North American Special - Trenchless International

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is a biaxial stress, which is a combination
of hoop stress due to internal pressure
and longitudinal stress due to bending
of the pipe to achieve the curved shape
of the HDD path added to the stress
imposed on the pipeline due to thermal
differential. A large positive thermal differential,
together with internal pressure
is typically the worst case.
Shortly after deciding to complete
the 36 inch ream the project team recognised
it may not be reasonable to
expect the 36 inch ream, to improve the
current minimum pilot hole radius from
270 metres to the minimum calculated
radius of 425 metres as calculated in
the Tresca analysis. This led to another
refinement of the combined stress calculation.
A more realistic approach is to
compute combined stress as per the Von
Mises formula, which uses similar inputs
as Tresca with modest time and effort
required. The Von Mises calculation
method is also known to be more realistic
for ductile steel such as employed in
modern oil and gas pipeline systems.
For the Pine Creek crossing the soil
within the drill path was assumed to
be a ‘soft formation’ based on feedback
from the drilling contractor, the
difficulty experienced with down hole
steering and the results of the geotechnical
investigation carried out by AMEC
Earth and Environmental. This resulted
in the use of soil category A.
Analysis of the drill path was completed
using the Von Mises method
with the above parameters. The resultant
minimum acceptable radius was
determined to be 385 metres, a marginal
improvement on the previous
425 metres. The 385 metre radius would
need to be compared to the calculated
minimum radius in the drill path after
completion of the 36 inch ream.
The ream was completed two days
later. Entec analysed the drill profile
data and calculated a minimum dogleg
radius of 284 metres, with a number
of other radii less than 385 metres.
The subsequent survey tool run was
completed later in the same day. Entec
analysed these results and calculated
a minimum radius of 170 metres
with a number of other radii less than
385 metres. The ever decreasing calculated
minimum radius confirms that the
progression of the calculations is from
conservative to more realistic.
Pembina and its construction manager
subsequently decided to pull the NPS24
pipe, as seen on page 55, into the hole
and the project team agreed to continue
analysing the available information to
determine if in fact the 36 inch reamed
drill path was acceptable.
First, the project team completed a
sensitivity analysis to determine if reducing
the MOP of the pipeline or increasing
the pipe grade would be a viable alternative.
After careful review, the required
MOP reduction to 4,400 kPa or pipe
grade increase to 545 MPa were still not
sufficient to provide acceptability for the
Pine Creek crossing.
The next and only viable alternative
for analysing the crossing was to determine
the absolute stress level within
the pipe at this location. The Tresca
and Von Mises methods are both stress
based, conservatively not accounting
for the nonlinear steel properties. Due
to tight bend radii determined in the
field, the group agreed to undertake a
third method of analysis; a nonlinear
strain analysis, which is permitted by
CSA-Z662 Annex C as a limit states
design approach. This strain analysis is
much more complex than either stress
method, but it also is a more rational
and usually less restrictive approach.
Determination of the longitudinal strain
with nonlinear steel properties and subject
to biaxial stresses is performed with
a finite-element computer program.
Subsequent to the very detailed review
of the stresses involved in this installation,
the current drill alignment was
supported by Worley Parsons Calgary,
based on the maximum calculated strain
being below the maximum allowable
strain for the crossing using limit states
analysis. Colt used the minimum drill
path radius of 170 metres and compared
the strain at this location to the maximum
strain allowable for the NPS24 crossing
pipe. The results were deemed acceptable
as per the above discussion.
In addition, Colt has recommended
running a geo-pig through the crossing
within two years of the start of pipeline
operation. This can be used to verify the
geometry of the pipe within the crossing
and reconfirm the results of the limit
states analysis. It is anticipated that the
final geometry of the drill profile will be
better than the lowest indicated radius
of 172 metres.
The purpose of this article is not to
support less conservative analysis as
a general rule for HDD installations but
rather, given the difficult circumstances
encountered at the site, to illustrate that
this method of analysis is applicable and
required in certain circumstances. HDD
installations of pipelines beneath pipeline
route obstructions are technically
challenging, as are the determination of
the construction stresses. These installations
are such that future repair and
cleanup if a problem develops, are
not possible or are extremely difficult
due to the location and depth of these
installations. Therefore, a conservative
approach to the design provides some
assurance that problems should not
occur at this type of crossing.
This article is an edited version of a paper entitled
Does the HDD reaming pass remove extreme
alignment dog-legs: Case History by James P.
Murphy, Glen Fyfe, Trevor Giesbrecht and Wes
Dyck. The paper is to be presented at No-Dig
2009 Toronto, Canada. Please refer to the paper
for references and acknowledgements.
Below: Crossing Alignment at Pine
Creek- ROW sandwiched between
adjacent ROWs.
Rehabilitating New
York City’s NCA
By A Noble, D Roberts and A Fareth
Constructed between 1885 and 1891, the New Croton Aqueduct
(NCA) in New York State is a 50 km tunnel conveying water from
the Croton watershed north of New York City to distribution
systems in the Bronx and Manhattan. The NCA has benefited from
an extensive design, inspection and rehabilitation program.
The NCA rehabilitation program
has been underway in phases since 1993,
with inspections scheduled to minimise
outages, allowing this strategic aqueduct
to remain in service during critical seasonal
periods. Between 1993 and 1997
a series of in-tunnel investigations were
performed in the open channel portion of
the NCA, consisting of field inspection,
non-destructive geophysical testing, and
coring of the brick liner.
Between November 2004 and
September 2005, a major inspection
program was conducted to assess the
condition of the 11 km long pressurised
section and all shafts, headhouses and
blow-off structures along the entire 50 km
alignment. Inspection methods included
using an underwater remote operated
vehicle (ROV) equipped with sonar and
cameras to inspect the deep siphon, fibre
optics examinations of probe holes drilled
through and beyond the brick lining, and
core holes and geophysical inspections to
assess properties of the liner and behindthe-liner
materials. A water pressure and
test grouting program was conducted
to assess methods and expected grout
takes for the rehabilitation work in the
pressurised sections. The rehabilitation
program is anticipated to be completed
in 2010.
Part 1: planning and design
The overall concept for rehabilitation
of the NCA was developed in the early
1990s, at which time a far-reaching program
for achieving a logical and practical
approach to the work was established.
Since outages could only take place seasonally,
contracts for any program had to
be appropriately sized to coincide with
the seasonal outages, while taking into
account the operational needs of other
water supply aqueducts owned and operated
by NYCDEP.
The design for the NCA rehabilitation
was initially based on the results of previous
investigations performed within the
aqueduct’s gravity flow section in 1993
and 1996. The field investigation program
in 1993 included:
• Visual inspection of 9.6 km of tunnel
• Geophysical surveys of 9.6 km of tunnel
• Remote operated vehicle (ROV) inspections
• Gould’s Swamp siphon
• Harlem River siphon
• Probe and core drilling
• Fibre optic scope inspection.
The field investigation program in 1996
included:
• Visual inspection of the remaining 29
km of gravity flow tunnel
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April 2009 - Trenchless International
The NCA is a 3.73 to 4.34 metre diameter, brick-lined circular, and horseshoe-shaped water tunnel.
Except at two siphons, the aqueduct operates as an open channel conduit from the New Croton Reservoir
downstream 39 km to the Jerome Park Reservoir in the Bronx. The remaining 11 km is operated as a
pressurised conduit and includes a major siphon, of circular cross-section, under the Harlem River. A total of
163 million bricks were used in the construction, enough to build a 48 km wall around Manhattan Island,
3 metres thick and 15 metres high.
The aqueduct has a flow capacity of approximately 1,100 million litres of water per and supplies on average
ten per cent of New York City’s drinking water. The City’s water supply system is operated and maintained
by the New York City Department of Environmental Protection (NYCDEP). Until a partial inspection in 1982,
the NCA had flowed continuously since its completion nearly 100 years before. Flow tests have confirmed
that the NCA can convey up to 290 million gallons per day.
April 2009 - Trenchless International
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