Patent Description:
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 platform or from an FPSO vessel. Most conventionally, such a riser is of steel - hence being known in the art as a steel catenary riser or SCR.

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 <NUM> and Recommended Practice <NUM>. 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.

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.

In recent years, the subsea oil and gas industry has begun to adopt rigid pipes of polymer composite materials in place of steel. Composite pipes have a tubular load-bearing structure that is principally of composite materials. This is to be distinguished from pipes having a composite structure, such as the various layered configurations of rigid and flexible pipes as mentioned above.

Typically, a composite pipe comprises a polymer resin matrix reinforced by fibres such as glass fibres or carbon fibres. The polymer matrix may be of thermoplastic or thermoset materials. The former results in what is known in the art as thermoplastic composite pipe or, more simply, as thermo-composite pipe (TCP). TCP is classed as a bonded composite pipe.

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, 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, where the riser extends beyond a sagbend to meet the seabed. Also, 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.

An SCR is joined at its upper end to a surface facility such as an FPSO by a connection device that provides some degrees of freedom, examples being a stress joint or a flexible joint or pivot as described in <CIT>. A drawback of flexible joints is their cost and difficulty of maintenance: the hang-off structure is located on the side hull of an FPSO or on a member of a floating platform.

A flexible joint often comprises an elastomeric element, as disclosed in <CIT> or <CIT>. Conversely, <CIT> discloses a gimballing SCR hang-off whereas in <CIT>, the riser is equipped with a half-sphere that can rotate within a complementary seat of a hang-off structure. However, such rotating devices cannot sufficiently accommodate SCR motion and fatigue when the surface facility is floating, for example when it is an FPSO.

In dynamic environments that suffer from high sea states and strong currents, FPSOs impart a large vertical motion at the riser balcony position. That motion is transmitted along the riser to the TDP and so can compromise riser integrity. Thus, a conventional SCR may not be appropriate for use in such 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. Thus, for some projects, available riser solutions are not viable or lead to a substantial increase in the field development cost.

It is known to decouple at least a portion of a riser from the motion of a supporting vessel. For example, degrees of freedom may be allowed at the connection between the riser and the vessel. This approach is used in hybrid risers such as that described in <CIT>, which effect a flexible connection to the vessel through a flexible pipeline or jumper pipe. However, hybrid risers require extra buoyancy to support the weight of the riser because that weight load is not supported by the surface facility. Buoyancy tanks are commonly used and are difficult to handle and to install because of their weight and size. The flexible pipe is also a critical part and is more expensive than a corresponding steel pipe.

An SCR can be a component of a hybrid riser, as disclosed in <CIT> in which an anchored sub-surface buoy supports an SCR and is connected to an FPSO by a flexible line.

Another known solution to the problem of fatigue is to use a fully flexible riser made of unbonded flexible pipe, which can be manufactured in lengths of hundreds of metres.

However, unbonded flexible pipe is very expensive, has limited resistance to pressure and temperature and is of limited diameter and hence flow capacity.

In another approach to reducing fatigue, a riser may itself be shaped to introduce compliance. For example, <CIT> discloses a lazy-wave steel catenary riser, which is characterised by a buoyantly supported hogbend between the surface and the TDP. The intermediate buoyancy around the hogbend also helps to support the suspended weight of the riser and so reduces its top tension.

The hogbend of a lazy-wave riser is defined and supported by adding external buoyancy modules to the riser. Multiple buoyancy modules are expensive; also, attaching them to the riser can be challenging and each attachment operation interrupts and hence delays the pipelaying process. Delay ties up valuable capital assets in the form of pipelaying vessels that are extremely expensive to operate. Delay also requires a longer weather window in which to complete the riser installation. The buoyancy modules must also be dimensioned and positioned with care to avoid sharp variations of curvature or regions where the effective tension is too low, or where there is a risk of inducing damaging dynamic compression.

<CIT> and <CIT> both describe risers that are suspended from a floating platform by means of a pivoting arrangement that includes a ball joint.

<CIT> relates to a hang-off adapter for use in an offshore riser system. The hang-off adapter is seated in a tension ring and allows for relative rotation between the riser and the tension ring.

<CIT> describes a riser system for connecting a subsea installation to a floating surface unit. A flexible pipe arranged in a catenary extends between the surface unit and a submerged buoy. A riser arranged in a catenary extends between the submerged buoy and the subsea installation.

<CIT> describes a riser that includes a flexible member near its upper end and a ball joint at its lower end.

<CIT> describes a flexible element in a basket-like structure that supports an SCR.

<CIT> describes an apparatus for installing workpieces on the seabed. The apparatus includes a conduit through which the workpieces can be lowered.

<CIT> describes a method for constructing a tubular connection assembly. It is described that a metallic riser, such as an SCR or a steel lazy wave riser, can be formed of a plurality of the tubular connection assemblies.

<CIT> relates to a method of stabilising a riser using one or more thrust units on the riser, so as to counteract oscillating movement of the riser.

The inventive concept embraces a buoy for a rigid subsea riser, the buoy comprising an inner part for attachment to a riser pipe, an outer part that is movable relative to the inner part, wherein the inner part defines a longitudinal axis and the outer part is pivotable about pivot axes that are orthogonal to each other and that intersect the longitudinal axis. For example, the inner part may comprise a part-spherical inner ball formation and the outer part may comprise a complementary socket formation engaged with the ball formation. The outer part may also be pivotable about the longitudinal axis.

A buoyant body of the buoy is spaced radially from the inner part, may be distributed angularly around the inner part, and may extend continuously around the inner part, for example as a toroid.

The invention also resides in 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 riser pipe is attached to the surface support by a hang-off interface that allows rotation of the riser pipe relative to the surface support about at least two horizontal axes. At least one subsea buoy as described in the preceding paragraphs is positioned on the riser pipe above the sagbend, the buoy being attached to the riser pipe by an attachment interface that allows rotation of the riser pipe relative to the buoy about at least two horizontal axes. The attachment interface may also allow rotation of the riser pipe relative to the buoy about a central longitudinal axis of the riser pipe.

The buoy is preferably attached only to the riser pipe and may be slidable along the riser pipe, in which case sliding movement of the buoy relative to the riser pipe may be limited by stopper formations that are spaced longitudinally along the riser pipe.

The buoy suitably comprises a buoyant body that is spaced radially from the riser pipe. For example, the buoyant body may be distributed angularly around the riser pipe and may extend continuously around the riser pipe, such as with a toroidal shape.

The attachment interface may comprise a part-spherical inner ball formation that is fixed relative to the riser, the ball formation being engaged with a complementary socket formation that is fixed relative to the buoyant body.

The riser may further comprise a pliant spool pipe extending between the hang-off interface and an upper connection structure of the surface support, the spool pipe being in fluid communication with an upper end of the riser pipe via the hang-off interface. A lower end portion of the spool pipe may be substantially aligned on a common longitudinal axis with an upper end portion of the riser pipe.

The inventive concept extends to a corresponding method of supporting a rigid subsea riser pipe that is suspended as a catenary from a surface support. The method comprises: applying buoyant upthrust force to the riser pipe from a subsea buoy that surrounds the riser pipe; and in response to movement of the surface support, deflecting the riser pipe to vary inclination, relative to the buoy, of a portion of the riser pipe extending through the buoy. The buoyant upthrust force is applied to the riser pipe via an attachment interface that allows rotation of the riser pipe relative to the buoy about at least two horizontal axes.

The deflection of the riser pipe may impart or vary S-shaped curvature in the riser pipe along its length, that curvature comprising mutually opposed curves respectively above and below the buoy, joined by a region of inflection that may coincide with the buoy.

An upper end of the riser pipe may be constrained while being allowed to pivot relative to the surface support in response to the deflection of the riser pipe. A pliant spool pipe extending upwardly from the upper end of the riser pipe may also be deflected in response to the deflection of the riser pipe. There could be limited longitudinal movement of the riser pipe relative to the buoy.

Thus, the invention provides an alternative solution to decouple the motion of a rigid riser. To do so, the invention adds an additional restraining element in the form of a special, preferably toroidal buoy that allows degrees of freedom relative to the riser extending within. In conjunction with rotation or pivoting allowed by a spherical hang-off system, motion of an FPSO and a riser can be accommodated by the riser bending freely along its length. Such bending takes place continuously and smoothly between the hang-off and the buoy and through the buoy toward the sagbend and the TDP.

Whilst <CIT> teaches a toroidal buoy for a transfer riser system, its disclosure only contemplates translation motion of the transfer riser through the central aperture of the torus.

The invention provides a riser solution with improved dynamic behaviour and reduced cost. The solution involves a combination of a hang-off joint and a buoy around a free-hanging riser. The riser is suspended from the hang-off joint at a floating support such as an FPSO and the buoy is attached to the riser in the water column above the sagbend, typically closer to the seabed than to the surface. The riser is movable relative to both the buoy and the hang-off joint. This arrangement creates two trigger points in the riser catenary, one at the top of riser and the other close to the sagbend.

Relative movement between the buoy and the riser, and between the riser and the hang-off joint, is possible about at least two mutually-orthogonal horizontal axes at each location. For example, a toroidal shell buoy may be attached to the riser via a ball joint, fixed to the riser, that allows the buoy to rotate around a centre of spherical curvature. This arrangement allows articulation with minimum stiffness due to very low friction force while the buoy supports some of the weight of the catenary and so reduces top tension. The buoy, and its buoyancy force, also increase inertia and drag forces and so damp oscillatory wave loads that are transmitted along the riser.

A riser in accordance with invention decouples loads that are transmitted along the riser from an FPSO, hence reducing the loads that are experienced at the TDP and addressing the problem of riser fatigue. The riser of the invention is less expensive than a lazy wave configuration that requires the installation of multiple buoyancy modules. Indeed, travelling down the riser from the upper end to the TDP, at no stage does the riser revert to an upward curve; this is unlike wave-configuration risers that have an upwardly convex hogbend disposed between the upper end and the TDP. The riser of the invention can be installed by any conventional installation technique such as J-Lay, S-Lay, or reel-lay.

The riser of the invention also provides an alternative solution to flexible joints conventionally used at the top of rigid risers, reducing the costs that relate to purchase and installation of the flexible joint.

Thus, a significant technical advantage of the invention is the improvement of loads around the riser sagbend section, close to the TDP. Other advantages include a reduction in top tension. In economic terms, it is possible to reduce capital expenditure and other costs, such as by removing the need to acquire and to attach buoyancy modules. As it enables installation by a regular multi-section pipelay procedure, the invention also reduces the operation time of installation vessels when compared with steep- and lazy-riser configurations.

Embodiments of the invention provide a buoy for supporting a rigid riser. The buoy comprises an interface with the riser such as a ball joint, the interface allowing free rotation of the riser around at least two axes. The buoy may have a toroidal shape. The buoy may not be anchored to the seabed, hence only being attached to the riser pipe.

The riser may be a catenary riser, which may be made of steel or of thermoplastic composite material. The riser could be able to rotate freely around any axis.

In some embodiments, the interface may allow the buoy to slide between two longitudinally spaced stoppers of the riser.

Embodiments of the invention also provide a riser, of the catenary type, for transporting fluids between the seabed level and a surface facility, the riser comprising: a riser pipe, which may be a rigid metal pipe or thermoplastic composite pipe; a hang-off interface on the surface facility to suspend the riser pipe; and an intermediate buoy between the surface and the main curvature or sagbend of the catenary.

The hang-off interface suitably comprises a housing and a seat and may allow at least rotation of the riser around two axes that may be in a substantially horizontal plane. The riser pipe may, for example, comprise a half-ball joint that is engaged rotatably into a complementary seat of the hang-off interface.

The riser may also comprise an upper pipe above the hang-off to connect the riser pipe to piping of the surface facility. The upper pipe may be bendable along its length, for example being a thermoplastic composite pipe or a flexible pipe.

The buoy is suitably located above the sagbend of the catenary riser, preferably between one third and two thirds of the depth of the water at that location and preferably at least <NUM> above the seabed.

In summary, a subsea catenary riser of the invention comprises a rigid riser pipe that is suspended from a floating support on the surface and that extends through a sagbend to a TDP on the seabed. A hang-off interface allows rotation or pivoting of the riser pipe relative to the support about mutually orthogonal horizontal axes.

In accordance with embodiments of the invention to be described, a subsea buoy is positioned on the riser pipe above the sagbend. The buoy applies buoyant upthrust force to the riser pipe via an attachment interface that allows rotation or pivoting of the riser pipe relative to the buoy about mutually orthogonal horizontal axes. In response to movement of the support, the riser pipe deflects with S-shaped curvature to vary the inclination, relative to the buoy, of the portion of the riser pipe to which the buoy is attached. The curvature comprises mutually opposed curves respectively above and below the buoy, joined by a region of inflection that coincides with the buoy.

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:.

In <FIG> and <FIG>, a riser <NUM> extending upwardly from the seabed <NUM> is suspended as a catenary from an upper support that is exemplified here by an FPSO <NUM> floating at the surface <NUM>. The riser <NUM> is of rigid pipe, for example of steel or of TCP. As explained above, a rigid pipe as understood in the art is capable of being bent elastically along its length in use.

At its lower end, the riser <NUM> extends around a sagbend <NUM> to meet the seabed <NUM> at a touch-down point (TDP) <NUM>. Beyond the TDP <NUM>, the riser is in fluid communication with a static flowline <NUM> that lies on the seabed <NUM>. The flowline <NUM> connects the riser <NUM> to a subsea source of hydrocarbons, such as a subsea well (not shown).

In these schematic views, which are not to scale, the typical depth of water between the surface <NUM> and the seabed <NUM> is greatly understated.

The riser <NUM> has negative buoyancy in seawater and so is held in tension by its suspended apparent weight. That weight, expressed in the art as top tension, is supported by a hang-off structure <NUM> protruding from the side of the hull of the FPSO <NUM>.

The riser <NUM> is connected to the hang-off structure <NUM> by an articulating joint <NUM>, exemplified here by a part-spherical ball-type joint. The joint <NUM> allows the riser <NUM> to pivot freely relative to the hang-off structure <NUM> about mutually orthogonal, substantially horizontal axes. Thus, as the riser <NUM> bends along its length, the top of the riser <NUM> can pivot relative to the hang-off structure <NUM> within a downwardly diverging cone <NUM> whose apex coincides with the centre of spherical curvature of the joint <NUM>. In this example, the riser <NUM> is also able to twist or turn about its longitudinal axis relative to the hang-off structure <NUM>.

An upper balcony or clamp <NUM> also protrudes from the side of the hull of the FPSO <NUM>, above and spaced vertically from the hang-off structure <NUM>. Typically, the vertical spacing between the hang-off structure <NUM> and the upper clamp <NUM> is up to about <NUM>.

A pliant spool pipe <NUM> in fluid communication with the upper end of the riser <NUM> is supported by, and extends between, the hang-off structure <NUM> and the upper clamp <NUM>. At the upper clamp <NUM>, the pliant spool pipe <NUM> connects the riser <NUM> to pipework (not shown) aboard the FPSO <NUM>, for example to convey hydrocarbon production fluids from the riser <NUM> for processing and storage. Thus, the upper clamp <NUM> serves as an upper connection structure for the pliant spool pipe <NUM> that connects the riser <NUM> to the FPSO <NUM>.

The pliant spool pipe <NUM> may, for example, be made of thermo-composite pipe (TCP). By virtue of its pliancy, the spool pipe <NUM> can deflect within an upwardly diverging cone <NUM> that is in mirror-image alignment and mutual opposition to the downwardly diverging cone <NUM> about the joint <NUM> of the hang-off structure <NUM>. Thus, the pliancy of the spool pipe <NUM> provides enough flexibility to support bending of the riser <NUM> involving rotation of the joint <NUM>.

The riser <NUM> is adapted in accordance with the invention by the attachment of a buoyancy module or buoy <NUM> to the riser <NUM>. The features of the buoy <NUM> and its interaction with the riser <NUM> are also evident in <FIG>.

The buoy <NUM> is attached to the riser <NUM> above the seabed <NUM>, typically at least <NUM> above the seabed <NUM> to place the buoy <NUM> above the sagbend <NUM> of the riser <NUM>. More generally, the buoy <NUM> is disposed in the water column between the seabed <NUM> and the surface <NUM> at a depth corresponding to, for example, between one third and two thirds of the height of the water column and hence of the overall depth of the water. Preferably the buoy <NUM> is at a depth that is closer to the seabed <NUM> than to the surface <NUM>.

The buoy <NUM> is connected to the riser <NUM> by an articulating joint <NUM>, again exemplified here by a part-spherical ball-type joint whose structure will be explained in more detail with reference to <FIG>. The joint <NUM> allows the riser <NUM> to pivot freely relative to the buoy <NUM> about mutually orthogonal, substantially horizontal axes. In this example, the riser <NUM> is also able to twist or turn about its longitudinal axis within and relative to the buoy <NUM>.

Thus, as the riser <NUM> bends along its length, the riser <NUM> can pivot relative to the buoy <NUM> within an upwardly diverging cone <NUM> and a downwardly diverging cone <NUM> in mirror-image alignment and mutual opposition about the joint <NUM>. The cones <NUM>, <NUM> converge so that their respective apices meet at the centre of spherical curvature of the joint <NUM>.

By virtue of the joint <NUM>, minimal friction between the riser <NUM> and the buoy <NUM> allows the riser <NUM> to deflect readily and rapidly relative to the buoy <NUM> without requiring the buoy <NUM> itself to deflect or tilt. Thus, the inertia and hydrodynamic drag of the buoy <NUM> does not hinder free deflection of the riser <NUM> in response to, for example, dynamic motion of the FPSO <NUM>. Nevertheless, beneficially, the inertia and drag of the buoy <NUM> help to damp oscillatory motion of the riser <NUM>.

<FIG> shows that when at rest, the riser <NUM> hangs in a smooth catenary curve that extends through the buoy <NUM> without inflection. Conversely, it will be apparent from <FIG> that the joint <NUM> allows the riser <NUM> to deflect rapidly relative to the buoy <NUM>, changing its longitudinal curvature in response to motion of the FPSO <NUM>. For example, the riser <NUM> may adopt the shallow S-shaped curvature of <FIG> in response to downward heave motion of the FPSO <NUM>.

The S-curve of the riser <NUM> extends through the buoy <NUM>, hence comprising opposed upper and lower curves <NUM>, <NUM> respectively above and below the buoy <NUM>. At a point of inflection coincident with or close to the buoy <NUM>, the curvature of the upper curve <NUM> reverses into the opposite curvature of the lower curve <NUM>. The curvature of the lower curve <NUM> reverses again toward the sagbend <NUM>.

The downward extent of the S-curve ends above the sagbend <NUM> of the riser <NUM>. By its deflection, alternately bending and straightening, the S-curve therefore substantially isolates the TDP <NUM> from forces in the riser <NUM> driven by motion of the FPSO <NUM>. For example, isolating the TDP <NUM> from compressive forces in the riser <NUM> driven by heave of the FPSO <NUM> beneficially reduces the effects of fatigue and the possibility of the riser <NUM> buckling.

Deflection of the top of the riser <NUM> is accommodated by the joint <NUM> of the hang-off structure <NUM> and also by opposite deflection of the pliant spool pipe <NUM> that extends from the joint <NUM> to the upper clamp <NUM>, as will be apparent in <FIG>.

In the example shown in <FIG> and <FIG>, the buoy <NUM> can also slide along and relative to the riser <NUM> within a longitudinal range of movement that is delimited by radially protruding stopper formations <NUM> spaced apart longitudinally along the riser <NUM>. This allows the riser <NUM> to move rapidly, within that limited range, through and relative to the buoy <NUM> in response to sudden vertical movements of the FPSO <NUM>, while corresponding movement of the buoy <NUM> can lag slightly due to inertia.

<FIG> show further details of the buoy <NUM>. Here, it will be apparent that the joint <NUM> comprises an outwardly convex, part-spherical inner ball <NUM> retained within a complementary inwardly concave annular hub <NUM> that defines a socket for the inner ball <NUM>. In this example, the buoy <NUM> further comprises a buoyant toroidal body <NUM> of circular plan shape that lies in a substantially horizontal plane. The body <NUM> is supported by spokes or struts <NUM> that extend radially from the annular hub <NUM> with equiangular spacing. The struts <NUM> hold the body <NUM> spaced radially apart from the hub <NUM>.

At least the body <NUM> of the buoy <NUM> may have its internal and external pressure equalised during the deployment of the riser <NUM>, using pressurised air, packed macrospheres, syntactic foam or other known pressure-compensation techniques.

The toroidal shape of the body <NUM> exemplifies how, beneficially, buoyancy and mass may be offset radially from the central longitudinal neutral axis <NUM> of the riser <NUM> and may be distributed circumferentially around that axis <NUM>. The offset and distributed buoyancy and mass offset the buoyancy forces exerted by the buoy <NUM> on the riser <NUM> and increase drag and the moment of inertia, which stabilise the buoy <NUM> and the riser <NUM>.

The local inclination of the inflection portion of the riser <NUM> extending through the buoy <NUM> increases relative to the vertical as the S-curve becomes more pronounced with continued downward motion of the FPSO <NUM>. As <FIG> shows schematically, changes in the inclination of this portion of the riser <NUM> are accommodated by rotation of the inner ball <NUM> within and relative to the annular hub <NUM> of the joint <NUM>.

Finally, <FIG> show further details of the pliant spool pipe <NUM> and its connections to the riser <NUM>, the hang-off structure <NUM> and the upper clamp <NUM>.

The pliant spool pipe <NUM> comprises upper and lower end fittings <NUM>, <NUM> respectively. The upper clamp <NUM> engages the upper end fitting <NUM> to transfer the bending moment from the spool pipe <NUM> to the FPSO <NUM>. A pipe section <NUM> surmounts the upper end fitting <NUM> to connect the riser <NUM> to pipework aboard the FPSO <NUM>.

The top of the riser <NUM> comprises a downwardly tapering pull head <NUM> that is received in a socket of the hang-off structure <NUM> in a conventional manner. The pull head <NUM> houses the joint <NUM>, which comprises a downwardly facing part-spherical ball formation <NUM> surrounding the riser <NUM>, received by a complementary upwardly facing part-spherical seat <NUM>. This is akin to the conventional hang-off arrangement used for flexible joints.

The ball formation <NUM> is surmounted by a tubular pup piece <NUM> that is in fluid communication with the riser <NUM> and with the lower end fitting <NUM> of the pliant spool pipe <NUM>. The lower end fitting <NUM> is attached to the pup piece <NUM> by welding or by a flanged connector.

Many variations are possible within the inventive concept. For example, it would be possible for the buoy <NUM> to be held at a fixed longitudinal position relative to the riser <NUM>. It would also be possible for the buoy <NUM> to be coupled to the riser <NUM> by a spring or damper system that resists, but does not prevent, relative longitudinal movement between the buoy <NUM> and the riser <NUM>.

Apart from TCP, the pliant spool pipe <NUM> could be made of bonded or unbonded flexible pipe, of a rigid pipe with deformable sections, or of coiled tubing.

Claim 1:
A buoy (<NUM>) for a rigid subsea riser, the buoy (<NUM>) comprising an inner part (<NUM>) for attachment to a riser pipe (<NUM>), an outer part (<NUM>) that is movable relative to the inner part (<NUM>), and a buoyant body (<NUM>) that is spaced radially from the inner part (<NUM>), wherein the inner part (<NUM>) defines a longitudinal axis and the outer part (<NUM>) is pivotable about pivot axes that are orthogonal to each other and that intersect the longitudinal axis.