GB2472644A

GB2472644A - Marine riser apparatus and method of installation

Info

Publication number: GB2472644A
Application number: GB0914249A
Authority: GB
Inventors: Jean-Luc Bernard Legras; Jean-Pierre Branchut
Original assignee: Acergy France SAS
Current assignee: Acergy France SAS
Priority date: 2009-08-14
Filing date: 2009-08-14
Publication date: 2011-02-16
Also published as: WO2011018713A2; WO2011018713A3; GB0914249D0

Abstract

Marine riser apparatus comprising at least one tensioned conduit 300/360 extending substantially vertically from the seabed towards the surface. The riser is supported at its upper end by at least one buoyancy device 340/395. One or more suspended conduits 380 are supported from an upper portion of the riser apparatus. The suspended conduits 380 may be catenaries. The tensioned conduits may be anchored 320/330 o the seabed. The tensioned conduit may be articulated or curved to change direction at the seabed. The apparatus can include a support frame 350 from which the suspended conduits are supported. A flexible line can connect each conduit to a surface facility 390. The suspended conduits may have further buoyancy devices near the top. A plurality of tensioned conduits may be provided in a single bundle.

Description

Marine Riser Apparatus and Method of Installation Thereof.

The present invention relates to methods and apparatuses for hydrocarbon production at sea, and especially to riser and pipeline apparatus and methods of installation thereof.

Direct connection of steel catenary risers or bundles to floating facilities is feasible.

Solutions such as Steel Catenary Risers (SCR5) or variations thereon (for example, "lazy wave"), are well-known and allow the pipeline to continue uninterrupted to the surface, following a catenary curve. However, these can have the problem of placing great stresses on the surface vessel and the joint between surface vessel and riser due to the considerable weight of the riser. Other problems that arise are that the riser must be installed after the floating production unit is on location, which is usually on the critical path for planning purposes, that disconnection takes a long time and is not feasible under adverse weather conditions, and that the floating facilities impose on the risers motions which are detrimental in terms of fatigue.

Other systems have been proposed to support risers. For large field developments, Hybrid Riser Towers are known and form part of the so-called hybrid riser, having an upper portion ("jumpers") made of flexible conduit and suitable for deep and ultra-deep water field development. US 6,082,391 (Stolt/Doris) proposes a particular Hybrid Riser Tower (HRT) consisting of an empty central core, supporting a bundle of riser pipes, some used for oil production some used for water and gas injection. This type of tower has been developed and deployed for example in the Girassol field off Angola. Further background has been published in paper "Hybrid Riser Tower: from Functional Specification to Cost per Unit Length" by J-F Saint-Marcoux and M Rochereau, DOT XIII Rio de Janeiro, 18 October 2001. Further developments of such risers have been proposed in WO 02/053869 Al. The contents of all these documents are incorporated herein by reference, as background to the present disclosure. These multibore HRTs are very large and unwieldy, cannot be fabricated everywhere, and reach the limit of the component capabilities. Also small to medium reserves fields require a small number of risers for which a riser tower may not be economical.

Another known solution is to use a number of Single Line Offset Risers (SLOR5) which are essentially monobore HRTs. A problem with these structures is that for a cluster of wells, a large number of these structures are required, one for each production line, each injection line and each gas line. This means that each structure needs to be placed too close to adjacent structures resulting in the increased risk of each structure getting in the way of or interfering with others, due to wake shielding and wake instability.

The aim of the present invention is to provide an alternative form of riser system in which some or all of the above mentioned problems are overcome or in the very least alleviated. In particular, for very deep water e.g. deeper than 800m, it would be desirable to have a multibore riser system without the complexity of conventional HRTs. Preferably the novel system can be installed offshore using a conventional installation vessel. It should be cost-effective. It should also eliminate or at least reduce clashing of risers. Preferably also most of the riser installation operational steps can be performed before installation of the floating production unit to which the risers are to be connected.

In a first aspect of the invention there is provided a marine riser apparatus comprising at least one tensioned conduit extending from the seabed toward the surface whereupon it has an upper end supported by at least one buoyancy device, such that said at least one buoyancy device maintains said at least one tensioned conduit in tension with a substantially vertical configuration, said apparatus further comprising one or more suspended conduits wherein said suspended conduits are all suspended from an upper portion of said marine riser apparatus.

At least some of said one or more suspended conduits may take a catenary configuration. Alternatively or in addition, at least some of said one or more suspended conduits may comprise buoyancy so as to take a different configuration such as a modified catenary. Examples of different configurations include the Lazy Wave and similar configurations.

Said at least one tensioned conduit may comprise an anchoring device anchoring it to the seabed. Said at least one tensioned conduit may comprise an articulation at its bottom to change its direction at the seabed. Alternatively it may comprise rigid or flexible pipe forming a curve.

At least some of said one or more suspended conduits may be suspended from the tensioned conduit. Alternatively or in addition, at least some of said one or more suspended conduits may be suspended from the at least one buoyancy device.

Alternatively or in addition, the device may comprise a support frame, wherein at least some of said one or more suspended conduits are suspended from said support frame.

At least some of said one or more suspended conduits may be attached to said apparatus via at least one chain and/or an accessory, said accessory for attaching to a respective receptacle attached to said apparatus.

Said apparatus may comprise a plurality of said suspended conduits.

Said apparatus may comprise only a single tensioned conduit, or more than one tensioned conduit in a single bundle.

Said apparatus may comprise connectors at the top of each conduit for connection to a flexible line. Said apparatus may further comprise said flexible lines for connection of each conduit to a surface facility.

At least some of said one or more suspended conduits may be provided with a further buoyancy device at or near its top.

Said apparatus may comprise two main and separable buoyancy devices tensioning said at least one tensioned conduit, a smaller one of said devices for providing buoyancy sufficient at least for suspending the at least one tensioned conduit alone.

Said tensioned conduit may be used solely for tensioning and not used for conveying fluid. Consequently it may not have any flow connections.

In a second aspect of the invention there is provided a method of installation of a marine riser apparatus according to the first aspect, said method comprising the steps of: * constructing on, and lowering from, a pipelay vessel the at least one tensioned conduit; * attaching an upper riser assembly to the at least one tensioned conduit; * attaching said at least one buoyancy device to said tensioned conduit to provide a tensioned riser assembly; * lowering the at least one tensioned riser assembly into position, so that it is tensioned by said buoyancy device; * constructing on, and lowering from, a pipelay vessel a first of the suspended conduits; * attaching an upper riser assembly to the first of the suspended conduits; * attaching said suspended conduit to the tensioned riser assembly; and * Repeating the previous three steps for each suspended conduit to be installed.

The above method steps are not necessarily in a fixed order, and where appropriate or possible, the order may change or steps may be done simultaneously.

The method may comprise the installation of an anchoring device to the seabed and attachment of said tensioned conduit to said anchoring device.

The installation of the suspended conduits may comprise the steps of restraining a section of the suspended conduit on the seabed and moving the pipelay vessel so as to obtain a catenary configuration in the suspended conduit.

The attaching of the suspended conduit to the tensioned riser assembly may comprise lowering the suspended conduit on a line to its installation position and placing a male support, forming part of the upper riser assembly of the suspended conduit, into a female support (receptacle) forming part of the upper riser assembly of the tensioned riser assembly.

The step of attaching the buoyancy device may comprise attaching a first buoyancy device, the method further comprising the installation of a further buoyancy device to the tensioned riser assembly prior to the attachment of the at least one suspended conduit.

A floating production unit may be installed, followed by the installation of flexible pipelines so as to connect the conduits to said floating production unit.

The installation may take place in water deeper than 800m.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Figure 1 shows a cut away view of a prior art seabed installation; Figure 2 shows another type of prior art seabed installation; Figure 3 shows a seabed installation according to a first embodiment of the invention; Figure 3a shows an alternative lower riser assembly for the seabed installation of Figure 3; Figure 4 shows detail of an arrangement near the top end of the riser, for a further embodiment of the invention; Figure 5 shows detail of one riser termination in the arrangement of figure 4; and Figure 6 shows an alternative form of riser termination, making a separate embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1, as the person skilled in the art will recognise, shows a cut-away view of a seabed installation comprising a number of well heads, manifolds and other pipeline equipment 100 to 108. These are located in an oil field on the seabed 110.

Vertical riser towers are provided at 112 and 114, for conveying production fluids to the surface, and for conveying, lifting and exporting gas, injection water and gas and treatment chemicals such as methanol from the surface to the seabed. The foot of each riser, 112, 114, is connected to a number of production well heads/injection sites 100 to 108 by horizontal pipelines 116 etc. Further pipelines 118, 120 may link to other well sites at a remote part of the seabed or to shore. Close to the sea surface 122, the top of each riser tower is supported by a buoy 124, 126. These towers are pre-fabricated at shore facilities, towed to their operating location and then installed to the seabed with anchors at the bottom and buoyancy at the top.

A surface support 128 is for example a floating production and storage vessel (FPSO) moored by means not shown, or otherwise held in place at the surface.

FPSO 128 provides production processing facilities, storage and accommodation for handling production from the wells 100 to 108. FPSO 128 is connected to the risers by flexible flow lines 132 etc arranged in a catenary configuration, for the transfer of fluids between the FPSO and the seabed, via risers 112 and 114. Such flexible flow lines do not allow for straightforward disconnection in difficult meteorological conditions. Also in such arrangements the FPSO itself cannot be easily removed from its anchoring system.

Individual pipelines may be required not only for hydrocarbons produced from the seabed wells, but also for various other fluids, which assist in the production and/or maintenance of the seabed installation. For the sake of convenience, a number of pipelines carrying either the same or a number of different types of fluid may be grouped in "bundles", and the risers 112, and 114 may comprise bundles of conduits for production fluids, lifting and exporting gas, injection water and gas, and treatment chemicals, such as methanol. In other known configurations, each riser comprises a single conduit.

The skilled person will appreciate that the riser bundle may be a few metres in diameter, but is a very slender structure in view of its length (height) of for example 500m, or even 1 km or more.

SCR5 are another form of riser more and more commonly used within the offshore oil and gas industry. They provide a fluid-conveying liaison between seabed and a (moored) vessel that can be used for oil production, water injection, gas injection, and as service lines. Conventionally the connection of the riser to the vessel features a flexible joint, attached to hull side. Location of the flexible joint on hull side is mainly dictated by architectural considerations (structure and process), as well as installation considerations. The departure angle of the SCR (in the horizontal plane) is generally restricted so that SCRs cannot head parallel to the hull side, to avoid collision with the hull.

Figure 2 shows the path of a catenary riser 200 connected to surface support 210.

The catenary riser 200, made of welded steel tubular sections, is coupled to the surface support 210 by means of a top flexible joint 220. Surface Support 210 is, for example, a floating production and storage vessel (FSPO).

Each of these systems have their drawbacks, as detailed in the introduction above.

Figure 3 shows a riser arrangement according to an embodiment of the invention. It comprises a first type of rigid conduit 300 in a first configuration similar to a Single Line Offset Riser (SLOR), which is essentially a monobore HRT. In this particular first configuration the vertical section of the first rigid conduit 300 is linked to the flowline/wellhead 310 via a catenary section, the top of which is tethered by tether 320 to foundation 330. In this first configuration the catenary section is short and the first rigid conduit 300 is therefore substantially vertical. The first rigid conduit 300 is held vertical by buoyancy element 340. Attached near the top of this first rigid conduit 300 is a support frame 350. The support frame 350 provides support for one or more (in this case two) further rigid or flexible conduits 360 arranged in a second configuration or type, which can be essentially that of a Steel Catenary Riser (SCR).

The support frame 350 of the first, substantially vertical, rigid conduit 300 includes: -An articulation to which one or several buoyancy elements 340 are attached, to keep the riser system under tension and ensure its stability. The riser arrangement optionally includes one or a series of buoyancy devices 395 for the further rigid conduits 360 of the second configuration to reduce the load on their support 355.

-Optionally, connectors 370, possibly of the "gooseneck" type, for connecting each conduit 300, 360 to a respective flexible conduit or jumper 380. These jumpers 380 links each conduit 300, 360 to a floating production unit 390 (or similar) in a catenary shape, so as to convey the riser fluid. The connectors may be of a type that is mechanically or hydraulically activated by a Remotely Operated Vehicle (ROy).

-Optionally, one or a series of supports 355 for the further conduits 360 of the second configuration. These supports 355 may be placed either at the top of the first conduit 300, or on the buoyancy element 340. This support may be fully rigid, but preferably articulated at least in two directions perpendicular to the first conduit 300.

The first conduit 300 may connect to the flowline/wellhead 310 via a rigid pipe laid in the continuity of the said flowline or pipeline and linked to the first conduit 300 by either a rigid connection or a flexible joint. This rigid connection may have a single catenary shape, or have a "lazy wave" shape obtained by one or several buoyancy modules attached to the pipe.

Figure 3a shows an alternative lower riser assembly having in place of the catenary section an articulation 375 to make the structural link with the foundation and a rigid spool (jumper) 385 with, at both ends, a connector similar to those mentioned above, so as to make the fluid link with the flowline/wellhead.

Each one of the further conduits 360 of the second type has its lower part on the seabed and extends to the support 355 in a single catenary shape or with a "lazy wave" shape obtained by one or several buoyancy modules attached to the conduit.

This concept therefore uses two types of rigid risers that extend from an elevation close to the seabed to an elevation above which the effect of environment (waves and current) is significant, corresponding to a water depth ranging approximately from SUm to 250m.

Figures 4 and 5 show detail of the arrangement near the top end of the conduits, for a particular embodiment of the invention. In this arrangement it can be seen that the further conduits 360 are individually attached to different parts of the first conduit 300, and/or buoyancy tank 340. Attachment may be via supports or "baskets" 400 and chain 420, or else directly, via a clamp, to hold the further conduit 360 laterally.

Each basket and chain or clamp could support only a single conduit of the second configuration, or more than one. There may be any number of further conduits 360, which could all be attached to the first conduit, all attached to a support frame, all attached to the buoyancy tank, all attached to any other suitable structure in the vicinity of the top of the first conduit, or attached to any combination of some or all of these possibilities.

The buoyancy tank can be seen to comprise of a short tank 440 and long tank 450.

This helps installation as the short tank 440 is sufficient to support and tension the first conduit 300 by itself, allowing that to be installed first, along with the required frames and attachments 350, 355, 410. The long tank 450 can then be attached, before attachment of the further risers 360 (which add very considerable weight).

Also shown is the articulation and structural connector 410 between buoyancy element 340 and first conduit 300. Another articulation and structural connector 430 can be found between the short tank 440 and long tank 450. These structural connectors may be of the type marketed as Rotolatch (RTM).

Installation of this riser arrangement is as follows: -The foundation 330 (e.g. a suction pile) is lowered down to the seabed and driven to the required penetration.

-The first conduit 300 is constructed from an offshore installation vessel: The lower end of a first section of the first conduit 300 is inserted in a lay tower, which handles the pipe string; and a next section of pipe is welded on this first section. The string is then lowered into the water below the lay tower as further sections are added. The column is completed by the addition of the upper riser assembly which includes goosenecks, connectors and supports for the flexible jumper 380 that completes the fluid link for conduit 300, supports 350 for the second type of conduits, and articulation for the connection 410 that makes the structural link with the buoyancy device 440.

When the conduit assembly is complete, the first conduit 300 is transferred to a support on the vessel edge where it is supported by "outriggers".

-The first buoyancy device 440 is taken by the crane on the installation vessel, upended and moved over the top of the conduit 300 for connection by means of the articulation.

-The whole conduit assembly plus buoyancy device is lifted by the crane and then lowered in water.

-The whole conduit assembly is connected to the foundation; this is achieved by placing a cable running in a sheave attached to the foundation is connected to the bottom of the conduit 300 and by pulling the cable from a service vessel and releasing the crane so that the conduit is lowered and forced to make connection with the foundation.

-The second buoyancy device 450 is towed to the site and upended by ballasting the tank. A cable running in a sheave attached to the top of the first buoyancy device is connected to the bottom of the second buoyancy device and by pulling this cable from a service vessel, the second device is lowered and forced to make connection with the first device. The tank of the second buoyancy device 450 is then de-ballasted by injection of nitrogen.

-Installation of a first further conduit 360 of the second type is initiated vertically from the lay tower. Construction proceeds by adding a section of pipe in the tower, welding it to the preceding section and lowering the string; when the latter reaches the seabed, it is connected to a restraint placed on the seabed and a catenary configuration is obtained by moving the vessel and paying out pipe.

-To complete construction of the conduit 360, the last section is added, which includes goosenecks and connectors and supports for the flexible jumper 380 making the fluid link for conduit 360, a chain 420 and articulation and male support to make the structural link with the basket (female) support on the first conduit 300.

-The conduit 360 is lowered by a cable from the installation vessel and positioned so that the male part of the support system can be placed in the basket; the cable from the vessel is released and removed from the top of the conduit 360.

-The other second type conduits 360 are installed similarly.

After installation of the floating production unit, the flexible jumpers are placed and connected to the conduits 300 and 360 and, at the other end, to the floating production unit 390.

Figure 6 shows an alternative termination for the second conduit 360, in which a Y-shaped framework 600 is interposed between the catenary riser 360, the flexible jumper 380 and a supporting linkage 602, for example a chain or buoy. This type of termination can be combined with that of Figure 5 in the same system, if desired.

Advantages of these risers arrangements include: -Suitable for very deep water, -Can be installed offshore using a conventional installation vessel, -It is a self standing system. Consequently, most of the riser installation operations can be carried before installation of the floating production unit on which conduits are to be connected.

-A number of conduits are structurally linked (at least) at the top, reducing or eliminating the risk of clashing between these conduits under the effect of currents.

-Uses a combination of riser systems that are individually field proven.

-Cost effective system, mainly in terms of installation time.

The embodiments mentioned above are for illustrative purposes only and other embodiments and variations can be envisaged with departing from the spirit and scope of the invention. For example, the first conduit may, in fact comprise more than one conduit, whether it be one or more conduits "piggy-backed" to a main conduit, or conduits bundled together, with or without a central structural core.

As mentioned in the introduction, the second conduits need not all have a simple caternary configuration. Modified catenary configuration such as the known Lazy Wave', Sleep Wave', Lazy 5' and Sleep 5' and Pliant Wave' variations are also known. These configurations, which are characterised by different forms of additional buoyancy devices carried on the riser conduit and/or separately anchored to the seabed, can be mixed and used on some or all of the second risers.

Claims

Claims 1. A marine riser apparatus comprising at least one tensioned conduit extending from the seabed toward the surface whereupon it has an upper end supported by at least one buoyancy device, such that said at least one buoyancy device maintains said at least one tensioned conduit in tension with a substantially vertical configuration, said apparatus further comprising one or more suspended conduits wherein said suspended conduits are all suspended from an upper portion of said marine riser apparatus.
2. A marine riser apparatus as claimed in claim 1 wherein at least one of said one or more suspended conduits takes a catenary configuration.
3. A marine riser apparatus as claimed in claim 1 or 2 wherein at least one of said one or more suspended conduits comprises buoyancy so as to take a different configuration such as a modified catenary.
4. A marine riser apparatus as claimed in any preceding claim wherein said at least one tensioned conduit comprises an anchoring device anchoring it to the seabed.
5. A marine riser apparatus as claimed in any preceding claim wherein said at least one tensioned conduit comprises an articulation at its bottom to change its direction at the seabed.
6. A marine riser apparatus as claimed in any of claims 1 to 4 wherein said at least one tensioned conduit comprises a rigid or flexible pipe forming a curve at its bottom to change its direction at the seabed.
7. A marine riser apparatus as claimed in any preceding claim wherein at least one of said one or more suspended conduits is suspended from the tensioned conduit.
8. A marine riser apparatus as claimed in any preceding claim wherein at least one of said one or more suspended conduits is suspended from the at least one buoyancy device.
9. A marine riser apparatus as claimed in any preceding claim wherein the device comprises a support frame, and wherein at least one of said one or more suspended conduits is suspended from said support frame.
10. A marine riser apparatus as claimed in any preceding claim wherein at least one of said one or more suspended conduits is attached to said apparatus via at least one chain and/or an accessory, said accessory for attaching to a respective receptacle being attached to said apparatus.
11. A marine riser apparatus as claimed in any preceding claim wherein said apparatus comprises a plurality of said suspended conduits.
12. A marine riser apparatus as claimed in any preceding claim wherein said apparatus comprises only a single tensioned conduit, or more than one tensioned conduit in a single bundle.
13. A marine riser apparatus as claimed in any preceding claim comprising connectors at the top of each conduit for connection to a flexible line.
14. A marine riser apparatus as claimed in claim 13 wherein said apparatus further comprises a flexible line for connection of each conduit to a surface facility.
15. A marine riser apparatus as claimed in any preceding claim wherein at least one of said one or more suspended conduits is provided with a further buoyancy device at or near its top.
16. A marine riser apparatus as claimed in any preceding claim comprising two main and separable buoyancy devices tensioning said at least one tensioned conduit, a smaller one of said devices for providing buoyancy sufficient at least for suspending the at least one tensioned conduit alone.
17. A marine riser apparatus as claimed in any preceding claim wherein said tensioned conduit does not have any flow connections, it being is used solely for tensioning and not for conveying fluid.
18. A method of installation of a marine riser apparatus of the type claimed in any preceding claim, said method comprising the steps of: * constructing on, and lowering from, a pipelay vessel the at least one tensioned conduit; * attaching an upper riser assembly to the at least one tensioned conduit; * attaching said at least one buoyancy device to said tensioned conduit to provide a tensioned riser assembly; * lowering the at least one tensioned riser assembly into position, so that it is tensioned by said buoyancy device; * constructing on, and lowering from, a pipelay vessel a first of the suspended conduits; * attaching an upper riser assembly to the first of the suspended conduits; * attaching said suspended conduit to the tensioned riser assembly; and * Repeating the previous three steps for each suspended conduit to be installed.
19. A method of installation of a marine riser apparatus as claimed in claim 18, the method comprising the installation of an anchoring device to the seabed and attachment of said tensioned conduit to said anchoring device.
20. A method of installation of a marine riser apparatus as claimed in claim 18 or 19 wherein the installation of the suspended conduits comprises the steps of restraining a section of the suspended conduit on the seabed and moving the pipelay vessel so as to obtain a catenary configuration in the suspended conduit.
21. A method of installation of a marine riser apparatus as claimed in claim 18, 19 or 20 wherein the attaching of the suspended conduit to the tensioned riser assembly comprises lowering the suspended conduit on a line to its installation position and placing a male support, forming part of the upper riser assembly of the suspended conduit, into a female support forming part of the upper riser assembly of the tensioned riser assembly.
22. A method of installation of a marine riser apparatus as claimed in any of claims 18 to 21 wherein the step of attaching the buoyancy device comprises attaching a first buoyancy device, the method further comprising the installation of a further buoyancy device to the tensioned riser assembly prior to the attachment of the at least one suspended conduit.
23. A method of installation of a marine riser apparatus as claimed in any of claims 18 to 22 wherein a floating production unit is installed, followed by the installation of flexible pipelines so as to connect the conduits to said floating production unit.
24. A method of installation of a marine riser apparatus as claimed in any of claims 18 to 23 wherein the installation is performed in water deeper than 800m.
25. A marine riser apparatus as hereinbefore described with reference to figures 3to 6 of the drawings.
26. A method of installation as hereinbefore described with reference to figures 3 to 6 of the drawings.