Patent Publication Number: US-3880102-A

Title: Method and apparatus for offshore submersible oil storage and drilling

Description:
United States Patent 1 Biewer METHOD AND APPARATUS FOR OFFSHORE SUBMERSIBLE OIL STORAGE AND DRILLING 1 Apr. 29, 1975 3.360.810 H1968 Busking 114/.5 T  
 Primary E.ruminerTrygve M. Blix Assistant E \&#39;aminerGregory W. OConnor Attorney. Agent. or Firm-Brown &amp; Martin {57] ABSTRACT An offshore submersible unit having a plurality of tanks in a hull that is submerged to a given depth by air in a first group of tanks and water ballast in a secand group of tanks that provides substantially neutral buoyancy and a given submerged draft. The hull has a central supporting column with platform for drilling and oil distribution. and has a plurality of individually articulating column stabilizers fixed by universal connections to the hull for stabilizing the hull in the submerged position, which articulated columns are passively stabilized by a water free surface effect at multiple levels.  
 29 Claims, 12 Drawing Figures GAS RELEASE 132 70 fiz- SEPARATOR I26 OIL :34 DEUVERY v 124 v 130 WATER AIR VENT T COLLECTING FROM TANK /B0 sounce Ila WATER OVERBOARD PATENTED Z H 3.880.102  
 SHEEI 16F a Fig. l  
  7 \2 44 2 as so 44 SHEET b [IF 4 WATER;  
  GAS RELEASE |2a SEPARATOR 2 O m 8 L I m H W A O I/ m 0 V 9 D F V m u v e V H m 9 M D M w m 3 n 6 I O 4 6 6 6 HM M W M M M g H 9 7 m u .H W U EA I T O I 2\ O 5 M1 5 I 7 C I I l PAIENIEIJnrnzsms OIL mom SOURCE METHOD AND APPARATUS FOR OFFSHORE SUBMERSIBLE OIL STORAGE AND DRILLING BACKGROUND OF THE ll\\&#39;l-IN HON The offshore production of oil has increasingly moved from wells in relatively shallow water and close in to shore, to areas having greater water depths and increasingly adverse surface wave conditions. For oil production in areas of relatively shallow water and close in shore areas. the most convenient and economical way to handle the oil produced is to install a fixed drilling and production platform at the site and to transfer the oil to shore facilities for further processing. storage and transfer. In more recent developments. the crude oil is being transferred directly to a tanker or storage barge on location. and some offshore tanks are being provided at the production site. These tanks usually either involve large storage units that rest on the bottom. or are tanks that float on the surface or very close to the surface of the water.  
  Deep water production in remote ocean areas poses different and considerable problems for the drilling. production and storage of crude oil. that are not satisfled by the prior approaches. ln such areas it is not economically or technically feasible to lay pipelines to facilities ashore. and a completely submerged sea floor tank installation requires some form of control. either local or remote, that is difficult to implement; and the water depth and sea conditions preclude the economical installation of fixed platforms or bottom mounted tanks. While the semi-submersible platform characterized by a submerged or a semi-submerged tank having a small water plane and a relatively large mass&#39;is adaptable to these conditions, the known tank structures do not have a sufficient degree of nonreaction to wave actions. So while such semi-submersible platforms have received some acceptance for exploratory offshore drilling. the platforms are single purpose units that are not effective for the storage of oil produced at the wells because such units are not stabilized. do not have an internal structure that is economically feasible and practical for the purpose of storing oil. and yet is mov able to and from the site with that stability necessary to insure the location and constant draft for the multiple purposes of drilling. production and storing of oil. Thus it is advantageous to have a new and improved method and unit for the offshore drilling. production and oil storage by a unit that is able to function in all water depths at any sea state. without regard to sea floor soil conditions. that will substantially preclude any chance of oil spills. that will permit the receiving. storage. and transfer of oil with a minimum of assist forces and ancillary systems. and that will be cost effective and yet provide sufficient storage capacity to permit the efficient utilization of large tankers.  
 SUMMARY OF THE INVENTION In a preferred embodiment of the method and unit of this invention, the offshore submersible drilling and oil storage unit comprises a hull enclosing a plurality of individual, fluid-tight tanks that are secured together in a side-by-side relationship forming a unitary submersible fluid storage hull with upper and lower water-tight hull surfaces. A supporting column is fixed to the hull and projects upwardly supporting an upper platform that is normally positioned above the surface of the water when the hull is at the desired submerged draft.  
 Articulated column stabilizers are secured around the outer edge of the hull by universal connections in spaced upright positions to provide righting moments to the hull in response to angular displacements of the hull. and to provide vertical restoring forces to the hull.  
  The tanks are divided into a first group of tanks for containing air and oil and a second group of tanks for containing water and oil. Each of the tanks has fluid input and discharge connections that allow oil. water and air to be inserted in or withdrawn from the respective tanks. Fluid control means controlling the admission and discharge of the respective fluids from the tanks.  
  In initial operation. the unit is towed to a desired lo cation and the required water ballast is admitted into the second group of tanks. This reduces the buoyancy of the unit to an extent sufficient to submerge the hull, center the column and stablizing columns to the desired draft, which draft places the upper surface of the hull well below the draft of any sea going tanker. At this given draft. oil is then admitted for storage into all of the tanks through the fluid control means. The oil is passed through a manifold that distributes the oil in predetermined portions to the water tanks and air tanks. displacing the water and air from the tanks in volumes equal to the admitted oil. The amount of oil distributed to the first and second groups of tanks is proportioned to that required to maintain the same buoyance. draft and trim in the hull. Oil is then pumped from the storage tanks through the center column and platform to, for example. ocean going tankers. with the oil being removed simultaneously from the first and second groups of tanks while simultaneously admitting air and water. in the correct proportions to maintain the given draft of the submerged hull. It has been found that the proper ratio of inserting or removing oil from the first and second groups of tanks is about two to nine in volume.  
  In the construction of the hull. the air tanks are concentric around the center column. The second group of tanks for containing water and oil is concentric around the first group of tanks. Radially extending bulkhead support members hold the tanks in the desired orientation.  
  The upper platform is centrally mounted on the support column and contains machinery such as pumps. prime movers. separation equipment and the like. with the column providing access to the lower hull. Positionable upon the upper platform may be a drilling rig for offshore drilling that is replaced after completion of the well by an extendable boom that holds an oil discharge line outwardly from the outer edge of the submerged tank for communication with a tanker. barge or the like.  
  The crude oil from subsea wells enters through. for example. a flexible riser with connections at the base of the central column. The oil then passes through a gas and water separator and is then transferred by the fluid control means to the respective storage tanks.  
  The articulating stabilizing columns extend upwardly from the surface of the hull with their tips projecting just above the normal water line. These stabilizing columns provide either an upward or downward force to the hull when the water line changes. but they are not wave followers. The columns may be subjected to collision with ships. ice floes, work boats or the like. However, they are attached to the hull by articulating joints and are free to pivot on such joints and are not subject to the same degree of damage from such collisions as would be a stationary vessel. Also, the articulating columns have internal stabilizers comprising liquid filled tanks with a fluid flow restrictors in the upper portions in the columns that lengthen the natural period of oscillation of the columns and serve to detune the period from the shorter period of the waves, thus reducing the forces transmitted to the hull through the articulating joints and further improving the stabilization of the submerged hull.  
  Thus in the preferred embodiment there is provided a submersible hull enclosing a plurality of tanks with a central column and support platform and articulating column stabilizers, for drilling, oil production and oil storage, having an oil storage and stabilizing hull that is submerged well below the significant wave action of the water, with a large, low center of gravity mass that has natural heave and pitch periods that are very long such that the hull cannot be excited at such natural periods by the ocean, which hull has a large underwater drag. maintains a steady, given submerged draft and trim. and is effectively stabilized by outer protective column stabilizers against heave, pitch and roll. The tanks of the hull are completely enclosed with means for separating oil from water jettisoned from the tanks. The hull can be moved off site in case of fires, blow outs or the like, and has the oil storage at a sufficient depth and with a sufficient strength of bull to minimize the danger of tank rupture and oil spillage from collision damage and the like.  
  It is therefore an object of this invention to provide a new and improved method and semi-submersible unit for offshore drilling, oil production and storage.  
  Other objects and many advantages of this invention will become more apparent upon a reading of the following detailed description and an examination of the drawings of a preferred embodiment, wherein like reference numerals designate like parts throughout and in which:  
 FIG. l is a perspective view of the complete unit.  
  FIG. 2 is a side elevation view illustrating the under sea connections.  
 FIG. 3 is a top plan view of the unit, partially cut away.  
  FIG. 4 is an enlarged sectional view taken on line 44 of FIG. 3.  
  FIG. 5 is a diagram of the tank connections to illustrate the proportional filling to maintain neutral buoyancy.  
  FIG. 6 is a diagram of the initial towing of the unit to a work site.  
  FIG. 7 is a diagram of the intermediate stage of submerging the unit.  
  FIG. 8 is a diagram of the unit in use with a drilling rig.  
  FIG. 9 is a diagram of the unit with a tanker receiving oil.  
  FIG. 10 is a diagram of the complete fluid handling system.  
  FIG. II is an enlarged sectional view taken on line 11-11 of FIG. 3.  
  FIG. 12 is an enlarged sectional view taken on line 12-12 of FIG. 11.  
  Referring now to the preferred embodiments illustrated in FIGS. 1, 2, 3 and 4, the unit 10 comprises a submersible fluid storage hull 12 that encloses a plurality of separate tanks. A supporting column 16 is fixed at one end to the upper surface of the hull 12 at the center point thereof and a platform housing 18 is supported on the upper end of the supporting column 16. A plurality of individually, articulating, column stabilizers 14 are at fixed by universal connections 22 to the hull 12.  
  The tanks enclosed by the hull 12 are configured as sector and portions are grouped into a first group of tanks for containing air and oil and a second group of tanks 84 for containing water ballast and oil. In the construction of the tanks, the inner tanks 80 comprise typically steel bulkheads 76, 77 and 78, top walls 79 and bottom walls 81, with the steel bulkhead 76 extending radially outwardly to the outer edge of the hull l2. Arcuate portions of the cylindrical bulkhead member 77 and 78, define the inner and outer ends respectively of inner air-oil tanks 80. The outer tanks 84 comprise pre-stressed concrete tanks. The upper and lower surfaces of the hull 12, as well as the outer edge surface 86 which is the outermost arcuate member, are of concrete. Thus the air tanks 80, that have a greater pressure differential relative to the outer water environment, have a stronger construction than the tanks 84 in which ballast water at ocean water pressure is stored. An opening at the inner portion of the hull 12 is defined by the vertical column member I6, concentrically within the cylindrical bulkhead 77. Positioned in column 16 is a shaft 20 that is supported by ring support members 21. Also positioned in column 16 is a collecting tank 72 and a plurality of interconnecting pipes and lines, as will be defined in more detail hereinafter. A hawsepipe 42 extends through tanks 80 as illustrated in FIGS. 2 and 4. The anchor chains or cables 44 pass through the hawsepipe 42 and through fairlead 40 and along side column I6 to the platform l8. A normal anchor operating mechanism, such as a winch or the like, on platform 18 controls each anchor chain 44. The anchors function to hold the unit at the desired location, such as in the proximity of oil wells 32 in the ocean floor 34.  
  Fluid control means include lines 98 and connections 100 for carrying oil to and from the respective tanks 84. Lines 106 with connectors 108 are inlet and discharge lines for the passage of sea water into and out of tanks 84. Lines 102 and connectors 104 admit air in and out of the tanks 84 with line 102 forming an air vent to the surface. Lines 110 and connectors I12 admit and discharge oil into and out of the tanks 80. Lines 110 and 98, see FIGS. 4 and 10, are connected to the respective sections 119 and 121 of manifold 118 through valves I38 and 136 respectively. Valves 136 are releasable check valves with flow control for purposes that will be explained in more detail hereinafter. Lines 106 communicate with the collecting tank 72 that in turn has a line 116 that communicates directly with the outer water environment.  
  In the operation of inserting air and water ballast into the tanks, and storing and discharging oil from the tanks 80 and 84, water is admitted into tank 84 through line 116, collecting tank 72 and lines 106. Air is admitted into tanks 80 through the open air vent 102. When oil is to be stored in the respective tanks, then oil is received through a source line 30, such as from an underwater oil producing well 32, which oil passes into a separator I27 that is positioned in the upper platform housing 18. The separator separates the water and gas from the oil, which water and gas are discharged through the respective lines 128 and 130. The oil then passes through a pump 132, valve 134 and line 124 to the one side 119 of manifold 118. A portion of the oil then flows through respective flow controll valves 136 and lines 98 to the second group of tanks 84, and another portion flows through manifold valve 123 and the other side 121 of manifold 118 and through respective flow control valves 138 and the lines 110 to the respective tanks 80. When the oil enters the tanks 84, water, in proportion to the volume of oil received, is displaced from the tanks through lines 106, valve 137, collecting tank 72 and line 116 to the outer water environment. The collecting tank 72 functions to remove oil by settling, from the water which is then jettisoned overboard. The air in tanks 80 is vented through the air vent 102. When it is desired to pump oil from the tanks for oil delivery, then pumps 120 and 125 pump oil from the respective tanks 80 and 84 through sections 121 and 119 of manifold 118, line 122, valve 124 and upper pump 126 to the oil delivery line 70. Booster pumps 120 and 125 function to raise the oil to the platform 18.  
  In the overall operation the ratio of air and water in the respective tanks 80 and 84 is such as to maintain a desired buoyancy of the hull 12, that in turn maintains the desired draft of the hull 12 in the water 36. This volume relationship is such that, when the water and air is replaced by oil in a given ratio of about 2 to 9, then the hull 12 maintains the desired constant buoyancy and submerged draft. The volume of the tanks 80 relative to the volume of the tanks 84 thus conveniently may have a ratio of about 2 to 9. The section 119 of manifold 118 through lines 98 direct oil to the tanks 84, as separate from oil inserted in the tanks 80, in the same ratio of about about 2 to 9. The valve 123 is representative of several valves 123 and lines interconnecting the two manifold sections 119 and 121 to substantially reduce flow restrictions therebetween when valves 123 are open. Valve 123 is controlled through line 173 by a known control mechanism 200. Thus the tanks 80 and 84 may be simultaneously filled with oil. This filling of the tanks in a ratio of about 2 to 9 can be accomplished by any suitable methods, such as by having multiple connections 100 and lines 98 for each of tanks 84, or by having lines 98 be sufficiently larger in diameter than lines 110 to tanks 80 to provide a nine to two oil flow relationship through the respective lines, or by using suitable means to achieve the correct flow relationship, such as controlling the flow through the respective valves 136 and 138. The valve openings through valves 136 and 138 are selectively controlled through the respective control lines 204 or 171 by a known control mechanism such as the central valve control unit 200. This same volume distribution relationship applies equally to the withdrawal of oil from the tanks through the distribution system by pumps 120 and 125, as controlled by valves 136 and 138 and valves 123.  
  The water replacing the oil in tanks 84 during the withdrawal of oil from the tanks is forced into the tanks through line 116 by the normal depth pressure on the water. Thus, when pump 132 is not in operation, check valves in valves 136 in line 98 function to prevent the back flow of water through tanks 84 and through the manifold 118 sections into the air tanks 80. In operating pumps 120 and 125 in pumping oil from tanks 80 and 84, it is necessary to disengage the check valve portions of valves 136 to permit the reverse flow of the oil. In this mode, valves 123 are then closed to prevent cross flow of water from tanks 84 to through manifold 118. Valves 136 and 138 are adjusted to control the flow. The operation of the pump 132 to pump oil into the tanks 84 generates sufficient oil pressure to force the water from tanks 84 through valves 137. The disengagement of the check valve portions of the valves 136 and the opening and closing of valves 123 may be controlled manually or by automatic control means through lines 171 and 173 from the central valve control unit. Valve 137 in line 106 functions to control the amount of water and its rate of flow into tanks 84.  
  With reference to FIG. 5, the ratio of nine tank volumes for water ballast in tanks 84 is illustrated relative to the two air tanks 80. This provides the constantly correct water and air versus oil ballast in the hull l2 and the unit 10 for normal specific gravities of sea water and crude oil. Known water and oil interface detectors 170 are positioned in each of the respective tanks 84 and provide signal information output through lines 202 to a central valve control unit 200. Similar air 160, oil 162 interfaces in tanks 80 are detected by probe 164 that supplies the signal information through line 204 to the central valve control unit 200. The central valve control unit 200 through return control lines 204 controls the flow through the respective valves 136 and 138 to maintain the correct relative oil volume in all of the respective tanks 80 and 84. The central valve control unit also allows the respective tanks 80 and 84 to be trimmed as necessary to trim the overall position of the housing 12 and the unit 10.  
  Each of the articulating column stabilizers 14 comprise a cylindrically shaped elongated column that is connected at one end by a universal joint 22 to the upper surface of the hull 12 at a location adjacent to the end of the bulkhead member 76. Referring to FIG. 11, the universal connector comprises a base member having a flange 97 secured to the upper surface of the bulkhead member 76 within the layer of concrete 96. A U-shaped saddle 90 receives a shaft on which is mounted connector 88 that is in turn connected by shaft 94, arranged in a normal angle to shaft 95, to connector 92 that is fixed to the column stabilizer 14. This connector 22 allows universal movement of the column stabilizer 14 in the articulating movement relative to the housing 12.  
  The column 14 has a plurality of vertically oriented, sealed compartments. The upper compartments have an outer ring shaped volume 144 and a center volume 140 that are separated into levels by bulkheads 142. Each of the compartments 144 have a fluid, such as water or the like 152, and an air space 150. Baffles, that comprise tubular pipes 146 are positioned in the center of the volume 144 at opposite sides thereof, to restrict the flow of the liquid 152 between the side volumes 156, 157, 158 and 159. This restriction of water flow stabilizes the columns and reduces the load force exerted by hinges 22 onto the hull 12. The articulated columns 14 are thus passively stabilized by using the free water surface effect at the multiple levels. The lower chamber 154 contains air. The baffle members 146 which are used in controlling the flow 148, are arranged to detune oscillations of the coulumns 14 caused by wave action, to reduce the hinge loads significantly.  
  In placing the unit 10 in operation. the tanks 84 are normally initially ballasted through valve 137 only the amount rcquired to provide the desired surface draft oi the hull 12. Valu: 135 is opened as necessary to hit. cd air or gas from the tanks. in this condition. the articulating columns. see FIGS. 6 and 7, rest on the upper surface of the hull l2 and may be tied down thereon. A suitable ship, such as a tug 52, connects onto the hull 12 by line 50 and connected 48 and pulls the unit 10 to the desired site. When the desired site is reached, the unit 12 is then anchored in place and the valves 137 are opened and sea water is allowed to flow into the tanks 84 filling the tanks 84. sufficiently to provide the desired neutral buoyancy of the unit 10. The housing 12 then submerges in the water to the desired draft where the buoyancy of the column 16 relative to the buoyancy of the hull l2 and articulating members 14 estab lishes the given draft of the hull 12. Valve 37 is then closed and the anchor lines 44 are then set out as necessary to hold the position of the unit 12 against drift. An oil rig 54 my be installed on the platform housing l8 with the drill string apparatus passing through shaft 20. When it is desired to convert the unit 12 into a production and storage unit, then the drilling rig 54 is removed and lines 30 to oil producing wells 32 are at tached, Oil then passes through the separator 125 and manifold 118 to the respective tanks 80 and 84 with the concurrent and simultaneous discharge of water and air from the respective tanks maintaining the given draft of the hull 12. When it is desired to offload oil from the storage tanks in hull 12, then a suitable boom 62 is mounted on the platform 18 and has a counter weight 66 and positioning rollers 64. An upper flow discharge connector housing 68 allows a discharge line 70 to be connected to the pump 126. The boom 62 is sufficient to carry the discharge line 70 to a tanker 60. The tanker may be connected to the hull l2 by mooring tether 48 and S0. A stern buoy 58 maintains a relative orientation of the tanker 60 relative to the unit 10. It should be recognized that the relative sizes of the boom 62 in FIG. 9 and the drilling rig 54 in FIG. 8 are for example purposes only, and do not reflect their true size, other than the boom 62 has sufficient length to extend beyond the outer edges of the submerged hull 12.  
  As an example of the size of the unit for a storage ca pacity of 1 million barrels. the outer diameter of the tank 12 is 450 feet, the diameter of the inner bulkhead 78 is 200 feet, and the height of the hull 12 is 50 feet. and the articulated columns are feet in diameter and 90 feet long.  
  Since the water in the tanks is always at the bottom of the sea water tanks 84, and since the oil is always at the bottom of the air tanks 80, the hull l2 always has the maximum low center of gravity providing the maximum stability to the unit 10.  
 Having described my invention, I now claim:  
  1. A semi-submersible drilling and oil storage unit for use offshore in a body of water comprising,  
 a plurality of individual, fluid-tight tanks secured together in a unitary, submersible fluid storage hull having upper and lower hull surfaces,  
 said tanks are separable into a first group of tanks for containing air and oil and a second group of tanks for containing water and oil,  
 each of said tanks having fluid input and discharge connections,  
 fluid control means tor inserting through said fluid connections :nr into said first group of tanks and water ballist into said second group of tanks giving said hull a g1 vcn uoyancy to submerge said hull to a given submerged draft.  
 said fluid control means including first means for inserting oil into said first and second group of tanks and simultaneously expelling air and water from said respective tanks in proportions to maintain the given buoyancy of said hull and the given submerged draft.  
 a supporting column fixed at one end to said hull and extending upwardly therefrom.  
 said column having sufficient length that the upper end projects above the surface of said body of water when said hull is submerged at said submerged draft,  
 and a plurality of individually articulating column stabilizers each fixed at one end by a universal connection to the upper surface of said hull, whereby said stabilizer columns may extend upwardly from said hull. said stabilizer columns being positioned around the outer edge of said hull.  
 2. An offshore semi-submersible drilling and oil storage unit as claimed in claim 1 wherein,  
 said hull has a center hole therethrough for communicating with said supporting column,  
 and said first and second group of tanks comprising substantially the entire volume of said hull, less said center hole.  
 3. An offshore semi-submersible drilling and oil storage unit as claimed in claim 1 including.  
 a platform fixed to the upper end of said supporting column,  
 boom means positionable on said platform and extending outwardly beyond the outer edge of said hull,  
 oil line means carried by said boom means for making an oil discharge connection with a ship,  
 and means for removing oil from said tanks and passing said oil to said oil line means.  
 4. An offshore semi-submersible drilling and oil storage unit as claimed in claim 1 including,  
 a platform being fixed to the upper end of said column.  
 means for positioning a drilling rig on said platform,  
 and said platform, column and said hull having openings to pass a drilling connection therethrough.  
 5. An offshore semi-submersible drilling and oil storage unit as claimed in claim 1 wherein,  
 said first and second groups of tanks are secured together in side-by-side relationship around a central point,  
 said first group of tanks being arranged ground the center portion of said hull with the inner ends of said tanks proximal said central point,  
 and said second group of said tanks disposed concentrically outwardly from said first group of tanks.  
 6. An offshore semi-submersible drilling and oil storage unit as claimed in claim 5 wherein,  
 the inner ends of each of said first group of tanks are spaced from said central point to provide an opening through said hull,  
 and said supporting column is positioned to communicate with said opening.  
  7. The method of maintaining a given submerged draft of a semi-submersible offshore drilling and oil storage and production unit comprising,  
 submerging a hull having a plurality of individual fluid-tight tanks secured together in side-by-side relationship and a central column fixed at the center of said hull,  
 admitting air at substantially atmospheric pressure into a first group of said tanks and admitting seawater ballast into a second group of said tanks until said unit has neutral buoyancy with said tanks submerged to a given submerged draft,  
 and inserting oil simultaneously into said first and second group of tanks in proportions to maintain said given submerged draft of said tanks 8. The method as claimed in claim 7 being characterized by,  
 said air and seawater being inserted into the first and second groups of tanks in a volume ratio of about 2 to 9.  
 9. In the method as claimed in claim 8 including the steps of,  
 drawing oil simultaneously from the first and second groups of tanks in a volume ratio of about two from the first group of tanks and about nine from the second group of tanks,  
 and simultaneously replacing the oil drawn from the first group of tanks with seawater and oil drawn from the second group of tanks with air at substantially atmospheric pressure.  
 10. A semi-submersible oil storage unit for use offshore in a body of water comprising,  
 a plurality of individual, fluid-tight tanks secured together in a unitary, submersible fluid storage hull having upper and lower hull surfaces,  
 said tanks being separable into a first group of tanks for containing air and oil and a second group of tanks for containing water and oil,  
 each of said tanks having connections for fluid input and discharge,  
 fluid control means for inserting through said fluid connections air into said first group of tanks and water ballast into said second group of tanks giving said hull a given buoyancy to submerge said hull to a given submerged draft,  
 said fluid control means including first means for inserting oil into said first and second group of tanks and simultaneously expelling air and water from said respective tanks in proportions to maintain the given buoyancy of said hull and the given submerged draft, and  
 a plurality of stabilizers positioned around the outer edge of said hull each said stabilizer being attached by an articulating connection to said hull and extending upwardly therefrom, whereby each said stabilizer may articulate with said hull.  
 11. An offshore semi-submersible oil storage unit as claimed in claim 10 wherein,  
 said fluid control means includes second means for withdrawing oil simultaneously from said first and second group of tanks and simultaneously inserting air in to said first group of tanks and water in to Said second group of tanks in proportions to maintain the given buoyancy of said hull and the given 6 submerged draft. 12. An offshore semi-submersible oil storage unit as claimed in claim 11 wherein,  
 said first group of tanks have a fluid volume ratio to said second group of tanks of about 2 to 9. 13. An offshore semi-submersible oil storage unit as claimed in claim 10 wherein,  
  said first means of said fluid control means has means for simultaneously inserting oil in to said first and second groups of tanks in volume proportion of about 2 to 9,  
 14. An offshore semi-submersible oil storage unit as claimed in claim 11 wherein,  
 said second means of said fluid control means includes means for simultaneously withdrawing oil from said first and second groups of tanks in vol ume proportions of the ratio of about 2 to 9. 15. An offshore semi-submersible oil storage unit as claimed in claim 10 wherein,  
  said first group of tanks have a fluid volume in ratio to the fluid volume of said second group of tanks of about 2 to 9. 16. An offshore semi-submersible oil storage unit as claimed in claim 10 wherein,  
 each of said stabilizers comprises an elongated watertight vessel having said articulating connection attached at one end thereof and having a plurality of separate water-tight compartments, at least one of said compartments having a stabilizer liquid therein, and means in said compartment for restricting the liquid flow from one portion of the compartment to the other portions of the compartment, providing fluid flow stabilization of the articulating movement of said stabilizer. 17. An offshore semi-submersible oil storage unit as claimed in claim 16 wherein,  
 said stabilizers have a cylindrical shape, said compartment comprises a ring shaped chamber around the outer surface of said vessel, and said fluid flow restricting means including at least three uniformly spaced restrictors in said ring 40 shaped chamber that restrict the flow of fluid therethrough. 18. An offshore semi-submersible oil storage unit as claimed in claim 17 wherein,  
 said compartment is positioned adjacent the free end of said stabilizer, said restrictors comprise tubular members positioned in substantially the radial center of said ring compartment and have a diameter less than the width of said compartment, and said stabilizer in the upright position have sufficient length that only the tips of the free ends thereof extend above the water surface when said hull is at the given submerged draft. 19. An offshore semi-submersible oil storage unit as claimed in claim 10 wherein,  
 said fluid control means includes means for selectively inserting suitable proportions of oil in selected tanks of said first and second group of tanks to maintain a desired trim to said hull. 20. An offshore semi-submersible oil storage unit as claimed in claim 10 including,  
 separator means for separating the oil from the water discharged from said second group of tanks. 21. An offshore semi-submersible oil storage unit as claimed in claim 10 wherein,  
 each of said stabilizers comprises an elongated sealed vessel having a plurality of sealed compartments for containing ballast providing each of said stabilizers with a substantially neutral buoyancy.  
 22. An offshore semi-submersible oil storage unit as claimed in claim l wherein.  
 said first and second groups of tanks are secured together in side-by-side relationship around a central point.  
 said first group oftanks are arranged around the cen&#39; tral portion of said hull with the inner ends of said tanks proximal said central point,  
 and said second group of said tanks disposed concentrically outwardly from said first group of tanks.  
 23. An offshore semi-submersible oil storage unit as claimed in claim 22 wherein,  
 said tanks are configured in radially outwardly ex tending sector portions whose ends are defined by concentric arcuate members,  
 said tanks comprise an inner tank of said first group and an outer tank of said second group.  
 and a circumferentially continuous bulkhead separates each of said first group of tanks from said concentrically outward second group of tanks.  
 24. An offshore semi-submersible oil storage unit as claimed in claim 23 wherein,  
 said inner tanks are constructed of steel bulkheads.  
 and said outer tanks are constructed of pre-stressed concrete.  
 25. An offshore semi-submersible oil storage unit as claimed in claim 23 including,  
 a plurality of bulkhead members radially extending from the innermost arcuate member to the outermost arcuate members,  
 and each of said stabilizers being fixed to said hull adjacent the outer end of at least one of said radially extending bulkhead members adjacent said outermost arcuate member.  
  26. The method of maintaining a given submerged draft of a semi-submersible offshore oil storage unit comprising,  
 submerging a hull having a plurality of individual fluid-tight tanks secured together in side-by-side relationship.  
 pressurizing a first group of tanks with air at substan tially atmospheric pressure and admitting seawater ballast into a second group of said tanks until said unit has neutral buoyancy with said tanks submerged to a predetermined submerged draft,  
 and admitting oil simultaneously into said first and second group of tanks in proportions to maintain said predetermined submerged draft of said tanks.  
  27. The method as claimed in claim 26 wherein the volumes of said air and said seawater are in the ratio of about 2 to 9 respectively.  
  28. The method as claimed in claim 26 further including the steps of,  
 withdrawing oil simultaneously from said first and second groups of tanks in proportions to maintain said predetermined submerged draft and simultaneously replacing the oil withdrawn from the first group of tanks with seawater and the oil withdrawn from the second group of tanks with air.  
  29. The method as claimed in claim 28 wherein the volumes of oil withdrawn from said first and second groups of tanks are in the ratio of about 2 to 9 respectively.