Notional Field Development Final Report - EBN

EBN Notional Field Development Plan
Figure 4-66. Equipment for boring on horizontal directional drilling (HDD) operations
Essentially, installing utility tunnels using the HDD technique comprises the following three stages:
• In the first stage a pilot bore is made from the point of attack in the direction of the exit point.
The steerable drill string made up of individual drill rods is conducted along the target route by a
guidance system located directly behind the cutterhead.
• During the drilling process a bentonite suspension is pumped to the nozzles fitted to the
cutterhead where it excavates the native soil hydraulically. The bentonite mixes with the
excavated soil and flows back through the annulus between the drill rods and the bore hole to
the point of attack at the surface, where it is processed by a separation plant and returned to
the drilling cycle.
• In order to increase the diameter of the pilot bore hole in the second stage, the pilot cutterhead
is removed at the exit point and a so-called muck bucket lips is attached to the drill rod. As the
drill string is retracted, the diameter of the bore hole is enlarged with the help of the muck
bucket lips. All the muck bucket lips are fitted with nozzles and excavation tools in order to
excavate the soil both hydraulically and mechanically. A mixture of water and bentonite or other
additives can be used dependent on soil conditions in order to stabilize the bore hole and
reduce friction forces. In the third stage the prefabricated and pipelines that have been checked
are retracted.
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