GB2500102A

GB2500102A - Riser tower with supplementary buoyancy modules

Info

Publication number: GB2500102A
Application number: GB1203849.3A
Authority: GB
Inventors: Jean-Francois Saint-Marcoux; Blaise Seguin
Original assignee: Acergy France SAS
Current assignee: Acergy France SAS
Priority date: 2012-03-05
Filing date: 2012-03-05
Publication date: 2013-09-11
Anticipated expiration: 2032-03-05
Also published as: GB201207050D0; GB2500102B; GB201203849D0; GB2500071A

Abstract

A hybrid riser tower comprises at least one support element 30 and at least one riser element 34 both attached to an upper structure. Those elements extend axially in parallel as a riser column that can be supported buoyantly in an upright orientation, with the riser element hanging from the upper structure. Buoyancy modules 38 are attached at respective load-bearing locations along the length of the support element 30 to impart axial upthrust to the support element when the riser column is upright. The invention also provides at least one supplementary buoyancy module 42 that is attached to the riser element 34 to impart axial upthrust to the riser element 34 when the riser column is upright. This reduces or eliminates weight loading applied by the riser element to the upper structure and so reduces or eliminates compression loading in the support element beneath the upper structure, as experienced particularly in the up-ending phase of installation.

Description

1

Buoyancy arrangements for hybrid riser towers

This invention relates to subsea risers, as used in the oil and gas industry to transport well fluids from the seabed to a surface installation such as an FPSO 5 vessel or a platform. The invention relates particularly to hybrid riser towers and more particularly to buoyancy arrangements for such towers.

Hybrid riser systems have been known for many years in the development of deepwater and ultra-deepwater fields. They comprise a subsea riser support 10 extending from a seabed anchor to an upper end held buoyantly in mid-water, at a depth below the influence of likely wave action. A depth of 70m to 250m is typical for this purpose but this may vary according to the sea conditions expected at a particular location.

15 An example of a hybrid riser system is a hybrid riser tower or 'HRT', used for instance in the Girassol and Greater Plutonio field developments lying in approximately 1200m to 1500m of water off Angola. An HRT is pivotably attached to its anchor and is held in tension by buoyancy. Riser pipes extend upwardly from the seabed to the upper end region of the riser support, as an upright bundle of generally 20 parallel pipes defining a riser column.

Flexible jumper pipes hanging as catenaries extend from the upper end of the riser column to an FPSO or other surface installation. The jumper pipes add compliancy that decouples the more rigid riser pipes from surface movement induced by waves 25 and tides. The riser pipes experience less stress and fatigue as a result.

Umbilicals and other pipes generally follow the paths of the riser pipes and jumper pipes to carry power, control data and other fluids. As a result, the bundle in the riser column may comprise some pipes used for oil production, some pipes used for 30 injection of water and/or other fluids, some gas-lift lines and/or some other pipes used for oil and gas export. Those pipes are clustered around a central core that may be a hollow, solely structural tube or pipe or that may convey fluids in use.

In the Girassol development, for example, each riser tower is a bundle comprising 35 one 22-inch (55.9 cm) structural core tube surrounded by four 8-inch (20.3 cm)

production risers; four 3-inch (7.6 cm) gas lift lines; two 2-inch (5.1 cm) service lines; and two 8-inch (20.3 cm) injection risers for either water or gas injection.

2

Buoyancy in an HRT is typically provided by a buoyancy tank or block at an upper end of the riser column. The upthrust of that top buoyancy tank or block may be supplemented by buoyancy modules distributed along the riser column. It has also 5 been proposed to rely solely upon buoyancy modules distributed along the riser column and to omit a buoyancy tank or block at the upper end of the riser column.

In some HRTs, the risers and other lines are guided and retained relative to the central core tube by foam elements that provide buoyancy and insulation. In the 10 Girassol development, for example, insulating material in the form of syntactic foam blocks surrounds the central core tube and the pipes and separates conduits carrying hot and cold fluids. The syntactic foam blocks also serve as buoyancy modules that add buoyancy to the HRT.

15 It is also known for HRTs to have a series of guide frames spaced along their length, to guide and retain the risers and other lines relative to the core tube. In some applications, the guide frames also transfer buoyancy loads from buoyancy modules to the core tube. In this respect, WO 02/053869 to Stolt and WO 2010/035248 to Acergy disclose some examples of guide frames for HRTs. Their content is 20 incorporated herein by reference.

WO 2010/035248, for example, teaches that a guide frame may be composed largely of a non-metallic material, for example a plastics material such as polyurethane (PU). However a guide frame may instead be composed largely of a metal such as steel or 25 a combination of metals and plastics.

The guide frame disclosed in WO 2010/035248 is in two halves that clamp together around a central core tube to grip the core tube with frictional engagement. Similarly, buoyancy modules may be placed around the core tube and bolted or strapped 30 together so that their buoyant load is also transmitted to the core tube by friction. It is also possible for buoyancy modules to be carried by a guide frame to impart their buoyant load to the guide frame and thus, indirectly, to the core tube to which the guide frame is attached. It should be noted that where two or more elements (whether a flowline or a structural core tube) of a riser column are bundled together, 35 buoyancy modules cannot generally be clamped to more than one of those elements due to differential thermal expansion between them.

3

Another prior art document, namely WO 2004/051051 to Stolt, discloses buoyancy arrangements for HRTs; again, its content is incorporated herein by reference. It notes that as the buoyancy of HRTs is achieved by fixing buoyancy modules to the 5 structure, the buoyant upthrust acts upon the structure where it is generated, i.e. at the fixed locations of the buoyancy modules.

Buoyancy is required to support an HRT in different orientations, particularly a generally horizontal orientation when being fabricated and towed to an installation 10 site and an upright, substantially vertical orientation when in operation. Distributed buoyancy forces acting on the structure where they are generated may be advantageous in one orientation, particularly when supporting horizontal risers during fabrication and installation, but they may become a hindrance when the tower is upended into an upright orientation for use.

15

Specifically, during the up-ending phase of installation, when the HRT is oriented substantially vertically for operation with the risers hanging freely from the top structure of the HRT, the induced thrust associated with weight compensation induces a large axial compressive load along the length of the element (typically the 20 core tube) to which the buoyancy modules are attached. This is because, in the upending phase, the buoyancy provided by the top buoyancy tank has not been fully mobilised: in the case of an integral top tank, de-ballasting has not yet taken place; or in the case of a separate top tank, the top tank has not yet been installed. Compression forces increase with increasing water depth for the same cross-section, 25 and particularly with heavy risers such as pipe-in-pipe (PiP) risers.

The compression forces are at their greatest where the core tube attaches to the top of the structure from which the surrounding risers hang, as the core tube at that point has to balance the weights of the risers against the thrust of the buoyancy modules 30 below.

The up-ending phase of installation is the critical loading for the risk of buckling of the core pipe, which needs to be oversized accordingly. However if the core tube is sized to resist axial compression caused by distributed buoyancy, this increases its 35 diameter and wall thickness. This, in turn, increases its susceptibility to fatigue during towing before installation, due to long waves.

4

WO 2004/051051 therefore aims to transfer the substantial compressive forces generated by the buoyancy modules directly to the top of the riser column, once installed, rather than via one or more of the elongate elements that make up the column. It does so by providing for the buoyancy modules to slide along the core 5 tube, such that when the HRT is deployed at sea in an upright orientation, the buoyancy modules are free to slide up the core tube.

In this way, the buoyant upthrust of each module acts upon the module above it rather than locally at distributed locations along the core tube. The result is that the 10 cumulative upthrust from the buoyancy modules acts substantially against the top of the HRT structure. This allows all of the suspended weight of the HRT to be taken from its top, where all of the buoyancy force is applied.

As a result, no single part of the structure of WO 2004/051051, in particular the core 15 tube, has to support too much of the structure's weight. Instead, the transfer of upthrust to the top of the structure is performed by the buoyancy modules, rather than via the core tube along which the buoyancy modules are distributed. The high compressive loads that would otherwise be imposed on the core tube are therefore avoided.

20

The arrangement disclosed in WO 2004/051051 presents some challenges. For example, high compressive loads must instead be borne by the upper buoyancy modules. Being made largely of foam, the mechanical properties of the buoyancy modules are limiting in this respect. Also the buoyancy modules must be fixed with 25 respect to the core tube in transit to an installation site and must then be released to enable them to slide relative to the core tube.

It is against this background that the present invention has been devised.

30 Broadly, the invention resides in a hybrid riser tower comprising at least one support element to be anchored to the seabed in service and at least one riser element both attached to an upper structure, those elements having substantial length and extending generally in parallel in an axial direction as a riser column that can be supported buoyantly in an upright orientation for use, whereupon the riser element 35 hangs from the upper structure. One or more buoyancy modules are attached at one or more respective load-bearing locations along the length of the support element to impart generally axial upthrust to the support element when the riser column is in the

5

upright orientation. At least one supplementary buoyancy module is attached to the riser element, preferably along the length of the riser element, to impart generally axial upthrust to the riser element when the riser column is in the upright orientation. This reduces or eliminates weight loading applied by the riser element to the upper 5 structure and so reduces or eliminates compression loading in the support element between the upper structure and the or each load-bearing location on the support element.

The riser element will generally serve as a flowline, which in this specification is 10 simply intended to mean a line arranged to convey fluids. The support element may also serve as a flowline although it could be a solely structural member such as a core tube instead. Where a buoyancy module or supplementary buoyancy module is attached to a flowline, it is preferably spaced from the flowline by one or more channels arranged for convective flow of water over the flowline.

15

The support element and the riser element are suitably linked to each other along the riser column in a manner permitting relative axial movement between those elements under differential extension. For example, a buoyancy module and/or a guide frame attached to the support element may restrain the riser element against cross-axial 20 movement with respect to the support element, while still permitting relative axial movement between those elements.

A supplementary buoyancy module attached to the riser element can move relative to the support element under differential extension between the support element and 25 the riser element. The supplementary buoyancy module may lie wholly or partially within a radius to which a buoyancy module and/or a guide frame attached to the support element extends cross-axially from the support element. In that case, axial clearance is advantageously maintained between the buoyancy module and/or the guide frame and the supplementary buoyancy module to allow differential extension 30 between the support element and the riser element.

A buoyancy tank or block may be integrated with, attached to or attachable to the upper structure, to tension the support element when the riser column is upright for use. It is also possible for buoyancy module(s) attached to the support element and 35 supplementary buoyancy module(s) attached to the riser element together to add sufficient buoyancy to keep the riser column upright in use.

6

The support element and/or the riser element suitably comprises a coated or sleeved pipe having at least one stop formation that is integral with an external coating or sleeve of the pipe or that is attached to the pipe via that external coating or sleeve. The stop formation may be predominantly of plastics and is arranged to restrain 5 movement along the pipe of a guide frame or a buoyancy module (including a supplementary buoyancy module) attached to the pipe. To avoid local cold spots, the stop formation is advantageously insulated from the pipe by the external coating or sleeve of the pipe.

10 The stop formation preferably comprises a shoulder that faces downwardly when the riser column is upright, to resist upthrust acting on a guide frame or buoyancy module when attached to a pipe. It is also possible for the stop formation to comprise an upwardly-facing shoulder to resist weight load acting on a guide frame or buoyancy module when attached to the pipe. A key formation may be provided to resist rotation 15 about the pipe of a guide frame or buoyancy module when attached to the pipe.

The stop formation may extend radially outwardly beyond a general outer diameter of the pipe; it is also possible for the stop formation to be machined from the external coating or sleeve of the pipe. In another arrangement, the stop formation is part of a 20 stop bracket attached to the external coating or sleeve of the pipe. Such a stop bracket may be predominantly of plastics and be welded or bonded to the external coating or sleeve of the pipe and optionally also to an exposed wall of the pipe.

In general, the guide frame or buoyancy module is preferably attached to the pipe by 25 a primary attachment provision such as clamping and the stop formation is a secondary, auxiliary or fall-back provision for attachment or location of the guide frame or buoyancy module.

The inventive concept extends to a method of up-ending a hybrid riser tower during 30 installation to define an upright riser column comprising at least one support element attached to an upper structure and at least one riser element hanging from the upper structure in an axial direction generally parallel to the support element, the method comprising:

35 using one or more attached buoyancy modules to impart generally axial upthrust to the support element when the riser column is upright; and

7

separately, using one or more attached supplementary buoyancy modules to impart generally axial upthrust to the riser element when the riser column is upright, reducing or eliminating compression loading in the support element beneath the upper structure by reducing or eliminating weight loading applied 5 by the riser element to the upper structure.

The method of the invention may further comprise adding buoyancy to the upper structure when the riser column is upright to reduce or eliminate compression loading in the support element beneath the upper structure, or to place that part of the 10 support element under tension.

As a general concept, tensioning by buoyancy to avoid compression of a riser is well known in the prior art. For example, compression of a bundle of risers or the core of an HRT is known to be solved by employing an overall buoyancy tank at the top of 15 the riser column, or distributed buoyancy modules that are attached to one pipe of the bundle although they may embrace and incorporate all of the pipes of the bundle. However, the prior art does not suggest the approach of the invention, which mitigates the cause of compression by using dedicated, localised buoyancy to counterbalance the individual weights of one or more (and preferably all) risers of the 20 HRT.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

25 Figure 1 is a much-simplified schematic perspective view of a subsea oil-

production installation including HRTs, to put the invention into context;

Figure 2 is a schematic part-sectioned side view showing a simplified riser column of an HRT in accordance with the invention, comprising a structural 30 core tube that need not be a flowline, between two riser pipes that are flowlines;

Figure 3 is a simplified force diagram superimposed onto a prior art HRT arrangement;

35

8

Figure 4 is a simplified force diagram corresponding to Figure 3 but in this instance superimposed onto an HRT arranged in accordance with the invention;

5 Figure 5 is an enlarged schematic view being Detail V in Figure 2, showing a riser pipe in longitudinal section in a plane containing the central longitudinal axis of the pipe, showing circumferentially-extending stop formations integral with a continuous 3LPP or 5LPP coating of the pipe; and

10 Figure 6 is an enlarged schematic detail view corresponding to Figure 5 but showing the riser pipe in this case coated with a 5LPP coating that is cut away and machined to define circumferentially-extending stop formations.

Referring firstly to Figure 1, this shows a subsea oil-production installation 10 15 comprising well heads, injection sites, manifolds and other pipeline equipment generally designated 12 located on the seabed 14 in an oil field. This drawing is not to scale: in particular, the water depth will be very much greater in practice than is suggested here.

20 Upright HRTs 16 convey production fluids from the seabed 14 to the surface 18 and convey lifting gas, injection water and treatment chemicals such as methanol from the surface 18 to the seabed 14. For this purpose, the base of each HRT 16 is connected to various well heads and injection sites 12 by horizontal pipelines 20. Further pipelines 22 optionally connect to other well sites elsewhere on the seabed 25 14.

The HRTs 16 are pre-fabricated at shore facilities, towed horizontally to their operating location and then upended and installed on the seabed 14 with anchors at the bottom and buoyancy at the top provided by a buoyancy tank 24. Optionally, 30 additional buoyancy is provided by buoyancy modules distributed along the riser column of each HRT 16. Such distributed buoyancy modules are omitted from Figure 1 for clarity but will be described in the context of the invention with reference to Figures 2 and 4 to 6 of the drawings.

35 Each HRT 16 comprises a bundle of pipes defining separate but parallel conduits for the various fluids that those pipes carry individually. Most or all of those pipes will

9

typically be of steel. One of more of the pipes will be a structural core tube that may or may not also serve as a flowline.

Polypropylene (PP) is commonly used to coat a steel pipe in subsea applications to 5 mitigate corrosion and to prevent mechanical damage; the coating also insulates the pipe to some extent. For example, a three-layer PP (3LPP) coating may be used for, predominantly, corrosion protection and mechanical protection, and a five-layer PP (5LPP) coating may be used for additional thermal insulation. Additional layers are possible.

10

A 3LPP coating typically comprises an epoxy primer applied to the cleaned outer surface of the pipe. As the primer cures, a second thin layer of PP is applied so as to bond with the primer and then a third, thicker layer of extruded PP is applied over the second layer for mechanical protection. A 5LPP coating adds two further layers, 15 namely a fourth layer of PP modified for thermal insulation e.g. glass syntactic PP (GSPP) or a foam, surrounded by a fifth layer of extruded PP for mechanical protection of the insulating fourth layer.

Guide frames are distributed along the riser column in a manner similar to that of WO 20 2010/035248. Buoyancy modules, where used, may apply upthrust directly to the riser column or indirectly via the guide frames. The guide frames may also be used to guide or support umbilicals, optical fibres, cables and the like included in the HRT 16.

Flexible flowlines 26 extend in a catenary configuration from the riser column of each 25 HRT 16 to a floating production, storage and offloading (FPSO) vessel 28 moored nearby at the surface 18. The FPSO vessel 28 provides production facilities and storage for the fluids coming from and going to the seabed equipment 12.

Referring now to Figure 2, a core tube 30 in the riser column of an HRT 16 as shown 30 in Figure 1 supports a guide frame 32 that extends generally in a plane orthogonal to a central longitudinal axis of the core tube 30. The guide frame 32 is clamped to the core tube 30 although such a fixing may be supplemented or replaced by stoppers on the core tube that effect mechanical engagement with the guide frame 32 in a manner known from the prior art.

35

Parallel riser pipes 34 in the riser column bundle surrounding the core tube 30 extend through openings 36 in the guide frame 32. Two such riser pipes 34 are shown in this

10

simplified view but as explained above, there will in practice be additional flowline pipes and also other elongate elements such as umbilicals or cables in the bundle. The riser pipes 34 may be insulated, non-insulated or of PiP construction as shown in this example.

5

The guide frame 32 holds the core tube 30 and the riser pipes 34 relative to each other against horizontal movement in the two lateral dimensions (X-and Y-axis movement). However the guide frame 32 does not constrain relative longitudinal (Z-axis) movement between the core tube 30 and the riser pipes 34 to allow for 10 differential elongation and contraction of the core tube 30 and the riser pipes 34 under pressure and temperature fluctuations in use. For this purpose, the guide frame 32 is fixed axially to the core tube 30 but is not fixed to the riser pipes 34,

which are therefore free to move axially through the openings 36 in the guide frame 32 as may be dictated by differential expansion between the core tube 30 and the 15 riser pipes 34.

In a manner known in the prior art, buoyancy modules 38 are positioned under the guide frame 32 to impart upthrust to the core tube 30. The buoyancy modules 38 may impart upthrust directly to the core tube 30 by being clamped to or mechanically 20 engaged with the core tube 30. Alternatively, or as a back-up in case of slippage, the buoyancy modules 38 may impart upthrust indirectly to the core tube 30 via the guide frame 32, by bearing against the underside of the guide frame 32.

Like the guide frame 32, the buoyancy modules 38 have openings 40 that allow the 25 riser pipes 34 to move axially through the buoyancy modules 38 as may be dictated by differential expansion between the core tube 30 and the riser pipes 34.

In accordance with the invention, at least one (and preferably, as shown, all) of the riser pipes 34 carry supplementary buoyancy modules 42 that impart upthrust directly 30 to the riser pipes 34. For this purpose, the supplementary buoyancy modules 42 are fixed to the riser pipes 34 and are not attached to the core tube 30. The supplementary buoyancy modules 42 are therefore decoupled from the core tube 30 and so are free to move with the riser pipes 34 with respect to the core tube 30 under differential expansion between the core tube 30 and the riser pipes 34.

35

The supplementary buoyancy modules 42 are preferably arranged as one module or block for each riser pipe 34. A supplementary buoyancy module 42 is designed to

11

balance the weight in water of the associated riser pipe 34 empty of fluid; the length and hence the buoyancy of the module 42 may be adjusted to allow for expansion of the riser pipe 34 under its weight, pressure and temperature.

5 If buoyancy foam is qualified for cyclic and impact loading, it is possible to enclose the riser pipes 34 with buoyancy. Alternatively, a guide frame 32 can support the buoyancy and take cyclic and impact load for the riser pipes 34.

Vertical spacing is maintained as an expansion clearance between the 10 supplementary buoyancy modules 42 on the riser pipes 34 and the guide frame 32. If the guide frame 32 is omitted, such clearance is maintained between the buoyancy modules 38 on the core pipe 30 and the supplementary buoyancy modules 42.

The effect of the invention will be apparent from a comparison of the force diagrams 15 in Figures 3 and 4 of the drawings. Figure 3 shows the problem of the prior art whereas Figure 4 shows how the invention solves that problem. Like numerals are used for like parts. In each case, a central core tube 30 is disposed between two riser pipes 34 in a parallel bundle. A top structure 48 at the upper end of the core tube 30 connects the core tube 30 and the riser pipes 34. That top structure 48 may or may 20 not have some added buoyancy of its own.

In Figure 3, substantially all added buoyancy 38 of the HRT 16 is distributed along the core tube 30. This added buoyancy 38 is shown schematically in Figure as an elongate feature extending along the core tube 30. In practice, of course, added 25 buoyancy will generally be distributed in discrete modules spaced along the core tube 30 as described previously. The riser pipes 34 in Figure 3 have no added buoyancy and so all of their weight down to the seabed hangs from the top structure 48. The result is a large compressive load in the top section of the core tube 30 above the buoyancy 38 and below the top structure 48.

30

In Figure 4, a lesser amount of added buoyancy 38 is distributed along the core tube 30. The aggregate buoyancy of the HRT 16 is maintained by the addition of supplementary buoyancy modules 42 to each riser pipe 34. This added buoyancy on the riser pipes 34 partially or fully offsets the weight load they apply to the top 35 structure 48. The result is a greatly reduced compressive load in the top section of the core tube 30 above the buoyancy 38 and below the top structure 48.

12

A small compressive load is shown in Figure 4, which may be attributed to the weight of the top section of the core tube 30. However, if the top structure 48 has sufficient buoyancy and the supplementary buoyancy 42 added to each riser pipe 34 is great enough, the top section of the core tube 30 may experience no compression at all 5 and may remain in tension. After the up-ending phase of installation, it is of course possible to mobilise buoyancy optionally associated with the top structure 48 - such as a top buoyancy tank or blocks integral with or attached to the top structure 48 - to apply tension to the core tube 30.

10 As the riser pipes 34 are flowlines that typically carry hot fluids in use, it is much preferred not to weld fastenings to them such as radially-projecting stoppers to resist upward movement of the supplementary buoyancy modules 42 under buoyant upthrust. There is a reluctance from some oil and gas operating companies to have structures welded to a pipe that serves as a flowline for hydrocarbon fluids. It is 15 common practice in the oil and gas industry to use forged fittings (such as hanger flanges or J-lay collars) in order to limit the manufacturing process to girth welds. In addition, for hot hydrocarbon fluids susceptible to hydrate formation, effective insulation must be ensured consistently along all points in the flowline, which means that localised 'cold spots' - such as may be caused by the thermal bridging effect of 20 structures such as connectors or stoppers attached to a pipe - have to be avoided.

Like the alternatives of a welded collar or a forged piece, a welded stopper cannot be used on flowlines in HRT bundles for other reasons. For example, it may interrupt the continuity of coating on the pipe. Also, fillet welds on fluid-carrying pipelines are best 25 avoided because they can locally jeopardise the properties of the steel from which the pipe is made. This would at least require specific welding and coating qualifications and intensive non-destructive testing during fabrication, which is time-consuming and costly.

30 For these reasons, it is preferred that a supplementary buoyancy module 42 is fixed longitudinally to the associated riser pipe 34 by radially-extending upper 44 and lower 46 stop formations of plastics that are integral with or attached to a PP coating 50 of that riser pipe 34. In this respect, reference is also made to Figures 5 and 6 of the drawings which show a coated riser pipe 34 supporting a supplementary buoyancy 35 module 42 that embraces the riser pipe 34. Other parallel pipes, umbilicals and cables are omitted from those drawings for clarity.

13

The possibility of plastics stop formations, optionally made from the coating surrounding the core tube 30 and the riser pipes 34, makes it easier to attach the foam to the core tube 30 and/or to the riser pipes 34. The attachment principles are similar to those discussed in our co-pending UK Patent Application No. 1201560.8, 5 the content of which is incorporated herein by reference and is reflected in the description that follows.

In the example shown in Figures 2 and 5, the stop formations 44, 46 are ridges or collars that extend circumferentially and continuously around each riser pipe 34.

10 Each stop formation 44, 46 comprises a shoulder 52 that lies in a plane orthogonal to a central longitudinal axis of the riser pipe 34, and a frusto-conical face 54 that faces generally away from the shoulder 52 and tapers down to the surrounding outer diameter of the riser pipe 34.

15 The stop formations 44, 46 are spaced axially from each other along each riser pipe 30 as a mutually-opposed pair. Their shoulders 52 are in facing relation, sandwiching an inwardly-facing flange 56 of each supplementary buoyancy module 42. The cross-sectional shapes of the stop formations 44, 46 are preferably mirrored about the central plane of that flange 56 as shown.

20

Thus, the shoulders 52 of the stop formations 44, 46 define between them a circumferential groove 58 that extends around the riser pipe 34. The groove 58 provides axial location for the inwardly-facing flange 56 of the supplementary buoyancy module 42 in opposite axial directions.

25

It will be apparent that the stop formations 44, 46 resist movement of the supplementary buoyancy module 42 in either axial direction along the supporting riser pipe 34, as could otherwise occur when the riser pipe 34 is horizontal during fabrication of the HRT 16 and when the HRT 16 is under tow before installation. Also,

30 the upper stop formation 44 resists upward axial movement of the supplementary buoyancy module 42 along the supporting riser pipe 34 under buoyant upthrust when the riser pipe 34 is upright in use of the HRT 16.

Whilst the stop formations 44, 46 could, in principle, together suffice to fix a

35 supplementary buoyancy module 42 axially with respect to the associated riser pipe 34, it is preferred that clamping is the primary means of attachment between the supplementary buoyancy module 42 and the supporting riser pipe 34. For example,

14

the supplementary buoyancy module 42 may be in two or more parts that are assembled around the riser pipe 34 to apply radially inward clamping force against the riser pipe 34 at the base of the groove 58.

5 Where the supplementary buoyancy module 42 clamps to the riser pipe 34, the stop formations 44, 46 are a back-up provision in case the clamping force slackens and the supplementary buoyancy module 42 could otherwise slip along the riser pipe 34 as a result. Nevertheless it is preferred that the inwardly-facing flange 56 of the supplementary buoyancy module 42 is a snug fit in the groove 58 to preserve 10 insulation. Insulation is also aided by the supplementary buoyancy module 42 being of foam material, as is preferred.

Turning finally to Figure 6 of the drawings, this illustrates an alternative fixing approach embraced by the inventive concept. Like numerals are used for like parts. 15 Figure 6 shows how stop formations 44, 46 may be machined or milled by removing material from the coating 50 of the riser pipe 34, so that the stop formations 44, 46 are defined wholly or partially within the surrounding outer diameter of the coated riser pipe 34. This approach is particularly apt to be used where a riser pipe 34 has a thicker 5LPP coating 50; it exploits the thickness of that coating 50 to allow stop 20 formations 44, 46 to be machined or milled from the coating 50 itself.

Radial cuts may be made at intervals through the coating 50 in planes orthogonal to a central longitudinal axis of the riser pipe 34. Between two such cuts, the coating 50 is machined or milled away to form a recess with a base profile defining axially-25 spaced, radially-extending circumferential stop formations 44, 46. A circumferential groove 58 between the stop formations 44, 46 is arranged to receive an inner flange 56 of a supplementary buoyancy module 42.

In this variant of the invention, radially-extending (but not radially protruding) upper 30 44 and lower 46 stop formations of plastics are integral with the coating 50 of the riser pipe 34, in this example comprising circumferentially-extending shoulders 52 within the outer diameter of the coated riser pipe 34. The shoulders 52 lie in parallel planes orthogonal to a central longitudinal axis of the riser pipe 34 and are spaced axially from each other along the riser pipe 34 as a mutually-opposed pair to 35 sandwich an inner flange 56 of a supplementary buoyancy module 42. Thus, the shoulders 52 define between them a circumferential groove 54 that extends around

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the riser pipe 34. The groove 54 provides axial location for the supplementary buoyancy module 42 in opposite axial directions with respect to the riser pipe 34.

Again, it is preferred that clamping is the primary means of attachment between the 5 supplementary buoyancy module 42 and the riser pipe 34, with the shoulders 52 being a back-up provision in case the clamping force slackens and the supplementary buoyancy module 42 could otherwise slip along the riser pipe 34.

In the example shown in Figure 2, each supplementary buoyancy module 42 is fixed 10 to the supporting riser pipe 34 at two locations. Specifically, two sets of stop formations 44, 46 are spaced axially from each other along the riser pipe 34, and each supplementary buoyancy module 42 has two inwardly-facing flanges 56 with corresponding spacing. However it would be possible to fix each supplementary buoyancy module 42 to the supporting riser pipe 34 at one location or at more than 15 two locations; it would also be possible to achieve fixing in other ways such as entirely by clamping or by mechanical engagement.

Similarly, although clamping is shown as the means of fixing the guide frame 32 and the buoyancy module 38 to the core tube 30 in Figure 2, it would be possible to adopt 20 an alternative fixing solution employing stop formations on the core tube 30

embracing the cross-sectional thickness of the guide frame 32. Also, it would instead be possible to fix the buoyancy module 38 to the core tube 30, or to fix a supplementary buoyancy module 42 to the supporting riser pipe 34, by a ring mounted on the core tube 30 or on the riser pipe 34 as appropriate.

25

As heat transfer by convection in seawater is known to be highly effective, water channels may be included between a supplementary buoyancy module 42 and a supporting riser pipe 34 to minimise heat transfer between the riser pipe 34 and the surrounding foam. Similarly, where the core tube 30 also serves as a flowline, water 30 channels may also be included between the core tube 30 and the buoyancy module 38 that is attached to the core tube 30.

For example, Figure 2 shows the supplementary buoyancy modules 42 spaced from the supporting riser pipes 34, in which case the resulting gaps may serve as water 35 channels. The inwardy-facing flanges 56 of the supplementary buoyancy modules 42 may be interrupted circumferentially and hence made discontinuous to allow water to flow into and out of those gaps.

16

Many other variations are possible within the inventive concept. For example, it would be possible to omit the lower stop formation, leaving the upper stop formation to resist upward movement of the guide frame or buoyancy modules along the 5 supporting flowline pipe under buoyant load.

Although not shown in Figures 2, 5 or 6, one or more key formations may be provided, for example between the stop formations, to resist rotational movement of a buoyancy module or a guide frame around the riser column element to which it is 10 attached. The plastics material around a stop formation and an optional key formation may be reinforced by, for example, metallic, composite or fibre inserts.

The stop formations may be integral with the coating of a flowline pipe or may be defined by injection-moulded plastics stop brackets of, for example, polyurethane 15 (PU) or PP attached to the coating of the flowline pipe by bonding and/or welding. A stop bracket may encircle a flowline pipe, in which case the bracket is suitably assembled from segments around the flowline pipe. Reinforcement such as straps, clamps or mechanical engagement may also be provided to support a stop bracket on a flowline pipe, as a back-up to bonded or welded attachment.

20

The coating may be machined away from the flowline pipe to expose the flowline pipe before a stop bracket is bonded or welded to the opposed coating portions on each side to span the exposed region. In the case of a PU stop bracket, a PU primer may be applied to the exposed surface of the flowline pipe to enable the stop bracket 25 to be bonded to that exposed surface; the PU primer also provides corrosion protection. In the case of a PP stop bracket, the exposed surface of the flowline pipe may be coated with a fusion-bonded epoxy coating to which the stop bracket is bonded.

30 It is also possible not to machine away the coating of a flowline pipe so that the coating instead extends continuously along the flowline pipe under a stop bracket bonded or welded to the coating. The lack of machining advantageously avoids a process step. It would be possible in this instance for a stop bracket to extend only part of the way around the circumference of the flowline pipe if desired.

35

Various factors may influence the choice of material for stop brackets, if used. PU is cheaper than PP, its fatigue behaviour is well understood, and good adhesion can be

17

achieved between a PU stop bracket and an exposed outer surface of a flowline pipe. However, the dissimilarity between a PU stop bracket and the PP coating reduces the bond strength between them: a PP to PP bond is preferred. A PP stop bracket also enjoys good adhesion to the exposed outer surface of a flowline pipe.

5

Further issues that may influence the choice of material for stop brackets in favour of PP are: PU is at risk of degradation due to hydrolysis under heat emanating from within the flowline pipe in use, which is a particular challenge under the high-pressure conditions of deep water. Also, mercury can no longer be used in PU catalysts for 10 environmental reasons, which may adversely affect the properties of the material.

The stop formations need not extend all of the way around the flowline pipe: they could be discontinuous or be discrete formations distributed around the circumference of the supporting flowline pipe. For example, as noted above, a stop 15 bracket defining stop formations may extend only part-way around a pipe. However, continuous circumferential stop formations are preferred as they maximise strength and are apt to be formed by machining.

Instead of steel, the core pipe could be made of a thermoset or thermoplastic 20 composite material such as a carbon fibre-reinforced epoxy or a carbon fibre-

reinforced PA (polyamide) or PEEK (polyetheretherketone). The plastics coating of the pipe described above could be replaced by an epoxy sleeve cast in situ.

18

Claims

Claims

1. A hybrid riser tower comprising at least one support element to be anchored to the seabed in service and at least one riser element both attached to an upper structure,

5 those elements having substantial length and extending generally in parallel in an axial direction as a riser column that can be supported buoyantly in an upright orientation for use, whereupon the riser element hangs from the upper structure; wherein:

10 one or more buoyancy modules are attached at one or more respective load-

bearing locations along the length of the support element to impart generally axial upthrust to the support element when the riser column is in the upright orientation; and

15 at least one supplementary buoyancy module is attached to the riser element to impart generally axial upthrust to the riser element when the riser column is in the upright orientation, to reduce or eliminate weight loading applied by the riser element to the upper structure and so to reduce or eliminate compression loading in the support element between the upper structure and

20 the or each load-bearing location on the support element.

2. The tower of Claim 1, wherein the support element serves as a flowline in addition to the riser element.

25 3. The tower of Claim 2, wherein the or each buoyancy module or supplementary buoyancy module attached to a flowline is spaced from the flowline by one or more channels arranged for convective flow of water over the flowline.

4. The tower of any of Claims 1 to 3, wherein the supplementary buoyancy module is

30 attached along the length of the riser element.

5. The tower of any preceding claim, wherein the support element and the riser element are linked to each other along the riser column in a manner permitting relative axial movement between those elements under differential extension.

35

19

6. The tower of Claim 5, wherein a buoyancy module and/or a guide frame attached to the support element restrains the riser element against cross-axial movement with respect to the support element.

5 7. The tower of any preceding claim, wherein the or each supplementary buoyancy module moves relative to the support element under differential extension between the support element and the riser element.

8. The tower of Claim 7, wherein the or each supplementary buoyancy module

10 attached to the riser element lies at least partially within a radius to which a buoyancy module and/or a guide frame attached to the support element extends cross-axially from the support element, and axial clearance is maintained between the buoyancy module and/or the guide frame and the supplementary buoyancy module to allow differential extension between the support element and the riser element.

15

9. The tower of any preceding claim, further comprising a buoyancy tank or block integrated with, attached to or attachable to the upper structure and being capable of tensioning the support element when the riser column is upright for use.

20 10. The tower of any of Claims 1 to 8, wherein the or each buoyancy module attached to the support element and the or each supplementary buoyancy module attached to the riser element together add sufficient buoyancy to keep the riser column upright in use.

25 11. The tower of any preceding claim, wherein the support element and/or the riser element comprises a coated or sleeved pipe having at least one stop formation that is integral with an external coating or sleeve of the pipe or that is attached to the pipe via that external coating or sleeve and is arranged to restrain movement along the pipe of a guide frame or buoyancy module attached to the pipe.

30

12. The tower of Claim 11, wherein the stop formation is insulated from the pipe by the external coating or sleeve of the pipe.

13. The tower of Claim 11 or Claim 12, wherein the stop formation comprises a

35 downwardly-facing shoulder to resist upthrust acting on a guide frame or buoyancy module when attached to the pipe.

20

14. The tower of any of Claims 11 to 13, wherein the stop formation comprises an upwardly-facing shoulder to resist weight load acting on a guide frame or buoyancy module when attached to the pipe.

5 15. The tower of any of Claims 11 to 14, further comprising a key formation to resist rotation about the pipe of a guide frame or buoyancy module when attached to the pipe.

16. The tower of any of Claims 11 to 15, wherein the stop formation extends radially 10 outwardly beyond a general outer diameter of the pipe.

17. The tower of any of Claims 11 to 16, wherein the stop formation is predominantly of plastics.

15 18. The tower of any of Claims 11 to 17, wherein the stop formation is machined from the external coating or sleeve of the pipe.

19. The tower of any of Claims 11 to 17, wherein the stop formation is part of a stop bracket attached to the external coating or sleeve of the pipe.

20

20. The tower of Claim 19, wherein the stop bracket is predominantly of plastics and is welded or bonded to the external coating or sleeve of the pipe and optionally also to an exposed wall of the pipe.

25 21. The tower of any of Claims 11 to 19, wherein the guide frame or buoyancy module is attached to the pipe by a primary attachment provision such as clamping and the stop formation is a secondary, auxiliary or fall-back provision for attachment or location.

30 22. A method of up-ending a hybrid riser tower during installation to define an upright riser column comprising at least one support element attached to an upper structure and at least one riser element hanging from the upper structure in an axial direction generally parallel to the support element, the method comprising:

35 using one or more attached buoyancy modules to impart generally axial upthrust to the support element when the riser column is upright; and

21

separately, using one or more attached supplementary buoyancy modules to impart generally axial upthrust to the riser element when the riser column is upright, reducing or eliminating compression loading in the support element beneath the upper structure by reducing or eliminating weight loading applied 5 by the riser element to the upper structure.

23. The method of Claim 22, further comprising adding buoyancy to the upper structure when the riser column is upright to reduce or eliminate compression loading in the support element beneath the upper structure, or to place that part of the 10 support element under tension.