Abstract: A centralizer comprises two annular bands which are spaced apart by a plurality of members extending there-between. The members are inclined to the longitudinal axis of the centralizer by an angle a of typically 30.degree. to 45.degree..

Abstract: A top drive is used to connect successive lengths of casing. The casing is gripped by a mechanism which comprises a support which is attached to the top drive by a drive plate. A circular plate is rotatably mounted in the support and is associated with four jaw assemblies. In use the mechanism is lowered onto a length of casing until the circular plate engages the top of the casing. When the top drive is rotated in one sense the support rotates relative to the circular plate. This causes toothed cylinders to rotate which, in turn, rotates eccentric members which advance the jaws into gripping engagement with the casing. Further rotation of the top drive rotates the casing and screws it into the casing below.

Abstract: A centraliser has been developed which in one aspect an upper ring and a lower ring connected by six rigid members. The centralizer can be mounted on casing to be lowered down deviated bores. The rigid members are designed to collapse under a lateral load of about 20 tonnes. Thus if, for example, part of the bore collapses and the casing has to be withdrawn the rigid members will collapse enabling the centralizer to be recovered on the casing, preferably in one piece.