US3508409A

US3508409A - Method and apparatus for handling tubular members at offshore locations

Info

Publication number: US3508409A
Application number: US693597A
Authority: US
Inventors: Neil H Cargile Jr
Original assignee: NEIL H CARGILE JR
Current assignee: NEIL H CARGILE JR
Priority date: 1967-12-26
Filing date: 1967-12-26
Publication date: 1970-04-28
Anticipated expiration: 1987-04-28
Also published as: JPS4841401B1; NL6818778A

Description

April 28, 1970 H. CARGILE, JR 3,508,409

METHOD AND APPARATUS FOR HANDLING TUBULAR-MEMBERS AT OFFSHORE LOCATIONS 1 Filed Dec. 26. 1967 2 Sheets-Sheet i M/m/A; mm: was (ONT/P04 BY iQMM A fro/m5 y 3,508,409 METHOD AND APPARATUS FOR HANDLING'TUBULAR MEMBERS Apri 8. 1970 N. H. CARGILE, JR

AT OFFSHORE LOCATIONS 2 Sheets-Sheet 2 Filed D90. 26. 1967 /e J). lNVEN/UR.

BY g

Arrow/3V nited States Patent 3,508,409 METHOD AND- APPARATUS FOR HANDLING TU- BULAR MEMBERS AT OFFSHORE LOCATIONS Neil H. Cargile, In, Cargile Road, Nashville, Tenn. 37205 Filed Dec. 26, 1967, Ser. No. 693,597 Int. Cl. E02d 21/00; B63b 35/44; E21b 15/02 US. Cl. 61-46.5 6 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for use with a workover or drilling rig, preferably water-borne, by a floatable structure with the rig normally cantilevered over the edge of the vessel above a well head or cased opening, said apparatus incorporating a means between the rig and the well for directing a generally upward force against the rig wherein the magnitude of the force is related to the hook load acting on the rig, to cancel the dynamic loading on the rig.

Summary of problem and invention Offshore petroleum work is generally divided into two classes of activities, the first of which is directed to the drilling of oil wells, and the second of which is the servicing of the well and completion. While the dividing line between the two activities is not hard and fast, for purposes of the present disclosure, workover and completion work are grouped together and generally defined as those activities involving a water-borne vessel or other fioatable structure having lighter derricks and draw works which are often cantilevered over the edge of the vessel. Drilling activities are normally understood. In the area of activities presently proscribed, problems exist in the use of a workover or drilling rig as will be noted.

Describing the problem in the context of a workover rig, the rig is floated to the site of the completed well or other well head installation, and the workover rig is positioned above the well head. If the task at hand is one of pulling the tubing string, typically the tubing string is pulled by the draw Works and derrick apparatus in a well known manner. In view of the depth of wells being drilled, it will be understood and appreciated that the hook load acting on the rig is substantial. In further view of the fact that the heavy load acting downwardly on the rig is displaced from the center of strength of the vessel or barge, often cantilevered to one side, it becomes readily apparent that enhanced strength and support on the vessel itself is required. One previous solution is use of an equal and diametrically opposite weight on the barge or vessel to prevent its tipping toward the well head. This is ordinarily obtained by the use of ballast, such as rocks loaded aboard the vessel, or the use of floodable tanks which are filled with a view of providing the counter-balance weight. Absent the counterbalance, the jack-up leg or other strutural support beneath the vessel is loaded heavily adjacent the cantilevered workover rig, whereas the remote legs are lightly loaded, if at all.

It will be appreciated that the disproportionate allocation of the load to the jack-up legs is undesirable, and generally requires perhaps twenty-five percent to fifty percent more structural steel in the jack-up legs. Because the entire vessel is placed under torque, the laterals or transverse members in the vessel itself must be reinforced to prevent warping. Therefore, the devices to which the present invention relate have only solved the problem of off-center loading in vessels by the brute force techniques of placing a disproportionately overbuilt structure in the water, or the addition of grossly heavy ballast which, while it counteracts the torque, creates a substantially inlice creased dead weight load acting on the underpinning or other supports of the barge structure.

Since the present problem occurs without regard to the mode of operation of the vessel, the device of the present invention is applicable to off-shore vessels of all designs, including drilling vessels, and for purposes of illustration and not as a limitation, it will be described hereinafter mounted on a jack-up barge. With a view of the foregoing problems in mind, the present invention is summarized as providing improved means and apparatus for use with a cantilevered vessel wherein a means providing a generally upwardly directed force acting on the derrick offsets the hook load, and, in effect, returns the load on the derrick, draw works and other equipment to the well head or casing, it being noted that the Well head or installed casing is typically constructed and arranged to support this weight at all other times. Therefore, one object of the device of the present invention is to provide a new and improved off-shore vessel method and apparatus which avoids cantilevered or off-center loading of the vessel when engaged in workover, completion, or drilling. operations Another object of the present invention is to provide a new and improved apparatus permitting the use of symmetrically built oif-shore barges which do not require counterbalancing ballast tanks or the like, and wherein the structural legs or other support means are designed to merely support the barge and not excessive weights or torques on the barge.

A related object of the present invention is to provide a new and improved apparatus which shifts the dynamic loads resulting from use of the rig carried on the barge and returns or restores such loading to the well head or well casing which normally carries the weight of the tubing string, and is sufficiently strong for such use.

Another object of the present invention is to provide a new and improved method of pulling a tubing or drill string from a water-located well whereby the dynamic load required for pulling the tubing or drill string is isolated at the well and not in the apparatus or vessel used for pulling the tubular members.

Other objects and advantages of the present invention will become more readily apparent from a consideration of the included drawings and specification wherein:

FIG. 1 is a side view of the present invention installed on an off-shore vessel; and

FIG. 2 is a side view of the present invention installed beneath a rotary table to support the weight of the drill string locked in the bowl by the slips.

In FIG. 1, attention is first directed to the off-shore vessel 10 which is shown supported on a jack-up leg or spud extending to the sub-soil. The numeral 14 indicates the well protector which, in the current example, extends to a point above the water line 16. The casing or protector 14 extends upwardly to a well head 18, the well head 18 being surrounded or even incorporated in a pile jacket 20 which includes a number of structural members extending to the soil beneath the 'body of water. A workover means indicated generally by the numeral 24 is carried by the barge or vessel 10, and is adapted to be positioned above the well head 18 for pulling a tubing string 26 from the well. Of particular interest to the present invention is the placement or location of force creating means indicated generally at 28, and extending between the well head 18 and the workover means 24 whereby a force is created acting upwardly on the workover means to reduce or even cancel the load of the tubing string 26 acting on the workover means 24.

As noted before, the workover means is shown in FIG. 1 and will be first described and thereafter, the invention in use with a drilling rig will be noted.

Considering the invention more in detail, it should be noted that the barge 10 s a conventional vessel known in the art, and is adapted to be moved, towed, or otherwise transported on the body of water 16 to the location of the well 14. The precise nature of the barge is of no particular significance to the present invention, and for this reason, many details are omitted. The illustrated embodiment is shown with jack-up legs 12 which elevate the barge above the surface of the water 16 whereby the weight of the barge is borne on several jack-up legs 12, not fully shown in the first illustration.

Normally, the jack-up legs 12 are symmetrically located about the vessel 10 to carry a proportionate load when the barge is elevated from the water surface. However, such symmetrical loading is not possible when the workover means 24 is cantilevered over the edge of the barge 10 without the utilization of the present invention. As noted hereinabove, the offset or cantilevered positioning of the workover rig places a non-symmetrical load on the barge legs which either creates bending torques in the long, slender legs, or which is only offset by the use of substantial balance weights at the other side of the vessel, thereby increasing the dead weight carried by the legs. Since neither circumstance is desirable, the device of the present invention is described hereinafter as providing a means whereby the hook load acting on the workover rig is cancelled by an approximately equal and opposite force applied to the rig, the force communicating the weight of the tubing string 26 back to the well head 18 which is routinely constructed and arranged to support this weight at all other times.

Note should be taken that the device of the present invention is not limited to the well head apparatus shown in FIG. 1. The view illustrates a pile jacket platform extending above the surface of the water 16. In actuality the well head 18 has many configurations located above or below the water line subject only to the requirement that a suitable structure support the standard well head equipment and provide support to the tubing hangers, both requirements being well known. In view of the variety of well head installations, the platform 20, its underpinnings, and the other structures are shown for purposes of illustration and not limitation in the present disclosure.

For convenience to the vessel 10 during navigation, the workover means 24 is preferably mounted on a pair of skid rails 32. The rails 32 are movable on the deck of the vessel 10 to permit the workover means 24 to be moved inboard of the vessel to improve its stability when the vessel is riding in the water. At a selected site, the rails 32 are moved with respect to'the vessel locked in position by suitable means such as clamps, skid beams,

barge superstructure or the like for extension out over 4 the water. The rails 32 are generally of I-beam construction construction with cross bracing (not shown) located there-between wherein the rails 32 form a platform clamped or secured with respect to the vessel and the platform is extended to carry the workover means over the water. The rails 32 are extended over the water to whatever length needed. This carries the device of the present invention to the illustrated position.

The workover means 24 includes a telescoping workover mast 36 which is supported on the platform and is movable therewith. The mast 36 is preferably erected to the desired height when positioned over the well head 18. The mast carries conventional pipe handling apparatus including a crown block 38, the traveling block 40, the appropriate swivel and hook; rotary drilling equipment, if needed, is included, and a rotary table, the Kelly drive, and other apparatus are placed on the platform. The precise nature of the apparatus is subject to variation and is noted herein to describe equipment known and understood by those skilled in the art. The workover or completion equipment is subject to variation.

A cable 44 is shown extending from a draw works 46 to the crown block 38 and is utilized in the normal mafiner to raise or lower the traveling block 40. The draw works 46 includes the conventional winch, motive apparatus, and other needed equipment. The cable 44, at its dead end, is tied to a hydraulic transducer 54, preferably a piston-cylinder arrangement secured to the platform and capable of forming a hydraulic pressure signal which measures the tension in the cable 44. The measurement of the tension in the cable 44 finds significant use in the present apparatus. It will be appreciated that the cable had is proportionately related to the hook load carried by the hook 42. Even when the hook 42 is not engaged with the tubing, and there is some tension in the cable 44, it will be appreciated that the corresponding weight is relatively small in comparison with the massive loads placed on the hook by the tubing string, most especially when the tubing string is struck in the well bore. As a matter of fact, hook loads as high as five hundred thousand pounds sometimes occur.

The load transducer 54 is communicated by a hydraulic line 56 to another line 62 input to a number of hydraulic jacks 64. As the pressure in the line 56 is increased by the hydraulic transducer 54, the pressure input to the jacks 64 is likewise increased. As suggested in the drawing, it is possible to use a hand control valve communicating with the main hydraulic system aboard the vessel 10 (at a suitable high pressure) wherein the outlet pressure of the valve 60 is regulated to approximate the pressure needed to offset the load on the derrick. Since the precise nature of the arrangement is subject to variation, the above noted arrangements are believed exemplary of apparatus suitable for installation in the present invention.

The several, preferably identical jack means 64 carried beneath the platform are adapted to extend to the Well head 18. The jack means 64 are secured at their upper end to the platform on which the workover means 24 is carried, and upon being fixedly attached thereto, access is permitted for connection of the hydraulic cylinders 64 to the well head 18. As a matter of convenience, it may be desirable to provide for permanent installation of the hydraulic jacks 64 to the rails 32. As a matter of flexibility, it may be helpful to provide a universal adaptor on the piston rods extending from the jack means 64 whereby connection with the well head 18 is achieved without regard to the variations in shape or location of the well head 18.

It should be noted that the hydraulic jacks 64 need only be single acting apparatus and capable of extension under pressure. Since weight is carried by the hydraulic jacks 64, the pistons are compressed into the cylinders of the jacks by the quiescent load.

The drawing illustrates two jack means 64, although a greater number may be used to maintain symmetrical loading on the well head 18. Because the details of the well head are dependent on many circumstances and factors, it may be necessary to provide as many as four or more jacks for transferring the load as will be described.

In many installations, the well head 18 is located beneath the surface of the water 16 while the platform of the barge 10 is sixty or more feet above the surface of the water. In this circumstance, it is necessary to secure suitable elongate members on the Well head (by welding or clamping) so that the extenders span the gap to the jacks '64. The jack means 64 do not need excessively long stroke because their primary function is to apply force to the platform on which the workover device rests, and not to move it.

While the foregoing describes the apparatus of the present invention, it is helpful to now describe the method of the present invention, assuming use of the above structure although others means are available. Assuming that the vessel 10 is in place, that the rails 32 are extended to carry the workover means 24 to the vicinity above the well head 18 and the hydraulic means 64 are extended between the workover rig and the well head 18. Access is gained by suitable techniques to the casing 14 and the tubing 26 is exposed. The pipe is hoisted upwardly by the hook mean in the well bore. The movement of the tubing 26 involves upward and downward movement of the traveling block 40 in response to thedraw works 46 which reels the cable 44 on the drum. It will be appreciated that the column of tubing 26 is substantially heavy for a deep well, and, the lifting force is often increased should the tubing string be stuck in the well bore.

After access is obtained to the upper end of the tubing string 26 and the traveling block 40 is lowered sufficiently to engage the hook 42 with the tubing string, the cable 44 hoists it upwardly and free of the casing 14. At this juncture, theload on the hook is transferred to the vessel through the supporting structure of the rails 32; as noted before, this load reacts to the detriment of the barge 10 by either unbalancing the stable footing of the barge 10 by putting too much weight on one leg, or alternatively, by requiring balancing ballast weights which increases the total dead weight of the vessel and therefore increases the structural requirements of the jack-up legs 12. But, with the present invention, load on the vessel 10 at its edge or corner as shown in the drawing is offset in the following manner.

As the cable 44 is pulled taut, indicative of an increased hood load, the load transducer means 54 senses the increase in tension and increases the pressure in the hydraulic line '56. The hydraulic line 56 communicates the control pressure to the hydraulic jacks 64. Since the increase in pressure in the line 62 is proportional to the tension in the line 44, the pressure in the hydraulic jacks 64 is likewise proportional to the tension in the cable 44. This means that the hydraulic jacks push upwardly on the rails 32 with a force approximately equal and opposite to the forerequired to unstick and lift the tubing string engaged by the hook means 42. The net result is that the forces on the platform that extends over the side of the barge 10 cancel one another. Moreover, the downward urging of the jack means 64 returns the weight of the tubing to the well head 18 and the well head 18, being constructed to carry the weight of the tubing string 26, amply supports the load transferred from the vessel 10. Noting the fact that the well head 18 is constructed to carry this weight at all times, the method of the present invention provides a force equivalent to the weight acting on the well head 18 to thereby maintain the normal static load of the well head and to remove the dynamic load from the workover rig 24.

The above described method is preferably dynamic in operation as will be noted. For instance, should the tubing string 26 hang in the well bore, and should the tension in the cable 44 be increased unusually high to pull the tubing string 26 free of the obstruction, the hook load is substantially increased. The transducer 54 is sensitive to the increase in tension in the cable 44, and the pressure in the line 62 is increased accordingly to thereby increase the pressure within the jack means 64. Since the jack means are constrained against physical move ment, the increased pressure therein increases the equal and opposite forces acting at the upper and lower ends of the jack means whereby the platform is reactively balanced against the cantilevered load, and the dynamic increase is communicated to the well head which carries the burden. Should the hook load drop suddenly, dynamic relief of the pressure build-up in the hydraulic system permits the platform carried on the rails 32 to continue stabilized without regard to its loading.

While the foregoing has been describing the preferred apparatus and method of the present invention, several alternatives should be noted with regard to both the device and method of the present invention. For instance, it may be more convenient to obtain a proportionate indication of the load acting on the hook by placing a load cell at some point in the derrick 36. For instance, strain gauges attached in the lower legs of the derrick 36 provide an electrical signal responsive to the stress acting in the derrick, a signal proportional to the load acting on the hook 42. Moreover, it might be convenient to sense the loading on the axle of the crown block 38 to cite a second alternative load sensing point. While many techniques of load sensing are available and may be adopted by one skilled in the art, the nature of the proportional signal should also be considered. It is acceptable to use av hydraulic signal as provided in the present system, or, if desired a proportional electrical signal from a strain gauge or the like. In the case of an electrical signal, suitable control amplifier means are known in the art to provide a control signal to a pressure regulator, pump, controlled valve, or the like, whereby the pressure in the conduit 62 is made proportional to the load acting on the hook of the draw works. Likewise, it is not absolutely essential to use hydraulic jacks as shown in the preferred embodiment. One suitable structure would be the implementation of mechanical jacks such as screw-type jacks, or other similar devices. However, it would appear that engineering economies are achieved by hydraulic jacks.

Attention is next directed to FIG. 2 of the drawings which illustrates a platform adapted to be cantilevered over the side of a vessel (not shown as a matter of clarity). Assuming the use of the vessel such as shown in FIG. 1, the platform 100 is preferably adapted for use with a drilling rig indicated in dotted line at 101. The drilling rig preferably has four load bearing upright supports which are each received on and supported by a load cell 102 positioned beneath each of the corner members. The load cells are deployed to support the entirety of the load acting on the drilling rig 101. As will be understood, the drilling rig 101 reactively returns the load to the load cells 102.

The view of FIG. 2 omits the rotary table, supporting deck work for the rotary table, and the bowl for receiving the slips for locking the drill string in position. These details are omitted to show the hydraulic system in accordance with the teachings of the present invention. However, FIG. 2 does include an I-beam 104 on which is received deck plating 105 which extends to the center of the platform 100 for supporting the rotary table.

It will be appreciated that the weight of the drill string is supported by the drilling rig 101 during drilling operations. However, during the interval which the drill string is supported by slips wedged into the bowl, the weight of the drill string is transferred from the drilling rig 101 to the rotary table, and thence to the I-beam supports 104. It will be understood that suitable beams are placed beneath the deck plating 105 to transfer the weight on the rotary table, including the drill string when at rest, to the load cells 102 which are deployed under the four corners of the drilling platform 105. The I-beam supports preferably extend to the four corners and cross-bracing is provided as needed. By the use of the described structure, it will be understood that the platform 105, which includes the rotary table, is, in effect, rested on the four load cells shown at the corners.

One factor which alters the load on the cantilevered platform 100 is the deployment of drill pipe or other tubular members resting in the derrick with the greater portion of their weight acting as the lower end in conventional pipe racks as will be understood by one skilled in the art. This is illustrated somewhat schematically in FIG. 2 wherein a plurality of pipes is indicated at 108, the intention being one of resting the dynamic load of the pipe on the present invention whereby the load is transferred from the platform 100 proper to the protector casing as will be noted.

Attention is next directed to the hydraulic apparatus shown in FIG. 2. A source of fluid under pressure is provided by means, not shown, through appropriate accumulators, valves, and the like to a hydraulic line 110. The hydraulic line communicates by way of a line 111 to a dead-end load sensor 112. As was shown in FIG. 1, one technique of obtaining the load acting on 7 the derrick 101 is to sense the tension in the dead end of the line indicated at 113. The cable 113 extends upwardly into the derrick and is connected through the crown block and traveling block to support the entirety of the weight of the tubing string. As the hook load increases, the tension in the cable increases and this is reflected at the dead end of the cable 113 wherein the increased tension pulls on the hydraulic .transducer shown at 112. The transducer means 112 converts the tension in the cable 113 to hydraulic pressure as communicated through the line 111. By way of example and not limitation, a single acting hydrauli cylinder filled with pressure fluid increases pressure in the line 111 in response to increased tension in the cable 113. The increase in pressure is communicated to the line for response as will be described momentarily.

A second branch line 115 has a manifold input to the lurality of load cells 102. The load cells 102 are connected in parallel with one another and to the line 115. Again, the load cells can be aptly described as single.- acting piston and cylinder arrangements wherein the weight or load acting. thereon increases pressure in the line 115.

The branch lines 111 and 115 supply control pressure through the line 110 and through the valve 16 to each of a number of downwardly extending hydraulic cylinder means 120. While FIG. 2 illustrates two hydraulic jacks indicated at 120, it will be appreciated that a symmetrical arrangement incorporating two or more hydraulic jack means 120 is desirable. For sake of clarity of the drawings, additional hydraulic jack means 120 have been omitted from FIG. 2; however, the disclosure relating to the illustrated hydraulic jack means is believed adequate.

The pressure in the line 110 causes the jack means 120 to extend in the manner described with respect to FIG. 1. It will be appreciated that the upper end of the jack means 120 is connected at 121 by suitable mounting brackets to the cantilevered platform 100. The lower end of the jack means 120 is connected with a load ring 122 which is secured about and relative to a protector casing 124. As shown in FIG. 1, it will be understood that the protector casing extends part-way into the water and soft bottom, and is sufliciently stout and stable to support the load placed thereon. Protector casings are customarily installed at most drilling installations to prevent pollution of fresh water formations and other undesired communications up or down the well bore. As was shown in FIG. 1, a platform may or may not be present surrounding the protector casing 124, and it will be further noted that a protector casing 124 and the load ring 122 are located below or above the surface of the water depending on factors not related to the present disclosure.

In operation, the structure shown in FIG. 2 functions somewhat in the manner as the structure shown in FIG. 1, the primary difference being that the derrick or rig shown in FIG. 1 is normally used for workover or completion operations, while the structure shown in FIG. 2 is primarily used for drilling wells. Of course, the two together indicate the use of the present invention with well bore activities of any description or nature, as will be understood. More specifically in FIG. 2, a load is sensed either by the dead-end transducer 112 or by the load cells 102 to communicate an increase in pressure to the conduit 110. The increase in pressure is communicated to the jack means 120 which are positioned below the platform, and which are connected to a fixed memher, the fixed member being the load ring 122 which is carried on the protector casing 124 and is fixed in position and adapted to support the load. As the pressure increases, the jacks 120 are extended. As the jacks extend, they apply a force to the lower side of the cantilevered platform 110 which is approximately equal and which has a vector direction approximately opposite to that of the dynamic load acting on the platform 100. It is immaterial that the load acting on the platform 100 is derived from the'hook load in the derrick 101 astransferred by the cable 113 to the dead end transducer 112 or through the legs of the derrick at 101. In either event, a load is transferred to appropriate transducers which increase the pressure in the line in relationship to the dynamic load. Moreover, the load of a full rack of pipe carried in the derrick is likewise detected and the hydraulic jack means are likewise extended in response to such a load. With the foregoing in view, and somewhat similar to the operation of the structure shown in FIG. 1, it will be understood that the drilling rig operates similar to that shown in FIG. 1.

With the foregoing in view, it will be appreciated that the scope of the present disclosure applies to any suitable means for deriving the force acting on the apparatus, using the signal indication thereof to thereby control the force applied generally upwardly and oppositely beneath the rig, and also co-acting on the well head 18.

While numerous changes or variations in the present invention are described above, and since others are known in the art, it is desirable to describe the invention broadly, limited only by the scope of the claims appended hereto.

What is claimed is:

.1. A method of dynamically loading a workover, completion or drilling rig which can be used when extended toward or over the side of a barge or vessel for carrying the rig; comprising the steps of:

(a) sufficiently aligning the rig with respect to a well thead;

(b) loading the rig by using same in a conventional manner;

(c) sensing the dynamic loading acting on the rig;

and

(d) applying a force proportionally related to the sensed dynamic loading and against the rig in a direction tending to cancel the dynamic loading thereon.

2. The method of claim .1 further including the step of coupling hydraulic force-applying means between the rig and well head before the rig is loaded whereby said proportionally related force is developed by applying a hydraulic pressure to the force-applying means that is proportionally related to the dynamic loading on the rig.

3. The method of claim -1 further including the steps of coupling hydraulic force-applying means between the rig and well head before the rig is loaded; sensing the tension in the cables of the rig; and developing a hydraulic pressure on the force-applying means that is proportionally related to the cable tension for applying said proportionally-related force between the rig and wellhead.

4. For use in offshore operations with a vessel carrying a rig, the improvement comprising:

(a) a platform adapted to be positioned over the edge of the vessel, the rig being mounted thereon;

(b) means for forming a force acting on the rig tending to counteract the load acting on the rig and including hydraulic jack means extending downwardly from said platform; and

(c) adaptor means for connection to said jack means and constructed and arranged to place a load relative to the Well head acting downwardly such that support is provided to said platform by said jack means.

5. The invention of claim 4, further including:

(a) transducer means for sensing the tension in the cables of the rig and for forming a signal proportional to the load thereon; and

, (b) means connected to said transducer means and to said hydraulic jack means for forming a hydraulic pressure therein for effectively returning the weight of the load acting on the rig to the well head.

6. The invention of claim 4, further including:

(a) load sensing means adapted to be positioned be- .neath the load acting on the derrick of a rig;

9 10 (b) means connected to said load sensing means and 3,201,945 8/1965 Sutton 61-465 to said hydraulic jack means for controlling a hy- 3,368,618 2/1968 Knox 166-.5 draulic pressure therein for effectively returning the 3,421,581 1/1969 Geijn 17527 X weight of the load acting on the rig to the well head.

5 STEPHEN J. NOVOSAD, Primary Examiner v References Cited UNITED STATES PATENTS US. Cl. X.R.

2,334,992 11/1943 Crake 61-465 166-.5; 175-5, 24 3,189,098 6/1965 Haeber 166-.6