Double Head Rotary Drilling System
Double head Rotary-Rotary systems are a preferred method for drilling straight, deep holes. These systems offer the driller a high level of flexibility and control to deal with varying ground conditions. In combination with a DTH system on the inner drill string, a Rotary-Rotary system is able to achieve deeper depths compared to drifter (top hammer) based systems.
The double head Rotary-Rotary drilling system utilizes two independent rotary heads to separately drive the outer casing and the inner drill string. This system uses a lower drill head to turn rotary casing equipped with a carbide casing crown. An upper rotary drill head turns the inner drill rod equipped with a down the hole hammer (DTH), drag bit, tri-cone, or an auger drill string.
The two rotary heads are typically mounted on separate sleds which can be adjusted independently to control the position of the inner string in relation to the casing bit. This means that the inner bit can be advanced in front of the casing bit to act as a pilot bit for the casing string or it can be retracted inside the casing crown to better contain flushing inside the casing.
When drilling with DTH hammers on the inner string it is important to include a shock absorber. The shock absorber is mounted below the upper rotary head for the inner string and protects this rotary head from the percussive energy of the DTH hammer. The elastometric elements in the shock absorber dampen the DTH vibrations protecting the internal gearing of the rotary head.
Components utilized in the Double Head Rotary Rotary System include the following
The flushing head in Rotary-Rotary drilling systems is mounted above the upper drilling head. It is designed to fit the specific drill head in the system.
The shock absorber in a double head rotary system mounts below the top rotary head and is connected to the inner drill string. The shock absorber protects the rotary head from damage due to vibrations from the DTH hammer.
The balance rod provides the link between the top drill head and the inner drill string. Balance rods are made to the exact length required by the installed drilling head system.
Casing flange & ejection bell
The casing flange connects the casing to the lower rotary head and provides an exit point for the flushing medium through the ejection bell.
Dual rotary drilling systems utilize male/female friction welded double-start rotary percussive casing. Rotary percussive single and triple-start casing can also be used.
Inner drill rod
The inner drill string on dual rotary drill systems are typically
API drill rods.
Casing bits for dual rotary systems are ring bits with tungsten carbide inserts. The type of carbide insert is dependent on the ground conditions being drilled but is typically a scraping type button form.
Inner string bit
Typically a down the hole hammer is used on the inner drill string of dual rotary systems. These systems can also utilize various types of rotary bits including tri-cone and drag bits depending on ground formations.
External Tube Ø Internal tube Ø
108 mm, LH 76.1 mm API 2 3/8” REG, RH
114.3 mm, LH 76.1 mm API 2 3/8” REG, RH
133 mm, LH 88.9 mm API 2 3/8” REG, RH
152.4 mm, LH 101.6 mm API 3 1/2” REG, RH
177.8 mm, LH 114.3 mm API 3 1/2” REG, RH