Foremost Dual Rotary drills feature two rotary drives, including a unique lower rotary drive that is used to advance steel casing through unconsolidated overburden such as sand, gravel, glacial till and boulders. The lower drive feeds and rotates the casing independently of the top drive. The casing is held securely in the lower drive by a set of power-operated jaws. A carbide-studded casing shoe, welded to the casing bottom, cuts through the overburden. Once the desired casing depth is reached, the DR continues drilling open-hole like a conventional top drive drill. There is no need to trip out or change tools when transitioning to open-hole drilling.
The independent rotary top drive simultaneously handles a drill string, which can be equipped with a down-the-hole hammer, tri-cone or drag bit. Cuttings are typically evacuated with air, but Foremost DR drills can also be configured with pumps for mud or flooded reverse circulation drilling.
The upper and lower rotary drives feed independently on separate hydraulic cylinders. This means that the bit position can vary in relation to the casing shoe. In most situations, the bit is advanced slightly ahead of the casing for best penetration rates. In heaving formations, or at times when it is desirable to obtain a clean sample of the formation, the casing can be advanced ahead of the drill bit.
Over the years, the Dual Rotary method has proven very effective for advancing steel casing in difficult unconsolidated overburden. In addition to the primary benefit of effective penetration, the rotation of the casing results in very straight holes and casing can be easily extracted using the lower drive, which is useful in well abandonment applications and for exposing well screens.