Horizontal Directional Drilling (HDD)

The Horizontal Directional Drilling method
The directional drilling technique is on the way up at the moment. The application of HDD units enables trenchless installations up to 500 m length. Crossings underneath rivers and other waters are frequently performed. The directional method even makes drillings underneath industry complexes possible. The spectrum of application includes all pipe construction measures within the bounds of gas, district heating and drinking water supply, the installation of pressure lines for sewers as well as cable protection pipes for television or telephone cables, traffic routing systems, emergency call boxes or low, medium, high voltage and optical fibre cables.

Schematic survey of the HDD method

The HDD method is extremely protective towards the environment, causing no ecological damage at all. Minimal damage to the area is only caused within the vicinity of the unit. Several reasons also speak for the application of the directional technique in central town areas. These mainly concern the construction costs, construction periods, permission procedures, soil movement, surface restoration and the traffic, compared to open trenching methods. The normal course taken by the drilling operation is described in the following:

1. Planning, preliminary survey
2. Selecting the drilling units and drilling tools
3. Pilot bore and detection
4. Backreaming(s) or upsizing bore(s)
5. Pulling in the pipe

A well planned HDD operation includes preliminary survey of the bore path area, concerning other existing external lines and the soil condition.
The choice of the drilling unit depends upon the bore length, the diameter of the pipe to be installed and the soil quality.

Keeping up a certain bore path and gradient course is the main problem of the pilot bore, when the fluid assisted technique fails because of mechanical soil resistance. To overcome this, high thrust and tension forces are required, reaching the limits of maximum capacity in many cases. Application of Bentonite might relieve the pilot bore and the pipe traction. But experience has proved that the technical and economical advantages of Soft Boring can hardly take effect in difficult soil conditions with coarse grained components and considerable rock inclusions or building rubble deposits. For these conditions, we have the TT drilling facilities with built-in impact unit, which can be switched on additionally, when the drilling progress becomes unsatisfactory due to problematic grounds. In other words, the drilling unit is equipped with the shattering force of a displacement hammer. It is the combination of fluid assisted drilling with an impact unit which makes propulsion and steerability possible in difficult soil qualities up to grade 5, sometimes even grade 6.

The high demands and expectations towards the quality of the drilling job waiting to be done require precise localisation and steering. Localisation is carried out following the transmitter-receiver principle. One operator follows the course of the bore head all the time. All measured values are taken to protocol, directional corrections are passed on to the machine operator via radiophone. It is also possible to directly store the data found in this way, printing them with the help of a PC or Laptop later on. Directional control works according to the "clock-hand principle" via slanted surface of the head; the head changes its direction in correspondence with the clockwise direction.

On arrival, the drilling head is replaced by an upsizing head (backreamer). One or more intermediate reaming operations follow, or the pipe is pulled in at once. Pipes up to (for the time being) OD 600 made of plastic, steel and cast iron can be pulled in, single or bundled.

Advantages of the HDD method:
- surfaces worth conserving are neither broken up nor damaged(road surface,
front gardens etc.), restoration and repair are not required - which leads to high
economical advantages
- low social costs, because detours are avoided
- short equipping times short drilling and construction times
- very economic for river crossings
- supported by the dynamic impact of the striking mechanism, the propulsion and
steerability are improved in soil qualities up to grade 5, sometimes even grade 6
- wide spectrum of application
- acknowledged procedure
- pulling force measurement and position determination are possible

Animation courtesy of Allen Watson Ltd.