Riser and method of installing same
A hybrid riser having a lower section (6) and an upper section (10), said upper section comprising a flexible pipe, and said lower section comprising a substantially rigid pipe in communication with the flexible pipe, said riser further comprising a buoyancy section (16) at or in the region of an upper end of said rigid pipe (13). Said buoyancy section (16) also comprises an elongate cylindrical buoyancy element, which may be of a coaxial compartmentalised tubular construction having valves such that it may be controllably flooded or evacuated. The hybrid riser may be tethered to a surface vessel or to the seabed. The hybrid riser may be constructed on land, and towed to the vicinity of the installation to which it is to be connected.
The present invention concerns a riser, for the transport of fluid hydrocarbons from an underwater wellhead.
Over recent years, as liquid and gaseous hydrocarbon fuel resources have become more scarce, there has been a tendency to search for and extract these resources in increasingly remote and inaccessible areas, and in particular in increasingly deep parts of the sea. As the depth of these sub-sea wells below sea level has increased, the demands on the technologies used to bring the oil or gas from the well to the sea surface have increased dramatically. As more and more sophisticated techniques for overcoming these difficulties are devised however, the costs and risks associated with the construction, installation and use of these structures has grown.
Risers of two different types are commonly used in bringing oil or gas from a sub-sea well to the surface. Flexible pipes have the advantages of being relatively easy to install, reducing the cost and risks involved in the installation process itself, and being resistant to work fatigue brought on by cyclic forces exerted on the pipe by the movement of the sea and other factors. On the other hand, solid steel pipes are considerably less costly to manufacture.
On the basis of these strengths and limitations, flexible pipes have tended to be chosen for use in shallow installations. This is the case firstly since the total length of the pipeline needed for a well positioned in shallow water is sufficiently short that the cost of the more expensive flexible pipe is not prohibitive.
As well as the two types of piping commonly used to serve as risers, there are furthermore two basic geometrical configurations which are frequently used. The first of these is the vertical riser, where the riser rises directly from the sub-sea well to a vessel on the surface of the sea. The second is a catenary configuration in which the riser starts from the well head running along the seabed, before rising up from the seabed towards a vessel floating or otherwise situated on the sea's surface, such that the riser forms a gentle curve in the shape of a catenary, or the riser catenary extends directly from the well head itself to the support vessel.
In situations where it is necessary to provide a riser between the sea surface and a wellhead in deep water, the price for the required length of flexible riser becomes less attractive. The use of a rigid riser thus becomes more desirable.
Although relatively cost effective, this configuration has the drawback that variations in the position of the vessel 3 result in cyclic stresses at the touch down point 6 of the catenary riser 9, such that the rigid riser is fatigued over time, and is prone to failure.
A first solution to this problem is known from U.S. Pat. No. 5,639,187, which discloses a configuration such as shown in
Thus, in operation, the riser bar 12 remains substantially stationary in the water, regardless of movements in the surface vessel 3, such that the steel catenary riser 13 is isolated from cyclic effects, and the touch down zone 6 is not fatigued. Further, the load on the surface vessel from the risers is reduced, since the buoy carries the weight of the lower part of the risers. The cost and effort involved in installing and maintaining the system, in particular the anchoring of the float arrangements, is substantial however.
An alternative solution to certain of the problems arising from deep sea wellheads is suggested by the Patent WO 00/53884, which describes a further hybrid arrangement as shown in
This arrangement naturally comes with certain of the drawbacks of the fully flexible catenary configuration discussed above however. Further, the load of the risers at the surface vessel is relatively high.
A further alternative arrangement is the tower riser, which comprises a rigid, vertical riser tower provided with air tanks at the upper extremity, and connected to a surface vessel by a flexible riser pipe.
The present invention seeks to overcome drawbacks of various riser configurations discussed above. Objects of the present invention thus include the provision of a riser configuration which is suitable for use in deep water, is less prone to fatigue effects or abrasion, and is of comparatively low cost and simple to construct and install.
According to the invention from a first aspect there is provided a riser having a lower section and an upper section, said upper section comprising a flexible pipe which may be made for example of standard flexible pipe, composite material or titanium and said lower section comprising a substantially rigid pipe in communication with the flexible pipe and forming a catenary, said riser further comprising a buoyancy section at or in the region of an upper end of said rigid pipe.
In a second aspect of the invention there is provided a riser having a lower section and an upper section, said upper section comprising a flexible pipe which may be made for example of standard flexible pipe, composite material or titanium and said lower section comprising a substantially rigid pipe in communication with the flexible pipe and forming a catenary, said riser further comprising a buoyancy section at or in the region of an upper end of said rigid pipe, said buoyancy section being tethered to a vessel on the sea's surface. This has the advantage that it is less costly to tether a riser to a surface vessel than to the seabed. It had not previously been thought possible to achieve satisfactory stability in the riser by this technique. The inventor has found that recent changes in anchoring techniques have made the required stability achievable.
According to the invention from a third aspect there is provided a riser having a lower section and an upper section, said upper section comprising at least one flexible pipe and said lower section comprising at least one substantially rigid pipe and forming a catenary, said riser further comprising a buoyancy section at or in the region of an upper end of said rigid pipe, said buoyancy section comprising an elongate buoyancy unit extending lengthwise of the rigid pipe. This configuration has the advantage of being relatively simple and cost effective to construct and install by a variety of methods as outlined further on.
According to a development of this third aspect of the invention said buoyancy section is tethered to a vessel on the sea's surface.
According to a further development of this third aspect of the invention said buoyancy section is tethered to the seabed.
A further tether may be provided between a point on the rigid pipe above the high bending area thereof, and the seabed.
The lower section may comprise a plurality of rigid pipes and said upper section may each comprise a corresponding plurality of flexible pipes. This construction is advantageous in that it provides for the transportation of different fluids in different directions through the same pipeline requiring a single installation process.
According to a further development, said plurality of rigid pipes may be arranged around the outside of said buoyancy section and may in addition be spaced apart evenly about the circumference of said buoyancy section.
Each of the plurality of rigid pipes may be fixed at or near an upper extremity thereof to said buoyancy section.
According to a further development, said buoyancy section may be further provided with a plurality of sleeves intended to slidingly receive said plurality of rigid pipes respectively.
According to a further development, the riser may further comprise a spacer connected to each of said plurality of rigid pipes, or to each of said plurality of flexible pipes so as to maintain said plurality of rigid pipes or each of said plurality of flexible pipes in a fixed position relative the other rigid pipes or flexible pipes.
According to a further development of any of the above aspects, said spacer is provided at a point on said rigid riser below a lower extremity of said buoyancy section.
According to a development of any of the above aspects each of said at least one flexible pipes is joined to a respective one of at least one rigid pipes at a substantially right angle. This configuration is advantageous in that the connection of a tether to the upper part of the buoyancy section is facilitated.
According to a development of any of the above aspects the substantially right-angled joins between respective rigid and flexible pipes are spaced apart from one another along the length of said buoyancy section. This configuration is advantageous in that the joints between the flexible and rigid sections can easily be separated from one another so as to facilitate connection of these joints during an installation process.
According to a development of any of the above aspects said buoyancy section is made of a foam. This has the advantage of not being affected by leaks in the structure of the riser.
According to a development of any of the above aspects said buoyancy section is a tube arranged such that the rigid riser runs therethrough. This construction is highly cost effective.
As a further development of this construction there is provided a riser wherein said buoyancy section is made of steel, titanium, aluminium or a composite material. A buoyancy unit according to this construction can be produced using the techniques conventionally used in the construction of rigid riser pipes, therefore further improving economy and autonomy.
As a further development of this construction there is provided a riser wherein said solid tube is arranged coaxially to said rigid riser. This configuration makes it easier to ensure a predictable spacing between the inner and outer tubes, with associated effects on the mechanical and thermodynamic properties of the riser as a whole. This is commonly called a Pipe-In-Pipe system
As a further development of the above constructions there is provided a riser wherein said buoyancy section further comprises a plurality of bulkheads dividing said buoyancy section into a plurality of closed chambers. This makes it possible to finely control the buoyancy of the buoyancy unit, and therefore the riser as a whole. It also introduces a degree of leak and damage resistance.
As a further development of this construction there is provided a riser wherein at least one valve is provided allowing flow of a fluid from inside said rigid riser to the interior of a respective at least one of said closed chambers. A further development involves the provision of at least one valve allowing flow of a fluid from inside a respective at least one of said closed chambers to the exterior of said buoyancy unit. By means of these valves it is possible to control the buoyancy of the buoyancy unit.
According to a development of any of the above aspects wherein an upper extremity of said rigid pipe is aligned away from the axis of the part of said riser immediately below said upper extremity. By this means it is possible to optimise the transmission of forces along the riser.
According to the invention from a fourth aspect there is provided a method of installing a riser having a lower section and an upper section, where said upper section comprises a flexible pipe, and said lower section comprises a rigid pipe in communication with the flexible pipe, and wherein said riser further comprises a buoyancy section at an upper end of said rigid pipe, said method involving the steps of;
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- i. constructing a rigid pipe and the buoyancy section on land,
- ii. at least partially flooding said buoyancy unit, such that the buoyancy of the riser is negative,
- iii. towing said rigid riser section and the buoyancy section out to sea to the location where the riser is to be installed by at least a first tug using a first tether,
- iv. allowing said rigid pipe and the buoyancy section to sink to the floor of the sea,
- v. connecting the end of the rigid pipe furthest from the buoyancy unit at a wellhead or flowline tie in,
- vi. expelling fluid from the buoyancy unit, such that a buoyancy force is exerted on the buoyancy section,
- vii. allowing the buoyancy section to rise towards the surface of the sea under the guidance of said at least one installation/surface vessel such that the rigid pipe bends upwards to form a catenary configuration, and
- viii. attaching a flexible pipe between said installation/surface vessel and the upper end of the rigid pipe.
This method is advantageous in that it can be carried out without specially adapted deployment vessels.
According to the invention from a fifth aspect there is provided a method of installing a riser having a lower section and an upper section, where said upper section comprises a flexible pipe, and said lower section comprises a rigid pipe in communication with the flexible pipe, and wherein said riser further comprises a buoyancy section at an upper end of said rigid pipe, said method involving the steps of;
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- i. constructing a rigid pipe and the buoyancy section on land,
- ii. weighting said buoyancy unit, such that the buoyancy of the riser is negative,
- iii. towing said rigid pipe section and the buoyancy section out to sea to the location where the riser is to be installed by at least a first tug using a first tether,
- iv. allowing said rigid pipe and the buoyancy section to sink to the floor of the sea,
- v. connecting the end of the rigid pipe furthest from the buoyancy unit at a wellhead or flowline tie-in,
- vi. removing said weighting from the buoyancy unit, such that a buoyancy force is exerted on the buoyancy section,
- vii. allowing the buoyancy section to rise towards the surface of the sea under the guidance of said at least one installation/surface vessel, such that the rigid pipe bends upwards to form a catenary configuration, and
- viii. attaching a flexible pipe between said installation/surface vessel and the upper end of the rigid pipe.
According to a development of this method one or more temporary buoyancy elements are connected to said rigid riser such that buoyancy is distributed along the length of the riser substantially evenly.
According to a development of this method, the lower end of said rigid riser may be connected to said well head by means of jumpers or rigid spools.
According to a development of the above method, said rigid pipe and the buoyancy section are towed out to sea to the location where the riser is to be installed further using a second tug and a second tether said second tether being connected to a point along the rigid riser behind the point to which said first tether is connected. By this means the riser can be steered and manoeuvred by the second tug while the first tug provides motive force.
According to a development of the above method, said rigid riser is pressurised with a gas prior to said step of expelling fluid from the buoyancy unit. This makes the provision of external pumping or pressurising means during the installation process unnecessary.
According to the invention from a sixth aspect there is provided a method of installing a riser configuration having a lower section and an upper section, where said upper section comprises a flexible pipe, and said lower section comprises a rigid pipe in communication with the flexible pipe, and wherein said riser further comprises a buoyancy section at an upper end of said rigid pipe, said method involving the steps of:
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- i. constructing the entire riser structure on land,
- ii. at least partially flooding said buoyancy unit, such that the buoyancy of the riser is negative,
- iii. towing the riser out to be installed at sea to the location where the riser is to be installed by at least a first tug (42) using a first tether (44),
- iv. allowing riser to land on the seabed,
- v. connecting the end of the rigid pipe furthest from the buoyancy unit (16) at a wellhead or flowline tie-in,
- vi. expelling fluid from the buoyancy unit, such that a buoyancy force is exerted on the buoyancy section,
- vii. allowing the buoyancy section to rise towards the surface of the sea under the guidance of said at least one installation/surface vessel, such that the rigid pipe bends upwards to form a catenary configuration, and
- viii. attaching a flexible pipe to a surface vessel.
This simplifies the installation process since it is no longer necessary to connect the flexible and rigid riser pipes underwater.
The above methods may also comprise the step of attaching a tether between the buoyancy unit and said surface/installation vessel or the seabed, so as to reduce the strains exerted on the riser.
The above methods may also comprise the step of attaching a tether between a point on the rigid pipe above the high bending area thereof, and the seabed.
According to the invention from a seventh aspect there is provided a method of installing a riser having a lower section and an upper section, said upper section comprising a flexible pipe and said lower section comprising a rigid pipe in communication with the upper section, said riser further comprising a buoyancy section at an upper end of said rigid pipe, said method comprising the steps of
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- i. lowering successive lengths of rigid pipe section in the sea, each length being connected endwise to the length of pipe section below it;
- ii. connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe;
- iii. connecting a flexible pipe to an upper end of said rigid pipe;
- iv. lowering the flexible pipe;
- v. allowing the buoyancy unit to sink;
- vi. adjusting the position of the floating vessel such that the rigid pipe assumes the configuration of a catenary in the sea water and,
- vii. connecting a lower end of the rigid pipe to a wellhead or flowline.
The buoyancy unit is many times its diameter in length, and is disposed along the length of the upper part of the rigid catenary section.
The buoyancy unit may float freely in the water, or may be tethered to an object on the surface of the sea or at the seabed.
According to the invention from an eighth aspect there is provided a method of installing a riser having a lower section (6) and an upper section (10), said upper section comprising a flexible pipe and said lower section comprising a rigid pipe in communication with the upper section, said riser further comprising a buoyancy section (16) at an upper end of said rigid pipe (13), said method comprising the steps of:
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- i. lowering a rigid pipe into the sea by reeling,
- ii. connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe;
- iii. connecting a flexible pipe to an upper end of said rigid pipe;
- iv. lowering the flexible pipe;
- v. allowing the buoyancy unit to sink;
- vi. adjusting the position of the floating vessel such that the rigid pipe assumes the configuration of a catenary in the sea water and;
- vii. connecting a lower end of the rigid pipe to a wellhead or flowline.
According to a development of either of the seventh or eighth aspects of the invention, the buoyancy unit is allowed to sink before said steps of: lowering a rigid pipe into the sea, and connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe.
According to a development of either of the seventh or eighth aspects of the invention the method may comprise the further step of attaching a tether (17c) between a point on said rigid pipe 13 above the high bending area thereof, and the seabed.
Under a tenth aspect of the invention there is provided a subsea riser comprising a first plurality of service pipes and a further pipe, the first plurality of service pipes and further pipe being in mutually spaced-apart relation, the further pipe being divided along its length into a second plurality of separate sections, each section having a vent valve communicating with the outside of the further pipe, one end of the further pipe having an inlet means for the introduction of a high-pressure gas and the further pipe comprising a third plurality of open-ended tubes extending between respective pairs of adjacent sections. This riser may be employed as part of the riser arrangements described earlier.
For a better understanding of the present invention as well as preferred and other embodiments thereof, reference is made by way of example to FIGS. 1 to 16, in which;
Thus, in operation, once installed, the rigid riser portion 13 is held substantially immobile in the sea by the buoyancy unit 16. The flexible riser portion 10 absorbs the motion of the surface vessel 3, and other forces exerted thereon, for example by the movement of the sea itself.
Although
According to a second embodiment of the present invention, as shown in
This arrangement is advantageous over the prior art as embodied for example by the tower riser as discussed above. in that the demands of anchoring the rigid riser to the seabed are reduced, there is no requirement for a flexible joint, and the degree of buoyancy required is reduced.
The positioning of the joint between the rigid and flexible riser parts so that the upper riser part intersects the buoyancy unit 16 at an angle exceeding 45 degrees thereto has the advantage of facilitating the attachment of a tether 17 to the upper end of the buoyancy unit 16 so that extreme movements of the surface vessel 3 will not exert undesirably large forces on the flexible 10, but will rather be absorbed by the tether 17.
According to a preferred realisation of this embodiment, a tether 17c is connected between the upper part of the lower rigid section 13, and the sea floor. It has been found to be highly advantageous in terms of the stability of the riser, and the limitation of fatigue thereto, to connect the seabed tether 17c to the riser 13 just over the high bending area, instead of at the buoyancy tank as described with regard to the tether 17b. It may nonetheless be found desirable to use this configuration in addition to the configuration of tethers 17b and 17. As an alternative, the tether position 17c may be transferred to 17d as shown, i.e. a long way down the riser 13.
A variation of this second embodiment is shown in
This has the effect of limiting the forces exerted on the buoyancy unit 16 to a substantially horizontal plane, thereby reducing the risk of inducing fatigue in the lower portion 6 of the rigid riser 13.
According to a third embodiment of the present invention as shown in
In operation, the provision of this tether has the effect of reducing the forces exerted on the flexible riser sections 10a, 10b, 10c, 10d, etc.
According to a preferred realisation of this embodiment, a tether 17c is connected between the upper part of the lower rigid section 13, and the sea floor. It has been found to be highly advantageous in terms of the stability of the riser, and the limitation of fatigue thereto, to connect the seabed tether 17c to the riser 13 just over the high bending area, instead of at the buoyancy tank as described with regard to the tether 17b. It may nonetheless be found desirable to use this configuration in addition to the configuration of tethers 17b and 17.
According to a fifth embodiment of the present invention shown in
Alternatively it may be desirable to pre-form the top of the rigid section 13 and the buoyancy unit 16, such that a top section of these two elements 16a deviates from the line defined by the essentially vertically rising section of the rigid riser.
In operation this may be found to be advantageous, in that the shear forces exerted on the joint between the flexible riser 10 and the rigid riser 13 present when the buoyancy unit 16 is not vertically below the surface vessel 3 are reduced.
In this embodiment, the buoyancy unit comprises an essentially tubular structure arranged coaxially with the rigid riser 13. The buoyancy unit is made of any suitable buoyant material, for example a foam. It may alternatively comprise a rigid hollow buoyant tank made of steel, composite material, aluminium or other materials as will readily occur to the skilled person, which is either an intrinsic part of the upper section of the rigid riser, or a separate tubular structure which is secured thereto. Such a tank may be filled with gas, or a foam or other buoyant material.
In operation, the ends of the rigid riser 13 are sealed, and the rigid riser is pre-pressurised, for example with nitrogen (N2) gas. First, third and fifth tubular spaces 20a, 20b and 20c are similarly filled with gas. The second and fourth tubular spaces 21a and 21b are filled with seawater, or another fluid having a higher density than the gas with which the rigid riser pipe 13 is filled. The buoyancy tank compartments may be filled with a light-weight fluid during tow out, which makes the buoyancy section close to neutrally buoyant. This fluid will be replaced by gas (compressed nitrogen or similar) during (or prior to or after) the upending operation (i.e. when the buoyancy tank is elevated into its final position). The gas is supplied from a surface vessel, or from compressed gas in the pipelines, or from compressed gas in the buoyancy tank.
Naturally, the number and configuration of the tanks into which the tubular space is divided may be varied as expedient. It may also be found advantageous to provide tubing connecting the different valves, or to connect one or more chambers together by tubing such that they can be vented through a common valve, or other arrangements so as to facilitate the transfer of fluids to and from the buoyancy unit. It may further be found to be advantageous to provide means such that the transfer of fluids to and from the buoyancy unit can be effected after the installation of the riser.
This has the effect of reducing the overall average density of the buoyancy unit 16.
This gap is provided to provide insulation between the fluids flowing through the rigid riser 13 and the surrounding seawater. The gap may be filled with a gas, a fluid or any other insulating material, and may further comprise spacers to maintain the coaxial configuration of the riser 13 and the buoyancy unit 16.
It is also possible to collect a plurality of riser pipes such as that shown in
It is also possible to collect a plurality of riser pipes such as that shown in
Naturally, the number and configuration of the riser pipes may be varied as expedient. For example, the rigid pipes may be arranged such that the bundle describes a circle in cross section, or a “flat pack” arrangement, in which the separate pipes are arranged in rows, or other arrangements as may be found to be advantageous.
The buoyancy unit is made of any suitable buoyant material, for example a foam. It may alternatively comprise a rigid hollow buoyant tank made of steel, composite material, aluminium or other materials as will readily occur to the skilled person, which is either an intrinsic part of the upper section of the rigid riser, or a separate tubular structure which is secured thereto. Such a tank may be filled with gas, or a foam or other buoyant material.
The upper part of each rigid riser (131, 132) may be connected to the buoyancy unit (16) by means of a hang-off arrangement or other bearing or fixture (251, 252), or may simply be welded or otherwise fixed thereto. A similar arrangement may be provided at other points along the length of the buoyancy unit (16). Furthermore, one or more sleeves (261, 262) may be attached to the buoyancy unit, with the rigid riser (131, 132) fitting slidingly through the sleeve. In this configuration, there is provided no permanent connection between the inside surface of the sleeve and the outer surface of the rigid riser, such that the mutual alignment of the buoyancy unit (16) and the rigid riser (131, 132) can be maintained, without exerting any substantial tensional force along the length of the riser pipe.
The mutual spacing of the riser pipes (131, 132) can be maintained below the lower extremity of the buoyancy unit (16) by means of a spacer (29) situated between two clamps (271, 272), each clamp connecting to the rigid riser (131, 132) respectively. The clamps may connect the riser fixedly or slidingly, so as to allow for expansion. By this means, the two rigid risers (131, 132) can be retained in a parallel or other desired configuration as they descend in a catenary manner to the sea floor. It may be desirable to employ spacers of different lengths along the length of the risers, so that the separation thereof changes as a function of distance from the sea bed.
Although as described above with reference to
Similarly,
It is also possible to construct the riser structure using conventional methods whilst at sea such as J-lay, reeling etc. For example, by adding pipe sections to the riser one by one as the pipe is deployed from a surface vessel. It may be appropriate to land the buoyancy tank 16 on the seabed and start reeling from there.
Furthermore, the invention is not limited to two riser pipes, but is further applicable to a larger plurality thereof. For example,
As will be clear to the skilled person, it is possible to combine the teachings of the above-described embodiments. For example,
All five risers (13, 131, 132, 133 and 134) can be retained in a desired configuration, for example one similar to that imposed by the arrangement of the risers through and around the buoyancy unit (16) respectively, by means of clamps (271, 272, 273 and 274). The central riser 13 may be replaced by a plurality of risers.
The installation of the riser is thus complete, and hydrocarbon transport may commence.
The skilled man will appreciate that variations may be made to the sequence in which the above steps are carried out. In particular, step d, where the buoyancy section is allowed to rise towards the surface of the sea, causing the rigid riser 13 to bend upwards to form a catenary configuration, may in fact take place before the end of the rigid riser 13 furthest from the buoyancy unit 16 is connected to the wellhead 41.
As a variation of the method described above with reference to
It is also possible to construct the riser structure using conventional methods whilst at sea such as J-lay, reeling etc. For example, by adding pipe sections to the riser one by one as the pipe is deployed from a surface vessel. In the case of reeling it may be appropriate to land the buoyancy tank 16 on the seabed and start reeling from there. More specifically, in accordance with such a constructional method successive lengths of rigid pipe section in the sea, each length being connected endwise to the length of pipe section below it. Then a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section is connected to an upper end of the length of rigid pipe section immediately below it, to form the rigid pipe. A flexible pipe is connected to an upper end of the rigid pipe, after which the flexible pipe is lowered, until the rigid pipe hangs suspended in the water from its upper end. Next the buoyancy unit is allowed to sink and a lower end of the rigid pipe is connected to a wellhead. Finally, to complete the installation, the position of the floating vessel is adjusted such that the rigid pipe assumes the configuration of a catenary in the seawater.
The opposite extremity of the rigid pipe 13, that is the end of this pipe furthest from the end of this pipe connected to the buoyancy unit 16, is released, and lowered to the seabed, where it is connected to the well head 41 or to another pipeline termination point, in a conventional manner. At this stage, the buoyancy of the buoyancy unit 16 can be increased, for example by evacuating sea water from the flooded tanks 21a and 21b, so that the buoyancy unit 16 floats towards the sea surface, as described with reference to
The upper, flexible pipe 10 may now be deployed in any conventional method, and connected to the upper end of the buoyancy unit 16 as described above. This process may be assisted by using the tether 44 as a guide.
The installation of the riser is thus complete, and hydrocarbon transport may commence.
The skilled man will appreciate that variations may be made to the sequence in which the above steps are carried out. In particular, the step where the buoyancy section is allowed to rise toward the surface of the sea, causing the rigid riser to bend upwards to form a catenary configuration may in fact take place before the end of the rigid riser 13 furthest from the buoyancy unit 16 is connected to the well head 41 or to another pipeline termination point.
According to an alternative method, it is also possible to construct the entire riser structure comprising at least one flexible pipe 10, at least one corresponding rigid pipe 13 connected thereto as described above and the buoyancy unit 16 disposed along and the lengthways of the rigid pipe nearest said flexible pipe on land, before towing the whole structure out to be installed at sea.
In all of the embodiments discussed in relation to FIGS. 6 to 19, and in particular those embodiments where there is provided no tethering means, as shown in, for example,
In all of the embodiments discussed in relation to FIGS. 6 to 19, the buoyancy unit 16 preferably has a length equal to at least twice its diameter. More preferably, the buoyancy unit 16 has a length equal to at least thirty times its diameter. Yet more preferably, the buoyancy unit 16 has a length equal to at least 100 times its diameter.
In any method of installing the riser of the present invention that involves towing all or part of the riser through the sea, the towed part may be towed using any conventional towing method. For example,
Any of the above three methods is appropriate for use in the installation methods of the present invention.
A further riser arrangement is now described with reference to
In this arrangement a number of service pipes (risers) 300 (each preferably, but not necessarily, rigid) are collected together into a bundle spaced preferably equidistantly from a further pipe (central core) 302 and held in spatially fixed relationship thereto by clamps 304 (see
Apart from any use of a separate foam buoyancy material 306 for the individual risers 300, it is necessary to provide further buoyancy for the whole riser arrangement. A conventional way of achieving this in the type of riser arrangement just described would be to employ a so-called “syntactic” foam material 308, shown in cross-section in
A way of achieving the above-mentioned aim will now be described, which allows the foam to be dispensed with completely (though, as mentioned, some supplementary foam can also be used, if desired). In this arrangement the core 302 is designed in the manner illustrated in
As regards the operation of the riser arrangement, it is firstly assumed that the arrangement illustrated in
When the liquid medium in any compartment has reached its minimum level, the valve associated with that compartment is closed. The pressure of the gas in that compartment has the same value as that of the surrounding water at that particular depth and therefore the pressure in each gas-filled compartment counterbalances the hydrostatic pressure of the water at the relevant depth. This means that the core need not be designed to withstand the maximum surrounding hydrostatic pressure, but a lesser value of pressure resistance can be tolerated, thereby saving costs in core materials. (As a rough figure, the core can be designed for typically less than 20% of the fully hydrostatic pressure). The cost reduction can take the form of a reduction in the thickness of the core wall and/or a downgrading of the core material. In addition, of course, there is the original saving in costs brought about by the elimination of the need to use a syntactic foam.
Where maximum buoyancy is required from the core, all the valves 330-335 will be opened at the appropriate time, as outlined above, in turn from the bottom to the top of the core. On the other hand, where only partial buoyancy is required, as soon as a sufficient number of compartments have been filled (or, more accurately, almost filled, due to the pressure of the residual liquid level) with gas, the valve associated with the compartment above the compartment which has last been filled remains closed, as do all the valves higher up the core.
As regards the installation of this riser arrangement, the arrangement is first fabricated on shore and then towed to the operating field in a horizontal attitude by means of tugs. At the field, the riser arrangement is up-ended and connected to the seabed foundation, after which flexible jumpers are attached to the upper ends of the risers 300.
While
The riser arrangement shown in
Although the embodiment so far described has assumed that the various valves will start off in a closed state, it is possible to arrange for them to remain open throughout the whole venting procedure just described. In this case, before the start of venting the liquid medium inside each compartment will not leak out due to the hydrostatic pressure exerted by the surrounding seawater. Also, during the venting process once a particular compartment has filled with high-pressure gas and the gas has been diverted to the next higher compartment, the gas will not escape to the surrounding environment due to the equal pressure outside and inside that compartment. Furthermore, once the core has reached its desired degree of buoyancy, i.e. a sufficient number of compartments have been gas-filled, the gas supply can be simply shut off at that point so that no further compartments are vented. Again, the valves associated with those further compartments can remain open.
In practice, however, it is preferred for the valves to be closed after the venting of their respective compartments. This is because, where the valves remain open, any appreciable vertical movement of the core could cause a transfer of fluid into/out of the compartments from/to the surrounding environment due to the finite pressure differences created by such movement, and this is considered to be undesirable.
Other ways of controlling the valves than have been described may be employed, while still allowing the desired degree of buoyancy to be created in the core.
The reduction in core wall-thickness which this invention allows can result in increased buoyancy solely by virtue of a decrease in weight in the core. However, buoyancy can also be enhanced by converting some of reduction in wall thickness to an increase in core diameter, which creates a higher internal core volume and hence an increase in buoyancy when gas is introduced. The advantage of this is that the individual supplementary foam material 306 provided for each service riser can be even further reduced in volume or even dispensed with altogether. The same applies to any foam that would normally be used to surround the core.
Although the above embodiments of the invention have been described in terms of a production riser connecting a surface vessel to a well head, the skilled person will realise that they are equally appropriate to other applications, for example tying a surface vessel in to a sea floor flowline, so as to function for example as an export riser. It will also be understood that the present invention is applicable to all types of riser. In particular, the present invention applies to water or gas injection risers.
Claims
1-55. (canceled)
56. A riser having a lower section and an upper section, said upper section comprising a flexible pipe and said lower section comprising a substantially rigid pipe and forming a catenary in communication with the flexible pipe, characterized in that
- said riser further comprises an elongate buoyancy section fitted around said rigid pipe at or in the region of an upper end thereof.
57. A riser according to claim 56 further comprising a tether.
58. A riser according to claim 57 wherein said buoyancy section is tethered by said tether to a vessel on the sea's surface.
59. A riser according to claim 57, wherein said buoyancy section is tethered by said tether to the seabed.
60. A riser according to any of claims 57 to 59 wherein a further tether is provided between a point on said rigid pipe above the high bending area thereof, and the seabed.
61. A riser according to any of claims 56 to 59 wherein said lower section comprises a plurality of rigid pipes and said upper sections each comprise a corresponding plurality of flexible pipes.
62. A riser according to claim 61 wherein said plurality of rigid pipes are arranged around the outside of said buoyancy section.
63. A riser according to claim 62 wherein said plurality of rigid pipes are spaced apart evenly about the circumference of said buoyancy section.
64. A riser according to claim 62 wherein each of said plurality of rigid pipes is fixed at or near an upper extremity thereof to said buoyancy section.
65. A riser according to claim 62, wherein said buoyancy section is further provided with a plurality of sleeves intended to slidingly receive said plurality of rigid pipes respectively.
66. A riser according to claim 61, further comprising a spacer connected to each of said plurality of rigid pipes, or to each of said plurality of flexible pipes so as to maintain said plurality of rigid pipes or each of said plurality pipes in a fixed position relative to the other rigid pipes or flexible pipes.
67. A riser according to claim 66, wherein said spacer is provided at a point on said rigid riser below a lower extremity of said buoyancy section.
68. A riser according to claim 61 wherein each of said at least one flexible pipes is joined to a respective one of at least one rigid pipes at a substantially right angle.
69. A riser according to claim 68 wherein the substantially right-angled joins between respective rigid and flexible pipes are spaced apart from one another along the length of said buoyancy section.
70. A riser according to any of claims 56 to 59, wherein said buoyancy section is made of a foam.
71. A riser according to any of claims 56 to 59, wherein said buoyancy section is a tube arranged such that the rigid pipe runs therethrough.
72. A riser according to any of claims 56 to 59, wherein said buoyancy section is made of one or more of: steel, titanium, aluminium and a composite material.
73. A riser according to any of claims 56 to 59, wherein said lower riser section, or parts thereof, is made of one or more of: steel, titanium, aluminium and a composite material.
74. A riser according to claim 71, wherein said tube arranged is arranged coaxially to said rigid pipe.
75. A riser according to any of claims 56 to 59, wherein said buoyancy section further comprises a plurality of bulkheads dividing said buoyancy section into a plurality of closed chambers.
76. A riser according to claim 75 wherein at least one valve is provided allowing flow of a fluid from inside said rigid pipe to the interior of a respective at least one of said closed chambers.
77. A riser according to claim 75, wherein at least one valve is provided allowing flow of a fluid from inside a respective at least one of said closed chambers to the exterior of said buoyancy section.
78. A riser according to any of claims 56 to 59, wherein an upper extremity of said rigid pipe is aligned away from the axis of the part of said riser immediately below said upper extremity.
79. A riser according to any of claims 56 to 59 wherein said buoyancy section is divided along its length into a second plurality of separate sections, each section having a vent valve communicating with the outside of the buoyancy section, one end of the buoyancy section having an inlet means for the introduction of a high-pressure gas and the buoyancy section comprising a third plurality of open-ended tubes extending between respective pairs of adjacent sections.
80. A riser according to claim 79, wherein the inlet means is at a lower end of the buoyancy section and the vent valves are disposed near correspondingly lower ends of their respective sections, each tube having a correspondingly lower end situated in a first of its pair of adjacent sections and aligned approximately with the vent valve associated with that first section and a correspondingly upper end situated in a second of its pair of adjacent sections and aligned approximately with the vent associated with that second section.
81. A riser according to claim 80, wherein the third plurality is equal to the second plurality, one of the tubes having a lower end which is located in the topmost section of the further pipe, and an upper end which communicates with the outside of the further pipe.
82. A riser according to claim 81, further comprising a control means connected to the vent valves and to the gas inlet and arranged to open each valve in turn starting with the lowest valve, the valve associated with each succeeding section being opened once a water level inside the preceding section has lowered to the point where it reaches the lower end of the tube associated with the relevant succeeding section and preceding section.
83. A riser according to claim 79, wherein the rigid pipes are arranged around the buoyancy section in an approximately circular configuration.
84. A riser according to claim 79, wherein each rigid pipe is provided with its own supplementary buoyancy means.
85. A riser according to claim 79, wherein the buoyancy section is provided with its own supplementary buoyancy means.
86. A riser according to claim 85, wherein the supplementary buoyancy means is constituted by a foam cladding surrounding the respective rigid pipe and/or buoyancy section.
87. A riser according to any of claims 56 to 59, wherein the control means is arranged to, in sequence:
- (a) close all the valves when the core is in a state of minimum buoyancy;
- (b) apply the gas supply to the gas inlet;
- (c) open the lowermost vent valve nearest the gas inlet, thereby to allow a liquid medium present in the section nearest the gas inlet to leave that section by way of said valve;
- (d) open the next valve up when gas starts to be introduced into the next section up;
- (e) repeat step (d) until the desired or a maximum degree of buoyancy has been attained;
- (f) stop the gas supply.
88. A riser according to claim 87, wherein the control means is configured so that the valve associated with each succeeding section is opened once a water level inside the preceding section has lowered to the point where it reaches the lower end of the tube associated with the relevant succeeding section and preceding section.
89. A method of installing a riser, said method comprising;
- i. constructing a rigid pipe and a buoyancy section on land,
- ii. weighting said buoyancy section, such that the buoyancy of the riser is negative,
- iii. towing said rigid pipe section and the buoyancy section out to sea to the location where the riser is to be installed by at least a first tug using a first tether,
- iv. allowing said rigid pipe and the buoyancy section to land on seabed,
- v. connecting the end of the rigid pipe furthest from the buoyancy section at a wellhead or flowline tie-in,
- vi. removing said weighting from the buoyancy section, such that a buoyancy force is exerted on the buoyancy section,
- vii. allowing the buoyancy section to rise towards the surface of the sea under the guidance of said at least one installation/surface vessel, such that the rigid pipe bends upwards to form a catenary configuration.
90. The method of claim 89, wherein said step of weighing involves at least partially flooding said buoyancy section, and said step of removing said weighting involves expelling fluid from the buoyancy section.
91. The method of claim 89 or 90, wherein one or more temporary buoyancy elements are connected to said rigid riser such that buoyancy is distributed along the length of the riser substantially evenly.
92. The method of claim 89 or 90, wherein the lower end of said rigid pipe is connected to said well head or flowline by means of jumpers or spools.
93. The method of claim 92, wherein a manifold, wellhead, riser base or further subsea structure is connected to the rigid pipe and towed to field together with the riser in one operation.
94. The method of claim 89 or 90, wherein said rigid pipe and the buoyancy section are towed out to sea to the location where the riser is to be installed further using a second tug and a second tether, said second tether being connected to a point along the rigid pipe behind the point to which said first tether is connected.
95. The method of claim 89 or 90, wherein said rigid pipe is pressurized with a gas prior to said step of expelling fluid from the buoyancy unit.
96. The method of claim 89 or 90 comprising the further step of a flexible pipe between said installation/surface vessel and the upper end of the rigid pipe.
97. The method of claim 89 or 90 comprising the further step of constructing the entire riser structure on land.
98. The method of any of claims 79 to 88, further comprising the step of attaching a tether between said buoyancy unit and said surface/installation vessel or the seabed.
99. The method of claim 79 or 89, further comprising the step of attaching a tether between a point on said rigid pipe above the high bending area thereof, and the seabed.
100. A method of installing a riser, said method comprising:
- i. lowering a rigid pipe into the sea by reeling or by lowering successive lengths of rigid pipe section into the sea, each length being connected endwise to the length of pipe section below it;
- ii. connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy section extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe;
- iii. connecting a flexible pipe to an upper end of said rigid pipe;
- iv. lowering the flexible pipe;
- v. allowing the buoyancy unit to sink;
- vi. adjusting the position of the floating vessel such that the rigid pipe assumes the configuration of a catenary in the sea water and;
- vii. connecting a lower end of the rigid pipe to a wellhead or flowline.
101. The method of claim 100, wherein said buoyancy unit is allowed to sink before said steps of:
- lowering a rigid pipe into the sea; and
- connecting a lower end of a further length of rigid pipe section having a buoyancy section comprising an elongate buoyancy unit extending lengthwise of said further length of rigid pipe section to an upper end of the length of rigid pipe section immediately below it, to form said rigid pipe.
102. The method of claim 100 or 101, further comprising the step of attaching a tether between a point on said rigid pipe above the high bending area thereof, and the seabed.
Type: Application
Filed: Oct 10, 2002
Publication Date: Mar 24, 2005
Inventor: Terje Clausen (Stavanger)
Application Number: 10/492,222