Centralizer

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 and a centre tube,

wherein the centralizer body is arranged to be rotatable around the split inner tube and around the centre tube,

wherein an inner surface of the centre tube facing an outer surface of the split inner tube and an outer surface of the centre tube facing an inner surface of the centralizer body comprise a low friction material, and

wherein the split inner tube comprises a longitudinal split and an inner surface made from a second material different from the low friction material, the second material having a higher coefficient of friction.

2. The centralizer according to claim 1 , wherein a stop collar is arranged at either end of said centralizer body, and the centralizer body is arranged to be rotatable relative to said stop collars, wherein at least one end surface on the centralizer body and/or the stop collar comprises a low friction material.

3. The centralizer according to claim 2 , 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.

4. The centralizer according to claim 2 , wherein the stop collar is provided with a bevel at the end remote from the centralizer body.

5. The centralizer according to claim 2 , wherein the ends of the stop collar facing the centralizer body are each accommodated within a recess formed at each end of the centralizer body.

6. The centralizer according to claim 2 , wherein the stop collar comprises a split main body with a longitudinal gap provided with a fastening device configured to fix the stop collar to the pipe string, and an inner surface made from a high friction material.

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

8. The centralizer according to claim 7 , wherein each protruding portion is provided with a coating made from low friction material.

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

10. The centralizer according to claim 7 , wherein each protruding portion is in the form of a roller.

11. The centralizer according to claim 10 , wherein each roller is located in a recess in the centralizer body and extends a predetermined distance radially outwards from said centralizer blades.

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

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

14. The centralizer according to claim 1 , wherein the split inner tube comprises:

the inner surface which in use is in contact with the pipe string; and

the outer surface which in use is in contact with the center tube,

wherein the inner surface has a higher friction coefficient than the outer surface.