A method for predicting a lock-in range of vortex-induced vibration of a wake riser

Abstract

The application discloses a method for predicting a vortex-induced vibration locking interval of a wake riser, and relates to the technical field of ocean engineering. The technical scheme solves the problem of whether the vortex-induced vibration locking of the upstream riser and the downstream riser is synchronous, and by introducing the spacing between the two risers in series into a reduced velocity calculation formula of the vortex-induced vibration locking interval of a cylinder, the vortex-induced vibration locking interval of the wake riser can be conveniently predicted, thereby providing a reference for a deepwater riser bundle designer.

Description

Technical Field

[0001] The present invention relates to the field of marine engineering technology, and more particularly to a method for predicting a vortex-induced vibration locking interval of a wake riser. Background Art

[0002] The riser bundle on a deepwater oil platform typically consists of six or more risers, usually arranged parallel to the two main axes of the platform plane. Due to platform space limitations, the spacing between risers is the same in both directions, with the minimum spacing being based on the principle of not causing interference sufficient to affect structural safety. Because the direction of the ocean current changes, the position of the risers relative to the flow field also changes, resulting in the formation of serial risers, parallel risers, and stepped risers. When the ocean current flows through these riser bundles, the riser that the current first flows through is called the upstream riser, and the risers located at the wake of the upstream riser are called downstream risers.

[0003] Vortex-induced vibration is the vibration of a cylinder parallel to its cross section and perpendicular to the direction of fluid flow, which occurs when the cylinder is subjected to the action of a fluid perpendicular to its axis. The vortex discharge generated at the cylinder's wake is a load that causes the cylinder to vibrate perpendicular to the direction of fluid flow, which is called vortex-induced lift. Vortex-induced motion and vortex-induced vibration describe the same physical phenomenon, but are used to describe the same physical phenomenon in different types of structures. Vortex-induced motion describes the transverse oscillation of a rigid cylinder under the action of a fluid; vortex-induced vibration describes the transverse bending vibration of a flexible cylinder under the action of a fluid.

[0004] When the vortex discharge frequency f s and the natural frequency f of the cylinder n When the flow velocity is equal to , the vortex-induced motion will be locked, that is, the vortex discharge frequency no longer increases with the increase of flow velocity according to the law of formula (1), but remains unchanged. When the flow velocity increases to a certain value, the vortex discharge frequency will change according to the law of formula (1) again, such as Figure 1 As shown;

[0005] in, (1)

[0006] Where: f s — vortex discharge frequency;

[0007] St — Strohal number;

[0008] U — flow velocity;

[0009] D — diameter of cylinder;

[0010] (2)

[0011] Where: f n— natural frequency of the cylinder;

[0012] EI —— bending stiffness of cylindrical section;

[0013] m — mass per unit length of the cylinder;

[0014] l —— length of cylinder.

[0015] Figure 1 In, f * is the ratio of the vortex discharge frequency to the natural frequency of the cylinder, that is: f * = f s / f n , called the frequency ratio, V r is the reduced speed, calculated according to formula (3).

[0016] (3)

[0017] The current method for determining the locking range of vortex-induced vibration of a single cylinder is to calculate the reduced speed of the vortex-induced vibration of a single cylinder entering and exiting the locking range through equations (4) and (5), and then adjust the mass and / or diameter of the cylinder so that the cylinder does not resonate when vortex-induced motion occurs, that is, the locking phenomenon does not occur.

[0018] (4)

[0019] (5)

[0020] Where: V r,min — the minimum reduced velocity at which vortex-induced vibration locking occurs in a cylinder;

[0021] V r,max — the maximum reduced velocity at which vortex-induced vibration locking occurs in a cylinder;

[0022] m * ——Mass ratio, the ratio of the mass of the cylinder to the mass of the water it displaces.

[0023] When vortex-induced vibration occurs in tandem risers, the vortex shedding and vortex-induced vibration properties of the downstream riser will be affected by the vortex discharged from the upstream riser. The degree of influence is related to the distance between the two risers and the vibration state of the upstream and downstream risers, such as Figure 2 Therefore, in order to reduce the mutual influence between upstream and downstream risers, designers need to reasonably design the riser spacing, taking into account the space limitations on the platform and minimizing the mutual influence between risers.

[0024] The current technical solutions are limited to solving the problems of vortex-induced vibration frequency and amplitude of the downstream riser. There is no experimental method to predict the locking range of vortex-induced vibration of the downstream riser, nor is there a theoretical method or empirical formula. Summary of the Invention

[0025] To solve the above problems, the present invention provides the following technical solution: a method for predicting the locking range of vortex-induced vibration of a wake riser, comprising the following steps:

[0026] S1. Fabricate two model risers from solid rods, each with a diameter of D. Securely mount them to the riser bundle vortex-induced vibration interferometry test apparatus. Install the two model risers parallel to the water flow, with a spacing of 1D between them. Install a displacement sensor along the longitudinal and transverse directions of the flume at the ends of the model risers exposed above the water surface. Install a flow meter at the front end of the test apparatus, 1 meter upstream of the flow.

[0027] S2. After causing a displacement greater than 1D in the model riser, the model riser is released without initial velocity, and the displacement data of the free vibration of the model riser is recorded;

[0028] S3. Turn on the flow pump and adjust the flow rate to the minimum flow rate of the flow pump. After the flow rate stabilizes, record the flow rate data and the displacement data of the model riser. Adjust the flow pump so that the flow rate increases by a certain value each time until the flow rate reaches the maximum flow rate of the flow pump. After each adjustment, record the flow rate data and the displacement data of the model riser.

[0029] S4, turn off the flow pump, increase the distance between the two model risers by a certain value, and repeat step S3;

[0030] S5, repeat step S4 until the distance between the two model risers reaches 10D;

[0031] S6. Calculate the reduced velocity V corresponding to each flow rate obtained in steps S3-S5 r ;

[0032] S7, performing fast Fourier transform on the displacement data of the free vibration of the model riser obtained in step S2 and the displacement data obtained in steps S3-S5, respectively obtaining the displacement spectrum of the free vibration and the displacement spectrum of the vortex-induced vibration; and then obtaining the natural frequency f of the model riser from the displacement spectrum of the free vibration and the displacement spectrum of the vortex-induced vibration. n and the vortex-induced vibration frequency f at different flow rates v , divide the two to get the frequency ratio ;

[0033] S8. Plot the wake riser frequency ratio f under different riser spacings of the two models. * Randomization speed V rThe curve of the change of the starting point and end point of the vortex-induced vibration locking interval with the spacing between the two model risers is obtained by connecting the two ends of the straight section of the curve at different spacings; then, according to the shape of the change curve, the functions f1(x) and f2(x) of the starting point and end point of the locking interval with the reduced velocity are selected;

[0034] S9, the functions f1(x), f2(x), and the reduced velocity V corresponding to the start and end points of the wake riser vortex-induced vibration locking interval r,min and V r,max By introducing the empirical formulas (6) and (7) for the start and end points of the vortex-induced vibration locking interval of the wake riser established in the present invention, the coefficients α and β are obtained by fitting:

[0035] (6)

[0036] (7)

[0037] S10. According to formulas (6) and (7), the lock range of the wake riser vortex-induced vibration can be predicted.

[0038] Furthermore, it also includes judging whether the test system is normal after step S2. The specific judgment method is: turn on the flow pump to 1 / 2 of the maximum flow rate, debug the displacement test system and the water flow test system, when the displacement sensor digital changes alternately and the flow meter digital remains stable, the test system is normal; then turn off the flow pump.

[0039] Furthermore, in step S3, the flow rate is increased each time by 1 / 10 of the difference between the maximum flow rate and the minimum flow rate.

[0040] Furthermore, in step S4, the distance between the two model risers is increased by 0.5D.

[0041] In summary, the present invention has the following beneficial effects: the method of the present invention solves the problem of whether the vortex-induced vibration locking of the upstream and downstream risers is synchronized, and by introducing the spacing between the two risers in series into the reduced velocity calculation formula for the vortex-induced vibration locking range of a single cylinder, the vortex-induced vibration locking range of the wake riser can be easily predicted for reference by deepwater riser bundle designers. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is the relationship curve between frequency ratio and reduced speed;

[0043] Figure 2 This is a schematic diagram of a tandem riser. DETAILED DESCRIPTION

[0044] The present invention is further described in detail below with reference to the examples.

[0045] The present invention proposes a method for predicting the vortex-induced vibration locking range of a wake riser. Based on model tests, the method fits an empirical formula for predicting the vortex-induced vibration locking range of a wake riser, providing a convenient and practical calculation method for riser research and design personnel.

[0046] The method of the present invention comprises the following steps:

[0047] S1. Fabricate two model risers from solid rods, each with a diameter of D. Securely mount them to the riser bundle vortex-induced vibration interferometry test apparatus. Install the two model risers parallel to the water flow, with a spacing of 1D between them. Install a displacement sensor along the longitudinal and transverse directions of the flume at the ends of the model risers exposed above the water surface. Install a flow meter at the front end of the test apparatus, 1 meter upstream of the flow.

[0048] S2. After causing a displacement greater than 1D in the model riser, the model riser is released without initial velocity, and the displacement data of the free vibration of the model riser is recorded;

[0049] Turn on the flow pump to 1 / 2 of the maximum flow rate, debug the displacement test system and the water flow test system. When the displacement sensor numbers change alternately and the flow meter numbers remain stable, the test system is normal; then turn off the flow pump;

[0050] S3. Turn on the flow pump and adjust the flow rate to the minimum flow rate of the flow pump. After the flow rate stabilizes, record the flow rate data and the displacement data of the model riser. Adjust the flow pump so that the flow rate increases by a certain value each time until the flow rate reaches the maximum flow rate of the flow pump. After each adjustment, record the flow rate data and the displacement data of the model riser. Each time the flow rate increases, the flow rate is increased by 1 / 10 of the difference between the maximum and minimum flow rates.

[0051] S4, turn off the flow pump, increase the distance between the two model risers by a certain value, and repeat step S3; in this step, the distance between the two model risers is increased by 0.5D;

[0052] S5, repeat step S4 until the distance between the two model risers reaches 10D;

[0053] S6. Calculate the reduced velocity V corresponding to each flow rate obtained in steps S3-S5 according to formula (3) r ;

[0054] S7, performing fast Fourier transform on the displacement data of the free vibration of the model riser obtained in step S2 and the displacement data obtained in steps S3-S5, respectively obtaining the displacement spectrum of the free vibration and the displacement spectrum of the vortex-induced vibration; and then obtaining the natural frequency f of the model riser from the displacement spectrum of the free vibration and the displacement spectrum of the vortex-induced vibration. n and the vortex-induced vibration frequency f at different flow rates v , divide the two to get the frequency ratio ;

[0055] S8. Plot the wake riser frequency ratio f under different riser spacings of the two models. * Randomization speed V r The curve of the change of the starting point and end point of the vortex-induced vibration locking interval with the spacing between the two model risers is obtained by connecting the two ends of the straight section of the curve at different spacings; then, according to the shape of the change curve, the functions f1(x) and f2(x) of the starting point and end point of the locking interval with the reduced velocity are selected;

[0056] S9, the functions f1(x), f2(x), and the reduced velocity V corresponding to the start and end points of the wake riser vortex-induced vibration locking interval r,min and V r,max By introducing the empirical formulas (6) and (7) for the start and end points of the vortex-induced vibration locking of the wake riser established in the present invention, the coefficients α and β are obtained by fitting:

[0057] (6)

[0058] (7)

[0059] S10. According to formulas (6) and (7), the lock range of the wake riser vortex-induced vibration can be predicted.

[0060] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art may make non-creative modifications to this embodiment as needed. However, as long as such modifications are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A method for predicting the locking interval of vortex-induced vibration of a wake riser, characterized in that: The following steps are involved: S1. Fabricate two model risers from solid rods, each with a diameter of D. Securely mount them to the riser bundle vortex-induced vibration interferometry test apparatus. Install the two model risers parallel to the water flow, with a spacing of 1D between them. Install a displacement sensor along the longitudinal and transverse directions of the flume at the ends of the model risers exposed above the water surface. Install a flow meter at the front end of the test apparatus, 1 meter upstream of the flow. S2. After causing a displacement greater than 1D in the model riser, the model riser is released without initial velocity, and the displacement data of the free vibration of the model riser is recorded; S3. Turn on the flow pump and adjust the flow rate to the minimum flow rate of the flow pump. After the flow rate stabilizes, record the flow rate data and the displacement data of the model riser. Adjust the flow pump so that the flow rate increases by a certain value each time until the flow rate reaches the maximum flow rate of the flow pump. After each adjustment, record the flow rate data and the displacement data of the model riser. S4, turn off the flow pump, increase the distance between the two model risers by a certain value, and repeat step S3; S5, repeat step S4 until the distance between the two model risers reaches 10D; S6. Calculate the reduced velocity V corresponding to each flow rate obtained in steps S3-S5 r ; S7, performing fast Fourier transform on the displacement data of the free vibration of the model riser obtained in step S2 and the displacement data obtained in steps S3-S5, respectively obtaining the displacement spectrum of the free vibration and the displacement spectrum of the vortex-induced vibration; and then obtaining the natural frequency f of the model riser from the displacement spectrum of the free vibration and the displacement spectrum of the vortex-induced vibration. n and the vortex-induced vibration frequency f at different flow rates v , divide the two to get the frequency ratio ; S8. Plot the wake riser frequency ratio f under different riser spacings of the two models. * Randomization speed V r The curve of the change of the start and end points of the vortex-induced vibration locking interval of the wake riser is obtained by connecting the two ends of the straight section of the curve at different spacings; and then the functions f1(x) and f2(x) of the start and end points of the locking interval changing with the reduced velocity are selected according to the shape of the change curve; S9, the functions f1(x), f2(x), and the start and end points of the wake riser vortex-induced vibration locking interval correspond to the reduced velocity V r,min and V r,max By introducing the empirical formulas (6) and (7) for the start and end points of the vortex-induced vibration locking interval of the wake riser established in the present invention, the coefficients α and β are obtained by fitting: （6） （7） S10. According to formulas (6) and (7), the lock range of the wake riser vortex-induced vibration can be predicted.

2. The method according to claim 1, characterized in that It also includes judging whether the test system is normal after step S2. The specific judgment method is: turn on the flow pump to 1 / 2 of the maximum flow rate, debug the displacement test system and the water flow test system, when the displacement sensor digital changes alternately and the flow meter digital remains stable, the test system is normal; then turn off the flow pump.

3. The method according to claim 1, characterized in that In step S3, the flow rate is increased each time by 1 / 10 of the difference between the maximum flow rate and the minimum flow rate.

4. The method according to claim 1, wherein The distance between the two model risers is increased by 0.5D.

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