US3664286A

US3664286A - Offshore storage system

Info

Publication number: US3664286A
Application number: US873291A
Authority: US
Inventors: Preston E Chaney
Original assignee: Sun Oil Co
Current assignee: Sunoco Inc
Priority date: 1969-11-03
Filing date: 1969-11-03
Publication date: 1972-05-23
Anticipated expiration: 1989-05-23

Abstract

A buoyant vessel for storing offshore oil and gas production having a ballast compartment for stability which can be raised and lowered relative to the vessel. The ballast compartment is in a lowered position when the vessel is in its offshore location, and it is in a raised position flush with the hull of the vessel when the vessel is being moved to and from its offshore position. This vessel is used in cooperation with wells completed on the floor of a body of water with flexible flow lines and hydraulic lines attached from the vessel to the wells by way of a float located on the water surface. The production is processed on the vessel and stored therein with ballast and stored fluids manipulated to provide appropriate buoyancy for the vessel.

Description

United States Patent Chaney [54] OFFSHORE STORAGE SYSTEM 51 May 23, 1972 Primary Examiner-Trygve M. Blix Attorney-George L. Church, Donald R. Johnson, Wilmer E. McCorquodale, Jr. and John E. Holder ABSTRACT A buoyant vessel for storing offshore oil and gas production having a ballast compartment for stability which can be raised and lowered relative to the vessel. The ballast compartment is in a lowered position when the vessel is in its ofi'shore location, and it is in a raised position flush with the hull of the vessel when the vessel is being moved to and from its ofl'shore position. This vessel is used in cooperation with wells completed on the floor of a body of water with flexible flow lines and hydraulic lines attached from the vessel to the wells by way of a float located on the water surface. The production is processed on the vessel and stored therein with ballast and stored fluids manipulated to provide appropriate buoyancy for the vessel.

19 Claims, 3 Drawing PATENTEDMAY 23 m2 3 664, 286

' SHEET 1 or 2 'I/VVEIVTOR PRESTON E. CHANEY K AT7OR/VE Y PATENTEDMAY 23 m2 3. 6642 86 sum 2 or 2 5 POWER J 0 SOURCE F IG 3 INVE/VTOR PRESTON E. CHANEY ATTORNEY OFFSHORE STORAGE SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to methods and apparatus for storing offshore produced fluids and more particularly to a system which can be practically used in areas where turbulent seas are not uncommon.

Currently, there are several methods for handling produced offshore fluids. The most commonly used method is to drill the wells from platforms fixedly attached to the ocean floor with the production equipment normally being on another centrally located platform or on the same platform usedfor the drilling operation. After the produced fluids have been processed through the productionequipment, the fluids are transported to shore by pipelines laid on the floor of the body of water. The fluids are then placed in on-shore storage facilities, and ultimately processed through a refinery. This system has been fairly satisfactory, except in times of very severe storms when the platforms have been known to collapse. Another system presently being used employs undersea chambers located on the floor of the body of water. These chambers store the produced fluids after having been processed in production equipment located on anabove-water platform. Thus again, the disadvantage is the failing of the production platform when a severe storm occurs. A recently used method for storing produced fluids from offshore locations involves a spherical chamber with an open bottom. The chamber is moved to its location and sunk so that the open bottom is in contact with the floor of the body of water, and is held there by pilings. The top of the chamber extends above the water. It is used in conjunction with platforms having production equipment thereon, which are fixedly attached to the floor of the body of water. This system is being used in the Persian Gulf, where the waters are fairly placid; however, it could not be used in deep or turbulent waters.

Another system known to have been employed in the Persian Gulf involved cutting and welding together two ships and attaching them to a platform. The production equipment was located on the platform, and the produced fluids were pumped to the ships after having been processed on the platform. Still another system involves simply anchoring a ship fairly close to the production platform and utilizing it for storage after the produced fluids have been processed. There are other systems which utilize slightly different concepts, but most if not all of these systems employ production equipment located on a fixed platform. There have been proposals related to significuntly different systems for storing offshore produced fluids. Some have both storage and production equipment located on the floor of the body of water. Such a system would be very expensive. Until now, most offshore drilling has been done in fairly shallow water where the seas are placid. Much of the search for oil and gas is now taking place in deeper and more turbulent waters, where most of the present storage systems cannot be utilized. It is therefore the object of this invention to provide a new and improved storage system which can be utilized in such an area.

SUMMARY OF THE INVENTION With this and other objects in view, the present invention contemplates utilizing a floating vessel maintained in position by dynamic positioning and having production equipment on board the vessel. The vessel is stabilized by a ballast compartment which is adjustable so that it can be raised and lowered, and when raised is flush with the vessel. The ballast compart ment is kept in a raised position when the vessel is being moved to and from its offshore location. When utilized in conjunction with wells completed on the floor of the body of water, there are no fixed platforms for the seas to collapse. When a storm occurs, the vessel can be moved to a safer location.

A complete understanding of this invention may be had by reference to the following detailed description, when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of the vessel when at its offshore location;

FIG. 2 is a bottom view of the vessel illustrating the positioning of the ballast compartments; and

FIG. 3 is a schematic view of a water level sensing device and related pumps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 of the drawings, a vessel 10 is shown at its offshore location in position to receive produced fluids from wells located on the floor of the body of water 6. The vessel 10 has a ballast compartment 22 which can be positioned flush with the vessel or can be extended well below the vessel as shown. It is positioned below the vessel 10. or raised so as to be flush therewith by jacking mechanism 24. The vessel 10 has production equipment 36 located on the deck of the vessel, used to process produced fluids. The vessel 10 is moved to the offshore location by a conventional propeller 26, and is maintained in position by reversible screws 32 which act in response to position indications from sound and radio receivers 30 or information from radar equipment 38 which indicate the distance to main float 18, as well as the vessels alignment related to main float l8.

Wellhead equipment 12 located on the floor of the body of water 6 is connected to flow and hydraulic lines bundle 16 between the floor of the body of water 6 and main float 18 by gathering lines 14. The flow and hydraulic lines bundle 16 is supported between the float and the vessel by subsidiary floats 20 and comprise at least one flow line, hydraulic lines for wellhead control, and a faiLsafe hydraulic line to shut in the wells if disconnection occurs. The float has a sound and radio sender 46, which transmits signals to receivers 30 located on the vessel 10. Difference in velocity of sound and radio pulses provides a measure or horizontal distance between float l8 and vessel 10.-The vessel 10 is separated into a multitude of compartments 40 as outlined by dash lines 41 which are storage compartment walls. The vessel also has a hull recess 42, for berthing the ballast compartment 22 when the ship is being moved to and from the vessel's offshore location. The

ballast compartment 22 is raised into the hull recess 42 by jacks 24.

In the use of the apparatus described above, the vessel 10 with ballast compartment retracted is moved to an offshore position related to wellhead equipment 12. The ballast compartment 22 is lowered by jacks 24 to act as a semi-submersible mass for resisting the vessels roll, pitch, and heave movements. Production is carried to the vessel by flow lines bundle l6 and the wellhead equipment is controlled by the hydraulic lines in the bundle. These lines are supported by floats l8 and 20 on the water surface 28. The production is then processed through production equipment 36 and stored in the vessel 10 or ballast compartment 22. By placing the produced fluids either above the water surface 28 for additional submergence, or by placing the produced fluids below the-water surface 28, proper draft requirements can be maintained for buoyancy. Care is taken in loading the vessel to keep the center of buoyancy above the center of gravity. Otherwise, when the vessel is out of its normal vertical position, the buoyant force would tend to aid wind and wave action in capsizing the vessel because the buoyancy is no longer acting upwardly through the vessel. If severe seas occur, the ballast compartment 22 is raised, hydraulic and flow lines bundle 16 is disconnected, and the vessel is removed to a safer location.

While on the offshore location, the vessel 10 is maintained within a set distance range of the float l8 and at right angles thereto by operation of the reversible screws 32 and propeller 26 acting in response to signals received from sound and radio sender 46 and float reflector 45. In practice, a radar signal initiated on the vessel would reflect from float reflector 45 and be received by radar equipment 38. On board computers record the travel time of the radar signal which is translated into distance by the computer using the speed of a radar signal through an environment of air. This distance calculation is compared to a preset distance range. If a variation occurs, a signal is generated in response thereto, which signal initiates controls for operating the propeller 26. This same procedure is employed to maintain the vessel radially from the float 18, except that a radar signal receiver is located on each side of the vessel and the distance determined from one receiver is compared with the distance calculated from the receiver on the other side of the vessel. If at variance, reversible screws 32 would be activated to correct the disparity.

An alternative vessel positioner employs sound and radio sender 46 and sound and radio receivers 30. The sender 46 simultaneously initiates both a sound and a radio signal which are received at different times by receiver 30. Because of the difference between the speed of travel of radio signals as opposed to sound signals, the reception time lag is proportional to the distance. A computer is used to translate such time lag into a signal proportional to the time lag. Once distance is determined, operation of the reversible screws 32 and propeller 26 is the same as described in the radar system previously discussed.

The maintenance of a free floating vessel in a specific location is referred to as dynamic positioning, and could be accomplished by either of a combination of the radar system or the sound and radio wave system. 7

The operation of the fail-safe hydraulic line located in bundle 16 is such that whenever the fail-safe line parts due to a severe storm, or for any other reason, the wells are automatically shut in. The pressure in the fail-safe line operates to keep a shut-in valve on each well open. Hydraulically operated shut-in valves are presently used on underwater wellhead systems to shut in wells in case production equipment fails at the surface. In the present system, a spring acts, when unimpeded, such that it tends to keep the shut-in valve closed. A hydraulic piston activated by the hydraulic pressure in the failsafe line acts to keep the spring in check so that it cannot close the shut-in valve. Thus, when pressure is lost in the fail-safe line, the spring is unimpeded by the piston, and acts to close the shut-in valve. This system prevents wells from producing through ruptured flow'lines when storms separate the lines from the vessel.

FIG. 2 is an illustration of the bottom of vessel of FIG. 1. The ballast compartment 22 fits in recess 42 located on the underside of the vessel 10. The ballast compartment legs 21 are shown as large circular dashed lines and storage compartment walls 41 are shown as straight dashed lines. Propeller 26 is also located on the underside of the vessel 10. Sound and radio receivers 30 project from either side of the vessel and from the end of the vessel opposite propeller 26. Also located on the underside of the vessel is the inlet of water height sensing tube 44, shown as small circular dashed lines as distinguished from the ballast compartment legs shown as large circular dashed lines. Sensing tube 44 is located in the recess at approximately the center of the vessel, and is further illustrated in FIG. 3. From FIG. 2 it can be seen that the ballast compartment fits in a recess on the underside of the vessel and that the ballast compartments 40 divide the vessel so that produced fluids can be stored in any quadrant of the vessel 10. Additionally, sound and radio receivers 30 are widely spaced so that the attitude of the ship can be accurately determined.

FIG. 3 schematically shows the water height sensing mechanism and its relationship to pumps for moving produced fluids and/or ballast from one compartment to another, through compartment walls 41 of FIG. 1. The inlet 34 located on the hull of the vessel 10 allows water to enter the vessel and rise to the height of the water outside of the vessel. Accordingly, water height 60 is equivalent to the height of water surface 28 of FIG. 1. Located in the water height sensing tube 44 is a damper and filter 48 located between the water level 60 and the water height sensing tube inlet 34. This damper and filter 48 reduces the minor fluctuations of the water surface 28 of FIG. 1, and filters out sediments embodied in the water. Located on top of water height sensing tube 44 is a chamber 63 housing

electrical contacts

62 and 64. These contacts are insulated from the sensing tube. An insulated electrical line 65 is attached to the water height sensing tube 44 near the damper and filter 48 and below the chamber 63. The other end of insulated line 65 is attached to battery 54 which in turn is attached to relay 56 and relay 68. Relay 56 operates switch 66 and relay 68 operates switch 70. The switch 66 is in the circuit containing a pump 52 and a power source 50, and switch 70 is in a circuit containing pump 74 and power source 72. When the water level reaches electrical contact 64, a circuit is closed consisting of the water in water height sensing tube 44, insulated line 65, battery 54, and relay 68. When this circuit is completed, the relay operates switch 70 which activates power source 72 to operate pump 74. When the water level 60 is in the position shown, a circuit is closed consisting of the water and water height sensing tube 44, insulated line 65, battery 54, and relay 56. This relay operates to keep the switch 66 open. When the water level 60 drops below electrical contact 62, this circuit no longer exists and switch 66 closes, which initiates power source 50 to operate pump 52.

Thus in operation of the water height sensing apparatus, when the water level exceeds electrical contact 64 or drops below electrical contact 62, either pump 52 or pump 74 operates to displace liquids from an upper storage compartment to a lower one or the reverse, depending on which pump circuit is in operation. This system can be arranged to operate on a single pump with a two way valve; i.e., pumping in one direction or another, depending on buoyancy requirements indicated by the water level 60. Additional water height sensing tubes and related pump circuits are used to determine whether front versus rear and side loading is uniform. Alternatively, a simple level device could be used for this purpose. Thus, when the water height sensors or level devices transmit indications that a side or end of the vessel is loaded improperly, the transmitted indications will activate pumps to shift the fluids in a direction to balance the load.

The action of the sound and radio receiving means and controls related to the reversible screws and the water height sensor maintains the vessel in position related to the main float 18 of FIG. 1, and in proper position related to the vessel s height in the water and load balancing, respectively. Additionally, the sound and radio receiver 30 (FIG. 1) not only keeps the vessel within a range of distance related to the float 18 (FIG. 1), but keeps its radial attitude regulated with respect to the float. When wave and wind action comes from a direction other than at right angles to the longitudinal axis of the vessel 10, it has a tendency to push the vessel out of line. Through a series of compensations, this wind and wave action will eventually work the vessel around so that its longitudinal axis is once again at right angles to the wind and wave action. The series of compensations comes about because the front reversible screw 32 (FIG. 1) operates to keep the nose ofthe vessel pointed toward the float and the front reversible screw 32 will correctively operate before the rear reversible screw 32, because the front of the vessel 10 will receive the force from a new wind and wave action before the rear of the vessel. Thus, by increments, the vessel will move around to a point at right angles to the wind and wave action.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects, and the aim in the above description is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

l. A process for storing production in an ofishore location comprising the steps of: locating a vessel in an offshore location relative to completed wells on the floor of a body of water; lowering a ballast compartment from the vessel; maintaining the vessel on location by a dynamic positioning; attaching production flow lines and hydraulic lines to each well and conducting them to the vessel; flowing production through the flow lines to the vessel; processing produced fluid reaching the vessel with production processing equipment located on the vessel; and storing the processed produce fluids in the vessel. w

2. The process of claim 1 including lowering the ballast compartment to semi-submerse the vessel and provide a drag against roll, pitch, and heave movement of the vessel.

3. The process of claim 1 including locating produced hydrocarbon fluids in the vessel above the water line so that it acts as a load on the vessel and locating produced hydrocarbon fluids below the water line so that it acts as buoyancy force with regard to the vessel.

4. The process of claim 1 including automatically shutting in v the wells when parting of the hydraulic lines occurs.

5. The process of claim 1 including raising the ballast compartment to a point flush with the vessel when the vessel is entering and exiting the production location.

6. The process of claim 1 including storing produced fluids in the ballast compartments.

7. A process for storing production in an offshore location comprising the steps of: locating a vessel in an offshore location relative to completed wells on the floor of a body of water; lowering a ballast compartment from the vessel; maintaining the vessel on location by dynamic positioning; attaching production flow lines and hydraulic lines to each well and conducting them to the vessel; supporting the hydraulic and production flow lines by a float located on the water surface and maintaining the vessel in the water so that its longitudinal axis extends radially from the float and at right angles to the water surface wave line; flowing production through the flow line to the vessel; processing produced fluid reaching the vessel with production processing equipment located on the vessel; and storing the processed produced fluids in the vessel.

8 The process of claim 6 including arranging the ballast and production in the vessel and the ballast compartment in such a manner that the vessels center of buoyancy is always above its center of gravity.

9. The process of claim 2 including sensing the water height relative to the vessel and automatically adjusting the buoyancy of the vessel to conform with draft requirements.

10. An apparatus for storing production from offshore wells comprising: a vessel having a hull and a deck, said vessel having a ballast compartment located on the underside thereof and said vessel having a recess formed in its hull for accommodating the ballast compartment such that the ballast compartment can be placed in a position flush with the hull of the vessel; a jacking system for raising and lowering the ballast compartment with respect to the vessel; pump means for changing the amount of ballast in the ballast compartment; and water height sensing means for controlling said pump means.

11. The apparatus of claim 10 including reversible screws located near each end of the vessel for providing lateral forces to the vessel.

12. The apparatus of claim 10 including production processing equipment located on the deck of the vessel.

13. The apparatus of claim 10 wherein the pump means is used for putting ballast and produced fluids in the ballast compartments and produced fluids in the vessel.

14. The apparatus of claim 13 wherein the water height sensing means senses water height relative to the vessel and controls said pump means to maintain proper buoyancy and loading of the vessel.

15. An apparatus for storing production from offshore wells comprising: a vessel having a hull and a deck, said vessel having a ballast compartment located on the underside thereof and said vessel being formed so that the ballast compartment can be placed in a position flush with the hull of the vessel; a jacking system for raising and lowering the ballast compartment with respect to the vessel; pump means for putting ballast and produced fluids in the ballast compartments and produced fluids in the vessel; and water height sensing means located in the vessel for sensing water height relative to the vessel and for controllin said purn means to maintain roper buoyancy and loading 0 the vesse wherein the water eight sensing means comprises a first electrical contact vertically spaced from a second electrical contact, a tube having an inlet on the underside of the vessel communicating with a chamber housing said contacts, said tube being for receiving water from the body of water surrounding the vessel, insulated electrical line attached to the tube and connecting a power source to relays, where one relay is attached to each of the electrical contacts.

16. The apparatus of claim 15 wherein water in the tube which seeks the level of the water outside the vessel, completes a circuit involving electrical contacts, water, electrical line, power source, and relay attached to each electrical contact and a switch.

17. The apparatus of claim 15 including a filter and damping means in said tube.

18. The apparatus of claim 12 wherein the production processing equipment is connected by hydraulic and flow line means to wellhead completions located in the floor of the body of water. v

19. The apparatus of claim 18 including float means attached to and supporting the hydraulic and flow line means; means located on the vessel for sensing distance from the vessel to the float; and means for aligning the vessel relative to the float acting in response to the distance sensing means.

Claims

1. A process for storing production in an offshore location comprising the steps of: locating a vessel in an offshore location relative to completed wells on the floor of a body of water; lowering a ballast compartment from the vessel; maintaining the vessel on location by a dynamic positioning; attaching production flow lines and hydraulic lines to each well and conducting them to the vessel; flowing production through the flow lines to the vessel; processing produced fluid reaching the vessel with production processing equipment located on the vessel; and storing the processed produced fluids in the vessel.

4. The process of claim 1 including automatically shutting in the wells when parting of the hydraulic lines occurs.

8. The process of claim 6 including arranging the ballast and production in the vessel and the ballast compartment in such a manner that the vessel''s center of buoyancy is always above its center of gravity.

15. An apparatus for storing production from offshore wells comprising: a vessel having a hull and a deck, said vessel having a ballast compartment located on the underside thereof and said vessel being formed so that the ballast compartment can be placed in a position flush with the hull of the vessel; a jacking system for raising and lowering the ballast compartment with respect to the vessel; pump means for putting ballast and produced fluids in the ballast compartments and produced fluids in the vessel; and water height sensing means located in the vessel for sensing water height relative to the vessel and for controlling said pump means to maintain proper buoyancy and loading of the vessel, wherein the water height sensing means comprises a first electrical contact vertically spaced from a second electrical contact, a tube having an inlet on the underside of the vessel communicating with a chamber housing said contacts, said tube being for receiving water from the body of water surrounding the vessel, insulated electrical line attached to the tube and connecting a power source to relays, where one relay is attached to each of the electrical contacts.

18. The apparatus of claim 12 wherein the production processing equipment is connected by hydraulic and flow line means to wellhead completions located in the floor of the body of water.