US3535883A - Apparatus for transporting fluids between a submerged storage tank and a floating terminal - Google Patents

Description

3,535,883 MERGED 2 Sheets-Sheet 1 RTING FLUIDS BETWEEN A SUB STORAGE TANK AND A FLOATING TERMINAL W. F. MANNING APPARATUS FOR TRANSPO Oct. 27, 1970 Filed Oct. 25. 1966 INVENTOR WILLIAM F. MANNING I BY ATTO RNEY 3,535,883 MERGED Oct. 27, 1970 w. F. MANNING APPARATUS FOR TRANSPORTING FLUIDS BETWEEN A SUB STORAGE TANK AND A FLOATING TERMINAL 2 Sheets-Sheet 2 Filed Oct. 25, 1966 INVENTOR WILLIAM F. MANNING za/nfi PM "'"U WI.

ATTORNEY United States Patent APPARATUS FOR TRANSPORTIN'G FLUIDS BE- TWEEN A SUBMERGED STORAGE TANK AND A FLOATING TERMINAL William F. Manning, Dallas, Tex., assignor to Mobil Oil Corporation, a corporation of New York Filed Oct. 25, 1966, Ser. No. 589,434 Int. Cl. B631) 35/00 U.S. Cl. 6146 6 Claims ABSTRACT OF THE DISCLOSURE The specification discloses offshore apparatus for storing material below the surface of a body of water and for transporting the material from the storage area to a point adjacent the surface. Structurally the apparatus comprises an open-bottom storage tank anchored in a submerged position. A rigid-walled tether pipe is attached to the tank and extends substantially vertically upward therefrom. Buoyant means is concentrically mounted on the upper end of the tether pipe to maintain it in a substantially vertical position. Material is transferred through the tether pipe between the tank and a point adjacent the surface.

The present invention relates to an improved offshore structure for the mooring and loading of vessels, particularly with crude oil and/ or liquefied gas accumulated in a submerged storage tank. More particularly, the invention is concerned with the surface and near-surface apparatus of a subsea producing system, a floating offshore terminal, commonly called a single point mooring system, wherein a vessel, such as a tanker, may be moored in the most advantageous direction, dictated by the wind and waves, during loading.

Present developments in the offshore oil and gas industry indicate that drilling and production efltorts will be extended to underwater areas, such as the outer fringes of the continental shelves and the continental slopes (defined as areas where the water depth is over 600 feet and less than 6,000 feet). At such depths, particularly where the subaqueous wellheads can be widely spaced, submarine production systems are presently considered the most practical method of producing hydrocarbons trapped in the subaqueous formations. Submerged storage facilities, able to accommodate at least ten days supply of accumulated fluid hydrocarbons, are an integral part of the system. With the wellheads, production satellites, gathering systems, and production storage all being supported on the ocean bottom, perhaps 600 feet or more below the surface, some means is necessary for transporting the accumulated hydrocarbons from these depths to a tanker which periodically visits the area. A single point mooring system is considered the most practical means for accomplishing this result. Such a system generally consists of a surface unit, means for anchoring the surface unit and at least one fluid passage between the surface unit and a submerged storage facility. The surface unit is provided with a central floating terminal and a loading and mooring means connected between the central floating terminal and the tanker. The loading and mooring means is designed to pivot around the terminal so that the tanker may be moored in the most advantageous direction at the time, which would usually be with the bow of the tanker pointed into the wind or waves.

A single point mooring system is also practical in shallower areas Where good harbors are not available or where the harbors are so extremely crowded that there is a considerable advantage in being able to load and unload offshore. Furthermore, tankers utilizing offshore loading facilities need not wait for a high tide to enter a harbor, which is the case in many areas of the world. In this in- 3,535,883 Patented Oct. 27, 1970 ice stance the single point mooring system could be connected to either onshore or offshore storage facilities.

Accordingly, it is an aspect of the present invention to provide a stable offshore floating terminal, connected to a point adjacent the ocean bottom, for mooring a tanker thereto and transporting fluids between bottom storage facilities and the tanker.

It is another aspect of the present invention to provide an improved means for connecting the floating terminal and storage facilities located at the marine bottom.

It is a further aspect of the present invention to provide a novel boom or support means extending part way between the floating terminal and the tanker to facilitate the tanker being moored at a safe distance from the terminal while being in fluid communication therewith.

Other aspects and advantages of the present invention will be readily apparent from the following description, when taken in conjunction with the accompanying drawings that illustrate useful embodiments in accordance with this invention, in which:

FIG. 1 is a schematic elevational view of the novel single point mooring system of the present invention;

FIG. 2 is a partial bottom plan view taken along line 22 of FIG. 1;

FIG. 3 is an enlarged partial sectional view of the loading boom of the present invention, as shown in FIG. 2, illustrating the structure for transporting fluids from the central floating terminal to the tanker through the loading boom;

FIG. 4 is a schematic elevational view of a second embodiment of the present invention; and

FIG. 5 is a top plan view of the second embodiment of the present invention.

Now looking at FIGS. 1 and 2, the single point mooring system of the present invention consists in part of a substantially vertical rigid-walled tether pipe 10 bridging the distance between a submerged facility comprising an open bottom storage tank 12 and a surface unit comprising a floating terminal 13. The tether pipe 10 is connected to the storage tank 12 by a collet connector 14- fastened to a short upstanding stub pipe 15 extending from the upper end of the storage tank 12 which is fixed in the formations underlying the marine bottom 16 by a plurality of piles 17. A universal or double pin joint 18, in the tether pipe 10 just above the collet connector 14, allows the upper end of the tether pipe 10 to be displaced from directly above the collet connector 14 without allowing rotation. In very deep water, the universal or double pin joint 18 can be dispensed with, and the inherent flexibility of the pipe can be taken advantage of, the flexibility of the tether pipe being proportional to its length. A buoyancy tank 19 for axially supporting the tether pipe 10 is fixed concentrically thereon near the upper end just below an upper collect connector 20 joining the upper terminus of the tether pipe 10, below the surface 28 of a body of water 30, to the apex of an A-frame forming a submerged tether arm 22 of the floating terminal 13. A concentric swivel joint 24 and a knee-action pin joint 26, in series, just above the collet connector 20, at the apex of the tether arm 22, allow the main portion of the tether arm 22 to be parallel substantially to the surface 28 of the body of water 30 while permitting the tetherarm 22 to rotate around and pivot about the center line of the tether pipe 10. At the inner end of the tether arm 22, a pair of legs 32 and 34 of the A-frame are joined by collet connectors 44 to conduits 36 and 38 depending vertically from the centers of parallel submerged supporting floats 40 and 42, respectively, of the floating terminal 13. The floating terminal 13 is a wave transparent structure consisting of an above surface platform 47 supported on a plurality of buoyant vertical sup- 3 port columns 48-54 which are connected in pairs by the submerged floats and 42. Pin joints and 46 in the legs 32 and 34, respectively, of the tether arm 22, just below the collet connectors 44 provide a knee-action movement between the tether arm 22 and the floating terminal 13.

The arrangement of the components above the swivel 24 allows all of the forced motions of the floating terminal 13. The pin joints 26, and 45 and 46 at either end of the tether arm 22, permit heave of the floating terminal 13 and also pitch in the plane of the tether arm 22. The swivel 24 at the outer end of the tether arm 22 and the universal joint 18 at the lower end of the tether pipe 10 permit the free movement of the floating terminal 13 around the tether pipe 10 and preclude a build-up of forces perpendicular to the plane of the drawing.

A loading boom, generally designated 56, is pivotally fastened to the lower end of the terminal 13 by means of knee-action pin joints 58. The boom 56 consists of a bent A-frame having a triangulated framework of support braces. The A-frame has an oil transporting leg 60 and a loading hose storage leg 62 with a flotation tank 64 fixed to the boom 56 at the apex of the legs 60 and 62. In the loading position of the boom 56, as shown in solid lines in FIG. 1, the outer ends of the legs :60 and 62 of the bent A-frame and the upper end of the flotation tank 64, having an upstanding mooring bollard 66 fixed thereto, are above the surface 28 of the body of water 30 whereby a tanker 67, being loaded, has its mooring line 68 fastened to the bollard 66. A buoyant loading hose 72 shown extending from the boom 56 to the tanker 67 is slidably connected in the outer end of the loading hose storage leg 62. The repositioning of the boom 56, from the storage position shown in phantom at 56 in FIG. 1, to the working position as shown in solid lines, is controlled by cable take-up means such as a winch 74, mounted on the deck 47 of the terminal 13, which has a cable 76 connected to the boom 56 outwardly of the pin joints 58 thereof. While the winch 74 is shown schematically as a manual device in FIG. 1 (and in FIG. 4), in actuality, the winch 74 would be driven from a power source on the deck 47.

FIG. 3 shows a partial sectional view of FIG. 2 illustrating the configuration of the boom 56 which permits the hollow legs .60 and 62 to double as fluid transportation and loading hose storage conduits, respectively. The loading hose 72, contained within the leg 62, has a rigid terminal pipe section 80, aflixed to the inner end thereof, adapted to slide in the hollow interior of the leg 62. The inner end of the pipe section 80 is closed oif by a blind flange 82,

of slightly larger diameter than the pipe section 80, permitting the flange 82 to abut a stop ring 84 fixed within the hollow conduit of the loading hose storage leg 62. The pipe section 80 also has a plurality of fluid entry ports 85 ringing the central portion thereof. The blind flange 82 fits in the hollow conduit of the leg 62 to permit fluid to bypass the blind flange 82 when storing the leg 62 as will be described in detail subsequently. A pair of circumferential expandable pneumatic gripper-seal arrangements 86 are affixed within the hollow conduit of the leg 62, one spaced at either end of the terminal pipe section 80 when the hose 72 is in the extended position shown. Supplying air under pressure to the gripper seals 86 is a rigid pressure line 88, fastened along the leg 62. The line 88 may be connected at its inner end to a flexible line (not shown) extending to the deck 47 of the floating terminal 13 where a compressor (not shown) is located. A retracting cable 90 extends axially through the hollow leg 62 and is connected at its outer end to a central ear 92 on the inner face of the blind flange 82.

The fluid transportation leg 60 has a drain valve 61 at the inner end thereof, providing selective fluid communication between the interior of said leg 60 and the body of water. For permitting fluid to be transferred from the outer end of the fluid transportation leg 60 to the outer end of the storage leg 62, a port 94 in the outer end of the fluid transportation leg 60 is connected to a similar port 96 in the storage leg 62 by a crossover pipe 98, the fluid transportation leg 60 having a shaped internal wall section 91 for directing the fluid smoothly into the port 94. A valve in the crossover pipe 98 is controlled from the valve actuator 70, shown in FIG. 1 as being accessible from the upper end of the flotation tank 64. In FIG. 3 it can be seen that with the 'hose 72 extended, the fluid entry ports 85 of the pipe section 80 are adjacent the port 96 in the leg 62. Compressed air, when applied to the gripper seals 86, expands them, forming a fluidtight connection between the hose 72 and the section of the conduit of the leg 62 between the gripper seals 86, and furthermore grips the pipe section 80 to prevent it from being retracted during loading. Fluid flows through the crossover pipe 98, if the valve 100 is open, and into the loading hose 72 and outwardly through the hose 72 to a mating connection with the tanker 67 moored to the bollard on the flotation tank 64 of the boom 56 by means of the line 68.

When the tanker 67 approaches the mooring and loading system with the intention of receiving the stored fluid therefrom, it is positioned in the most advantageous direction for mooring with respect to the tether pipe 10; The boom 56 is then lowered from the position shown in phantom at 56', in which it is stored, to the partially submerged position illustrated in solid lines, in which it is utilized for transferring the fluids to, and mooring, the tanker 67. A mooring line 68 of the tanker 67 is looped over the bollard 66 on the flotation tank 64 by personnel stationed on the deck of the flotation tank 64. The upper portion of the single point mooring system including the tether arm 22, the floating terminal 13, and the loading boom 56, should be rotated around the tether pipe 10 (by means of the swivel connection 24) into an approximate position in a line between the tanker 67 and upper end of the tether pipe 10. This may be done while the tanker 67 is still operating under its own power by drawing the mooring line 68 taut to bring the floating terminal 13 around. If some motive means, mounted directly on the floating terminal 13 is desirable, propulsion motors (not shown) may be fixed on the floating terminal 13 beneath the surface 28 for positioning the floating terminal 13 into a vertical plane between the tanker 67 and the tether pipe 10. When the tether pipe 10, the terminal 13, and the tanker 67 are substantially lined up, within 20 or 30 for example, the action of the wind and/or ocean currents will tend to bring the floating structures into close alignment and hold the tanker 67 away from the terminal 13 within the limits of the mooring line 68. A second line (not shown) from the tanker 67 would be attached to a blind flange (not shown) on the outer end of the loading hose 72 for withdrawing the loading hose 72 from storage in the leg 62 of the boom 56. The various means for retrieving the outer end of the loading hose 72 from storage in the leg 62 of the boom 56. The this invention and will not be specifically described. Such discussions may be found in the Ault US. Pat. No. 2,701,375, issued Feb. 8, 1955, and the Eiji Suzuki U.S. Pat. No. 3,204,658, issued Sept. 7, 1965.

When the present mooring and loading system is to be utilized in conjunction with a subaqueous oil field, a long distance from shore, the fluid to be delivered to the tanker 67 is conducted to the storage and loading facilities by a number of subsea pipelines 102 (one shown), each connected with a plurality of subsea wellheads by a subsea gathering station (not shown). The tether pipe 10 provides fluid communication between the pipeline 102 and the floating terminal 13. For purposes of this illustration, the tether pipe 10 is constructed as a double-Walled conduit, forming concentric flow passages, with the subsea pipelines 102 being connected to the annulus of the tether pipe 10, above the universal joint 18, through a flexible section 104. The fluid is forced up the tether pipe and through the swivel 24, which also has concentric passages therethrough. The fluid leaves the swivel 24 through the annulus of a short section of concentric piping 106 and is directed around the pin joint 26 and into a hollow conduit forming a leg 34 of the tether arm 22 by flexible tubing 109 directing the fluid around the pin joint the deck 47 of the floating terminal 13 through at least one of the hollow support columns 52 and 54 by way of flexible tubing 109 directing the fluid around the pin joint 46, into the depending pipe section 38, and from there through the hollow buoyant float 42 and the interconnected hollow buoyant support columns. The fluid, delivered to the deck 47 of the floating terminal 13, is processed before being pumped back down into the storage tank 12. If the system is designed for the production of oil, the fluid is directed through separation equipment (not shown) on the deck 47, the oil being separated from any included gas and the pressure being reduced to atmospheric. The resulting gas can then be flared, or, if it is economical, it can be liquefied and stored.

A path for fluid communication is provided from the deck 47 of the floating terminal to a storage area within the storage tank 12. The separated oil is conducted down beneath the sea through the hollow buoyant column 50', a portion of the submerged hollow float 40 (the left-hand end of the float 40 and the hollow buoyant column 48 being blocked by a plate 111 welded across the interior of the float 40), the depending pipe section 36, and the leg 32 of the tether arm 22, bypassing the pin joints 45 and 26 by flexible tubing sections 110 and 112, respectively. The oil is directed through the central passages of the pipe section 106, swivel 24, and the tether pipe 10. The flowing oil bypass the universal joint 18 by a flexible line 114 connecting the central passage within the tether pipe 10 with storage area within the storage tank 12 through the interior of the upstanding stub pipe 15. Since the tank 12 is open on the bottom any suspended solids will settle out, as will included water. It is well within the skill of the art to design pin joints, for use with this system, which can carry the fluid internally. If such joints are used, the flexible tubing, illustrated and described as bridging the joints shown, can be eliminated.

When the processed oil is to be loaded onto a waiting tanker, a three-way, two-position valve 116 in the lower end of the column 50 is positioned to block the flow of oil from the column 50 into the float 40 and instead connects the interior of the hollow float 40 with the interior of the fluid transportation leg 60 of the boom 56 through a section of flexible tubing section 118. The oil then is delivered from the storage tank 12 to the tanker 67 through the central passage of the tether pipe 10 and is directed through the leg 32 of the tether arm 22, the float 40, and the leg 60 of the boom 56. During the loading of the tanker 67 the separation of the oil can continue, the oil being stored in the remaining buoyant columns and the walled-off portion of submerged float 40, as well as any portion of the float 42, not being already used as fluid conduits. All of the support columns 48-54 and the floats 40 and 42, comprising the buoyant structure of the floating terminal 13, can be used for fluid storage if the preprocessed oil from the subsea wells is conducted from the tether arm 22 to the deck 47 through closed conduits, either within the support columns and floats or along the side of the structure, rather than flowing freely through the hollow structure of the terminal 13.

When the loading hose 72 is connected to the tanker 6'7 and personnel located on the upper end of the flotation tank 64 open the valve 100 through the actuator 70 to permit the oil to flow through the loading hose 72 into the tanker 67, the diflerential head of the sea water outside and the oil inside the open bottom of the storage tank 12 provides an adequate supply of oil on the suction side of pumps (not shown) mounted in the floating terminal 13 for water depths greater than about 450 feet, the pumps on the terminal 13 providing high pressure for quickly loading the tanker 67. For depths of less than approximately 450 feet, subsurface pumps will be included in the submerged portion of the floating terminal 13. Pumps that may be used for transferring the oil from the bottom storage tank 12 to the terminal 13 are illustrated in more detail in the British Pat. No. 1,023,085, published Mar. 16, 1966.

Since the boom 56 is to be raised above the surface 28 of the body of water 30 for the storage thereof, the residual oil should be drained to reduce the handling weight of the boom 56 and therefore reduce the loads thereon. Prior to draining, at the completion of the loading operation, the oil is flushed by pumping water through the boom 56 and the loading hose 72. As the pumped water reaches the loading hose connection at the tanker, the connection is broken and a blind flange is installed on the outer end of the hose 72. The gripper seals 86 are released and the loading hose 72 is then withdrawn into the storage leg 62 of the 'boom 56 by the line 90 shown running through the interior of the storage conduit in the leg 62. A submersible electric motor and windup reel (not shown) may be mounted in the inner end of the leg 62 or the cable 90 may run up through the boom 56 and one of the buoyant columns, with the windup reel mounted on the deck 47. After the hose 72 is stored, the drain valve 61 is opened, the mooring line 68 is cast off, and the boom 56 is withdrawn up out of the Water into the phantom line position shown at 56 by winding up the cable 76 onto the drum of the winch 74. The water in the fluid transportation leg 60 is drained through the open valve 61 at the inner end of the leg 60 while the water in the loading hose storage leg 62 drains out the open inner end of that conduit, the water in the stored loading hose 72 draining out through the fluid entry ports in the pipe section 80, past the loosely fitting terminal flange 82, and out the open inner end of the leg 62. The drainage is made easier by having a one-way valve in the blind flange, attached to the outer end of the loading hose 72, which will permit air to enter the outer end of the leg 62, and the leg 60 through the crossover pipe 98.

A similarly functioning modified structure is shown in FIGS. 4 and 5. The lower portion, beneath the tether arm 22, is identical to that shown in FIGS. 1 through 3. The tether arm 22 is connected to the tether pipe in the same manner as shown in FIGS. 13. The other end of the tether arm 22', however, is connected to the floating terminal 13' by means of knee-action pin joints 45 and 46' at approximately the surface 28 of the body of water 30. The boom 56" is also similar in structure to that of the previous embodiment except that it is also designed to be hinged at the surface 28 of the body of water 30 by pin joints 58'. The boom 56" is, in this embodiment, not complicated by the oil conduits therethrough having to transport the oil from a point beneath the surface 28 to the inner end of the loading hose 72' above the surface 28. Therefore, the A-frame is in a single plane. The procedure for transporting fluid from the modified structure to a tanker 67 is similar to that described in conjunction with the previously discussed embodiment. The main advantages of the modified unit of FIGS. 4 and 5 reside in the location of the pin joints 45', 46, and 58' at the surface 28 which provides ease of maintenance and reduces the heeling tendency of the floating terminal 13' because the reaction provided by the horizontal component of the forces on the arm 22' is nearer the same elevation as the horizontal forces imposed by wind and current.

The wind and wave forces acting on the components of the surface unit, in each of the embodiments of the novel single point mooring system herein disclosed, such as the floating terminal 13 (13), the flotation tank 64 when it is in the loading position, as well as the tanker 67 when moored thereto, will cause the upper portion of the system to be laterally displaced, and in so doing will pivot the tether pipe about the universal joint 18, out of the vertical position shown in FIG. 1. The movement of the tether pipe 10 away from its vertical position is opposed by the upwardly directed restorative force resulting from the submerged buoyancy tank 19 concentrically fixed to the pipe 10. The magnitude of the restorative force will be determined by the dimensions of the tank 19 and its axial location on the tether pipe 10, the closer to the top of the pipe, the greater the resultant couple acting to right the tether pipe 10.

The single point mooring system of the present invention could serve other functions in conjunction with a subsea production system. Electrical generators, and the prime power source therefor, could be located on the deck 47 of the terminal 13 for supplying necessary power to the pumps and controls of the storage tank 12 and the bottom-mounted satellite gathering system and therethrough to the subsea wellheads. The tether pipe 10 would support the interconnecting power lines as well as electrical readout and command lines for the subsea system. These various lines would be supported along the outside of the tether pipe 10, bridging the swivel 24 by flexible lines, if the tether pipe 10 is not allowed to rotate more than one revolution, or rotatable electrical connections may also be formed within the swivel 24.

What is claimed is:

1. A subsea hydrocarbon producing system comprising: a storage tank anchored in formations underlying the marine bottom beneath a body of water at an offshore site; a substantially rigid-walled tether pipe extending substantially vertically from said storage tank to a point at least adjacent the surface of said body of water for providing fluid communication between a storage area within said storage tank and a point at least adjacent said surface of said body of water; an upstanding stub pipe extending from and rigidly fixed in the upper end of said storage tank; a universal joint between the lower end of said tether pipe and said stub pipe; flexible hose means bridging at least said universal joint to connect the interior of said tether pipe with the interior of said stub pipe which in turn is in fluid communication with said storage area within said storage tank; and buoyant means fixed to said tether pipe at least adjacent the upper end thereof and beneath said surface of said body of water at least partially supporting said tether pipe in a substantially vertical position in said body of water and opposing movement of said tether pipe away from a vertical position thereof.

2. A subsea hydrocarbon producing system as recited Cir in claim 1 wherein there are at least two fluid passages extending the length of said tether pipe, said flexible hose means connecting a first of said fluid passages within said tether pipe with said interior of said stub pipe and a pipeline for connecting the second of said fluid passages within said tether pipe with a subsea gathering system collecting the produced hydrocarbons from a plurality of subsea wells.

3. A subsea hydrocarbon producing system as recited in claim 2 wherein said tether pipe comprises concentric conduits.

4. A subsea hydrocarbon producing system as recited in claim 1 wherein said tether pipe terminates beneath said surface of said body of water, said buoyant means comprises a concentric buoyancy tank fixed on the tether pipe adjacent the upper end thereof; a first end of a concentric swivel joint fixed to the upper end of said tether pipe, a pivotable joint fixed between the second end of said swivel joint and a first end of a tether arm; a floating terminal being provided with at least an above-surface deck, and connected to the second end of said tether arm by a pivotable joint; and means for providing fluid communication between said upper end of said tether pipe and said floating terminal.

5. A subsea hydrocarbon producing system as recited in claim 4 wherein said tether arm is substantially parallel to and beneath said surface of said body of water, said second pivotable joint connecting said tether arm to said floating terminal beneath said surface of said body of water.

6. A subsea hydrocarbon producing system as recited in claim 4 wherein said second pivotable joint, connecting said second end of said tether arm to said floating terminal, is connected to said floating terminal substantially at said surface of said body of water.

References Cited UNITED STATES PATENTS 2,731,168 1/l956' Watts l37236 X 3,409,055 11/1968 Bily 137-236 X 3,292,695 12/1966 Haeber 166.5

LAVERNE D. GEIGER, Primary Examiner E. J. EARLS, Assistant Examiner

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