WO2008035114A1

WO2008035114A1 - Fairing

Info

Publication number: WO2008035114A1
Application number: PCT/GB2007/050535
Authority: WO
Inventors: David Michael Ruthven Somerville; Nicholas W. Byrne
Original assignee: Trelleborg Crp Limited
Priority date: 2006-09-21
Filing date: 2007-09-11
Publication date: 2008-03-27
Also published as: GB0618588D0; GB2442003A

Abstract

A fairing is disclosed which is used to suppress vortex induced vibration of a supporting member such as a marine riser used in offshore oil extraction. The fairing is adapted to be mounted on the supporting member and to pivot about it in order to align itself with the fluid flow. The fairing has a front portion and a trailing edge (12). Li accordance with the invention it is provided with at least one forward inlet (44, 48, 50, 63) and at least one rearwardly directed outlet (46, 63). An internal space or passage leads rearwardly from the inlet to the outlet and in a fluid flow the fairing takes in fluid through the inlet and exhausts it through the outlet, increasing the effectiveness of the fairing and suppressing vortex induced vibration.

Description

DESCRIPTION FAIRING

The present invention relates to a fairing for mounting upon a cylindrical underwater member such as a marine riser.

Specifically, the present invention has been developed for use in connection with marine risers used in offshore oil extraction, although it has potential applications in other situations in which a submersed, elongate member is exposed to water flow and is to be protected from the effects thereof.

Water currents impinging on marine risers and other immersed members create two particular problems. Firstly such currents produce drag — i.e. a lateral loading upon the riser, creating undesirable bending stresses and potentially also increasing loads at the riser's point of suspension. Secondly, they can create vortex induced vibration ("VIV"). Vortices are in some circumstances shed alternately from opposite sides of the riser and the effect can be to produce vibration which is potentially damaging, particularly if accompanied by resonance effects which can increase the amplitude of the vibration.

It is known to address both problems by placing around the submerged member a streamlined, teardrop shaped fearing which is free to pivot about the longitudinal axis of the member and so to "weathervane" — that is, to align itself with the direction of the current.

Trelleborg CRP Limited's pending United Kingdom patent application GB0603040.7, published under number 2421751, describes such a fairing. It has a generally wedge shaped body which tapers from front to rear and is mounted upon the submerged member through a narrow collar which allows pivotal motion. A teardrop shape is formed by the combination of the submerged member i tself, w hich i s c ylindrical a nd forms t he 1 eading e dge, a long with t he wedge shaped body, which forms the trailing edge.

Despite the streamlined, teardrop shape of a typical fairing, there is typically a region of turbulent flow at its trailing edge and the possibility that vortices in this region will produce VIV under c ertain conditions. One way to improve performance in this regard is to increase the effective length of the fairing by means of a "splitter plate" which is essentially a fiat plate projecting from the trailing edge of the teardrop profile and serving to separate flows over the opposite sides of the fairing for some distance downstream of it.

Splitter plates inevitably increase the size and bulk of fairings, which is an important issue given that they must often be carried on board ship, and that the length of submerged members such as risers can be very large, requiring a correspondingly large number of individual fairing units. In this context, the facility to stow multiple units in a compact configuration is highly desirable.

United States patent 6517289 (Coakley Et Al) concerns an inflatable fairing for suppressing vortex induced vibration. It has inlets to admit fluid to inflate a fairing shroud, but no suggestion that the fluid should be exhausted from the fairing.

Published US patent application 2002/0066570 (Shu Et Al) describes a structure in which risers or spars of circular cross section are provided with through-going passages to suppress VIV. The items in question are not fairings as such and are not intended to weathervane to face a fluid flow. Instead, the passages are placed at intervals around the circumference of the relevant item, presumably so that some of the passages are expected to face the fluid flow regardless of its direction.

In accordance with the present invention, there is a fairing having a front portion and a trailing edge, the faring being adapted to be mounted upon a supporting member and to pivot thereabout to align itself with a fluid flow passed the supporting member such that its trailing edge is downstream of its front portion, the fairing being characterised in that it has at least one forward inlet and at least one internal spacor passage which extends rearwardly from the inlet and through which the inlet communicates with at least one rearwardly directed outlet, so that in a fluid flow the fairing takes in liquid through the forward inlet and exhausts it rearwardly from the fluid outlet.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: -

Figure 1 is a perspective illustration of a first fairing unit embodying the present invention, viewed from the front;

Figure 2 is a perspective illustration of the same fairing unit, viewed from the rear;

Figure 3 is a perspective illustration of a stack of fairing units;

Figure 4 is a perspective illustration of a riser joint with fairings embodying the present invention installed upon it;

Figure 5 shows a pair of the fairings to an enlarged scale;

Figure 6 is a perspective illustration of a further fairing unit embodying the present invention, viewed from the front; and Figure 7 is a perspective illustration of the same fairing unit, viewed from the rear.

The fairing unit 10 illustrated in figures 1-3 is a unitary moulding, formed in the present example by rotational moulding although other moulding processes could in principle be used. It can be described as "wedge shaped", having a broad, open front face and a narrow rear portion or trailing edge 12. This overall shape is defined by left and right side panels 14, 16 arranged in a "V", these panels being slightly convex and being joined at top and bottom through respective end panels 18, 20. The shape of the end panels approximates to a triangle, but their front edges 22 are concave, being in fact an arc of a circle, while their two remaining edges 24, 26 are slightly convex, to match the side panels 14, 16. A projecting, part-circular flange 28, 30 extends from the front edge 22 of each end panel. The fairing unit 10 is hollow, an interior space being defined by the side and end panels 14, 16, IS, 20. The side panels 14, 16, rather than simply being a curved plane, have shaped features which serve to improve rigidity. In the illustrated embodiment, these take the form of shallow troughs 32 extending along the fore-and-aft direction, three such troughs being provided in each side panel in the illustrated example.

Figures 4 and 5 show multiple fairing units mounted upon an elongate member, which in this case is a riser joint used in oil extraction. Two bands 34, 36 are attached to respective flanges 28, 30 of each fairing unit and extend around the elongate member 38, thereby mounting the fairing unit 10 with its front face toward the elongate member. The bands 34 and 36 are each provided with a buckle mechanism 40 to facilitate installation. In principle the flange is 28, 30 and the bands 34, 36 may ride directly upon an elongate member 38, where this is of cylindrical shape. However in the present embodiment self-lubricating bushes 42 a re p rovided b etween t he e longate m ember 38 a nd t he r elevant p arts o f t he fairing unit. The mounting permits the entire fairing unit 10 to rotate about the elongate member 38, for weathervaning. Note from figure 5 that neighbouring fairing units 10 move independently, so that changes of flow direction along the length of the elongate member 38 can be accommodated. Note also that the elongate member 38 and the fairing unit 10 together form a streamlined teardrop shape, the leading edge of which is formed by the elongate member 38 and the trailing edge of which is formed by the fairing unit 10. hi accordance with the present invention, the fairing unit 10 has forward fluid inlets 44 and rearward fluid outlets 46, so that in use fluid (typically of course water, although in principle the invention could be applied to fairings used in other fluids) flows through the interior of the fairing unit and is exhausted rearwardly through the outlets 46. This exhaustion of fluid from the fairing modifies its hydro dynamic properties in a desirable manner.

In the example illustrated in figures 1 -3, the forward fluid inlets 44 are formed as convex scoops at the front edges 48, 50 of the side panels 14, 16. Three of these scoops are provided on each front edge, in this example. It will be appreciated that when the fairing unit 10 is installed upon an elongate member 38, these scoops form openings, directed toward the fluid flow over the fairing, through which fluid tends to flow into the interior of the fairing unit 10. The rearward fluid outlets 46 are positioned at the trailing edge 12 of the fairing unit 10. In this example they take the form of a set of openings along the length of the trailing edge 12. They serve to exhaust fluid from the trailing edge at a speed higher than that of the surrounding fluid passing over the outer surfaces of the fairing unit 10. The effect is thought to be akin to that of a splitter plate, increasing the effective length of the fairing and reducing the incidence of VIV.

The cross sectional area of the forward fluid inlets 44 is significantly larger than the cross sectional area of the rearward outlets 46. This produces a "jetting" effect, increasing the speed of the exhausted fluid. hi principle, the shape and position of the forward fluid inlets may be considerably altered. As one example of an alternative construction, figures 6 and 7 show a variant of the fairing unit 10 in which the scoop formations 52 are dispensed with. Instead, front portions 60, 62 of the side panels 14, 16 are extended forwardly and outwardly to form wings which serve to define slots 63 on either side of the elongate member 38 into which fluid is diverted. The rearward fluid outlets can also take a number of different forms, and in the embodiment seen in figures 6 and 7 they comprise elongate slots extending along the trailing edge of the fairing unit 10.

The illustrated fairing units all have drainage holes 64 in their end panels 18, 20 but it is thought that it may prove beneficial to dispense with these, thereby increasing flow through the rearward fluid outlets 46, 63.

The c onstruction o f the fairing units 1 O p ermits them to b e stacked one upon another in a highly compact configuration as illustrated in figure 3. It will be appreciated that to facilitate this, the end panels 18, 20 converge slightly from front to rear of the fairing unit 10. It must be emphasised that the illustrated fairing units serve only as an example of the manner of implementation of the present invention. The invention could however be applied to fairings which are wholly different in construction. For instance, whereas the examples described above have a wedge-shaped fairing unit to form the trailing edge of a teardrop shape, the leading edge of this shape being defined by the elongate member upon which they ride, the present invention could equally be applied to a fairing of the type which surrounds the entire elongate member to define both leading and trailing edges. In this case, the inlets might for example be positioned at or near the leading edge of the fairing and lead through its body to outlets at the trailing edge.

Claims

1. A fairing having a front portion and a trailing edge, the faring being adapted to be mounted upon a supporting member and to pivot thereabout to align itself with a fluid flow passed the supporting member such that its trailing edge is downstream of its front portion, the fairing being characterised in that it has at least one forward inlet and at least one internal spacor passage which extends rearwardly from the inlet and through which the inlet communicates with at least one rearwardly directed outlet, so that in a fluid flow the fairing takes in liquid through the forward inlet and exhausts it rearwardly from the fluid outlet.

2. A fairing as claimed in claim 1, in which the rearward outlet is at the trailing edge.

3. A fairing as claimed in claim 2, comprising multiple rearward outlets disposed along substantially the entire length of the trailing edge.

4. A fairing as claimed in claim 1 or claim 2, in which the rearward outlet extends along substantially the entire length of the trailing edge.

5. A fairing as claimed in any preceding claim comprising a wedge-shaped body for mounting against a cylindrical supporting member.

6. A fairing as claimed in claim 5, in which the wedge-shaped body is hollow in order to define the internal space through which the forward inlet communicates with the rearward outlet.

7. A fairing as claimed in claim 5 or claim 6, in which the forward inlets are defined by front edges of the wedge-shaped body.

8. A fairing as claimed in any preceding claim, which has a plane of symmetry coinciding with the trailing edge and in which the rearward outlets are disposed on and directed along this plane of symmetry.

9. A fairing as claimed in claim 6, in which the hollow wedge-shaped bodies are adapted to be stacked one upon another.

10. A fairing as claimed i n claim 6 o r c laim 9 , in which the w edge-shaped bodies are provided with bands for attached to a cylindrical supporting member.

11. A fairing substantially as herein described with reference to, and as illustrated in, figures 1, 2 and 3 or figures 6 and 7.