A dynamic cable damping ratio test method

Abstract

The application discloses a kind of dynamic cable damping ratio test methods, and its steps are as follows: according to the production test laying cable of dynamic cable laying environment, according to the laying state of dynamic cable in seawater, using multiple support frames to support dynamic cable, mark multiple test points on dynamic cable, connect tension rod at test point, make test point offset to simulate the displacement of test point vortex-induced vibration by tension rod, record, loosen tension rod, and dynamic cable carries out free vibration, record and save the displacement-time curve of multiple test points, calculate its logarithmic decay rate by single frequency free decay vibration waveform, and obtain damping ratio.The application provides a kind of more accurate damping ratio test to dynamic cable, to ensure that the calculated use fatigue life is more accurate, avoid to increase the suppression equipment to dynamic cable in application process to reduce a kind of dynamic cable damping ratio test method of project cost.

Description

Technical Field

[0001] This invention belongs to the field of dynamic cable testing, and specifically relates to a method for testing the damping ratio of dynamic cables. Background Technology

[0002] As marine energy development gradually extends to the deep sea, and marine operations shift from fixed platforms to floating platforms, the dynamic cable serves as the vital artery for energy and signal transmission on these deep-sea floating platforms. As a slender, flexible structure connecting surface production systems to underwater equipment, the dynamic cable is constantly subjected to ocean loads during operation. In particular, under the influence of ocean currents, alternating vortices form on both sides of the umbilical cable. The shedding of these vortices causes periodic vibrations in the umbilical cable structure; this phenomenon, known as vortex-induced vibration, is one of the main causes of dynamic cable fatigue. The damping inherent in the dynamic cable can suppress vortex-induced vibration to a certain extent, improving its fatigue performance.

[0003] The most commonly used model currently couples the structural equations with the wake oscillator equations. The damping term in the structural equations affects the predictive accuracy of the wake oscillator model. However, the damping ratios in existing wake oscillator models are overly conservative, much smaller than the actual structural damping value of the dynamic cable. Furthermore, the dynamic cable system includes numerous accessories besides the cable body, which also affect the overall damping. Using smaller damping in calculations easily leads to large calculated vortex-induced vibration amplitudes, and overly conservative fatigue results result in calculated values ​​that fail to meet fatigue life requirements. Therefore, in engineering applications, vortex-induced vibration suppression equipment is needed to ensure the service life of the dynamic cable, significantly increasing project costs. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a method for testing the damping ratio of dynamic cables to ensure a more accurate calculation of their fatigue life, thereby avoiding the need to add suppression devices during application and reducing project costs.

[0005] The technical solution adopted by this invention to solve the above problems is a dynamic cable damping ratio testing method, the steps of which are as follows: S1: Produce test cables according to the dynamic cable laying environment. The length of the test cable meets the dynamic margin length and bottoming stability length of the dynamic cable. S2: Based on the laying status of the dynamic cable in seawater, multiple support frames are used to support the dynamic cable so that the dynamic cable forms a waveform with dynamic margin. S3: Mark multiple test points on the dynamic cable; S4: Connect a tension rod at the test point and use the tension rod to shift the test point to simulate the displacement of the test point under vortex-induced vibration; S5: Start the non-contact three-dimensional dynamic displacement measurement system to record; S6: Release the tension rod, and the dynamic cable will vibrate freely; S7: Record and save the displacement-time history curves of multiple test points; S8: Calculate the logarithmic decay rate from the single-frequency free decay vibration waveform to obtain the damping ratio.

[0006] Compared with the prior art, the advantages of this invention are as follows: Through steps S1 and S2, the laying state of the dynamic cable is simulated, thereby ensuring that the test results are more accurate when the damping ratio of the dynamic cable is tested. This avoids the situation in traditional test methods where the simulated environment is severely deviated, making it difficult for the test results of traditional test methods to meet the test requirements. Through the design of step S3, in the laying state of the dynamic cable, there are many structural joints and turning points such as crests and troughs. The damping ratios of these test points are different. If only traditional test methods are used, these differences of test points cannot be detected at all. However, in this invention, these special points can be isolated for testing, thereby obtaining more accurate test results and calculating more accurate fatigue life. Through the design of steps S4-S8, a relatively free test environment is provided for each test point of the dynamic cable. By allowing each test point to be damped by the damping force of the dynamic cable itself, a more accurate dynamic cable damping ratio can be measured.

[0007] As an improvement of the present invention, the waveform of the dynamic cable includes a trough and a peak. The end of the dynamic cable away from the bottom section is fixedly connected to the end support frame. The trough is movably connected to the trough support frame, and the peak is movably connected to the peak support frame. Through this improvement, the usage state of the dynamic cable can be simulated.

[0008] As an improvement of the present invention, a wave limiting rope is connected to one end of the dynamic cable near the bottom section, and the end of the wave limiting rope away from the dynamic cable is fixed. With this improvement, in the case of simulating the laying state of the dynamic cable, the dynamic cable used for testing cannot be the same length as the dynamic cable in actual use. Therefore, the length of the static cable is relatively short, which cannot achieve the stability of the static cable in actual use. During the damping ratio test, the vibration of the dynamic cable can easily cause the synchronous vibration of the static cable, which can easily affect the test results. Therefore, the design of the wave limiting rope is added to limit the vibration range of the alternating area of ​​the dynamic cable and the static cable, thereby simulating the stability of the actual static cable and ensuring the accuracy of the damping ratio test of the dynamic cable.

[0009] As an improvement of the present invention, the dynamic cable includes a wave crest section where the buoyancy is greater than the weight. The wave crest is located at the highest point of the wave crest section. The connection end of the wave limiting rope and the dynamic cable is located on the bottoming section and is spaced apart from the wave crest section. Through this improvement, the wave crest section mainly floats in seawater. When the wave crest section is stable in seawater, it will drive part of the bottoming section to rise. During the test, the stability of the floating wave crest section is lower than that of the bottoming section, and the fatigue life of the wave crest section is also significantly lower than that of the bottoming section. In addition, the overall service life of the dynamic cable during use will be based on the fatigue life of the shortest area. Moreover, the test accuracy requirements for the wave crest section are higher than those for the bottoming section. Therefore, the wave limiting rope cannot be connected to the wave crest section, and the wave crest section needs to form a sufficient gap to reduce the impact.

[0010] As an improvement of the present invention, the end support frame is movably connected along a horizontal line perpendicular to the laying direction of the dynamic cable. With this improvement, since the connection structure with the landing end of the dynamic cable is a floating platform, the end support frame needs to simulate the use state of the floating platform. The end support frame cannot be fixed to ensure the accuracy of the damping ratio test.

[0011] As an improvement of the present invention, the support surface of the wave crest support frame is arranged in a concave curved surface with a high center and low sides along the longitudinal plane perpendicular to the laying direction of the dynamic cable. Furthermore, the support surface of the wave crest support frame is provided with a wave crest friction surface. The coefficient of friction between the wave crest friction surface and the dynamic cable increases as the height of the wave crest support frame decreases. Through this improvement, in practical applications, the dynamic cable is not only subjected to vortex-induced vibration, which is caused by the flow of ocean currents bypassing the dynamic cable. The ocean currents themselves also cause the dynamic cable to deviate. Therefore, while simulating vortex-induced vibration, the deviating effect caused by ocean currents must also be considered. When the dynamic cable deviates, it is not a simple horizontal lateral displacement. It also forms a tensioning process, that is, the peaks of the dynamic cable descend and the troughs rise. In extreme cases, the midpoint of the dynamic cable will be at half the depth of the static cable. Therefore, it is necessary to simulate the height change of the dynamic cable during the offset process. At the same time, during the offset process of the dynamic cable, as the offset distance increases, the offset difficulty also increases, that is, the resistance of the dynamic cable also increases, making it less likely to cause vortex-induced vibration. Therefore, it is also necessary to change the friction coefficient between the peak friction surface and the dynamic cable according to the offset distance to achieve more accurate test results. The friction coefficient change curve can be calculated by using the self-resetting force of the dynamic cable in the horizontal bending state of the same amplitude as the friction force.

[0012] As an improvement of the present invention, the support surface of the trough support frame is arranged as a convex curved surface with a low center and high sides along the longitudinal plane perpendicular to the laying direction of the dynamic cable. Furthermore, the support surface of the trough support frame is provided with a trough friction surface. The friction coefficient between the trough friction surface and the dynamic cable increases with the height of the crest support frame. Through this improvement, in practical applications, the dynamic cable is not only subjected to vortex-induced vibration, which is caused by the flow of ocean currents bypassing the dynamic cable. The ocean currents themselves also cause the dynamic cable to deviate. Therefore, while simulating vortex-induced vibration, the deviating effect caused by ocean currents must also be considered. When the dynamic cable deviates, it is not a simple horizontal lateral displacement. It also forms a tensioning process, that is, the peaks of the dynamic cable descend and the troughs rise. In extreme cases, the midpoint of the dynamic cable will be at half the depth of the static cable. Therefore, it is necessary to simulate the height change of the dynamic cable during the offset process. At the same time, during the offset process of the dynamic cable, as the offset distance increases, the offset difficulty also increases, that is, the resistance of the dynamic cable also increases, and it is less likely to cause vortex-induced vibration. Therefore, it is also necessary to change the friction coefficient between the trough friction surface and the dynamic cable according to the offset distance to achieve more accurate test results. The friction coefficient change curve can be calculated by using the self-resetting force of the dynamic cable in the horizontal bending state of the same amplitude as the friction force.

[0013] As an improvement of the present invention, the support point of the wave crest support frame is the midpoint of the dynamic cable. Through this improvement, during the process of supporting the dynamic cable with wave crests and troughs, the structure that mainly causes the change of the dynamic cable curve is the wave crest, while the support effect of the wave trough is relatively low. It mainly simulates the buoyancy of seawater on the wave trough area and facilitates the friction of the wave trough friction surface on the dynamic cable. The midpoint of the dynamic cable is the area with the least constraint force in the entire dynamic cable, and the vortex-induced vibration it experiences is also the most intense. By selecting the midpoint of the dynamic cable as the support point of the wave crest support frame, it is easier to accurately select the test point, which is beneficial to the accuracy of the measurement results. Moreover, the change curve of the midpoint of the dynamic cable during the offset deformation process is easier to draw, which helps to manufacture and shape the support surface in the wave crest support frame, making the vibration trajectory of the dynamic cable more consistent with the vibration trajectory in the sea.

[0014] As an improvement of the present invention, the dynamic cable, from the fixed end to the bottoming section, sequentially includes a landing section, a crest section, and a bottoming section. The trough is located on the landing section, and the crest is located on the crest section. The test points include a trough test point near the trough, an ascending test point at the connection between the landing section and the crest section, a crest test point near the crest, and a descending test point at the connection between the crest section and the bottoming section. Through this improvement, during use, the trough and crest test points are two points with relatively high degrees of freedom, and therefore are more susceptible to vortex-induced vibration, making them two points more likely to reach fatigue life. The ascending and descending test points are points where the dynamic cable structure changes... The two points of change in structure are affected by many factors, such as the connection strength and uniformity of the structure. As a result, the structural stability of the rising and falling test points is low, and they are also the two points that are more likely to reach fatigue life. Therefore, these four points need to be tested in detail. However, during the test, due to the influence of the support surface of the trough support frame and the support surface of the crest support frame, the vortex-induced vibration at the crest and trough positions is difficult to perform free vibration. Therefore, only the crest test point close to the crest and the trough test point close to the trough can be selected, and the crest and trough cannot be directly selected as test points. Other areas can be selectively tested.

[0015] As an improvement of the present invention, in step S8, the peak value A of any two vibration waveforms is taken. n A n+m Its logarithmic decay rate δ is calculated using the following formula: ; Where m is the number of periods in the interval; The formula for calculating the damping ratio ζ is: ; The aforementioned improvement enables the calculation of the damping ratio at each test point. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the overall structure of the present invention.

[0017] Figure 2 This is a schematic diagram of the end support frame structure of the present invention.

[0018] Figure 3 This is a schematic diagram of the wave crest support frame structure of the present invention.

[0019] Figure 4 This is a schematic diagram of the peak curve of the peak test point of the present invention under simulated vortex-induced vibration.

[0020] Figure 5 This is a schematic diagram of the valley support frame structure of the present invention.

[0021] Figure 6This is a schematic diagram of the peak curve of the trough test point of the present invention under simulated vortex-induced vibration.

[0022] Figure 7 This is a schematic diagram of the peak curve of the rising test point of the present invention under simulated vortex-induced vibration.

[0023] Figure 8 This is a schematic diagram of the peak curve of the descent test point under simulated vortex-induced vibration.

[0024] The diagram shows: 1. Dynamic cable; 1.1. Landing section; 1.1.1. Trough; 1.2. Crest section; 1.2.1. Crest; 1.3. Bottom section; 1.4. Trough test point; 1.5. Ascending test point; 1.6. Crest test point; 1.7. Descent test point; 2. Three-dimensional dynamic displacement measurement system; 3. End support frame; 3.1. Slide rail; 3.2. Slider; 4. Trough support frame; 4.1. Trough friction surface; 5. Crest support frame; 5.1. Crest friction surface; 6. Wave limit rope. Detailed Implementation

[0025] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0026] like Figure 1 As shown, a dynamic cable damping ratio testing method comprises the following steps: S1: Produce test cables according to the laying environment of dynamic cable 1. The length of the test cable meets the dynamic margin length and bottoming stability length of dynamic cable 1. S2: Based on the laying state of dynamic cable 1 in seawater, multiple support frames are used to support dynamic cable 1 so that dynamic cable 1 forms a waveform with dynamic margin. S3: Mark multiple test points on dynamic cable 1; S4: Connect a tension rod at the test point and use the tension rod to shift the test point to simulate the displacement of the test point under vortex-induced vibration; S5: Start the non-contact SMT-3D three-dimensional dynamic displacement measurement system 2 to record; S6: Release the tension rod, and dynamic cable 1 will vibrate freely; S7: Record and save the displacement-time history curves of multiple test points; S8: Calculate the logarithmic decay rate from the single-frequency free decay vibration waveform to obtain the damping ratio.

[0027] like Figure 1As shown, the waveform of the dynamic cable 1 includes a trough 1.1.1 and a crest 1.2.1. The end of the dynamic cable 1 away from the bottoming section 1.3 is fixedly connected to the end support frame 3. The trough 1.1.1 is movably connected to the trough support frame 4, and the crest 1.2.1 is movably connected to the crest support frame 5. A wave limiting rope 6 is connected to the end of the dynamic cable 1 near the bottoming section 1.3. The end of the wave limiting rope 6 away from the dynamic cable 1 is fixed. The dynamic cable 1 includes a crest section 1.2 where the buoyancy is greater than the weight. The crest 1.2.1 is located at the highest point of the crest section 1.2. The connection end of the wave limiting rope 6 and the dynamic cable 1 is located on the bottoming section 1.3 and is spaced apart from the crest section 1.2.

[0028] The SMT-3D three-dimensional dynamic displacement measurement system 2 moves along the laying direction of the dynamic cable 1 and can move to an area less susceptible to interference for testing, depending on the selection of the test point.

[0029] like Figure 2 As shown, the end support frame 3 is movably connected along a horizontal line perpendicular to the laying direction of the dynamic cable 1. The end support frame 3 has a low-friction coefficient slide rail 3.1 and a slider 3.2 that moves on the slide rail 3.1. The end of the dynamic cable 1 away from the bottom-contact section 1.3 is fixedly connected to the slider 3.2. The movement of the slider 3.2 on the slide rail 3.1 can simulate the floating of a floating platform on the sea surface. Since this invention mainly tests vortex-induced vibration, only horizontal movement design is required, not omnidirectional movement design.

[0030] like Figure 3 As shown, the support surface of the wave crest support frame 5 is set as a convex curved surface with a high middle and low sides along the longitudinal surface perpendicular to the laying direction of the dynamic cable 1, and the support surface of the wave crest support frame 5 is provided with a wave crest friction surface 5.1. The friction coefficient between the wave crest friction surface 5.1 and the dynamic cable 1 increases as the height of the wave crest support frame 5 decreases.

[0031] like Figure 5 As shown, the support surface of the trough support frame 4 is a concave curved surface with a low middle and high sides along the longitudinal surface perpendicular to the laying direction of the dynamic cable 1, and the support surface of the trough support frame 4 is provided with a trough friction surface 4.1. The friction coefficient between the trough friction surface 4.1 and the dynamic cable 1 increases as the height of the crest support frame 5 increases.

[0032] The convex and concave surfaces are obtained by measuring the lateral forces acting on the wave crests and troughs in the water perpendicular to the laying direction of the dynamic cable 1, which cause the dynamic cable 1 to deform accordingly, and the movement trajectories of the wave crests and troughs.

[0033] The surface roughness of the crest friction surface 5.1 and the trough friction surface 4.1 can be gradually varied to achieve a change in the coefficient of friction. Alternatively, they can be made of elastic material with a gradually varying thickness. When the spring material is compressed, the thicker the area, the greater the frictional force, thus achieving a change in the coefficient of friction. Figure 3 and Figure 5 The mid-wave crest friction surface 5.1 and the wave trough friction surface 4.1 adopt a thickness gradient design using elastic material.

[0034] When constructing the thickness curve of the crest friction surface 5.1, five points on the motion trajectory of the crest are selected. When the crest is at each of these five points, the restoring force at these points is measured using a pressure testing instrument. The restoring force is then taken as the maximum static friction force. Based on the weight of the support unit area of ​​the crest and the static friction force, the friction coefficient of the crest friction surface 5.1 at these five points is calculated. Then, based on the elastic material properties, the thickness value of the crest friction surface 5.1 at these five points is calculated, or the thickness value of the crest friction surface 5.1 at these five points is obtained experimentally. An approximate thickness curve is then plotted using these five thickness values. Similarly, the thickness curve of the trough friction surface 4.1 is constructed.

[0035] The support point of the wave crest support frame 5 is the midpoint of the dynamic cable 1. When the dynamic cable 1 undergoes extreme deformation, the height of the wave crest is exactly half the depth of the seawater.

[0036] like Figure 1 , Figure 4 , Figure 6-8 As shown, the dynamic cable 1, from the fixed end towards the bottoming section 1.3, sequentially includes a landing section 1.1, a crest section 1.2, and a bottoming section 1.3. The trough 1.1.1 is located on the landing section 1.1, and the crest 1.2.1 is located on the crest section 1.2. The test points include a trough test point 1.4 near the trough 1.1.1, an ascending test point 1.5 at the junction of the landing section 1.1 and the crest section 1.2, a crest test point 1.6 near the crest 1.2.1, and a descending test point 1.7 at the junction of the crest section 1.2 and the bottoming section 1.3. Figure 4 , Figure 6-8 In each diagram, there is a solid-line dynamic cable 1 structure and a dashed-line dynamic cable 1 structure. The solid-line dynamic cable 1 represents the position of the dynamic cable 1 after it is pulled by the tension rod, which simulates the amplitude of vortex-induced vibration. The dashed-line dynamic cable 1 structure represents the peak position on the other side of the dynamic cable 1 after it returns to its original position due to inertia after the tension rod is released. The solid and dashed coils in the diagram represent the peak points of the test points in different periods. In the calculation, the peak values ​​in the corresponding two periods are taken, and the damping ratio ζ is measured using the formula.

[0037] In step S8, the peak value A of any two vibration waveforms is taken. n A n+mIts logarithmic decay rate δ is calculated using the following formula: ; Where m is the number of periods in the interval; The formula for calculating the damping ratio ζ is: .

[0038] After the test is completed, the damping ratio ζ of at least four test points will be obtained, and the fatigue life of the corresponding test points will be calculated using the damping ratio. The final service life of dynamic cable 1 will be based on the minimum fatigue life.

[0039] During the test, the wave crest support frame 5 cannot perfectly demonstrate the effect of ocean buoyancy. That is, the submarine cable at the highest point of the convex surface is the point of force equilibrium in the ocean, but it cannot be stationary on the wave crest support frame 5, which has obvious replication defects. Therefore, it is difficult to perfectly replicate the test. Test items that cross the highest point of the convex surface in wave crest segment 1.2 should be avoided as much as possible. It is more accurate to conduct the test only on one side of the convex surface.

[0040] Meanwhile, the position of the dynamic cable 1 on the end support frame 3, the trough support frame 4, and the crest support frame 5 can be moved to simulate the laying pattern of the dynamic cable 1 under different ocean current environments. The damping ratio ζ can be tested under this laying pattern to obtain more accurate and comprehensive fatigue life data.

[0041] The above description only illustrates the preferred embodiments of the present invention and should not be construed as limiting the scope of the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All modifications made within the scope of the independent claims of this invention are also within the scope of protection of this invention.

Claims

1. A method for testing the dynamic cable damping ratio, characterized in that... The steps are as follows: S1: Based on the laying environment of the dynamic cable (1), the test cable is produced and laid. The length of the test cable meets the dynamic margin length and bottoming stability length of the dynamic cable (1). S2: Based on the laying state of the dynamic cable (1) in the seawater, multiple support frames are used to support the dynamic cable (1) so that the dynamic cable (1) forms a waveform with dynamic margin. S3: Mark multiple test points on the dynamic cable (1); S4: Connect a tension rod at the test point and use the tension rod to shift the test point to simulate the displacement of the test point under vortex-induced vibration; S5: Turn on the non-contact three-dimensional dynamic displacement measurement system (2) to record; S6: Release the tension rod, and the dynamic cable (1) will vibrate freely; S7: Record and save the displacement-time history curves of multiple test points; S8: Calculate the logarithmic decay rate from the single-frequency free decay vibration waveform to obtain the damping ratio; The waveform of the dynamic cable (1) includes a trough (1.1.1) and a crest (1.2.1). The end of the dynamic cable (1) away from the bottom section (1.3) is fixedly connected to the end support frame (3). The trough (1.1.1) is movably connected to the trough support frame (4), and the crest (1.2.1) is movably connected to the crest support frame (5). The support surface of the crest support frame (5) is set as a concave curved surface with a high middle and low sides along the longitudinal surface perpendicular to the laying direction of the dynamic cable (1). The support surface is provided with a crest friction surface (5.1). The friction coefficient between the crest friction surface (5.1) and the dynamic cable (1) increases as the height of the crest support frame (5) decreases. The support surface of the trough support frame (4) is set as a convex curved surface with a low middle and high sides along the longitudinal surface perpendicular to the laying direction of the dynamic cable (1). The support surface of the trough support frame (4) is provided with a trough friction surface (4.1). The friction coefficient between the trough friction surface (4.1) and the dynamic cable (1) increases as the height of the crest support frame (5) increases.

2. The dynamic cable damping ratio test method according to claim 1, characterized in that: The dynamic cable (1) is connected to a wave limiting rope (6) at one end near the bottom section (1.3), and the wave limiting rope (6) is fixed at the other end away from the dynamic cable (1).

3. The dynamic cable damping ratio test method according to claim 2, characterized in that: The dynamic cable (1) includes a wave crest section (1.2) where the buoyancy is greater than the gravity. The wave crest (1.2.1) is located at the highest point of the wave crest section (1.2). The connection end of the wave limiting rope (6) and the dynamic cable (1) is located on the bottom section (1.3) and there is a gap between them and the wave crest section (1.2).

4. The dynamic cable damping ratio test method according to claim 1, characterized in that: The end support frame (3) is movably connected along a horizontal line perpendicular to the laying direction of the dynamic cable (1).

5. The dynamic cable damping ratio test method according to claim 1, characterized in that: The support point of the crest support frame (5) is the midpoint of the dynamic cable (1).

6. The dynamic cable damping ratio test method according to claim 1, characterized in that: The dynamic cable (1) includes a landing section (1.1), a crest section (1.2), and a bottoming section (1.3) in sequence from the fixed end to the bottoming section (1.3). The trough (1.1.1) is located on the landing section (1.1), and the crest (1.2.1) is located on the crest section (1.2). The test points include a trough test point (1.4) near the trough (1.1.1), an ascending test point (1.5) at the connection between the landing section (1.1) and the crest section (1.2), a crest test point (1.6) near the crest (1.2.1), and a descending test point (1.7) at the connection between the crest section (1.2) and the bottoming section (1.3).

7. The dynamic cable damping ratio test method according to claim 1, characterized in that: In step S8, the peak value A of any two vibration waveforms is taken. n A n+m Its logarithmic decay rate δ is calculated using the following formula: ； Where m is the number of periods in the interval; The formula for calculating the damping ratio ζ is: 。

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