and suspended pipe section C each constitute an independent single-span pipe.

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