Method for measuring vibration characteristics of low-temperature stainless steel pipe of LNG (liquefied natural gas) ship

Abstract

A method for measuring vibration characteristics of a low-temperature stainless steel pipe of an LNG (liquefied natural gas) ship comprises the following steps: selecting a shortest pipeline span l1 and a longest pipeline span l2 required by measurement, not filling any medium into a measurement pipeline, filling high-pressure gas into the shortest pipeline, filling working liquid into the shortest pipeline, and measuring maximum amplitude points of a first-order vibration mode and a second-order vibration mode of the pipeline in the three states, so as to measure the vibration characteristics of the low-temperature stainless steel pipe of the LNG ship. According to the method, 1 / 4, 1 / 2 and 3 / 4 points of the pipeline are measured, vibration characteristics of the pipeline are obtained by analyzing measurement results, vibration reasons are determined, and design of the pipeline layout is guided according to actual conditions. According to the invention, the number of measuring points can be greatly reduced while high reliability is maintained, vibration can be detected, and besides, when resonance occurs in actual work, the source of vibration can be analyzed, and a targeted solution is provided.

Description

Technical Field

[0001] The invention belongs to the field of offshore LNG ship design and construction, and in particular relates to a method for measuring vibration characteristics of a low-temperature stainless steel pipe of an LNG ship. Background Art

[0002] The main product of Dalian Shipbuilding in 2023 is a 175,000 cubic meter LNG carrier. During the sea trial, the vibration level of the LNG cryogenic piping system was measured, but the magnetic sensor cannot be directly adsorbed on the wall of the information security cryogenic stainless steel pipe. During the sea trial, due to the limited time for the main engine to run under MCR conditions, it is very difficult to complete the inspection within a limited time for LNG ships with a large number of cryogenic piping systems. In addition, due to the large number of pipelines, a method is needed to quickly measure the vibration characteristics of the cryogenic stainless steel pipes of LNG ships to guide the design and layout of the cryogenic stainless steel pipes of LNG ships.

[0003] During the sea trial, the vibration of the LNG cryogenic piping system of the LNG carrier is measured, and the focus of the measurement is the GVU valve box of the LNG carrier. The GVU valve box is equipped with a solenoid valve assembly, which is used as a valve and pipeline device before controlling or adjusting the gas fuel engine on the LNG-powered ship.

[0004] Due to the large vibration of ships during navigation, the gas supply pipe end of the solenoid valve assembly in the GVU valve box of LNG-powered ships and the main engine are currently directly connected by thread, and this connection structure is easy to fall off. The gas supply pipe end of the solenoid valve assembly in the GVU valve box and the main engine connection are prone to LNG gas leakage after long-term vibration, and the leakage cannot be eliminated in time, affecting the normal navigation and accident safety of LNG-powered ships. Therefore, during the ship trial, the low-temperature piping vibration level test must be completed under the main engine MCR working condition.

[0005] Under the main engine MCR condition, the vibration excitation source is only the main engine, and the excitation frequency is single. When measuring only this condition, the full picture of the cryogenic stainless steel pipe of the LNG carrier cannot be reflected. The real vibration condition of the cryogenic stainless steel pipe of the LNG carrier can only be obtained through testing during the sea trial. Due to the limited time and number of sea trials, it is difficult to deal with it in a targeted manner after the vibration exceeds the standard, and there is no time and opportunity to verify it after the treatment. Therefore, it is necessary to consider the vibration characteristics of the cryogenic stainless steel pipe of the LNG carrier during the design to avoid the occurrence of dangerous situations caused by the vibration of the stainless steel pipe.

[0006] At present, in the construction process of LNG carriers, since it is necessary to carry out vibration level measurement of cryogenic piping system under the main engine MCR condition and measure the acceleration values ​​in multiple directions within the same circle, it is necessary to measure the acceleration values ​​in multiple directions within the same circle in a short time and take the average acceleration value in multiple directions within the same circle as the final measurement value.

[0007] Since the cryogenic stainless steel pipe of LNG ship is a symmetrical double-walled circular pipe, the supporting points between the two pipes are "up, down, left, and right". According to the different vibration directions, its vibration characteristics are different in different positions. At the same time, the vibration characteristics of the pipeline are different when it is filled with air, high-pressure gas and liquid. Therefore, for the cryogenic stainless steel pipe of LNG ship (hereinafter referred to as pipeline), its vibration characteristics include at least the following categories: (1) Different vibration characteristics caused by different filling media in the pipe: air, high-pressure gas and working liquid. Working liquid refers to the liquid medium that may be filled into the pipe during the operation of the ship; (2) Different vibration characteristics caused by different pipeline rotation directions: the typical positions are 0 degrees and 45 degrees [*0.1].

[0008] like Figure 1 As shown, the first section and the second section have significantly different cross-sectional rigidity, and therefore the vibration characteristics are also significantly different.

[0009] (3) Different vibration characteristics caused by different pipeline spans: the test can be carried out at intervals of 0.1 m (0.5-2 m), and interpolation calculations can be performed at other locations.

[0010] Therefore, we found that in order to initially obtain the detailed vibration characteristics of the cryogenic stainless steel pipes of LNG ships, at least 3*2*15=90 tests are required, and each experiment requires repeated adjustments to the pipeline and comparisons. The measurement is time-consuming and laborious. Summary of the invention

[0011] In order to solve the above problems, the present invention provides a method for measuring the vibration characteristics of the cryogenic stainless steel pipe of the LNG ship, aiming to measure the vibration of the cryogenic stainless steel pipe of the LNG ship at different frequencies, analyze the vibration characteristics through the measured data, greatly reduce the number of measurements and measurement points, avoid the occurrence of resonance, and guide the pipeline design layout with the vibration characteristics of the cryogenic stainless steel pipe of the LNG ship. The technical solution adopted is: A method for measuring the vibration characteristics of low-temperature stainless steel pipes of LNG ships, characterized in that the specific operations are as follows: S1: Take the first measurement The shortest pipeline span l1 required for measurement is selected, and the pipeline is fixed with clamps at both ends of the span. At this time, both ends of the pipeline at the clamps can be regarded as simply supported conditions at the ends, and no medium is filled in the pipeline.

[0012] Record the fixed points at both ends of the low-temperature stainless steel pipe to be measured, take the midpoint, 1 / 4 point and 3 / 4 point between the two fixed points, and mark them.

[0013] The newly invented vibration measuring device is clamped on the low-temperature stainless steel pipe, connected to the vibration measuring device, and prepared for measurement. Measure at the midpoint, 1 / 4 point and 3 / 4 point simultaneously, and record the vibration waveform.

[0014] The vibration signal is converted into time-frequency and modal analyzed to obtain the frequency-amplitude waveform diagram and the first and second order vibration modes and the corresponding natural frequencies, which are recorded. The first order natural frequency is recorded as p1.1 and the second order natural frequency is recorded as p1.2.

[0015] S2: Perform the second measurement The pipeline span l1 remains unchanged, and high-pressure gas is filled in. The operation of the previous step S1 is repeated to obtain the frequency-amplitude waveform and the 1st and 2nd order vibration modes for the second time. The natural frequencies corresponding to the 1st and 2nd order vibration modes are recorded, where the 1st order natural frequency is recorded as p2.1 and the 2nd order natural frequency is recorded as p2.2.

[0016] Record a coefficient x1, which is defined as follows: x1= ; S3: Perform the third measurement The pipeline span l1 remains unchanged, and the working fluid is filled in. Step S1 is repeated to obtain the frequency-amplitude waveform and the 1st and 2nd order vibration modes for the third time. The natural frequencies corresponding to the 1st and 2nd order vibration modes are recorded, where the 1st order natural frequency is recorded as p3.1 and the 2nd order natural frequency is recorded as p3.2.

[0017] Record a coefficient x2, which is defined as follows: x2= ; S4: Perform the fourth to sixth measurements S4.1: Select the required pipeline spans for measurement and measure them separately. The spans are: l1+1 / 3*(l2-l1), l1+2 / 3*(l2-l1), l2, where l2 refers to the longest available pipeline span.

[0018] First, measure the situation of l1+1 / 3*(l2-l1). Fix the pipeline with clamps at both ends of the span. At this time, both ends of the pipeline at the clamps can be regarded as simply supported conditions at the ends, and no medium is filled in the pipeline.

[0019] Repeat the operation of step S1 to record the natural frequencies corresponding to the first and second order vibration modes, wherein the first order natural frequency is recorded as p4.1 and the second order natural frequency is recorded as p4.2.

[0020] S4.2: Change the span to l1+2 / 3*(l2-l1), repeat the operation of step S1, record the first-order natural frequency as p5.1, and the second-order natural frequency as p5.2.

[0021] S4.3: Change the span to l2, repeat the operation of step S1, record the first-order natural frequency as p6.1, and the second-order natural frequency as p6.2.

[0022] S5: Data Analysis A coordinate system is established with the vertical axis representing the natural frequency and the horizontal axis representing the span length. p1.1, p4.1, p5.1, and p6.1 are all represented on the coordinate system. The four points described in S4.1 to S4.3 are connected using a spline curve.

[0023] When no medium is filled in the pipeline, the natural frequency pi of pipeline vibration at any span li can be obtained from the difference on the curve.

[0024] S6: Get the vibration characteristics when filled with high pressure gas: The curve obtained in step S5 has its abscissa unchanged and its ordinate multiplied by the coefficient x1 obtained in step S2, which is the vibration characteristic when the pipeline is filled with high-pressure gas.

[0025] S7: Get the vibration characteristics when filled with working fluid The curve obtained in step S5 has its abscissa unchanged and its ordinate multiplied by the coefficient x2 obtained in step S2, which is the vibration characteristic when the pipeline is filled with high-pressure gas.

[0026] According to the vibration characteristic curves obtained from S6 and S7, the natural frequency pi of pipeline vibration at any span li under the two working conditions can be obtained from the difference of the curves.

[0027] S8: Change the first-order natural frequency in steps S1 to S7 to the second-order natural frequency pi.2 (i=1, 2, 3, 4, 5, 6), repeat steps S5 to S7, and obtain the correlation curve between the second-order natural frequency and the span.

[0028] S9: Rotate the pipeline 45 degrees, repeat S1 to S8, and perform the seventh to twelfth measurements to obtain the vibration characteristics when the pipeline is rotated 45 degrees.

[0029] S10: Compare the natural frequencies corresponding to all the 1st and 2nd order vibration modes obtained in S1~S9 with the main engine and auxiliary group of the ship to determine the cause of vibration and guide the design of pipeline layout according to the actual situation.

[0030] The beneficial effects of the present invention are: (1) The number of tests required to measure pipeline vibration characteristics has been greatly reduced from at least 90 measurements to 12.

[0031] (2) Significantly reduce the number of measurement points while maintaining high reliability.

[0032] (3) In previous patent searches, no relevant detection methods for the vibration characteristics of low-temperature stainless steel pipes on LNG ships were found.

[0033] (4) The prior art only measures the vibration amplitude as a means of vibration inspection. In addition to detecting vibration, the present invention can also analyze the source of vibration when resonance occurs in actual work and provide targeted solutions.

[0034] (5) Through prior measurement, the vibration characteristics of low-temperature stainless steel pipes of different spans can be analyzed and taken into consideration in the design. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is the cross section of low temperature stainless steel pipe; Figure 2 It is the basic vibration waveform of the 1st and 2nd order vibration modes; Figure 3 This is a schematic diagram of the shortest pipe span l1 required for measurement; Figure 4 It is a schematic diagram of the natural frequency pi of pipeline vibration at any span li when no medium is filled in the pipeline; Figure 5 It is a schematic diagram of the natural frequency pi of pipeline vibration at any span li when the pipeline is filled with medium; Figure 6 It is a schematic diagram of the relationship between the second-order natural frequency and the span; Figure 7 It is a schematic diagram of the position relationship between the sensor and the pipeline cross section after the pipeline is rotated 45 degrees; Among them, 1- sensor. DETAILED DESCRIPTION

[0036] The present invention will be further described in conjunction with the accompanying drawings.

[0037] A method for quickly measuring the vibration level of low-temperature stainless steel pipes of LNG ships, taking a 175,000 cubic meter LNG transport ship as an example, includes the following steps: S1: First take the first measurement S1.1: Select the shortest pipe span l1 required for measurement, and fix the pipe with clamps at both ends of the span. At this time, the two ends of the pipe at the clamps can be regarded as simply supported at the ends, and no medium is filled in the pipe. Figure 3 shown.

[0038] S1.2: Determine the measurement position, record the two fixed points of the low-temperature stainless steel pipe to be measured, and mark the midpoint between the two ends and the 1 / 4 point close to the measurer.

[0039] S1.3: Clamp the newly invented vibration measuring device onto the low-temperature stainless steel pipe, connect the vibration measuring device, and prepare for measurement.

[0040] S1.4: Measure at the midpoint, 1 / 4 point, and 3 / 4 point simultaneously, and record the vibration waveform. The vibration waveform consists of time and amplitude.

[0041] S1.5: Perform time-frequency conversion and modal analysis on the vibration signal (the time-frequency conversion and modal analysis mentioned here are fast Fourier transform and modal calculation, which are all textbook knowledge) to obtain the frequency-amplitude waveform diagram and the first and second order vibration modes and the corresponding natural frequencies.

[0042] S1.6: Record the natural frequencies corresponding to the 1st and 2nd order vibration modes, where the 1st order natural frequency is recorded as p1.1 and the 2nd order natural frequency is recorded as p1.2.

[0043] S2: Perform the second measurement S2.1: Keep the pipeline span l1 unchanged, fill it with high-pressure gas, and repeat the previous steps S1.1 to S1.5.

[0044] S2.2: Record the natural frequencies corresponding to the first and second order vibration modes, where the first order natural frequency is recorded as p2.1 and the second order natural frequency is recorded as p2.2.

[0045] S2.3: Record a coefficient x1, which is defined as follows [*1.1]: x1= ; [*1.1]: According to the natural frequency formula of a simply supported beam in vibration mechanics, when the span and support conditions remain unchanged, its natural frequency is proportional to the stiffness coefficient (in this article, E is Young's modulus and I is the section inertia moment). Therefore, the physical meaning of the coefficient x1 is: the ratio of the stiffness coefficients under two working conditions, that is, the ratio of the stiffness coefficients when the pipeline is not filled with high-pressure gas and when it is filled with high-pressure gas.

[0046] S3: Perform the third measurement S3.1: Keep the pipe span l1 unchanged, fill it with working fluid, and repeat the previous steps S1.1 to S1.5.

[0047] S3.2: Record the natural frequencies corresponding to the first and second order vibration modes, where the first order natural frequency is recorded as p3.1 and the second order natural frequency is recorded as p3.2.

[0048] S3.3: Record a coefficient x2, which is defined as follows: x2= ; S4: Perform the fourth to sixth measurements S4.1: Select the required pipe spans for measurement and measure them separately. The spans are: l1+1 / 3*(l2-l1), l1+2 / 3*(l2-l1), l2, where l2 refers to the longest pipe span that can be selected.

[0049] S4.2: First measure the situation of l1+1 / 3*(l2-l1). Fix the pipeline with clamps at both ends of the span. At this time, both ends of the pipeline can be regarded as simply supported at the clamps, and no medium is filled in the pipeline.

[0050] S4.3: Repeat steps S1.1 to S1.5.

[0051] S4.4: Record the natural frequencies corresponding to the first and second order vibration modes, where the first order natural frequency is recorded as p4.1 and the second order natural frequency is recorded as p4.2.

[0052] S4.5: Change the span to l1+2 / 3*(l2-l1), repeat the previous steps S4.2~S4.4, and record the first-order natural frequency as p5.1 and the second-order natural frequency as p5.2.

[0053] S4.6: Change the span to l2, repeat the previous steps S4.2 to S4.4, and record the first-order natural frequency as p6.1 and the second-order natural frequency as p6.2.

[0054] S5: Data Analysis: S5.1: Establish a coordinate system with the vertical axis representing the natural frequency and the horizontal axis representing the span length. Express p1.1, p4.1, p5.1, and p6.1 on the coordinate system.

[0055] S5.2: The four points described in S4.7 are connected using a spline curve in order to fit a quadratic curve. Since the physical characteristics of the vibration characteristics show that the natural frequency is inversely proportional to the square of the span [*4.1], the spline curve composed of four points is used to fit a quadratic curve [*4.2].

[0056] [*4.1]: Vibration Mechanics, textbook [*4.2]: Gao Guangjian: A simple quadratic B-spline curve fitting algorithm, Microcomputer Applications, 2010, 1007-757X(2010)10-0037-03 S5.3: When no medium is filled in the pipeline, the natural frequency pi of the pipeline vibration at any span li can be obtained from the difference of the curve. Figure 4 As shown, S6: Get the vibration characteristics when filled with high pressure gas: S6.1: The curve obtained in S5.3 is multiplied by the coefficient x1 obtained in S2, with the horizontal coordinate unchanged, to obtain the vibration characteristics of the pipeline when it is filled with high-pressure gas. The reason for this use is shown in [*1.1], which is determined by the physical properties of the simply supported beam.

[0057] S7: Get the vibration characteristics when filled with working fluid: S7.1: The curve obtained in S5.3 is kept constant on the horizontal axis and multiplied on the vertical axis by the coefficient x2 obtained in S2, which is the vibration characteristic when the pipeline is filled with high-pressure gas.

[0058] S7: According to the vibration characteristic curves obtained from S6 and S7, the natural frequency pi of pipeline vibration under any span li under two working conditions can be obtained from the difference of the curves. Figure 5 shown.

[0059] S8: Change the first-order natural frequency in S1~S7 to the second-order natural frequency pi.2 (i=1,2,3,4,5,6), repeat S5~S7, and obtain the correlation curve between the second-order natural frequency and the span. Figure 6 shown.

[0060] S9: Rotate the pipeline 45 degrees, repeat S1 to S8, and perform the seventh to twelfth measurements to obtain the vibration characteristics when the pipeline is rotated 45 degrees. Figure 7 shown.

[0061] S10: Compare the natural frequencies corresponding to all the 1st and 2nd order vibration modes obtained in S1~S9 with the main engine and auxiliary group of the ship to determine the cause of vibration and guide the design of pipeline layout according to the actual situation.

[0062] Through the present invention, the number of tests can be compressed to 12 times.

[0063] According to the laws of physics, the complex vibration of the pipeline can be regarded as the superposition of multiple simple forms. These forms are called vibration modes, and each vibration mode corresponds to a natural frequency. When the vibration frequency reaches a certain natural frequency, the pipeline will vibrate according to the waveform shown by the vibration mode.

[0064] Among them, the vibration energy is mostly concentrated in the first and second order vibration modes, and its basic vibration waveform is as follows: Figure 2 As shown, the corresponding vibration frequencies are called the first-order natural frequency and the second-order natural frequency. It can be seen that the maximum amplitude points of the first and second-order vibration modes, that is, the key points are 1 / 4, 1 / 2, and 3 / 4 points. Measuring these three points can significantly reduce the number of measurement points while maintaining high reliability. This is one of the advantages of this patent.

Claims

1. A method for measuring the vibration characteristics of low-temperature stainless steel pipes of LNG ships, characterized in that: The specific operations are as follows: S1: Take the first measurement Select the shortest pipe span l1 required for measurement, and fix the pipe with clamps at both ends of the span. At this time, both ends of the pipe can be regarded as simply supported at the clamps, and no medium is filled into the pipe. Record the fixed points at both ends of the low-temperature stainless steel pipe to be measured, take the midpoint, 1 / 4 point and 3 / 4 point between the two fixed points, and mark them; The newly invented vibration measuring device is clamped on the low-temperature stainless steel pipe, connected to the vibration measuring device, and prepared for measurement. Measure at the midpoint, 1 / 4 point and 3 / 4 point simultaneously, and record the vibration waveform; Perform time-frequency conversion and modal analysis on the vibration signal to obtain the frequency-amplitude waveform and the first and second order vibration modes and the corresponding natural frequencies, and record them. The first order natural frequency is recorded as p1.1, and the second order natural frequency is recorded as p1.

2. S2: Perform the second measurement The pipeline span is kept constant at l1, and the high-pressure gas is filled in. The operation of the previous step S1 is repeated to obtain the frequency-amplitude waveform and the first and second order vibration modes for the second time. The natural frequencies corresponding to the first and second order vibration modes are recorded, wherein the first order natural frequency is recorded as p2.1, and the second order natural frequency is recorded as p2.2; Record a coefficient x1, which is defined as follows: x1= ； S3: Perform the third measurement The pipeline span is kept constant at l1, and the working fluid is filled in. Step S1 is repeated to obtain the frequency-amplitude waveform and the first and second order vibration modes for the third time. The natural frequencies corresponding to the first and second order vibration modes are recorded, where the first order natural frequency is recorded as p3.1, and the second order natural frequency is recorded as p3.2; Record a coefficient x2, which is defined as follows: x2= ； S4: Perform the fourth to sixth measurements S4.1: Select the required pipeline spans for measurement and measure them separately. The spans are: l1+1 / 3*(l2-l1), l1+2 / 3*(l2-l1), l2, where l2 refers to the longest available pipeline span; First, measure the situation of l1+1 / 3*(l2-l1), and fix the pipeline with clamps at both ends of the span. At this time, both ends of the pipeline can be regarded as simply supported at the clamps, and no medium is filled in the pipeline. Repeat the operation of step S1 to record the natural frequencies corresponding to the first and second order vibration modes, wherein the first order natural frequency is recorded as p4.1 and the second order natural frequency is recorded as p4.2; S4.2: Change the span to l1+2 / 3*(l2-l1), repeat the operation of step S1, record the first-order natural frequency as p5.1, and the second-order natural frequency as p5.2; S4.3: Change the span to l2, repeat the operation of step S1, record the first-order natural frequency as p6.1, and the second-order natural frequency as p6.2; S5: Data Analysis Establish a coordinate system, with the vertical axis being the natural frequency and the horizontal axis being the span length. Express p1.1, p4.1, p5.1, and p6.1 on the coordinate system, and connect the four points in S4.1 to S4.3 using a spline curve. When no medium is filled in the pipeline, the natural frequency pi of pipeline vibration under any span li can be obtained from the difference of the curve; S6: Obtain vibration characteristics when filled with high pressure gas The curve obtained in step S5 has its abscissa unchanged and its ordinate multiplied by the coefficient x1 obtained in step S2, which is the vibration characteristic when the pipeline is filled with high-pressure gas; S7: Get the vibration characteristics when filled with working fluid The curve obtained in step S5 has its abscissa unchanged and its ordinate multiplied by the coefficient x2 obtained in step S2, which is the vibration characteristic when the pipeline is filled with high-pressure gas; According to the vibration characteristic curves obtained by S6 and S7, the natural frequency pi of pipeline vibration under any span li under two working conditions can be obtained from the difference of the curves; S8: Change the first-order natural frequency in steps S1 to S7 to the second-order natural frequency pi.2 (i=1, 2, 3, 4, 5, 6), repeat steps S5 to S7, and obtain the correlation curve between the second-order natural frequency and the span; S9: Rotate the pipeline 45 degrees, repeat S1 to S8, and perform the seventh to twelfth measurements to obtain the vibration characteristics when the pipeline is rotated 45 degrees; S10: Compare the natural frequencies corresponding to all the 1st and 2nd order vibration modes obtained in S1~S9 with the ship's main engine, auxiliary group, etc., determine the cause of vibration, and guide the design of pipeline layout according to actual conditions.

Images