| 摘要 |
<p>A steerable drilling assembly (1) for mounting on the end of a drill string (11) serves to drill a horizontal borehole (2) in an underground formation (3). The drilling assembly comprises a mud motor (12) and a drill bit (4) driven by said mud motor and having an axis of rotation which forms an angle with the axis of the drilling assembly, as well as a self-propelling drive assembly (8) for substantially translatorily advancing the drilling assembly in the formation and exerting on the drill bit the pressure necessary for the drilling, said drive assembly being provided with driven wheels (15) which are pressed against the wall of the borehole by the mud pressure and are pulled free from the wall when the pressure is relieved. A swivel (6) is inserted between the drive assembly (8) and the mud motor (5), said swivel consisting of successively - seen in the flow direction of the mud - arranged first and second mutually rotatable swivel parts (16, 17). The swivel moreover incorporates a brake (21) to impede the rotation of the two swivel parts with respect to each other. Furthermore, the swivel incorporates a coupling arrangement which is so adapted as to be capable of locking the two swivel parts (16, 17) together by varying the volume flow of the mud in a predetermined manner, determining whether the drilling assembly is to drill straight ahead or change direction.</p> |
