FLEXIBLE RISER SYSTEM AND METHOD FOR INSTALLING THE SAME

Abstract: A flexible riser l0 has a lower connector ll which is moved laterally over the seabed by a pull-in sled 7 under a mid-water support buoy 3 to connect it to a connection point l7 on subsea equipment l. The riser l0 is attached at a point part way along its length to buoy 3. This is done at one side of the buoy prior to construction vessel l3 laying the riser over the buoy and connecting the upper connector 24 to a production vessel 2 positioned over the subsea equipment. Attachment is done by guiding the riser pipe into a channel 36, a projection 20 on the riser engaging the channel against lowering of the pipe. An upper connector of the riser may include a single pivotal arm and a single offset guide post to maintain its orientation while it is being pulled into engagement with a mating part on the production vessel.

Description: This invention relates to systems of flexible risers (within which we include control umbilicals) extending from subsea oil or gas production equipment positioned on the seabed, over a mid-water support and up to a vessel on the surface of the sea, and to methods of installing them. A system wherein flexible ducts from different directions are brought over a low support before rising is seen in GB-A-20ll506. Other designs and configurations for flexible risers have been described by Coflexip in their brochure "Dynamic Flexible Risers" printed in September l985. These include five configurations named as follows: (a) free-hanging (b) lazy S (c) steep S (d) lazy wave (e) steep wave. The last four all have mid-water supports. In addition "Offshore Engineer" of February l985 shows a form of lazy S configuration wherein the lower leg, below the mid water support, runs back under that support i.e. towards the region under the vessel. It is likely that many semi-submersible floating production systems will in future have at least some of their production wells positioned directly beneath the vessel for two reasons:- produced fluids can flow more readily without suffering from the slugflow, hydrate, wax and emulsion problems associated with subsea flowlines; workover and other subsea equipment maintenance can be performed directly without requiring a second vessel. The steep S and steep wave configurations, which are designed to be used when wells are beneath the vessel, are difficult to install and require extra connection points at the riser base. It is desired therefore to reduce the number of connections to a minimum and to make installation as simple as possible. It may be necessary to perform the installation without using divers. The present invention is concerned to allow lazy S installations to be made comparatively readily on a pre-positioned midwater support. The key to the successful operation is the realisation that both the upper and lower legs may be precisely designed to optimum lengths if the riser is restrained by the support before the upper leg is laid and if that restraint is provided at the lateral side of the support away from the direction in which the upper part of the riser will then be moved to lay it over the support and from the slack catenary of the upper leg which is then attached to the production vessel. The restraint is preferably provided by a channel into which the riser is guided and is then prevented from downward movement by engagement of a stop on the riser with a detent associated with the channel. The channel may be part of a guide channel extending over the support and receiving the riser as it goes into the loop of the slack catenary of the upper leg. The support may be a tethered buoy or may be a rigid structure. The free end of the riser at the seabed may be drawn back under the support to achieve connection with equipment vertically under the vessel: this may be achieved by a sled associated with the free end, either temporarily (the end being lowered into a container on the sled borne on a base of the support) or permanently, the sled then being lowered with the end from a construction vessel and preferably then being received on a specially conformed base with orienting guide ribs or channels. Furthermore the base may have stops whereby the sled, initially vertically oriented, may be tipped over by appropriate movement of the construction vessel, so as to present the free end appropriately towards the equipment. Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:- Figure l is a diagrammatic side view showing a riser being unreeled and suspended vertically from a construction vessel; Figure 2 shows a later stage in the operation; Figure 3 shows a third stage after laying the upper section over the buoy and attaching the upper connector to the production vessel. Figure 4 is an isometric view of a completed nine-riser system using a buoyant mid-water support. Figure 5 is an isometric view of a completed system using a fixed mid-water support; Figure 6 is a detail perspective view of a possible arrangement at the seabed; Figures 7,8 and 9 show different forms of catches for attachment of the risers to the side portion of the mid-water support, Figures 7a,8a and 9a being sectional views looking along the support and Figures 7b,8b and 9b being views on the arrows VII, VIII and IX respectively; Figure l0 is an isometric view of another embodiment of mid-water support; Figures ll and l2 are sectional views of the embodiment of Figure l0 looking along the support; Figure l3a is a sectional view analogous to Figure ll but of a modified embodiment; Figures l3b-d being sections on the planes B,C and D respectively in Figure l3a; Figure l4 is a sectional view of a further embodiment of mid-water support looking along the support; Figure l5 is a plan view of the embodiment of Figure l4; Figure l6 is a diametrical section through an upper-end connector; and Figure l7 is a section on the plane XVII-XVII of Figure l6. Referring to Figure l, the subsea production equipment l (which can be template-mounted) is shown positioned on the seabed and a production vessel 2 is floating directly above. A mid-water support in the form of a buoy 3 is tethered to a seabed base 4 by a main tether system 5 and may include a subsidiary tether system 6 to deflect the buoy. The base 4 must be accurately positioned relative to the subsea production equipment l and the connection point l7. This may be done by a sonar positioning system which should be able to work to a tolerance of +/- l metre, or preferably better, over a distance of l00 metres. Alternatively the base 4 can be positioned relative to the subsea equipment l by using a seabed spacing frame or mudline cover which could also be used to provide a stable working surface between the base 4 and subsea equipment l in soft soil areas. A receptacle sled 7 initially positioned on the base 4 comprises a runner and a horizontally-pivotable receptacle 8 mounted on the runner by pivot 9. Items 3,4,5,6,7 may be installed simultaneously on the seabed in a single lift by a crane vessel. A flexible pipe l0 with a lower connector ll is lowered vertically from a reel l2 on a construction vessel l3. A remotely-operated vehicle (ROV) l4 is used to monitor the approach of connector ll towards receptacle 8 and construction vessel l3 manoeuvres, for example using dynamic-positioning, to enable the connector ll to enter the receptacle 8 as the connector is lowered. Pipes having multiple bores must be correctly oriented, possibly by incorporating a helical cam (not shown) in the receptacle 8 and a cam follower on the connector ll. Alternatively, the sled 7 could be lowered down from vessel l3 together with the connector ll already positioned in it, but the sled would require orientation as it approached base 4 (see Figure 6). A second ROV l5 launched from vessel 2 has the ability to attach a pulling line l6 to sled 7 and then to land on the subsea equipment l and pull the sled 7 towards a connection point l7 on the equipment l. As it does so and the riser runs along the seabed behind it the receptacle 8 pivots, to offer the connector ll horizontally to the connection point l7. A stop prevents the receptacle pivoting beyond the horizontal. Figure 2 shows the sled 7 after having been pulled into the subsea equipment l and connection having been made between the connector ll and the connection point l7. Before the upper leg of the riser, i.e. the part above the mid-water support, is brought back towards the vessel 2 it is attached to the support 3 in such a way that the weight of the lower leg is supported by it. This ensures that the catenaries of the legs of the riser are as designed; without the attachment the weight of the lower leg could cause the riser to slide over the support during the laying-over of the upper leg, resulting in a coil of pipe forming on the seabed and shortening the upper one. Shortening the upper leg would deprive the system of its optimum acceptance of surface vessel motion and would undesirably diminish the radius of curvature of the lower leg where it turns as it meets the seabed. To attach the riser, after the connection of the lower leg to the equipment l7, the vessel l3 manoeuvres to position the riser l0 adjacent to the mid-water support 3 prior to attachment and then lays it back over the support in guide cradles 23. The ROV l4 allows visual monitoring of the relationship between the support 3 and riser during that approach. One method of ensuring that the free-hanging catenary l8 between the mid-water support 3 and the seabed is shaped as required would be to clamp a marked position of the flexible pipe to the side of the support 3. However this would require manual intervention, and preferred means for achieving the attachment will be described later. Figure 3 shows the upper riser connector 24 being pulled in to make connection with the production vessel at a connection point 25, the pipe length in the upper leg of the riser being such as to form a catenary 26. If more than one flexible riser pipe is to be connected to the upper connector 24, all the flexible pipes would first be connected to the seabed equipment l and attached to the support 3. Then all the pipes would be laid over the support 3 together and attached there prior to attachment to the upper connector 24. Figure 4 is an isometric view showing a cylindrical buoy 3 carrying eight riser pipes (or umbilicals) l0. An export riser 27 could be installed in a lazy S configuration away from the base 4. Riser 27 shown in Figure 4 does not pass back under the mid-water buoy 3 but extends across the seabed away from the production vessel 2 and subsea production equipment l. It could connect for example to an export single point mooring. Similar risers to riser 27 could extend across the seabed as lines to satellite wells or manifolds or to an onshore terminal. To connect the upper end of riser 27 to the production vessel 2 the construction vessel l3 must advance towards the mid-water buoy 3. In order for the lower catenary of riser 27 beween the seabed and mid-water buoy to take up the correct geometry, it is essential to transfer the lower riser catenary weight to the buoy 3 at the correct position along the riser length. Use of sonar measuring methods as described in Offshore magazine, July l979 issue, page 45, will enable catch means (to be described) to be added to riser 27 in the correct position. Referring now to Figure 5, the required mid-water support could be provided by a fixed rigid structure 35 which resists all applied loads without incorporating buoyancy. In this case, detents for the risers (to be decribed later) could be rigidly attached to the cradles 23 or on a nearby point of structure 35 without being suspended on chains or cables. As mentioned previously, part of the means to laterally move the lower connector across the seabed (i.e. the sled) could be attached to the lower connector before the riser pipe is lowered from the construction vessel l3. Figure 6 shows means of assisting orientation of the lower connector and an associated sled as it approaches the base. Orientation of the lower connector ll attached with sled 7′ (see Figure 6) is assisted by channels or grooves 28 on the base 4 which define the correct direction (shown by arrow A) towards the subsea equipment l and connection point l7. A bump or slight rise 29 in each channel guide restrains movement of the nose 30 of the sled 7′ along the channel selected as the support vessel l3 moves away from the production vessel 2. The sled 7′ and riser l0 with it will then tip over away from the subsea equipment so that as extra flexible riser is unreeled at the surface, it takes up the correct position for pull-in by line l6 of the sled, which will then run on the runners 7a. An integral sled may be constructed to tilt spontaneously onto its runners. Making the sled permanently associated with the lower connector ll allows later recovery of a faulty riser and installation of a replacement riser by the same method. Various means for attaching the riser to the support will now be described. Figure 7 shows how a steel spool l9 may be let in part-way along flexible pipe l0. The spool l9 has a narrow tubular stem which can enter into the mouth of a carrier 2l and an enlarged portion 20 which can nestle into a detent cup at the upper end of the carrier 2l. The carrier 2l is suspended from the support, below the end of the cradle 23, by for example chains 22. When the attachment has been made, some of the weight (i.e. all of the free weight from the support down to the seabed) of the riser pipe l0 can be carried by support buoy 3. The construction vessel l3 can then move towards the production vessel 2 and more flexible pipe l0 can be unwound from the reel l2 so that pipe can be laid across the support into the cradle 23 without slipping. Figure 8 shows another means of transfer of lower riser catenary weight to the mid-water buoy 3. A steel stop 3l is clamped around the riser at the correct position using screws 32. The stop 3l is manufactured as one piece and then split in half diametrically. The enlarged portion 20′ of the stop can nestle into a detent in a carrier 33 similarly as in carrier 2l. The carrier 33 may be suspended on wire rope slings 34. Flexible suspension is advantageous in that during wave-induced motion, riser flexing occurs mainly at the touch-down point on cradle 23 rather than at the catenary weight support position. Referring now to Figure 9, the arched cradle 23 is shaped to include a protruding guide fork 36 at its lower end, to guide the riser l0 into the cradle 23 and the flanged clamped stop 3l into a recess 37 in the cradle 23 as the riser is lowered into position (see Figure 2). The flange 20′′ of the stop rests on the top edges 36a of the fork which also assist in sustaining the weight of the riser even when the stop 3l is engaged in the recess 37. The recess 37 is at a point lying at about 30 degrees above horizontal to remove the stop 3l away from the region of riser flexure in service. The guide fork has outwardly divergent lips 36b to assist entry of the riser pipe into the fork. This detent means for weight transfer has the advantages of being integral with the cradle 23 and being easier to protect against corrosion. In any of the systems described, the stop or catch on the pipe, e.g. the enlarged portion 20, may be part of a separable connection, either flanged or hydraulically-actuated, such that the upper catenary 26 can be separated from the lower catenary l8. A hydraulic connector included in the enlarged portion 20 could be actuated by a pressure source carried by a diver or ROV such as l4. Any of the systems can be used either with risers l0 or risers 27. The base 4 may extend across the seabed and link with the subsea equipment l, thus incorporating the spacing frame or mudline cover previously described within itself, or in the variant described with reference to Figure 6 providing grooves or channels for guidance all the way to the connection point l7. The construction of the support 3, when it is a buoy can be advantageously carried out in the ways to be described with reference to Figures l0 to l5. In the embodiment of Figures l0 to l3 a modular system is shown wherein buoyancy units 39 of variable buoyancy are attached to a beam 3′; the units are not laterally symmetrical and in that way can compensate for the laterally unequal loading supported by a buoy in the present types of systems. The cradles 23′ are part wheel-like objects attached as required to flanges on the beam 3′. Advantageously, while mobilised to install the equipment l, the crane vessel can also install the buoyant beams 3 with their associated tethers 5,6 and base 4. In this case, the crane vessel lifts a base 4 with a beam 3 and tethers 5 lying in contact with it. This assembly may exceed 30 metres in length and l00 tonnes in weight when in air. The buoyant beam 3 should be held in place by releasable latches or straps (not shown) so that on entering the water the beam 3 remains in contact with the base 4 and does not interfere with the crane rope. Later, possibly after drilling wells and installing the flowlines to satellite wells (if any), the latches or straps are released and the buoyant beam 3 will rise through the water until reaching the limit of the tethers 5. The beam needs only sufficient buoyancy to overcome its own weight and the weight of the tethers. This permits the beam to be fabricated from tubular steel about l to 2 metres in diameter. To develop buoyancy, the ends must be sealed to prevent ingress of water. By thus minimising the diameter of the tubular structure there is good ability to resist collapse due to external pressure. It may also be filled with pressurised gas to permit installation in deeper water. Figure l2 is a section through the buoyant beam 3 with riser support arch 23 bolted on its flange 38. Figure ll is a section through the buoyant beam 3 and buoyancy unit 39. The unit 39 has an open bottom so that gas can be added as required by holding a gas line underneath or by delivery through special piping (not shown). The unit 39 may be divided into two by a partition 40, the divisions having vents 4l so that buoyancy can be adjusted on each lateral side of the buoyant beam 3. Buoyancy adjustment can be made to match the distribution of riser load each side of the beam 3. The module 39 may be fabricated from sheet metal or plastic or fibreglass or similar, as there is equal pressure inside and outside. A sight glass or a transparent section (not shown) would allow monitoring of the amount of gas trapped inside the module 39. When the buoyant beam 3 reaches its mid-water position at the limit of the tethers 5, it will usually be less than 200 metres below sea surface. The support arches 23′ and buoyancy units 39 could be added as required by divers, or they could be pre-installed. A single riser pipe extending from the seabed to the mid-water support arch can weigh around 5 tonnes or greater. This part of the riser weight, plus part of that of the upper leg can be supported by the adjustable buoyancy units 39 each side of the support arch 23′; thus minimising the buoyancy required of the beam 3′. Not only can total buoyancy be adjusted but also its lateral distribution. If all the required buoyancy were to be designed into a beam 3, this item has to be around 3 to 4 metres in diameter and 60 mm to 80 mm thick with associated fabrication difficulties and high cost. Also, if buoyancy is constant, the position of the beam 3 and angle of the tethers 5 will vary more widely as riser weight is added during the setting up of the system. So far, we have shown mid-water arch supports 23 or 23′ which have part-circular section with slightly greater internal radius than the external radius of the flexible riser pipes l0,27 which rest in them. This is adequate to accommodate movement of the risers in the case of weather (wind, waves, current) acting largely in the line or in the plane of the risers. However, the weather direction may be at 90 degrees to the plane of the risers. Typically, this may cause the vessel 2 to move up to 30 metres off the common equilibrium (zero weather) centre plane extending through the vessel 2, the supports 3, bases 4 and equipment l. In the equilibrium condition, the horizontal distance between the vessel connection point of a riser l0 and its corresponding mid-water arch support may be 60 metres Therefore the angle of approach of a riser l0 to the beam 3 in transverse weather conditions could be around 30 degrees to the normal or equilibrium approach line. Unless the U-shaped catenary of the upper leg of the riser from the vessel to the arch support 23,23′ is steep, this 30 degree angle of approach could cause the riser to climb out of the arch support internal radius. Advantageously, the cross-section of the arch support will vary as shown for support 23′′ in Figure l3 where section D (Figure l3d) is positioned on the side of the support nearer to the vessel 2. This flaring of the groove in the arch support 23′′ spreads the touch-down load point of the U-shaped catenary and reduces the intensity of riser pipe flexure. A similar though lesser outward flaring at the end of the support nearer the lower leg (section C, Figure l3c) has the same effect. In the region of section B, Figure l3b, the support has a cross section much as described for earlier embodiments. Alternatively, especially if the flexible riser pipe has low resistance to torsional load or the outer covering has low wear resistance, the mid-water support system shown in Figures l4 and l5 can be used. Of course, channels of these or similarly varying sections can be used in all the embodiments of system which have been described. Twin beams 3′′ are joined together at intervals by structures which incorporate bearings 42. The bearings must operate freely in a seawater environment and may e.g. be aluminium-bronze bearings with integral graphite lubricant as used for SALM (single anchor-leg mooring) and articulated column joints. Buoyancy can be made integral with the arch support by providing sheet material 44 on both sides of the support while still allowing the angle of pivotal movement to be about 35 degrees each side of the centre. In a transverse weather condition, the arch support 23′′′ will align itself with the U-shaped catenary of the upper leg and slight torsion will be applied to the J-shaped catenary of the lower leg extending down to the seabed. Multi-line pipe connectors (which have been used previously) make installation and individual line replacement more difficult and two-wire pull-in systems occupy more space and take longer to rig than single line systems. It is desired therefore to enable heavy flexible pipes to be individually connected to a floating vessel using a relatively small, lightweight connector and to require only a single wire to pull the lower part of the connector into engagement with the upper part. We therefore provide a connection system for the flexible riser pipe comprising an upper part of a connector rigidly attached to the production vessel with a single guide funnel adjacent to the connector part,and a single pull-in cable extending from a winch, also mounted on the production vessel, and passing through the guide funnel; a lower part of a connector attached to one end of the flexible riser pipe and having a single guide post mounted with its axis offset from the pipe and lower connector part axis by a similar distance to that between the guide funnel and upper connector part axis, and the said post including means for co-axial attachment of the pull-in cable such that the cable extends from the top of the post; and a removable lever arm attached at one end near to the base of the guide post by a pivot pin whose axis is at right angles to the pipe/connector axis and spaced away from it by a similar but not necessarily equal distance to the guide post axis, and having means at the other end for attaching a handling and transfer cable extending from a second vessel or structure, such that this attachment can swivel at least about an axis which is at right angles to both the pivot pin axis and to the lever arm. Figure l5 is an elevation of the upper and lower connector parts after engagement of the guide post and funnel. A plan view of the lower connector part is also shown. Figure l5 shows the upper connector engagement in greater detail. The pull-in cable 45 is attached co-axially to guide post 46 which extends at an offset position from the connector part 24. Cable 45 pulls the guide post 46 into engagement with the funnel 47 mounted adjacent to the upper part of the connector 25. Tension in cable 44 when the pipe 26 and connector part 24 are lowered into the water from vessel l3 will be 5 to l0 tonnes or greater. When the weight of pipe catenary 26 and connector part 24 are transferred to cable 45, (which may be attached to post 46 on the deck of vessel l3, or by divers when submerged) the tension in this line will rise to a similar value, but slight tension (less than l tonne) is held on cable 44 which is attached through lever arm 48. In the position shown with connector and pipe axis close to vertical, the axis of retractable pivot pin 49 which attaches lever arm 48 to connector part 24 is horizontal and the axis of pin 50 is close to vertical to allow the cable 44 to pull directly ont he lever arm 48 without bending or buckling the cable 44. Tension in cable 44 tends to maintain the connector part 24 in the current orientation with respect to connector part 25 prior to and after the guide post 46 enters funnel 47. Connection will be visually monitored, and if required, tension in cable 44 can be applied at an angle to the vertical plane to generate additional torque around the guide post axis. Flexible pipe catenary 26 usually hangs away from the vessel 2 and the top angle of the catenary at the point of connection is a few degrees off vertical. If the pull-in cable 45 is outboard of the connector parts 24 and 25 as shown, the natural hanging angle of the pipe 26 due to the offset between guide post 46 and connector part 24 will also be a few degrees of vertical in the same direction. This will assist entry of connector part 24 into part 25. When fully entered, part 24 is hydraulically clamped in engagement with part 25. There may be hydraulically operated shut-off valves in both part 24 and part 25. A buoyancy collar 5l is mounted outside the pipe bend restrictor 52. This collar could have a square or rectangular periphery in section to allow closer spacing between adjacent connectors. After connection has been made, lever arm 48 and cables 44 and 45 are released and recovered. Removal of an individual pipe would begin by attaching cables 45 and 44 (possibly via lever arm 48) and releasing the connector such that part 24 and the pipe 26 could be transferred back to an installation vessel, and then winding the pipe on a reel. A replacement pipe could be installed by repeating the initial installation procedure. Supports such as 3,35 may be provided at only one side of the vertical below the production vessel, or at more than one. Preferable would be not more than two sides, diametrically opposed. The detents may be provided anywhere on the relevant lateral side of the support, and on an arcuate support can be between the horizontal radius and a radius making an angle of about 45° above the horizontal, more preferably of about 30° above the horizontal.

Claims: A method of installing a flexible riser system in water to bring flexible risers (l0,27)from seabed equipment (l) to a production vessel (2) at the sea surface, including providing a support (3,3′,35) to one lateral side of a line running vertically from the production vessel (2) and at a level intermediate the seabed and the sea surface, characterized by a) lowering at least one flexible pipe (l0,27) from the sea surface, b) bringing the pipe (l0,27) to the support at a lateral side portion thereof remote from the said line, c) attaching said pipe (l0,27) and said support (3,3′,35) together to sustain on the support at least the weight of the pipe below the support (3,3′,35), and thereafter d) laying an upper portion of the pipe over the support towards said line to form a catenary upper leg of a riser. A method as claimed in claim l including after step a and before step b; e) drawing the pipe along the seabed below said support (3,3′,35) to bring a free end of the pipe to said equipment (l). A method as claimed in claim 2 including in step a, lowering the free end of the pipe (l0,27) together with an attached sled (7,7a) in a vertical attitude, and tipping the sled to a horizontal attitude when at the seabed, and in step e pulling the tipped sled and the free end of the pipe to the equipment. A method as claimed in any one of the preceding claims wherein steps b and c are performed by bringing the pipe (l0,27) laterally towards the support (3,3′,35) to enter a laterally directed channel projecting therefrom, and engaging a projection (20,20′,20′′) on the pipe (l0,27) with a detent (2l,33,36a,37) associated with the channel against relative downward movement of the pipe. A method as claimed in any one of the preceding claims wherein the support is a tethered buoy (3,3′), including the step of adjusting the buoyancy of the buoy. A method as claimed in claim 5 wherein the adjustment step includes adjusting the relative buoyancy of the lateral sides of the buoy (3′). A method as claimed in claim 5 or claim 6 wherein the adjustment step includes adding at least one buoyancy module (39) to the buoy (3′). A method as claimed in any one of the preceding claims wherein step d includes moving a construction vessel (l3) with the pipe (l0,27) attached towards the production vessel (2) and the method includes thereafter attaching a traction line (45) from the production vessel to pull the pipe towards an upper connector part (25) on the production vessel, maintaining contact between a lower connector part (24) on the pipe (l0,27) and the construction vessel (l3) by a traction line between a radial arm (48) on the lower connector part (24) whereby to maintain an orientation of the lower connector part, and engaging first a single guide post and socket means (46,47) on the connector parts further to orientate and align the parts and then engaging mating pipeline male and female connections (24,25) on the connector parts whereby to form a fluid-conducting connection between the pipe and the production vessel. A flexible riser system comprising a) a production vessel (2) at the sea surface b) underwater equipment (1) at the seabed c) a support (3,3′,35) for at least one riser (10,27) at a level between the seabed and the sea surface, the support having a first and second lateral side d) at least one flexible riser joining the equipment to the vessel said at least one riser lying over said support to form a catenary upper leg of the riser from said support to the vessel at the first lateral side of the support and hanging therefrom at the second lateral side thereof to define a catenary lower leg of the riser; characterized in that the pipe (l0,27) is engaged to the second lateral side portion of the support by engagement means (20,2l,33,36a,37) to prevent downward movement of the riser (l0,27) at that side and to transfer at least the weight of the lower leg to the support (3,3′35). A system as claimed in claim 9 wherein the engagement means comprise a projection (20,20′,20′′) on the riser and an outwardly directed channel on the support, the channel being dimensioned to receive the riser and comprising a detent (21,33,36a,37) means at the second side portion of the support for engagement with the projection to prevent movement thereof in the downward direction lengthwise of the channel. A system as claimed in claim l0 wherein said detent means lies in a region subtending an angle of arc of 45° upwardly from the horizontal. A system as claimed in claim l0 wherein said channel and detent are provided by a carrier flexibly attached to said one side of said support. A system as claimed in claim l0 or claim ll wherein said channel and detent (36a) are provided by addition to an end portion (36) of an arcuate guide channel (23) extending over the support. A system as claimed in any one of claims l0-l3 wherein the upper leg is a slack catenary with a length such that at a design maximum wave height movement of the production vessel (2) the lowest point of the catenary remains above the seabed. A system as claimed in any one of claims l0-l4 wherein a free end of the riser at the seabed is attached to a sled (7a), and means (l6) are provided for attachment to the sled to pull it towards the equipment (l). A system as claimed in any one of claims l0-l5 wherein to form a connection between the production vessel (2) and an upper end of the riser pipe (l0,27) a connection system comprises an upper part (25) of a connector rigidly attached to the production vessel with a single guide funnel (47) adjacent to the connector part, and a single pull-in cable (45) extending from a winch, also mounted on the production vessel, and passing through the guide funnel (47); a lower part (24) of the connector attached to one end of the flexible riser pipe (l0,27) and having a single guide post (46) mounted with its axis offset from the pipe and lower connector part axis by a similar distance to that between the guide funnel and upper connector part axis, and the said post including means for co-axial attachment of the pull-in cable (45) such that the cable extends from the top of the post; and a lever arm (48) removably attached at one end near to the base of the guide post (46) by a pivot pin (49) whose axis is at right angles to the pipe/connector axis and spaced away from it by a similar distance to the guide post axis, and having means (50) at the other end for attaching a cable (46) extending from a second vessel (l3) or structure.

Inventors: SHOTBOLT, KEITH
Applicants: BECHTEL LIMITED
Application year: 1987
Main classification: E21B43
IPC codes: E21B43, F16L37, E21B17