GB2473018A

GB2473018A - Hydrocarbon production system

Info

Publication number: GB2473018A
Application number: GB0914940A
Authority: GB
Inventors: Stephen Anthony Hatton
Original assignee: 2H Offshore Engineering Ltd
Current assignee: 2H Offshore Engineering Ltd
Priority date: 2009-08-26
Filing date: 2009-08-26
Publication date: 2011-03-02
Also published as: GB0914940D0

Abstract

An underwater hydrocarbon production system (101) comprises a riser system (102) and a submerged buoyancy module (110). The riser system comprises an upper riser assembly (106), a lower riser assembly (104) and a base foundation (116) for fixing the lower riser assembly to a floor (122) of a body of water. The lower riser assembly (104) comprises a flowline (136) having an upstream end and a downstream end. The flowline upstream is connected to a production flowline (119). The base foundation (116) and the lower riser assembly (104) are articulated (114) to allow for relative movement. The buoyancy module (110) is engaged with an upper end of the upper riser assembly (106) in order to tension the riser string (108). Further connectors permit the bottom end of the riser string to be remotely connected from the lower riser assembly without disturbing the lower riser assembly or connection to the production flowline (120).

Description

Hydrocarbon Production System

BACKGROUND

a. Field of the Invention

The present invention relates to hydrocarbon production systems with a substantially vertical riser and a submerged buoyancy module for use in the extraction of hydrocarbons and in particular to such systems that are used to extract oil or gas from offshore and deepwater fields.

b. Related Art Risers are high pressure dynamic tubular structures used in the extraction of oil and gas from offshore fields. They extend from the seafloor to the surface production vessel and are used to transport oil, gas and injection fluids.

In deep water (for example a depth of greater than 1000 metres) there are often a limited number of feasible riser solutions for a particular field development. This is 2Q due to the many design, operational, commercial and contractual constraints. This *..

limitation is particularly evident on developments in ultra deep water (a depth of .. : typically between 1500 and 3000 metres) which typically require a large number of risers, utilise dynamic production vessels such as turret and spread moored :.. Floating Production, Storage and Off-loading (FPSO) vessels and are often located in an environment that has significant wave, current and wind loading. For these applications there is a demand for improved riser technology and system configurations to assist future developments.

Figure 1 shows a schematic depiction of a conventional free standing Single Line Offset Riser (SLOR) system 1 commonly used in deepwater oil and gas development and production and which is recognised as a field proven deepwater riser arrangement. A full description of such a system is given in "The Roncador P- 52 Export System -Hybrid Riser at 1800m Water Depth" by Roveri, F.E. et al, OTC 19336, 2008 Offshore Technology Conference, Houston Texas, 5-8 May 2008, published as http://e-book. lib. sjtu.edu.cn/otc-2008/pdfs/otcl 9336.pdf, the contents of which are incorporated herein by reference.

Typically, a Free Standing Hybrid Riser (FSHR) assembly 2 is formed from a base foundation 16, a lower riser assembly, a pipe string 8 and a top riser assembly 24.

The pipe string 8 is sometimes referred to as an intermediate riser assembly, but in this patent specification, the riser assembly above the lower riser assembly consisting of the top riser assembly 24 and the pipe string 8 will be referred to as an upper riser assembly 6. The pipe string 8 is near-vertically configured and is supported and over tensioned by a near surface buoyancy module 10 (also called a buoyancy can) via a chain linkage 12 that is connected to the top of the top riser assembly 24.

At the base of the lower riser assembly 4 an articulating connector 14 is connected to the base foundation 16.The foundation may be a driven pile, suction pile or gravity base structure. S.. * .. *. S

2Q The lower riser assembly is permanently connected to the upper riser assembly at I.'.

a stress joint 21, which secures the riser string 8 to an offtake spool 36 that serves I...

.. : as a flowline into the riser string 8. An upstream end of the offtake spool 36 terminates in an upwardly facing mandrel 25. An induction formed steel flow :.. spool, known as a Rigid Base Jumper (RBJ) 18 has a downwardly facing mandrel 27 connected to the offlake spool mandrel 25 and provides a compliant expansion facility between the base foundation 16 mounted lower riser assembly 4 and a production flowline termination 20 from at least one hydrocarbon well (not shown) in the seafloor 22. The production flowline termination 20 and the spool pipe 18 together form a production flowline 19 for connection to the hydrocarbon well.

The top riser assembly 24 incorporates a flow bend 29 for fluid offlake and a Y-spool 31. The flow bend 29 provides the facility to connect a compliant jumper 26, also referred to as a flexible pipe catenary section between the riser and the surface production facility 28, normally a floating production vessel. The connection to the flow spool 29 is made at a point on the top riser assembly 24 below the buoyancy module 10. The Y-spool 31 is a branch in the production line permitting tools to be inserted into the production bore. This particular SLOR configuration has been used exclusively in multiple deep water locations and has proven to be structurally reliable.

Seafloor mooring lines may be used to stabilise the buoyancy module 10, but normally this is not necessary.

It is anticipated that this SLOR arrangement will be used on future worldwide deepwater developments. However, the above arrangement presents a number of significant installation challenges since it requires the handling of three major structural assemblies whilst fabricating the vertical riser pipe. The three main structural assemblies are: Lower riser assembly 4; * * . ** * 2'Q * Upper riser assembly 6, including top riser assembly 24; and **** **** * Buoyancymodulelo.

:.. Each of these assemblies has a significant size and weight such that their **2 handling dimensions, the installation vessel specification and the necessary handling procedures dominate much of the installation time, cost and installation risk.

Additionally, this conventional arrangement, in which the flexible jumper 26 is attached below the buoyancy module 10 causes significant challenges in the installation and connection of the flexible jumper since the riser connection point is hidden' at the surface below the buoyancy module. This means that vertical access using lifting equipment cannot be used, thereby complicating the installation procedure.

A further issue is that of providing vertical access into the riser bore for the purposes of remediation using coiled tubing. In the standard SLOR arrangement 1 described above this is provided by the Y-spool 31 located in the top riser assembly 24 providing the facility to connect a lubricator assembly. However, despite these facilities being commonly provided they are practically useless since accessibility to the Y-spool is very poor due to the presence of the buoyancy module 10 located immediately above the upper riser assembly 6 and general congestion around the upper riser assembly structure.

These issues have resulted in the conventional SLOR having a high capital equipment cost and high installation cost, driven by the need to use a highly capable, and therefore expensive, installation vessel.

The present invention therefore seeks to simplify the SLOR arrangement specifically for ease of installation by smaller lower cost vessels whilst maintaining *:::: design and equipment simplicity which is a key driver in achieving a robust and "2Q reliable design. *.. **** * * * ** *

SUMMARY OF THE INVENTION *q.* * , *

According to a first aspect of the present invention there is provided an underwater hydrocarbon production system comprising a riser system and a submerged buoyancy module, wherein: -the riser system comprises an upper riser assembly, a lower riser assembly and a base foundation for fixing the lower riser assembly to a floor of a body of water; -5.- -the lower riser assembly comprises a flowline, said flowline having an upstream end and a downstream end, said upstream end having means for connection to a production flowline; -the base foundation and the lower riser assembly are articulated so that when the upper riser assembly is connected to the lower riser assembly relative movement between the upper riser assembly and the base foundation is accommodated; -the upper riser assembly comprises a substantially vertical riser string and at an upstream end of the riser string a riser connector portion for connection to and disconnection from the lower riser assembly; -the buoyancy module is engaged with an upper end of the upper riser assembly in order to tension the riser string; and -the lower riser assembly further comprises a flowline connector portion for connecting and disconnecting by means of the riser connector portion said downstream end of the flowline to said upstream end of the riser string.

This arrangement permits remote connection and disconnection of the riser string from the flowline I the lower riser assembly. The remote connecting and *r: disconnecting may be facilitated by a remotely operable vehicle (ROV) and is therefore remote in the sense that the operation is performed remotely from the surface. There is therefore no need to make the connection prior to assembly of the lower riser assembly with the base foundation or to raise the lower riser assembly when there is a need to disconnect the riser raiser string from the lower riser assembly. dl

* The riser system is preferably a single line offset riser system.

The buoyancy module most preferably has a channel, with the riser pipe string extending through the channel and the channel having dimensions sufficient to allow passage of the riser string and the riser connector portion through the channel. The channel most preferably extends through a central portion of the buoyancy module, which in a preferred embodiment of the invention is substantially cylindrical.

A particular advantage of the system is that the bottom end of the riser pipe string can be remotely connected and disconnected from the upper end of the lower riser assembly. This can be achieved subsea so it allows the riser to be removed without disturbing the lower riser assembly or base jumper spool.

According to a second aspect of the invention, there is provided an underwater hydrocarbon production system comprising a submerged buoyancy module and a riser system, the riser system including a substantially vertical riser string, wherein: -the buoyancy module is engaged with an upper end of the riser system in order to tension the riser string; -a lower portion of the riser string terminates with a riser connector portion for connection to and disconnection from a production flowline; and -the buoyancy module has a channel, the riser string extending through the channel and the channel having dimensions sufficient to allow passage of the riser string and the riser connector portion through the channel. * * * S. S

In a preferred embodiment of the invention, the riser system is a single line offset (SLOR) riser system. 5.1 * S

In use, the buoyancy module is engaged with an upper end of the upper riser assembly in order to tension the riser string. A lower portion of the riser string may terminate with a riser connector portion for connection to a flowline. When the buoyancy module has a channel, the riser string extends through the channel with the channel having dimensions sufficient to allow passage of the riser string and the riser connector portion through the channel.

According to a third aspect of the invention, there is provided an underwater hydrocarbon production system comprising a submerged buoyancy module and a riser system, the riser system including a substantially vertical riser string, wherein: -the buoyancy module is engaged with an upper end of the riser system in order to tension the riser string; -the buoyancy module has a channel, the riser string extending upwards through the channel to an upper portion of the buoyancy module; -an upper end of the riser string terminates at a first upper connector portion; -the buoyancy module supports a substantially vertically extending sleeve, said sleeve extending upwards to said upper portion of the buoyancy module; -an upper end of said sleeve terminates at a second upper connector portion; -the riser system includes a jumper pipe that extends between said first and second connector portions so that production flow from the riser system is directed towards said sleeve; and -the sleeve has at a lower end an opening for receiving an end of a flexible jumper pipe and within the sleeve means for connecting to said jumper pipe so that production flow from the riser system is conveyed into said flexible jumper pipe.

The substantially vertically extending sleeve may be supported on a side surface of the buoyancy module. Alternatively, the sleeve may be supported within the *Q module. * S S...

The riser connector portion and flowline connector portion preferably form a stab connector assembly. S. S. *

In a preferred embodiment of the invention, the riser connector portion is provided * at a lowermost portion of the upper riser assembly, and may be downwardly facing, with the flowline connector portion being upwardly facing.

The flowline connector portion is then upwardly directed in order to engage with downward movement of the riser connector portion.

The flowline connector portion may include a collet for receiving the riser connector portion, in which case, the riser connector portion includes a ring that extends around the upstream end of the riser string.

The ring may be a mandrel.

The base foundation and the lower riser assembly are preferably articulated by means of an articulated joint, which may therefore be positioned between the flowline connector and the base foundation.

The production flowline may be joined to the lower riser assembly at the flowline connector portion.

According to a fourth aspect of the invention, there is provided an installation system for an underwater hydrocarbon production system, comprising an installation vessel and a submerged buoyancy module for tensioning a riser system, the buoyancy module having a channel therethrough for receiving a substantially vertical riser string, wherein: -the buoyancy module has means for controlling the buoyancy of the module 50 that the module may be heavier or lighter than the water displaced by the buoyancy module when submerged; * S * -an upper end of the buoyancy module has a first mounting means for supporting the weight of buoyancy module when the module is heavier than said :::: displaced water; -the installation vessel has a second mounting means for bearing said weight of buoyancy module; and *. ** * S S * -the installation system comprises additionally a suspension means that is : connected to both the first and second mounting means to support said weight of the buoyancy module so that the buoyancy module is suspended beneath the installation vessel during installation of an underwater hydrocarbon production system.

In a preferred embodiment of the invention, the buoyancy system is for tensioning a single line offset (SLOR) riser system.

The suspension means may include a motion damper.

The installation vessel will normally have a passage extending through a deck of the vessel for lowering a riser string into the water. The buoyancy module most preferably has a substantially vertically extending channe' so that the installation vessel may lower a riser string into the water through the passage in the deck and through the channel in the buoyancy module.

According to a fifth aspect of the invention there is provided a method of installing an underwater hydrocarbon production system, the system being according to the first aspect of the invention, wherein the method comprises the steps of: -fixing the base foundation to the floor of a body of water; -connecting the lower riser assembly to the base foundation so that the base foundation and the lower riser assembly are articulated; -connecting a production flowline to the flowline of the lower riser assembly; -submerging the buoyancy module in position above the lower riser assembly; -lowering the riser string including the riser connector portion towards the * S S ::..: lower riser assembly; -connecting the riser connector portion to the flowline connector portion of *: the lower riser assembly; and -increasing the buoyancy of the buoyancy module and engaging the buoyancy module with the riser string in order to tension the riser string.

* .. The buoyancy module may have a channel that extends substantially vertically through the buoyancy module, wherein the method comprises prior to connection of the riser connector portion to the flowline connector portion, the steps of: -lowering the riser string including the riser connector portion through the channel through the buoyancy module. -10-

In a preferred embodiment of the invention, riser string has a load shoulder. The method may then comprise, prior to the step of increasing of the buoyancy of the buoyancy module, the steps of: -positioning the riser string so that the load shoulder is above the channel through the buoyancy module; and -allowing the buoyancy module to rise as the buoyancy is increased until the buoyancy module comes into contact with the load shoulder and so tension the riser string.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic depiction of a known hydrocarbon production system in which a single line offset riser (SLOR) is tensioned by a submerged buoyancy module, with a flexible jumper pipe leading from an upper riser assembly to a production vessel for offloading of produced hydrocarbons; S..' Figure 2 shows a schematic depiction of a base foundation for securing a riser system to a floor of a body of water; Figure 3 is a schematic depiction of a lower riser assembly for use in a * hydrocarbon production system according to a preferred embodiment of the ** . S ** * invention, secured around the base foundation of Figure 2, and having an upwardly directed flowline connector portion for connection to a riser pipe string; Figure 4 shows a schematic depiction of an installation vessel in the process of installing a hydrocarbon production system according to a preferred embodiment of the invention, during which a riser string including a lowermost riser connector portion has been lowered through a central channel in a submerged buoyancy module; Figure 5 shows a schematic depiction of how the riser connector portion is lowered to connect with the flowline connector portion; Figure 6 shows a schematic depiction of the installation process, after connection of the riser connector portion to the flowline connector portion, and with the buoyancy of the buoyancy module being increased by pumping air or nitrogen into the module; Figure 7 shows a schematic depiction of the installation process, with a flexible jumper pipe catenary section connected to an I tube fixed to a side wall of the buoyancy module; and Figure 8 shows a schematic depiction of the completed hydrocarbon production system, including also a rigid jumper pipe above the buoyancy module that connects to an upper connector of the riser string and to a connection at the top of the Itube.

SSS isS* * * * S. *

DETAILED DESCRIPTION *. **

** Figure 2 shows a base foundation 116 for securing a riser system to a floor 122 of * a body of water, which will normally be a seafloor, although the invention is also applicable to deep freshwater. The term "seafloor" as used herein is therefore intended to encompass also the floors of bodies of freshwater.

-12 -The base foundation, which is typically 4 m to 5 m across, is secured to the seafloor 122 typically by a suction pile (not shown), although a driven pile or gravity base structure (not shown) could alternatively be used. The base foundation 116 has an upwardly directed receptacle in the form of a collet 130.

As shown in Figure 3, a lower riser assembly 104 is then built up around the base foundation 116. The lower riser assembly includes a flowline connection portion 132 that incorporates an offtake spool 136 that provides a flowline into the riser string 108 and which has an upstream end that terminates in an upwardly facing mandrel 125. Although not illustrated, during installation this lower riser assembly 104 would be made ready and tested on a back deck of an installation vessel, rigged with lifting wire and then lowered into the water.

The flowline connection portion 132 has a downwardly directed projection 134 that engages with a mechanical latch mechanism 133 inside the base foundation funnel 130 and incorporating an articulation connection 114 that permits the flowline connection portion 132 to rotate away from a vertical axis. The wire used to install the flowline connection portion 132 is tensioned after installation as part of an overpull test to ensure that the base foundation 116 is secure on the seafloor 122. * S S

:.. The flowline connection portion 132 has an upwardly directed flowline hydraulic S.. S connector mechanism 138 that is connected to an end of the offtake spool 136. *S * ** S. *

An induction formed steel flow spool pipe forms a Rigid Base Jumper (RBJ) 118 has a downwardly facing mandrel 127 that is connected to the offlake spool jumper 125 and provides a compliant expansion facility between the flowline * connector portion 132 and a production flowline termination 120 from a hydrocarbon well (not shown) in the seafloor 122. The production flowline termination 120 and the spool pipe 118 together form a production flowline 119 for connection to the hydrocarbon well. -13-

Figure 4 shows how the upwardly directed flowline connector mechanism 138 is used to make a connection to a riser connector portion in the form of a mandrell40 at a lower end of a near-vertically configured steel riser pipe string 108. The flowline connector mechanism is surrounded by an upwardly directed funnel or collet 142 that guides the riser connector portion 140 to connect with the flowline connector mechanism 138 when the riser connector portion is dropped in a stabbing connection operation, as shown in Figure 5.

During this process, an installation vessel 128 is used to support a submerged buoyancy module 110. Although not illustrated, the module 110 may be transported to the well site by barge, and then lowered into the water near the installation vessel 128. The buoyancy module 110 is then rigged with wires 150, including motion dampers 152, fitted to mounts 154 on the keel of the installation vessel 128 and to mounts 155 on an upper surface 157 of the buoyancy module.

The module 110 has an internal volume 144 that is initially partially flooded with water 146 to make the module heavier than the displaced water so that the module sinks and is suspended directly beneath the installation vessel 128, close to the centre of a moonpool 156 in a passage 158 that extends beneath a deck 160 of the vessel. The amount of water 146 inside the module 110 can be controlled by pumping air or nitrogen 148 into the module. Initially, the module 110 is suspended at a depth 10 m to 15 m below a final desired installation depth.

: Motion compensation devices (not shown) may be used in the connection between the vessel 128 and the buoyancy module 110 to stabilise its motion relative to the seafloor 122. S. *S * S * * * S. S * S S

* The formation of the riser pipe string 108 may then begin in a conventional manner, for example by supporting the first riser joint in a tower and by J lay welding of the riser string (not illustrated).

As the riser string 108 is formed, this is lowered to pass through a channel 162, formed by a central structural tube that extends centrally through the buoyancy module 110 which is preferably substantially cylindrical in form. As with some other processes described above, such as the formation of the lower riser assembly 104, this may require initial guidance using a remotely operable vehicle (ROV) (not shown). During this process, the module suspended position may be optimised to ensure smooth passage of the riser string 108.

This process is continued as the full length of the riser string is built up, including riser pipe components such as the lower riser connector portion 140, a keel taper joint 164, a pair of centraliser flanges 166, an upper load shoulder 112 and an upper connector in the form of an upper mandrel assembly 168. As shown in Figure 6, the tapered keel joint 164, which bears against a lower end of the channel 162 through the module 110, serves to spread loads at the interface between the riser and the base of the buoyancy module associated with lateral movement of the buoyancy module relative to the base foundation 116. The joint 164 is preferably a forged back to back taper joint. The centraliser flanges 166 are provided around the riser pipe string 108 inside the channel 162 to ensure that the top end of the riser pipe remains central inside the channel 162. These flanges 166 also control the riser deflection and stresses.

*.. During the final lowering of the riser string 108, a riser running string 170, which may be pipe or wire cable, is attached to the top end of the riser pipe string 108.

*: As the riser string 108 is lowered, the lower mandrel 140 is landed into the upward looking connector collet 142 and mechanism 138 at the top of the lower riser assembly 104. The connector mechanism 138 is hydraulically operated so that once the mandrel enters the connector mechanism, these are automatically S. * engaged together. At a later time, these may be disengaged hydraulically, for

example by an ROy.

The running string 170 is then used to perform an overpull test to confirm the structural integrity of the riser string assembly 108 and lower riser assembly 104.

A

After this testing is complete, the buoyancy module 110 is filled with air or nitrogen, as shown in Figure 6. The module 110 then rises until the upper surface 157 of the module comes into contact with the load shoulder 112. The buoyancy 172 of the module then tensions the riser pipe string 108.

Figure 7 shows how a flexible jumper 126 is connected to the buoyancy module 110. A sleeve 174 in the form of a vertical I tube is used for connecting the flexible jumper 126 to the riser pipe string 108. The I tube 174 is fabricated as an integral part of the buoyancy module 110 and may be included on the outside diameter of the module as illustrated, or alternatively within the buoyancy module, for example against an inner wall (not illustrated).

The I tube 174 extends along the length of the buoyancy module 110 to the upper face 157. The lower termination of the I tube can be selected depending on the structural response of the SLOR system. It is likely that in most cases the I tube will extend to a bottom face 177 of the module 110 where it will be terminated with a suitable bend radius and I tube connector to suit the pull in and latching of a bend stiffener portion 180 of the flexible jumper 126.

The I tube 174 has a conventional upper termination 182. This is a reliable method of pulling in and latching a flexible jumper. * S ****

Figure 8 shows how, after the flexible jumper 126 has been pulled in and latched : in place, a rigid jumper spool connection 124, of conveniently simple form, is used at the top of the buoyancy module 110 to complete an upper riser assembly 106, and hence the riser string system 102. This is achieved by fabrication of an * induction flowbend complete with hydraulic connector 184, 186 at both ends to * remotely and simultaneously connect onto the top connector mandrel 168 of the main riser pipe string 108 and the jumper termination 182 of the I tube 174. The simplicity of this connection is due to the fact that it is easily accessible vertically from the surface and the precise locations and tolerances of the two mating hubs -16- 168,182 are well defined and controlled through the fabrication phase of the buoyancy module 110.

Compared to the conventional arrangement shown in Figure 1, this has a number of benefits. There is a vertical pull-in of the jumper to a point where there is easy access at the top 157 of the buoyancy module 110. Also the flexible jumper high bend area is kept remote from the end termination.

It is noted that the I tube 174 need not be straight along its length but may be curved, within the bending constraints of the flexible pipe and installation loads to optimise the locations of both the upper jumper termination with respect to the central riser pipe string 108 and the lower offlake 180.

It is to be noted that in the installation process described above, more installation operations are conducted from vertically above the buoyancy module that in the prior art arrangement 1. The design will therefore preferably incorporate suitable dropped object protection and installation procedures should be developed to minimise the risk of damage due to a dropped object.

A key feature of the SLOR configuration 101 described above is the provision of a :::* remote connect/disconnect facility of the riser string 108 immediately above the lower riser assembly 104. This is provided by the hydraulic connection mechanism 138 at the top of the lower riser assembly. This has three main purposes: S * S. S * Allows the lower riser assembly 104 to be pre-installed and if necessary hooked up to the flowline 120 and gas lift system; * * Reduces the size of the rise pipe connection portion 140 to that of a standard hub so that this can easily pass the bore of the buoyancy main tube 162; and Allows disconnection of the riser string 108 without disturbing the flowline connections or allows vertical access to pull and replace the lower riser assembly 104 without having to retrieve the upper riser assembly 106.

The latter point is considered a key benefit since the complexity of the lower riser assembly 104 is such that its long term reliability can be uncertain. Therefore this approach facilitates easy access for removal using a small vessel rather than a complete riser system 102 removal and reinstallation.

The remote connection will consist of a standard proprietary collet style connector arrangement 138, 140, 142 used regularly in drilling and spar/TLP deep water production risers. These are rugged, have a high structural capacity and proven metal sealing capability. The connector mechanism 138 will be incorporated into the design of the lower riser assembly 104, including a large upward facing funnel 142 for alignment during stabbing.

At the top end of the riser pipe string 108, above the top face 157 of the buoyancy module 110, the riser pipe string 108 is terminated with a vertically orientated mandrel 168 to accept a hydraulic connection. This mandrel is machined with a seal pocket, complete with stainless steel inlay to accept a conventional AX type metal to metal seal. * S * ** * S.,.

The incorporation of a hydraulic connector 138 in the lower riser assembly 104 *S* . . . . *** : facilitates the installation of the lower riser assembly independent of the upper riser assembly 108, hence it can be pre-installed. This activity can be conducted offline to the main riser installation and typically can be achieved using a smaller :*.: construction vessel rather than a large riser installation vessel, providing some * cost savings. Further advantages are: * Reduces the weight of the riser string 108 during installation; -18- * Simplifies lowering and landing of the riser string 108 by virtue of improved visibility and reduced component weight; * Allows rigid jumper connections 118 to the flowline to be preinstalled and without congestion an the associated risk of an adjacent riser string; * Allows the riser string 108 to be disconnected and the lower riser assembly 104 to be removed for its repair; and * Allows the riser string 108 to be disconnected and lifted close to the surface for buoyancy module repair without disturbing flowline connections.

Furthermore, because the riser pipe string 108 is continued up through the central structural tube 162 of the buoyancy module 110, rather than being terminated below, the riser string 108 continues to the upper face 157 of the buoyancy module where the tension from the module is reacted via a simple load shoulder 112. This through can' arrangement eliminates the need for an articulated connection or tether chain typically used on conventional SLOR systems in order to connect the buoyancy module to the riser stung.

*:::: The rigid jumper spool 124 used above the buoyancy module 110 to connect between the top 168 of the riser string 108 and the flexible jumper end termination **.* 182 is easily configured using a piggable induction flow bend and two remote : hydraulic connectors 184, 186 or alternatively by diver made flanges.

It is proposed that the piggable flowbend incorporates a Y' junction 190 to allow vertical access into the main bore of the riser string 108 using coiled tube of * wireline. This may be necessary for inspection of remediation of wax and hydrates.

As the Y-spool 190 is located uppermost on the riser string 108, it is easily accessible to an over head vessel which eases the ability to land a lubricator or similar for pressure control. -19-

The Y-spool 190 will incorporate a gate valve and isolation plug 192 completed with pressure test facilities between the two to confirm sealability prior to plug removal. The upper end of the Y-spool 190 is machined with a mandrel profile for attachment of a lubricator or pressure rated conduit to the surface vessel.

After installation of the vertical riser and buoyancy module 110, the flexible jumper 126 can be installed in a vertical arrangement hung from the bottom face 177 of the module, where its weight is efficiently supported. The riser string 108 may remain in this preinstalled condition for many months until the arrival of the Floating Production Storage and Off-loading (FPSO) unit at the well site.

Once the project is ready for hook-up of the jumper to the FPSO, the jumper end termination is rigged, using the triplate connection to the final pull-in wire. This is used to make the final pull-in to the I tube and can be done by a small construction vessel rather than a more expensive, fully capable vessel. This provides good operational flexibility.

It will be understood that the preceding references to vertical risers are not intended to act as a strictly geometrical limitation. In use, a vertical riser will define a vertical or substantially vertical path. * S s55*

The preferred embodiment of the SLOR system 101 described above provides S...

: significant economies over prior art SLOR systems. The SLOR system buoyancy module 110 be installed from a much smaller and lower specification installation * vessel 128, and also provides other benefits such as improved vertical access to the riser bore with coiled tubing and easier replacement of the key critical * components, including the flexible jumper 126 and the lower riser assembly 104.

It should be noted that although the invention has been described in the context of a single line offset riser (SLOR) system, the invention is also applicable to hybrid line offset riser (HLOR) systems and also concentric offset riser (COR) systems.

-20 -As a result, the underwater hydrocarbon production system described above not only offers improved installation, which is the main cost driver but also offers improvements in long term reliability and maintenance. * . , ** *4'*

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Claims

CLAIMS1. An underwater hydrocarbon production system comprising a riser system and a submerged buoyancy module, wherein: -the riser system comprises an upper riser assembly, a lower riser assembly and a base foundation for fixing the lower riser assembly to a floor of a body of water; -the lower riser assembly comprises a flowline, said flowline having an upstream end and a downstream end, said upstream end having means for connection to a production flowline; -the base foundation and the lower riser assembly are articulated so that when the upper riser assembly is connected to the lower riser assembly relative movement between the upper riser assembly and the base foundation is accommodated; -the upper riser assembly comprises a substantially vertical riser string and at an upstream end of the riser string a riser connector portion for connection to and disconnection from the lower riser assembly; -the buoyancy module is engaged with an upper end of the upper riser assembly in order to tension the riser string; and -the lower riser assembly further comprises a flowline connector portion for connecting and disconnecting by means of the riser connector portion said downstream end of the flowline to said upstream end of the riser string.

* .* * :
2. An underwater hydrocarbon production system as claimed in Claim 1, in which the riser connector portion and flowline connector portion form a stab *. connector assembly. *0 * * ** * *

*
3. An underwater hydrocarbon production system as claimed in Claim 1 or Claim 2, in which the riser connector portion is provided at a lowermost portion of the upper riser assembly.

-22 -
4. An underwater hydrocarbon production system as claimed in any preceding claim, in which the riser connector portion is downwardly facing and the flowline connector portion is upwardly facing.
5. An underwater hydrocarbon production system as claimed in any preceding claim, in which the flowline connector portion includes a collet for receiving the riser connector portion.
6. An underwater hydrocarbon production system as claimed in Claim 5, in which the riser connector portion includes a ring that extends around the upstream end of the riser string.
7. An underwater hydrocarbon production system as claimed in any preceding claim, in which the base foundation and the lower riser assembly are articulated by means of an articulated joint.
8. An underwater hydrocarbon production system as claimed in any preceding claim, in which the production flowline is joined to the lower riser assembly at the flowline connector portion.

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9. An underwater hydrocarbon production system as claimed in any preceding * claim, in which the buoyancy module has a channel, the riser string extending through the channel and the channel having dimensions sufficient to allow passage of the riser string and the riser connector portion through the channel.
10. An underwater hydrocarbon production system comprising a submerged *:*. buoyancy module and a riser system, the riser system including a substantially vertical riser string, wherein: -the buoyancy module is engaged with an upper end of the riser system in order to tension the riser string; -a lower portion of the riser string terminates with a riser connector portion for connection to and disconnection from a flowline; and -23 - -the buoyancy module has a channel, the riser string extending through the channel and the channel having dimensions sufficient to allow passage of the riser string and the riser connector portion through the channel.
11. An underwater hydrocarbon production system as claimed in Claim 10, in which: -the riser system comprises an upper riser assembly and a lower riser assembly, the upper riser assembly comprising said substantially vertical riser string and said riser connector portion; -the riser system comprises a flowline for connection to a hydrocarbon well, and a base foundation for fixing the lower riser assembly to a seafloor; -the lower riser system comprises a flowline connector portion for connecting and disconnecting by means of the riser connector portion a downstream end of the flowline to the upstream end of the riser string; and -the base foundation and the flowline connector portion are articulated with respect to each other so that when the upper riser assembly is connected to the lower riser assembly relative movement between the upper riser assembly and the base foundation is accommodated.
12. An underwater hydrocarbon production system as claimed in Claim 10 or * I*.: Claim 11, in which the channel extends through a central portion of the buoyancy * ..* module. ***
13. An underwater hydrocarbon production system as claimed in Claim 12, in which the buoyancy module is substantially cylindrical. S. ** * S a * S

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14. An underwater hydrocarbon production system comprising a submerged buoyancy module and a riser system, the riser system including a substantially vertical riser string, wherein: -the buoyancy module is engaged with an upper end of the riser system in order to tension the riser string; -24 - -the buoyancy module has a channel, the riser string extending upwards through the channel to an upper portion of the buoyancy module; -an upper end of the riser string terminates at a first upper connector portion; -the buoyancy module supports a substantially vertically extending sleeve, said sleeve extending upwards to said upper portion of the buoyancy module; -an upper end of said sleeve terminates at a second upper connector portion; -the riser system includes a jumper pipe that extends between said first and second connector portions so that production flow from the riser system is directed towards said sleeve; and -the sleeve has at a lower end an opening for receiving an end of a flexible jumper pipe and within the sleeve means for connecting to said jumper pipe so that production flow from the riser system is conveyed into said flexible jumper pipe.
15. An underwater hydrocarbon production system as claimed in Claim 14, in which the substantially vertically extending sleeve is supported on a side surface of the buoyancy module.
16. An underwater hydrocarbon production system as claimed in Claim 14, in : which the substantially vertically extending sleeve is supported within the module. S... * .

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17. An underwater hydrocarbon production system as claimed in any of Claims 14 to 16, in which: -the riser system comprises an upper riser assembly and a lower riser .. assembly, the upper riser assembly comprising the substantially vertical riser *:*. string and the lower riser assembly comprising a flowline; -the buoyancy module is engaged with an upper end of the upper riser assembly in order to tension the riser string; -a lower portion of the riser string terminates with a riser connector portion for connection to and disconnection from said flowline; and -25 - -the buoyancy module has a channel, the riser string extending through the channel and the channel having dimensions sufficient to allow passage of the riser string and the riser connector portion through the channel.
18. An installation system for an underwater hydrocarbon production system, comprising an installation vessel and a submerged buoyancy module for tensioning a riser system, the buoyancy module having a channel therethrough for receiving a substantially vertical riser string, wherein: -the buoyancy module has means for controlling the buoyancy of the module so that the module may be heavier or lighter than the water displaced by the buoyancy module when submerged; -an upper end of the buoyancy module has a first mounting means for supporting the weight of buoyancy module when the module is heavier than said displaced water; -the installation vessel has a second mounting means for bearing said weight of buoyancy module; and -the installation system comprises additionally a suspension means that is connected to both the first and second mounting means to support said weight of the buoyancy module so that the buoyancy module is suspended beneath the installation vessel during installation of an underwater hydrocarbon production : system. * *
19. An installation system as claimed in Claim 18, in which the buoyancy module has a substantially vertically extending channel, and the installation vessel has a passage extending through a deck of said vessel for lowering a riser string into the water and through said channel in the buoyancy module. I. * * * * * S.
20. A method of installing an underwater hydrocarbon production system, the system being as claimed in any of Claims I to 9, wherein the method comprises the steps of: -fixing the base foundation to the floor of a body of water; -26 - -connecting the lower riser assembly to the base foundation so that the base foundation and the lower riser assembly are articulated; -connecting a production flowline to the flowline of the lower riser assembly; -submerging the buoyancy module in position above the lower riser assembly; -lowering the riser string including the riser connector portion towards the lower riser assembly; -connecting the riser connector portion to the flowline connector portion of the lower riser assembly; and -increasing the buoyancy of the buoyancy module and engaging the buoyancy module with the riser string in order to tension the riser string.
21. A method as claimed in Claim 20, in which the buoyancy module has a channel that extends substantially vertically through the buoyancy module, wherein the method comprises prior to connection of the riser connector portion to the flowline connector portion, the steps of: -lowering the riser string including the riser connector portion through the channel through the buoyancy module.
22. A method as claimed in Claim 21, in which the riser string has a load S..: shoulder, the method comprising prior to said step of increasing of the buoyancy of *** the buoyancy module, the steps of: -positioning the riser string so that the load shoulder is above the channel through the buoyancy module; and -allowing the buoyancy module to rise as the buoyancy is increased until the *:* buoyancy module comes into contact with the load shoulder and so tension the riser string.
23. An underwater hydrocarbon production system, substantially as herein described, with reference to or as shown in the accompanying drawings.

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24. An installation system for an underwater hydrocarbon production system, comprising an installation vessel and a submerged buoyancy module for tensioning a riser system, substantially as herein described, with reference to or as shown in the accompanying drawings.
25. A method of installing an underwater hydrocarbon production system, substantially as herein described, with reference to or as shown in the accompanying drawings. * * * ** * * . * ** S. * *. ** * . S * * * . * . S **