GB2475108A

GB2475108A - Methods of constructing and installing rigid riser structures and associated apparatus

Info

Publication number: GB2475108A
Application number: GB0921214A
Authority: GB
Inventors: Jean-Pierre Branchut
Original assignee: ACERGY US Inc; SUBSEA 7
Current assignee: ACERGY US Inc; SUBSEA 7
Priority date: 2009-11-05
Filing date: 2009-12-03
Publication date: 2011-05-11
Also published as: GB0921214D0

Abstract

The invention relates to a method of fabricating and installing a riser tower structure comprising the steps of installing a central core 210 and means 220 to tension the core, such that the central core extends from the seabed toward the surface and is suspended by the tensioning means at, or near the sea surface, thereby forming a central core structure. An elongate conduit 285 is then lowered on a line 290 and suspended from an upper part of the central core structure. The suspended elongate conduit is located into guide means 240 located at one or more points along the length of the central core. The previous steps are then repeated for successive conduits. Following installation of the elongate conduits, the method further includes lowering a flexible conduit (700 figure 7) from the surface to the top end of one of the elongate conduits, the flexible conduit thereby connecting a respective elongate conduit to a surface facility. This last step is repeatable for the other elongate conduits to connect all of them to a surface facility. The invention also relates to a buoyancy device that enables the step-by-step location of the elongate conduits, a guide frame mountable to the central core to guide the suspended elongate conduits and a pulley system to aid locating the conduits into the body of the guide frame.

Description

METHODS OF CONSTRUCTING AND INSTALLING RIGID RISER

STRUCTURES AND ASSOCIATED APPARATUS

This invention is in the general field of riser fabrication and installation, and in particular, fabrication and installation of Hybrid Riser Tower structures.

Hybrid Riser Towers are known and form part of the so-called hybrid riser, having an upper portions ("jumpers") made of flexible conduit and suitable for deep and ultra-deep water field development. US-A-6082391 (Stolt/Doris) proposes a particular Hybrid Riser Tower (HRT) consisting of an empty central core, supporting a bundle of (usually rigid) riser pipes, some used for oil production some used for injection of water, gas and/or other fluids, some others for oil and gas export. 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. Updated versions 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.

At present, Hybrid Riser Tower structures need to be fabricated close to the installation site, as the towing of an assembled Hybrid Riser Tower over significant distances carries with it many risks. In particular the surface waves and currents may result in significant fatigue damage to the structure. Also, the simple act of transporting such a large structure proposes logistical difficulties.

As a result of this, it is necessary to have a fabrication yard close to the installation site. Furthermore, the fabrication yard also requires a site having a long sheltered body of water directly in line with it, so that the Hybrid Riser Tower structure can be progressively fabricated and assembled. Such a suitable location is generally difficult to find.

Furthermore, even where such a fabrication yard exists close to the installation site, the towing of the competed tower structure as well as the upending process, in which the tower is installed in its operational, vertical configuration, places great stresses on the structure, and consequently incurs risks which include damage to the structure and even complete loss of the structure to the seabed.

Also, after a Hybrid Riser Tower has been installed, it is often difficult or impossible to replace a damaged riser while leaving the tower in place.

It is an aim of the present invention to address one or more of the above mentioned issues.

In a first aspect of the invention there is provided a method of fabricating and installing a riser tower structure, said method comprising the steps of: installing a central core structure comprising a central core rising from the seabed and having an upper end supported at a depth below the sea surface by a tensioning means; lowering an elongate conduit in a vertical configuration on a line from a surface vessel, and suspending it from an upper part of said central core structure; locating said suspended elongate conduit into each of one or more guide means located at one or more points along the length of the central core, repeating the lowering, suspending and locating steps for each elongate conduit; lowering a flexible conduit from said vessel and installing it to the top end of one of said elongate conduits, said flexible conduit for connecting its corresponding elongate conduit to a surface facility; and repeating the previous step for some or all of the other elongate conduits.

It should be noted that the above steps do not necessarily need to be performed in the above order, and where possible, this order may change. For example, the installation of the elongate conduits and the flexible conduits may be performed alternately or in another order.

Said central core structure may further comprise suspension means for suspending said elongate conduits.

Where said suspension means is located at a point vertically directly below said tensioning device, said tensioning device may be provided with at least one open vertical channel, said lowering and suspending step comprising: locating said line in one of said channels, so as to allow the elongate conduit to be manoeuvred into position onto said suspension means without said tensioning device obstructing said line.

Said connecting of said flexible conduit to its corresponding elongate conduit may be performed by suspending said flexible conduit via a line from said vessel, said method including locating said line in one said channels in said tensioning device so as to allow the flexible conduit to be manoeuvred into position and connected to said elongate conduit without said tensioning device obstructing said line. The same channel may be used during installation of an elongate conduit and the corresponding flexible conduit to which it will be connected. Said flexible conduits may be connected to said elongate conduits via a remote connector of a type connectable without the need for diver intervention. Said connection may be made using a running tool with a remotely operated vehicle.

Alternatively, said elongate and flexible conduits may be manoeuvred into their respective positions and installed while a force is applied to said tensioning device, so as to prevent said tensioning device from obstructing the line suspending said elongate and flexible conduits. Said force may be applied by two vessels pulling onto said tensioning device in opposite directions, one vessel applying tension to an upper part of said tensioning device, the other vessel applying tension to a lower section of said tensioning device.

Said guide means may comprise a plurality of closable recesses for the securing therein of each elongate conduit.

Said elongate conduits may be located into said one or more guide means by attaching a line to said elongate conduit in the region of said guide means and applying to said elongate conduit, using said line, a force towards and into an open recess. Said guide means may further comprise pulley means between each of said closable recesses and said central core, said method comprising applying said force to said elongate conduit via said pulley.

Said suspension means may further comprise docking means for docking said flexible conduit substantially in position while performing said connection step, said method further comprising the step of docking said flexible conduit into said docking means prior to performing the connection step.

Said central core may be attached to the seabed by a Rotolatch (RTM) connection to the top of a suction pile (as described in previously mentioned patent US-A-6082391).

Said locating step may further comprise securing said elongate conduit into place at each of said guide means.

In a further aspect of the invention there is provided a buoyancy device, said buoyancy device being provided with at least one open vertical channel along its length, such that a winch line may be located in the channel and pass through it.

A plurality of channels may be provided.

In a further aspect of the invention there is provided a guide frame for location at one or more points along the length of a riser tower structure of a type having an upper end supported at a depth below the sea surface and comprising a central core and one or more conduits extending from the seabed toward the surface, said conduit(s) being arranged around said central core, such that in use, said guide frame guides the conduit(s) relative to the central core, wherein said guide frame comprises pulley means for pulling said conduits into said guide frame.

Said guide frame may comprise closable recesses for the guiding of said conduits wherein a pulley is provided for each of said recesses said pulleys each being located between the central core and its corresponding recess, when part of said riser structure.

In a further aspect of the invention there is provided a riser tower structure comprising a plurality of elongate conduits arranged around a central core, said central core being suspended by a tensioning means at, or relatively near to, the sea surface, wherein said tensioning means comprises a buoyancy device having a plurality of open vertical channels along its length, such that a winch line may be located in the channel and pass through it; there being provided one of said channels for each of said elongate conduits, so as to enable the installation of said elongate conduits via said line, subsequent to installation of said central core and buoyancy device, each of said channels enabling a corresponding elongate conduit to be installed into a position directly below said buoyancy device.

Said riser structure may further comprise the guide frame of the previously described aspect of the invention.

Said riser tower structure may further comprise flexible conduits for connecting the top end of each of said elongate conduits to a surface facility, wherein said flexible conduits are connected to said elongate conduits via a remote connector of a type operable such that connection is made without the need for diver intervention. Said connectors may be of a type operable such that connection is made using a running tool.

In a further aspect of the invention there is provided a riser tower structure comprising a plurality of elongate conduits arranged around a central core, said central core being suspended by a tensioning means at, or relatively near to, the sea surface; and flexible conduits for connecting the top end of each of said elongate conduits to a surface facility, wherein said flexible conduits are connected to said elongate conduits via a remote connector of a type operable such that connection is made without the need for diver intervention.

Said connectors may be of a type operable such that connection is made using a running tool, A method of removing and replacing an elongate conduit from a riser tower structure of the type comprising a plurality of elongate conduits arranged around a central core, said central core being suspended by a tensioning means at, or relatively near to, the sea surface, said method comprising: disconnecting the flexible conduit that is connected to the elongate object being replaced; attaching a line from a winch on a vessel to the elongate object to being replaced and manoeuvring the elongate object clear of the tower structure; recovering said elongate object; lowering a replacement elongate conduit, in a vertical configuration, from a surface vessel, and installing it in place of the elongate object being replaced; locating said replacement elongate conduit into one or more guide means located at one or more points along the length of the central core, and securing into place; and installing a flexible conduit to the top end of the replacement elongate conduit Said replacement elongate conduit may be the same as the elongate conduit removed for repair, after said repair, or else it may be a new elongate conduit.

Equally the flexible conduit disconnected in the first step may be the same flexible conduits connected in the final step, or a different flexible conduit, as required.

Said tensioning device may be provided with at least one open vertical channel, wherein said attaching a line and manoeuvring the elongate conduit to be repaired step and said lowering and suspending step each comprises locating a line that is suspending said elongate conduit from said vessel, in one of said channels, so as to allow the elongate conduit to be manoeuvred into or out of position at a position directly below said tensioning device, without the tensioning device obstructing said line.

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: Fig. 1 shows a known type of hybrid riser structure in an offshore oil production system; Fig. 2 shows a first stage of an installation method according to an embodiment of the invention; Fig. 3 shows a detail of the top end of the risers during the step depicted in Fig. 2; Fig. 4 shows a further step of the installation method according to an embodiment of the invention wherein a first riser is locked in place; Fig. 5 shows an optional step of the riser installation method according to an embodiment of the invention showing the assisted docking of a riser into a guide frame; Fig. 6 shows the lower end of the hybrid riser tower structure, having been installed according to an embodiment of the invention with single riser docked into place; Fig. 7 shows an additional step of a method according to an embodiment of the invention showing the docking of a jumper conduit to the installed riser conduit; Fig. 8 shows the upper part of the hybrid riser tower structure being installed according to an embodiment of the invention having two risers and two flexible jumpers in place; Fig. 9 shows a close up detail of one of the jumper connections depicted in Fig. 8; and, Fig. 10 shows a method of replacing a riser according to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to Figure 1, the person skilled in the art will recognise 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 gas, injection water 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 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. At 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 floating production unit (FPU) 128 is moored by means not shown, or otherwise held in place at the surface. FPU 128 provides production facilities, storage and accommodation for the fluids from and to the wells 100 to 108. FPU 128 is connected to the risers by flexible flow lines 132 etc. arranged in a catenary configuration, for the transfer of fluids between the FPU and the seabed, via riser towers 112 and 114.

Individual pipelines may be required not only for hydrocarbons produced from the seabed wells, but also for various auxiliary 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 are grouped in "bundles", and the riser towers 112, and 114 in this embodiment comprise each one a bundle of conduits for production fluids, lifting gas, water and gas injection, oil and gas export, and treatment chemicals, e.g. methanol. All the component conduits of each bundle are arranged around a central core, and are held in place relative to each other (in the two lateral dimensions, longitudinal movement not being prevented) by guide frames attached to the central core.

Individual sections of riser tower structures, or bundles are conventionally fabricated such that individual sections of pipe, umbilicals, etc. are made and arranged around similar length sections of central core, the pipes and umbilicals being held in place around the core by one or more guide frames. As such, each bundle section is simply a short version of the whole riser structure, having the same cross section, such that the whole riser tower structure can be assembled by assembling together similar bundle sections, end on end (The top and bottom bundle sections will differ slightly in that they will have provisions for attachment to a top buoyancy module or anchor, as appropriate). This assembly is conventionally done as each section is fabricated, each section then being attached to the main riser tower structure extending out from the fabrication yard towards the nearby installation site.

The above conventional construction and installation method for hybrid riser towers has a number of drawbacks. First of all the completed structure must either be towed long distances, which results in huge logistical problems or a suitable fabrication site has to be found very close to the installation site. However, for a fabrication site to be suitable, in needs to be very long, with a long sheltered body of water directly in line with it, which is difficult to find.

In order to address these issues, it is proposed to assemble the riser tower vertically, at the installation site.

Fig. 2 shows an initial step of such a method. It shows a riser tower core structure having already been installed in its vertical configuration. It comprises a central core 210, a buoyancy tank 220, riser hang-off platforms 230, riser guide frames 240, and a top funnel of the Rotolatch (RTM) connector system 250. It can be seen that the buoyancy tank 220 has a plurality of open vertical channels 260, while the hang off platforms 230 have in total a corresponding amount of hang off slots 270.

Similarly, each of the guide frames 240, (only one is shown) also have a corresponding number of guide slots 280. Also shown is a first riser 285, being suspended from a line or wire 290 from an abandonment and recovery (A&R) winch located on an installation vessel from which the riser 285 has been lowered from.

The core pipe 210 has been manufactured from core pipe sections on board the vessel, after which all the ancillaries such as guide frames 240 and hang off platforms 230 are attached. The buoyancy tank 220 is then attached, so as to complete the riser tower core structure 200. This riser tower core structure 200, is then deployed by pulling at the bottom of the core pipe to engage it into a Rotolatch receptacle located at the top of the suction anchor. This riser tower core structure is then anchored to the seabed and the water in the buoyancy tank 220 is evacuated thereby enabling it to apply its upward tensioning force on the central core 210. The buoyancy tank 220 is attached to the top of the riser tower core structure 200 by the same method as the bottom is attached to the suction pile, that is by via Rotolatch receptacle. It should be noted that little rotation (around a vertical axis) is allowed by this system.

Fig. 3 shows a close up of the top assembly just prior to the first riser 285, being deployed. As well as the features described previously, this figure also shows, on the hang off platform 240, the jumper docking device 310, and hang off slot closing clamp 320.

In order to deploy the riser 285, it is lowered on wire 290 from a vessel, until the top of the riser is lower than the buoyancy tank 220 and in the region of the hang off platform 240. The riser 285 can then be manoeuvred towards the riser hang off platform 240, and into the hang off slot 280, by locating the A & R winch wire 290 into the appropriate channel 260 of the buoyancy tank 220. It is the channel 260 that allows the riser 285 to be positioned at a point under the buoyancy tank 220 without the tank 220 obstructing the line 290.

It should be noted that there is a buoyancy channel 220 for each of the hang off slots 280, and therefore for each riser, assuming that they are all used.

Fig 4 shows the riser 285, having been docked to the hang off slot 280, and locked into place by closing clamp 320. The riser is now suspended from both the hang off platform 240 and A & R winch wire 290. As can the seen, the A & R winch wire 290 now passes through channel 260 in the buoyancy tank 220, through which it is able to move vertically. With the riser 285 docked in this manner, and having been successfully located into the correct guide slots 280 in each guide frame 240 along the length of the core 210, then each one of the corresponding guide frame closing clamps 500 can be locked into place to secure the riser 285.

In many cases, the riser 285 will need assistance in being successfully docked into the guide frame slot 280. In particular, this may be the case where the riser tower core structure 200 is not actually vertical. It should be appreciated that, while riser tower structures of this type are essentially vertical (in comparison with other riser designs, such as catenary risers), they will rarely, if ever, be at true vertical due to the sea currents and stresses, and the fact that the riser is a very tall and very slender structure.

In order to assist with this docking, a number of pulleys 510 are located on the guide frame 240, one for each guide slot 280. A further line 520 tied to the riser 285, in the vicinity of the guide frame 240, can then be pulled around this pulley 510 by a winch on, for example, a remotely operated vehicle (ROV) so as to pull the riser 285 into the slot 280. Such an ROV can also be used to close closing clamps 500, as well as the hang off closing clamps 320. Fig. 5 shows this operational step in action.

Fig. 6 shows the lower end of the properly docked riser 285. This shows the bottom guide means 240, with riser 285, locked into place by closing clamp 500. Also shown is a special docking port 600, which allows the MATIS (RTM) remote bolted flange connection apparatus to be docked and used to connect the bottom end of the riser 285, to a well head or similar. This connection apparatus is described in PCT publication W003/040682 which is incorporated herein by reference.

Fig. 7 shows the installation of the flexible connectors or jumpers 700 which connect the top of the risers 285, to a surface installation, such as an FSPO or platform. This shows, in two views, the jumper 700 attached to A & R winch wire 290. In this regard the jumper 700 is deployed in the same way as the riser 285, being lowered from a vessel until it is in the vicinity of the top of the riser 285, whereupon the A & R winch wire is introduced into the channel 260 of the buoyancy tank 210. The A & R winch is then further lowered until abutments 720 on the jumper 700 are successfully docked into the docking slots 310 on the hang off platform 240. This positions the jumper connectors 710 in line with the top of the riser 285 so that the connection can be made.

The connectors used may be diverless, non self-installable, connectors which are connected using a running tool being operated by a remotely operated vehicle (ROy).

Fig. 8 shows this top arrangement with jumpers in place in both the upper and lower hang off platforms 240.

Fig. 9 shows a close up of the hang off platform 240, with one jumper 700 and one riser 285 successfully docked and connected together via connector 710.

There are a number of advantages of the methods disclosed herein over and above the conventional installation methods for these types of risers. They allow the construction of the riser tower offshore without the need of a huge onshore construction site nearby. Furthermore the methods negate the need to withstand the towing and up-ending stresses incurred in a conventional installation. Further still, installation of the risers can be performed directly from a construction barge, with all construction and installation performed on/from this single barge. The disclosed methods also allow connection of the jumpers with direct docking to the hanging platform which supports the risers, this docking being done using a normal well proven procedure. The jumper connector can also be installed using a "running tool", which means cheaper, non self-installable connectors can be used.

One other feature that this method and particular HRT arrangement allows is the possibility of removing and replacing risers, which can be particularly useful when a riser is damaged, for example. This is normally difficult with conventional HRTs, due to the buoyancy tank not allowing access to the riser for an A & R winch line.

Fig. 10 illustrates the riser changeover procedure. The A & R winch line 290 is lowered from the vessel through the appropriate channel 260 in the buoyancy tank 220 and is attached to the appropriate jumper 710b. This can be lifted and then manoeuvred clear of the HRT structure 200, by manoeuvring the wire 290 out of the channel 260. This jumper 710b can then be recovered. The line 290, can then be attached to the top of the riser 285b, that is to be replaced. The closing clamp 320, can then be unlocked (by an ROy, for example) and the wire and riser manoeuvred outwards, so that the riser is clear of the hang off slot 270 and the wire 290 clear of the channel 260. The damaged riser 285b can then be recovered.

A new riser 285a, can then be installed using the installation method already described, as can a new flexible jumper 71 Oa, (or else the same flexible jumper can be used).

The above embodiments are for illustration only and other embodiments and variations are possible and envisaged without departing from the spirit and scope of the invention. For example, the riser arrangements depicted are simply for illustration and may be varied, including provision of less or more conduits than shown. The steps of any methods described may also be performed in orders different to those described where possible, while remaining within the scope of the invention.

Claims

Claims 1. a method of fabricating and installing a riser tower structure, said method comprising the steps of: installing a central core structure comprising a central core rising from the seabed and having an upper end supported at a depth below the sea surface by a tensioning device; lowering, on a line from a surface vessel and in a vertical configuration, an elongate conduit, and suspending it from an upper part of said central core structure; locating said suspended elongate conduit into each of one or more guide means located at one or more points along the length of the central core, repeating the lowering, suspending and locating steps for each elongate conduit; lowering a flexible conduit from said vessel and installing it to the top end of one of said elongate conduits, said flexible conduit for connecting its corresponding elongate conduit to a surface facility; and repeating the previous step for some or all of the other elongate conduits.
2. A method as claimed in claim I wherein said central core structure comprises suspension means for suspending said elongate conduits.
3. A method as claimed in claim 2 wherein said suspension means is located at a point vertically directly below said tensioning device, said tensioning device being provided with at least one open vertical channel, said lowering and suspending step comprising: locating said line in one of said channels, so as to allow the elongate conduit to be manoeuvred into position onto said suspension means without said tensioning device obstructing said line.
4. A method as claimed in claim 3 wherein said connecting of said flexible conduit to its corresponding elongate conduit is performed by suspending said flexible conduit via a line from said vessel, said method including locating said line in one said channels in said tensioning device so as to allow the flexible conduit to be manoeuvred into position and connected to said elongate conduit without said tensioning device obstructing said line.
5. A method as claimed in claim 4 wherein the same channel is used during for location of said line during installation of an elongate conduit and the corresponding flexible conduit to which it will be connected.
6. A method as claimed in claim 2 wherein said suspension means is located at a point vertically directly below said tensioning device and said elongate and flexible conduits are manoeuvred into their respective positions and installed while a force is applied to said tensioning device, so as to prevent said tensioning device from obstructing the line suspending said elongate and flexible conduits.
7. A method as claimed in claim 6 wherein said force is applied by two vessels pulling onto said tensioning device in opposite directions, one vessel applying tension to an upper part of said tensioning device, the other vessel applying tension to a lower section of said tensioning device.
8. A method as claimed in any of claims 2 to 7 wherein said suspension means comprise docking means for docking said flexible conduit substantially in position while performing said connection step, said method further comprising the step of docking said flexible conduit into said docking means prior to performing the connection step.
9. A method as claimed in any preceding claim wherein said flexible conduits are connected to said elongate conduits via a remote connector of a type connectable without the need for diver intervention.
10. A method as claimed in claim 9 wherein said connection is made using a running tool with a remotely operated vehicle.
11. A method as claimed in any preceding claim wherein said guide means comprises a plurality of closable recesses for the securing therein of each elongate conduit.
12. A method as claimed in claim 11 wherein said elongate conduits are located into said one or more guide means by attaching a line to said elongate conduit in the region of said guide means and using said line to apply a force to said elongate conduit, towards and into an open recess.
13. A method as claimed in claim 12 wherein said guide means further comprises pulley means between each of said closable recesses and said central core, said method comprising applying said force to said elongate conduit via said line and pulley.
14. A method as claimed in any preceding claim wherein said central core is attached to the seabed by a Rotolatch (RTM) connection to the top of a suction pile.
15. A method as claimed in any preceding claim wherein said locating step comprises securing said elongate conduit into place at each of said guide means.
16. A buoyancy device comprising at least one open vertical channel along its length, such that a winch line may be located in the channel and pass through it.
17. A buoyancy device as claimed in claim 16 comprising a plurality of channels.
18. A guide frame for location at one or more points along the length of a riser tower structure of a type having an upper end supported at a depth below the sea surface and comprising a central core and one or more conduits extending from the seabed toward the surface, said conduit(s) being arranged around said central core, such that in use, said guide frame guides the conduit(s) relative to the central core, wherein said guide frame comprises pulley means for pulling said conduits into said guide frame.
19. A guide frame as claimed in claim 18 comprising closable recesses for the guiding of said conduits, wherein a pulley is provided for each of said recesses, said pulleys each being located between the central core and its corresponding recess, when part of said riser structure.
20. A riser tower structure comprising a plurality of elongate conduits arranged around a central core, said central core being suspended by a tensioning means at, or relatively near to, the sea surface, wherein said tensioning means comprises a buoyancy device having a plurality of open vertical channels along its length, such that a winch line is located in the channel and pass through it; there being provided one of said channels for each of said elongate conduits, so as to enable the installation of said elongate conduits via said line, subsequent to installation of said central core and buoyancy device, each of said channels enabling a corresponding elongate conduit to be installed into a position directly below said buoyancy device.
21. A riser structure as claimed in claim 20 comprising a guide frame as claimed in claim 18 or 19.
22. A riser structure as claimed in claim 20 or 21 comprising flexible conduits for connecting the top end of each of said elongate conduits to a surface facility, wherein said flexible conduits are connected to said elongate conduits via a remote connector of a type operable such that connection is made without the need for diver intervention.
23. A riser structure as claimed in claim 22 wherein said connectors are of a type being operable such that connection is made using a running tool.
24. A riser tower structure comprising a plurality of elongate conduits arranged around a central core, said central core being suspended by a tensioning means at, or relatively near to, the sea surface; and flexible conduits for connecting the top end of each of said elongate conduits to a surface facility, wherein said flexible conduits are connected to said elongate conduits via a remote connector of a type operable such that connection is made without the need for diver intervention.
25. A riser structure as claimed in claim 24 wherein said connectors are of a type being operable such that connection is made using a running tool.
26. A method of removing and replacing an elongate conduit from a riser tower structure of the type comprising a plurality of elongate conduits arranged around a central core, said central core being suspended by a tensioning means at, or relatively near to, the sea surface, said method comprising: disconnecting the flexible conduit that is connected to the elongate object being replaced; attaching a line from a winch on a vessel to the elongate object to being replaced and manoeuvring the elongate object clear of the tower structure; recovering said elongate object; lowering a replacement elongate conduit, in a vertical configuration, from a surface vessel, and installing it in place of the elongate object being replaced; locating said replacement elongate conduit into one or more guide means located at one or more points along the length of the central core, and securing into place; and installing a flexible conduit to the top end of the replacement elongate conduit
27. A method as claimed in claim 26 wherein said replacement elongate conduit is the same as the elongate conduit removed for repair, having been repaired.
28. A method as claimed in claim 26 wherein said replacement elongate conduit is a new elongate conduit.
29. A method as claimed in claim 26, 27 or 28, said tensioning device being provided with at least one open vertical channel, wherein said attaching a line and manoeuvring the elongate conduit to be repaired step and said lowering and suspending step each comprises locating a line that is suspending said elongate conduit from said vessel, in one of said channels, so as to allow the elongate conduit to be manoeuvred into or out of position at a position directly below said tensioning device, without the tensioning device obstructing said line.