Centralizer

Abstract

A centralizer includes a centralizer body to be situated at the outer surface of a pipe string in the form of casing, liner, or the like used while drilling, the centralizer body being formed with a plurality of outer centralizer blades arranged in an inclined manner to the longitudinal axis thereof, wherein the centralizer body has an separate split inner tube secured to the pipe string by means of a press fit, and low friction inner surface of the centralizer body and separate center tube facing each other are made from low friction material.

Claims

1. A centralizer comprising a centralizer body to be situated at the outer surface of a pipe string in the form of casing or liner used while drilling, the centralizer body being formed with a plurality of outer centralizer blades arranged in an inclined manner to the longitudinal axis of the centralizer, wherein the centralizer comprises a split inner tube secured to the pipe string, wherein the centralizer body is arranged to be rotatable around the split inner tube, wherein the centralizer further comprises a centre tube provided between the centralizer body and the split inner tube, wherein facing surfaces on the centre tube and the centralizer body and the split inner tube comprise a low friction material, wherein the centralizer further comprises a first stop collar and a second stop collar, the first and second stop collars being arranged at opposite ends of said centralizer body, the centralizer body being arranged to be rotatable relative to said first and second stop collars, wherein at least one end surface on the centralizer body and/or the first and second stop collars comprises a low friction material, and wherein the centralizer body includes a first recess and a second recess at opposite ends thereof, ends of each of the first and second stop collars facing the centralizer body are accommodated within the first and second recesses, respectively, and the first and second stop collars are separately secured about the pipe string to hold the centralizer in place.

2. The centralizer according to claim 1, wherein an annular ring coated with or made of the low friction material is situated between the centralizer body and at least one stop collar of the first and second stop collar.

3. The centralizer according to claim 1, wherein each of the first and second stop collars is provided with a bevel at the end remote from the centralizer body.

4. The centralizer according to claim 1, wherein each of the first and second stop collars comprises a split main body with a longitudinal gap provided with a fastening device configured to fix the first and second stop collars to the pipe string, and an inner surface made from a high friction material.

5. The centralizer according to claim 1, wherein the outer centralizer blades are provided with at least one outer protruding portion.

6. The centralizer according to claim 5, wherein each protruding portion is provided with a coating made of low friction material.

7. The centralizer according to claim 5, wherein each protruding portion is in the form of a pad made of low friction material.

8. The centralizer according to claim 5, wherein each protruding portion is in the form of a roller.

9. The centralizer according to claim 8, wherein each roller is located in a recess in the centralizer body and extends a predetermined distance radially outwards from said centralizer blades.

10. The centralizer according to claim 8, wherein each roller extends radially outwards through an opening in the outer circumferential surface of the centralizer blades, and that the opening is smaller than the roller outer diameter at each point along its length.

11. The centralizer according to claim 8, wherein each roller has an oval basic shape.

12. The centralizer according to claim 1, wherein the outer centralizer blades are formed with a curved or helical configuration.

13. The centralizer according to claim 1, wherein the separate split inner tube comprises a longitudinal split and an inner surface made from high friction material allowing for the press fit against the pipe string.

14. The centralizer according to claim 1, wherein each of the first and second stop collars comprises a first surface arranged to face the pipe string and a second surface arranged to face the centralizer body, wherein the first surface is made of a first material and the second surface is made of a second material, and wherein the first material has a higher friction coefficient than the second material.

15. The centralizer according to claim 1, wherein the split inner tube comprises an inner surface made of a first material, wherein the split inner tube comprises an outer surface made of a second material, and wherein the first material has a higher friction coefficient than the second material.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will be described in detail with reference to the attached figures. It is to be understood that the drawings are designed solely for the purpose of illustration and are not intended as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to schematically illustrate the structures and procedures described herein.

(2) FIGS. 1a-d show four different side elevations of the centralizer comprising a centralizer body provided with a centre tube, a split tube, outer centralizer blades comprising rollers, and two stop collars in perspective, exploded and sectional views, respectively;

(3) FIG. 1e shows an end elevations of the centralizer comprising a centralizer body provided with an separate split inner tube, centre tube, outer centralizer blades comprising rollers, and two stop collars in perspective, exploded and sectional views, respectively;

(4) FIG. 1f shows a side elevation of a oval roller arrangement inside a recess in the centraliser protruding blade

(5) FIG. 2 shows the same view as FIG. 1b, where the rollers have been replaced by a low friction coating or pads; and

(6) FIG. 3 shows a side elevation of the stop collar in perspective view.

EMBODIMENTS OF THE INVENTION

(7) As illustrated in FIG. 1a-e, the present invention comprises four main components, that is, a centralizer body 2, a separate split inner tube 8, a centre tube 9 which can as an alternative be an integrated part of the centraliser body 2, and a stop collar 3. As shown in FIG. 1a, identical stop collars 3 are positioned at each end of the centralizer body 2.

(8) The centralizer body 2 is fixedly mounted around a casing 1 located in a borehole during a drilling operation. The centralizer body 2 comprises a separate split inner tube 8 and is provided with a plurality of outer centralizer blades 4 arranged in an inclined, helical manner to the longitudinal axis thereof. In FIG. 1e four centralizer blades 4 are shown but this number may be varied according to the application of the centralizer. The separate inner tube is secured to the casing by means of a press fit. As already mentioned above, the inner surface of the centralizer body 2, the entire centre tube 9 and the outer surface of the separate split inner tube 8 facing each other are made from a suitable low friction material, such as Teflon or nylon. This arrangement allows for a reduction of rotational friction between the centralizer body 2, the centre tube 9 and the split inner tube 8 and ensures that no rotation occurs between the casing 1 and split inner tube 8, thereby preventing undesired wear on the pipe string outer diameter.

(9) The split inner tube 8 is provided with a longitudinal split 12 allowing it to be expanded and placed over the casing 1 into the selected position. The split inner tube 8 is made slightly under gauge and clamps around the outer diameter of the casing by a force caused by the resilient properties of split inner tube 8 as the force expanding the split inner tube 8 is released. The grip of the split inner tube 8 is enhanced by an inner surface made from high friction material, such as brake band materials or a soft metal (e.g. aluminium). This low friction tube is placed around the casing to act as a bearing face about which the centralizer body and the low friction centre tube 9 in can be rotated. The intention is to protect from wear during rotation and create a rotational surface outside the casing with very low resistance to the revolving inner centralizer surface. Having positioned the split inner tube 8 in the correct area, the centralizer body 2 and the centre tube 9 is slid over the split inner tube 8 and secured in place by a stop collar 3 on each side of the centralizer body 2.

(10) Each end of the centralizer body 2 is equipped with an annular low friction ring 7 in order to reduce the rotational friction between the centralizer body 2 and the stop collars 3. The low friction material centre tube 9 is formed by a cylindrical body placed between the split tube 8 and the centralizer body 2. A stop collar overlap 6 is formed by allowing each end of the centralizer body 2 to extend axially past the ends of the cylindrical low friction centre tube 9. The ends of the cylindrical low friction centre tube 9 forms an annular stop, against which the respective annular low friction rings 7 and the stop collars 3 are positioned. The stop collar overlap 6 reduces the amount of particles to enter the bearing faces between the inner split tube 8, the centre tube 9 and the centralizer body 2, and the stop collars 3 and the low friction end rings 7, respectively.

(11) According to a further example, the ends of the stop collar 3 facing the centralizer body 2 can be provided with a low friction coating or be made from a suitable low friction material (not shown).

(12) The centralizer body 2 is equipped with protruding portions 5 formed in the helical centralizer blades 4. The curved blades give better circular coverage which makes stand off less dependent on the position of the centralizer in the borehole. The protruding portions reduce the sliding resistance and each are made in the form of an oval formed roller 10 to avoid point loading on the edge of the roller in a curved borehole. The roller 10 is supported by means of an axle (see FIG. 1f) mounted in a recess in the centralizer body 2 and extends a predetermined radial distance out of a roller opening. The outer diameter of the roller 10 is larger than the width of the roller opening in the axial direction of the centralizer body. This arrangement removes the possibility of the rollers falling out of the centralizer body into the borehole and becoming an operational hazard.

(13) A bevel 11 at each end of the centralizer functions as a guide if encountering cuttings beds and reduces the risk of hanging up on ledges or sharp edges while running in or pulling out of a borehole.

(14) As indicated in FIG. 1f, each protruding portion can be in the form of a roller, such as a failsafe oval formed roller with an outer surface having a minimum diameter at each end and a maximum diameter at its middle section. The roller is journalled in bearings at each end and is arranged with its central axis at right angles to the central axis of the centralizer. The rollers are installed in recesses inside the blades and extend radially outwards through openings in the outer circumferential surface of the blades. These openings are smaller than the roller outer diameter at each point along its length. The roller can be located in a recess machined into the inner surface of the centralizer body, which recess opens out through the outer surface of the centralizer blade. The shape of the opening is arranged to conform with the outer contour of the roller extending a predetermined radial distance out of the opening.

(15) FIG. 2 shows an alternative embodiment of the invention as shown in FIG. 1b. In FIG. 2 each of the protruding portions indicated by reference numeral 5 in FIG. 1b are replaced by a pad 25 made from a suitable low friction material sunk into the outer surface of the blade 24 or a raised surface provided with a coating made from low friction material. The remaining numerals are identical to those used in FIG. 1b. The pad 25 or the raised, coated surface protrudes a predetermined radial distance from the outer surface of the blade 24. The low friction pad or coating reduces the sliding resistance between the centralizer and the borehole. The pad or coating is given an oval shape conforming to the diameter of the borehole to avoid a point loading from being applied on the pad or coating by the wall of the borehole.

(16) The outer diameter and inner diameters of the centralizer in the above embodiments are matched to the casing size. The centralizer inner diameter normally ranges from 4 to 20 and the centralizer outer diameter ranges from 6 to 24. The length of the centralizer can vary somewhat with its inner diameter but will typically range from 15 to 30.

(17) As shown in FIG. 3, the stop collar 3 comprises a split main body 31, which is to be slid on to the casing, and a number of fastening screws 34 arranged in a tangential direction at right angles to the central axis of the main body 31. The fastening screws 34 extend across a gap 33 formed by the split portion of said main body 31, but do not protrude outside the outer diameter thereof. A stop collar is slid onto the casing the end of a casing section to a selected position. When the fastening screws 34 are tightened, the inner diameter of the stop collar 3 is reduced and the stop collar is clamped around the casing to ensure that the contact between the pipe string casing and stop collar is as tight as possible. A high friction internal surface 32 of the stop collar is provided to increase the resistance to movement between stop collar and the casing after the stop collar has been fixed firmly in place. The stop collar 3 is also formed with a bevel configuration 36 at the end remote from the centralizer body and has a low friction ring 35 at the end surface facing the centralizer body. The low friction ring 35 can be used in place of or as a supplement to the low friction ring 7 mentioned in connection with FIGS. 1a-1e above. The low friction ring 35 is installed against the centralizer and ensures that the friction between the stop collars and the centralizer assembly is kept as low as possible when the pipe string casing is rotated. The bevel 36 on the stop collar is arranged to guide the pipe string if ledges are encountered in the borehole and reduces the risk of hang up during operations.

(18) The outer diameter and inner diameters of the stop collar in the above embodiments are matched to casing size. The stop collar inner diameter normally ranges from 4 to 20. The length of the stop collar can vary somewhat with its inner diameter but will typically range from 2 to 6.