Methodology of Horizontal Directional Drilling

INTRODUCTION

Horizontal directional drilling (HDD) has emerged as a preferred crossing method in many situations for the installation of various pipelines as well as other utilities under watercourse, roads, rail lines, steep slopes and other obstacles.

This technology has been enthusiastically embraced by proponents, contractors and regulators as a low impact construction technique.

Description of HDD

Horizontal directional drilling is a trenchless construction method utilizing equipment and techniques from horizontal drilling technology and conventional road boring. HDD construction is used to install pipelines (steel or plastic), fibre optic and electric cables, water and waste water pipelines where conventional open trench construction is not feasible or will cause adverse disturbances to environmental features, land use or physical obstacles.

HDD technology is used in many situations, including the following:

• Lake crossings,
• Wetland crossings,
• Canal and watercourse crossing,
• Valley crossing,
• Sensitive wildlife habitat,
• Road and railway crossing.

HDD installation involves four main steps:

1. Pre-site planning,
2. Drilling and pilot hole,
3. Expanding the pilot hole by reaming,
4. Pull back of pre-fabricated pipe.

1. Pre-site Planning

If a HDD is determined to be feasible, a drill path is designed to meet the requirements of the crossing and appropriate drill entry and exit locations are selected. Prior to selecting the drill entry point, the width and the depth of the river or stream are measured to determine the position of the drill entry point. Depending on the depth of the river or stream, the drill entry point is positioned at a minimum distance of 5m from the river bank or the edge of the stream with a 1.5m clearance under the river bed and the maximum tilt angle is 12 degree to the horizontal.

2. Drilling the Pilot Hole

The HDD drill rig and supporting equipment is set-up at the drill entry location determined during the pre-site planning phase. A pilot hole is drilled along the predetermined drill path.
Periodic readings from a probe situated close to the drill bit are used to determine the horizontal and vertical coordinates along the pilot hole in relation to the initial entry point; the pilot hole path may also be tracked using a surface monitoring system that determines the down hole probe location by taking measurements from a surface point.

3. Reaming of the Pilot Hole

The down-hole assembly is removed from the drill upon breaking the ground surface at the exit location and is replaced with a back reamer. The drill is pulled back through the bore hole and the back reamer enlarges the diameter of the drill hole.

4. Pipe Pull Back

Pipe is then attached, that is slightly longer than the length of the drill, on the exit side of the bore hole. The pipe is pulled into the exit hole and pull-back continues until the entire pipe has been pulled into the bore hole. Upon successful pull-back of the pipe, the drilling equipment is dismantled and demobilized. This pipe is then connected to the conventionally laid pipe and work areas are reinstated.

HDD Design Consideration

When designing a project for directional boring, it is important to have accurate geo-technical data, sufficient space for the bore rig and support equipment. It is best to allow extended work-hours for boring operations and is essential for pull-back. Additional considerations may be required for specific projects.

a) Utility detection and mapping is necessary for the designation of HDD profile.
b) Local authority and related permit must be approved before entrance to site.
c) The minimum bending radius of steel pipe is 1000 times of the pipe diameter. (250mm x 1000 = 250 meters)
d) The entry pit and exit pit must be ensured that no existing utility, structure, pilling or drainage along the proposed HDD alignment.
e) The soil condition is normal with N values less than 30 blows/30cms.