US 3692109 A
A wire line centralizer designed for fast assembly to and disassembly from an oil well line. The centralizer body is slotted laterally to receive the wire line and includes fastener means for holding the centralizer in a selected place along the line with the aid of stop means fixed to the line crosswise of the centralizer slot.
Claims available in
Description (OCR text may contain errors)
D United States Patent  3,692,109 1 Grayson 14 1 Sept. 19, 1972  WIRE LINE CENTRALIZER ASSEMBLY 2,793,917 5/1957 Ward ..l66/l76 X [72 Inventor; Bob], Gm Son 7306 P'embmk 2,944,608 I 7/1960 Rush ..l66/24l 1 Avenyofldale, 93308 6 3,047,073 7/1962 Fry ..l66/l76 3,282,344 ll/1966 Tripplehorn ..l66/241 X  Filed: Oct. 28, 1970 21 A N 84 738 Primary Examiner-David H. Brown I 1 pp 0 Attorney-Sellers and Brace Related US. Application Data  Division of Ser. No. 27,214, April 10, 1970,  ABSTRACT Pat. No. 3,572,245. A wire line centralizer designed. for fast assembly to and disassembly from an oil well line. The centralizer  US. Cl ..l66/241, 308/4 A body is slotted laterally to receive the wire line and in-  Int. Cl. ..E2lb 17/10 cludes fastener means for holding the centralizer in a  Field of Search ..l66/24l, 170,172, 173,176; selected place along the line with the aid of stop 308/4 A; 102/2l.6 means fixed to the line crosswise of the centralizer slot.  References Cited v UNITED STATES PATENTS 8 Claims, 13 Drawing Figures 2 ,702,033 2/195; wa i rm og 4 PATENTEDSEP 19 m2 SHEET 1 [1F 3 INVENTOR 5056 W 654 VSd V BY A mm v PATENTEDSEP 19 I972 sum 2 or 3 INVENTOR away 14 eeAysa/v BY ATTGE/Vk PATENTEnsEP 19 m2 3.692.109
INVENTOR 5055 V W 66/? V50/\/ Fwd l WIRE LINE CENTRALIZER ASSEMBLY This application is a division of my application for US. Pat. Ser. No. 27,214, filed Apr. 10, 1970 now Pat. No. 3,572,245.
This invention relates to wire lines of the type extended into earth bores, and more particularly to a centralizer designed for expeditious assembly to and disassembly from a wire line at the mouth of the bore in order that the line may be handled efficiently by reeling equipment.
The string shot technique employed to perform servicing operations in oil wells and to improve the flow by clearing away material tending to clog the flow perforations is subject to many shortcomings and disadvantages avoided by the present invention. This technique utilizes an explosive cord supported along the side of a wire line and lowered to position the cord opposite a section of perforated casing to be cleared of debris. It is not possible to determine the cause or seriousness of the clogged condition and more particularly how strong an explosive charge is required to clear the perforationsyet avoid risk of damage to thecasing. It is costly and time consuming to carry out a string shot operation and this encourages the operators to use an excessively powerful charge to avoid having to perform several run in operations. Not infrequently, the extra strong charge seriously damages the casing.
Attempts to use explosive cords of low power or strength are uncertain and often result in failure or produce a serious damage'to the casing or parting of the wire line and lossof the weight required to pull the line into the well for quite different reasons. For example, in certain instances the cords fail to explode or explode over only a small portion of their full length due to a failure of flame propagation along the cord. For reasons that have not been understood prior to the present invention, these lower'strength lines would explode over only a short portion of their upper lengths. In other instances either the wire line was severed or serious damage caused to the casing, or both, for reasons not understood or readily explained. It has been discovered that, in most instances, these accidents are produced by stretching of the cord due to a number of factors including the adverse effect on the cord of the very substantial sub-surface ambient temperatures customarily encountered at lower levels in oil wells and the high drag forces acting on the cord as it is lowered through the mud and other fluids customarily present in a well. As a result of this stretching the distance between adjacent grains leads to propagation failure. It can also result in excess portions of the stretched cord collecting in a loop adjacent an anchorage connection between the cord and the wire line. Upon detonation of the loop, an unusually powerful blast is produced in a localized area of the well which not infrequently cuts the wire line and releases the line weight but can cause serious damage to the casing itself.
It has been proposed heretofore to assemble a pair of explosive cords in parallel along the wire line and to explode them separately either by using a single or separate pairs of electrical leads. If the same pair is used, then both cords are set off in fast unregulatable sequence. The use of separate lead wires for each explosive cord enables the operator to vary the interval between detonations but is subject to the serious disadvantage of having to provide and protect separate leads for each charge. This complicates the equipment and servicing operation and involves the risk of severing one or more of the lead in wires and greatly increases the risk of malfunctions as well as the maintenance and handling costs.
Other problems associated with string shot operations as heretofore conducted involve the problem of equipping the wire line with centralizer devices to hold the line out of contact with the casing side wall and couplings during the run in and pull out cycle as well as to safeguard the explosive cord against damageduring the lowering operation. These centralizers are required at frequent intervals along the wire line and their presence on the line interferes with reeling and storage of the line. I
The foregoing and other serious disadvantages accompanying string shot operations as heretofore conducted are avoided by the present invention using the equipment and the improved techniques provided by this invention. A simple wire line is employed having a central conductor and a second conductor comprising the metallic sheath of the line which are utilized to supply the electrical energy required to detonate one or a series of explosive cords in any desired time interval or simultaneously in one or more groups if so desired. Explosive cording incorporating means safeguarding against stretch and malfunctioning of the cord makes possible the use of lower strength cord than heretofore possible and numerous other advantages.
An improved centralizer construction permits the centralizer to be installed and removed from the wire line expeditiously at the well head. After the main body of the centralizer has been detached, the wire line can be compactly reeled on the take up spool or storage reel. As soon as the centralizer is'installed on the wire line one or more lengths of explosive cord can be secured to the periphery of its main body following which the wire line is lowered into the well until in position to receive the next centralizer.
The selective detonation of explosive cords via a single pair of conductors is made possible by unique electrical discriminator means connected between one of the conductive paths and the detonators for each cord. Typically the discriminator utilizes semi-conductors arranged to pass electrical energy only to a particular cord or to a set of cords selected for detonation at a particular time. Greater versatility is achieved by utilizing the discriminator in combination with a reversing switch and means for controlling the applied potential.
Another feature of the invention is the provision of a blowout protector of simple unique design across the well head. This protector includes a cover having a non-conductive gasket which fits tightly about the wire line in the closed position of the protector and which opens for the full diameter of the casing whenever the wire line is being lowered or withdrawn.
Another object of the invention is the provision of a wire line centralizer having a radial slot from end to end sized to seat readily over a wire line from the side thereof and cooperable with means fixed to the line to restrain the centralizer against movement along the wire line.
Another object of the invention is the provision of a rugged wire line centralizer designed to embrace the line and to be applied thereto intermediate its opposite ends and equipped with simple quickly manipulated means for securing the centralizer in place thereon without pinching the line and while preventing relative rotary movement.
Another object of the invention is the provision of an improved centralizer readily installed on and removed from a wire line at intervals therealong as the line is being lowered into or withdrawn from a well.
Another object of the invention is the provision of a simple, easily operated blow out protector installable on a well head and scalable against a wire line having an electrical conductor along its exterior.
These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate.
Referring now to the drawing in which a preferred embodiment of the invention is illustrated:
FIG. I is a vertical sectional view of a well having a perforated casing and showing an illustrative embodiment of the invention string shot apparatus in position for detonation of its separate cords;
FIG. 2 is a top plan view on an enlarged scale of the blowout protector closed against the well head;
FIG. 3 is a cross sectional view taken along line 3-3 on FIG. 2;
FIG. 4 is a cross sectional view taken along line 44 on FIG. 3; o
- FIG. 5 is a diagramatic elevational view at the well head showing details of the means for installing the explosive along the wire line at a desired uniform tension;
FIG. 6 is a schematic view of an illustrative embodiment of the electrical firing circuit for a plurality of explosive cords;
FIG. 7 is an elevational view on an enlarged scale of means for securing the explosive cords to an adjacent portion of the wire line;
FIG. 8 is a cross sectional view taken along line 8--8 on FIG. 7;
FIG. 9 is a fragmentary enlarged view of the explosive cord;
FIG. 10 is an elevational view of one preferred embodiment of the wire line centralizer;
FIG. 11 is a top plan view taken on FIG. 10;
FIG. 12 is a cross sectional view taken along line l212 on FIG. 10; and
FIG. 13 is an elevational view of a second preferred from of the centralizer assembled to the wire line.
Referring initially and more particularly to FIG. 1, there is shown an oil well casing 10 having a multiplicity of flow perforations 1 1 distributed along its side wall at the production flow level. It will be understood that these perforations may be distributed over many feet of the casing and in one or more areas of the casing.
Secured to the top of the casing is a radial flange 12 to which a blot out protector mounting ring 13 is detachably secured, as by bolts 15. This ring has a central opening 16, corresponding to the ID. of the casing so as not to restrict the entrance to the well. Opening 16 is provided with a cover 17 here shown as formed in a single part and equipped with a hinge 18 secured to mounting ring 13. However, it will be understood that cover 17 may be formed in two halves each having its own hinge connection with mounting ring 13 if so desired. Threaded studs 20 welded or otherwise secured to mounting ring 13 pass through openings in cover 17 and support thumb nuts 22 for clamping the cover firmly sealed closed prior to a string shot firing operation.
Suitably secured to cover 17 and mounting ring 13 is a two part or split sealing gasket 23,24 having its edge portions shaped-to clamp snugly about the wire line 25 when cover 17 is closed. Each of the gasket halves 23,24 are suitably and firmly anchored to the cover and to the mounting ring as in the manner indicated in FIGS. 2 and 4. These non-conductive sealing gaskets not only provide a fluid-tight seal about the wire line but isolate the line and its conductors electrically from the casing.
Wire line 25 includes a central conductor 26 separated from the helical sheath conductors 27 which also serve as the second conductive path employed as will be explained presently to provide one portion of the electrical circuit between the direct current power source 28 (FIG. 6) and one or more string shots. Typically wire line 25 passes over a guide pulley 30 overlying the well and thence to-a storage reel, not shown, but of any suitable well known construction driven by reversible power means for handling the wire line.
Referring now more particularly to FIGS. 1, l0, l1 and 12, it will be understood that wire line 25 includes a weight 35 at its lower end of adequate size to facilitate the expeditious lowering of the wire line into the well through the thick fluids generally present in the well. Distributed along the lower portion of the wire line at suitable intervals, as 10 to 25 feet, are a plurality of centralizers 40,40. As is best shown in FIGS. 10 to 12, each centralizer includes a mandrel 41 provided at its opposite ends with stop means or enlargements in the form of yokes 42. These yokes are formed with aligned openings 43 each equipped with a pivot pin 44 suitably secured in place and extending through a loop 45 of the wire line or of a similar cable 46 interconnecting adjacent ones of the centralizer mandrels.
Loosely fitting over the mid-portions of mandrels 41 is the main body 48 of the centralizer. This main body is generally cylindrical in contour and has a long deep slot 49 opening'through one side wall and sized to fit readily over mandrel 41. Main body 48 is slightly shorter than the distance between the adjacent ends of yokes 42, and is formed with a threaded bore seating a thumb screw 50 having a knurled head. Its inner end extends into slot 49 in position to engage the mandrel and hold the centralizer body firmly locked in assembled position upon any attempt for disassembly movement in the manner made clear by FIG. 12.
Extending crosswise of the opposite ends of the centralizer bodies 48 are pairs of passages 51 loosely seating highly tempered wire loops 53. As herein shown, each centralizer is provided with a single pair of these wire loops shaped as best shown in FIG. 10 and each including oppositely directed semi-cylindrical loops projecting radially from the side wall of main body 48. Since each of these loops lies at an angle to the portions located in passages 51, the loops are held against sliding movement lengthwise of these passages. The radial extent of each of the loops is such that they cooperate in holding the wire line loosely centralized in the well casing.
A second and simpler embodiment of the invention centralizer 40', illustrated in FIG. 13, may be mounted directly on wire line 25 at any suitable spacing therealong. For this purpose the wire line is provided with stop collars 60 telescoped over one end of the wire line and then subjected to high pressure contraction forces adequate to squeeze the ductile collar material into interlocking gripping action with the wire sheath of the wire line. It will also be understood that the collars may be brazed or otherwise firmly secured to the wire line. These stop collars pass readily over the storage reel for the wire line.
The main body 48' of the centralizer is formed with slot 49' similar to the corresponding slot in FIGS. to 12 and sized to fit readily over the wire line. Intermediate the opposite ends of this slot there is an enlarged U-shaped recess 61 shaped to fit loosely about and accommodate collar 60 in the manner made clear by FIG. 13. As is evident from that figure, the end walls of enlargements 61 provide stop surfaces cooperating with the stop surfaces at the opposite ends of stop collar 60 in preventing any but very restricted movement of the centralizer lengthwise of the wire line. A thumb screw 50' is carried in main body 48' and serves similarly to the corresponding thumb screw in FIGS. 10 to 12 to hold the centralizer assembled to and against lateral detachment from the wire line. Additionally main body 48' is equipped with wire loops 53' in the same manner described above in connection with FIGS. 10 to 12. It is therefore evident that the thumb screw 50' holds the centralizer detachably assembled to the wire line whereas collar 60 cooperates with recess 61 to hold the centralizer against movement longitudinally of the wire line. The simplified centralizer avoids the need for the yoke-equipped mandrels as well as for separate lengths of cabling 46 interconnecting adjacent centralizers.
The assembly of the explosive cord through the wire line will now be described with the aid of FIGS. 1 and 5. The crew attaches weight 35 to the lowermost end of the wire line and inserts this weight into the top of the well. As shown in FIGS. 1 and 5, only a pair of explosive cords 65,66 are shown. In many operations a single pair suffices but in others it may be desirable to fit the wire line with a larger number of string shots. In this case, it is usually preferable to use explosive cord of considerably smaller size and explosive power than that heretofore employed. For example, it has been common practice heretofore not to use explosive cord smaller than 25 grain size in string shot operations. However, by this invention it is feasible and advantageous to use cord of the smallest commercial size, namely four grain as well as various sizes intermediate four grain and b 25 grain provided adequate precatuions are taken against elongation of the cord while being lowered into the well. Such precautions may comprise firm anchorage of the cord to the wire line at closely spaced intervals and/or the use of cord in accordance with the present invention incorporating built-in stretch resisting means such as non-stretching small wires or other high strength non-stretching filaments 68,68. These may take the place of a corresponding number of filaments of the outer protective sheath of the cord in the manner shown in FIG. 9. As there shown, the non-stretching filaments 68 are wrapped spirally in opposite directions between adjacent spiral wrappings 69,69 of the conventional cord sheath. It will be understood that the explosive cord is otherwise of any suitable conventional design including a central core of explosive material 70 encased within a tough flexible tubular shroud 71 of plastic or like. The latter is embraced by the wax covered braided sheath 69,69.
65,66 be assembled lengthwise of the wire line in a taut condition and preloaded to a suitable uniform tension. According to this invention, this is accomplished in the manner illustrated in FIG. 5 and showing the tensioning equipment for one only of the cords, it being understood that cord 66 passes over similar automatic tensioning equipment. The cord passes from a supply reel 73 mounted on a suitable support 74, and thence over an idler pulley 75. The cord is held firmly seated on pulley 75 by a pivotally supported idler pulley 76 and a tension spring 77. The cord then passes over a pair of idlers 78 between which there is mounted a pulley 79 on which any suitable member of removable tension-regulating weights 80,80 can be mounted. The cord then passes over an idler 82 carried by the hanger 83 for wire line guide pulley 30, pulley 82 being appropriately positioned close to one side of the wire line at the point of entry into the well head.
It will be understood that the wire line leaves the power-driven wire line reel carrying only mandrels 41 or their equivalent such as collars 60 (FIG. 13)]As these components approach entry into the well head, the wire line is stopped while the crew inserts the main body 48 or 48' of the centralizer and secures it in place by tightening thumb nut 50 or 50'. At the same time the explosive cords are securely anchored to the main body of the centralizer by serving the same with tapes or tiebands 85 (FIG. 1). The crew then proceeds to lower the wire line along with the attached explosive cords under appropriate tension as determined by the weights 80 carried by the pulley 79. Centralizers continue to be assembled to the wire line in succession and anchored to the explosive cords as described.
If the centralizers are spaced considerable distances apart or in cases where smaller size explosive cord is being used, it is desirable to anchor the cords to the wire line at one or more points between adjacent centralizers. This is done using cylinders constructed as shown in FIGS. 7 and 8. These anchor members are secured to the wire line by swaging, brazing or other suitable means. Each member 90 is formed with long grooves 91 of V-shape in cross section extending lengthwise of the body and sized to grip cords 65,66 firmly irrespective of the particular size being used. A deep annular groove encircling the mid-portion of member 90 accommodates tape or banding 93 applied over the cords 65,66 to 'clamp the latter firmly within grooves 91.
The equipment employed in connection with the two conductors 26,27 of the wire line to detonate the explosive cords according to one preferred embodiment is illustrated schematically in FIG. 6. The upper ends of conductors 26,27 are connected to battery '28 by way of a double throw reversing switch 95 and a rheostat 96. The lower end of the wire line closest to the upper end of the explosive cords is connected to the cord by It is important and desirable that the explosive cord wayof an electrical discriminator 97. This discriminator preferably comprises a thick-walled tubular housing 98- of non-conductive material charged with potting compound encapsulating at least one and preferably several semi-conductors 99a,99b, connected in series parallel with one of the wire line conductors as conductor 27. The other conductor 26 is connected to a common bus 100 connected in turn to one terminal of each of the cord detonators 101. The other lead 102 of each detonator is connected to a terminal post 103 in the side wall of discriminator 98 and connected as shown in FIG. 6 to a respective one of the semi-conductors 99a,99b.
The detonators 101 are preferably arranged in vertically staggered relation as indicated in FIGS. 1 and 6 and are sufficiently spaced from one another that the detonation of one does not cause detonation of any other detonator, or the premature ignition of any explosive cord. Zener-type b In some service operations only two or three explosive cords are secured to the wire line during single run in operations. In this case, ordinary diodes may be used. However when employing a larger number of string shots, such as is indicated in FIG. 6 at 65',66', 65",66",65', then it is desirable to employ Zenertype diodes 99a,99 of an appropriate voltage rating to be triggered by a particular voltage applied thereto by adjusting the contactor of rheostat 96. It will be noted that diodes 99a are connected in series to pass current of a particular polarity whereas diodes 99b are connected in series with one another in a parallel circuit and are triggered to pass current only by voltage of an opposite polarity.
The operation of the equipment will now be described, it being assumed that perforations 11 of well casing are plugged with sand or other debris interfering with the well flow. To remove this debris and restore full flow conditions, the service crew connects the blow out preventor assembly to the top of the well casing usingassembly bolts 15 and then proceed to lower weight 35 of the wire line toward the well head. As this weight is about to disappear into the casing the main body assembly 48 of the first centralizer is inserted crosswise of the wire line and locked in place by tightening thumb screw 50. The selected number of string shot cords are then threaded through the tension control equipment shown in FIG. 5 and their free ends are firmly secured to the opposite sides of the first centralizer by banding 85. The wire line is then lowered into the well and successive ones of the centralizer bodies are assembled to the line and taped in place. If the intermediate cord anchorages 90, such as those shown in FIGS. 6 and 7 are employed, the explosive cords are inserted into the V-shaped slots 91 of these anchorages and taped firmly in place.
After all of the centralizers have been installed and the tensioned explosive cords have been secured to the sides of the wire line in the manner just described, their upper ends are cut off in echelon and each is provided with a detonator cap 101 and connected in circuit with a respective terminal 103 of discriminator 97. The lead wires to the discriminator and to the detonator bus 100 are also completed following which the wire line is lowered into the well until the string shots are located directly opposite the perforations to be cleared of debris. Cover 17 of the blow out preventor is then closed and locked in place by thumb nuts 22 to avoid any possibility that the string shot operations will open a high pressure area of the earth formation into the well and initiate high pressure flow from the top of the well.
Before closing switch 95, the operator makes certain that rheostat regulator 96 is adjusted to its upper end or a harmless position ineffective to activate any detonator. The switch is then closed and the first cord is fired by adjusting rheostat 96 downwardly until the voltage increases sufficiently to fire the first string shot 65. No one of the other string shots 66,66, or 66" will fire because the voltage is inadequate to trigger any one of the Zener diodes 99a. The second string shot 66 will be detonated simply by shifting the rheostat in a direction to increase the applied voltage to a value triggering the upper one of diodes 99a. In this same manner the operator can proceed to detonate cords 66' and 66" in any desired time'interval by regulation of rheostat 96.
He may then proceed to detonate the remainder of the cords by returning the rheostat to its initial position and close switch in the opposite direction thereby reversing the polarity of the applied voltage. The operator then proceeds to adjust the voltage in steps as before to values adequate to fire cords 65', 65 65" Should the operator desire to fire more than one cord simultaneously, he adjusts the voltage regulator 96 to the appropriate voltage'known as required to assure triggering the desired number of diodes and then closes the switch in the proper direction to trigger these selected diodes.
It will therefore be apparent from the foregoing that a highly versatile apparatus and technique has been provided for performing string shot service operations on oil wells having faulty flow characteristics. The technique makes it possible to employ any desired number of string shots using the smallest available grain rating and including a series of larger grain ratings and to explode these in succession or in any desired groupings until full flow conditions are restored. The technique permits individual detonations to determine the effectiveness thereof, following which additional explosions of any selected size and number may be performed or, if tests show that full flow has been restored, the servicing operation can be immediately discontinued and the wire line withdrawn from the well.
The withdrawal operation is performed after opening cover 17 and then reeling in the line until the discriminator 97 and the centralizer exit from the well head. These are removed as are successive other centralizers in a simple and expeditious manner until the withdrawal operation has been completed.
While the particular wire line centralizer assembly herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention, and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims.
1. A wire line centralizer assembly adapted to be quickly attached to and detached from wire line means expeditiously with positive assurance against slippage of the centralizer assembly therealong when in attached position and leaving the wire line means proper free for compact stowage on a storage reel when detached, said centralizer assembly comprising: a main body having a passage therethrough to receive wire line means from one side of said main body, said main body and passage being designed for assembly to the wire line means from one lateral side of the latter with the wire line means seated in and-extending lengthwise of said passage, readily operable retainer means carried by said main body and'extending into said wire line seating passage in a position adapted tohold a wire line captively seated therein, and said centralizer assembly including stop means constructed and arranged to engage cooperating stop means on said wire linemeans and adapted to mutually cooperate with one another to hold said centralizer assembly secured to said wire line means and to limit and restrict movement lengthwise thereof.
2. A centralizer assembly as defined in claim 1 characterized in that said passage through said main body includes an enlargement between the opposite ends of said slot adapted to fit loosely about collar stop means fixed to wire line means, said enlargement and said collar stop means and said stop means on said main body cooperating to prevent displacement of the cen tralizer axially of said wire line means after being assembled thereto.
3. A centralizer assembly as defined in claim 1 characterized in the provision thereon of a plurality of stiff wire loops lying in generally radial planes dis tributed about the periphery of said main body and shaped to have a loose sliding fit with the interior of an oil well casing.
4. A centralizer as defined in claim 1 characterized in that said stop means on said wire line means comprises coupler means having enlarged ends securable to the adjacent ends of aligned sections of said wire line means and a reduced mid portion sized to have a loose fit within said passage through said main body and with the adjacent surfaces of the enlarged ends of said coupler means.
5. A centralizer assembly as defined in claim 1 characterized in that said retainer means extends into said passage in an area spaced from the bottom of said passage adequate to accommodate said wire line means and effective when in one adjusted position thereof to hold said main body captively assembled to a wire line means.
6. A wire line centralizer assembly adapted to be quickly attached to and detached from a wire line ex peditiously leaving the wire line proper free for compact stowage on a storage reel when not in use, said centralizer assembly comprising a mandrel having enlargements at its opposite ends for connecting said mandrel between the oppositely extending portions of a wire line of the type used to service oil wells and the like earth bores, centralizer means having an elongated main body formed with an open ended slot opening laterally through its side wall and sized to fit loosely over the mid portion of said mandrel, and readily operable retainer means carried by one of said first mentioned means and effective to lock said main body in assembled position with said mandrel in one position thereof and to leave said main body free for disassembly from said mandrel in another position of said retainer means.
A centralizer assembly as defined in claim 6 characterized in that said retainer means comprises thumb screw means adjustable crosswise of said slot and in position to lock said main body assembled to said mandrel.
8. A centralizer assemblyas defined in claim 6 characterized in that the leading end of said retainer means extends into said slot along a lateral side of said mandrel remote from the inner end of said slot and thereby effective to hold said main body against disassembly from said mandrel until said retainer means is retracted.