CA1163552A - Self-standing marine riser - Google Patents
Self-standing marine riserInfo
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- CA1163552A CA1163552A CA000376111A CA376111A CA1163552A CA 1163552 A CA1163552 A CA 1163552A CA 000376111 A CA000376111 A CA 000376111A CA 376111 A CA376111 A CA 376111A CA 1163552 A CA1163552 A CA 1163552A
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- riser
- column
- chains
- base
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Abstract
A B S T R A C T
"Self-standing marine riser"
A self standing marine riser is provided which comprises a base, a riser column, a flexible joint between the base and the riser column, and means for providing a loose coupling between the top of the riser column and a vessel, rig or platform on the surface above the location of the riser. The riser column comprises an upper column section which includes at least one buoyancy chamber, and a lower, relatively slender column section. The riser includes, or is adapted to support, at least one conduit for the conveyance of a fluid (e.g. oil or gas) or a control line. The buoyancy provided by the upper section of the riser column is preferably variable, and this facilitates the connection and use of the riser. The riser may be used for drilling operations or for production operations. By employing a riser in accordance with the invention, is is not necessary to use large riser tensions in order to maintain the position and structural integrity of the riser in deep water and rough weather.
"Self-standing marine riser"
A self standing marine riser is provided which comprises a base, a riser column, a flexible joint between the base and the riser column, and means for providing a loose coupling between the top of the riser column and a vessel, rig or platform on the surface above the location of the riser. The riser column comprises an upper column section which includes at least one buoyancy chamber, and a lower, relatively slender column section. The riser includes, or is adapted to support, at least one conduit for the conveyance of a fluid (e.g. oil or gas) or a control line. The buoyancy provided by the upper section of the riser column is preferably variable, and this facilitates the connection and use of the riser. The riser may be used for drilling operations or for production operations. By employing a riser in accordance with the invention, is is not necessary to use large riser tensions in order to maintain the position and structural integrity of the riser in deep water and rough weather.
Description
t 1 63~5~
"Self-standing marine riser"
.. _ . . .
This invention relates to a self-standing marine riser suitable for use in drilling, and in serni submersible production operations and with a dynamically positioned oil/
gas production ship, a chain moored ship with a spindle or with a tension leg platform.
Various aspects of the invention are as follows:
A self-standing marine riser which comprises a base, a riser column, a flexible joint between the base and the riser column, and a plurality of chains attached to the top of the riser column for providing a loose coupling between the top of the riser column and a structure selected from the group consis-ting of vessels, rigs and platforms on the surface above the location of the riser, wherein (1) the riser column comprises a lower, relatively slender column section and an upper column section which includes at least one buoyancy chamber, the buoyancy of which is variable, and (2) the riser supports at least one conduit for the conveyance of a fluid.
A self-standing marine riser which comprises a base which is adapted to be mounted above a sea bottom drill-ing template, a riser column, a flexible joint between the base and the riser column, and a plurality of chains attach~
ed to the top of the riser column for providing a loose coupling between the top of the riser column and a struc-ture selected from the group consisting of vessels, rigs and platforms on the surface above the location of the riser, wherein (1) the riser column comprises a lower relatively slender column section and an upper column section which includes at least one buoyancy chamber the ~r~
-la-buoyancy of which is variable and is such as to enable neutral buoyancy to be achieved for the riser; (2) the riser supports at least one conduit for the conveyance of a fluid;
and (3) said plurality of chains are attached to a plate which carries a plurality of sockets which are positioned and shaped so as to mate with a corresponding number of posts formed on the top of the upper column section of the riser column to facilitate quick release of the chains from the riser.
The fluid can be oil, gas, water, or drilling mud.
Optionally there may be provided a conduit for conveying solid objects, such as tools, from the top of the riser to the base. One or more control lines (e.g. electrical or hydraulic lines) may be housed in the or one of the con-duits. A fle~ible joint may be provided at the top of the riser column between the column itself and a riser bundle connecting with the surface structure and through which the conduit for the conveyance of a fluid passes.
As previously indicated, the buoyancy provided by the upper column section should be variable. This arrange--~ 1 ~3~5~
ment ~acilitates the emplacement of the riser and its loose coupling to a surface vessel or structure. The buoyancy chamber(s) are advantageously such as to enable neutral buoyancy to be achieved; in preferred embodi-ments of the invention, positi~e and/or neutral buoyancycan be achieved for the riser alone, submerged and unattached to the surface vessel, or for the combination of the riser and the means by which it is loose coupled to the surface vessel or structure, or when coupled to the sur~ace vessel or structure. This loose coupling is advantageously effected by chains which are removably attached to the top of the riser column. Conveniently there can be four such chains when the riser is secured to a semisubmersible rig or platform, If the riser is secured to a ship, there may be two or four such chains attached to a swivelling ring to allow for azimuth varia-tions.
The riser can include one or more pipes attached to the exterior of the riser column at least in the region of the lower section thereof. One or more flexible hoses may be provided at the lower end o~ the riser to connect the or each of said pipes to a ~e~head or a production outlet at, or in the vicinity of, the base of the riser, The base itself can be a gravity base or a piled base. Generally, the base will be aligned next to or pvsitioned aroùnd a sea bottom drilling template. If t~e base is a piled base, it is preferable to install the base at the same time as the drilling production template, for reasons of wellhead safety. The riser column and flexible joint may then be linked to the base with a connector on completion of the well drilling.
The riser is self-standing and buoyant when stand-ing alone and submerged. Two or more wirelines are prefer-ably attached between the upper column section a~d thebase. These initially are used as guideline wires to emplace the riser and connect it to the base; subseque~tly .
.
~ ~ 635S2 when terminated and fastened to the upper section of the riser after its emplacement and attachment to inter-mediary points and to the base they act as safety wires to avoid accident should the riser break, ~erving to prevent the riser accelerating to the sea surface. The necessary slack in the w~res to allow freedom of riser column angular movement is advantageously taken up by weighted lever de~ices at the base. In preferred embodi-ments the riser is chained by four chains to a semi-submersible structure through pontoon mounted fairleads orto a ship either through spaced hull-mounted ~airleads (when the ship is dynamically positioned over the riser) or in the case of a spindle-moored ship through fairleads which form part of the spindle structure. Each chain may be connected to the riser via a swivel chain fastener or fasteners that are part of a swivelling ring located about the riser top. The first arrangement is preferred for connection to semisubmersibles and to spindle-moored vesssls while the second is preferable for connection to ship-shaped surface structures which are dynamically positionedand must weathervane. Preferably, the chains can be "quick released" from the riser, the action necessary to achieve this being effected on the semi-submersible platform or o~ the vessel to which the riser is loose coupled.
The conduits for conveying fluids, e.g. oil or gas, along the riser can terminate in a submerged riser top manifoldtstabbing block. The connection between the manifold/stabbing block and the production facility should be of the quic~:-release type, so that in an emergency the well(s) may be shut-in, after whi~h the connectio~ to the riser conduit~s) may be "~uick-released" from the top of the submerged riser, followed by "quick release" of the chains from the submerged riser top. The connection from the surface vessel to the riser manifold/stabbing block can be in the form of a flexible riser bundle. ~hen the self-standing riser is emplaced, it will ~enerally be fully -"` i ;~ 6~5~
9~ .
submerged. ~ith an arrangement such as that just described, the tensioned riser bundle will be supported by tensioners which compensate for vessel motion, draught and changes in sea level.
With the preferred ~tructure described above~ i~
the self-standing production riser should break loose while still connected to a semi-submersible or vessel production facility, the riser will not loat to the surface because of the restraining safety wires and the weigh-t imposed by the catenary chains.
In general, the riser will maintain a substantially upright configuration. ~ovement of the vessel, rig or platform to which it is loose coupled as well as tidal and current effects may result in the riser moving away from the vertical. The riser will accommodate a maximum of 15 tilt from the vertical, but it is preferred that the riser should not deviate from the vertical by more than 10 . Under normal operating conditions, the maximum in-clination of the riser is expected to be about 7 or less from the vertical.
A riser constructed in accordance with the present invention does not require a complicated tensioning system to hold it in place. The buoyancy and stiffness pro-vided in the submerged riser mean that the riser is not subjected to stresses as severe as those normally associated with an equivalent length tensioned riser.
Furthermore, the design is such that if the riser breaks fre~e at the bottom while connected to a production facility, it will not inevitably come to the surface and/or collide with th`e production facility. Also, i~
a piled or gravity base is employed, it can straddle the well; head template (without contacting it) thereby providing protection for the well "trees".
For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:
~ :~ 6355~
FIGURE 1 shows a front view of a sel~-standing production riser in accordance with the inve~tion loosely coupled to a semi-submersible production plat~orm;
FIGUR~ 2 shows a side view of the apparatus of Figure l;
FIGURE 3 shows a ~ront view of the top of a self-standing production riser in accordance with the invention loosely coupled to a production/storage ship-shaped vessel;
~IGURE 4 shows a front view of the top of riser in accordance with the in~ention loosely coupled to a dynamically assisted ship or barge having a turret/chain mooring arrangement;
FIGURE 5 is a schematic plan view of the arrangement shown in Figure 4; and FIGURE 6 illus-trates one method of connecting a sur-face structure and the marine riser.
Referring now to the drawings, the riser 1 shown in Figures 1 and 2 is loosely coupled to a semi-submersible production platform 2 via a plurality of chains 3. There are four such chains in the embodiment illustrated in the drawings. As illustrated, the chains are attached to the inboard area of the pontoon; alternatively they may be attached to the outboard area. The riser 1 comprises a piled or gravity base 4~ e.g. a piled steel base which can have two basic configurations. In the first, it is mounted over but is not in contact with a circular well-head template (not shown). In the second 7 the base is connected to one end or to the middle of a rectangular or square wellhead template. The circular template can accommodate ten wells with one spare slot in its presently envisaged form. The number of wells which can be accommo-dated depends on the capability of the riser and manifold system to handle the fluids. In the circular wellhead template, the production trees are protected by the base 4.
The riser may also be connected to a satellite production tree or trees or a separate manifold well template adjacent J ~ ~3552 the base 4, as indicated by line 5 in Figure 1.
The riser column comprises a lower slender part 6 connected to the base 4 by a universal, ball or flex joint 7. Pipe conduits 8 are mounted on the outside of lower section 6 of the riser column, ~ach o~ conduits 8 is connected at its lower end to a flexible hose 9 which in turn is connected to the well production tree 10.
The lower part 6 of the riser column occupies the greater proportion of the total length of the riser. The upper portion 11 of the riser column includes both a fixed and a variable buoyancy system. Conduits 8 pass through the interior of upper riser column section 11, At the top of the riser column, there is a riser top manifold/stabbing block 12 by means of which a flexible riser bundle 13 may be connected to the upper termination of conduits 8.
The length of each of chains 3 is adiustable. Under normal operating conditions, each chain will generally have substantially the same length. The connection be-tween the chains 3 and riser 1 is effected at swivelling chain fasteners 14 which are attached to the outside of upper riser column section 11 at the top part thereof. The length of each chain catenary between connectors 14 and the pontoon ~airleads of the production platform 2 will normally be in the range from 20 to 60 metres, preferably about 45 met~es; the length may occasion-ally be as little as 10 metres. The loose chain con-nections may be made either to the insides or to the outsides of the pontoons, and the chain will generally run through fairleads whose positions are such as to afford the optimum scope ratio for control of the sub-merged riser, The scope ratio will depend on environ-men-tal conditions, rig layout, depth of the riser top below sea level and pontoon depth for an optimum operation, Referring now to Figuxe 3, there is shown a mooring arrangement suitable for use when a self-standing pro--- ~ 1 63552 duction riser in accordance with the present invention is loosely coupled to a dynamically-assisted vessel, i.e.
a ship or barge whose mooring position is maintained with ~ynamic assistance. The two chains 3 are attached to the upper section 11 o~ the riser column at a slewing ring 16 which is fi*ted about the top part of column .
section 11. The chains 3 pass over chain sheaves 17 which pre~erably can be raised or lowered by a predeter-mined amount in order to adjust the vertical/horizontal chain catenary ratio to the optimum ~or any given cir-cumstance. The mooring chains -then pass upwardly into chain tubes 18 within the vessel 20. Alternatively, if four chains 3 are employed, there may be four chain tubes 18 positioned on the outside of the hull of the~vessel.
The ~lexible riser bundle 13 passes through a moonpool 19 and terminates at a fluid swivel 21 to which tensioners 22 are connected via cables 23. A guide frame 24 holds fluid swivel 21 in position in a horizontal plane, and also ~unctions to rotate it.
Referri~g now to Figures 4 and 5, an arrangement is shown for connecting a free-standing marine riser in accordance with this invention to a vessel having a turret/chain mooring arrangement. In this case, the mooring chain 3 can be connected to the top of riser section 11 either by two, three or four swivelling chain fasteners or by chain ~asteners which are part of a slewing ring attached to the outside of upper riser sectior, 11. The arrangement illustrated in Figure 4 shows the first of these two possibilities, there being two swivelling chain faste~ers 14 attached to the out-side of upper riser column section ll. The choice between these two possible confi~urations will be decided according to the method of equipment installation relative to acceptable weather conditions, The outboard ends of mooring chains 3 are connected to wires 30 which pass over fairleads 31 held by spreader arms 32. The length of each chain catenary between fasteners 14 and the first of the iairl~ads ~l will generally be about 23 to 27 I ~ ~3~5~
metres in the present~y pre~erred arrangement. The spreader arms 32 are structurally connected to a cylindrical body 34 ~orming part o~ the vessel 33, the interior of body 34 constituting a spindle or turret. This turret also houses winches and mooring line equipment ~not shown) and anchoring windlasses one o~ which is shown at 35. When the mooring lines are in place, the turret 34 remains on a consistent heading while the vessel itself can weathervane about the turret.
The flexible riser bundle 13 passes through turret 34 and terminates at a multi-fluids swivel 21a mounted above the vessel deck. This swivel is held in a gimballing table guided by frame 24 attached to riser tensioning wires 23 which terminate in tensioning means, such as pneumatic or hydraulic tensioners or weights 22.
The schematic arrangement shown in ~igure 5 illustrates the positionin~ of four double-drum mooring winches or windlasses (MW) mounted on top of turntable 36 which is, in effect, the topmost part of turret body 34.
The riser 1 can be used in deep water conditions, for example at depths of 90 metres ~300 feet) or greater.
An emergency release system (not shown) is provided to enable chains 3 to be separated ~rom riser 1 quickly.
The system can comprise a wire attached to a locking arm which, when the wire is pulled taut, will cause a locking pin holding a respective chain to connector 14 to shear and allow the chain to fall free of the riser.
The loose coupling between riser 1 and platform shown in Figures 1 and 2 may be effected as follows.
Initially, the buoyancy of the riser is adjuste~ so that it is slightly positive. With the riser in this con-dition, the semisubmers~ble is moored with its moonpool centered over the riser. When all is ready for effecting the connection, the buoyancy in upper section 11 of the riser column is increased and the chains 3 are lowered from the semisubmersible for connection to the top of riser 1.
This can be done by attaching strayline wires to a point a given number of links above the hanging chain ends, and ~ ~ ~35~2 _ 9 _ paying out the chains as the wires are pulled towards the moonpool. The end links or shackles of the chains will be locked into the riser swivelling chain fasteners 14, opposing chains preferably being connected simultaneously.
The strayline wires may then be let out and de-tached from the chain; they can later be used as guidelines for guiding the riser sections from the surface to the stab-bing manifold block at the top of the submerged riser.
Next, the chains will be tightened to give the desired catenary chain lengths. When the first two chains are connected, the procedure will then be repeated for the other two opposing chains. When all four chains are connscted J the combined weight of the coupled chains, .. ..
the riser and the maximum,vertical wave force is~buoyed, which results in an overall marginally positive buoyant system.
The loose coupling between riser 1 and the vessel 20 shown in Figure 3 may be effected as follows, The vessel ~0 is positioned with its moonpool centered over the riser 1. When all is ready for effec-ting the connection, the buoyancy in the upper section 11 of the riser column is increased and the chains 3 are lowered from the sides of the vessel ~hrough bilge-mounted fairleads for connection to the top of the riser. The procedure for effecting this co~nection may be substantially the same as that described above with reference to Figures 1 and 2.
However, instead of attaching the chains to swivelling chain fasteners 14, they are attached to connectors mounted on the slewing ring 16 which is capable of rotation about the top of riser section 11.
The coupling between riser 1 and the turret/chain moored vessel shown in Figures 4 and 5 may be effected generally as described above with reference to Figures 1 and 2. The flexible riser bundle 13 is connected to the submerged riser section 11 at a stabbing block manifold show diagrammatically at 13a in Figure 4. Sections of the flexible riser bundle 13 pass through the *urret 34 to the deck area of the vessel, where a multi-fluid 1 ~ 5 2 swivel 21a is provided. The top section of swivel 21a is af-Eixed to a gimballed plate -forming part of the frame 24 and having wire connections 23 to tensioning means e.g.
weights 22 which are suspended via pulleys from a support-ing frame 37, This frame is also used to pull and lowerthe riser sections as required. ~ard piping or hose 25 having terminal swivel joints are connecte~ to the multi-fluid swivel 21a, there being a separate piping line for each fluid which is carried in the system. The hard piping is arranged so as to allow the heave of the vessel to be accommodated. The pitch and roll of the vessel, and the angular offset of the riser sections caused by vessel movement, is accommodated by the gimballed plate which ~orms part of the frame 24. Where line 25 is i~ the form of hard piping, it may advantageously be guided by a sleeve-like structure for support (such as that shown in ~igure 3), since a certain amount o~ torque at the multi-fluid swivel will develop with change of vessel heading.
To protect the riser sections from torque build-up, pressure sensing transducers may be employed in con-junction with fluid swivel turning motors mounted on the multi-fluid swivel 21a; these are not shown in the drawings.
An alternative method of attaching the mooring chains 3 to the top of the submerged buoyant riser will now be described with reference to Figure 6. In this figure, four chains 3 are attached to a circular plate 40 which is provided with three or four tapered sockets 41. The plate 40 is suspended by wires 42 (conveniently the same in number as sockets 41) which wires may be passed through a vessel chain tube or turret as shown in Figures 1 to 5.
The top o~ the riser section 11 is formed with an appropriate number of upstanding, fluted posts 43 which are desi~ned to mate with the sockets 41. The posts 43 may be mounted on a slewing ring (not shown in Figure 6).
As plate 40 is lowered, the fluted posts 43 penetrate into sockets 41 from which water is forced out. This evacuation of water from within the sockets 41 causes an ` ~ `1 63~52 automatic cushioning effect which increases in magnitude as the plate 40 approaches surface 44 of riser section 11.
This passive cushioning ef~ect assists the steady location of the plate 40 onto the riser section 11. When the fluted posts 43 are fully engaged in sockets 41, plate 40 may be locked hydraulically to the top of the sub-merged riser.
After connection in the manner just described, the riser bundle with its centering probe and a hydraulic connector, flexible joint and riser flowline tubes, is lowered and positioned, locked and tensioned, for example by use of the riser tensioners 22 as illustrated in Figures 3 and 4. Buoyancy in the submerged riser section 11 is adjusted when the riser bundle is connected thereto.
A modified arrangement may be adopted at the lower end of the submerged riser in order to facilitate well entry through the top of the well tree(s). In this modification, a circular well template is provided inside the riser base and the flowlines connected from each tree pass up along a bell-shaped, gimballed structure attached to the lower riser section at a point high up enough to allow as slight an angle of flowline deviation as possible; the gimballed structure is also attached low enough on the riser so as not, with changing riser angles, to cause too much deflection of the flowlines. A flex joint will be provided atop each well tree in order to accommodate the changing flowline angles caused by move-ment of the bell-shaped structure as it follows the riser deflections.
"Self-standing marine riser"
.. _ . . .
This invention relates to a self-standing marine riser suitable for use in drilling, and in serni submersible production operations and with a dynamically positioned oil/
gas production ship, a chain moored ship with a spindle or with a tension leg platform.
Various aspects of the invention are as follows:
A self-standing marine riser which comprises a base, a riser column, a flexible joint between the base and the riser column, and a plurality of chains attached to the top of the riser column for providing a loose coupling between the top of the riser column and a structure selected from the group consis-ting of vessels, rigs and platforms on the surface above the location of the riser, wherein (1) the riser column comprises a lower, relatively slender column section and an upper column section which includes at least one buoyancy chamber, the buoyancy of which is variable, and (2) the riser supports at least one conduit for the conveyance of a fluid.
A self-standing marine riser which comprises a base which is adapted to be mounted above a sea bottom drill-ing template, a riser column, a flexible joint between the base and the riser column, and a plurality of chains attach~
ed to the top of the riser column for providing a loose coupling between the top of the riser column and a struc-ture selected from the group consisting of vessels, rigs and platforms on the surface above the location of the riser, wherein (1) the riser column comprises a lower relatively slender column section and an upper column section which includes at least one buoyancy chamber the ~r~
-la-buoyancy of which is variable and is such as to enable neutral buoyancy to be achieved for the riser; (2) the riser supports at least one conduit for the conveyance of a fluid;
and (3) said plurality of chains are attached to a plate which carries a plurality of sockets which are positioned and shaped so as to mate with a corresponding number of posts formed on the top of the upper column section of the riser column to facilitate quick release of the chains from the riser.
The fluid can be oil, gas, water, or drilling mud.
Optionally there may be provided a conduit for conveying solid objects, such as tools, from the top of the riser to the base. One or more control lines (e.g. electrical or hydraulic lines) may be housed in the or one of the con-duits. A fle~ible joint may be provided at the top of the riser column between the column itself and a riser bundle connecting with the surface structure and through which the conduit for the conveyance of a fluid passes.
As previously indicated, the buoyancy provided by the upper column section should be variable. This arrange--~ 1 ~3~5~
ment ~acilitates the emplacement of the riser and its loose coupling to a surface vessel or structure. The buoyancy chamber(s) are advantageously such as to enable neutral buoyancy to be achieved; in preferred embodi-ments of the invention, positi~e and/or neutral buoyancycan be achieved for the riser alone, submerged and unattached to the surface vessel, or for the combination of the riser and the means by which it is loose coupled to the surface vessel or structure, or when coupled to the sur~ace vessel or structure. This loose coupling is advantageously effected by chains which are removably attached to the top of the riser column. Conveniently there can be four such chains when the riser is secured to a semisubmersible rig or platform, If the riser is secured to a ship, there may be two or four such chains attached to a swivelling ring to allow for azimuth varia-tions.
The riser can include one or more pipes attached to the exterior of the riser column at least in the region of the lower section thereof. One or more flexible hoses may be provided at the lower end o~ the riser to connect the or each of said pipes to a ~e~head or a production outlet at, or in the vicinity of, the base of the riser, The base itself can be a gravity base or a piled base. Generally, the base will be aligned next to or pvsitioned aroùnd a sea bottom drilling template. If t~e base is a piled base, it is preferable to install the base at the same time as the drilling production template, for reasons of wellhead safety. The riser column and flexible joint may then be linked to the base with a connector on completion of the well drilling.
The riser is self-standing and buoyant when stand-ing alone and submerged. Two or more wirelines are prefer-ably attached between the upper column section a~d thebase. These initially are used as guideline wires to emplace the riser and connect it to the base; subseque~tly .
.
~ ~ 635S2 when terminated and fastened to the upper section of the riser after its emplacement and attachment to inter-mediary points and to the base they act as safety wires to avoid accident should the riser break, ~erving to prevent the riser accelerating to the sea surface. The necessary slack in the w~res to allow freedom of riser column angular movement is advantageously taken up by weighted lever de~ices at the base. In preferred embodi-ments the riser is chained by four chains to a semi-submersible structure through pontoon mounted fairleads orto a ship either through spaced hull-mounted ~airleads (when the ship is dynamically positioned over the riser) or in the case of a spindle-moored ship through fairleads which form part of the spindle structure. Each chain may be connected to the riser via a swivel chain fastener or fasteners that are part of a swivelling ring located about the riser top. The first arrangement is preferred for connection to semisubmersibles and to spindle-moored vesssls while the second is preferable for connection to ship-shaped surface structures which are dynamically positionedand must weathervane. Preferably, the chains can be "quick released" from the riser, the action necessary to achieve this being effected on the semi-submersible platform or o~ the vessel to which the riser is loose coupled.
The conduits for conveying fluids, e.g. oil or gas, along the riser can terminate in a submerged riser top manifoldtstabbing block. The connection between the manifold/stabbing block and the production facility should be of the quic~:-release type, so that in an emergency the well(s) may be shut-in, after whi~h the connectio~ to the riser conduit~s) may be "~uick-released" from the top of the submerged riser, followed by "quick release" of the chains from the submerged riser top. The connection from the surface vessel to the riser manifold/stabbing block can be in the form of a flexible riser bundle. ~hen the self-standing riser is emplaced, it will ~enerally be fully -"` i ;~ 6~5~
9~ .
submerged. ~ith an arrangement such as that just described, the tensioned riser bundle will be supported by tensioners which compensate for vessel motion, draught and changes in sea level.
With the preferred ~tructure described above~ i~
the self-standing production riser should break loose while still connected to a semi-submersible or vessel production facility, the riser will not loat to the surface because of the restraining safety wires and the weigh-t imposed by the catenary chains.
In general, the riser will maintain a substantially upright configuration. ~ovement of the vessel, rig or platform to which it is loose coupled as well as tidal and current effects may result in the riser moving away from the vertical. The riser will accommodate a maximum of 15 tilt from the vertical, but it is preferred that the riser should not deviate from the vertical by more than 10 . Under normal operating conditions, the maximum in-clination of the riser is expected to be about 7 or less from the vertical.
A riser constructed in accordance with the present invention does not require a complicated tensioning system to hold it in place. The buoyancy and stiffness pro-vided in the submerged riser mean that the riser is not subjected to stresses as severe as those normally associated with an equivalent length tensioned riser.
Furthermore, the design is such that if the riser breaks fre~e at the bottom while connected to a production facility, it will not inevitably come to the surface and/or collide with th`e production facility. Also, i~
a piled or gravity base is employed, it can straddle the well; head template (without contacting it) thereby providing protection for the well "trees".
For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:
~ :~ 6355~
FIGURE 1 shows a front view of a sel~-standing production riser in accordance with the inve~tion loosely coupled to a semi-submersible production plat~orm;
FIGUR~ 2 shows a side view of the apparatus of Figure l;
FIGURE 3 shows a ~ront view of the top of a self-standing production riser in accordance with the invention loosely coupled to a production/storage ship-shaped vessel;
~IGURE 4 shows a front view of the top of riser in accordance with the in~ention loosely coupled to a dynamically assisted ship or barge having a turret/chain mooring arrangement;
FIGURE 5 is a schematic plan view of the arrangement shown in Figure 4; and FIGURE 6 illus-trates one method of connecting a sur-face structure and the marine riser.
Referring now to the drawings, the riser 1 shown in Figures 1 and 2 is loosely coupled to a semi-submersible production platform 2 via a plurality of chains 3. There are four such chains in the embodiment illustrated in the drawings. As illustrated, the chains are attached to the inboard area of the pontoon; alternatively they may be attached to the outboard area. The riser 1 comprises a piled or gravity base 4~ e.g. a piled steel base which can have two basic configurations. In the first, it is mounted over but is not in contact with a circular well-head template (not shown). In the second 7 the base is connected to one end or to the middle of a rectangular or square wellhead template. The circular template can accommodate ten wells with one spare slot in its presently envisaged form. The number of wells which can be accommo-dated depends on the capability of the riser and manifold system to handle the fluids. In the circular wellhead template, the production trees are protected by the base 4.
The riser may also be connected to a satellite production tree or trees or a separate manifold well template adjacent J ~ ~3552 the base 4, as indicated by line 5 in Figure 1.
The riser column comprises a lower slender part 6 connected to the base 4 by a universal, ball or flex joint 7. Pipe conduits 8 are mounted on the outside of lower section 6 of the riser column, ~ach o~ conduits 8 is connected at its lower end to a flexible hose 9 which in turn is connected to the well production tree 10.
The lower part 6 of the riser column occupies the greater proportion of the total length of the riser. The upper portion 11 of the riser column includes both a fixed and a variable buoyancy system. Conduits 8 pass through the interior of upper riser column section 11, At the top of the riser column, there is a riser top manifold/stabbing block 12 by means of which a flexible riser bundle 13 may be connected to the upper termination of conduits 8.
The length of each of chains 3 is adiustable. Under normal operating conditions, each chain will generally have substantially the same length. The connection be-tween the chains 3 and riser 1 is effected at swivelling chain fasteners 14 which are attached to the outside of upper riser column section 11 at the top part thereof. The length of each chain catenary between connectors 14 and the pontoon ~airleads of the production platform 2 will normally be in the range from 20 to 60 metres, preferably about 45 met~es; the length may occasion-ally be as little as 10 metres. The loose chain con-nections may be made either to the insides or to the outsides of the pontoons, and the chain will generally run through fairleads whose positions are such as to afford the optimum scope ratio for control of the sub-merged riser, The scope ratio will depend on environ-men-tal conditions, rig layout, depth of the riser top below sea level and pontoon depth for an optimum operation, Referring now to Figuxe 3, there is shown a mooring arrangement suitable for use when a self-standing pro--- ~ 1 63552 duction riser in accordance with the present invention is loosely coupled to a dynamically-assisted vessel, i.e.
a ship or barge whose mooring position is maintained with ~ynamic assistance. The two chains 3 are attached to the upper section 11 o~ the riser column at a slewing ring 16 which is fi*ted about the top part of column .
section 11. The chains 3 pass over chain sheaves 17 which pre~erably can be raised or lowered by a predeter-mined amount in order to adjust the vertical/horizontal chain catenary ratio to the optimum ~or any given cir-cumstance. The mooring chains -then pass upwardly into chain tubes 18 within the vessel 20. Alternatively, if four chains 3 are employed, there may be four chain tubes 18 positioned on the outside of the hull of the~vessel.
The ~lexible riser bundle 13 passes through a moonpool 19 and terminates at a fluid swivel 21 to which tensioners 22 are connected via cables 23. A guide frame 24 holds fluid swivel 21 in position in a horizontal plane, and also ~unctions to rotate it.
Referri~g now to Figures 4 and 5, an arrangement is shown for connecting a free-standing marine riser in accordance with this invention to a vessel having a turret/chain mooring arrangement. In this case, the mooring chain 3 can be connected to the top of riser section 11 either by two, three or four swivelling chain fasteners or by chain ~asteners which are part of a slewing ring attached to the outside of upper riser sectior, 11. The arrangement illustrated in Figure 4 shows the first of these two possibilities, there being two swivelling chain faste~ers 14 attached to the out-side of upper riser column section ll. The choice between these two possible confi~urations will be decided according to the method of equipment installation relative to acceptable weather conditions, The outboard ends of mooring chains 3 are connected to wires 30 which pass over fairleads 31 held by spreader arms 32. The length of each chain catenary between fasteners 14 and the first of the iairl~ads ~l will generally be about 23 to 27 I ~ ~3~5~
metres in the present~y pre~erred arrangement. The spreader arms 32 are structurally connected to a cylindrical body 34 ~orming part o~ the vessel 33, the interior of body 34 constituting a spindle or turret. This turret also houses winches and mooring line equipment ~not shown) and anchoring windlasses one o~ which is shown at 35. When the mooring lines are in place, the turret 34 remains on a consistent heading while the vessel itself can weathervane about the turret.
The flexible riser bundle 13 passes through turret 34 and terminates at a multi-fluids swivel 21a mounted above the vessel deck. This swivel is held in a gimballing table guided by frame 24 attached to riser tensioning wires 23 which terminate in tensioning means, such as pneumatic or hydraulic tensioners or weights 22.
The schematic arrangement shown in ~igure 5 illustrates the positionin~ of four double-drum mooring winches or windlasses (MW) mounted on top of turntable 36 which is, in effect, the topmost part of turret body 34.
The riser 1 can be used in deep water conditions, for example at depths of 90 metres ~300 feet) or greater.
An emergency release system (not shown) is provided to enable chains 3 to be separated ~rom riser 1 quickly.
The system can comprise a wire attached to a locking arm which, when the wire is pulled taut, will cause a locking pin holding a respective chain to connector 14 to shear and allow the chain to fall free of the riser.
The loose coupling between riser 1 and platform shown in Figures 1 and 2 may be effected as follows.
Initially, the buoyancy of the riser is adjuste~ so that it is slightly positive. With the riser in this con-dition, the semisubmers~ble is moored with its moonpool centered over the riser. When all is ready for effecting the connection, the buoyancy in upper section 11 of the riser column is increased and the chains 3 are lowered from the semisubmersible for connection to the top of riser 1.
This can be done by attaching strayline wires to a point a given number of links above the hanging chain ends, and ~ ~ ~35~2 _ 9 _ paying out the chains as the wires are pulled towards the moonpool. The end links or shackles of the chains will be locked into the riser swivelling chain fasteners 14, opposing chains preferably being connected simultaneously.
The strayline wires may then be let out and de-tached from the chain; they can later be used as guidelines for guiding the riser sections from the surface to the stab-bing manifold block at the top of the submerged riser.
Next, the chains will be tightened to give the desired catenary chain lengths. When the first two chains are connected, the procedure will then be repeated for the other two opposing chains. When all four chains are connscted J the combined weight of the coupled chains, .. ..
the riser and the maximum,vertical wave force is~buoyed, which results in an overall marginally positive buoyant system.
The loose coupling between riser 1 and the vessel 20 shown in Figure 3 may be effected as follows, The vessel ~0 is positioned with its moonpool centered over the riser 1. When all is ready for effec-ting the connection, the buoyancy in the upper section 11 of the riser column is increased and the chains 3 are lowered from the sides of the vessel ~hrough bilge-mounted fairleads for connection to the top of the riser. The procedure for effecting this co~nection may be substantially the same as that described above with reference to Figures 1 and 2.
However, instead of attaching the chains to swivelling chain fasteners 14, they are attached to connectors mounted on the slewing ring 16 which is capable of rotation about the top of riser section 11.
The coupling between riser 1 and the turret/chain moored vessel shown in Figures 4 and 5 may be effected generally as described above with reference to Figures 1 and 2. The flexible riser bundle 13 is connected to the submerged riser section 11 at a stabbing block manifold show diagrammatically at 13a in Figure 4. Sections of the flexible riser bundle 13 pass through the *urret 34 to the deck area of the vessel, where a multi-fluid 1 ~ 5 2 swivel 21a is provided. The top section of swivel 21a is af-Eixed to a gimballed plate -forming part of the frame 24 and having wire connections 23 to tensioning means e.g.
weights 22 which are suspended via pulleys from a support-ing frame 37, This frame is also used to pull and lowerthe riser sections as required. ~ard piping or hose 25 having terminal swivel joints are connecte~ to the multi-fluid swivel 21a, there being a separate piping line for each fluid which is carried in the system. The hard piping is arranged so as to allow the heave of the vessel to be accommodated. The pitch and roll of the vessel, and the angular offset of the riser sections caused by vessel movement, is accommodated by the gimballed plate which ~orms part of the frame 24. Where line 25 is i~ the form of hard piping, it may advantageously be guided by a sleeve-like structure for support (such as that shown in ~igure 3), since a certain amount o~ torque at the multi-fluid swivel will develop with change of vessel heading.
To protect the riser sections from torque build-up, pressure sensing transducers may be employed in con-junction with fluid swivel turning motors mounted on the multi-fluid swivel 21a; these are not shown in the drawings.
An alternative method of attaching the mooring chains 3 to the top of the submerged buoyant riser will now be described with reference to Figure 6. In this figure, four chains 3 are attached to a circular plate 40 which is provided with three or four tapered sockets 41. The plate 40 is suspended by wires 42 (conveniently the same in number as sockets 41) which wires may be passed through a vessel chain tube or turret as shown in Figures 1 to 5.
The top o~ the riser section 11 is formed with an appropriate number of upstanding, fluted posts 43 which are desi~ned to mate with the sockets 41. The posts 43 may be mounted on a slewing ring (not shown in Figure 6).
As plate 40 is lowered, the fluted posts 43 penetrate into sockets 41 from which water is forced out. This evacuation of water from within the sockets 41 causes an ` ~ `1 63~52 automatic cushioning effect which increases in magnitude as the plate 40 approaches surface 44 of riser section 11.
This passive cushioning ef~ect assists the steady location of the plate 40 onto the riser section 11. When the fluted posts 43 are fully engaged in sockets 41, plate 40 may be locked hydraulically to the top of the sub-merged riser.
After connection in the manner just described, the riser bundle with its centering probe and a hydraulic connector, flexible joint and riser flowline tubes, is lowered and positioned, locked and tensioned, for example by use of the riser tensioners 22 as illustrated in Figures 3 and 4. Buoyancy in the submerged riser section 11 is adjusted when the riser bundle is connected thereto.
A modified arrangement may be adopted at the lower end of the submerged riser in order to facilitate well entry through the top of the well tree(s). In this modification, a circular well template is provided inside the riser base and the flowlines connected from each tree pass up along a bell-shaped, gimballed structure attached to the lower riser section at a point high up enough to allow as slight an angle of flowline deviation as possible; the gimballed structure is also attached low enough on the riser so as not, with changing riser angles, to cause too much deflection of the flowlines. A flex joint will be provided atop each well tree in order to accommodate the changing flowline angles caused by move-ment of the bell-shaped structure as it follows the riser deflections.
Claims (13)
1. A self-standing marine riser which comprises a base, a riser column, a flexible joint between the base and the riser column, and a plurality of chains attached to the top of the riser column for providing a loose coupling between the top of the riser column and a structure selected from the group consisting of vessels, rigs and platforms on the surface above the location of the riser, wherein (1) the riser column comprises a lower, relatively slender column section and an upper column section which includes at least one buoyancy chamber, the buoyancy of which is variable, and (2) the riser supports at least one conduit for the conveyance of a fluid.
2. A riser as claimed in claim 1, wherein said upper column section is such as to enable neutral buoyancy to be achieved for the riser.
3. A riser as claimed in claim 1, characterised in that the chains can be attached to the riser by swivelling chain fasteners.
4. A riser as claimed in claim 1, characterised in that the chains can be attached to the riser by a connector mounted on a slewing ring rotatably held on the top section of the riser.
5. A riser as claimed in claim 1, characterised in that the riser includes one or more pipes attached to the riser column on the exterior thereof at least in the region of the lower section of the column.
6. A riser as claimed in claim 5, characterised in that one or more flexible hoses are provided to connect the or each of said pipes to a production outlet at, or in the vicinity of, the base of the riser.
7. A riser as claimed in claim 1, characterised in that the base of the riser is, or is adapted to be, mounted above a sea bottom drilling template.
8. A riser as claimed in claim 1, characterised in that a riser top manifold is provided at the top of the riser to permit the or each fluid conduit to be connected to the vessel, rig or platform.
9. A riser as claimed in claim 1, characterised in that the riser is connected, or is adapted to be connected, by four chains to a semi-submersible production platform.
10. A riser as claimed in claim 9, characterised in that the overall buoyancy of the riser with the chains attached thereto can be adjusted to neutral.
11. A riser as claimed in claim 9 or claim 10, characterised in that means is provided to enable the chains to be released quickly from the riser.
12. A riser as claimed in claim 1, characterised in that the flexible joint between the base and the riser column is a universal joint, a ball joint or a flex joint.
13. A self-standing marine riser which comprises a base which is adapted to be mounted above a sea bottom drill-ing template, a riser column, a flexible joint between the base and the riser column, and a plurality of chains attach-ed to the top of the riser column for providing a loose coupling between the top of the riser column and a struc-ture selected from the group consisting of vessels, rigs and platforms on the surface above the location of the riser, wherein (1) the riser column comprises a lower relatively slender column section and an upper column section which includes at least one buoyancy chamber the buoyancy of which is variable and is such as to enable neutral buoyancy to be achieved for the riser; (2) the riser supports at least one conduit for the conveyance of a fluid;
and (3) said plurality of chains are attached to a plate which carries a plurality of sockets which are positioned and shaped so as to mate with a corresponding number of posts formed on the top of the upper column section of the riser column to facilitate quick release of the chains from the riser.
and (3) said plurality of chains are attached to a plate which carries a plurality of sockets which are positioned and shaped so as to mate with a corresponding number of posts formed on the top of the upper column section of the riser column to facilitate quick release of the chains from the riser.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000376111A CA1163552A (en) | 1981-04-23 | 1981-04-23 | Self-standing marine riser |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000376111A CA1163552A (en) | 1981-04-23 | 1981-04-23 | Self-standing marine riser |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1163552A true CA1163552A (en) | 1984-03-13 |
Family
ID=4119813
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000376111A Expired CA1163552A (en) | 1981-04-23 | 1981-04-23 | Self-standing marine riser |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1163552A (en) |
-
1981
- 1981-04-23 CA CA000376111A patent/CA1163552A/en not_active Expired
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