North American Special - Trenchless International

asset management
April 2009 - Trenchless International
An example graph.
but did not have sufficient failure history to
warrant a standalone analysis.
Sample results of the calibration process
are shown in table 1. Results show a
relatively long mean time to first failure for
ductile iron pipes compared to cast iron
pipes. However, for mean time to subsequent
failures (especially after the 4th
failure) cast iron pipes tend to outperform
ductile iron. No significant differences
were noticed in the performance of spun
and pit cast iron pipes.
The performance model was used to
predict the following:
• Long-term funding requirements for
repair and replacement/rehabilitation
under different level of service scenarios.
Level of service was measured
by number of failures experienced on a
water main segment.
• Using a Monte Carlo simulation to predict
future failure patterns on various
water main vintages, the optimum time
to replace/rehabilitate was computed
as a function of the ratio to repair
to replacement cost. This minimum
expected economic loss (MEEL) was
found to be somewhat large for typical
cost ratios (8-12 failures on a water
main segment). This indicated that the
governing criterion is more likely to be
a level of service indicator set by the
municipality rather than pure economics.
In addition to the use of the model at the
strategic planning level, the performance
model was used to develop two important
tools for water main management at the
tactical/operational level. The coordinated
infrastructure renewal tool and the early
replacement tool set.
Critical Water Main Management
The approach for managing critical
water mains differs considerably from that
for noncritical mains. Some of these key
differences include:
• Repair policy: With noncritical water
mains, breaks can be tolerated and
hence a run-to-end of service life
approach can be accepted. Conversely,
critical water mains with zero tolerance
for failure, a proactive maintenance and
rehabilitation policy should be sought.
• Tolerance to uncertainty: Whereas
with noncritical water mains and their
run to failure management approach
uncertainty in condition state can be
tolerated, no such tolerance can be
allowed for critical water mains. This
has ramifications on the amount of
information being collected and the
level of detail at which this information
should be stored.
The critical water main management
framework consists of three main tool
sets.
1. Assessment rationalisation framework:
2. Condition rating consolidation
framework: This tool set attempts to
standardise the way the results of
assessment techniques are interpreted
and subsequently used to drive decisions.
The framework is developed
for ductile iron and cast iron water
mains as they compose the majority
of the city’s critical inventory. The
framework utilises Fuzzy Logic and
the Analytical Hierarchy Process to
combine condition rating results into an
overall rating for the condition state as
well as the expected deterioration rate
of the pipe.
3. Planning cycle decision analysis tool:
The purpose of this tool is to equip
the asset manager with a consistent
methodology for decision-making
during each planning cycle. Within
a planning cycle, the asset manger
must make one of three decisions
for the critical water main inventory;
schedule intervention, schedule
inspection and revisit at next planning
cycle.
In order to make an informed decision,
the asset manager must consider the following
aspects:
• Condition State. This is analogous to
the probability of failure.
• Pipeline Risk. This corresponds to the
consequence of failure.
• Extent of condition/deterioration
information currently available. The
tool performs a trade-off between
the available amount of condition/
deterioration information and the risk
associated with operating the pipeline.
• Level of uncertainty associated
with inferring the condition state.
Associated with the condition state
that is inferred from the consolidation
tool will be a measure of uncertainty.
This factor must be considered in the
decision process.
This study is still ongoing and aims
to re-evaluate what information is collected,
and the way information is stored
and handled throughout the lifecycle of
the water main assets.
Conclusion
With these tools built, the city has established
the foundation for the effective
management of its watermain infrastructure.
Furthermore, as these tools are
integrated into the daily business decision
process, they will be refined and
improved to reflect the increasing knowledge
growth within the city. They will also
form the basis for clearly articulating the
ramifications of decisions. This includes
the need to focus resources, particularly
financial, on the assessment of critical
infrastructure, which in many cases has
not yet failed or otherwise caused operational
issues. These types of studies are
often expensive and do not result in new
tangible assets, but rather an improved
understanding of the probability of failure.
As such, these types of expenditures can
often be challenging for cities and utilities
to get funding approval for with out being
able to demonstrate the non tangible
benefits.
This article is an edited version of a paper entitled Water Main Asset Management in the City of Hamilton: A Comprehensive Overview of Policies, Practices, Tools,
and Technology by Hesham Osman and Kevin Bainbridge. The paper is to be presented at No-Dig 2009 Toronto, Canada. Please refer to the paper for more detailed
information, references and acknowledgements.
Delivering desalinated
water to Sydney
The $US419.5 million water delivery infrastructure project, part of
the Sydney’s Desalination Project, aims to secure the city's water
supply for future generations by constructing about 24 km of
pipelines across Botany Bay, Australia.
The new pipeline and its associated
infrastructure and systems will carry the
desalinated water from Kurnell, across
Botany Bay, to Sydney's main water supply,
the City Water Tunnel at Erskineville.
Three tunnel boring machines (TBMs)
will be employed to minimise the disturbance
to residents and also to protect
unique tracts of seagrass on the floor of
Botany Bay.
TBM: managing the environment
The southern shore of Botany Bay contains
extensive seagrass beds, which
are a valued and protected part of the
estuarine environment. Three species of
seagrass are present off Silver Beach at
Kurnell: zostera capricorni or eelgrass;
posidonia australis or strapweed; and
halophila ovalis or paddleweed. Posidonia
requires the greatest consideration due to
its slow reproduction and poor propagation
by seed.
Stretching about 6,500 metres in a westerly
arc from Silver Beach at Kurnell to
Lady Robinson’s Beach at Kyeemagh, the
twin and single steel pipelines will impact
approximately one per cent of the overall
area of Botany Bay. Along the entire route,
however, less than half of one per cent
of existing seagrass along the southern
shore (0.45 per cent) and Botany Bay
(0.42 per cent) will be removed as a result
of pipeline construction.
Trenching through these seagrass beds
would have required a seagrass management
plan to be implemented during and
after construction, and a compensatory
seagrass package involving steps like
transplantation. Instead, Sydney Water
has chosen to microtunnel the pipeline
from its Silver Beach construction area
under Botany Bay for a distance of about
800 metres in order to protect the seagrass.
The Water Delivery Alliance will join
the single 1,800 mm diameter pipeline
from Silver Beach to the twin 1,400 mm
diameter pipeline about 800 metres from
the shoreline, and (as always) protection
of the environment will be a key
consideration. Pit construction is nearing
completion at the Silver Beach site. This
pit is supported by secant piling, has
internal jet grouting and is around ten
metres deep. Land-based sections of the
pipeline will be constructed first. Material
that has been dug up from the pit is being
used on site where possible, in order to
minimise truck movements. Continuous
water quality monitoring is being carried
out around the Silver Beach construction
area. Recent monitoring of the site has
indicated good water quality conditions,
with similar results both inside and outside
the silt curtain.
TBM onshore
A TBM is also an essential tool to minimise
disruption onshore. The bore passes
beneath Tasman and Dampier Streets,
Kurnell, to install pipeline. The model
shown is (page 45) a Herrenknecht earth
pressure balance and AVN machine,
weighing approximately 100 tonnes.
The first microtunnelling drive through
a residential area is now complete. The
TBM tunnelled 640 metres from the
launch pit in Cook Park, under General
Holmes Drive and under Tancred Avenue
to the receival pit at Muddy Creek. The
TBM used was chosen specifically for the
conditions at Cook Park. The pipe liner
and services will be installed on this section
of pipe in the coming months.
The TBM has also finished tunnelling
645 metres from Canal Road to
Botany Freight Rail Line, which was the
first tunnel completed on the project.
Microtunnelling from Marsh Street
Arncliffe, under the Cooks River to Tempe
Recreation Reserve will begin in March.
Choosing the route
The pipeline route across Botany Bay
was chosen because it avoids known
areas of contamination. Every practical
effort is being made to protect the
Bay environment during construction.
Water quality monitoring is ongoing
during construction activities, in accordance
with the Construction Water Quality
Management Plan. The results of this
monitoring will help the project team
manage their work.
environment
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