摘要 |
Drilling apparatus comprises: a body; a pilot hole-drilling assembly mounted to the body for drilling a pilot hole of a first diameter; and a hole-opening assembly mounted to the body for opening the pilot hole to a larger second diameter. The apparatus is configured such that the pilot hole-drilling assembly will drill at a faster rate than the hole-opening assembly for a given unit loading. As the pilot hole- drilling assembly has the capability to drill at a faster rate than the hole-opening assembly, the pilot hole-drilling assembly will tend to experience relatively low weight-on-bit (WOB). The pilot hole-drilling assembly will thus tend to maintain a vertical orientation. The hole-opening assembly will follow the path of the pilot hole, while opening the hole to the required diameter. |