Flexible pipe body and method of providing same

Abstract

A flexible pipe body and method of producing a flexible pipe body are disclosed. The flexible pipe body includes a collapse resistant layer comprising a radially inner surface and a radially outer surface, the radially inner surface comprising, in cross section, a substantially flat portion and at least one cavity extending from the flat portion radially outwards, and the radially inner surface further comprising, in cross section, at least one aerodynamic feature extending from the flat portion for breaking up a boundary layer of fluid flowing along the flexible pipe body in use.

Claims

1. Flexible pipe body for transporting fluids from a sub-sea location, comprising: a collapse resistant layer comprising a radially inner surface and a radially outer surface, the radially inner surface comprising, in cross section, a substantially flat portion and at least one cavity extending from the flat portion radially outwards, and the radially inner surface further comprising, in cross section, at least one protrusion extending from the flat portion for breaking up a boundary layer of fluid flowing along the flexible pipe body in use; wherein the collapse resistant layer is a carcass layer; wherein the or each at least one protrusion is provided upstream of the or each at least one cavity in terms of a flow direction of fluid through the pipe body in use; wherein the at least one protrusion is formed integrally with the collapse resistant layer wherein the collapse resistant layer comprises a helically wound elongate tape element and the at least one cavity is between adjacent windings of the tape element; wherein the at least one protrusion helically extends continuously around the inner surface of the carcass layer.

2. Flexible pipe body as claimed in claim 1 wherein the protrusion has a height of between about 1/50th to about the depth of the cavity.

3. Flexible pipe body as claimed in claim 2 wherein the protrusion has a height of between about 1/10th and 1/15th the depth of the cavity.

4. Flexible pipe body as claimed in claim 1, further comprising a barrier layer, a hoop strength layer and an outer fluid-retaining layer provided radially outwards of the collapse resistant layer.

5. Use of the flexible pipe body as claimed in claim 1 for the transportation of production fluids from a sub-sea location.

6. A method of providing a flexible pipe body, comprising: forming an elongate tape element and helically winding the tape element to form a collapse resistant layer comprising a radially inner surface and a radially outer surface, the radially inner surface comprising, in cross section, a substantially flat portion and at least one cavity extending from the flat portion radially outwards, and the radially inner surface further comprising, in cross section, at least one protrusion extending from the flat portion for breaking up a boundary layer of fluid flowing along the flexible pipe body in use; wherein the collapse resistant layer is a carcass layer; wherein the or each at least one protrusion is provided upstream of the or each at least one cavity in terms of a flow direction of fluid through the pipe body in use; wherein the at least one protrusion is formed integrally with the collapse resistant layer; wherein the at least one protrusion helically extends continuously around the inner surface of the carcass layer.

7. A method as claimed in claim 6, further comprising providing a barrier layer, a hoop strength layer and an outer fluid-retaining layer provided radially outwards of the collapse resistant layer.

Description

(1) Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates a flexible pipe body;

(3) FIG. 2 illustrates a riser assembly;

(4) FIG. 3 illustrates a cut away view of a known carcass layer;

(5) FIG. 4 illustrates a cross sectional view of the carcass layer of FIG. 3;

(6) FIG. 5 illustrates fluid flow through a known flexible pipe body;

(7) FIG. 6 illustrates a cross sectional view of the carcass layer of FIG. 5;

(8) FIG. 7 illustrates a cross section of a carcass layer of the present invention;

(9) FIG. 8 illustrates fluid flow through a flexible pipe body including the carcass layer of FIG. 7;

(10) FIG. 9 illustrates an enlarged view of a portion of FIG. 8; and

(11) FIGS. 10a, 10b and 10c illustrate portions of various alternative carcass layers.

(12) In the drawings like reference numerals refer to like parts.

(13) The present inventors have performed a 2-D LES CFD simulation (2-Dimensional Large Eddy Simulation Computational Fluid Dynamics) to identify shear layer oscillations in the flow of fluid travelling along a flexible pipe. In particular, the inventors have studied the fluid flow along the longitudinal direction of the bore of a flexible pipe by viewing a 2-dimensional cross-section though a flexible pipe during fluid transport. As used herein, the term shear layer will be used to denote an interface between portions of flowing fluid where a large velocity gradient is present (for example from a flow velocity of 30 m/s down to zero within the carcass interstices). A skilled person will realise that fluid flowing along the bore of a pipe will have a portion of fluid adjacent the pipe wall that is subject to shear forces, thus causing a more turbulent flow (vorticity) with fluid flowing at different velocities. Another portion of fluid, generally in the central portion of the pipe, will be relatively unaffected by forces from the pipe wall. In particular, when an inner surface of a pipe is not smooth the vorticity may be great. The boundary between the area that is affected by shear forces and the area that is unaffected by shear forces is termed the boundary layer.

(14) FIG. 5 illustrates vorticity contours of fluid flowing from left to right as shown in the diagram along the bore 520 of a flexible pipe. Whilst of course a longitudinal cross section through a pipe will show two walls of the pipe enclosing a pipe bore, only one wall of the carcass layer 501 and a part of the bore is shown in the figure. In this case the flexible pipe body tested includes a known carcass layer 501 with a cross sectional profile as shown in FIG. 6 (the same as that shown in FIG. 4).

(15) The carcass layer 501 was formed from a helically wound elongate metallic tape element, where windings are interlocked with adjacent windings to form the interlocked tubular construction. The carcass layer 501 was formed by folding an elongate strip of stainless steel, for example, with a rectangular cross section, to have an approximate reverse S-shaped cross section, with a first end folded over to form a first hooked portion and a second end folded in the opposite direction to form a second hooked portion, and a diagonally formed central body portion. The strip was helically wound such that the first hooked portion locates over and nests into a corresponding valley of the second hooked portion of an adjacent winding. As per FIG. 4, the dimensions of the hooked regions allow for a degree of movement between adjacent windings in the axial direction.

(16) Because of the particular cross-sectional profile of the carcass layer 501, there exists a plurality of substantially flat portions 505.sub.1-4 and cavities 503.sub.1-3 forming the radially inner surface 511 of the carcass layer 501. The cavities are a void area of unfilled space and effectively extend radially outwardly in an approximate rhomboid-type shape 507 in cross section (shown in dotted lines in FIG. 6). Of course the cavities shown in the cross section will actually be a single cavity that helically extends continuously around the inner surface of the carcass layer.

(17) The different areas of shading shown in the bore 520 of FIG. 5 illustrate fluid flowing along the bore at different velocities. It can be seen that the fluid flow is affected by the presence of the cavities 503.sub.1-3, which create vortices of fluid flow as fluid enters and leaves the cavities. The result is a turbulent fluid flow along the carcass inner surface to the boundary layer 509. It is this turbulent fluid flow that can lead to VIVs and riser singing, as discussed above.

(18) FIG. 7 illustrates a cross section of a carcass layer 701 according to the present invention. It can be seen that the carcass layer is similar in some respects to the carcass layer 501 described above and for brevity the same features will not be discussed. However, the carcass layer 701 includes protrusions 715.sub.1-3 formed on portions of the radially inner surface 711 of the layer. The protrusions 715.sub.1-3 are an aerodynamic feature formed as an approximate semi-circle or hump extending from a respective substantially flat portion 705.sub.1-3 of the carcass layer. Of course, when not looking at a cross section, the protrusions shown will actually be a single protrusion that helically extends continuously around the inner surface of the carcass layer.

(19) The protrusions 715.sub.1-3 are formed integrally with the carcass layer, i.e. an integral part of the carcass layer. As such the aerodynamic feature is unitary with the collapse resistant layer.

(20) As shown in the figure, the protrusion 715 is provided at an end of the substantially flat portion 705, adjacent the cavity 703. The protrusion 715 may be formed during manufacture of the elongate tape element feed sheet as an integral portion of the tape element feed sheet (for example by using a specifically textured surface on a roll or set of rolls used to thin, elongate and form the tape element), which is then bent into the appropriate shape as shown in the figure.

(21) The tape element is metallic, entirely of stainless steel in this example.

(22) Aptly the protrusion has a height of between about 1/50.sup.th and one quarter the depth of the cavity. The height of the protrusion would be measured from the base at the substantially flat surface 705 to its peak. The depth of the cavity would be measured from the same point at the substantially flat surface to the base (the inner edge of the other hook portion). More aptly, the protrusion has a height of between about 1/10.sup.th and 1/15.sup.th the depth of the cavity, and more aptly the protrusion has a height of about 1/12.sup.th the depth of the cavity.

(23) Aptly the protrusion has a height of greater than 200 m. Aptly the protrusion has a height of less than 1.5 mm.

(24) Aptly the cavity has a depth of around 2.5 mm. In this case the protrusion may aptly have a height between 0.208 mm and 0.833 mm.

(25) The protrusion 715 is provided to be upstream of the cavity 703 when the pipe body is in use transporting fluid. The protrusion is arranged to break up the boundary layer of fluid flowing along the flexible pipe body.

(26) FIG. 8 illustrates vorticity contours using a 2-D LES CFD simulation of fluid flowing from left to right as shown in the diagram along the bore 720 of a flexible pipe. Again only one wall of the carcass layer 701 and a part of the bore is shown in the figure. In this case the flexible pipe body tested includes the carcass layer 701 with a cross sectional profile as shown in FIG. 7. FIG. 9 illustrates an enlarged view of the area around a protrusion 715.sub.1.

(27) From FIGS. 8 and 9 it can be seen that the turbulence and vorticity of fluid adjacent the wall of the pipe body (carcass layer) is very much reduced compared to the known structure shown in FIG. 5. The fluid flow is more laminar compared to FIG. 5. The fluid flowing along the pipe body is affected in the region of the protrusion, creating a tail of vortices 717. The tail 717 acts to effectively cover over the main part of the cavity 703 such that fluid is not directed directly into the cavity 703. As such the resulting flow pattern is improved, since it is mainly fluid flowing into the cavities that creates the most turbulent flow patterns.

(28) Various modifications to the detailed designs as described above are possible. For example, although the protrusions 715 have been described above as generally semi-circular, the protrusions may take any shape, e.g. as rectangular (as shown in FIG. 10a) or oval or triangular, or a polygon or other convex protrusion or concave depression, or be of different sizes relative to the depth of the cavity (as shown in FIGS. 10b and 10c). The protrusion may be any suitable ridge or convexity formed so as to help break up the boundary layer and reduce vorticity in fluid flowing through the pipe body. For a semi-circular or oval shaped protrusion, the radius of curvature of the protrusion may be predetermined to give a suitable effect on the fluid flow.

(29) Although the protrusion has been described to be located at the end of the substantially flat portion on the inner surface of the carcass layer, the protrusion may be part way along the substantially flat portion. The inner surface of the carcass layer should be investigated and in particular the relative lengths of the substantially flat portions and the cavities determined so as to optimise the tail provided by the aerodynamic feature so as to substantially prevent fluid from directly entering the cavities.

(30) Although the embodiment of FIG. 7 has protrusions formed integrally with the collapse resistant layer, the protrusion may be fixedly attached to a regular collapse resistant layer, during manufacture of the tape element (prior to winding), e.g. by welding or adhering with suitable adhesive, by bolting on, or other method.

(31) Although the aerodynamic feature for breaking up the boundary layer above has been described as one or more protrusions, it is also possible for the carcass layer to have a depression or hollow incorporated into the radially inner surface for breaking up the boundary layer. The depression should be formed so as to cause fluid flow to be directed away from cavities. Thus, the aerodynamic feature is configured to cause fluid flow along the pipe body to be directed away from a cavity of the collapse resistant layer.

(32) Any of the above variations may be used in combination, for example as a series of features for breaking the boundary layer. The dimensions of the aerodynamic feature may change along the length of the pipe body.

(33) Although the carcass layer described above has been described to include helical windings of an elongate tape element, the carcass layer may be formed in other manners. For example, the carcass layer may be formed from a plurality of discrete annular elements that have connecting portions so as to interlock with adjacent annular elements. The adjacent annular elements may have one or more cavity extending from the inner surface of the carcass layer, such as at the point between adjacent annular elements, or in other areas of the inner surface of the carcass layer.

(34) Although the carcass layer described above is of stainless steel, the carcass layer could be formed from any suitable material, e.g. carbon steel, other metal, composite, polymer, or other material, or a combination of materials.

(35) With the above-described arrangement it has been found that the provision of the protrusion or other aerodynamic feature on the innermost layer of pipe body is effective to reduce shear layer oscillations and vorticity of fluid flowing through the pipe body compared to known designs. As such, the overall velocity and pressure oscillations at the cavity face are greatly reduced in amplitude and severity leading to improved flow with no risk of high frequency vibrations causing a risk of fatigue failure of pipe components or equipment in the locality compared to the known design. This leads to smaller amplitude of shear layer flow oscillations and weaker acoustic sources, reducing or eliminating acoustic pulsation at the flow velocities typical in gas production.

(36) That said, the provision of the aerodynamic feature is intended to intentionally disrupt the fluid flow along the flexible pipe. This actually reduces shear layer oscillations and vorticity of the fluid.

(37) The physical location of shear layer oscillations associated with the carcass layer are also moved further from the cavity, thus avoiding impingement of fluid vortices with the cavity.

(38) The invention described above should therefore help in the prevention of unwanted riser singing, which will in turn improve fatigue life and increase the lifetime of a flexible pipe.

(39) Reduction in shear layer oscillations may also lead to a lesser pressure drop in fluid flow through a flexible pipe. Increased production rates through the pipe may therefore be possible.

(40) It will be clear to a person skilled in the art that features described in relation to any of the embodiments described above can be applicable interchangeably between the different embodiments. The embodiments described above are examples to illustrate various features of the invention.

(41) Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(42) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

(43) The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.