Horizontal Directional Drilling

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What is HDD ?

Horizontal Directional Drilling (HDD) is a technique that comes from the oil field, but it is applied to the crossing of rivers, railways, and other obstacles. The drilling assembly has a bent sub for steering purposes, and is equipped with an electronic probe to continuously report the position of the pilot hole to the driller. Interpreting this information allows the pilot hole the follow the designed path.

- Step 01

Pilot Hole

The first step in a HDD installation is to drill a carefully guided pilot hole that delivers the drill bit and bore head to the surface at the specified exit point. The bore is launched from the surface, and the pilot bore proceeds downward at an angle until the necessary depth is reached. A small-diameter drill string penetrates the ground at a prescribed entry point and the design entry angle, normally between 8 and 16 degrees.

During the drilling process the bore path is traced by interpreting electronic signals sent by a monitoring device located near the head of the drilling string. At any stage along the drilling path the operator receives information regarding the position, depth, and orientation of the drilling tool, allowing him or her to navigate the drill head to its target.

- Step 02


Once the pilot hole is successfully drilled, the hole is often enlarged to a suitable diameter for the product pipeline. For instance, if the pipeline to be installed is 18 inches in diameter, the hole may be enlarged to 30 inches or more. This is accomplished by reaming the hole to successively larger diameters. Generally the reamer is attached to the drill string on the bank opposite the drilling rig, rotated, and pulled (pushed in some instances) back through the pilot hole. Joints of drill pipe are added as the reamer makes its way back to the drilling rig.

Large quantities of slurry are pumped into the hole to maintain its integrity and to flush out cuttings. While soil conditions do have an impact, the required number of reaming runs is mainly dependent on the diameter of the product pipe and the diameter of the pilot hole. It may vary from no reaming runs to several for large-diameter product pipes.

- Step 03

Pull Back

Once the drilled hole is enlarged, the product pipeline can be pulled through it. The pipeline is prefabricated and usually tested on the bank opposite the drilling rig. A reamer is attached to the drill string and then connected to the pipeline pull head via a swivel. The swivel prevents any translation of the reamer’s rotation into the pipeline string, allowing for a smooth pull into the drilled hole. The product pipe has to be supported for the pullback operation. This is usually accomplished on rollers or with some type of crane. The drilling rig then begins the pullback operation, rotating and pulling on the drill string and once again circulating high volumes of drilling slurry.

The pullback continues until the reamer and pipeline break ground at the drilling rig. If possible, there should be enough work space on the pull side so that the product pipe can be assembled in one continuous length. This reduces the chance that the pipe may get stuck during the pullback operation.

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