Horizontal directional drilling (HDD) technology is a steerable trenchless construction technique
that offers an alternative for installing underground pipes, conduits and cables (Wang and
Sterling, 2007; Onsarigo, Adamtey and Atalah, 2014; Krechowicz and Krechowicz, 2021) . HDD
technology was first introduced in the 1970s in California, USA and has since grown to a yearly
multi-billion-dollar industry that has found applications across the globe. The advantage of the
HDD trenchless technology over conventional pipe installation methods is that it requires a
minimum amount of excavation to attain complete installation (Yan et al., 2018) . HDD has
turned out to be a versatile technique becoming increasingly popular for installing pipes in
locations where trenching is difficult, for example, under rivers or railways (Royal, Riggall and
Chapman, 2010) .
Indeed, over the past years, HDD technology has emerged as the most preferred construction
technique for installing new underground infrastructure (Lueke and Ariaratnam, 2005; Yan et al.,
2018) . The technology uses subsurface-mounted rigs to install small to medium-sized pipelines
and conduits at relatively shallow depths (Wang and Sterling, 2007; Krechowicz and
Krechowicz, 2021) . The first step in HDD involves drilling a guided pilot hole along the bore
path by cutting and mixing geological formations with drilling fluids to make a flowable slurry
(Wang and Sterling, 2007). It then uses down-hole bits to produce the bore and expand it with
back-reamers to enable installation of the pipe. The ability to control the path of the bore has
ensured the success and attractiveness of the technology.
Despite the success and popularity of HDD technology, there are several issues pertaining to its
installation which are still poorly understood. For instance, the evaluation of the stability of the
borehole wall especially when HDD is applied to very loose sand or gravel-sand mixtures
presents a big challenge to industry (Wang & Sylvester, 2007). Borehole stability is a critical and
key issue during horizontal directional drilling projects (Zhou et al., 2013) . Consequently,
borehole stability must be given due consideration in HDD, because borehole instability
challenges usually lead to non-productive time. For instance, cases of borehole instability
problems have been reported in shale formations in Zubair in Southern Iraq (Mohammed and
Selman, 2020) . A proper balance of several critical soil parameters like soil stress and strength,
pore pressure, drilling fluid pressure and drilling mud chemical composition are necessary for
borehole stability. In contrast, borehole instability is influenced by chemical effects (filter cake
formation) and mechanical effects (soil sloughing and hydraulic fracturing) (Wang & Sylvester,
2007). Recent studies have applied numerical modeling methods to investigate borehole stability
in horizontal directional drilling operations (Wang and Sterling, 2007) .
Typical borehole instability problems include stuck pipe, caving, lost circulation, and the tight
hole, requiring more time to treat and therefore additional costs (Allawi and Al-Jawad, 2021) .
Studies have shown that borehole instabilities present significant problems to drilling, especially
in areas with weak bedding planes and pre-existing fractures where formations have strong
anisotropies (Zhang, 2013) . Generally, borehole instability can be caused by both natural and
artificial factors. Borehole instability occurs when there is disturbance and change of natural
equilibrium state of the surrounding rock and soil mass causing loss of support for the formation
of the hole (Zhou et al., 2013) . Borehole shrinkage or enlargement may cause the actual borehole
track to deviate from the designed track and could result into too large torque, too large pulling
force, difficult or even failure in pipeline pullback (Hawkes, 2007; Zhou et al., 2013) . Lack pf
borehole stability may also lead to the drilling pipe sticking and drilling tools being buried in the
process of drilling and reaming. Also, large amount of mud pressure can lead to rupture of the
bore wall, and mud leakage may induce ground surface splitting and result in serious
environmental pollution and economic losses. Failure to undertake accurate borehole stability
analysis may pose problems such as borehole washouts, breakout, collapse, stuck pipes and drill
bits, and losses of boreholes (Zhang, 2018). It is known that borehole instability increases
drilling time and costs and may lead to well abandonment before it reaches its objective. It is
estimated that the cost of these issues is about 10% of the total drilling time (Li, George and
Purdy, 2012; Darvishpour et al., 2019) .
Steering in Horizontal Directional Drilling
Methodology and Equipment
Horizontal Directional Drilling (HDD) operates with a steerable system that is useful in
installing pipes, conduits, and cables in a shallow arc using a surfaced launched drilling rig
(Jariwala et al., 2013) . The HDD technology has been used in large scale crossings (e.g. rivers)
where fluid-filled pilot hole is drilled without rotating the drill string and is then enlarged by a
wash over the pipe and back reamer to the size required. Underground steering is a critical
component of HDD because it assures navigation past underground utilities and obstacles. The
steering system also helps keep records of the actual underground route, thereby simplifying the
project operations because it enables all the underground obstacles to be identified (Kendon,
2019). The ability of the steering to control the path of the bore is essential for the success of
many horizontal directional drilling installations. Indeed, the potential inability to maintain and
control the direction of the bore hinders the widespread adoption of the technique in place of
traditional open-cut methods (Royal, Riggall and Chapman, 2010) .
The HDD installation process involves drilling a pilot hole along the desired directional path,
enlarging it to the desired diameter to accommodate the product pipe, and pulling the product
pipe back (Royal, Riggall and Chapman, 2010) . The rig in HDD technology makes a pilot bore
by forcing a drilling head with a tracking system at the surface. Steering around natural or
anthropogenic obstacles in horizontal directional drilling is achieved by changing the direction of
the drilling head in the pilot hole drilling phase. In drilling the pilot hole, the HDD rig uses a
non-rotating drill string with a drill bit that has an asymmetrical leading edge. The drill string is
turned to change the direction so that the leading-edge faces in the new direction (Kendon,
2019). The technique is simple and allows drillers to change the direction of the drill at any time
during the process. The drilling head has an angled cutting edge at the front end, which allows
the steering of the drilling rig by pushing and rotating the drill rods (Onsarigo, Adamtey and
Atalah, 2014) . The drilling fluid is pumped through nozzles in the drill head to cut and displace
the soil. When the pilot bore is completed, a back reamer enlarges the hole. After that, larger
back-reamers are progressively used until the hole is big enough to accommodate the product
pipe (Royal, Riggall and Chapman, 2010; Krechowicz and Krechowicz, 2021) . The more
challenging task in this process is identifying the actual position of the drill and knowing the
degree of changing the direction of the drill. In practice, the parameters used to set the direction
of the drill are Azimuth and Inclination (Kendon, 2019). In HDD, Azimuth is a measure of the
direction of the drill relative to the North, and inclination measures the extent of the drill
pointing up or down (Royal, Riggall and Chapman, 2010) .
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