RISER

The present invention relates to a riser for conveying fluids during production of hydrocarbons, and particularly, but not exclusively, to flexible risers.

The present invention also provides a method for reducing risk of failure of a riser system. The present invention also relates to a method and apparatus for installing an coiled internal member into a riser, preferably when the riser is installed in a subsea wellbore system.

During production of hydrocarbons, one or more risers are typically installed between a well in the seafloor and an installation or floating vessel at or the near the sea surface. The risers may be flexible to accommodate relative motions between the installation or floating vessel and the well in the sea floor, such motion may be induced by waves. The types of motions typically encountered are heave and sway. Typically, but not always, a wellhead is located on the sea floor and the riser extends from the wellhead. Risers may comprise a hose, tubular, or series of interconnected tubulars, used to convey fluids, such as liquids, gases, and plasmas, between the wellhead and the surface installation or floating vessel. One or more flow lines may then be used to convey the fluids from the installation or floating vessel to land, tanker ship, other storage vessel, processing plant or the like. The flow line may be a few hundred metres long or may be several kilometres long. The pressure of a gas in the riser may be 200 Bar.

During operation of a riser, fluid flows within the riser, generally from the well to the installation or floating vessel. Force is exerted by the fluid flowing through the riser on the inner surface of the riser. Water pressure is exerted on the outside of the riser. The inventors observed that in circumstances where corrugations or other discontinuities are formed along the inner surface of the riser, contact between the fluid and the corrugations may induce vortex shedding, which in turn, may induce vibration in the riser. Such discontinuities in the inner surface of a riser are found in flexible risers, where the inner surface is made up of a coil of wire or flat material, the discontinuities occurring between adjacent flights of the coil, which allows for flexing of the riser.

Vibration of the riser may necessitate a reduction in the flow rate of fluid through the riser, which is commercially undesirable and may be difficult to achieve. Over time, the vibration may also cause fatigue damage in the riser and other components of the riser system and shorten its service life. If the frequency of the induced vibrations coincides with a resonant frequency of the riser system, large amplitude vibrations are induced in the riser system. This may induce failure of a component of the riser system, such components are: a connection of the riser between the wellhead and the riser; a connection between the riser and a component at the top of the riser; or in the riser itself.

The connection of the riser to the wellhead typically comprises a bolted flange connection. The bolted flange connection may comprise a rigid neck portion which is then joined to the flexible riser.

The inventors observed that there is a risk of failure of a riser system due to fatigue and particularly but not exclusively, in the connection apparatus of the riser system. The inventors observed that fatigue may be induced by pulsations in flow of gas through a riser of the riser system. The inventors observed that low frequency pulsation in the fluid may induce fatigue failure, such as below 400 Hz. The inventors observed that the pulsations in the fluid may be induced by vortex shedding, the vortex shedding being induced by a rough bore.

In accordance with the present invention, there is provided a method for reducing the risk of failure of a riser system comprising a riser arranged between an installation and a subsea well the riser having a bore for conveying fluids therebetween, the method comprising the step of providing at least one vortex shedding member in said bore of said riser and flowing the fluid between the subsea well and the installation.

Advantageously, the installation is a drilling rig, FPSO, submerged platform or other vessel. Preferably, the fluid is a gas, such as natural gas or shale gas. For the avoidance of doubt, the term subsea is used to mean under any kind of water, fresh, brackish or salty. The bore may be discontinuous and rough, but may be smooth. Preferably, the vortex shedding member lies along at least a substantial portion (length) of the bore of the riser. Fatigue may occur in a connection connecting the riser to the subsea well or at a connection between the riser and a flowline. Fatigue may also occur in the riser itself. Preferably, the riser is flexible.

Advantageously, the method further comprises the step of deploying said at least one vortex shedding member into the bore whilst the riser remains installed on said subsea well. Preferably, the at least one vortex shedding member comprises a coil biased against an inner surface of said riser. Advantageously, the vortex shedding member is injected through an opening in an injecting head and expands to hold itself against said bore. Preferably, the step of deploying the at least one vortex member is carried out by unfurling the coil in the bore. Advantageously, the step of deploying the at least one vortex member is carried out by unfurling the coil in the bore with a constant pitch. Thus, advantageously, leaving the vortex shedding member in the form of a helix biased against the bore. Preferably, the VSM is deployed at a constant rate. Preferably, the method further comprises the step of dragging said coil through said bore to install the at least one vortex member in said bore, wherein preferably, the dragging is carried out from the top of the riser to the bottom.

Preferably, the step of dragging said coil through said bore is carried out using a coiled tubing injector. Preferably, said coiled tubing injector comprises a reel with coiled tubing thereon, the method comprising the step of unreeling the coiled tubing to drag the coil through the bore of the riser. Advantageously, the injector comprises an advancing mechanism, such a caterpillar chain drive, the method comprising the step of advancing the coiled tubing down through the bore using the advancing mechanism. Advantageously, an injector head is provided on a free end of the coiled tubing. Preferably, the injector head comprises a gripper for gripping the lower end of the coil. Advantageously, a communication path and power supply are provided to activate the gripper to release the lower end of the coil when the bottom of the riser is reached, which may be the coupling, coupling the riser to the subsea well. Preferably, the injector head is provided with at least one camera, so that the operator can see the vortex shedding member being deployed.

Advantageously, the vortex shedding member comprises a tube filled with fluid, the method further comprising the step of monitoring the fluid in the tube to assess the integrity of the vortex shedding member. Preferably, the fluid in the tube is pressurized and the pressure of the fluid therein monitored to assess the integrity of the vortex shedding member.

The present invention also provides a riser comprising a hollow tubular body having an inner surface defining a bore through which fluids may flow, the riser further comprising an internal member arranged to follow a spiral path within the bore.

Preferably, the internal member forms a continuous spiral. Preferably, the spiral is a helix, having constant pitch. Advantageously, the pitch is between one and twenty times the diameter of the bore. Advantageously, between three and seven times the diameter of the bore and most preferably five times the diameter of the bore. Alternatively, the spiral may be discontinuous, formed of discrete fins projecting from the bore into the centre of the bore.

Advantageously, the internal member is biased against or fixed to an inner surface of the riser. Preferably, the internal member comprises a tube. Advantageously, the tube is between 4 mm and 20 mm in diameter and preferably a coil of hydraulic tubing. Preferably, the tube is between 6 mm and 12 mm in diameter. Preferably, the internal member comprises wire. Preferably, the internal member comprises a plurality of fins projecting from the internal surface of the riser.

Advantageously, an inner liner is provided having fins projecting into the bore (115) of the riser. Preferably to form a continuous spiral, but may be non-continuous, having gaps therebetween. Preferably, the inner liner is formed from a coiled strip having said fins arranged thereon. Preferably, the fins are arranged at an angle to the length of the strip.

The present invention also provides a method for installing a coiled internal member into a riser, the method comprising the step of dragging said coiled internal member through said bore on the end of a coiled tubing deployed along the riser with a coiled tubing injector.

The present invention also provides a riser comprising a hollow tubular body having an inner surface defining a bore through which fluids may flow, the riser further comprising a vortex shedding member along a substantial portion thereof, preferably, of the length.

Preferably, the riser comprises a plurality of sleeves, wherein said inner liner forms one of said sleeves. Other preferable and advantageous layers are set out in the description with reference to FIGS. 3 and 4.

The present invention also provides a method of manufacturing a flexible tubular, the method comprising: disposing a plurality of spaced apart members along a face of a substantially flat plate; bending the flat plate in a spiral fashion to form a tubular body, the tubular body having an inner surface over which the members are disposed and bounding a fluid flowbore; wherein said bending aligns the members to form a helix along the inner surface.

The present invention also provides a method for inhibiting pulsations of a potentially damaging frequency in a fluid flowing in a flexible riser using the above methods and apparatus.

The methods and apparatus may also be used in flow lines or other tubulars for facilitating the conveying of fluids from a wellbore.

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings in which:

FIG. 1 is a schematic view of an offshore drilling platform coupled to a subsea well by a riser system in accordance with the invention;

FIG. 2 is a schematic view of a riser system coupled between a Floating Production, Storage, and Offloading (FPSO) vessel and the subsea well;

FIG. 3 is a perspective view, with layers cutaway, of a prior art riser;

FIG. 4 is a perspective view, with layers cutaway of a riser in accordance with the present invention with some hidden parts shown in dashed line, the riser comprising an internal member;

FIG. 5A is a graph indicating flow pulsation against amplitude;

FIG. 5B in a schematic diagram showing eddie currents induced by a discontinuous inner surface of a riser;

FIG. 5C is a schematic diagram showing an internal member of the riser shown in FIG. 4 before installation shown in a compressed coil, an expanded coil and an unfurled coil;

FIG. 6A is a perspective view of an injector used in a method of installing an internal member in accordance with the present invention;

FIG. 6B is a perspective view of an injector head of the injector shown in FIG. 6A;

FIG. 7 is a perspective view of a lining member being formed into a liner for use in another embodiment of a riser in accordance with the present invention;

FIG. 8A is enlarged view of the lining member shown in FIG. 7;

FIG. 8B is an axial cross-sectional view of the lining member shown in FIG. 7;

FIG. 9 is an end view of a strip of the lining member shown in FIG. 7; and

FIG. 10 is an end view of an alternative strip for use in forming a lining member.

The following description is directed to exemplary embodiments of a flexible riser having an internal contour system preferably for mitigating vortex shedding during conveyance of a fluid through the tubular. One skilled in the art will understand that the following description has broad application, and that the discussion is meant only to be exemplary of the described embodiments, and not intended to suggest that the scope of the disclosure, including the claims, is limited only to those embodiments. The embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. For example, in the exemplary embodiments described below, the flexible tubular is a component of an offshore riser system. However, the flexible tubular may also be utilized in other types of systems where it is desirable to mitigate vortex shedding.

Certain terms are used throughout the following description and the claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. Moreover, the drawing figures are not necessarily to scale. Certain features and components described herein may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, the connection between the first device and the second device may be through a direct connection, or through an indirect connection via other intermediate devices and connections.

Referring now to FIG. 1, there is shown a Small Water Plane Area Twin Hull (SWATH) type drilling platform 1 floating at the sea surface 11. The drilling platform 1 is coupled to a subsea well 7 located in the seafloor 9 by a riser system 5 in accordance with the present invention. The riser system 5 has a riser 13 coupled to a tensioning device 15. A riser extension 17, including a joint 19 and a manifold 21, is coupled between the tensioning device 15 and the drilling platform 1. The riser 13 is coupled at its lower end 22 to a wellhead 23. A flanged coupling 24 is fitted to the end 22 of the riser 13 for connection to a flange fitting on top of the wellhead. It will be appreciated that the wellhead 23 is optional and that the coupling 24 could be connected directly to a template and the wellhead fitted elsewhere, such as on the drilling platform 1. The riser system 5 may be used inter alia during an exploratory phase, drilling phase, production phase, work-over phase and/or re-injection phase.

Alternatively or latterly, preferably during a production phase, as illustrated by FIG. 2, a riser system 5 may be installed between a buoyant body 12 and the subsea well 7. The buoyant body 12 is arranged within 100 metres of the sea surface to enable easy access thereto. A flowline or a continuation of the riser system 14 continues the flowpath of production fluids to a floating production, storage, and offloading (FPSO) vessel 20 or other storage vessel or the like. The production fluid is then transported to point of use or for further processing on land. Such transportation may be by tanker or by a subsea pipeline. In either case, the riser system 5 enables fluid conveyance between the subsea well 7 and a structure at the sea surface 11. As used herein, the term “fluid” includes a liquid, a gas, a plasma, and a mixture of any of one or more liquids, gases, and plasmas.

FIG. 3 shows a prior art flexible tubular 100 which forms part of a riser system, such as the riser system 5 shown in FIGS. 1 and 2. The flexible tubular 100 comprises an inner sleeve 101, known as a carcass which preferably inhibits collapse of a fluid tight liner 103 and advantageously protects against abrasive particles and/or pigging tools (not shown). The inner sleeve 101 is constructed from an interlocking conduit 102, preferably made a stainless steel strip. The interlocking conduit 102 has a substantially flat portion and interlocking elements formed integrally therewith on either side thereof such that upon coiling, the interlocking elements interlock to form the inner sleeve 101. The inner sleeve 101 thus has a rough discontinuous inner surface 115 defining a bore 116 through which fluids flow from the subsea well 7 to the surface installation 1. The flexible tubular 100 also comprises a fluid tight liner 103 made from an extruded polymer. The flexible tubular 100 also comprises a first armour layer 104 made from a helically wound abutting C-shaped steel wires and/or steel strips 108 to preferably provide resistance to radial loads, such as water pressure. A pair of further armour layers 105 and 106, preferably comprising a helically wound rectangular steel wire 109, 110 which may be counterwound to provide additional resistance to axial tensile loads. Anti-wear layers (not shown) may be provided between each of the layers, sleeves and liners to provide wear resistance therebetween. An outer sheath 107, advantageously made from an extruded polymer, preferably shields the other layers from the outer environment and provides mechanical protection. An insulation layer (not shown) may be provided internal or external to the outer sheath 107. Furthermore, a buoyancy jacket (not shown) may be provided along at least a portion of the length of the flexible tubular 100 to provide buoyancy.

The flexible tubular 100 may be sufficiently flexible to be wound on to a reel. The reel may be 9.2 m in diameter. The internal diameter of the flexible tubular is typically from 2.5″ to 16″ (50 mm to 410 mm).

The riser system 5 in accordance with the present invention as shown in FIG. 4, comprises a flexible tubular 100 of the type shown in FIG. 3 with the addition of an internal member 111. The internal member 111 may be a solid wire, but is preferably a tube 112 having a substantially circular cross-section wound into a coil and abutting the internal surface of the inner sleeve 101. The coil preferably forms a helix having a pitch five times the internal diameter of the flexible riser 100, although may be of an alternative pitch, such as between one and ten times internal diameter of the riser. The tube 112 is preferably between 6 mm and 12 mm in diameter, although may be for example of between 2 mm and 50 mm in diameter. The cross-sectional shape of the hydraulic tube may be oval, square, triangular or other suitable shape. The tube may be made from a metal, such as stainless and may be made from steel, which may match the material of the inner liner. The tube 112 is advantageously hydraulic tubing. Hydraulic tubing is commonly available with pressure ratings comparable to those need for use as an internal member 111 of a riser system 5.

The tube 112 is preferably filled with a fluid, such as hydraulic fluid and pressurized. The tube 112 may be suitably capped at a distal end and a proximal end 114 connected to a pressure gauge 113. The distal end is arranged at a bottom of the riser 100 and the proximal end 114 is arranged at the top of the riser 100. If the pressure gauge 113 sees a drop in pressure, a user can assume that the integrity of the tube 112 has been compromised. The internal member 111 can thus be removed from the flexible riser 100 and replaced.

The internal member 111 is preferably continuous along the length of the riser 100.

FIG. 5A shows a graph showing amplitude against flow pulsation frequency. The inventors observed that the riser can “sing” when subjected to certain frequencies. The inventors observed that the flow of fluid, such as natural gas, through the riser and over the discontinuous surface of the inner liner 101 formed by the interlocking conduit 102 induce vortices 120 (see FIG. 5B) and audible frequencies. This range of frequencies is shown in FIG. 5A. The inventors believe that it is the low frequency range which may induce fatigue failure in a component of the riser system. Furthermore, it is resonant frequencies which may also induce rapid fatigue failure. Line 121 is a trace of frequency against amplitude for a flexible riser, such as the riser shown in FIG. 3. As can be seen from the graph in FIG. 5A, the line 121 shows that below 400 Hz there is a sharp increase in amplitude and thus energy in the this low frequency range. Line 122 is a trace of frequency against amplitude for a flexible riser with an internal member 111, such as the riser shown in FIG. 4. As can be seen from line 122, the amplitude and thus energy in the low frequency range below about 350 Hz has been significantly reduced with the addition of the internal member 111. The inventors believe the internal member significantly changes the pressure pulsations and/or the vortices induced by these pulsations.

The internal member 111 is produced in the form of an expanded coil 130, as shown in FIG. 5C. When the internal member 111 is produced it lies at rest having a natural pitch shown as expanded coil 130. The coil is axially compressed into a compressed coil 131 for transport. The compressed coil 131 is then placed above a mouth 133 of a flexible riser 100 and dragged down the inner liner 101 until the internal member 111 is unfurled to form an unfurled coil 134. The unfurled coil 134 preferably has a pitch of approximately five times the internal diameter of the flexible riser 100. The unfurled coil 134 preferably has a natural diameter which is slightly larger than the internal diameter of the flexible riser 100. The expanded coil 130 thus also has at rest is slightly greater diameter than the internal diameter of the inner liner 101. Thus the unfurled coil 134 biases itself under a radially outwardly spring force pressing against the inner liner 101, inhibiting the coil from falling down the riser 100. An internal shoulder (not shown) in the coupling 24 will also inhibit the internal member 111 from falling down into the well 7 in the seafloor 9. The coil is provided with a locking coil 135 at the distal end of the tube 112, which includes a reverse bend 136 and a reverse directed coil 137. The locking coil 135 inhibits the coil from being pulled upwardly through the riser upon installation.

The internal member 111 can be installed into a riser 100 which is already installed on a well in the sea. A suitable injector, such as the injector 200 shown in FIG. 6A is used. The injector 200 comprises a frame 201 and a chain mechanism 202 for pulling coiled tubing 204 (shown in dashed lines) from a reel (not shown) through a slot 207 in the top of the frame 201 between two chain drives 205, 206 through a slot 208 in a skid 209 and into the riser 100.

A push rod in the form of coiled tubing 204 is provided with a head 210 having a gripping mechanism 212 for gripping the distal end of the tube 112 of the compressed coil 131. The distal end of the tube 112 of the compressed coil 131 is pulled through the inner liner 101 of the flexible riser 100. A wire frame 211 facilitates unfurling of the compressed coil 131. Cameras 213 and 214 and appropriate lighting are provided on the head 210 to provide a visual inspection of the unfurling of the hydraulic tube 112. A communication bus (not shown) which may be in the form of wires, extends up through the coiled tubing 204. The communication bus provides a data path to the surface for video footage from the cameras 213 and 214 and a signal path for operating a latch 215 of the gripping mechanism 212 to selectively grip and release the distal end of the tube 112.

The coiled tubing is preferably of a large diameter, preferably of 4″ (110 mm) diameter for use in large internal diameter risers. This size coiled tubing is extremely rigid and will deploy the coil without flexing, thus giving a consistent feed out during unfurling of the coil in the flexible riser 100.

The internal member 111 may also be formed integrally with the riser 100 as part of the riser's construction in a factory environment. FIGS. 7 and 8 shows an inner liner 301 which may replace or fit inside of inner liner 101 in the embodiment of FIG. 4. The inner liner 301 comprises a continuous strip of flat plates 305 bent, folded, moulded, drawn (such as through a die) or otherwise formed into a spiral such as a helix to form a tubular body 315. The tubular body 315 has an inner surface 320 defined by a substantially constant diameter and bounding a flowbore 325 through which a fluid may be conveyed. The continuous strip of flat plate 305 may be formed of discrete section of flat plate joined end to end to form a continuous strip of flat plate.

The inner liner 301 further comprises a plurality of spaced apart vortex shedding mitigation (VSM) members 330. The VSM members 330 are disposed along a face 335 of the continuous strip 305. The VSM members 330 may be a series of discrete angled fins spaced along the continuous strip 305, the spacing selected such that after the continuous strip 305 is bent, folded, moulded or drawn to form the tubular body 315, the VSM members 130 align to form a spiral, preferably a helix 340 along the inner surface 320 of the tubular body 315, as best viewed in FIG. 8. In the illustrated embodiment, the spiral 340 formed by the VSM members 130 is non-continuous, having spaces 345 between adjacent VSM members 330. In alternative embodiments, the spacing of the VSM members 330 along the continuous strip 305 may be selected such that after the continuous strip 305 is bent, folded, moulded or drawn to form the tubular body 315, the VSM members 130 align to form a continuous spiral, preferably a continuous helix along the inner surface 320 of the tubular body 315, having negligible space between adjacent VSM members 330 or indeed overlapping. Further, the angular orientation of the angled fins relative to the continuous strip 310 is selected such that after the continuous strip 305 is bent, folded, moulded or drawn to form the tubular body 315, the spiral, preferably helix 340 formed by the VSM members 330 along the inner surface 320 of the tubular body 315 has a desired pitch P.

In preferred embodiments, each VSM member 330 is a lengthwise discontinuity extending from the face 335 of the plate 310, as illustrated by FIG. 5. In some embodiments, the VSM member 330 may be a weld seam, which may project preferably at least 5 mm from the inner surface 320 and advantageously be at least 2 mm wide. Alternatively, the VSM member 130 may be joined to the face 335 by gluing, bonding, welding, folded, bent, pinched or other equivalent methods known in the art. Further, the VSM member 330 may formed in the strip 305 through localized compression, or pressing, of the strip 305. In such embodiments, the VSM members 330 as well as the spiral, preferably helix 340 formed by them along the inner surface 320 of the inner liner 315 extends radially inward from the inner surface 320 into the flowbore 345 of the tubular body 315.

In alternative embodiments, the VSM members 330 may formed in the strip 310 through localized compression, or pressing, of the plate 110 such that each VSM member 330 becomes essentially a depression, or recessed region, in the inner surface 320 of the tubular body 315.

Regardless of their method of formation on the inner liner, which may be a tubular body 315, the VSM members 330 form a spiral such as a helix 340, or discrete sections aligned to pass through a spiral path with optional gaps therebetween which preferably disrupts and mitigates vortex shedding during conveyance of fluid through the flexible riser 100.

While various embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings herein. The embodiments herein are exemplary only, and are not limiting. Many variations and modifications of the apparatus disclosed herein are possible and within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.

	Number	Date	Country
	61467799	Mar 2011	US
	61490846	May 2011	US

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