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A downhole device, adapted for use in wireline or slickline application, is provided for incorporation into a downhole string and movement in a wellbore. The device comprises means to orient the device in a wellbore, and one or more conveying means arranged to engage the inner surface of the wellbor

A downhole device, adapted for use in wireline or slickline application, is provided for incorporation into a downhole string and movement in a wellbore. The device comprises means to orient the device in a wellbore, and one or more conveying means arranged to engage the inner surface of the wellbore and reduce the frictional coefficient. It may also incorporate an eccentric weight and/or a vibrator.

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1. A roller sub for incorporation into a downhole string and movement in a well-bore, the sub comprising at least one swivel device, an orientation device to rotationally orient the sub in a well-bore, and at least one conveyor selected from the group consisting of wheels and rollers arranged on the

1. A roller sub for incorporation into a downhole string and movement in a well-bore, the sub comprising at least one swivel device, an orientation device to rotationally orient the sub in a well-bore, and at least one conveyor selected from the group consisting of wheels and rollers arranged on the sub to engage the inner surface of the well-bore. 2. A roller sub as claimed in claim 1, wherein the orientation device comprises an eccentric weight. 3. A roller sub as claimed in claim 1, wherein the orientation device comprises an eccentric shape of a portion of the sub. 4. A roller sub as claimed in claim 1, wherein the orientation device comprises a ballast mechanism eccentrically positioned in the sub to favour adoption by the sub of at least one orientation rather than others. 5. A roller sub as claimed in claim 1, wherein the orientation device is adapted to orient the sub such that the at least one conveyor is in contact with the inner surface of the well-bore. 6. A roller sub as claimed in claim 1, wherein the orientation device comprises a portion of the sub having a variable radius that favours orientation of the sub on a portion of the sub having a relatively larger radius. 7. A roller sub as claimed in claim 6, wherein the at least one conveyor is associated with the portion of the sub having a relatively larger radius so that it is brought into contact with the well-bore surface when the sub orientates itself on the portion. 8. A roller sub as claimed in claim 1, wherein at least one conveyor is provided on opposing sides of the sub. 9. A roller sub as claimed in claim 1, wherein the at least one conveyors is provided in at least one pair of wheels or rollers, each wheel or roller in the pair being laterally spaced from the another in a side-by-side arrangement. 10. A roller sub as claimed in claim 1, wherein the conveyor is provided on the lower surface of the sub. 11. A roller sub as claimed in claim 1, wherein the conveyor is arranged to contact the well-bore inner surface substantially at the outer circumference of the sub. 12. A roller sub as claimed in claim 1, wherein the sub is provided with apertures from which the conveyor protrudes to engage the inner surface of the well-bore. 13. A roller sub as claimed in claim 12, wherein the protruding portions of conveyor are shaped to match the outer surface of the sub adjacent to the apertures. 14. A roller sub as claimed in claim 12, wherein the protruding portions of the conveyor are shaped to match the inner surface of the well-bore through which the sub will be moving. 15. A roller sub as claimed in claim 1, wherein the conveyor includes wheels of large diameter arranged to extend through the body at opposite sides of the sub. 16. A roller sub as claimed in claim 1, wherein the conveyor includes wheels positioned in pairs side-by-side, with each pair on the same axis through the sub. 17. A roller sub as claimed in claim 1, wherein two or more conveyors are provided, and at least some of the conveyors are axially, circumferentially or laterally offset with respect to one another. 18. A roller sub as claimed in claim 1, incorporating a vibrator. 19. A roller sub as claimed in claim 18, wherein the vibrator comprises an eccentric motor. 20. A roller sub as claimed in claim 18, incorporating a motor to drive at least one of the group consisting of the conveyor and the vibrator. 21. A roller sub as claimed in claim 18, wherein the frequency of vibrations induced by the vibrator can be varied. 22. A roller sub as claimed in claim 1, having a sensor coupled to a switch for controlling the movement of the sub in the well-bore. 23. A roller sub as claimed in claim 22, wherein the sensor is adapted to sense the attitude of the sub. 24. A roller sub as claimed in claim 22, wherein the sensor detects the vertical attitude of the sub and sends a signal to the switch controlling the conveyor, triggering the conveyor to initiate movement of the sub if the sub deviates more th an a fixed amount beyond the vertical. 25. A roller sub as claimed in claim 24, wherein the conveyor is triggered to initiate movement of the sub if the deviation of the sub from the vertical exceeds a value above 50°. 26. A roller sub as claimed in claim 1, having a power source. 27. A roller sub as claimed in claim 1, having an axial passage for at least one of the group consisting of power cables, data cables, control wires, coiled tubing and other elongate members. 28. A roller sub as claimed in claim 27, wherein the axial passage is held stationary with respect to the downhole string. 29. A downhole tool comprising a roller sub as claimed in claim 1. 30. A downhole string comprising a roller sub as claimed in claim 1.