US3519034A - Submerged storage and floating terminal loading assembly - Google Patents

Description

United States Patent 3,519,034 SUBMERGED STORAGE AND FLOATING TERMINAL LOADING ASSEMBLY William F. Manning, Dallas, Tex., assignor to Mobil Oil Corporation, a corporation of New York Filed Nov. 15, 1966, Ser. No. 594,461 Int. Cl. B63b 35/00; B65b 3/00 U.S. Cl. 141-387 11 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to an improved offshore structure for the mooring and loading of vessels, particularly with crude oil and 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, 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 efforts 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 six hundred feet and less than six thousand feet). At these 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 tanks, able to accommodate at least ten days supply of accumulated fluid hydrocarbons, are an integral part of the system. With wellheads, production satellites, gathering systems, and production storage, all being supported on the ocean bottom, perhaps six hundred 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 advantageous -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 submerged storage facilities. 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 tank 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 and Waves.

A single point mooring system is also advantageous in 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 3,519,0'34 Patented July 7, 1970 ICC instance the single point mooring system could be con` nected 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 to 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; and

FIG. 2 is an enlarged view of the portion of the floating terminal illustrating an arrangement for providing fluid communication between apparatus on the floating terminal and a tanker, through a rotatably and pivotally mounted boom, that can be extended outwardly from the terminal in anydirection.

Now looking at FIG. 1, the single point mooring system of the present invention consists of a substantially vertical rigid-walled tether pipe 10 bridging the body of water between a submerged facility, comprising at least an open bottom storage tank 12, located on the marine bottom 13, and a surface unit `comprising a buoyant central floating terminal 14 universally fixed to the upper end of the tether pipe 10, and a loading boom 11 rotatably and pivotally mounted on the tether pipe 10 and selectively connected to the terminal 14. The floating terminal 14 is a wave transparent structure consisting of a deck 16 supported, above the surface 18 of a body of water 20 on buoyant horizontal submerged floats 22, by four buoyant peripheral vertical columns 24. The upper end of the tether pipe 10 is connected by a collet connector 28 to the lower end of a universal joint 26, depending centrally from the floating terminal 14 lat approximately the center of buoyancy thereof. The floating terminal 14 provides support for an above-surface deck, for mounting equipment, as well as a buoyant support for the tether pipe 10. The universal joint 26 allows the deck 16 to hold a constant orientation even though the upper end of the tether pipe 10 is displaced from over the fixed lower end thereof. The tether pipe 10 is anchored at its lower end by a collet connector 30 fastened to the upper end of a short. upstanding stub pipe 32 extending from the upper end of the storage tank 12 which is firmly fixed in the formation underlying the marine bottom 13 by piles 34. As shown, the flexibility of the tether pipe 10 will compensate for any movement of the floating terminal 14 from directly over the storage tank 12. Since such an arrangement is only practical in very deep water, the flexibility of the tether pipe 10 being proportional to its length, a universal connection between storage tank 12 and the tether pipe 10 is normally advisable. Such a connection is shown in the present inventors similarly titled c0- pending application Ser. No. 589,434, filed on Oct. 25, 1966.

The loading boom 11, consisting of an open framework of braces, has, at its outer end, a flotation tank 36 which floats partially above the surface 18 of the body of water 20 when the boom 11 is in the position shown in full lines in FIG. 1. The flotation tank 36 is large enough for the landing of personnel and has an upstanding mooring bollard 37 lixed on its upper planar surface. The outer end of the boom 11 is buoyantly supported by the tank 36, the inner end of the boom being pivotally supported on the tether pipe by a triangular conguration of struts 38 depending from the boom 11 and connected at an apex, spaced below the boom, by a kneeaction pin joint 40, to a subsurface swivel 42 journaled on the tether pipe 10, near the upper end thereof. The swivel 42 consists of a pair of spaced rotatable collars 44 mounting the arrangement on the tether pipe 10, and a pair of interconnecting struts 46 extending therefrom and converging at the pin joint 40. Another swivel 48, journaled lower down on the tether pipe 10, consists of a pair of spaced rotatable collars 50l and 52 journaled on the tether pipe 10 with a horizontal beam 54 extending outward from the upper collar 50 and being supported by a diagonal strut 56 fixed to the lower collar 52 at its inner end. On the outer end of the beam 54 is a seat 58, for supporting the outer end of the boom 11, or the otation tank 36 thereof, when the boom is stored beneath the surface 18 when not in use as shown in phantom.

When the boom 11 is unrestrained it tends to rise to the surface 18 due to the buoyancy of the flotation tank 36. To store the broom 11 beneath the surface 18 of the body of water 20, a handling line 59 is connected from the lower end of the flotation tank 36, through a pulley 60 spaced out from the tether pipe 10 on the horizontal beam 54 of the swivel 48 and hence, up substantially parallel to the tether pipe 10 to the edge of the deck 16 of the terminal 14 where a power winch, shown schematically at 61, is mounted on a trolley 62 between the C rails of a circular track 63 running along the perimeter of the deck 16. Reeling up the handling line 59 draws the outer end of the boom 11 down, pivoting it about the pin joint 40, beneath the surface 18, until the flotation tank 36 at the outer end of the loading boom 11 is located in the seat 58 on the swivel 48. By this arrangement the loading boom 11 is located far beneath the surface 18 of the body of water 20 when not in use so that it cannot be damaged during storms and will not itself present a surface for wind and waves to act upon, and therefore permitting only minimal surface wind and wave forces to be applied to the tether pipe 10 through the above-surface components of the terminal 14 when the system is not 1n use.

When the present mooring and loading system is to be utilized in conjunction with a subaqueous oil eld, a long distance from shore, the lluid, to be delivered periodically to a tanker 63 moored by a line 64 to the bollard 37, is conducted to the storage and loading facilities by a number of subsea pipelines 65 (one shown), each connected with a plurality of subsea wellheads through a subsea gathering station (not shown). The tether pipe 10 provides fluid communication between the subsea pipeline 65 and the floating terminal 14. The tether pipe 10, for purposes of this illustration, is constructed as a doublewalled conduit, forming concentric flow passageways, with the subsea pipelines 65 being connected to the annulus of the tether pipe 10 through the outer wall thereof just above the collet connector 30.

Crude oil and/or gas is transferred from the upper end of the tether pipe 10 to the deck 16 of the floating terminal 14 through the hollow interior of at least one of the supporting columns 24 and the submerged floats 22, the universal joint 26 being bypassed by a ilexible line 66 connected to the annulus of the tether pipe 10. The crude oil may be carried directly within the hollow interiors of the columns 24 in the floats 22 or may be contained within a closed conduit 67, as shown in FIG. 2, carried within these components. The specific system under discussion, being designed for the production of oil, dictates that the produced fluid is directed through processing equipment 68 (schematically shown in FIG. 2), including a separator on the deck 16, the oil being separated from any included gas and water and the pressure being reduced to atmospheric. The resulting gas can then be llared, or, if economical, it can be stored. The separated oil is then conducted back down to the bottom of the floating terminal directly through certain of the hollow buoyant columns 24 and the submerged floats 22, or by enclosed conduits 69, through a two-way, three-position valve (also shown in FIG. 2). A path of lluid communication is provided from the floating terminal 14 to a storage area within the storage tank 12 through the central passage in the tether pipe 10 and a flexible tube 68 bridging the universal joint 26. The oil is conducted down through the center of the tether pipe 10, passing through the collet 30, and into the storage tank 12. Any suspended solids will settle out through the open bottom of the tank 12. Included water will also separate out from the oil.

When the tanker 63' arrives to receive the oil stored in the tank 12, the connection between the processing equipment 68 on the deck 16 of the oating terminal 14 and the storage tank 12 on the marine bottom 13 is broken, and the storage tank 12, through the tether pipe 10, is then connected to the tanker loading apparatus, to be discussed later. As shown in the embodiment of FIG. 2, the valve 70 is repositioned to direct the oil from the upper end of the fluid conduit 69 into the loading apparatus, while blocking the outlet of the processing equipment 68. The differential 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 14 for water that is greater than about four hundred fty feet, the pumps on the terminal 14 providing high pressure for quickly loading the tanker 63. For depths of less than approximately four hundred fty feet, subsurface pumps will also be included in a submerged portion of the floating terminal 14. Pumps that may be used for transferring oil from the bottom storage tank 12 to the terminal 14 are well known in the art and are illustrated in more detail in the British Pat. No. 1,023,085, published Mar. 16, 1966.

The boom 11, constituting a portion of the abovementioned tanker loading apparatus, serves the double purpose of supporting a fluid transferring conduit along its length and providing means at its other end for mooring the tanker 63 at any point on the circumference of a circle around the tether pipe 10. A hollow conduit 72 extending the length of the iboom 11 contains the oil being transferred from the inner to the outer end of the boom 11 while functioning as the backbone for the framework making up the boom itself. At the outer end of the boom 11, the conduit 72 extends through the flotation tank 36 and is connected to the inner end of a buoyant loading hose 74. A hose retrieving line 76 is connected between the outer free end of the buoyant loading hose 74 and a schematically illustrated winch 78 mounted at the inner end of the boom 11. Prior to storing the boom 11, the winch 78 (which would probably be power driven) would be actuated to reel up the retrieving line 76, wrapping the buoyant loading 'hose 74 back around boom 11 to keep it from becoming entangled in the water during storage.

There is no permanent fluid connection between the rotatable and pivotable boom 11 and the stationary terminal 14. The inner end of the lboom 11 terminates in a standpipe section 80. A flange 82 on the upper end thereof is designed to mate, when the boom 11 is in the solid line position of FIG. 1, with a complementary ilange 84 formed on the outer end of a transition arm 86 (FIG. 2) suspended beneath the platform 16 of the floating terminal 14. Since the boom 11 must be able to rotate :approximately 360 around the tether pipe 10, the transition arm 86 must also be able to rotate 360. To avoid a collision with one of the four support columns 24 of the terminal 14 during rotation, the transition arm 86 must also be retractable completely under the platform 14 and so it is pivotally mounted on a reciprocatable underhung carriage 88 suspended from a track `comprising a pair of parallel rails 90 on a turntable 92 rotatable on a circ-ular rail 94 located centrally to the underside of the platform 16. The pivoting of the transition arm 86 is controlled by a winch 96 mounted on the carriage 88 by a line 98 connected therebetween and entrained over a pulley 100 fixed, between the rails 90, on the turntable 92. As shown in FIG. 2, the carriage 88 is as far over to the left as allowed by the rails 90, within the support columns 24. This permits the right end of the transition arm 86 to be also within the support columns 24 so that the turntable 92 may be rotated to line the transition arm up with the boom 11. Once the boom has been rotated to a position where it is in line between the tanker 63 and the tether pipe 10, the turntable 92 is rotated into the same position. The carriage 88 is then moved in on the rails 90 until it is centrally located beneath the platform 16. The transition :arm 86 in this position extends out :beyond the platform support columns 24 as shown in FIG. 1. As the carriage 88 moves in on the rails 90, the distance between the winch 96 and the pulley 100 shortens, causing the transition arm 86 to drop down toward the boom 11.

When the carriage 88 reaches the center of the platform 16 on the rails 90 and is locked in place, the winch 96 is actuated to continue the lowering of the outer end of the transition arm 86 until the flange 84 thereof mates with the flange 82 on the inner end of the standpipe 80 of the conduit 72. Personnel on the inner end of the boom 11 bolt the flanges together to form a tluidtight seal. With the flanges 82 and 84 bolted, a flange 102, on the inner end of the transition arm 86, is now in a horizontal plane directly beneath the terminus of a fluid conduit 104 selectively connected to the storage tank 12 through the valve 70 and having its downstream end depending centrally through the platform 16 to a point just above the transition arm 86, when the transition arm 86 is in the position shown in FIG. l. The terminal portion 106 of the conduit 104 is a rotatable slip coupling biased upwardly out of connection with the transition arm 86 by a pair of springs 8 connected between the under neath of the platform 16, on opposite sides of the conduit 104, and a thrust or throw-out bearing 110 for allowing the terminal portion to rotate while being biased upward. By applying a downward force on the terminal portion 106, a terminal flange 112 on the end thereof may be lowered into conjunction with the llange 102 on the inner end of the transition arm and personnel in the carriage 88 can bolt the flanges 112 and 102 together to form also a fluidtight seal. The terminal portion 106 would be of a flexible pressure resistant material to compensate for any misalignment of the conduit 104 and the transition arm 86. A flexible section 114 at the other end of the transition arm 86 compensates for the individual movement on the terminal 14 and the boom 11 caused by surface wave action. With such an arrangement the oil ilowing up from the storage tank 12 through the line 69, within one of the support columns 24, is directed through the two-position, three-way valve 70 into the line 104, down through the transition arm 86, and into the uid conduit 72 of the boom 11, the oil being conducted through the loading hose 74 and into the tanker 63.

The turntable 92 should not be rigidly locked in position in the circular track 94 so that any slight movement of the boom 11 caused by shifting in the position of the tanker 63, moored thereto, will cause the turntable to rotate slightly on the rails rather than causing a distortion of the transition arm 86. The rotation of the boom 11 will also cause the winch trolley 62 to roll along its track to realign the Winch with respect to the boom 11 due to the interconnecting handling line 59.

The wind and wave forces acting on the surface components of the system, such as the floating terminal 14 and the boom 11 when it is in the loading position, as

well as the tanker 63 when moored thereto, will cause the upper portion of the system to be laterally displaced and, in so doing, will force the tether pipe 10` out of the vertical position shown in FIG. 1. The tether pipe would either bend, due to its inherent flexibility, or it would pivot around a lower universal joint just above the collet connector 30, if such a joint were included (as previously described). The movement of the tether pipe 10 away from its vertical position is opposed by the upwardly directed forces resulting from the submerged buoyant liloats 22 and the buoyant support columns 24 of the terminal 14. Since the tether pipe 10 is of constant length, the terminal 14 will be drawn down further into the water by its lateral displacement. As the terminal 14 is drawn down, the additional submergence of the support columns 24 results in an increasing buoyant force tending to restore the tether pipe to its vertical position. The magnitude of the restorative force will be determined by the combined buoyancy of the oats 22 and the support columns 24. Therefore, these buoyant elements are designed to producea buoyant force much greater than that necessary to support the terminal 14 and the equipment thereon. If, in a particular instance, a small terminal 14 is to fbe used, having buoyant sections only large enough to support the equipment, one or more cylindrical buoyancy tanks can be mounted concentrically on the upper portion of the tether pipe 10` between the two swivels 42 and 48, or the swivel 42 and pivot Ijoint 40 can `be redesigned to be lower on the tether pipe 10 and the cylindrical buoyancy tank can be mounted between the swivel 42 and the collet connector 28. The closer to the top of the tether pipe 10 that the buoyancy tank is mounted, the greater the resultant couple acting to right the tether pipe 10. Without a buoyancy tank on the tether pipe 10, it would be necessary to provide auxiliary support for the tether pipe 10 any time the terminal 14 was removed for repairs.

The single point mooring system of the present invention could serve other functions in conjunction with a subsea production system. yElectrical generators, and the prime power source therefor, would be located on the deck 16 of the terminal 14 for supplying necessary power to the pumps and controls of the storage tank 12 and the ibottom-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 26 by flexible lines, if the loading boom 11 is not allowed to rotate more than one revolution, or rotatable electrical connections may be formed within the swivel 26.

What is claimed is:

1. A subsea material producing system comprising:

a storage tank anchored in formations underlying a marine bottom beneath the surface of a body of water at an offshore site;

a tether pipe attached to said storage tank and eX- tending substantially vertical to a point at least adjacent said surface of said body of water;

a lloating terminal for mooring a Ivessel thereto and for loading produced material onto the vessel;

means for fastening said iloating terminal to the upper end of said tether pipe to locate said lloating terminal at said olfshore site, said floating terminal maintaining said tether pipe in its substantial vertical position in said body of Water;

means for fluid communication between said storage tank and said floating terminal, said communication means being at least in part supported by said tether pipe for directing material from said storage tank to said floating terminal; and

means for conducting said material from said lloating terminal to a vessel moored thereto.

2. An offshore structure as recited in claim 1 wherein means for buoyantly supporting said floating terminal extend through said surface of said body of water whereby the further said upper end of said tether pipe is displaced from directly above said fixed lower end of said tether pipe, the deeper said terminal is drawn down into said body of water, further submerging said buoyant means and creating a larger restorative buoyant force tending to move said floating terminal back over said lower fixed end of said tether pipe.

3. An offshore structure as recited in claim 2 wherein said tether pipe is fastened to said liioating terminal at substantially the center of ibuoyancy thereof.

4. An offshore structure as recited in claim 2 wherein said floating terminal has a portion thereof normally positioned above the surface of the body of water and wherein said buoyant means of said oating terminal comprises at least one horizontal submerged oat and at least one vertical column extending between said submerged oat and said above-surface portion of said oating terminal to produce a substantially wave transparent structure.

5. A subsea material producing system comprising a storage tank anchored in formations underlying a marine bottom beneath a body of water at an offshore site; a rigid-walled tether pipe having the lower end thereof fixed with respect to said marine bottom, said tether pipe extending substantially vertically to at least a point near said surface of said body of water; a floating terminal having at least an above-surface deck, said tether pipe being fastened to said terminal to locate said oating terminal at said offshore site; a first iiuid communication means, at least supported by said tether pipe, for conducting material from at least one pipe line extending across said marine bottom to said above-surface deck of said floating terminal; and a second fluid communication means, at least supported by said tether pipe, for conducting material from said deck of said floating terminal to a storage area within said storage tank anchored far beneath said surface of said body of water.

46. A subsea material producing system as recited in claim S wherein the lower end of said storage tank is in open communication with said body of water adjacent said marine bottom.

7. A subsea material producing system as recited in claim 5 wherein at least one of said first and second fluid communication means comprises an axial passageway within the rigid outer wall of said tether pipe.

8. A subsea material producing system as recited in claim 5 wherein said first and second fluid communication means comprises said tether pipe formed of a concentric double-walled conduit to provide a central fluid passageway and an annular fluid passageway extending the length of said tether pipe.

9. A subsea material producing system as recited in claim r8 wherein said pipe line, extending across said marine bottom, is in fluid communication with said annular passageway of said tether pipe, and said storage area of said storage tank is in fluid communication with the central passageway of said tether pipe.

10. A subsea material producing system as recited in claim 5 wherein said rst and second Ifluid communication means extend from said upper end of said tether pipe to said above-surface deck of said floating terminal through at least one column supporting said terminal deck above said surface of said body of water on at least one submerged oat, said floating terminal forming a wave transparent structure.

11. A subsea material producing system comprising:

a storage tank anchored in formations underlying a marine bottom beneath the surface of a body of water at an offshore site;

a tether pipe extending from a ,fixed point at least adjacent said marine bottom to a point at least adyjacent said surface of said body of water;

a floating terminal for mooring a. vessel thereto for loading material onto the vessel;

means for fastening said floating terminal to said tether pipe to locate said floating terminal at said offshore site;

rst means for fluid communication between a storage area within said storage tank and said oating terminal, said rst means being at least in part supported by said tether pipe and adapted to allow ow of material between said tank and said terminal;

means for conducting material from said floating terminal to a vessel when moored thereto;

a pipe line extending at least in part across said marine ibottom and adapted to be connected at one end to a material producing source; and

second means attached to the other end of said pipe line for uid communication ibetween said pipe line and said floating terminal, said second means being at least in part supported by said tether pipe and adapted to allow flow of material from said pipe line to said oating terminal.

References Cited UNITED STATES PATENTS 2,731,168 l/1956 Watts 137-236 X 3,103,200 9/ 1963 Fulkerson et al. 9-8 X 3,311,142 3/1967 Bergstrom 137-236 X 3,359,741 12/1967 Nelson 166-.5 X 3,409,055 11/1968 Bily 137-236 X LAVERNE D. GEIGER, Primary Examiner E. I. EARLS, Assistant Examiner U.S. Cl. X.R.

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