FLOATING PRODUCTION SYSTEM AND VESSEL FOR UNDERSEA OIL WELL

Abstract: A semi-submersible vessel (24) for a floating oil production system comprises a superstructure (62) supported by legs (60) which are joined by a base (58) at their lower ends to form a rigid structure. The vessel is made from steel boxes welded together and square or rectangular in horizontal cross-section. Oil processing facilities on the vessel are connected by flexible risers (22) to the upper ends of rigid or semirigid risers (12) secured to a buoy (18) held against movement at a depth of some 50 m. The vessel is thus free to move relative to the risers (12) which are held at a level such that they are substantially unaffected by wind, wave and tidal movements.

Description: FLOATING PRODUCTION SYSTEM AND VESSEL FOR UNDERSEA OIL WELL This invention relates to a simplified floating production system for an undersea oil well, and in particular to a system which seeks to minimise the amount of equipment on the sea bed. The invention also relates to a semisubmersible vessel for use with such a system. With falling oil prices, and with subsea oil reserves becoming increasingly difficult to extract, it has become important in recent years to minimise the cost of extraction in order to prevent fields from becoming unprofitable. In particular, attempts have been made in recent years to minimise the amount of equipment on the seabed, where it is expensive to install, inspect and main¬ tain, and to have as much equipment as possible at or near the surface. According to one proposal, pipes from the undersea wells extend straight up to a fixed platform on the surface, with the well heads on the main deck. A floating production vessel is moored alongside to reduce the size, and hence the cost, of the platform. Nonetheless, the cost of such a fixed platform is high. Another factor which increases the cost of known systems of this type is that the vessels used are based on mobile semi-submersibles and the codes developed for their design, construction and operation. Considerable savings could be obtained using a fixed floating structure. Conventional mobile semi-submersibles are of the 'catamaran' type, with a superstructure supported by four buoyant legs extending below the surface. The severe bending stresses on each o these legs mean that heavy reinforcement is needed at the upper ends of the legs. This adds considerably to weight, and to the expense, of such structures and also tends to make them top heavy. GB-A-2159468 discloses a vessel in which four legs supporting the platform are connected together at their lower ends by elongate cylindrical box structures. It has however always been assumed that the legs must be of rounded cross-section in view of the action of waves and currents on the structure. In GB-A-2159468, the legs and the elongate bottom connecting structures are of cylindrical cross-section which means that complicated connecting sections are required at the bottom corners of the vessel if the required rigidity is to be obtained. The present inventors have now found that a simple structure having excellent hydrodynamic properties can be obtained by building a vessel wherein the legs are of square or rectangular cross section. Accordingly, the present invention provides a semi-submersible vessel for a floating oil production system, comprising the superstructure supported by legs, the lower end of the legs being connected together to form a rigid buoyant structure, character¬ ised in that the legs are square or rectangbular in horizontal cross-section and in that the vessel is constructed from boxes butt-welded together. Preferably the bottom connections between the legs are also of square or rectangular cross-section, so that a simple structure can be built up entirely of square or rectangular steel boxes with butt-welded connections between them and forming continuous straight walls. Known floating production systems use flexible hoses of composite construction to bring oil from the seabed wells as well as for export, for choke and kill lines, water injection and gaslift. The flexibility is required because even a "fixed" floating structure on the surface is subject to considerable movement because of wind, waves and tides. Such hoses are however expensive and greatly increase the overall cost of the installation. The flexible hoses are laid under sufficient tension to ensure that the minimum radius in the catenary is exceeded by a safe margin to avoid buckling of the hose and consequent blockage. The present invention also aims -to provide a floating production system wherein the amount of subsea equipment is minimised and wherein relatively rigid pipes can be " used not only for flow lines but for risers and other purposes. According to a further aspect of the invention there is provided a floating oil production system wherein a fixed buoy, normally anchored below the sea surface, is connected to the well or wells by one or more relatively rigid catenary risers and to a semi-submersible vessel by one or more flexible risers. The semi-submersible vessel is equipped with process facilities. Wellheads may be provided on the vessel or on the buoy. Because the fixed buoy is, in its working postion, anchor¬ ed below the surface (preferably about 50 metres below the surface) , it is largely unaffected by waves and will only move significantly in the heaviest weather. Since its position relative to the wells remains fairly constant, it can be conn¬ ected to these wells with pipes made from a relatively rigid and cheap material such as steel. On the other hand, the semi- submersible vessel is free to move to some extent relative to the buoy because of its flexible connection to the buoy. A preferred embodiment of the invention will now be des¬ cribed with reference to the accompanying drawings:- Figure 1 shows a perspective view of a floating production system in accordance with the present invention; Figure 2 shows a lateral elevation of the system shown in Figure 1 ; Figure 3 is a lateral elevation in the direction of the arrow III in Figure 2; Figures 4 to 8 show the stages of installation of the system of Figures 1 to 3; Figure 9-shows the structure of one embodiment of the semi- submersible vessel of the invention; Figure 10 shows a semi-submersible vessel in accordance with a further embodiment; Figure 11 is a schematic representation of an on-board electrical generating system for the vessel of Figure 9 or Figure 10; Figure 12 is a partial view of an undersea cable for the export of electricity from the vessel; Figure 13 shows diagrammatically how part of the vessel of Figure 9 is assembled and Figure 14 shows in cross-section the configuration of the boxes making up the leg in Figure 13. Referring first to Figure 1, a number of risers 12, pref¬ erably made from steel, connect a set of wellheads 10 on the bottom of the seabed to a submerged buoy 18. On the sea¬ bed, the wellheads 10 each comprise simply a master valve and an annulus valve. The risers 12 curve tangentially from the buoy 18 down to the seabed and then along the seabed to the The buoy 18 comprises a number of buoyancy tanks 26 and is anchored to the seabed by four anchor chains 20 which hold it against any substantial movement. The tanks can be filled or emptied to adjust the depth of the buoy. Also connected to the buoy 18 is a steel pipe 14 which also curves tangentially down to the seabed to convey oil or gas to a pipeline 16 leading to the shore. From the buoy 18, crude from the risers 12 passes to a semi-submersible barge 24 through flexible hoses 22. The hoses 22 are connected to the risers 12 by respective valves 28 on the buoy. These hoses are connected to inlets on the barge 24 and convey the crude to the various process facilities on the barge. The oil and gas separated from the crude are exported to the shore, for example through the pipelines 14 and 16. Not all the catenary steel pipes 12 are used for conveying oil or gas. Some- of them will be used for choke and kill lines. The installation of the system is illustrated in Figures 4 to 8. In Figure 4, a steel riser 12 is conveyed to the • site and laid in position by a pipelaying barge 34, anchored adjacent the buoy 18 which at this stage is on the surface. The upper end of the riser is then transferred to a crane 32 and lifted into position on the buoy 18 as shown in Figure 5. The upper end of the riser is secured to the buoy. A flexible riser 22, with one end connected to the crane 32, is then secured by that end to the buoy 18. Its other end (the lower end as shown in Figure 6) is connected by a cable 36 to the barge 24. This end is then winched by a derrick 30 to be connected up at 31 to the required processing facilities on the barge. As shown in Figure 7, the adjacent ends of the flexible riser 26 and the steel riser 12 are then connected on the buoy 18. Finally, as shown in Figure 8, the buoy 18 is lowered to its final position, suitably 50 metres below the surface. At that depth, the buoy is not subject to any substantial movement resulting from wind, waves or tide and the risers 12 are therefore held steady relative to the well heads at their lower ends. On the other hand, the barge 24 is free to move substantially relative to the buoy without breaking or blocking the connection through the flexible riser 26. The flexible risers 26 can be of a conventional composite construction made from polytetrafluoreothylene stainless steel etc., of the type used hitherto for the flow lines, choke and kill lines etc., connecting the surface installation to the seabed. Such hoses are much more expensive than the relatively rigid steel risers, so that by using steel risers to cover the greater part of the distance between the surface installation and the well heads on the seabed, and using the flexible risers for only " a small proportion of that distance, considerable cost savings can be effected. Furthermore, the buoy 18 and the risers 12 can be left in position in virtually any weather conditions and can be easily attached to, and detached from, the barge 24 through the flexible riser 26. The latter can be detached from the barge and left to hand from the buoy when not required, the free ends being attached to marker buoys. The steel risers 12 are tensioned to form a catenary between the buoy 18 and the seabed. At the seabed the pipes (and control cables) which descend tangentially to the seabed, are routed to an anchor point and thereafter lift up off the seabed before reaching the respective well heads, passing over an arch in each case to enter the well vertically. Maintenance of the wells is generally performed from the barge 24. For a minor workover tools can be introduced via a swab valve on the barge las is conventionally carried out on shore) . The tools pass down the line to the seabed and " into the wells. An alternative possibility, for a major workover, is to moor the barge above the field, on the side of the buoy 18 opposite to that on which the barge is shown in Figure 1. The vessel can then be manoeuvred into a position on each of the well centre lines and can use a work over rig for pulling tubing. Figure 9 illustrates in more detail the semi-submersible barge shown schematically in Figure 1. The barge is constructed from a large number of hollow boxes 50, made from mild steel or the like. These are simply butt-welded together to form the desired structure. Typically, two to three hundred of these boxes will be used to construct the vessel. Unlike a conventional semi-submersible platform, the lower ends of whose legs are free, the barge according to the invention has a square base 58 from which the four upright legs 60 project upwards to support the deck 62. As can be seen in Figure 9, the legs 60 each have a cross section made up of four boxes welded together, imparting considerable strength and rigidity. The base 58 is made up of pairs of boxes welded together side by side and the deck is constructed from a single layer of boxes. Becuase the legs of the vessel are connected together at their lower ends, they are less subject to bending movements caused by wind, waves and tides. The stresses on the vessel are more evenly spread throughout it, so that the heavy reinforcement at the tops of the legs below the deck, which would normally be necessary in a conventional vessel, can be dispensed with. This considerably reduces the overall cost of the vessel and also lowers its centre of gravity so that it rides in the water with increased stability. Also, the deck can carry a greater load. Additional bow and stern pontoon sections 53,57 may be added to the base to enable the vessel to be more easily towed from one site to another. The deck is provided with conventional equipment for a semi-submersible vessel of this type, including a deck crane 52, accommodation 54, a helicoptor landing pad 56 and winches 63. The steel boxes from which the vessel is constructed will generally be from 5 to 10 metres along each edge and will typically be 7.5 metre cubes. Some of the boxes may be in the form of buoyancy tanks or oil storage tanks. As well as being relatively cheap, this vessel according to the invention is easy to construct and launch. The financial savings involved will help to render commercially viable some undersea oil fields which have hitherto been regarded as marginal. Referring now to Figure 10, there is shown a semi- submersible barge the upper part of which is similar in shape to that shown in Figure 9 and which has an upper deck 70 which can be equipped in the same way as that of Figure 9. The equipment need not therefore be described again. The upper deck is supported on four piers or legs 71 which project upwardly from a subsea housing 73. The housing 73 is largely hollow to provide the required buoyancy, and houses buoyancy tanks and oil or liquefied gas storage tanks 75. The latter can be reached via hatches 74 giving access throuh the piers 71. A delivery and pump¬ ing system 76 is also provided at the bottom ends of the tanks. It is further envisaged in accordance with the invention that two or more semi-submersible barges, such as those shown in Figure 9 and 10, may be secured together, for example by bolting or welding, to form larger platforms. Figure 11 shows schematically an on—board gas-fired generating system which may be provided on the vessels of Figures 10 and 11. Gas from the risers is fed via a riser balcony toa processing plant 81 which renders the gas suitable for combustion. The gas is then fed to a gas turbine 80 which drives an electrical generator 86 via a reduction gear system 84. The generator ' 86 supplies to main power board 88 of the vessel which supplies the power requirements of the vessel such as heating, lighting, pumps etc. Power can also be exported to the shore via a submarine cable such as that 5 illustrated in part in Figure 12. This comprises an insulated cable 98 with a protective sheath 99 of helically wound wire made from, or coated with, a suitably non-corroding alloy. The plant may optionally also include an auxiliary 10 generating system 90 comprising a waste heat boiler 92 which uses heat from the gas turbine exhaust to generate steam to drive a steam turbine 94 and hence a generator 96 which also supplies power to the main board. The semi-submersible vessels of the invention may include storage 15 facilities for a variety of liquefied gases such as natural gas (methane), propane, butane etc. as well as crude oil, condensates, and gas/oil mixtures and indeed for all the components, normally found in undersea oil and gas deposits as produced at the well head. 20 The vessel may incorporate facilities for processing crude oil as well as gas, and the generating plant described above could indeed be oil-fired instead of using a gas turbine. Further embodiments are also envisaged within the scope ~^-~ of the invention. Instead of being square, the platform could for example be triangular and supported by three legs rather than four. In this case, the base will norm¬ ally also be triangular in plan view. The shape of the boxes can also be adapted so that simple butt welds will 0 still give continuous walls without the need for complicated connecting sections. Referring now to Figure 13, one of four legs 60 support¬ ing the platform 62 is made up of steel boxes 50 welded together in layers of four. The platform 62 and base 104 are also square so that simple abutting connections can be made between the leg, the platform and the base to form continuous flat structural surfaces without the need for complicated connecting structures as in the cylindrical- columned vessels of the prior art. As shown in Figure 14, each of the boxes 15 has one open vertical side the edges of which abut, and are welded to a vertical side of an adjacent box. There is thus prod¬ uced a simple structure having high rigidity, and in part¬ icular having a strong central connection up the middle of the leg. The base 104 is made up of rectangular boxes 102, butt- welded together in much the same way as the boxes 50 making up the leg. The boxes may have elongate reinforcing members such as 100,101 on their inner sides. These reinforcing members may support inner walls, imparting further structural rigidity. The insides of the boxes may define separate compart¬ ments or continuous shafts or passages, depending on whether the boxes have one, two or three open sides. These shafts, passages and compartments can be used for access, ducting, pipes, cables, accommodation or storage as desired.

Claims: CLAIMS 1. A semi-submersible vessel (24) for a floating oil production system, comprising a superstructure (62) sup¬ ported by legs (60), the lower ends of the legs being con¬ nected together to form a rigid buoyant structure, charac- terised in that the legs (60) are square or rectangular in horizontal cross-section and in that the " " vessel is con¬ structed from boxes (50) butt-welded together. 2. A semi-submersible vessel according to claim 1 which has four said legs at the corners of a square or rectangle. 3. A semi-submersible vessel according to claim 2 wherein the lower ends of the legs are connected by a base (58) in the form of a square. 4. A semi-submersible vessel according to any preceding claim wherein the lower ends of the legs (60) are connected together by elongate connecting structure (58) of square or rectangular cross-section. 5. A semi-submersible vessel according to claim 1 or claim 2 wherein the legs (71) project upwardly from a rigid under¬ sea housing (73). 6. A semi-submersible vessel according to claim 5 wherein the undersea housing contains oil and/or gas storage tanks. 7. A semi ^ submersible vessel according to any preceding . claim which has wellheads provided thereon. 8. A floating oil production system wherein a fixed buoy (18), normally anchored below the sea surface, is connected to at least- one undersea well by at least one relatively rigid catenary riser (12) and by at least one flexible riser (22) to a semi-submersible vessel (24) equipped with processing facilities. 9. An oil production system according to claim 8 wherein the rigid catenary risers (12) curve tangentially down from the buoy to the seabed: 1 , and then along the seabed to respective wellheads (10). 10. A method of installing a floating oil production system comprising ' the steps of:- a) lowering at least one rigid catenary riser (12) to the seabed and connecting one end thereof to a wellhead (10) on the seabed; b) secured the other end of the catenary riser (12) to a moored buoy (18) on the surface; c) connecting the upper end of the catenary riser (12) on the buoy to a flexible riser (22) which extends from the buoy to a semi-submersible production vessel (24) and d) lowering the buoy (18) with the riser connection thereon and anchoring it at a depth at which it is substantially unaffected by wind, waves or tidal movements.

Inventors: LANG ALEXANDER CRAIG, RYAN BARRY RICHARD, LANG, ALEXANDER, CRAIG, RYAN, BARRY, RICHARD
Applicants: SEAWAYS ENG UK, SEAWAYS ENGINEERING (U.K.) LIMITED
Application year: 1988
Main classification: B63B35
IPC codes: B63B9, B63B35, E02B17, E21B43