| 摘要 |
Drilling apparatus comprises: a body; a pilot hole-drilling assembly (14) mounted to the body for drilling a pilot hole of a first diameter; and a hole-opening assembly (16) 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 (14) will drill at a faster rate than the hole-opening assembly (16) for a given unit loading. As the pilot hole-drilling assembly (14) has the capability to drill at a faster rate than the hole-opening assembly (16), the pilot hole-drilling assembly will tend to experience relatively low weight-on-bit (WOB). The pilot hole-drilling assembly (14) will thus tend to maintain a vertical orientation. The hole-opening assembly (16) will follow the path of the pilot hole, while opening the hole to the required diameter. |
