GB2162610A

GB2162610A - Regulation of response of bodies to a fluid flow

Info

Publication number: GB2162610A
Application number: GB08501247A
Authority: GB
Inventors: Thomas Henderson; Peter Kenneth Stansby
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-02
Filing date: 1985-01-18
Publication date: 1986-02-05
Also published as: GB8501247D0; GB2162610B

Abstract

Submerged elongate elements of a marine structure are fitted with devices comprising a ring (2; 102) carrying vanes or fins (4; 104) such as to produce vortices when water flows past the elements of a kind to mitigate the effect on the regular line vortices produced by the elements themselves. <IMAGE>

Description

SPECIFICATION Regulation of response of bodies to a fluid flow description The invention relates to means for regulating the response of a body in a fluid to relative movement between the body and the fluid.

Relative movement between a body and a fluid in which it is received tends to produce a turbulent wake in the fluid downstream of the body, the forces of which can cause the body to respond.

The effects of the turbulence can be minimised by streamlining the body but this possibility is not available for bodies which are fixed in position and subjected to flows in various directions. The effects of turbulence in the wake can be particularly severe when the body is a submerged part of a marine structure, exposed to waves and/or currents.

Vortex shedding occurs downstream of the body, and a body for example of cylindrical form can respond substantially, if its natural frequency is close to the frequencies of the fluid dynamic forces induced by unsteadiness in the wake caused by the vortex movement. The response of the body to these forces depends on its mass and damping, but the effects are in general undesirable in as far as they necessarily impose fatigue on the body, so shortening its life and even resulting in its destruction.

The invention accordingly provides means for use with a body subjected to ambient fluid flow, the means being arranged to produce turbulence downstream thereof such as to modify downstream turbulence due to the body in a manner to minimise the effect thereof on the body. The invention thus provides a device for association with a body submerged in a liquid, the device being shaped to cause disruption of vortices in the wake of the body, so that vibrations in the body due to the vortices is minimised.

Devices embodying the invention are intended for use in particular with elongate bodies, in particular cylindrical bodies, such as the risers of offshore platforms, which are subjected to fluctuating forces due to coherent vortices formed downstream in flowing water the vortices having axes parallel to the axes of the bodies. The device of the invention is thus arranged to be secured to such a body and to then produce streamwise vortices, that is, vortices in a direction perpendicular to the axis of the body. The streamwise vortices interact with the axial vortices to produce a highly three-dimensional flow field with poor spanwise coherence. Vortex-induced forces are consequently weakened and so consequently are vibrational effect on the body.

A device of the invention can thus comprise a partial or complete hoop or collar which can be placed around the submerged cylindrical body, the hoop or collar having elements or formations, such as outwardly extending fins or blades, at right angles or inclined to the plane of the hoop or collar, and of a shape to produce streamwise vortices in the water or the like moving past the body. The device is preferably so configured that is can be placed around an existing elongate body. The position of the device axially of the body can be controlled by selection of material to afford a predetermined buoyancy and/or by physical restraining means.

The invention will be more fully understood from the following illustrative description and accompanying drawings, in which: Figure 1 is a schematic isometric view of a first vortex shedding device in accordance with the invention; Figure 2 is a side elevation of the first device; Figure 3 is a plan view of the first device; Figure 4 schematically illustrates the use of a modified form of the first device; Figure 5 is a schematic isometric view of a second vortex shedding device in accordance with the invention; Figure 6 is a sideelevation of the second device; Figure 7 is a plan view of the second device; and Figure 8 schematically illustrates the use of the second device.

The device 1 illustrated in Figures 1-3 comprises a hoop portion 2 in the form of an axially short circular sleeve, provided externally with streamwise vortex forming formations, each formation having the form of a fin 4 extending in plane inclined at an appropriate angle, for example, 45 , to the axis or centre line of the hoop portion. The fins 4 are approximately triangular or delta shaped, being bounded at their inner edges by the outer periphery of the hoop portion 2, and externally by two edges a+ right angles, but differently shaped fins capable of forming streamwise vortices can be employed. Four of the fins 4 are equally spaced around the hoop portion so that the edges of the fins, seen axially of the device, form a square, as appears from Figure 3, but different numbers of fins can be fitted to the hoop portion.For example, the modified device shown in Figure 4 corresponds to that of Figures 1-3 but has two diametrically opposed fins only.

In use, the device 1 is placed around a body or member of which the response to fluid flow is to be modified. As shown in Figure 4, a device 11, which is a modified form of the device 1 having only two of the fins 4 at diametrically opposed positions, extends around and at least partially submerged upright pipe member 10, for example, a riser of an offshore platform. To permit installation of the device 1 or the device 11 on an existing structure, the device can be constructed with the hoop portion separated at one position between the fins 4, means being provided for securing the separated ends together after installation. The diameter of the hoop portion 2 of the device is slightly greater than the diameter of a member 10, as apppears from Figure 4, so that the device is capable of movement so as to remain free of local marine growths.

The second device 100 shown in Figures 5-8 comprises a hoop portion 102, for example of square or circular cross-section, having equally spaced around it four outwardly projection stream wise vortex forming formations. Each formation has the form of a flat blade 104 extending at least approximately in a plane containing the axis of the hoop portion 102, that is, at right angles to the plane of the hoop portion. The blades 104 are approximately square with a side dimension of about one third the diameter of the pipe or other member with which the device is to be used. The structure of the device 100 can be strengthened by reinforcement wires 105 between the centres of the blades 104 and the hoop portion at positions on either side of the connection of the blade to the hoop portion.The device 100 can be assembled with an underwater member in the same way as the devices 1 and 11 and it functions in the same way as those devices.

A device of the invention is preferably located at a selected position lengthwise of the member 10 and because the device is submerged in liquid this position can be obtained by selection of the material of the device so as to have a preferred buoyancy. Thus the device may have a near buoyancy or may be slightly buoyant, so as to tend to take up a position near the surface of the liquid, where the member 10 projects above this, at which position response of the member to flow induced vibrations tends to be maximised.

A plurality of the devices can also be mechanically spaced along the member 10. Preferably, the devices have an axial spacing along the member of about one half to one diameter. The positions of the devices can be maintained by making the devices of material such that they are heavier than water or of neutral buoyancy and linking them together by means of an elongate spacer or connector, such as the connector 20 shown in Figure 4.

The connector 20 comprises an elongate strip of plastics material with appropriately spaced slots 21 through which the hoop portions of the devices 11 extend.

As shown in Figure 8, a plurality of the devices 100 can be spaced axially along the member 10, with adjacent devices angularly spaced by 45 , that each blade 104 of each device is midway between an adjacent pair of the blades of the device immediately above and below. This orientation of the devices 100 is maintained by a connector 120 which resembles the connector 20 except in that the hoops 102 are secured to the connector so as to be restrained against relative rotation about their common axis.

in Figures 4 and 8, the member 10 is shown as experiencing a transverse flow in the direction indicated by the arrows 12. The consequence of this flow is the formation of parallel vortices 14 due to the far side of the pipe, together with parallel vortices 15 produced by the near side of the pipe. The effect of these vortices 14, 15 is to induce undersirable vibrations in the member 10, but this effect is mitigated by the fins or blades of the devices 11 and 100 because these cause streamwise vortices 16 to be created, the effect of which is to disrupt the parallel vortices 14, 15, thereby reducing the dynamic response and drag forces on the member 10.

In Figures 4 and 8, devices embodying the invention have been shown in use with a vertical pipe member 10, but it will be understood that the devices are not confined to use with vertical members, but can be equally well applied to inclined or horizontal members, not necessarily hollow or of circular cross-section.

Vortex shedding devices embodying the invention may be made of a suitable plastics material, for example polypropylene, and may be integrally moulded.

The invention can of course be embodied in a variety of ways than as specifically described and illustrated.

Claims

1. A body shaped to form vortices in a liquid in which the body is received on relative movement of the body and the liquid and means associated with the body shaped to disrupt the vortices.

2. An elongate body movable relative to a liquid in which the body is received whereby vortices parallel to the length df the body are produced in the liquid, the body having associated therewith means shaped to produce in the liquid streamwise vortices to disrupt the parallel vortices.

3. A body as claimed in claim 1 or 2 wherein the associated means comprises a plurality of blade or fin-like elements spaced around the body.

4. A body as claimed in claim 3 wherein the blade or fin-like elements are carried by a support engaged at least partly around the body.

5. A body as claimed in claim 4 wherein the support is releasably engaged around the body.

6. A device for reducing the effect on a body submerged in a fluid of turbulence due to relative movement of the body and the fluid, the device comprising a plurality of blade or fin-like elements carried by a support engageable at least partly around the body.

7. A device as claimed in claim 6 wherein the support has the form of a ring or hoop and each element extends outwardly therefrom at least approximately at right angles to the plane of the ring or hoop.

8. A device as claimed in claim 6 wherein each element is a substantially rectangular plate.

9. A device as claimed in claim 6 wherein the support comprises a ring or hoop and each element extends outwardly therefrom in a plane inclined to the plane thereof.

10. A device as claimed in claim 8 wherein the elements extend in planes inclined at least approximately at 45" to the plane of the ring or hoop.

11. A device as claimed in claim 9 or 10 wherein each element is substantially triangular seen axially of the device.

12. A device as claimed in any one of claims 611 wherein the support comprises an elongate member with releasably connectable ends whereby the device can be fitted around the body.

13. A device as claimed in any one of claims 612 wherein the elements are carried by the support so as to be substantially equally spaced around the body.

14. A device as claimed in any one of claims 613 having four of the elements.

15. A device as claimed in any one of claims 614 moulded in plastics material.

16. A device as claimed in any one of claims 615 for use with a body of elongate form, the device being connected to at least one like device so that the supports of the devices can extend around the body at spaced positions therealong.

17. A device as claimed in claim 16 connected to the at least one like device by an elongate member engaging the support of each device.

18. A device as claimed in claim 17 wherein the elongate member is secured to the device and to the at least one like device so as to maintain the elements of the devices in a predetermined annular orientation about the body.

19. A device as claimed in claim 18, maintained by the securement thereof to the elongate member with each element thereof at least approximately equally spaced from adjacent elements of the or each axially adjacent device.

20. A device for reducing the effect on a body received in a fluid of turbulence due to relative movement of the body and the fluid substantially as herein described with reference to Figures 1-3, Figure 4, or Figures 5-8 of the accompanying drawings.

21. An elongate body having the device of any one of claims 6-23 placed around it.