Introduction
Horizontal Directional Drilling (HDD) is a trenchless construction method used to cross obstacles along a pipeline route, such as roads, rivers, and sensitive areas. This method minimizes environmental impact and avoids interruptions to the obstacles being crossed.
HDD applies proven directional drilling techniques from electricity, oil, and gas well drilling to steer a drill bit beneath an obstacle along a planned pathway. This pathway is later enlarged to install a pipeline or conduit under the obstacle.
The Pit Construction Layout
A cross-sectional diagram illustrates the layout of pits required for HDD work. Pit sizes are calculated based on the bore length and size.
The Main Stages of HDD Technology
- Pilot Hole Drilling: Drilling a small diameter pilot hole along a curved profile using a remotely controllable assembly.
- Reaming: Enlarging the pilot hole to a diameter suitable for pipeline installation.
- Pipe Pull Back: Installing the pipeline into the reamed hole.
Stage 1: Pilot Hole Drilling
A small-diameter pilot hole is drilled with directionally controllable tools, following a planned profile. Specialized cutting heads and directional control devices ensure precision.
Stage 2: Pre-Reaming & Reaming
The pilot hole is enlarged using reamers to allow for the installation of the product pipe. This process may involve multiple passes with different reamers suited for varying ground conditions.
Stage 3: Pipe Pull Back
The pipeline is prepared as a continuous string, laid on a launching ramp with roller supports to minimize friction. After jointing and coating, the pipe is pulled back into the reamed hole.
Surveying Technique
A radiosonde system continuously transmits the cutting head's position and orientation using electromagnetic waves, assisting the drilling crew in steering the drill.
Drilling Fluid
The drilling fluid, a water and bentonite mix, is prepared in a mixing unit with double tanks holding up to 4000 liters of fluid. High-pressure pumps ensure efficient circulation during drilling.
HDPE Welding
HDPE (High-Density Polyethylene) pipes are joined using the butt-welding process, which involves:
- Alignment of pipe ends with clamps.
- Milling and smoothing the ends for clean coupling.
- Heating the ends to create molten rims for welding.
- Fusing and removing external protrusions for a smooth finish.
Bundle Formation
HDPE pipes can be pulled in bundles if all segments have similar physical properties. Bundles are strapped securely to withstand tension during installation. Pressure tests are conducted after bundling to ensure the integrity of the strapping.