Horizontal directional drilling (HDD) is a steerable trenchless method of installing underground pipelines in a shallow arc along a prescribed bore path by using a surface-launched drilling rig, with minimal impact on the surrounding area. Directional boring is used when trenching or excavating is not practical. The tools and techniques used in the HDD process are an outgrowth of the oil well drilling industry.
The components of a horizontal drilling rig used for pipeline construction are similar to those of an oil well drilling rig with the major exception being that a horizontal drilling rig is equipped with an inclined ramp as opposed to a vertical mast. HDD pilot hole operations are not unlike those involved in drilling a directional oil well. Drill pipe and downhole tools are generally interchangeable and drilling fluid is used throughout the operation to transport drilled spoil, reduce friction, stabilize the drilled hole, etc. Because of these similarities, the process is generally referred to as drilling as opposed to boring.
The horizontal directional drilling process represents a significant improvement over traditional trenching & backfill methods for installing pipelines beneath obstructions, such as rivers or shorelines, which warrant specialized construction attention.
Installation of a pipeline string by HDD is generally accomplished in following six stages:
- Geo-technical Investigation
- Drill path design
- Pilot hole drilling
- Boring/ reaming operation
- Preparation of pipe string
- Pipe pull-back
HDD crossing of pipeline primarily consists of drilling a small diameter pilot hole (≈ 6 inch to 8 inch) along the drilling path and then enlarging/ reaming the pilot hole upto a diameter which can facilitate the pipe string pull-back (generally 1.5 times pipe diameter). While boring/ reaming operation is being performed, pipe string preparation i.e. welding of line pipes, NDT of weld joints, field joint coating, pre-hydrostatic testing of the prepared pipe string etc. is done opposite to rig side of the crossing simultaneously. After the successful completion of hydrostatic testing, pipe string is pulled back into the enlarged hole.