Asset management of wastewater networks
Creators: name: Ugarelli, Rita Maria <1973>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: ICAR/01 Idraulica

description: Asset Management (AM) is a set of procedures operable at the strategic-tacticaloperational
level, for the management of the physical asset’s performance, associated
risks and costs within its whole life-cycle. AM combines the engineering, managerial and
informatics points of view. In addition to internal drivers, AM is driven by the demands of
customers (social pull) and regulators (environmental mandates and economic
considerations). AM can follow either a top-down or a bottom-up approach.
Considering rehabilitation planning at the bottom-up level, the main issue would be to
rehabilitate the right pipe at the right time with the right technique. Finding the right pipe
may be possible and practicable, but determining the timeliness of the rehabilitation and
the choice of the techniques adopted to rehabilitate is a bit abstruse. It is a truism that
rehabilitating an asset too early is unwise, just as doing it late may have entailed extra
expenses en route, in addition to the cost of the exercise of rehabilitation per se. One is
confronted with a typical ‘Hamlet-isque dilemma’ – ‘to repair or not to repair’; or put in
another way, ‘to replace or not to replace’. The decision in this case is governed by three
factors, not necessarily interrelated – quality of customer service, costs and budget in the
life cycle of the asset in question.
The goal of replacement planning is to find the juncture in the asset’s life cycle where the
cost of replacement is balanced by the rising maintenance costs and the declining level of
service. System maintenance aims at improving performance and maintaining the asset
in good working condition for as long as possible. Effective planning is used to target
maintenance activities to meet these goals and minimize costly exigencies.
The main objective of this dissertation is to develop a process-model for asset
replacement planning. The aim of the model is to determine the optimal pipe replacement
year by comparing, temporally, the annual operating and maintenance costs of the
existing asset and the annuity of the investment in a new equivalent pipe, at the best
market price. It is proposed that risk cost provide an appropriate framework to decide the
balance between investment for replacing or operational expenditures for maintaining an
asset.
The model describes a practical approach to estimate when an asset should be replaced.
A comprehensive list of criteria to be considered is outlined, the main criteria being a visà-
vis between maintenance and replacement expenditures. The costs to maintain the
assets should be described by a cost function related to the asset type, the risks to the
safety of people and property owing to declining condition of asset, and the predicted
frequency of failures. The cost functions reflect the condition of the existing asset at the
time the decision to maintain or replace is taken: age, level of deterioration, risk of failure.
The process model is applied in the wastewater network of Oslo, the capital city of
Norway, and uses available real-world information to forecast life-cycle costs of
maintenance and rehabilitation strategies and support infrastructure management
decisions. The case study provides an insight into the various definitions of ‘asset lifetime’
– service life, economic life and physical life. The results recommend that one common
value for lifetime should not be applied to the all the pipelines in the stock for investment
planning in the long-term period; rather it would be wiser to define different values for
different cohorts of pipelines to reduce the uncertainties associated with generalisations
for simplification.
It is envisaged that more criteria the municipality is able to include, to estimate
maintenance costs for the existing assets, the more precise will the estimation of the
expected service life be. The ability to include social costs enables to compute the asset
life, not only based on its physical characterisation, but also on the sensitivity of network
areas to social impact of failures.
The type of economic analysis is very sensitive to model parameters that are difficult to
determine accurately. The main value of this approach is the effort to demonstrate that it
is possible to include, in decision-making, factors as the cost of the risk associated with a
decline in level of performance, the level of this deterioration and the asset’s depreciation
rate, without looking at age as the sole criterion for making decisions regarding
replacements.