US3438204A

US3438204A - Underwater storage reservoir

Info

Publication number: US3438204A
Application number: US673756A
Authority: US
Inventors: James M Cleary
Original assignee: Atlantic Richfield Co
Current assignee: Atlantic Richfield Co
Priority date: 1967-10-09
Filing date: 1967-10-09
Publication date: 1969-04-15
Anticipated expiration: 1986-04-15

Description

April 15, 1969 J. M. CLEARY UNDERWATER STORAGE RESERVOIR Sheet Filed Oct. 9, 1967 INVENTbR. James M. Cleory April 15, 1969 J. M. CLEARY UNDERWATER STORAGE RESERVOIR Z of 2 Sheet Filed Oct. 9, 1967 lN\/'E1 ITOR. James M. Cleory m w FE w w @MJM Attorney United States ABSTRACT OF THE DISCLOSURE Offshore underwater storage is created by forming a pile of objects heavier than water on the bottom of a body of water and by forming a thick impermeable upper barrier, preferably moldable, covering and supported by the pile. Two or more conduits are placed in the pile. A water-immiscible liquid lighter than water is added to, stored in and removed from the pile. The objects for the pile are preferably transported over water to location and may be deposited on a previously formed impermeable lower barrier. Pumping liquid from the pile consolidates the upper barrier and pile. Heavy material may be spread over the upper barrier. The pressure inside the pile may be maintained lower than water pressure on the upper barrier.

Background of the invention This invention pertains to a method of forming an underwater storage reservoir and of storing a waterimmiscible liquid having a specific gravity less than water in the storage reservoir.

There is a need for a less expensive, easy and quick way to construct large hydrocarbon storage reservoirs in offshore oil producing areas. The storage requirement for offshore production is very large. Standard offshore production practice requires a minimum storage capacity of ten days production and even greater storage volumes are required in the less accessible offshore producing areas. In order to be economical, offshore fields are generally produced at very high daily rates; consequently, a storage volume holding ten or more days production can involve hundreds of thousands to millions of barrels of storage capacity. In addition, in offshore areas where the oil is produced to storage to later be transferred to a tanker, the storage volume should be at least as great as the capacity of the tanker, A common guide for establishing the capacity of storage for tankers is to have enough capacity to fill the tanker plus five days production. A small T2 tanker holds 120,000 barrels and five days production would be likely to require an additional one hundred thousand barrels. Tankers having a capacity of over a million barrels are already in use and tankers holding several million barrels are soon to be used.

The most accepted method of providing large storage capacities is to provide tanks on short and pipeline the oil to these tanks. This conventional method requires a long time to construct, is most expensive and involves the many problems related to coastal land use. Rigid tanks also have a maximum practical size limited by the strength of large sections of the rigid tank materials and the inability to handle and transport these large rigid sections; consequently, the cost per barrel of storage does not decline as the size of the storage facility is increased beyond this maximum practical size.

The increase in size of tankers has created a further limitation to the use of onshore storage. Many waterways cannot handle the new large tankers. In such areas, it is necessary to build an offshore island or loading dock in water suitable for these large tankers.

atent O "ice Various types of offshore storage facilities have been proposed. For example, it has been proposed to use cement or steel tanks weighted by sand or rock, and to use flexible bagsmoored to the ocean floor, and to use anchored tankers or floating tanks. Most of these proposals are not feasible for anything but small amounts of hydrocarbon storage. It would be feasible to use a large tanker as a storage facility, but the expected cost per barrel of such storage would exceed twelve dollars and there are many risks to this type of offshore storage which are not justified in view of the already high cost per barrel of storage. Providing mooring and piping to these large floating tankers poses additional problems.

Offshore submerged storage facilities involve other unique design and operational problems. There are several sources of stress on the walls and roof of the submerged storage reservoir. These stresses vary in direction and magnitude. There are buoyant forces inside the storage reservoir created by the oil being lighter than water and the fact that the oil in the storage reservoir to tensile stresses. An opposite or compressive stress is created when the storage reservoir is unloaded because, in order to minimize tanker loading time, storage reservoirs are unloaded at a rate sufiicient to collapse the walls of a rigid tank. Thus, the walls of the underwater storage reservoir undergo cyclic compressive and tensile forces which expand and contract the walls of the storage reservoir. These cyclic forces fatigue rigid materials. It should be noted that standard methods of ballasting tanks do not overcome the effects of these cyclic forces. It would, therefore, be beneficial to provide a submerged storage container that will either withstand both compressive and tensile forces, or in the alternative, one that is operated in a manner to avoid the effects of one or more of the forces.

In addition to the tensile and collapsed forces just described, submerged storage reservoirs must withstand large water thrusts or scouring action which forces vary in direction and magnitude. These water forces peak when there is storm at sea and standard storage reservoirs are designed for the largest peak forces since there is no ready way to increase the resistance to wave action for only the short peak periods. It would be additionally advantageous if the design of the storage reservoir were such as to better withstand Water thrusts and if the strength and stability of the storage reservoir could be increased during periods of peak wave forces.

Under normal operating conditions, oil will leak from a submerged or substantially submerged tank into the Water and it is difficult to repair the leak as long as oil is present. It would be advantageous if the storage reservoir could be operated in a manner that avoids leakage into the water and at the same time permits ready repair of the leak While oil is being stored.

When a tanker ties up to a submerged storage reservoir to load, there must be provided a secure way of mooring the tanker. It would be advantageous if the storage reservoir could also act as a stable mooring for the tanker or a loading platform.

Offshore reservoirs should be easy to construct and made of materials that can be handled and shipped in large quantities and that are readily available in most areas. Offshore reservoirs should also be near the producing wells.

Accordingly, there is a genuine need for a method of constructing and operating a large offshore storage facility.

Summary of the invention This invention provides a method of creating and operating an offshore, underwater storage reservoir for water-immiscible liquids having a specific gravity less than water. The light, water-immiscible liquid is stored in the voids of a pile of high bulk porosity objects which in addition to supplying the storage volume also provides support and stability to the reservoir. The system provided herein is chiefly noted for its ease of construction, practically unlimited size, ruggedness and applicability to any depth Water.

The underwater storage reservoir is created by forming a pile of high bulk porosity on the bottom of a body of water with the top of the pile being below the surface of the water. The pile is composed of a quantity of objects heaped together in multiple vertical and horizontal layers thereby providing a pile having an upper portion and a lower portion. The objects are composed of material having a density greater than the density of water. For example, oyster shells, large sized heavy objects surrounded by finer objects, or large sized rocks having a size at least as great as 0.5 foot. A thick upper barrier being sufliciently impermeable to contain the water-immiscible liquid within the pile is formed over and supported by the pile. At some time during construction of the storage reservoir, a minimum of two flow conduits are placed in a manner such that when the storage reservoir is completed the fiow conduits extend from inside the pile to outside the upper barrier with one conduit in communication with the upper portion of the pile and the other conduit in communication with the lower portion of the pile. Usually, an impermeable lower barrier will be formed over a portion of the bottom of the sea and the pile formed on top of this lower barrier. Preferably, the objects for the pile will be transported to location and deposited either on the sea bottom or the lower barrier. It is also preferred that the upper barrier over the pile be moldable. The upper barrier, the pile and the bottom of the body of water beneath the pile may be consolidated by pumping liquid from beneath the upper barrier at a rate sufiicient to reduce the pressure inside the pile. A material heavier than water may be spread over the top of the upper barrier. This heavy material may be large sized objects or layers of small sized objects covered with larger sized objects. The top of the heavy material may be above water; however, the top will usually be below the surface of the water. The amount of heavy material may be suflicient to place a net downward force on the exterior of the upper barrier. A water-immiscible liquid lighter than water is added to the pile to occupy the upper portion of the pile. A portion of this water-immiscible liquid is later removed. An especially unique feature of this storage system is that the pressure of the stored liquid inside the storage reservoir with or without heavy material over the upper barrier may be maintained lower than the water pressure on top of the upper barrier for any predetermined period without collapsing the storage reservoir. This pressure reduction greatly improves the sta bility and operation of the storage system. For example, a lower pressure inside the storage reservoir may be maintained when water-immiscible liquid is being added to the pile, or whenever wave action is likely to damage the pile, or when a tanker is loading from the pile, or when a leak develops, or throughout an entire storage cycle.

The foregoing summary and the following description show that this invention provides a new, less expensive method for constructing and operating a large, rugged, submerged storage reservoir. The storage reservoir is made up of common, readily available materials and is constructed in a manner such that the size of the storage reservoir is practically unlimited and the cost per barrel of storage declines as the size of the storage reservoir is increased. Moreover, the storage reservoir may be constructed in any depth water so as to be readily accessible to large tankers or to be adjacent production equipment.

Brief description of drawings Description of the preferred embodiments This invention pertains to a method of forming and operating an underwater storage reservoir. The drawings are designed to assist description of the method of constructing and operating this underwater storage system and are neither to scale nor exact in detail. In the figures, similar items have the same reference numeral and the features of any one figure could be used in any other figure when desired.

The underwater storage system is for storage of a waterimmiscible liquid having a specific gravity less than the water in which the light, water-immiscible liquid is stored. For purposes of description, this light liquid is herein also referred to as crude oil or oil and the body of water in which the oil is stored is also called sea water or the sea.

As illustrated in the drawings, the storage system is made by forming pile 11 on bottom 13 of body of water 15 with the top of the pile being below surface 17 of the body of water as shown in FIGURE 1, or on lower barrier 19 as shown in FIGURE 3. Lower barrier 19 is formed by covering a substantially greater portion of the bottom of body of water than is to be covered by the storage reservoir. The pile would then be formed on top of this lower barrier. Lower barrier 19 is designed to be substantially impermeable and to greatly improve operation of the storage system especially that part of the storage method which relates to a reduction in pressure inside of the pile as hereinafter described. It is essential that pile 11 be supported by the sea floor. This is accomplished by forming the pile either on the sea floor or on something supported by the sea floor, e.g., the lower barrier. If the sea floor is stable and sufficiently level, the lower barrier may be formed simply of concrete poured on the sea fioor. Usually, however, the sea floor will settle and it is much preferred that the lower barrier be flexible and moldable for reasons hereinafter explained in conjunction with an upper barrier placed over the pile. In such case, the lower barrier will be made of similar materials as the upper barrier.

Pile 11 is composed of a quantity of objects heaped together in multiple horizontal and vertical layers thereby providing the pile with upper portion 21 and lower portion 23 The objects are chunks, lumps, pieces, rubble, scrap or other loose objects. The objects are composed of material insoluble to the water-immiscible liquid and having a density greater than the density of the water. The density of this material is at least 1.1 times as great as the density of the water. In addition to being heavy, the objects forming the pile have sufficient strength and size relation to create a pile having highly conductive voids comprising a bulk porosity of at least thirty percent under the operating conditions hereinafter set forth.

Preferably, the objects for forming the pile are transported by Water transportation to the location for the pile and the pile is formed by depositing the transported objects on the bottom of the body of water or lower barrier 19, and that portion of the objects previously de posited to form a mound or a truncated pyramid or coni cal structure. Use of specially transported objects is in contrast to merely dredging a pile made up of mud or sand from the ocean floor or to merely blasting portions of rock on the ocean floor to form the pile. It is by water transporting select material that a truly high bulk porosity pile of strong objects is formed in a uniform, structurally sound arrangement. Dredged mud or sand would be totally unsuitable for this purpose as these materials would compact under load and form low conductivity pore spaces. The most readily available and suitable objects for the pile are large rocks without fines having a minimum cross-sectional dimension of at least 0.5 foot with fairly uniform rocks between 1 and 2 feet or larger in size being preferred. Oyster shells without a large amount of fines would also provide an excellent material for forming the pile. By way of example, a half million barrel, truncated pyramid rock pile storage with walls having a 3 to 1 slope and 50 feet high would have a base of 550 feet and and a top 250- feet wide. An '80 foot high pile of the same storage volume would have a 533 foot base and a 53 foot top. A 2.5 million barrel pile 50 feet high would have a base of 1065 feet and a top of 765 feet.

It is advantageous to form the pile using transported objects and form a series of first layers of small sized rocky objects 25, such as gravel, sand or clay on lower barrier 19 to act as a cushioning and protective layer for the lower barrier as shown in FIGURE 3. A heap of larger sized rocky objects 27 are then formed on the series of first layers of small sized rocky objects. This heap of larger sized rocky objects forms most of the bulk of pile. A series of second layers of small sized rocky objects 29 is formed over the heap of larger sized rocky objects. The second series of layers forms the sea side rim of the pile and is especially suited for completion of the storage reservoir as hereinafter shown. Second series of small objects 29 smooth the contours of the large objects, provide a protective layer of small objects, act as filter bed and may provide a filler aggregate for the cover for the pile.

As will hereinafter be made more clear, the objects provide highly conductive voids which are the storage volume, support the seal and storage reservoir from all downward and inward forces, give weight to the storage system, act as a structure on which a seal may be formed and as a filter bed and filter strengthening material for assisting in sealing the sea side rim of the pile, provide self-adjusting support for the seal allowing for variations in the sea bottom, and provide a way of consolidating the storage system.

After pile 11 is formed, there is formed thick upper barrier 31 which covers the pile, that is, the upper barrier spreads over the top of the pile and extends down on all sides of the pile and outward onto bottom 13 of the body of water or lower barrier 19 thereby forming an inverted container whose bottom rim will seal against the ocean floor or the lower barrier. The top of the upper barrier is below surface 17 of the body of water so that the pressure inside the storage system may be lowered without operating the storage reservoir under a vacuum or at a pressure less than the vapor pressure of the oil. Upper barrier 31 must also be supported by the objects in pile 11. The objects directly under and in contact with the upper barrier provide a highly wave resistant structure with many ways to increase the strength of the storage reservoir, to overcome cyclic forces and to control and repair leaks. The upper barrier is sufficiently impermeable to contain oil within the pile. Upper barrier 31 will be at least three inches thick with a thickness at least as great as one foot being preferred. As illustrated in FIGURE 2, much greater thicknesses are contemplated. The upper barrier is of suificient thickness to permit the pile covering seal to take the shape of the supporting pile.

The upper barrier may be composed of many different materials with or without reinforcing and any number of layers, provided that the barrier forms a strong. thick, longlasting seal over and supported by the objects in the pile. The upper barrier needs to be much more rugged than the lower barrier and should maintain its properties for periods of from ten to thirty years. It is highly advantageous and essential to some embodiments hereof that the barriers, especially the upper barrier, be moldable. A moldable barrier is readily deformed without rupture and is flexible, or pliable and resilient enough to fit the contours of the objects in the pile or the contours of the sea floor and be readily supported thereby. This elastic or deformable behavior in contradistinction to nondeforrnable or rigid barriers, such as cement aggregates or thick steel sheet, readily transfers forces through the barrier to the support structure thereby compensating for pressure differentials, cyclic forces, periodic thrusts, uneven compaction of support structure, and voids in the support structure adjacent either side of the barrier. The support structure for the lower barrier will be the sea floor on one side and the objects in the pile on the other side. The support structure for the upper barrier will be the objects in the pile and the lower barrier or sea fioor around the outer rim of the pile. If a heavy material is placed on the upper barrier, this material may act as support structure against the buoyant force on the upper barrier created by oil in the pile. The principal advantages of this method of constructing and operating the storage reservoir are obtainable because of the tihck, moldable character of the upper barrier and the way that the barrier is loaded and supported. A moldable upper barrier is particularly suitable to compaction and consolidation of the pile to increase the stability of the reservoir and improve the seal between the upper and lower barriers.

Preferably, the upper barrier is composed of a series of separately formed layers. This laminated construction of layers superimposed on one another provides a more flexible load-supporting barrier with self-healing, lea-k preventive properties. A laminated construction also permits an optimum combination of materials of dilferent costs and properties.

The upper barrier will be composed of oil insoluble materials that will form a strong, thick cover that is suitable for being supported by loose objects in the pile and that possesses resistance to erosion, light, oxidation, age hardening, plant and marine growth, and flow at high angles of repose. Usually, these materials will be selected from the group consisting of pourable time-setting materials or flexible sheets, or mixtures thereof. Suitable sheets are the thick, strong plastic or neoprene film materials used in lining water disposal tanks in oil production areas. These liners have been readily joined under water. For example, an excellent sheet material that has been used in water is a neoprene sheet having a thickness of oneeighth to three-sixteenths of an inch, with a specific gravity of 1.4 to 1.5, tensile strength of about 2,000 psi. and an elongation of 300 to 400 percent. Examples of pourable materials are asphalts and special Portland cements.

Asphalts with or without fillers or reinforcing provide a universally available pliable and resilient material which has been extensively tested and developed for paving and marine uses such as lining canals and preventing soil erosion. For a fuller discussion of techniques and materials, refer to Bitumen in Hydraulic Engineering, Baron W. F. van Asbeck, volume Il, 1964, Elsevier Publishing Company, Library of Congress Catalog Card Number 56- 3649, especially at pages 15, 70, and 89. These asphalt lining materials are available in both prefabricated and pourable forms. Asphalts are readily mixed with resins, plasticizers, fillers, aggregate, solubilizers, adhesives, wetting agents, and the like, to improve desirable properties such as adhesion, cohesion, plasticity, chemical and oxidation resistance, reduced shrinkage, resistance to age hardening, and the like. Asphalts have the further advantage of being already available in penetrating, time-setting mixtures which can be used to penetrate the ocean floor or layers of objects in the pile. In this manner, a very thick, strong and heavy upper barrier may be formed. The

drawings show the upper and lower barriers formed with penetrating asphalt.

As shown in FIGURE 1, heavy material 33, which is heavier than water, is spread or distributed over the sea side or top of upper barrier 31. It is preferred that this heavy material be formed by placing over the upper barrier a series of layers of small sized heavy material 35 to act as a cushioning and protective layer covering the upper barrier and as a filter bed for repairing leaks. A series of layers of large sized heavy material 37 is then placed over the series of layers of said small sized material. Preferably, the large and small sized materials will be objects like those used in forming the pile. Heav material 33 functions as a protective covering for the upper barrier, as support for the upper barrier when the oil in the pile pushes upward against the upper barrier, as a material for placing a net downward force on the upper barrier and as a filter bed or protective cover for repairing leaks. When heavy material 33 is used to place a net downward force over the upper barrier, the amount of heavy material spread over the upper barrier will counteract buoyant forces and be essentially proportional to the amount of oil stored in the pile.

Emplaced in the pile are at least two fiow conduits. These flow conduits may be placed in location at any stage of construction of the storage system, that is, either before or after the pile, the upper barrier or heavy material 33 are formed. For example, the flow conduits could be the legs of a platform or piling about which the pile is formed. The flow conduits are made in a manner such that the flow conduits extend from inside pile 11 to outside the storage reservoir or to the sea side of upper bar rier 31 and are adapted for alternately adding or removing oil and sea water from the pile.

There are many well-established Ways to accomplish placement and connection to the flow conduits. One way is shown in FIGURE 1 wherein first flow conduit 39 is in communication with upper portion 21 of pile 11 and extends upward to platform 41 with which the storage reservoir Will normally be associated and which may be separate from or a part of the storage pile. This platform will normally carry support equipment and perhaps several wellheads which are not shown. Second flow conduit 43 is in communication with lower portion 23 and extends upward to the platform. First flow conduit 39 is comprised of oil inlet 45 and oil outlet 47. Oil outlet 47 is connected to pump 49. With this arrangement, oil may be pumped into inlet 45 to the pile and may be removed from the pile by way of pump 49 and outlet 47. Second flow conduit 43 is comprised of sea water inlet 51 and sea water outlet 53. Sea Water outlet 53 is connected to pump 55. Sea water inlet 51 has sea water dump valve 57 which is below surface 17 of the water and which when opened allows water to flow into the pile. This arrangement allows sea water to be added to or removed from pile 11 wherever desired as hereinafter set forth. For example, when oil is being removed by way of oil outlet 47, sea water is allowed to flow by way of sea water inlet 51 into the pile. In a similar manner, sea water is removed from the pile by way of pump 55 and sea water outlet 53 when oil is added to the pile.

FIGURE 3 shows a second and special arrangement for the flow conduits which is designed to maintain a reduced pressure inside the rock pile when oil is being stored in the pile. The purpose of this arrangement will hereinafter be discussed in detail. In FIGURE 3, second flow conduit 43, which extends up to the platform is comprised of sea water inlet-outlet 59 and gas lift inlet 61. Sea water is either added to or removed from the pile by way of sea water inlet-outlet 59. In sea water inlet-outlet 59 is sea water inlet valve 63 which is below surface 17 of the water, sea water outlet 65 which is above water surface 17. Gas lift inlet 61 has gas valve 67 and extends downward to the pile. Sea water inlet valve 63 and gas valve 67 will in standard fashion be operable automatically 75 and be connected through suitable means (not shown) to oil level sensor 69 which is placed in oil inlet at a level below water surface 17. Oil level sensor 69 is any type of well-known liquid level sensor and will not be described in detail. Similarly, the oil level sensor is adapted in the usual manner to open and close gas valve 67 and sea water inlet valve 63 thereby controlling upper level 71 of the oil in oil inlet 45 to a level at water surface 17 or below water surface 17.

Before any oil is placed in the pile, it would be best to compact the storage pile and sea floor to its most stable arrangement and to test the upper barrier for leaks. This can readily be accomplished by removing water from the pile at a rate sufficient to reduce the pressure inside the pile when the barrier is impermeable. This may be done after the upper barrier is formed, but it will frequently be preferred to perform this step after only a portion of the upper barrier is formed and while this portion is still moldable. This step is carried out by removing water from the pile by way of either or both of the flow conduits. If the upper barrier leaks water into the pile, the relation between the rate of water removal and pressure inside and outside the pile will indicate the size of the leak. The inward flow of water through the upper barrier can readily be detected and located from the exterior or sea side of the upper barrier. The leak is readily repaired by either increasing the overall thickness of the upper barrier or by plugging the leak or leaks. The upper barrier or the pile of objects under the upper barrier, especially the small sized layers of rocky objects, readily act as a filter bed upon which bridging and sealing materials may be screened out. The same techniques used in plugging zones of lost circulation during Well drilling, or in plugging porous or fractured formations, may be used. When the barriers are sufficiently impermeable to allow the pressure inside the pile to be reduced to a level below the pressure on the sea side of the upper barrier, the pressure differential across the large area of the barrier creates a large inward and downward force on the pile. A downward force is required and this is one reason why the pile must rest on the lower barrier or the ocean floor. This force compacts the sea floor and the pile of objects to a very stable position. This force also causes the moldable barrier to snugly conform to the contour of the pile and fill voids just below the upper barrier. Both of these results greatly strengthen the whole system. Under certain conditions it may be desirable to dehydrate the sand in the sea floor and better stabilize the foundation for the pile by performing the same step on the lower barrier before the pile is formed. In which case, the Water would be pumped from the sand below the lower barrier through either specially laid piping or through the legs of the platform. Usually this will not be necessary since the large weight of the pile on the lower barrier greatly compacts the sea floor making the sea floor and the lower barrier impermeable and stable.

When operating the pile as an oil storage reservoir, the water-immiscible oil is added to pile through first flow conduit 39 :by way of oil inlet 45 during a first period. The oil in the pile, being lighter than water, collects in upper portion 21 just under upper barrier 31. Whenever desired, a portion of the oil stored in the pile is removed from the pile through first flow conduit 39 by way of oil outlet 47 and pump 49. The voids in the pile are highly conductive so that oil may be removed just as rapidly as if the storage reservoir were a standard oil tank. Since this is important, 1t should be apparent that several flow conduits could be spaced about the storage reservoir if needed.

During the periods when oil is being removed from or added to the pile, sea water is added to or removed from the pile. Sea water may be removed from the pile when 011 is being added to the pile through second flow conduit 43 by way of pump and sea water outlet 53 or through second fiow conduit 43' by way of sea water inlet outlet 59 and sea water outlet by injecting gas into gas lift inlet 61. Removing Water as oil is added prevents overpressurization of the pile and allows the oil to be produced into the pile by gravity flow from above water separators or other production equipment if desired. In a similar manner, when oil is being removed from the pile, sea water may be added to the pile through second flow conduit 43 by way of sea water inlet 51 and sea Water dump valve 57 or through second flow conduit 43 by Way of sea water inlet valve 63 and sea water inlet-outlet 59.

It has been pointed out that it is advantageous to place a net inward and downward force on the exterior or sea side of the upper barrier. Due to the way that the storage system is constructed, the unique relationship between the upper barrier and pile and the location of these in relation to the surface and bottom of the water, this force is readily created and transmitted from the upper barrier to the objects below this upper barrier without collapsing the storage system. The advantages derived from being able to place a large net inward force on the upper barrier without damaging or collapsing the storage system or greatly altering the storage volume are many. For example, this permits the operator to lower the pressure inside the storage pile and strengthen and anchor the pile against peak wave forces and to provide stable tanker mooring or a firm foundation for loading or production equipment. Reducing the pressure also provides a way of repairing leaks from the exterior of the reservoir and of preventing leakage of the lighter-than-water, stored liquid into the surrounding water.

One way to accomplish a net, but permanent and costly, downward force is to spread heavy material 33 over upper barrier 31 as shown in FIGURE 1. The amount of heavy material spread over the upper barrier is sufiicient to create a downward force on the upper barrier greater than the upward force on the upper barrier created by the waterimmiscible being lighter than water. The amount and distribution of heavy material required to at least balance this buoyant force is readily calculated from such information as the depth of the upper barrier below water surface 17, the specific gravities and densities of the oil and water and the volume of oil to be placed in the pile. The amount of heavy material spread over the upper barrier will be substantially greater than the minimum amount necessary to counteract the buoyancy of the oil.

An especially advantageous way for placing a net downward force on the upper barrier is to create a pressure differential across the upper barrier by reducing the pressure inside the pile to a level below the water pressure on the sea side or exterior side of the upper barrier. This way of creating the desired force has the advantages of being readily adjusted to varying conditions, of creating a truly inward force as well as a downward force, of improving the seal between the rim of the upper barrier and the sea floor or lower barrier, of preventing leakage of oil into the water, of assisting in repairing leaks, of being less costly than other methods and other similar advantages. The pressure inside the pile may be reduced without creating a vacuum or operating the pile below the vapor pressure of the oil because the barrier is below the surface of the water. This pressure reduction places a large inward, downward force on the storage system. Thus, after the storage reservoir has been formed and oil added to the pile, the pressure inside the reservoir may be reduced for any predetermined period whenever desired. For example, the pressure inside the pile may be reduced to strengthen and anchor the reservoir against peak wave forces, to provide stable tanker mooring or a firm foundation for other equipment or a platform, and to repair or prevent leaks. This pressure reduction may be readily accomplished by removing sea water from the pile using techniques previously mentioned.

It will also be advantageous in some instances to use a combination of heavy material spread over the upper barrier and a lower pressure inside the pile. The heavy material acts as a protective layer over the barrier, or as a 1O safety control in the event that control of the lower pressure inside the pile is lost, or as a support structure against upward rupture of the upper barrier, or a combination of these functions.

It is advantageous to reduce the pressure inside the pile throughout the first period when oil is being added to the pile. One method of accomplishing this is shown in FIGURE 3. During the first period when oil is being added to the pile by way of oil inlet 45, oil level sensor 69 is used to determine upper oil level 71 of the oil in the oil inlet. This upper level of oil is maintained at a level at least as low as surface 17 of body of water 15, and will usually be maintained substantially below surface 17. As illustrated, this is accomplished by using oil level sensor 69 to open gas valve 67 whenever the upper level of the oil rises above a predetermined level. When gas valve 67 opens, gas lift gas passes through gas lift inlet 61 and lifts sea Water out of the pile by way of sea water inlet-outlet 59 and sea water outlet 65. The rate of seawater removal is such that upper oil level 71 does not rise above the surface of the water. The oil level sensor may also be used to shut off production of oil when necessary. For example, it may be necessary to stop production if a leak develops and the oil level rises above a preset level. Under these conditions, the pressure inside the pile is maintained below the pressure on the exterior or sea side of the upper barrier because the oil is lighter than water and the head of water is either equal to or greater than the head of oil.

When the pressure inside the pile is to be maintained below the water pressure on the sea side of the upper barrier during oil removal from the pile, oil level sensor 69 controls the rate of sea water entrance through sea water inlet valve 63. In a similar manner, the pressure inside the pile may be maintained below the water pressure on the sea side of the upper barrier throughout the periods when oil is being added to, stored in and removed from the pile. Oil level sensor 69 may be used to open and close gas valve 67 and sea water inlet valve 63 so that upper oil level 71 stays relatively constant for any preset period.

Certain modifications of the invention will be apparent to those skilled in this art. For example, what is illustrated'in one figure could have been placed in the other figures. A pressure sensing system could be used in place of a liquid level sensing system for maintaining a lower pressure in the pile. Any form of pump suitable for removing liquids from the pile could be used in place of the gas lift system of FIGURE 3. A material heavier than water could be placed on the sea floor ahead of the lower barrier or as a part of forming the lower barrier. The flow conduits for adding and removing oil and for adding and removing water could be in one or more pipes and a series of pipes could be spread about the pile to provide for uniform pressures and to prevent any possibility of water coning. The pipes could be the legs of the platform. The illustrative details disclosed are therefore not to be construed as imposing unnecessary limitations on the invention.

What is claimed is:

1. An underwater storage method for a water-immiscible liquid having a specific gravity less than said water, which method comprises forming a pile on the bottom of a body of water with the top of said pile being below the surface of said body of water, said pile being composed of a quantity of objects heaped together in multiple horizontal and vertical layers thereby providing a pile having an upper portion and a lower portion, said objects being composed of material having a density greater than the density of said water, forming a thick upper barrier covering said pile and supported by said objects in said pile, said upper barrier being sufficiently impermeable to contain said water-immiscible liquid within said pile, and at some time during construction of said storage placing first and second flow conduits in a manner such that when said storage is completed said flow conduits extend from inside said pile to outside said upper barrier with said first flow conduit in communication with said upper portion of said pile and said second fiow conduit in communication with said lower portion of said pile.

2. The method of claim 1 wherein a material heavier than water is spread over the exterior of said upper barrier.

3. The method of claim 1 wherein the method includes the steps of adding the water-immiscible liquid to said pile during a first period, and thereafter removing a portion of said water-immiscible liquid from said pile.

4. The method of claim 3 wherein the method includes the steps of removing water from said pile at the same time as Said water-immiscible liquid is added to said pile and adding water to said pile at the same time as said portion of said water-immiscible liquid is removed from said pile.

5. The method of claim 4 wherein during said first period the water-immiscible liquid is added by way of said first flow conduit and the method includes the steps of determining the upper level of said water-immiscible liquid in said first flow conduit and maintaining said upper lever of said water-immiscible liquid at level at least as low as said surface of said body of water.

6. The method of claim 3 wherein for a predetermined period the pressure of said water-immiscible liquid in said pile is maintained at a pressure below the water pressure on the exterior of said upper barrier.

7. The method of claim 6 wherein the predetermined period extends throughout said first period.

8. The method of claim 6 wherein a material heavier than water is spread over the exterior of said upper barmen 9. The method of claim 3 wherein a material heavier than water is spread over the exterior of said upper barrier, the amount of said material being sufificient to create a downward force on said upper barrier greater than the upward force on said upper barrier created by said waterimmiscible liquid being lighter than said water.

10. The method of claim 1 wherein after at least a portion of said upper barrier is formed, water is removed from said pile at a rate sufiicient to reduce the pressure inside said pile.

11. The method of claim 1 wherein the objects for construc ion of said pile are transported by water transportation to the location for said pile and said pile is formed by depositing said transported objects on the bottom of the body of water and that portion of said objects previously deposited.

12. The method of claim 11 wherein a material heavier than water is spread over the exterior of said upper barrier.

13. The method of claim 11 wherein the method includes the steps of adding the water-immiscible liquid to said pile during a first period, and thereafter removing a portion of said water-immiscible liquid from said pile.

14. The method of claim 13 wherein the method includes the steps of removing water from said pile at the same time as said water-immiscible liquid is added to said pile and adding water to said pile at the same time as said portion of said water-immiscible liquid is removed from said pile.

15. The method of claim 14 wherein during said first period the water-immiscible liquid is added by way of said first flow conduit and the method includes the steps of determining the upper level of said water-immiscible liquid in said first flow conduit and maintaining said upper level of said water-immiscible liquid at level at least as low as said surface of said body of water.

16. The method of claim 13 wherein for a predetermined period the pressure of said water-immiscible liquid in said pile is maintained at a pressure below the water pressure on the exterior of the upper barrier.

17. The method of claim 16 wherein the predetermined period extends throughout said first period.

18. The method of claim 16 wherein a material heavier than water is spread over the exterior of said upper barner.

19. The method of claim 13 wherein a material heavier than water is spread over the exterior of said upper barrier, the amount of said material being sufficient to create a downward force on said upper barrier greater than the upward force on said upper barrier created by said waterimmiscible liquid being lighter than said water.

20. The method of claim 11 wherein after at least a portion of said upper barrier is formed, water is removed from said pile at a rate sufiicient to reduce the pressure inside said pile.

21. The method of claim 11 wherein a major portion of said objects forming said pile is composed of large rocks having a minimum cross-sectional dimension of at least 0.5 foot.

22. The method of claim 11 wherein a major portion of said objects forming said pile is composed of oyster shells.

23. The method of claim 11 wherein the pile is formed by forming a series of first layers of small sized rocky objects on the bottom of the body of water, forming a heap of larger sized rocky objects on said first layers of said small sized rocky objects, and forming a series of second layers of small sized rocky objects over said heap of said larger sized rockv objects.

24. The method of claim 23 wherein after the upper barrier is formed, there is placed over said upper barrier a series of layers of small sized material and a series of layers of large sized material is placed over said series of layers of said small sized material, said small sized material and said large sized material being heavier than water.

25. The method of claim 11 wherein before the pile is formed, there is formed a lower barrier covering a portion of the bottom of the body of water and the pile is formed on top of said lower barrier, said lower barrier being designed to be substantially impermeable.

26. The method of claim 25 wherein a material heavier than water is spread over the exterior of said upper barrier.

27. The method of claim 25 wherein the method includes the steps of adding the Water-immiscible liquid to said pile during a first period, and thereafter removing a portion of said water-immiscible liquid from said pile.

28. The method of claim 27 wherein the method includes the steps of removing water from said pile at the same time as said water-immiscible liquid is added to said pile and adding water to said pile at the same time as said portion of said water-immiscible liquid is removed from said pile.

29. The method of claim 28 wherein during said first period the water-immiscible liquid is added by way of said first flow conduit and the method includes the steps of determining the upper level of said water-immiscible liquid in said first flow conduit and maintaining said upper level of said water-immiscible liquid at level at least as low as said surface of said body of water.

30. The method of claim 27 wherein for a predetermined period the pressure of said water-immiscible liquid in said pile is maintained at a pressure below the water pressure on the exterior of the upper barrier.

31. The method of claim 30 wherein the predetermined period extends throughout said first period.

32. The method of claim 30 wherein a material heavier than water is spread over the exterior of said upper barrier.

33. The method of claim 27 wherein a material heavier than water is spread over the exterior of said upper barrier, the amount of said material being sufiicient to create a downward force on said upper barrier greater than the upward force on said upper barrier created by said waterimmiscible liquid being lighter than said water.

34. The method of claim 25 wherein after at least a portion of said upper barrier is formed, water is removed from said pile at a rate sufficient to reduce the pressure inside said pile.

35. The method of claim 25 wherein a substantial portion of said material forming said pile is composed of large rocks having a minimum cross-sectional dimension of at least 0.5 foot.

36. The method of claim 25 wherein a major portion of said objects forming said pile is composed of oyster shells.

37. The method of claim 25 wherein the pile is formed by forming a series of first layers of small sized rocky objects on the lower barrier covering a portion of the body of water, forming a heap of larger sized rocky objects on said first layers of said small sized rocky objects and forming a second series of layers of said small sized rocky objects over said heap of said larger sized rocky objects.

38. The method of claim 37 wherein after at least a portion of said upper barrier is formed, water is removed from said pile at a rate sufiicient to reduce the pressure inside said pile.

39. The method of claim 37 wherein after the upper barrier is formed, there is placed over said upper barrier a series of layers of small sized material and a series of layers of large sized material is placed over said series of layers of said small sized material, said small sized material and said large sized material being heavier than water.

References Cited UNITED STATES PATENTS 2,536,320 1/1951 Smith 61-.5 X 2,747,774 5/ 1956 Breitenbach 6l.5 X 2,879,646 3/ 1959 Brandt 61-.5 3,003,322 10/1961 Jordan 61-.5 3,068,654 12/ 1962 Warren 61.5 3,113,699 10/ 1963 Crawford et a1. 3,152,640 10/ 1964 Marx 6l-.5 X

FOREIGN PATENTS 769,764 3/ 1957 Great Britain.

EARL J. WITMER, Primary Examiner.

US. Cl. X.R. 6 1-1