US3553969A

US3553969A - Submerged oil storage structure

Info

Publication number: US3553969A
Application number: US786281A
Authority: US
Inventors: Robert S Chamberlin; William A Davis; James E Stevens
Original assignee: Chicago Bridge and Iron Co
Current assignee: Chicago Bridge and Iron Co
Priority date: 1968-12-23
Filing date: 1968-12-23
Publication date: 1971-01-12
Anticipated expiration: 1988-01-12
Also published as: NL6917496A; DE1956328A1; JPS5018846B1; FR2026906A1; GB1266247A

Abstract

AN OFFSHORE STRUCTURE HAVING AN ELONGATED SHAFT VERTICALLY POSITIONED IN A BODY OF WATER AND PIVOTALLY CONNECTED AT ITS BOTTOM END TO A WEIGHT MEANS ON THE FLOOR OF THE BODY OF WATER. THE WEIGHT MEANS CAN BE A TANK FOR STORING OIL.

Description

Jan. 12', 1971 R. s.cHAMBl-:RL|N Erm.

A SUBMERGED OIL STORAGE STRUCTURE 6 Sheets-Sheet l Filed Decpza, 1968 /NrfA/rags 205er@ 5 Cd//PzeWJ/k/ WM M6.; cf fiez/19724 #rrazA/fr Jam 12,131971 R.`s.cHAMBERL|N ETAL 3,553,969

` SUBMERGEDVOIL STORAGE STRUCTUREV Y 6y Sheets-Sheet 2 Filed Ded-T 23. '196e 'er @www y fling.

R. S. CHAMBERLIN I'AL SUBMERGED OIL STORAGE STRUCTURE I \\\7// 77, \`y/ f//\\\y// w L` w L 5 w fp` Enea neef. 25. 1968 Jan. 12,1971

' Jan. 121971 R,SCHVAMBERL|N ErAl-.VA' 3,553,969

Y Y Y SUB'MERGED OILv STORAGE STRUCTURE Y 'Filed Dec. 23,V 196e.

6 Sheets-Sheet 4 SUBMERGED OIL STORAGE STRUCTURE 'Filed necf. .23, 1968 f 6 Sheets-Sheet 5 /NVE/VTOES R. s.` CHAMBERLIN ET AL 3,553,969 SUBMERGED on. STORAGE STRUCTURE rumbas. 23,'1968 6 sheets-sheet s /NVA'A/ 70495 United States Patent O 3,553,969 SUBMERGED OIL STORAGE STRUCTURE Robert S. Chamberlin, Western Springs, William A. Davis, Glen Ellyn, and James E. Stevens, Palos Park, Ill., assignors to Chicago Bridge & Iron Company, Oak

Brook, Ill., a corporation of Illinois Filed Dec. 23, 1968, Ser. No. 786,281

Int. Cl. E02d 27/52 U.S. Cl. 61-46 4 Claims ABSTRACT F THE DISCLOSURE An offshore structure having anelongated shaft vertically positioned in a body of water and pivotally connectedV at its bottom end to a weight means on the oor of the body of water. The Weight means can be a tank for storing oil.

This invention relates to apparatus and equipment used in the production of oil. More particularly, this invention is concerned with devices and structures useful for offshoreexploration for oil, the drilling of wells and the storage of oil produced from such wells.

To meet the ever increasing need for petroleum products, the exploration for and production of oil has been extended from land to offshore locations. Exploration and production of oil from offshore locations requires the drilling of wells. While the drilling can be effected from floating platforms it is generally more eicient to support the drilling platform above the water surface on members which rest on the lioor of the body of water, 'which normally is the ocean oor. Offshore drilling platforms supported `by the ocean floor are in extensive use around the world. However, most such structures are employed where the water is comparatively shallow, such as upto about 100 to 150 feet in depth.

There are many offshore locations where it is believed oil could be produced but where it would be necessary to effect drilling in comparatively deep water, such as up to 600 or more feet in depth. The very depth of the water in such locations renders unsatisfactory the use of most drilling rigs or towers presently employed in shallow water offshore locations. Most of the structures now in use could not readily and safely be installed at deep water sites because of inherent engineering problems as well as the lack of structural strength of equipment used in the shallow sites. Accordingly, there is a substantial need for apparatus and equipment which can be readily positioned at deep water offshore sites for use in drilling for oil and for producing and storing the oil obtained from such Wells.

There is accordingly provided by the subject invention novel structures for use in olshore deep water drilling and production of oil. The structures provided by this invention rest on the floor of the body of water, such as an ocean, and extend upright to a height above the surface of water commensurate with the need for supporting drilling and pumping equipment as Well as auxiliary apparatus used therein.

VThe novel structure of this invention is characterized by an elongated shaft set vertically when the structure is in position offshore. The lower end of the shaft is pivotally connected to a suitable anchoring means which rests on, and is in contact with, the floor of the body of water.

The elongated shaft can be a tubular member, a trussed member or any combination of such members. Thus, each end of the shaft can be a tubular member joined to an intermediately positioned truss member. Also, the shaft can comprise a plurality of tubular members or truss 3,553,969 Patented Jan. 12, 1971 ice members or a combination of such members vertically placed in proximity to one another.

Pivotal connection of the lower end of the shaft to the anchoring means is advisably made by a connection which has little if any lateral drift or movement. A universal joint is particularly useful for making the pivotal connection although an elastomeric member which permits pivotal movement is also satisfactory. The pivotal connection must be of suflicient strength to withstand the forces applied thereto during assembly of the structure, transportation to an offshore site, positioning on the ocean oor and subsequent use as well as all weather conditions.

The anchoring weight resting on the iloor of the body of water can be in the form of a pile driven into the floor, a simple Weight of sufficient size to securely hold the lower end of the shaft in pivotable position, or the anchoring weight can comprise a tank, joined at a suitable location thereon, to a pivotal connection such as a universal joint. When a tank is used as the anchoring weight to which the shaft is joined by means of the pivotal connection, the tank can be used for the storage of oil produced from offshore wells.

The elongated shaft is maintained substantially vertical by buoyancy means associated with, or an integral part of, the shaft, or by means of guy lines, or by a combination of buoyancy means and guy lines. Because of the forces applied to the shaft by waves, ocean current and wind it is not reasonably possible to prevent excursion of the shaft from true vertical position. The invention however encompasses as an important feature the use of buoyancy means and/or guy lines to restrict the angle of excursion from the Vertical of the elongated shaft, so that conductor pipes and well pipes supported by the shaft and driven into the ocean floor will not be overstressed and rupture.

When guy lines are used they can be joined at one end to the elongated shaft and at the other end to anchor means either in the oor of the body of water or to other suitable means which restricts undue tilting or excursion of the elongated shaft.

The structure of this invention also advisably includes pile means which prevents lateral movement of the anchoring means on the door of the body of water. It is advisable, and often essential, to prevent lateral movement of the anchoring means to avoid rupturing or overstressing conductor pipes and well pipes. Accordingly, piles are generally used to secure the anchoring means in lixed position ou the ocean floor. The conductor pipes, if driven into the floor of the body of water sulliciently far, can function as the piles or separate piles, apart from the conductor pipes, can be used.

Other features and aspects of the invention will be described hereinafter in conjunction with the attached drawings, in which:

FIG. l is an elevational view showing one embodiment of the invention positioned at an offshore site FIG. 2 is an enlarged partial vertical sectional view of th lower part of the structure of FIG. l and illustrates the universal joint connecting the supporting weight and the elongated vertical tube shaft, as well as a conductor pipe for well drilling and oil production;

FIG. 3 is an isometric view showing a weight bearing inflexible universal joint such as used on the structure of FIGS. 1 and 2;

FIG. 4 illustrates another embodiment of the invention in which the elongated tube extending above the surface of the water is joined at its lower end through a weight bearing inflexible universal joint to the top portion of a conical tank which rests on the floor of the body of water;

FIG. 5 is an enlarged side View, partially in section, of the lower portion of the structure of FIG. 4 and illustrates the conductor pipes used for well drilling and oil production;

FIGS. 6 and 7 illustrate how the structure of FIGS. 4 and 5 can be transported on the water surface to an offshore site and then submerged at the desired location;

FIG. 8 illustrates another embodiment of the invention in which a lower tank with a separate internal vessel or bottle aids submergence of the structure. The figure also shows float means which applies additional tension to the guy lines with tilting of the shaft thus stabilizing the elongated hollow tube shaft part of the structure;

FIG. 9 illustrates the offshore structure of FIG. 8 with a different system for applying additional tension to the supporting and stabilizing guy lines;

fFIG. l illustrates still another embodiment of the invention, in many ways similar to that of FIG. 8 but having part of the elongated shaft as a trussed member with tubular portions at each end;

FIG. l1 is an enlarged elevational view of the lower part of the structure of FIG. 10 and illustrates the universal joint connecting the tank to the shaft, the internal vessel or bottle in and forming part of the tank, and also conductor pipes and supporting members for oil well drilling and production;

FIG. 12 is a sectional View taken along the line 12-12 of FIG. ll and shows the spoke-like arrangement of pipes which stabilize the internal vessel or bottle relative to the walls of the tank;

FIG. 13 is an elevational view showing a supporting ring and related braces for holding the conductor pipes in place in the structure of lFIGS. 8 to 10 and 11;

FIG. 14 is a front elevational view of the inexible trunnion type universal joint used in the structure of FIGS. 8 to 10 and ll;

FIG. l is a side elevational view of the universal joint of FIG. 14;

FIG. 16 illustrates still another embodiment of the invention in which the tank at the bottom of the structure is similar to that shown in FIGS. 8, 9 and l0 but the elongated hollow tube shaft is made in two sections joined together by a pivotal connection with the lower section joined at its lower end to the tank by a pivotal connection;

FIGS. 17 to 21 show an operating sequence for submerging the structure of FIGS. 8 to 10 and positioning it on a site offshore.

So far as is practical, the subsequent discussion will employ the same descriptive number for elements which are identical or similar appearing in one or more of the figures comprising the drawings. It should be understood furthermore that the embodiments of the drawings are merely illustrative of the invention and that the scope of the claims is not to be restricted to these embodiments unless required by specific language forming part of such claims.

FIG. 1 illustrates a structure according to the invention in which the shaft is an elongated hollow tube 10 joined by a pivotal connection in the form of a universal joint 11 to anchor means 12, which may either rest on the oor 13 of the body of water 14 or which may be driven into the oor 13 and thus form a supporting pile. The elongated hollow tube 10 can be open or closed at the bottom depending on the use to which the structure is to be put. To avoid the necessity of using heavy metal Walls for tube 10 it is generally advisable to always have the inside lower space of tube 10 filled with a liquid when subjected to substantial water pressures. The empty upper part can provide -buoyancy in an amount suiiicient to limit excursion of the shaft. The liquid in the lower part can be either water from the surrounding sea or oil produced from wells in the area. However, entire t-ube 10 can be pressurized with air provided suitable bulkheads and/or end closures are used to prevent the pressurized air from escaping. Pressurizing the tube in this manner, or lling it with oil or water, serves to counteract the hydraulic pressure exerted on the tube by the sea and thereby permits construction of the tube of thinner walled metal than would otherwise be necessary.

To minimize the excursion or tilting of elongated tube 10 by wave, wind and current forces, a series of guy lines can be joined at their upper ends to the tube and at their lower ends to suitable anchoring structures, such as a series of piles driven into the floor of the body of water. The guy lines can be joined to tube 10 considerably below the surface of the water so that they are not struck and cut by servicing tugs, barges and ships.

If advisable, any suitable platform can be mounted on top of tube 10. The type of platform used, if any, will depend on the operations to be performed at the offshore site. If a platform is used it generally will be mounted sufficiently high on tube 10 so that it is not affected by waves, whether the tide is high or low. The structure of FIG. l, while shown as a tube, can be an open bridged or trussed shaft if desired. A tube is much more satisfactory, however, since it aids in submerging the structure and can be used for oil storage.

FIG. 2 of the drawings shows the lower part of the structure of FIG. l in greater detail. Thus, the anchoring weight 12 is provided with a plurality of vertically positioned pipe sections 16 which extend completely through the anchor base 12. Similarly, pipe sections 17 extend through the end of tube 10. In addition, pipe sections 19 are supported by flanges 20 mounted in alignment periodically on the inside of tube 10. After the structure has been placed in position offshore, conductor pipe 21 is slid through guiding pipe section 19, through pipe section 17 and through pipe section 16 into the oor of the body of water for a substantial distance. After that has been done, grout 22 is placed in pipe section 17 and grout 23 is placed in pipe section 16 to securely hold conducting pipe 21 in position and to prevent flow of liquids therebetween. Conductor pipe 21, of which there will be a plurality in the structure, can be used for the drilling of wells and for the removal of oil once a well has been completed.

The internal space of hollow tube 10 can be used, if desired, for the storage of oil produced by Wells associated with the structure or from other nearby Wells. Oil can be pumped into tube 10 and, if water is therein, it will be displaced as the amount of oil increases. Suitable valve means can be provided as required to permit water to be expelled from the bottom portion of the tube as it is filled with oil. Such valves can of course be used to let water into the tube as oil is removed therefrom.

The drawings in FIGS. 3 to 5 illustrate another embodiment of the invention. This embodiment is characterized by an elongated tube 30 joined at its lower end to tank 36 through a universal joint 32. Thus, the elongated tube 30 is provided with extending members 31 which are connected to a trunnion member 33 which in turn is connected to trunnion members 34 associated with the top 35 of the vessel or tank 36. A conventional platform 37 is positioned at the upper end of tube 30. The tank or vessel 36 is provided with vertical weighted walls 38 (FIG. 5) such as concrete covered with steel plate. Bottom 39 extends to the walls and thereby closes 01T the interior of the tank from the water. A plurality of conductor pipes 40 extend from platform 37 down through sleeves 41 in the roof 35 of tank 36 and through sleeves 42 positioned in the bottom 39 of the tank. Sleeves 42 extend through the bottom 39 in the tank 36. Suitable grout is placed in

sleeves

41 and 42 after conductor pipes 40 have been put into position. If tube 30 does not provide buoyancy needed to maintain the excursion of the tube within reasonable limits, a plurality of guy wires connected at their upper ends to tube 30 and at their lower ends to a plurality of piles 44 driven into the floor of the body of water can be added. The guy wires will serve to limit tilting of the tube due to Wave, wind and water current forces.

The universal joint illustrated in greatest detail in FIG. 3 is inflexible, meaning that it can withstand both compressive and tensile forces without varying the spacial relationship, either vertically or laterally, between the tank 36 and the lower end of tube 30. However, the universal joint permits pivotable movement of the lower end of tube 30 while the tank 36 is in stationary position on the floor of the body of water".

FIGS. 6 and 7 illustrate a technique used for transporting the structure of FIGS. 3 to 5 to an offshore site and its submergence at such location. Because of the dfliculty of assembling the universal joint while at sea to connect tube 30 to tank 36, it is advisable to join these components by means of the universal joint while they are on shore or stabilized in shallow water. The assembled structure is then floated with the tube 30 and the tank 36 empty and towed out to the site where it is to be submerged. During towing the structure will be positioned as shown in FIG. 6. When the offshore site has been reached, Water is pumped, into tank 36 and air vented therefrom. The tank 36 sinks slowly as it is filled with water. After tank 36 has been lled withwater, water is pumped into tube 30 which can be bafed or have bulkheads positioned therein to form distinct compartments which can be selectively flooded with water to achieve controlled submergence. Thus, a lower compartment in tube 30 would be filled with water followed by a consecutive lling of compartments thereabove until the structure has been lowered to rest on the floor of the body of Water. One or more upper compartments in the tube can provide buoyancy to hold the tube vertical and restrict its angle of excursion by wind, waves and current. After the structure has been so submerged, guy wires also can be used to hold tube 30 vertical.

The structures for olfshore drilling and oil production shown in FIGS'. 8 to 12 are different froml those previously described primarily in that the lower tank 50 is provided with an internal vessel or bottle 51, as shown more clearly in FIG. ll. The bottle 51 is positioned vertically, has a cylindrical body 43, a hemispherical bottom 52 and a truncated conical upper end 53. The lower end of vessel or bottle 51 has a cylindrical skirt 54 to which the inner ends of reinforcing pipes 55 are connected and which run outwardly therefrom like spokes, as shown in FIG. 12,. to join concrete weighted walls 56 of tank 50. A ringlike member 57 (FIGS. 11 and 12) surrounds the upper part of bottle 51 and to it is joined conical roof 58 of tank 50. The lower end or edge of roof 58 is joined to cylindrical walls 56. The bottle or vessel 51 is primarily employed during submergence of the structure at an olfshore site as will be described hereinafter.

[The'conical roof.58 is provided with reinforcing T- beams 59 (FIG. 11), In addition7 the upper portion 60 (FIG. 1l) of wall 56 of tank 50 is4 left hollow to provide buoyancy when submerging the structure. l

The tank 50, in FIGS. 8, 9, 1l and '12, has upper elongated hollow tubular shaft 65 on which a platform 66 is mounted above the water level. The tube 65 is joined to tank 50 yby means of a double trunnion type universal joint 67 as is shown in more detail in FIGS. 14 and l5.

FIG. shows an embodiment of the invention having an elongated shaft 101 supported by universal joint 67 on tank 50. Shaft 101 has a lower tubular portion 102, an intermediate trussed portion 103 and an upper tubular portion 104. Tubular portion 102 can be used to store oil or it can be weighted, such as with concrete, to help hold tank 50 in position when lled with oil. Upper tubular portion 104 can be left empty to provide needed buoyancy to maintain shaft 101 vertical with limited pivotal excursion.

The universal joint 67 shown in FIGS. 14 and 15, and used in the structures of FIGS. 8 to 1l, comprises a. trunnion block 70 which has two pairs of opposing pins with the pins of each pair being axially positioned. Opposing pins 71a and 71b comprise one pair and opposing pins 72a and 72b comprise another pair. A pair of arms 73 connected to the conical end 74 of tube 65 project downwardly and have receiving holes which surround and hold the opposing pair of pins 71a and 71b. Similarly, upwardly projecting legs 75, which are joined to the upper conical part 53, project upwardly and by means of holes near the end thereof receive the pair of opposing pins 72a and 72b thus completing the trunnion based universal joint.

Once the structures as shown in FIGS. 8 to ll have been submerged and positioned at an offshore site, conductor pipes are fed downwardly from the platform 66 through guides 81 as shown more fully in FIGS. l1 and 13. Each guide 81 comprises a metal ring 82 which is supported around tube 65 by means of

braces

83 and 84. Ring 82 contains a number of tubular sleeves 86 placed therein to receive and hold conductor pipes 80 in position. The holes formed by sleeves 86 must be aligned so conductor pipe 80 can be threaded therethrough from the top downwardly and into the series of corresponding holes 87 positioned in ring 57 at the top of bottle 51. As shown in FIG. 1l, conductor pipes 80 extend through tank 50 and into the floor of the body of water.

If guy lines are needed to stabilize the elongated shaft, it is advisable to use guy lines which will permit an increase in tension thereon only up to a predetermined limit after which the tension on the guy line will not be increased with further reasonable increase in tilt of the shaft. The guy line systems which 'will now be described, which however are not our invention, permit substantially constant and controlled guy line loads after a predetermined tension is reached. This avoids forming high loads induced by wave action which might otherwise cause breaking of the guy lines or inducement of destructive vibratory action in the structure.

One such guy line system which gives comparatively constant and controlled loads is shown in FIG. 8 where the guy line 90 is fastened at one end to tube 65 and at the other end to pile 91 driven into the floor of the body of water. Intermediate the ends of guy line 90 is attached line 92 which extends upwardly to hollow sphere 93. A plurality of such float controlled guy lines is employed arranged around the structure. When stron-g wind or wave action is applied to tube 65 it will tilt such as to position 95, shown in phantom in FIG. 8, causing the guy line to move to position 90a and float 93 to submerge to position 93a. Because of the water displacement by oat 93 when it submerges, it induces a positive buoyancy force in the guy line and pulls it upwardly thereby maintaining a relatively constant load on tube 65 so long as the float is submerged. Of course, during submergence of float 93, tension will be increased proportional to the water displaced by the oat.

Another guy wire arrangement for achieving constant and controlled guy line loads is shown in FIG. 9. In this gure, `guy line is fastened at its uper end to tube 65 and at its lower end to pile 101 driven into the oor of the body of water. Intermediate the ends of guy line 100 is positioned heavy weight 102 which rests on the floor of the body of Water when tube shaft 65 is in vertical position. Wave action, water currents and wind can cause tipping of the tube 65 and a resulting increase in tension on guy line 100, which when sufliciently large, will result in lifting of weight 102 from the Hoor of the body of water. Thus, tension can increase in guy line 100 by such tilting until it reaches a value suiciently high to cause lifting of weight 102 and thereafter the increase in tension is minimal, resulting in a rather constant and controlled guy line load.

FIGS. 17 to 21 of the drawing show sequentially a recommended procedure for submerging the structures described in FIGS. 8, 9, 10 and 11.

The structures of these figures can have a tube or shaft 65 about 50() feet long and 30 feet in diameter connected by a trunnion type universal joint 67 to tank 50 having an overall height of about feet and a diameter of about 200 feet. The bottle 51 can have a diameter of about 50 feet. As shown in FIG. 17, the structure is assembled on shore or in a graving dock and then floated to a shallow area from which it can be towed. The bottom of tank 50 is left open to the water and is left open at all times even after the structure has been submerged. A baille 120 is placed in the upper part of tube 65 and the portion 121 thereof is flooded with water. The structure is then towed to location. Once the structure has reached the site, air is removed from tank 50, with bottle 51 and air spaces 60 (FIG. 11) in the walls of the tank providing buoyancy (FIG. 18). Then space 60 in wall 56 is flooded, water is emptied from space 121 in tube shaft 65 and water added to bottle 51 to about half fill it therby causing the structure to submerge to a depth of about 200 feet and take the position shown in FIG. 19. The addition of water to fill bottle 51, and with shaft 65 under air pressure, lowers the structure further such as to a depth of about 400 feet as shown in FIG. 20. Water is then added to shaft 65 until the structure lands on the fioor of the body of water at a depth of about 600 feet as shown in FIG. 21.

FIG. 16 illustrates a structure, employing two universal joints, which could be used even more readily in deeper water than some of the structures previously described. This structure has a lower tank 190, like tank 50 previously described, a universal joint 191 joining tubular shaft 192 to the tank, a tubular shaft 193 joined to shaft 192 by a universal joint 194, and platform 195 placed on the top of shaft 193. Each of

tubular shafts

192 and 193 can be used to store oil or they can be used for buoyancy by partially or completely filling them with air. Guy lines 196 and 197 can be employed if needed to stabilize the structure. Instead of a tank 190, a heavy weight can be substituted therefor as described in connection With FIGS. 1 and 2. Also if desired shaft 193 can be made in two or more sections joined by universal joints.

The structures of this invention having an elongated tubular shaft can be provided with a buoyancy section in the shaft to offset part or all of the weight of the shaft, platform and platform equipment thereby lowering the forces on the universal joint and the tank.

In constructing a structure according to this invention variables such as the weight of the structure, depth of water, amount of buoyancy and magnitude of forcing functions must be taken into account. These variables affect the natural period of the structure and the angle of excursion. Therefore, any particular structure should be designed so that it has a natural period greater than the expected natural period of the forcing function. It is also advisable to limit the angle of excursion to 6 either side of the vertical so that conductor pipes are not overstressed.

The foregong detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. A structure for use offshore supported by the floor of a body of water comprising:

an elongated shaft positioned upright in a body of water;

a roofed tank defining a volume therein placed on the floor of the body of water;

a pivotal connection joining the lower end of the shaft to the roofed tank, said pivotal connection being capable of supporting the shaft While permitting pivotal movement of the shaft with the tank stationary;

means to maintain the shaft essentially vertical; and

at least one conductor pipe supported by the shaft and extending vertically through the tank and into the floor of the body of water and projecting above the surface of the water for receiving and supporting a well drilling shaft.

2. A structure for use offshore supported by the floor of a body of water comprising:

an elongated shaft positioned vertically in a body of water;

an anchoring weight placed on the floor of the body of water;

a pivotal connection joining the lower end of the shaft to the anchoring weight, Said pivotal connection being capable of supporting the shaft while permitting pivotal movement of the shaft with the anchor stationary;

a plurality of conductor pipes supported by the shaft extending from above the surface of the water down into the floor of the body of water;

means to prevent lateral movement of the anchoring weight on the floor of the body of water; and

the upper part of the shaft having buoyancy means for maintaining the shaft upright and being the sole means for restricting wind, wave and current nduced excursion of the elongated shaft from vertical position to an angle of tilt which does not overstress the conductor pipes, said angle of tilt being limited to 6 either side of vertical and said buoyancy means offsetting less than all of the weight of the shaft.

3. A structure according to claim 2 in which the shaft comprises at least two sections joined together by a universal joint.

4. A structure according to claim 2 in which the lower portion of the shaft has weight receiving means to apply a downward force to the shaft.

References Cited UNITED STATES PATENTS 3,154,039 10/1964 Knapp 6l-46.5X 3,165,898 l/l965 Elliott 61-465 3,355,899 12/1967 Koonce et al. 6l-46.5

FOREIGN PATENTS Ad. 86,230 11/1965 France 6l-46.5

OTHER REFERENCES Robert Vilain: World Oil, Feb. l, 1968.

I. KARL BELL, Primary Examiner