HDD Construction Method
The HDD rig is positioned at the start or end of the route along which the pipe will be pulled. The drill bit is advanced by pushing and rotating in turn, according to the ground conditions and the designed underground profile, until it surfaces on the far side.
As in every stage, drilling fluid is injected into the hole under pressure during these operations.
When the steering head emerges on the far side it is removed and replaced by the reamer that will enlarge the hole. Depending on the pipe diameter, this may be done in several passes.
In the final stage the pipe to be installed is connected behind the reamer by means of a swivel (a double ring, one side of which rotates on a spindle), and the pipe is pulled underground.
The longest crossing built this way to date is the project connecting the Mittelplate oil field in Germany's North Sea to Dieksand. A 406 mm steel oil pipeline was installed underground over a distance of 7,500 metres. The total project took 2 months. 1600 mm is the largest pipe diameter installed by HDD.
Summary of the method and planning
The horizontal directional drilling construction method consists of three main stages: the pilot bore, reaming and pullback. During the pilot bore, the sonde inside the steering head allows the depth, inclination and direction of the bit to be monitored continuously; when the path deviates from the design profile, the rig stops rotating and pushes only, turning the head in the required direction. Reaming is carried out in stages until the hole reaches 1.2–1.5 times the pipe diameter; at each stage the drilling fluid both carries the cuttings out and keeps the hole stable. During pullback the pipe is connected to the reamer through a swivel; because the swivel does not transfer the reamer's rotation to the pipe, the pipe is not twisted.
Site planning is an inseparable part of the method. Entry and exit angles are usually chosen between 8° and 18°, depending on the rig class and pipe material; the allowable bending radius of the pipeline determines the deepest point of the profile and its total length. No route is designed without a geotechnical survey, the groundwater level and a map of existing utilities. Deltek shares this profile with the client on every HDD project; rig capacity, drill rod set, reaming stages and mud recipe are all determined from the ground data. See also the HDD rig selection and drilling fluid pages.
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