Horizontal directed drilling (HDD)
HDD is an extremely versatile trenchless technology that is used for the installation of everything from service connections to residences and buildings, to pipes and cables under roadways and rivers. HDD is best suited for installing pressure pipes and conduits where precise grades are not required. The main components of HDD are:
A directional drill rig sized for the job at hand.
Drill rods linked together to form a drill string for advancing the drill bit and for pulling back reamers and products.
A transmitter/receiver for tracking and recording the location of the drill and product;
A tank for mixing and holding drilling fluid.
A pump for circulating the drilling fluid.
Other components of an HDD operation include bits, reamers, swivels and pulling heads. (ISTT)
The microtunnel execution process by means of a Horizontal Directed Drilling equipment comprises 3 different stages which are directly related to the excavation direction along the microtunnel alignment.
1. The first stage is the pilot drilling, using a small diameter bit, from the entrance trench to the exit trench. To allow the sustain of the ground within the perforation hole, the entire space is filled with a bentonitic fluid. This fluid is injected through the back of the drilling bit and allows both the sustain of the hole and the extraction of the cuttings from the excavation.
2. The second stage consist on the enlargement of the pilot drill by means of back-reaming, meaning that the reamer is installed at the end of the alignment and then pulled back from the starting trench.
3. Finally a bigger reamer is connected to the alignment, with the appropiate diameter for the final pipeline to be installed. Pulling back the pipeline, it gets into the borehole through the alignment until reaching the starting pit. During this process the bentonitic fluid will provide the necessary lubrication for the pipe, helping to avoid most of the friction with the borehole perimeter.
The guidance of the pilot drilling usually is performed with 3 different technologies:
Walk-over: Radio based system consisting on the installation of a transmitter into the borehead of the pilot drilling which signals are received on the surface along the alignment route.
Wire-line: In cases when the surface above the drilling path is not accessible it is usual to apply this system which consist on transmitting the borehead position data through a wire inside the drilling string.
Gyro compass: Other guidance systems use magnetometry for the borehead tracing, working separately form the earth magnetic field and, therefore, defining accurately the drilling axis direction.
The drilling string is comprised by pipes under big stresses both traction and compression due to the push and thrust of the machine as well as torsion due to the rotation torque. Moreover the pipes must be flexible, in order to adapt to the direction changes within the alignment, and light enough to improve transportation and installation. And, of course, resistant to abrasion and wear damages. (Victor Yepes)
Therefore, the HDD technology can be implemented into pipelines with the following lining systems:
Metallic pipes (Cast iron, steel)
Plastic pipes (HDPE, PVC, GRP…)
