As an exploratory tool, it can be desirable to drill very small diameter boreholes, hereinafter referred to as "slim hole" drilling. Whereas a conventional wellbore may start with a diameter of about 20 inches, a slim hole wellbore would start with a diameter of no greater than about 6 inches. By drilling small diameter boreholes, larger numbers of exploratory wells can be drilled more economically. In addition, the drilling rig necessary for these smaller diameter boreholes is lighter thereby allowing for more economical moving of the rig from one drilling location to another.
With slim hole drilling much greater than conventional rotation rates for the drill pipe and bit are employed. Whereas conventional rotary drilling uses rotation rates no greater than about 350 rpm, and most usually in the range 50-150 rpm, slim hole drilling is preferably carried out at no less than about 500 rpm.
Heretofore in offshore rigs in order to gain more working room on the working floor of the rig, the rotary table has been replaced by an electric motor hung in the derrick, U.S. Pat. No. 3,426,855. However, the method and equipment used in such a drilling setup was for conventional size boreholes drilled at conventional rotation rates and therefore was not suggestive for slim hole drilling. Nor was it known whether such equipment would or could be made to stand up under the unique rotation and vibration conditions encountered only with slim hole drilling techniques.