Compact floating production, storage and offloading facility

Abstract

An oil storage apparatus (111) comprising a buoyant hull (102) comprising a single column of circular or polygonal cross-section. The interior of said hull (102) comprises at least one oil-over-water tank (103), and said oil storage apparatus (111) further comprises means for maintaining said tank in pressed full condition.

Claims

1. An oil storage apparatus, comprising: a buoyant hull having a single column of circular or polygonal cross-section, said hull having a diameter and an interior that includes at least one oil-over-water tank, means for maintaining said tank in pressed full condition, wherein a first region of the hull is, in use, proximate the waterline, and a second region of the hull is, in use, distal from the waterline, wherein the diameter of said hull reduces from a maximum at the first region to a minimum at the second region, further comprising at least one oil-water separation cell positioned proximate the upper most portion of the oil-over-water storage tank, wherein the at least one separation cell is of sufficient volume to ensure, in use, separation of the oil and water therein through gravity, at least one baffle or weir arranged in the separation cell to form at least one zone through which, in use, oil and water may flow, and an oil caisson fluidly connecting the at least one separation cell with the oil-over-water storage tank, wherein the separation cell further comprises a heating arrangement and insulation.

2. An oil storage apparatus according to claim 1, wherein the single column comprises two or more vertically stacked cylinders.

3. An oil storage apparatus according to claim 1, wherein the apparatus comprises a center of buoyancy and a center of mass, wherein the apparatus is arranged such that the center of buoyancy and center of mass are substantially coincident.

4. An oil storage apparatus according to claim 1, wherein the at least one oil-over-water tank has a center of mass and is enclosed and isolated from the external atmosphere.

5. An oil storage apparatus according to claim 4, wherein at least one water ballast tank is provided proximate the center of mass, below the center of mass, or above the center of mass of the oil-over-water tank.

6. An oil storage apparatus according to claim 1, wherein the oil-over-water tank is provided about the central vertical axis of said hull.

7. An oil storage apparatus according to claim 1, further comprising a water caisson which is in fluid communication with the oil-over-water tank, the water caisson is arranged so as to enable control of the pressure in the oil-over-water tank, wherein the water caisson extends through an upper most deck of the hull to connect to water treatment facilities located thereon.

8. An oil storage apparatus according to claim 7, wherein the water caisson is arranged such that, in use, water displaced from the oil-over-water tank is not exposed directly to the sea, wherein the displaced water is cleaned and or used for water injection.

9. An oil storage apparatus according to claim 1, further comprising a crude oil caisson which fluidly connects the oil-over-water tank to a main process topsides facility.

10. An oil storage apparatus according to claim 1, wherein the buoyancy of the hull is provided by internal spaces arranged around the oil-over-water tank.

11. An oil storage apparatus according to claim 10, further comprising collision bulkheads provided within the internal spaces such that watertight subdivision of said spaces is possible.

12. An oil storage apparatus according to claim 1, further comprising at least one water ballast tank provided around the oil-over-water tank.

13. An oil storage apparatus according to claim 1, further comprising a keel provided beneath the hull, at least one substantially horizontally oriented heave plate adapted to be perforated arranged on or proximate to the keel, and fixed solid ballast positioned proximate the keel.

14. An oil storage apparatus according to claim 1, wherein, in use, the draft from the water surface to the base of the keel is between substantially 60 and 90 meters, wherein the maximum width of the hull is between substantially 38 and 45 meters; wherein the freeboard of the apparatus in use is between substantially 15 and 20 meters; wherein the oil-over-water tank runs from proximate the base of the FPSO to, in use, substantially 10 meters above the waterline; wherein the oil-over-water tank is approximately 20 to 25 meters in width when viewed from above; and wherein the height of an upper most deck of the hull is between substantially 75 and 110 meters.

15. An oil storage apparatus according claim 1, wherein the water-plane area of the hull is such that, in combination with the displaced mass, hydrodynamic mass, and damping, natural periods in heave and pitch degrees of freedom are maintained at or above substantially 20 seconds.

16. A method of using an oil storage apparatus according to claim 1, including a buoyant hull having a single column of circular or polygonal cross-section and an interior including at least one oil-over-water tank, the method comprising the step of: maintaining the oil drawn into the oil-over-water tank in pressed full condition so that a first region of the hull is, in use, proximate the waterline, and a second region of the hull is, in use, distal from the waterline, the diameter of said hull reducing from a maximum at the first region to a minimum at the second region.

17. A method according to claim 16, further comprising the step of using an internal water caisson to manage the pressure in the oil-over-water tank.

18. A method according to claim 16, further comprising the steps of: cleaning the water displaced from the oil-over-water tank; using the cleaned water for water injection.

Description

(1) In order that the invention may be more readily understood, reference will now be made, by way of example, to FIGS. 1 to 4, in which:

(2) FIG. 1 is a side view of a first embodiment of the present invention;

(3) FIG. 2 is a cut-away side view of the embodiment of FIG. 1;

(4) FIG. 3 is a quarter plan view of section A-A of FIG. 2; and

(5) FIG. 4 is a partial side view of a second embodiment of the present invention.

(6) A first embodiment of the invention will now be described with reference to FIGS. 1 to 3. Referring firstly to FIG. 1, a side elevation of the first embodiment of the invention is shown, with a mooring system, flexible risers, flow-lines and umbilicals attached.

(7) The FPSO 100 comprises four sections. The first section is an inwardly tapering column and sits atop a second, cylindrical, section with substantially vertical sides. In use, the first section and a portion of the second section protrude from the water above the water surface 101 and comprise an upper buoyant hull 102. The hull 102 sits atop a further inwardly tapering frusto-conical section from which a further substantially vertical cylindrical column extends vertically, to form an internal crude oil cargo tank 103. The cargo tank 103 extends, in use, vertically down into the water from beneath the hull 102 to form the ballast keel 104. One or more heave plates 105 extend horizontally on the outside surface of the keel 104 and are horizontally spaced apart from each other. A first heave plate is proximate the bottom of the keel 104 such that the wave loading on said keel is reduced whilst retaining the added mass and motion damping effects. Further heave plates 105 may be positioned above said first heave plate. Damping effects may be enhanced by perforating the plates 105 and ensuring there is a gap between the plate 105 and the hull 102 such that water can flow through and or around said plate 105. Flow in this manner generates turbulent vortices from the edges of the plates 105 which provide a damping effect.

(8) Mooring equipment 106 may be located on the upper most deck 107 of the hull 102 with mooring lines 108 running vertically alongside said hull 102 and through external fairleads 109 located at some convenient vertical position. Flow-lines and riser bundles 110 rise from the seabed to the production facility located on the upper deck 107 and, in the first embodiment, attach to a manifold arrangement on said upper deck 107. A main process topsides facility 111 is located above the upper deck 107.

(9) A single oil-over-water storage tank 112 is housed internally, within the FPSO 100 and about the axial centre of the second, third and fourth sections of said FPSO 100. The storage tank 112 extends from the keel 104, substantially the height of said FPSO 100. This is shown in FIG. 2.

(10) The storage tank 112 forms a single tank volume. A central water caisson 113 runs vertically through the axial centre of the storage tank 112 and is open at its lowest level to said storage tank 112. One or more submerged water pumps and risers 114 may be used to pump or inject water at the lowest point of the water caisson 113 via a lower sump (not shown). Said risers 114 are positioned such that the tank may be completely emptied for inspection.

(11) In use, the oil-over-water storage tank 112 contains oil 112a and seawater 112b. The oil 112a sits atop the water 112b by virtue of said oil 112a having a lower specific gravity. Accordingly, an interface 115, between the two liquids is formed which moves up and down as the oil 112a is drawn into the storage tank 112, or offloaded therefrom.

(12) The central water caisson 113 is open to the atmosphere at the end proximate the upper deck 107 and said central water caisson 113 is open to the lower portion of the storage tank 112 at the end proximate the keel 104. The caisson 113 contains water to a level suitable to maintain the desired pressure in the storage tank 112.

(13) The buoyant hull 102 further comprises internal spaces 116 which surround the upper portion of the storage tank 112. The internal spaces 116 are formed by the outer skin 122 of the storage tank 112 and surrounding outer hull shell plating 117 of the hull 102. These internal spaces 116 may be further divided with internal collision bulkheads 118 to assist with watertight subdivision of said spaces 116 in the event of ship collision and breach of the hull 102.

(14) A crude oil caisson 119 extends vertically upwards from the top of the oil storage tank 112 to the main process topsides facility 111 located above the upper deck 107 and is substantially open to atmosphere. In this way, the levels of liquid in the oil caisson 119 and water caisson 113 are connected hydrostatically. The water caisson 113 and risers 114 also extend through the upper deck 107 and connect to produced water treatment facilities (not shown).

(15) The third section of the FPSO comprises water ballast tanks 120. Said tanks 120 are provided around at least the storage tank 112 and are used to compensate for variations in the mass, and centre of mass, of fluids within said storage tank 112, caused by the difference in density between crude oil 112a and sea-water 112b. The water ballast tanks 120 are provided proximate the height of the centre of mass of the storage tank 112 and define the lowest point of the hull 102. The centre of mass of the oil storage tank 112 moves up and down as the level of the interface 115 between the oil and water moves from the top to the bottom of said tank 112. When the storage tank 112 is full of oil or full of water, the centre of mass of said tank 112 is approximately in the middle of said tank 112. Therefore, operation of the tank 112 in the pressed full condition minimises movement of the centre of mass. The change in centre of mass is only affected by the difference in the specific gravity between the oil 112a and the water 112b which is relatively small. This enables ballast tanks 120 positioned at approximately half the height of the tank 112 to be used at first as the oil 112a displaces the water 112b and the centre of mass of the tank 112 moves downwards. Ballast tanks 120 positioned below the half height of the oil storage tank 112 are used once the centre of mass of the tank 112 starts to rise back to the mid-point of said tank 112 later during the filing cycle.

(16) Fixed solid ballast 121 is located proximate the keel 104, beneath the first crude oil storage tank 112. The solid ballast 121 is located proximate the lowest point of the keel 104 to compensate for the mass of the topsides located at the highest point of the FPSO 100. The amount of fixed ballast is such that the centre of mass of the FPSO 100 is at or just below the centre of bouyancy. Too much fixed ballast, i.e. overcompensating for the mass of the topsides, would create a need for more buoyancy, which increases the water-plane area and decreases the natural heave period. It would also increase the steel weight and cost.

(17) FIG. 3 shows a quarter plan view of section A-A of the FPSO 100. It can be seen that the upper deck 107 is octagonal and symmetric about axis 140, with circular fairings between flat sections. It will, of course, be apparent to the skilled person that the upper deck may have an alternative number of sides, for example, twelve or sixteen, whilst remaining substantially symmetric about axis 140. The oil storage tank 112, also known as an oil-over-water tank, is shown in the centre of the plan view of FIG. 3 around the water caisson 113 and riser 114. The outer skin 122 and the hull shell plating 117 form the internal spaces 116 as described above and the inner collision bulkheads 118 are also shown in plan view in FIG. 3. Further spaces 123, 124 are arranged about the oil storage tank 112 to aid buoyancy, provide general access and equipment storage space, and or provide routes for piping, chain lockers, or similar.

(18) A second embodiment of the invention is shown in FIG. 4. In this embodiment, the upper portion of the central oil storage tank 112 is used to provide space for an oil-water separation cell or plurality of cells. The top of the storage tank 112 forms the base of the space provided for the separation cell or cells. The space extends up to proximate the upper deck 107 and across the width of the tank 112.

(19) Degassed production fluids are transferred from the topsides facility 111, as before, via the caisson 119 through which fluid levels and pressures are managed. The cell or cells are of sufficient overall volume to provide a long enough residence time based on the daily fluids production rate—to ensure the separation of the oil 112a and water 112b through gravity. Insulation and or heating elements 126 may be provided such that the temperature of the contents of the cell or cells may be controlled according to the specific needs of the fluids being separated. Each cell may be comprised of a series of baffles and weirs 127, arranged to form a sequence of zones 128 through which the production fluids may be directed such that the oil 112a has sufficient time to rise and separate from the water 112b and may be skimmed off the top and transferred to a further oil caisson 129 used to supply the storage tank 112 beneath. The water is taken off at the lower level, shown in FIG. 4 as via water caisson 130, and transferred to a suitable water clean-up facility on the topsides 111. Such an arrangement enables oil 112a to spill from the cell or cells into the oil filled caisson 129, with an equivalent amount of water 112b being removed from the water caisson 130 to allow the oil 112a to fill the tank 112.

(20) Spaces may in addition be provided to provide over-flow or slops tank in case of production interruptions or fluctuations that may from time to time need to be managed.

(21) One or more such cells may be in use at any one time, with the facility to take cells in and out of production to allow cleaning, inspection or other activities as may be required from time to time.

(22) In this way, the amount of topsides oil-water separation equipment may be reduced, and/or fields that have a very high proportion of water to oil, may be exploited more readily.

(23) For certain oil-fields, it may be that the bulk of oil and gas processing is carried out elsewhere on a fixed or floating platform, or subsea. A third embodiment of this invention may therefore be so arranged as to provide an oil storage and export facility only, or, in combination with the second embodiment, minimal processing or conditioning such as water removal and treatment.

(24) In this third embodiment, there may be no requirement to provide a topsides structure or permanent manning, with all equipment located within the upper deck structure of the facility. However, in all other aspects of function and design of this embodiment, it is the same as described above.

(25) In each embodiment, the dimensions of the FPSO are tailored to the required oil storage capacity, topsides weight, static stability requirements and operability constraints on motions in waves. For example, for 200 k to 250 kbbl oil storage, the total draft from the water surface to the lowest point would typically vary between approximately 60 m to 90 m, and the maximum width of the upper hull section would vary between approximately 38 m and 45 m. The freeboard (i.e. the waterline to the deck edge) of the apparatus in use would be chosen to be between approximately 15 m and 20 m. The internal oil storage 112 tank runs from approximately the lowest point to around 10 m above the external waterline and, when viewed from above, is approximately 20-25 m across.

(26) The present invention is not limited to the specific embodiments described above. Alternative arrangements will be apparent to a reader skilled in the art.