This invention relates to subsea risers as used in the offshore oil and gas industry to convey hydrocarbons and sometimes other fluids and data from the seabed to the surface. Risers may also be used reciprocally to convey other fluids, power and data from the surface to the seabed. The invention is particularly concerned with catenary risers of rigid steel pipe that are apt to be installed by the reel-lay method.

Various riser configurations are known, including those known in the art as free- hanging, steep, lazy wave and weight-distributed risers. The riser is typically suspended between a floating upper support and the seabed, the support being a surface facility such as a platform or an FPSO (floating production, storage and offloading) vessel.

A riser moves in multiple directions on various timescales and frequencies throughout its operational life. Motion of the riser is driven by multiple inputs, notably: motion of the floating upper support expressed as heave, pitch, roll and yaw; seawater motion caused by currents, tides and waves, including flows that promote vortex-induced vibration (VIV); and pipeline motion across the seabed, known in the art as walking. Repetitive or oscillatory motion generates fatigue in a riser that may, over time, cause its failure and rupture.

A common free-hanging riser comprises a rigid pipe that hangs freely as a catenary from a floating upper support. Most conventionally, such a riser is of steel, hence being known in the art as a steel catenary riser or SCR. SCRs have been used around the world for more than fifty years to transport oil and gas between the seabed and surface facilities. The SCR is a field-proven solution that offers lower CAPEX and OPEX than other riser options.

Those skilled in the art know that nominally rigid pipes are not devoid of flexibility. Indeed, SCRs exploit the bending behaviour of rigid pipes in the elastic domain. However, whilst they have flexibility, ‘rigid’ pipes do not fall within the definition of ‘flexible’ pipes as understood in the art. Conventional rigid pipes used in the subsea oil and gas industry are specified in the American Petroleum Institute (API) Specification 5L and Recommended Practice 1111. A rigid pipe usually consists of, or comprises, at least one pipe of solid steel or steel alloy. However, additional layers of other materials can be added, such as an internal liner layer or an outer coating layer. A rigid pipe may also have a concentric pipe-in- pipe (PiP) structure. Rigid pipe joints are terminated by a bevel, a thread or a flange, and are assembled end-to-end by welding, screwing or bolting them together to form a pipe string or pipeline.

The allowable in-service deflection of rigid pipe is determined by the elastic limit of steel, which is around 1% bending strain. Exceeding this limit caused plastic deformation of the steel. It follows that the minimum bend radius or MBR of rigid pipe used in the subsea oil and gas industry is typically around 100 to 300 metres. However, slight plastic deformation can be recovered or rectified by mechanical means, such as straightening. Thus, during reel-lay installation of a rigid pipeline made up of welded rigid pipes, the rigid pipeline can be spooled on a reel with a typical radius of between 8 and 10 metres. This implies a bending strain above 2% for conventional diameters of rigid pipes, requiring the pipe to be straightened mechanically during unreeling.

Conversely, flexible pipes used in the subsea oil and gas industry are specified in API Specification 17J and Recommended Practice 17B. The pipe body is composed of a composite structure of layered materials, in which each layer has its own function. In particular, bonded flexible pipes comprise bonded-together layers of steel, fabric and elastomer and are manufactured in short lengths in the order of tens of metres. Typically, polymer tubes and wraps ensure fluid-tightness and thermal insulation, whereas steel layers or elements provide mechanical strength.

The structure of a flexible pipe allows a large bending deflection without a significant increase in bending stresses. For example, the MBR of flexible pipe used in the subsea oil and gas industry is typically between 3 and 6 metres. The bending limit of the composite structure is determined by the elastic limit of the outermost plastics layer of the structure, typically the outer sheath, which limit is typically 6% to 7% bending strain. Exceeding that limit causes irreversible damage to the structure. A simple free-hanging rigid riser such as an SCR has advantages of low cost, a short catenary length and ease of installation. For example, such risers may be installed by conventional pipelaying vessels using well-proven installation techniques such as S- lay, J-lay or reel-lay. However, the tension load at the top of a simple catenary riser increases with depth due to the weight of the riser that is suspended in the water column between the surface and the seabed. Also, a free-hanging rigid riser is particularly susceptible to fatigue-inducing motion being transmitted directly from a floating upper support toward the touch-down point or TDP. There, the riser extends around a sagbend being an upwardly-concave section with increased curvature disposed between the main upper section of the riser and the TDP.

Vessel motion is the primary driver of fatigue-inducing motion in a riser that is freely suspended from a vessel. In dynamic environments that suffer from high sea states, a floating upper support such as an FPSO imparts a large repetitive vertical motion that is transmitted along the riser toward the TDP and so can compromise the integrity of the riser. For example, wave-driven movement of an FPSO may cause dynamic compression-wave pulses to travel downwardly along an attached free-hanging riser, rather like a wave travelling along a whip. Such pulses travel from the top joint connection, where the riser is connected to the FPSO, and down the riser to the TDP. At the TDP, the seabed can reflect the pulses back up the riser in reaction, thereby creating secondary compression waves that can amplify the primary compression waves by constructive interference. If the resulting transient compressive loads reach a critical limit, the structure of the riser can buckle, rupture and collapse.

Thus, a conventional SCR may not be appropriate for use in some environments. This creates a problem because more complex riser systems that meet all technical challenges are much more expensive, especially if they cannot be installed using techniques for which appropriate installation vessels are widely available. Consequently, available riser solutions are not viable for some projects or at least can lead to a substantial increase in the field development cost. For example, one way to address the problem of fatigue would be to use a fully flexible riser made of unbonded flexible pipe. Whilst unbonded flexible pipe can be manufactured in lengths of hundreds of metres, it is very expensive, has limited resistance to pressure and temperature and is of limited diameter and hence flow capacity. It therefore remains strongly preferable to make a riser from rigid steel pipe where possible.

Various approaches can be used to minimise fatigue of an SCR, such as: changing the type of steel alloy (for example using X80 steel, being an API classification of high- strength steel); increasing the thickness of steel along part or all of the length of the riser; anchoring the touch-down point; or decoupling motion between the surface and the SCR.

The most common approach to controlling fatigue is to decouple at least a portion of a riser from the motion of a floating upper support. For example, degrees of freedom may be allowed at the connection between the riser and the support. This approach is used in hybrid risers that effect a flexible connection to the support through a flexible pipe or jumper pipe. However, hybrid risers require a large amount of buoyancy to support the weight of the riser because that weight load is not supported by the surface facility. Sub-surface buoyancy tanks are commonly used but are expensive to make and difficult to handle and to install because of their weight and size. The flexible pipe is also a critical part and is much more expensive than an equivalent length of steel pipe.

BR PI0602675 teaches the addition of a pliant section at a specific location along an SCR. Similarly, the applicant’s gimbal joint riser disclosed in WO 2019/051576 proposes another solution to decouple motion of upper and lower sections of a riser. However, both of these solutions require complex bespoke structures to ensure continuity of the riser and mechanical strength and flexibility.

In WO 2006/073887, weights or buoys are added at relevant locations along a riser to modify the dynamic response of the riser to force inputs. BR PI0804577 discloses a combination of anchoring and dynamic decoupling in which a flexible section is present between the floating upper support and a subsurface buoy. In WO 2011/028432, ballast is distributed along the riser. In some cases, typically as disclosed by WO 2008/036728, weights, buoys and anchors are combined to create a lazy wave configuration comprising one or more extra bend sections. In WO 2011/041860, one or more sections of an SCR are surrounded with hydrodynamic dampers to hinder propagation of compression waves along the riser. The or each damper section is initially nominally straight or follows the general smooth catenary curvature of the SCR as a whole. When a compression wave propagates, the damper section is deformed into a loop to absorb the compression wave. In other words, the damper section is preferentially deflected laterally at the loop to adopt a smaller radius of curvature than the adjoining undamped sections of the riser.

Against this background, the invention resides in a method of installing a steel catenary riser, the method comprising: progressively unspooling and launching the riser into water from a reel-lay vessel; plastically deforming the riser in a straightening process aboard the vessel downstream of unspooling and upstream of launching the riser; and adjusting the straightening process to form a series of two or more residual curvature loops of locally increased curvature in a length of the riser that will be suspended in the water above a touch-down point in use, successive loops of the series being separated and joined by a straighter portion of the riser of lesser curvature than those loops.

The straightening process may also be adjusted to form upper and lower straighter portions of the riser respectively above and below the series of loops, those straighter portions also being of lesser curvature than the loops of the series. The straightening process may substantially fully straighten the or each straighter portion of the riser. Nevertheless, in the installed riser, the or each straighter portion of the riser may substantially follow a catenary curve that extends to a touch-down point of the riser.

In the installed riser, successive loops of the series may be downwardly convex and vertical clearance between the bottom of the series of loops and the seabed may be less than 5% of the water depth.

One or more buoyancy elements may be attached to the riser above at least one of the loops of the series, for example between successive loops of the series, or above the series of loops. The or each buoyancy element may be attached to a point on the riser at any stage after that point undergoes the straightening process, for example before that point is launched into the water. Conversely, one or more ballast weights may also be attached to the riser, for example to at least one of the loops of the series. Similarly, one or more chains may be suspended from at least one of the loops of the series.

Correspondingly, the inventive concept may also be expressed as a steel catenary riser comprising a series of pre-formed portions that are plastically formed to different extents in longitudinal succession along a length of the riser that is suspended in water above a touch-down point. Those portions comprise two or more residual curvature loops of locally increased curvature, successive loops of the series being separated and joined by a straighter portion of the riser of lesser curvature than those loops.

The inventive concept also embraces a subsea installation comprising at least one riser of the invention.

Thus, the invention contemplates a subsea riser comprising a rigid riser pipe that is suspended from a surface support as a catenary extending from the surface support through a sagbend to a seabed touch-down point. The catenary shape of the riser extending between the surface and the seabed is interrupted by deflected sections or loops of locally greater curvature than the adjoining sections of the riser above and below them. The loops depart laterally from the underlying catenary curvature to add axial flexibility to the riser and hence to maximise its fatigue life by absorbing and damping compression and tension motions.

In the invention, the loops are pre-formed in the riser by controlling the shape of the pipe wall of the riser itself during straightening, hence changing the curvature with which the pipe is bent along its length. Thus, the locally deflected discontinuous shape of the riser is intrinsic to the main pipe of the riser itself rather than being imparted to that pipe by means of other structures or attachments only after straightening. This is distinguished from the prior art in which deflected sections of a riser pipe are defined by forces applied locally to the pipe underwater, after straightening, by external influences or attachments such as ballast, buoyancy, anchors, moorings or hydrodynamic dampers. To impart this intrinsic shape to a riser pipe, the invention employs principles of the ‘residual curvature method’ (RCM) when installing the riser by a reel-lay method. The RCM, based on the teachings of EP 1358420 and further exploited in WO 2013/126251, was developed as a buckle control technique to create thermal expansion loops in reel-laid subsea pipelines or flowlines. The purpose of the thermal expansion loops is to reduce the longitudinal stiffness of selected portions of the pipeline corresponding to the loops, compared to the longitudinal stiffness of straighter portions of the pipeline disposed between the loops. This ensures that thermal elongation of the pipeline as a whole will occur in a distributed and controlled manner, causing the loops to deflect laterally without generating excessive compressive forces in the pipe wall.

The RCM exploits the conventional straightener system of a reel-lay installation vessel aboard which a pipeline is spooled and transported in a plastically-deformed state as noted above. The pipeline passes through the straightener system, which generally comprises rollers, after being unspooled from a reel or carousel of the vessel. The action of the rollers reverses the plastic deformation that was imparted to the pipeline upon spooling.

In accordance with the RCM, the radius of curvature of the pipeline is modified locally by changing the straightening force that is applied to the pipeline. Typically, the pipeline is under-straightened locally at longitudinal intervals as the pipeline is launched into the sea. Thus, bending stress remains present in some sections of the pipeline. This forms a series of laterally-extending thermal expansion loops of locally increased curvature - that is, with a locally reduced radius of curvature - that are distributed longitudinally along the pipeline between straighter portions of lesser curvature.

The invention arises from the insight that residual curvature, preferably two or more loops of residual curvature, can drastically improve the dynamic response of the riser and therefore greatly reduce fatigue. The residual curvature loop or loops may optionally be combined with known ancillary equipment such as buoys and weights, with the beneficial effect varying depending on the location of such ancillary equipment. Calculations show that the best trade-off against fatigue without buoys is to incorporate three loops. With a few extra buoys, two loops are enough but one loop may be insufficient.

Embodiments of the invention implement a method to install an SCR by the reel-lay method to provide the SCR with improved resistance to fatigue. The method comprises: starting to lay the riser by unspooling a pipeline from a reel; and generating at least two residual curvature loops in the section of the pipeline that will remain permanently above the seabed in the riser, the or each adjacent pair of those loops being separated by a section of the pipeline that is straightened or straightened to a greater extent than the loops. The method may also comprise adding at least one buoy or a set of distributed buoys between, or directly above, the residual curvature loops.

The invention proposes a rigid riser solution that enables a free hanging riser configuration to be suspended, or hung off, in deep or ultradeep water from a surface floater that will experience large vertical motions during its operational life. The riser solution requires few modifications and little, if any, extra equipment. The riser of the invention is a free-hanging catenary of steel pipe divided into two sections, or upper and lower risers, by introducing two or more pre-bent segments or loops of the same steel material into a region of the riser close to, but not touching, the seabed. The prebent loops are formed by the RCM. As noted above, the RCM is a proven technology that is already in use to install flowlines that lie on the seabed. Use of the RCM need not increase the time required to install a riser once the straightener of the installation vessel has been calibrated appropriately.

The invention provides various benefits, enabling a free-hanging riser configuration to be used in deep-water production systems located in harsh environments by reducing dynamic loads around the TDP and, in particular, drastically reducing or avoiding compressive waves reaching the TDP. The invention provides these benefits at a lower cost than available alternative solutions such as steel lazy wave risers (SLWRs). In particular, buoyancy modules cost money, and their installation takes time and costs money too. SLWRs therefore suffer from the cost and logistics involved in procuring, handling and installing the numerous buoyancy modules they require, and the consequential operational risk of dealing with so many lifts and assembly operations aboard the installation vessel. Nevertheless, the invention may employ a relatively small number of buoyancy modules positioned on the riser above one or more of the residual curvature loops, which improves performance in some cases. A pipe end fitting or additional coating thickness may also be applied to the pipeline around the TDP to improve fatigue life.

RCM has been proposed for the installation of SCRs on a theoretical, academic basis. Specifically, https://pantheon.ufrj.br/bitstream/11422/12728/1/AndreRamiro Amorim- min.pdf is a link to a Masters Degree dissertation entitled Study of the Residual Curvature Applied to a Rigid Riser Under Dynamic Compression in Free Hanging Configuration. The dissertation was presented in 2018 by one of the inventors of the present invention to COPPE, the engineering institute of the Federal University of Rio de Janeiro. It proposes inserting one loop of residual curvature into an SCR. Whilst some of the drawings in the dissertation suggest the presence of multiple loops, those loops alternate in opposite lateral directions in the manner of a sinusoidal wave rather than extending in the same lateral direction separated by fully straightened sections of lesser curvature.

In summary, a method of installing a steel catenary riser in accordance with the invention comprises progressively unspooling and launching the riser from a reel-lay vessel. The riser is plastically deformed in a straightening process aboard the vessel downstream of unspooling and upstream of launching the riser. The straightening process is adjusted to form a series of two or more residual curvature loops of locally increased curvature in a length of the riser suspended above a touch-down point. Successive loops of the series are separated and joined by a straighter portion of the riser of lesser curvature than those loops. Buoyancy elements are attached to at least one straighter portion of the riser above at least one of the loops of the series, or between successive loops of the series, or above the series of loops.

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: Figure 1 is a schematic plan view of a steel catenary riser of the invention being reel-laid from a pipelay vessel that employs an RCM technique to impart locally increased curvature to portions of the riser spaced at longitudinal intervals;

Figure 2 is a schematic side view corresponding to Figure 1 ;

Figure 3 is a schematic side view corresponding to Figure 2 but showing a variant in which a crane of the vessel is adding buoyancy modules to the riser;

Figure 4 is a schematic side view of various risers of the invention with buoyancy modules disposed between residual curvature loops of a series;

Figure 5 is a schematic side view of various risers of the invention with buoyancy modules disposed above a series of residual curvature loops;

Figure 6 is a schematic side view of various risers of the invention shown in conjunction with a conventional SCR;

Figure 7 is a chart of load and resistance factor design (LRFD) values for the various risers shown in Figure 6, the risers in each case having a thermal insulation coating or a 3LPP coating;

Figures 8a and 8b are charts showing, respectively, LRFD values and compressive forces for risers with one, two and three residual curvature loops subject to a net buoyancy force of from six to twelve tonnes, the risers in each case having a thermal insulation coating;

Figures 9a and 9b correspond to Figures 8a and 8b but represent risers that have a 3LPP coating;

Figures 10a and 10b are charts showing, respectively, LRFD values and compressive forces for risers with two, three and four residual curvature loops subject to a net buoyancy force of from one to fourteen tonnes, the risers in each case having a thermal insulation coating; Figures 11a and 11b correspond to Figures 10a and 10b but represent risers that have a 3LPP coating and are subject to a net buoyancy force of from one to sixteen tonnes;

Figures 12a and 12b are charts showing, respectively, LRFD values and compressive forces for a conventional SCR, a riser with a series of residual curvature loops, a riser with buoyancy modules and a riser with a combination of buoyancy modules and a series of residual curvature loops, in each case with a thermal insulation coating or a 3LPP coating;

Figure 13 is a side view of a riser having two residual curvature loops and a touch-down portion that extends across the touch-down point;

Figure 14 is a perspective view of an upset-end pipe that may be incorporated in the touch-down portion of Figure 13;and

Figure 15 is a side view of a riser having three residual curvature loops and a touch-down portion comprising a coating or layer applied to the outer surface of the riser.

Referring firstly to Figures 1 and 2 of the drawings, which are not to scale, a conventional reel-lay vessel 10 is shown here advancing across the surface 12 of the sea while installing a steel catenary riser 14 extending from the surface 14 to a touchdown point (TDP) 16 on the seabed 18. The riser 14 is nominally rigid, having been fabricated onshore from lengths of steel pipe. However, the riser 14 has sufficient flexibility to bend along its length. This bending deformation remains in the elastic domain provided that an appropriate minimum bending radius (MBR) is observed.

By way of example, the riser 14 may have an inner diameter of eight inches (203.2mm), a wall thickness of one inch (25.4mm) and a top angle of 10° at the floating upper support when fully installed. The riser 14 is apt to be installed in deep to ultradeep water, for example in a water depth of 2100m. The riser 14 may have a thick coating of thermally insulating material, for example with a thickness of 75mm, or a thinner anti-corrosion coating such as three-layer polypropylene (3LPP) of, typically, 3mm in thickness.

The vessel 10 carries a reel 20, in this example turning about a horizontal axis, onto which the riser 14 is spooled during or after fabrication for transport to the installation site. The bending deformation involved in spooling the riser 14 onto the reel 20 exceeds the MBR and hence the elastic limit, thus imparting plastic deformation to the pipe wall of the riser 14. Consequently, after being unspooled from the reel 20 and before being launched into the sea, the riser 14 is guided through a straightener system 22 that imparts a suitable degree of reverse plastic deformation to the pipe wall.

The straightener system 22 is mounted on an inclined laying ramp 24 that extends over the stern of the vessel 10. The laying ramp 24 also comprises a hold-back system 26 that typically comprises tensioners and clamps for supporting the weight of the riser 14 suspended as a catenary between the vessel 10 and the seabed 18.

In the invention, the straightener system 22 is controlled in accordance with the residual curvature method (RCM), periodically to reduce the straightening force that imparts reverse plastic deformation to the riser 14. As a result, the riser 14 is understraightened locally at longitudinal intervals while being launched into the sea. This creates a series of loops 28 in accordance with the principles set out in EP 1358420 as noted above.

The loops 28 are portions of the riser 14 whose curvature is increased locally relative to adjoining straighter portions 30 of substantially lesser curvature. In other words, the loops 28 have a substantially smaller radius of curvature than that of the straighter portions 30. Consequently, the straighter portions 30 have a substantially greater radius of curvature than that of the loops 28. Indeed, the radius of curvature of a straighter portion 30 may approach infinity to the extent that the portion 30 is substantially straight.

The straighter portions 30 of the riser 14 extend between the loops 28 as intervening intermediate portions that extend continuously from one loop 28 to the next loop 28. The straighter portions 30 of the riser 14 also include upper and lower portions of the riser 14 that extend respectively above and below the series of loops 28. Thus, the loops 28 alternate with the straighter portions 30 along the length of the riser 14. Transition sections effect a smooth transition of curvature between the straighter portions 30 and the loops 28 that adjoin them.

The loops 28 aside, the riser 14 follows an underlying conventional catenary path 32 that curves smoothly with progressively increasing curvature approaching the TDP 16. The straighter portions 30 of the riser 14 lie substantially on that underlying path 32 whereas the loops 28 depart laterally or downwardly from the underlying path 32.

The series of loops 28 is very close to the seabed 18 relative to the length of the riser 14. For example, the vertical clearance between the seabed 18 and the bottom of the series of loops 28 may be less than about 5%, for example 2.75%, of the water depth. Thus, in a water depth of 2100m, the bottom of the series of loops 28 may be only about 58m above the seabed 18.

In view of the path of the riser 14 from the reel 20, over the laying ramp 24 and through the straightener system 22, the loops 28 are typically upwardly convex in a vertical plane before being launched into the sea. As the riser 14 is lowered toward the seabed 18 and twists about its central longitudinal axis, the loops 28 tilt from their initial orientation to become downwardly convex eventually, hence hanging beneath the underlying catenary path 32 of the riser 14. The loops 28 may then lie in a vertical plane or at an acute angle to either side of the vertical plane.

Figure 3 corresponds to Figure 2 but shows buoyancy elements in the form of buoyancy modules 34 being lifted by a crane 36 of the vessel 10 and fixed to the riser 14 before being launched with the riser 14 into the water. The buoyancy modules 34 are fixed to the riser 14 at any suitable location downstream of the straightener system 22, for example downstream of the hold-back system 26 on the laying ramp 24 or otherwise above where the riser 14 enters the water.

In this example, one or more buoyancy modules 34 are disposed on the straighter portions 30 of the riser 14 between the loops 28 of the series. In other examples, one or more buoyancy modules 34 are disposed on the straighter upper portion 30 of the riser 14 above a single loop 28 or a series of loops 28, instead of or additional to any buoyancy modules 34 between the loops 28 of the series. Any suitable number of buoyancy modules 34 may be used on the riser 14, from one upwards, but significantly fewer buoyancy modules 34 will be required than in SLWRs of the prior art.

In this respect, Figures 4 and 5 show variants of the riser 14 with different numbers of residual curvature loops 28, namely riser 14A with a single loop 28, riser 14B with a series of two loops 28, riser 14C with a series of three loops 28 and riser 14D with a series of four loops 28. In each of risers 14B, 14C and 14D, the loops 28 of the series alternate with straighter portions 30 of the riser 14.

In Figure 4, which shows risers 14B, 14C and 14D, one or more buoyancy modules 34 are disposed on the straighter portions 30 between the loops 28 of the series and one or more buoyancy modules 34 are disposed on the straighter upper portion 30 of the riser 14 above the series of loops 28. In other words, each loop 28 has at least one buoyancy module 34 on the straighter portion 30 of the riser 14 immediately above it. In this example, a pair of buoyancy modules 34 is provided directly above each loop 28.

Conversely in Figure 5, which shows risers 14A, 14B and 14C, no buoyancy modules 34 are disposed between the loops 28 but again, one or more buoyancy modules 34 are disposed on the straighter upper portion 30 of the riser 14 above the loop 28 or the series of loops 28. In this example, a set of multiple buoyancy modules 34 is provided on each riser 14A, 14B and 14C.

Figure 6 also shows the riser variants 14A to 14D, in this case without buoyancy modules 34, in addition to a conventional SCR 38 that is devoid of residual curvature loops 28.

Figure 7 shows the beneficial effect of the residual curvature loops 28 on load and resistance factor design (LRFD) values for the various risers 14A to 14D shown in Figure 6, relative to the conventional SCR 38 also shown in Figure 6. LRFD is a design approach based upon a limit state and partial safety factor methodology and is used in the DNV standard relevant to riser systems, namely DNV-ST-F201. The number of residual curvature loops 28 is represented in Figure 7 as ‘NRC=1’ for riser 14A, ‘NRC=2’ for riser 14B and so on. In each case, separate LRFD values are shown for the SCR 38 and the risers 14A to 14D with a thermal insulation coating or with a 3LPP coating.

It will be apparent from Figure 7 that LRFD values for the risers 14A to 14D are beneficially lowered in comparison to the SCR 38 and that the benefit is especially clear for risers 14B, 14C and 14D with two, three and four loops 28, hence NRC=2, NRC=3 and NRC=4. For those risers 14B, 14C and 14D, the LRFD values are close to 1 or in the case of risers 14C and 14D (NRC=3 and NRC=4) with a 3LPP coating, below 1.

Figures 8a, 8b, 9a and 9b illustrate the dynamic response of risers 14 with one, two and three residual curvature loops 28 subject to a net positive buoyancy force of from six to twelve tonnes. In these examples, this upthrust is provided by buoyancy modules 34 disposed on the straighter upper portion 30 of the riser 14 above the loop 28 or series of loops 28. As in Figure 5, no buoyancy modules 34 are disposed between the loops 28 in these examples.

Figures 8a and 9a show LRFD values and Figures 8b and 9b show compressive forces. Figures 8a and 8b represent risers with a thermal insulation coating whereas Figures 9a and 9b represent risers that have a 3LPP coating. It will be noted that LRFD values fall with increasing buoyancy. Beneficially, as shown in Figure 8a, LRFD values are below 1 for risers 14 coated with thermal insulation and with two or three residual curvature loops 28 subject to buoyancy of eight tonnes or more. LRFD values are also below 1 for risers 14 with a 3LPP coating and one, two and three residual curvature loops 28, at least when subject to buoyancy of more than six tonnes. A reduction of compressive forces is also evident with increasing buoyancy, that reduction being especially marked in the case of risers 14 with a 3LPP coating as shown in Figure 9a.

Figures 10a, 10b, 11a and 11b illustrate the dynamic performance of risers 14 with series of two, three and four residual curvature loops 28. In these examples, as in Figure 4, buoyant upthrust is provided by buoyancy modules 34 disposed between and above the loops 28 of each series. Figures 10a and 10b represent risers with a thermal insulation coating, subject to a net positive buoyancy force of from one to fourteen tonnes, whereas Figures 11a and 11b represent risers that have a 3LPP coating and are subject to a net positive buoyancy force of from one to sixteen tonnes.

Figures 10a and 11a show LRFD values and Figures 10b and 11b show compressive forces. Again, it will be noted that the LRFD values for most of the riser configurations and for most of the buoyancy provisions remain below 1 , especially in the case of risers 14 with a 3LPP coating as shown in Figure 11a.

Finally, Figures 12a and 12b show, respectively, LRFD values and compressive forces for a conventional SCR, a riser 14 with a series of residual curvature loops 28 (‘RC- SCR’), a riser 14 with buoyancy modules (‘BM-SCR’) and a riser 14 with a combination of buoyancy modules 34 and a series of residual curvature loops 28 (‘RCBM-SCR’).

In each case shown in Figures 12a and 12b, values are shown for riser 14 with a thermal insulation coating or with a 3LPP coating. In the case of thermal insulation coating with buoyancy modules 34, the riser 14 is subject to a net positive buoyancy force of six tonnes. In the case of 3LPP coating with buoyancy modules 34, the riser 14 is subject to a net positive buoyancy force of five tonnes. In the case of thermal insulation coating with residual curvature loops 28, the riser 14 has three such loops 28. In the case of 3LPP coating with residual curvature loops 28, the riser 14 has two such loops 28.

It will be noted from Figure 12a that the combination of buoyancy modules 34 and a series of residual curvature loops 28 (‘RCBM-SCR’) benefits from an LRFD value below 1 for risers 14 with a thermal insulation coating and for risers 14 with a 3LPP coating. Figure 12b also shows that compressive forces in the riser 14 are minimised by the combination of buoyancy modules 34 and residual curvature loops 28.

Other variations are possible within the inventive concept. For example, one or more ballast weights could be attached to the riser 14 at one or more residual curvature loops 28. One or more chains could also be attached to the riser 14 at one or more residual curvature loops 28. Such chains could be attached to and suspended from respective ballast weights or could instead be attached directly to the residual curvature loop 28 of the riser 14, hence serving as ballast weights themselves.

One or more ballast weights could be attached to a straighter lower portion 30 of the riser 14 at a location beneath the series of residual curvature loops 28. Similarly, the riser 14 could be moored at that location to a subsea foundation.

In some embodiments, a touch-down portion 40 of the riser 14 includes at least one section that is stiffer than other parts of the riser 14.

A riser 14 incorporating such a touch-down portion 40 is shown in Figure 13. The touch-down portion 40 extends across the TDP 16, such that a first portion 46 to one side of the TDP 16 rests on the seabed, and a second portion 48 to the other side of the TDP 16 extends to a point along the riser 14 that is suspended in the water column. In this example, the lengths of the first and second portions 46, 48 are substantially equal, but in others they may differ from one another. It should also be noted that the total length of the touch-down portion 40 may differ across embodiments.

The touch-down portion 40 may include at least one upset-end pipe 50, for example of the type described in WO 2008/111828. An example of an upset-end pipe 50 that may be incorporated in the touch-down portion 40 of Figure 13 is shown in isolation in Figure 14.

As will be understood by the skilled person, an upset-end pipe 50 is formed using forging to create thickened end portions 54 through heating and compression. The upset-end pipe 50 of Figure 14 includes thickened end portions 54 that join to a central body 56 via transition portions 58 that taper radially inwardly towards a longitudinal mid-point of the pipe 50. In this way, the outer diameter of each end portion 54 is greater than the outer diameter of the central body 56. Furthermore, the thickness of the wall 60 of each end portion 54 is greater than the thickness of the wall 60 of the central body 56.

The touch-down portion 40 may be formed by joining together a string of upset-end pipes 50 end-to-end, for example using welding. The outermost upset-end pipes 50 located at ends of the touch-down portion 40 may be joined to neighbouring riser pipe sections outside the touch-down portion 40 using welding or any other appropriate technique.

In a riser 14 such as that shown in Figure 13, the outer diameter of the central body 56 of each upset-end pipe 50 of the touch-down portion 40 is substantially equal to the outer diameter of neighbouring sections of the riser 14 outside the touch-down portion 40. As such, the outer diameter of the end portions 54 of each upset-end pipe 50 is greater than the outer diameter of neighbouring sections of the riser 14.

As discussed already, fatigue inducing motion may be transmitted along a riser 14 from a floating support towards and across the TDP 16. For example, wave-driven movement of a floating support may cause dynamic compression-wave pulses to travel downwardly along the riser 14, as well as resulting in periodic impact of the riser 14 against the seabed 18.

Steel catenary risers are known to experience high levels of fatigue in the region at and around the TDP 16 in particular, such that the portion of the riser 14 around the TDP 16 is more susceptible to fatigue-induced damage than other riser portions.

Furthermore, welds between neighbouring pipe sections of a riser 14 define zones of stress concentration that are more likely to experience failure through fatigue than other portions of the riser 14.

Through the use of upset-end pipes 50 in the touch-down portion 40, the joining welds between pipe sections that generally experience the highest level of fatigue along the riser 14 are made at thickened end portions 54. These thickened end portions 54 are stiffer than surrounding portions of the riser 14, and reduce stresses experienced by the welds in the touch-down portion 40. This reduces the risk of fatigue-induced damage in the touch-down portion 40, thus improving the fatigue resistance of the riser 14 as a whole.

Turning now to Figure 15, in some embodiments the touch-down portion 40 is defined by a length of riser 14 having a thicker wall 60 than sections of the riser 14 outside the touch-down portion 40. This thicker section of wall 60 may be formed, for example, through applying layers or coatings of material 52 to the outer surface 64 of the riser 14, or through use of thicker-walled pipe sections in the touch-down portion 40.

In the example of Figure 15, material 52 is wrapped around the riser 14 to define a wrapped layer or sleeve. In other examples material may be deposited on the outer surface 64 of the riser 14 to form a coating. The thickness of the added material 52 is 10mm in the embodiment of Figure 15, although this thickness may vary. For example, the thickness of the added material layer 52 may be 20mm, 30mm, 40mm or 50mm.

When installed for use, the first portion 46 of the touch-down portion 40 that rests on the seabed to one side of the TDP 16 has a length of 200m in the example of Figure 15, and the second portion 48 that is suspended in the water column has a length of 50m. In other examples the overall length of the touch-down portion 40 may vary, as may the ratio of the lengths of the first and second portions 46, 48.

The thicker and stiffer wall 60 of the riser 14 across the touch-down portion 40 reduces fatigue-inducing stresses on the welds between riser pipe sections of the touch-down portion 40 in a similar manner to the thickened end portions 54 of the upset-end pipes 50 discussed above.

It will be appreciated that the stiffer wall 60 of the riser 14 along some or all of the touch-down portion 40 may be achieved in ways other than those described above. For example, a stiffer steel alloy may be used for pipe sections in the touch-down portion 40, or a stiffer material other than steel may be used. Furthermore, material processing such as heat treatment may be used to alter the mechanical properties, specifically the stiffness, of pipe sections incorporated in the touch-down portion 40.

In some embodiments, the riser 14 may include one or more pipes of titanium or titanium alloy, in particular along the touch-down portion 40 or a residual curvature loop 28. Titanium has a higher strength-to-weight ratio than steel, which allows for use of thicker titanium pipes without increasing the weight of the riser 14. It will also be appreciated that although the touch-down portion 40 has been described with reference to the risers 14 of Figures 13 and 15, such a touch-down portion 40 could be applied to any riser 14 of the invention.