GB2544766A

GB2544766A - Underwater cable fairing

Info

Publication number: GB2544766A
Application number: GB1520821.8A
Authority: GB
Inventors: Hayman Jason; Jeffcoate Penny; Gardner Alexander
Original assignee: Sustainable Marine Energy Ltd
Current assignee: Sustainable Marine Energy Ltd
Priority date: 2015-11-25
Filing date: 2015-11-25
Publication date: 2017-05-31
Anticipated expiration: 2035-11-25
Also published as: GB201520821D0; GB2544766B

Abstract

A cable fairing 120 suitable for removable mounting to an umbilical cable (126, Fig 6) comprises a tube (100, Fig 1) comprising a central bore 102 shaped to restrict the bend of the umbilical cable and a tail fin (112, Fig 1) extending substantially radially from the tube. The tail fin is shaped to reduce drag on the tube when immersed in a flowing liquid. In use the cable fairing both reduces drag on and restricts the bending of underwater cables or pipes to prevent over bending of the cables or pipes. The cable fairing has a lower drag in a fluid flow than the cables or pipes themselves in order to reduce vortex formation. The central bore may have a circular cross-section and may have a maximum dimension at an upper and lower end of the bore with a minimum dimension (114, Fig 2) at a mid-point of the bore. The cable fairing may be formed from a buoyant material such as plastic foam.

Description

UNDERWATER CABLE FAIRING

Field

The present invention relates to cable fairings for use on underwater cables or pipes. More particularly, the present invention relates to modular cable fairings that both reduce drag on and restrict the bending of underwater cables or pipes to prevent overbending of the cables or pipes.

Background

The use of flexible underwater cables and pipes is well known for a variety of applications, such as towing a payload, providing power to underwater equipment, oil exploration and drilling and underwater power generation.

When such cables are subject to a fluid flow (either from the background flow of the fluid they are immersed in or as a result of being towed through the fluid), the general cylindrical shape of these cables cause vortices to form in the fluid as it passes around the cable. These vortices can induce vibrations in the cable, which are a source of fatigue and damage, and which may reduce the useful lifetime of the cables.

Furthermore, when underwater cables are subject to different flow velocities at different points along their lengths (for example if the fluid velocity varies with depth), the cables can bend, which inhibit their function or even cause them to break should the cables bend in excess of their tolerances.

Summary of the Invention

Aspects and/or embodiments seek to provide an improved cable fairing for underwater cables or pipes that has a lower drag in a fluid flow than the cables or pipes themselves in order to reduce vortex formation and which also restricts bending of the cables or pipes.

According to a first aspect of the present invention, there is provided a cable fairing suitable for removable mounting to an umbilical cable, the cable fairing comprising: a tube, the tube comprising a central bore shaped to restrict the bend of the umbilical cable; and a tail fin extending substantially radially from the tube, the tail fin being shaped to reduce drag on the tube when immersed in a flowing fluid.

By providing a cable fairing that both reduces drag and restricts bending of an enclosed cable, the enclosed cables can be protected from being bent further than the bending tolerance for the cables and can thus be at a reduced risk Of damage due to over-bending while the reduction in drag on the cable can avoid the creation of vortices that can exert undesirable and potentially damaging localised forces on portions of the cable.

Optionally, the central bore is shaped to restrict the bend of the umbilical cable to the minimum bend of the umbilical cable.

By providing a central bore that is shaped to restrict bending of the enclosed cable to only the minimum tolerance of bending of the cable, the risk of damage by over-bending to the cable can be reduced through the use of a known safe tolerance of the cable.

Optionally, the central bore has a substantially circular cross section.

By providing a central bore with a substantially circular cross section, cables or pipes having a substantially circular or circular cross section can be enclosed by the cable fairing. By providing a substantially circular cross section, the cable fairing can move to rotate with the direction of fluid flow, for example to move with a change in tidal flow.

Optionally, the radius of the circular cross section varies along the length of the bore.

By providing a circular cross section having a radius that varies along the length of the bore, the narrowest portion of the bore can be configured to grip an enclosed cable and retain the cable fairing in place on the cable.

Optionally, the radius of the circular cross section has a maximum at or near the upper and/or lower end of the tube.

By providing a circular cross section having a maximum radius at the upper or lower (or both) end(s) of the tube, the widest radius portion(s) of the bore can be configured not to grip an enclosed cable to allow for movement of the cable fairing relative to the cable, for example in conditions of turbulent flow.

Optionally, the radius of the circular cross section has a minimum at approximately midway along the length of the tube.

By providing a circular cross section having a minimum radius at approximately the midway point in the tube, the narrowest portion of the bore can be configured to grip an enclosed cable at approximately the midway point.

Optionally, the tail fin extends substantially continuously from the tube.

By providing a tail fin that extends substantially continuously from the tube, the fairing can reduce the hydrodynamic drag acting on the enclosed cable.

Optionally, the tail fin has a length that extends in the axial direction of the tube, the length varying in the radial direction from the tube.

By providing a tail fin that has a length that extends in the axial direction of the tube where the length varies in the radial direction from the tube, the fairing can further reduce the hydrodynamic drag acting on the enclosed cable.

Optionally, the length of the tail fin decreases along the radial direction from the tube.

By providing a tail fin that decreases in width with radial distance from the tube, when installed next to other cable fairing this can prevent an overlap with a tail fin of a neighbouring substantially identical cable fairing also attached to the umbilical cable.

Optionally, the length of the tail fin has a maximum length approximately equal to the length of the tube.

By providing a tail fin that has a maximum length that is limited to being approximately equal to the length of the tube, neighbouring cable fairings can remain with the neighbouring tail fins being spaced apart even when the enclosed cable is subject to bending.

Optionally, the maximum length of the tail fin occurs where it joins the tube.

By providing a tail fin having a maximum length at the portion that it joins the tube, neighbouring cable fairings can remain with the neighbouring tail fins being spaced apart even when the enclosed cable is subject to bending.

Optionally, the tube can be split along its length.

By providing a tube that can be split along its length, it can be straightforward for the cable fairing to be installed and removed from the cable.

Optionally the tube further comprises at least one strap securing point suitable for securing the umbilical cable to the cable fairing.

By providing a strap securing point, a strap can be used to fasten the cable fairing.

Optionally, the tube is constructed from a material having a buoyancy greater than water. Optionally, the material used to construct the tube is buoyant plastic foam.

By providing a cable fairing having a tube made from a material having a positive buoyancy in water, the cable fairing can act as a buoyancy device to support an enclosed portion of cable in water.

Optionally, the tube is constructed from a material with a buoyancy less than water.

By providing a cable fairing having a tube made from a material having a negative buoyancy in water, the cable fairing can act as a weight to submerge an enclosed portion of cable in water.

As used herein, the term cable or umbilical cable preferably connotes any flexible underwater cable or pipe.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only and with reference to the accompanying drawings having like-reference numerals, in which:

Figure 1 illustrates a perspective view of a cable fairing;

Figure 2 illustrates a side view of a cable fairing;

Figure 3 illustrates a top view of a cable fairing;

Figure 4 illustrates a front view of a cable fairing;

Figure 5 illustrates multiple cable fairings attached to an umbilical cable;

Figure 6 illustrates a schematic diagram of cable fairings in use;

Figure 7 illustrates a schematic diagram of an alternative use of the cable fairings;

Figure 8 illustrates a perspective view of an alternative embodiment of a cable fairing; and

Figure 9 illustrates a top view of an alternative embodiment of a cable fairing. Specific Description

Referring to Figures 1 to 9, an exemplary embodiment of the cable fairing will now be described.

Figure 1 illustrates a perspective view of a cable fairing. The cable fairing comprises a substantially cylindrical tube 100, with a central bore 102 extending along its length. The tube is split along its length for the insertion of an umbilical cable. It may be split along one line along its length or two, the latter allowing part of the tube to be separated into two removable pieces for easier umbilical cable insertion. Cutaway sections 104 act as strap securing points. The strap securing points divide the tube into an upper section 106, a middle section 108 and a lower section 110. Extending outwardly and substantially continuously from the tube is a tail fin 112 shaped to give the fairing a low drag profile in a flowing fluid, such as sea water. The cable fairing can be manufactured from a material that is buoyant in the fluid in which it is to be immersed, in order to provide positive buoyancy to the umbilical cable to which they are attached, or may be manufactured from a material that is less buoyant than the fluid in order to provide a negative buoyancy to the umbilical cable.

Figure 2 illustrates a side view of a cable fairing. The edges of the central bore 102 are shown as dashed lines. The radius of the central bore varies along the length of the tube 100 in order to restrict the bend of cables that pass through it. The radius is at a maximum at the top of the upper section 106 of the tube and the bottom of the lower section of the tube 110, and has a minimum radius 114 approximately midway along the tube 100.

The tail fin 112 extends substantially radially from the tube 100. It has a length that decreases in the outward radial direction from the tube, giving the tail fin 112 an approximately trapezoid cross section in the vertical plane. This shape is provided by cut-out sections 116 at the top and bottom of the tail fin.

Figure 3 illustrates a top view of a cable fairing. The tail fin 112 extends outwardly from the tube 100 in a substantially continuous way in order to lower the drag on the fairing. In this embodiment, the cross section of the tail fin 112 in the horizontal plane is substantially triangular, though it will be recognised by those skilled in the art that many other cross sectional shapes are possible that will provide the drag reducing effect.

Figure 4 illustrates a front view of a cable fairing.

Figure 5 illustrates multiple cable fairings attached to an umbilical cable. An umbilical cable passes through the central bore 102 of each cable fairing 120 in turn. The curvature of the sides of the central bores 102 along the length of each cable fairing prevents the umbilical cable from bending more than a predetermined amount.

The cutaways 104 in each cable fairing 120 allow a strap to be attached around each cable fairing 120 once the umbilical cable has been inserted into the central bore 102. This prevents the cable fairings 120 from becoming detached from the umbilical cable via the split along which the umbilical cable was inserted, or alternatively, if there are two splits along the tube of the cable fairings 120, securing the two parts of the cable fairings 120 together.

Each cable fairing 120 is separated from the next along the cable by a float spacer 122. A friction collar 124 is attached to the umbilical cable above the highest cable fairing. This prevents the cable fairings 120 from sliding along the umbilical cable beyond where the friction collar 124 is attached. A friction collar may also be attached below the lowest cable fairing to prevent the cable fairings sliding downward along the umbilical cable.

Figure 6 illustrates a schematic diagram of cable fairings in use. An umbilical cable 126 descends beneath the surface of a fluid 128, such as water, from a floating object 130, such as a vessel or buoy. The direction of fluid flow in this example is shown by the arrow 132. Cable fairings with negative buoyancy 134 are attached to the umbilical cable 126 and are prevented from sliding down the cable beyond a predetermined point by a first friction collar 136. The cable fairings with negative buoyancy 134 provide a downward force on the section of umbilical cable 126 above where the first friction collar 136 is attached. Cable fairings with positive buoyancy 138 are attached to the umbilical cable 126 below a second friction collar 140, which is further along the umbilical cable 126 than the first friction collar 136. The cable fairings with positive buoyancy 138 provide an upward force on the sections of umbilical cable 126 below the second friction collar 140.

The fluid flow 132 applies a force to the tail fins 112 of the cable fairings 134 & 138. This causes the cable fairings 134 & 138 to rotate around the umbilical cable 126 until they are all substantially aligned. The low drag profile provided by the tail fins 112 then acts to inhibit the formation of vortices as the fluid flows past the cable fairings, which reduces vortex induced vibrations (VIVs) on the umbilical cable 126. These are a source of damage to the umbilical cable 126.

Variations in the fluid flow 132 velocity with depth may act to bend the umbilical cable 126. The central bore of the cable fairings 134 & 138 acts to restrict this bend in the umbilical cable 126 to a certain maximum bend. The cut-out sections 116 at the top and bottom of the tail fins 112 allow the umbilical cable 126 to bend by this maximum amount without the tail fins 112 of adjacent cable fairings overlapping, which would cause them to become unaligned with the direction of the fluid flow 132.

Figure 7 illustrates an alternative use of the cable fairings. An umbilical cable 126 is attached to a buoy or surface vessel 130 by a tether 142. A series of surface floats 144 are attached to the umbilical cable 126 which cause a section of the umbilical cable 126 to float on the surface of the fluid 128. Cable fairings with positive buoyancy 138 are attached to the umbilical cable 126 below a friction collar 146. The friction collar 146 prevents the cable fairings 138 from rising along the umbilical cable 126 beyond the section of cable it is attached at. The tail fins 112 align the cable fairings with the direction of the fluid flow 132, and act to reduce vortex formation due to the low drag profile of the tail fins.

The cable fairing can be manufactured from buoyant plastic foam.

Figure 8 illustrates a perspective view of an alternative embodiment of a cable fairing. Figure 9 illustrates a top view of an alternative embodiment of a cable fairing. The cable firing comprises an elongate tube 148 with a central bore 150. The top and bottom of the elongate tube 150 protrude from a jacket 152 which is shaped to have a hydrodynamic profile. A tail fin 112 extends substantially continuously from the jacket 152 and is tapered in the outward direction from the elongate tube 148. The tail fin has cut-out sections along its top and bottom to give it a substantially trapezoidal cross section in the vertical plane. The elongate tube 150 and jacket 152 are split along their length to allow the insertion of an umbilical cable.

In use, this embodiment of the cable fairing functions in the same way as the embodiment shown in the previous figures.

Adaptable inserts for the cable fairings may be provided that allow for hanked additional cables and lines to run through the central bores of the cable fairings.

Any system feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to system aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

Claims

CLAIMS:

1. A cable fairing suitable for removable mounting to an umbilical cable, the cable fairing comprising: a tube, the tube comprising a central bore shaped to restrict the bend of the umbilical cable; and a tail fin extending substantially radially from the tube, the tail fin being shaped to reduce drag on the tube when immersed in a flowing fluid.

2. The cable fairing of claim 1, wherein the central bore is shaped to restrict the bend of the umbilical cable to the minimum bend of the umbilical cable.

3. The cable fairing of claim 1 or 2, wherein the central bore has a substantially circular cross section.

4. The cable fairing of claim 3, wherein the radius of the circular cross section varies along the length of the bore.

5. The cable fairing of claim 4, wherein the radius of the circular cross section has a maximum at or near the upper and/or lower end of the bore.

6. The cable fairing of any of claims 3 or 4, wherein the radius of the circular cross section has a minimum at approximately midway along the length of the bore.

7. The cable fairing of any preceding claim, wherein the tail fin extends substantially continuously from the tube.

8. The cable fairing of any preceding claim, wherein the tail fin has a length that extends in the axial direction of the tube, the length varying in the radial direction from the tube.

9. The cable fairing of claim 8, wherein the length of the tail fin decreases along the radial direction from the tube.

10. The cable fairing of any of claims 8 or 9, wherein the length of the tail fin has a maximum length approximately equal to the length of the tube.

11. The cable fairing of any of claim 10, wherein the maximum length of the tail fin occurs where it joins the tube.

12. The cable fairing of any preceding claim, wherein the tube can be split along its length.

13. The cable fairing of any preceding claim, wherein the tube further comprises at least one strap securing point suitable for securing the umbilical cable to the cable fairing.

14. The cable fairing of any preceding claim, wherein the tube is constructed from a material having a buoyancy greater than water.

15. The cable fairing of claim 14, wherein the material used to construct the tube is a buoyant plastic foam.

16. The cable fairing of any of claims 1 to 13, wherein the tube is constructed from a material with a buoyancy less than water.

17. An apparatus substantially as hereinbefore described in relation to the Figures.

18. A method substantially as hereinbefore described in relation to the Figures.