GB2210334A - Floating production system and vessel for undersea oil well - Google Patents

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 may be made from steel boxes welded together or from reinforced concrete. 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. <IMAGE>

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 semi submersible 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 be@oming 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 maintain, and to have as much e@uipment as possible at or near the surface.

According to one pr@posal, 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.

An@t@er factor which in@rea@es the @@st of known syste@s of this type is that the vessels @sed 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-s@b@ersibles are of the 'catamaran' type, with a superstr@cture supported by four buoyant legs extending below the surface. The severe bending stresses on each of these legs mean that heavy reinforcement is needed at the upper ends of the legs.

This adds considerably to weight, end to the expense, of such structures and else tends to make them top heavy.

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 over211 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 aims to provide a floating production system wherein the amount of subses equipment is minimised and wherein relatively rigid pipes c-n be used not only for flow lines bt for risers and other rurposes. The invention also aims to provide a semi-sub- @rsible vessel of simplified construction which is cheaper to build than conventional vessels.

According to a first aspect of the invention there is provided a semi-submersible vessel for a floating oil production system comprising a superstructure supported by buoyant legs, the lower ends of the legs being connected to c-ne another to form a r.C.d underwater base struoture.

The preferred configuration has four such 'egs at the corners of a square or rectangle. The vessel i:ay be made up of buoyant steel boxes welded together to give a rigid buoyant structure. Alternatively, the vessel may be built from reinforced concrete.

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 fecilities. Wellheads mey be provided on the vessel or on the buoy.

Because the fixed buoy is, in its working position, anchored below the surface ,preferably about 50 metres below the surface), it is largely unaffected by waves and will only movie significantly in the heaviest weather.

Since its position relative to the wells remains fairly constant, it can be connected 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 mcve to some extent relative to the buoy because of its flexible connection to the buoy.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings: Figure 1 shows a perspective view of a floating production ystem 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 11 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-s@@mersible vessel of he invention; Figure 10 shows a semi-submersible vessel in accordance ith a further embodiment;; Figure 1 li is a schematic representation of an on- board electrical generating system for the vessel of Figure 9 cr Figure 10 and Figure 12 is a partial view of an undersea cable for the export of electricity from the vessel.

Referring first to Figure 1, a number of risers 12, preferably de from steel, connect a set of wellheads 10 on the bottom of the seabed to a submerged buoy 18.

On the se-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 respective wellheads 10.

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 2t 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 ana 111 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 thren 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 214.

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 construotion 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 th=n 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 te effected.Furthermore, the buoy 18 and the risers 12 tan 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 whet t required, the free ends being attached to marker buoys.

The steel risers 12 are tensioned to form a catenary between the buoy S 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 @as is conventionally carred 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.

alike 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, iprting 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 5t, a helicoptor landing pad 56 and winches 63.

The steel boxes fre 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 Fibre 10, there is shown a semisubmersible barge the upper part of which is similar in shape to that shown in :-ure 9 but which is built from reinforced concrete.

The barge has an uppe deck 70 which can be equipped in the same way as that 5 Figure 9, and the equipment need not therefore be described again. The upper deck is supported on four piers sr legs 71 which project upwardly from a subsea housing 73, also of reinforced concrete.

The housing 73 is largely hollow to provide the required buoyancy, and h---ses buoyancy tanks and oil or liquefied gas storage tanks 75. The latter can be reached via hatches 74 giving ac@ess through the piers 71. A delivery and pursing 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 Figures 9 and 1, may be secured together, for example by bolting or welding, to form larger platforms.

Figure 11 shows scheratically an on-board gas-fired generating system which ray be provided on the vessels of Figures 10 and 11. Gas cm the risers is fed via a riser balcony to a processing plant 81 which renders the gas suitable for combustion. The gas is then fed to a gas turbine 80 which drives Cr 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 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 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 facilities for a variety of liquefied gases such as natural gas methane) propane, butane etc. as well as crude oil, condensates, and gas/cilmixtures and indeed for all the components normally found in undersea oil and gas deposits as produced at the well head.

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 enx7isaged within the scope of the invention. For example, the legs of the platfcr may have any desired geometric shape such as square, rectangular, triangular, tapered or polygonal. 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 normally also be triangular in plan view.

Claims (13)

1) A semi-submersible vessel for a floating oil production system, comprising a superstructure supported by legs, the lower ends of the legs being connected together to for: a rigid buoyant structure.

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 in te form of a square.

4) A semi-submersible vessel according to claim 1 or claim 2 wherein the legs project upwardly from a rigid undersea housing.

5) A semi-submersible vessel according to claim 4 wherein the undersea housing contains Oil and/or gas storage tanks.

E) A semi-submersible vessel according to any preceding claim which is constructed from steel boxes butt-welded together.

7) ss semi-submersible vessel according to any one of claims 1 to 5 which is race rom reinforced concrete.

8) A semi-submersible vessel according to any precedin- claim which has wellheads provided thereon.

9) A floating oil production system wherein a fixed b@@y, normally anchored below the sea surface, is connected to at least one undersea well by at least one relatively rigid catenary riser and by at least one flexible riser to a semi-submersible vessel equipped with processing facilities.

10) An oil production system according to claim 9, wherein the rigid catenary risers curve tangentially down from the boy to the sea bed and then along the sea bed to respective wellheads.

11) ss method of installing a floating oil production system comprising the steps of: a) lowering at least one rigid catenary riser to the seabed and connecting one end thereof to a wellhead on the seabed; b) secured the other end of the catenary riser to a moored buoy on the surface; c) ccnnecting the upper end of the catenary riser on the buoy to a flexible riser which extends from the buoy to a semi-submersible production vessel and d) @wering the bucy with the riser connection thereon and anororing it at 2 depth at which it is substantially unaffected by wind, waves or tidal movements.

12) A semi-submersible vessel substantially as herein described with reference to Figure 9 or Figure 10 of the accompanying drawings.

13) A floating oil production system substantially as herein described with reference to Figures 1 to e, in conjun@tion with Figure 9 or Figure 10, of the accompanying 14) A rethod of installing a floating oil production system, substantially as herein desoribed with reference to Figures 4 to 8 of the accompanying drawings.