Boring - Trenchless International

gas
April 2010 - Trenchless International
information was examined that showed
surface gravels to a depth of approximately
12–15 metres. The gravels were
deposited above claystone, shale and
sandstone bedrock, which was expected
to provide a good path for the HDD. This
information had been gathered for a previous
project which was over 100 metres
away from the chosen crossing route.
Additional geotechnical boreholes were
Entec Trenchless Engineering is:
HDD cranes on Peace River.
then planned to confirm these results
while preliminary designs were prepared.
Due to the presence of gravels on
both the entry and exit side, a pilot hole
intersect was proposed to allow the installation
of surface casing on both sides of
the crossing. Given the consistency of
the bedrock materials, determination of
the intersect location was left up to the
drilling contractor. Since an intersect was
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required to complete the crossing, drilling
contractors with successful pilot hole drilling
intersect experience were selected as
potential bidders for the project.
The preliminary design called for entry
and exit angles of twelve degrees to balance
the need for keeping the exit side
casing as short as possible with the need
to reduce the pipe lifting requirements for
pullback. Due to the diameter of the pipeline
a minimum 60 inch diameter surface
casing was required. The large diameter,
when combined with the length of the casing,
required planning to install the casing
in two telescoped stages. Ultimately a
final diameter of 76 inch casing being
placed through a shorter length of 84 inch
diameter casing was selected to allow
the casing to reach a target length of
78 metres. The target depth was selected
to allow the casing to seat into bedrock
at a vertical depth of 12–15 metres with
some extra length for contingency. A
final diameter of 76 inches was proposed
to allow a smaller casing to be used as
contingency against failure to reach the
target depth.
Due to the long lead time for large diameter
casing pipe, the materials had to be
ordered prior to the completion of the new
geotechnical boreholes. Unfortunately
results from the geotechnical boreholes
drilled at the actual crossing location
showed gravel to a depth of 19 metres.
Due to this change in design constraints
the exit angle of the crossing needed to
be increased to 15 degrees to allow the
use of the previously ordered materials at
the expense of greater pipe lifting requirements
for pullback. Given the new exit
angle of 15 degrees and a target depth
of 19 metres the exit side surface casing
needed to isolate 76 linear metres of
the drill path from unconsolidated sand,
gravel and cobbles.
The entry side gravel thicknesses were
confirmed by the new geotech and the
entry casing was left at its original angle
of 12 degrees and proposed length of
53 metres.
Entec was concerned that the large
buoyant forces exerted by the 42 inch
pipeline during pullback could damage
the pipe coating as it passed through
the long steel exit and entry casings.
To address this concern, a buoyancy
control plan was designed to ensure the
pipeline passed through the casings and
the borehole with near neutral buoyancy,
minimising the risk of coating damage.
Also of concern was the possibility that
the pipeline coating could be damaged
by a misalignment between the pipeline
pull section and the exit casing during
pullback. A misalignment could result in
binding between the pipeline and casing.
For this reason a detailed lifting plan
was prepared to ensure the position and
heights of the lifting equipment allowed
the pipeline to precisely match the exit
angle without placing excessive stresses
on the pipeline during installation.
The final design consideration was
the possibility that the surface casing
might not be extracted or could damage
the pipeline during extraction and could
accelerate corrosion of the pipeline. To
mitigate against this risk, casing insulators
were to be installed in the event the casing
could not be removed after pipe pullback.
Surface casing and pilot hole
construction
Entry side surface casing was installed
in November 2008 and reached refusal
at 24 metres in length or 5 metres vertical
depth. This was well in advance of
the 53 metres of casing expected to
be installed according to the geotechnical
investigation that found bedrock
at 9 metres vertical depth. Subsequent
excavation to confirm the bedrock depth
revealed the bedrock to be at five metres
below ground surface and the casing was
re-seated into bedrock.
Exit side surface casing installation
began once frozen conditions were
present in early December. Installation
began with the excavation of a launch
pit and hammering of the 84 inch surface
casing. The 84 inch casing reached
refusal at 41 metres. The 84 inch casing
was then augered clean and the 76
inch casing installation was started. The
76 inch surface casing reached refusal
at a final depth of 74 metres but was not
yet seated into the bedrock needed for
the HDD drilling and reaming operations.
Successful contingency planning allowed
the installation of 60 inch diameter casing
that was installed to a final length of
88 metres and firmly sealed into bedrock
at 22 vertical metres.
While the final metres of exit side
casing were being installed the pilot
hole was started on the entry side of
the crossing using Direct Horizontal’s
American Augers DD1100 drilling rig
which reached a final drilled depth of
980 metres where it waited for the exit
side pilot hole to begin.
The casing installation faced several
challenges including long welding times,
slow penetration rates and extreme cold,
but the exit side HDD rig finally started
drilling the remaining 130 metres of the
pilot hole in early February. The pilot
hole intersect was successfully completed
on 14 February 2009. A slight
correction to the steering in the pilot hole
was successfully performed prior to the
start of reaming.
The reaming operations preceded
smoothly using two drilling rigs in tandem
to complete the 30 inch, 42 inch and
gas
April 2010 - Trenchless International
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