The technique of directional drilling begins with the boring of a small diameter pilot hole to the design profile, incorporating monitoring and adjustment of the alignment as the bore is advanced. The head assembly contains instrumentation to transmit the magnetic bearing of the drilling direction, the angle of the inclination of the drilling and the established position. Adjustment of the small offset section of the head assembly known as the “bent” allows steering corrections throughout the bore.
The pilot hole is enlarged by reaming in stages as required until the final diameter is achieved. The final pipeline or casing is then pulled through the hole into position. This method has been designed for long crossings of pressure pipes or cables in a curved profile beneath rivers or similar sized obstructions.