SCRs are more and more commonly used within the offshore oil and gas industry. They provide a fluid-conveying liaison between seabed and a (moored) vessel, that can be used for oil production, water injection, gas injection, and as service lines.

Conventionally the connection of the riser to the vessel features a flex-joint, attached to hull side. Location of the flex-joint on hull side is mainly dictated by architectural considerations (structure and process), as well as installation considerations. The departure angle of the SCR (in the horizontal plane) is generally restricted so that SCRs cannot head parallel to the hull side, to avoid collision with the hull., SCRs are known to be fatigue-prone. Moored vessels experience swell-induced motions, which, in turn, generate displacement (in the 6 degrees of freedom) of flex- joint or other connection at the top extremity of the SCR. The vertical motions of this joint depend on the heave, roll and pitch motions of the vessel. It is known that these top motions induce displacements of the touch-down point (where the curve of the riser meets the seabed) and that this area is the most exercised mechanically. Roughly speaking, the longer the distance from the joint location to the pitch or roll axis of the vessel, the greater the displacements of SCR top, and the greater the impact on fatigue in the touch-down point area. In some cases, it may prove difficult to design an SCR that will meet the fatigue criterion (lifetime requirement). The design of the riser relies on all parts being in tension, and it may also prove difficult to avoid effective compression in some parts of the riser, in all the relevant environmental conditions.

The present invention originates in development efforts focused on lowering the fatigue damage experienced by steel catenary risers when mechanically excited by the swell- induced motions of a floating vessel. The connection design which is proposed aims at suppressing, or at least greatly reducing, roll and pitch influences in the displacements of SCR top connection, thus reducing stress and fatigue in the touch down area (and also reducing the risk of effective compression).

The invention provides a marine riser installation comprising a length of steel conduit suspended from a floating vessel so as to hang in tension between seabed and surface, the installation including a flexible upper conduit extending between an upper end of said steel conduit and a fluid connection point on the vessel, and an upper non-conduit portion extending between the upper end of said steel conduit and a hanging point elsewhere on the vessel, wherein the length of the flexible upper conduit portion is such that substantially all the tension in the riser is transferred via the upper non-conduit portion to the hanging point, and wherein the location of the hanging point is significantly closer to at least one of a pitch axis and a roll axis of the vessel than the fluid connection point.

By this construction, the two functions of the connection device, hanging and fluid transmission, are segregated. The former can then be relocated on the hull in an optimised way, that is, to a location displaying the smallest, or at least smaller, swell- induced motions for the relevant environmental conditions.

The upper non-conduit portion may comprise chain suitable for carrying the weight and any additional tension in the riser. Besides having the necessary strength, chain by its nature will readily adapt to the direction and curvature necessary to continue the catenary path of the steel conduit.

The upper end of the steel conduit may be located at a depth of 20-100m below the sea surface, for example around 50m.

Where the vessel has a single hull, the hanging point may be on or beside the keel line.

The hanging point may similarly be at or near a midship position between bow and stern. For a traditional hull design, a location on the keel line at midship will be substantially calmer than any location on the side of the vessel where the fluid connection point can be located.

For other hull designs, or multi-hulled vessels, the hanging point may be located between hulls, or in an opening similar to a moonpool.

It will be understood that the best effect can be obtained by positioning the hanging point on both the roll axis and the pitch axis of the vessel. However, these axes may not intersect exactly, and in any case it may be difficult to form an attachment point at that precise location. The skilled reader will appreciate that a degree of benefit can be achieved even if the hanging point is only a little nearer to the axis than the fluid connection point.

Without limiting the scope of protection, the hanging point may be, say, twice as near as the fluid connection point to one or both of said axes. Roughly speaking, the contribution of vessel roll and pitch to the overall motion of the riser will be halved.

Proportionally greater benefit is gained if the hanging point can be positioned four times or eight times nearer, for the sake of example. Since heave motion, as opposed to rolling and pitching will still be experienced by the riser, there is little point striving to eliminate the roll and pitch components entirely. Depending on the design of the vessel and the location of the fluid connection point, one of pitch and roll motion may be significantly more of an issue than the other. A fluid connection point at the bow of the vessel, for example, may already be very close to the roll axis, but far from the pitch axis.

In one class of embodiments, the riser extends from the hanging point in a direction whose horizontal component is substantially parallel to a side of the vessel at which the fluid connection point is provided. This permits a layout of vessel, riser and seabed

installations that is not permitted when connecting the riser conduit directly to the side of the vessel.

The invention further provides a method of installing a marine riser, the method comprising: - providing a floating vessel; - providing a length of steel conduit adapted to be suspended from the floating vessel so as to hang in tension between seabed and surface; - providing a flexible upper conduit extending between an upper end of said steel conduit and a fluid connection point on the vessel; and - providing an upper non-conduit portion extending between the upper end of said steel conduit and a hanging point elsewhere on the vessel, wherein the length of the flexible upper, conduit portion is such that substantially all the tension in the riser is transferred via the upper non-conduit portion to the hanging point, and wherein the location of the hanging point is significantly closer to at least one of a pitch axis and a roll axis of the vessel than the fluid connection point.

While the above steps can in principle be performed in almost any order, various measures can be taken to sequence the steps so as to simplify installation. Thus it is possible to preinstall the risers, while the production vessel is not yet on site.

Furthermore, the steps can be arranged so as to minimise or even eliminate the need to make fluid or mechanical connections underwater. The following features in particular may be advantageous in this regard.

The upper non-conduit portion may be attached to the hanging point prior to attachment of the non-conduit portion to said steel riser conduit. The upper non-conduit portion may be attached to the vessel while the vessel is at a location remote from the location of the completed riser installation, for example in a shipyard.

The upper non-conduit portion may be flexible. The upper non-conduit portion may for example comprise a chain.

The flexible conduit portion may be attached to the steel riser conduit at a time before the steel riser conduit is connected to the upper non-conduit portion.

The fluid connection between steel riser conduit and flexible conduit portion can be made while the upper end of the steel riser conduit is held above the sea surface.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawing, in which: Figure 1 is a schematic elevation of a marine riser installation in accordance with the present invention, viewed from the stern of the floating vessel; and Figure 2 is a schematic plan view of the same installation.

DETAILED DESCRIPTION OF THE EMBODIMENTS Referring to Figures 1 and 2, a riser installation extends from the sea surface 10 to an oil well installation (not shown) on the seabed 12. The depth of the water in many modern oilfield developments is very great, routinely exceeding 1000m, so that a platform fixed on the seabed is not practicable. A floating production and storage and offloading (FPSO) vessel 14 is therefore provided, with all the processing, storage and personnel accommodation facilities that its name implies.

A steel riser conduit 16 made from welded sections of rigid steel pipe is suspended from vessel 14, and may have a diameter for example of 8-12 inches (approximately 20-30cm).

As with a conventional SCR, vessel 14 is moored (and/or driven by thrusters) so as to maintain a horizontal tension in the riser. The upper end of the installation is distinguished from conventional SCRs in that the rigid steel conduit 16 terminates at a junction point J approximately 50m below the surface 10. For the purpose of fluid flow,

the riser continues in the form of a flexible conduit 18, which is connected to the steel conduit 16 and which is terminated at fluid connection point F at one side of the vessel's topsides. (Flexible conduit 18 is not shown in Figure 2, for clarity. ) From point F, fixed pipework conveys the fluid to processing equipment and storage tanks within vessel 14. A flex-joint may be provided here, if necessary.

For the purpose of supporting the tension in the riser, a section of chain 20 is connected between the upper end of steel conduit 16 and a hanging point H separate from the fluid connection point F. Chain 20 can be connected to conduit 16 at or just below the connection to flexible conduit 18, for example by a collar (not shown). As explained in the introduction, hanging point H is chosen so as to experience relatively little motion due to the roll and pitch components of the vessel's motion induced by sea swell. In the example, point H is located on the keel beneath the vessel, at a midship position (approximately half way between bow and stern). For the sake of illustration, R indicates the location of the roll axis of the vessel. Although point H on the keel is below this axis somewhat, it will be apparent that the distance between R and H is much less than the distance between R and F. Accordingly, motion of point H, and hence of the whole riser, in response to rolling motion of the vessel is considerably less than would be the case if riser 16 were suspended directly from the fluid connection point F in the conventional manner. The flexible conduit 18 does not carry substantial tension, and can readily be provided with sufficient length to absorb expected displacements of the fluid connection point F relative to junction point J, caused by rolling motion of the vessel.

Similarly, in relation to pitch motion, it can be seen from Figure 2 that the hanging point H is close to the pitch axis P (roughly at the midship position), whereas the fluid connection point F is to one side and somewhat forward. The fluid connection point F can be positioned relatively freely along the length of the vessel, without incurring a penalty in terms of pitch-induced fatigue at the touch-down point T.

Another degree of freedom which is provided for the designer concerns the orientation of the vessel relative to the riser, as it extends in a horizontal direction from the vessel

to the touch-down point T. The solid arrow labelled 16 in Figure 2 shows the riser extending almost perpendicularly from the beam of the vessel. The alternative configuration labelled 16'and shown in broken lines shows the riser extending in a direction substantially parallel to the vessel's side, where fluid connection point F is located. In a conventional arrangement, where the steel riser conduit 16 would be suspended directly from the fluid connection point, it is not normally possible to have the riser extending parallel to the side, as the flex-joint would have to be located at or near keel level to ensure that there was no interference of the hull with the riser during vessel pitch and roll. The problem is increased where multiple SCRs are involved, as to avoid interference between each other they would have to extend from the flex-joint at significantly different departure angles, which is not a simple engineering task. Use of the improved marine riser installation overcomes the difficulties associated with such a configuration, as significantly more departure angles in the horizontal plane are offered.

A proposed method for installation of the improved marine riser is as follows : - - in the shipyard/dry dock, provide the hanging point H (such as a padeye) at the desired location on or near the keel of the vessel.

- attach chain 20 to the hanging point, and temporarily strap the chain to the side of the hull of the vessel 14, where it can be accessed when the vessel is afloat.

- separately install riser conduit 16.

- locate the vessel at the oilfield, un-strap the chain and attach it to the upper end of the steel conduit, as described previously.

- connect flexible conduit 18 between a top-side fluid connection point F and steel riser 16.

The riser conduit (s) 16 can be pre-installed on the seabed, to be lifted to the vessel when the latter arrives on site. Alternatively, the riser can be laid subsequently, and handed over from a lay vessel to the vessel 14. Similarly, it may be most convenient if the flexible conduit 18 is attached to the top of the steel riser conduit 16 before the riser is suspended from the chain 20, so that the fluid connection can be made above water, either in the yard or on a laying vessel. For plural risers, the number of padeyes, chains

and so forth can be adapted as necessary, although it is possible for two risers to share a padeye, for example.

The skilled person will appreciate that other methods that achieve the same effect may be used to connect the risers to the hanging points under the vessel.

The skilled person will appreciate the utility of the proposed design, for example when : - it is difficult to avoid effective compression by conventional techniques - fatigue damage for an entirely rigid riser is predicted to be high, and/or - layout would be enhanced with SCRs departing from floater in or near the axis of vessel.

The invention is not limited to the simple catenary arrangement illustrated in the example, and may for example be used in combination with any of the wide range of constructions for the lower end of a catenary riser that are known in the art, and which are designed to accommodate motion or various types. Although in appropriate cases the chain-hung construction may eliminate the need for a special construction at the lower end of the riser, in other cases special constructions at both top and bottom may be advantageous. By using the invention in combination with other measures, for example, a riser may be designed for use in a harsher environment than by using either measure alone. Alternatively, or in addition, parameters of the surface vessel and its moorings may be relaxed. The different measures may serve to accommodate movements due to other causes such as changes in buoyancy, or movements at different frequencies from the vessel wave motion.

Thus, in addition to the invention, for example, buoyancy may be used to provide a "lazy wave"or similar configuration, rather than a simple catenary. According to another patent application of the present applicant, a flex-joint may be included at the lower end of the riser to effectively fix the touch-down point. The invention may be used in combination with such an arrangement to limit the expected excursions of the riser within the range of the flex-joint.

In practice, several risers may come to a single vessel, where only one has been shown and described, for simplicity. Even for a single well, there may be several conduits required, for production flowlines, water injection lines, gas lift and so forth, as is well known to those skilled in the art. These may be hung separately, particularly if they are substantial like water injection lines, while smaller service lines may be"piggy-backed" onto the larger line. The arrangement described permits these risers to be brought to fluid connection points convenient for the topsides design, without worsening the motions experienced by the steel riser. The different risers may be provided with their own chains suspended from the same or separate hanging points, according to requirements.