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Beyond pipe failure data in Japan, Sweden and the Netherlands: enabling cross-national comparison, analysis and action
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Abstract
Pipe failure data provides valuable information on the mechanisms and conditions which cause drinking water pipes to break or leak. Such data feeds asset management models, helping water utilities to decide future interventions at different timelines. Among countries, and even within countries, utilities apply different pipe failure data collection methods, for a-posteriori analyses. This contribution focuses in the comparison of pipe failure data collection methods and practices in three countries: Japan, Sweden and The Netherlands. We compare their methodologies and the use that such data is given. We identify the potential for knowledge transfer of methods between the three countries. This paper stems from the Watershare knowledge sharing platform, where the three organizations are active members, along with 17 other knowledge institutes spread across the world.
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Comsima is a mechanical model that calculates stresses and joint rotations in drinking water distribution pipes based upon several loadings on the pipe (soil, traffic, water pressure, differential settlements). Pipe degradation mechanisms (slow crack growth resistance for PVC and calcium leaching for AC) were added to the model. A comparison with failure registration for an area in the Netherlands using satellite data to determine differential settlements shows that pipes with higher stresses or higher joint rotations in general have a higher failure rate.
The integrity of drinking water distribution systems (DWDS) may be influenced by climate change. Using the statistical relations between failure frequencies and weather conditions described in our previous work (Wols & Van Thienen 2014a), a methodology is proposed to assess the effect of climate change on future DWDSs. The effect of climate change is combined with the evolution of the DWDS. This analysis can be conducted for any DWDS, for which historical failure registrations and weather parameters are available. The proposed methodology can therefore assist in the construction and maintenance planning of DWDSs. The methodology has been worked out for the Dutch drinking water distribution network. The results show that failures in networks with high AC proportions will increase as a result of expected climate change in the Netherlands, whereas failures in networks with high PVC and GCI proportions will even slightly reduce.
Understanding pipe failure is essential for effective asset management. Buried drinking water pipes are exposed to several types of external loads, e.g. soil weight, loads due to soil settling differences and traffic loads. The hypothesis that traffic loads positively affects the number of failures was statistically tested. For three out of four studied water companies significant higher failure frequencies than average were found at road crossings. Frequencies equal to average were found for pipes which are installed under other road sections. Frequencies higher than average–but not statistically significant–were found around speed bumps. The results of the multiple regression analyses show that the overall contribution of the parameter ‘road classification’ to pipe failure is small compared to the influence of pipe diameter, pipe material and year of installation.
The occurrence of pipe failures in drinking water distribution networks may be influenced by weather conditions. In this work, a statistical analysis is performed to study the correlations between weather parameters and pipe failure in the Netherlands with the ultimate aim to predict the effects of climate change on network integrity. Failure data from a Dutch national failure database were divided into different cohorts, depending on type of pipe material, year of installation, and diameter class. Weather data related to temperature, drought, and wind were collected. Relationships between weather data and failure data were sought using a linear regression analysis and a frequency analysis. The latter analysis results in a weather variable dependent pipe failure frequency. The most obvious relationships were found between pipe failure and temperature. Failures in asbestos-cement (AC) and steel pipes increased during warm periods, which often simultaneously occurred when water consumptions were high. For cast iron pipes, failures increased at low temperatures. Drought parameters had a smaller effect on pipe failure than temperature, but still an increase in pipe failure was observed during dry periods for AC and steel pipes. No effect of weather conditions on pipe failure were observed for poly(vinyl chloride) and polyethylene pipes.
Failure data are a valuable source of information on the condition of underground assets. Systematic registration of relevant items on cause, nature and circumstances of each occurring failure is, however, complicated in day-to-day practice. Moreover, in a network of limited size it takes a long time to obtain a statistically relevant amount of data. Dutch water companies in collaboration with the KWR Watercycle Research Institute have joined forces and developed a data standard and central database to store and analyse failure data. As of early 2012, 66,000 km of network are monitored, providing around 2,500 records on failure annually. All in all it provides solid information needed for strategic asset management that helps to support well established asset management plans.
Ageing drinking water, stormwater and sewer pipe networks imply an increased degree of rehabilitation. The need for rehabilitation can be predicted using lifetime distribution functions together with current network age and material distribution. In Sweden, current age and material distribution is neither documented on a national level, nor for many water utilities on a local level. In this study, current network age and material distribution was provided through a questionnaire sent to Swedish water and wastewater utilities and the data provided were extrapolated to cover the whole of Sweden. The data were then combined with lifetime distribution functions to provide predictions. One limitation is that for newer materials the lifetime is still uncertain. Predictions were made for different scenarios to reflect local differences and the medium scenario shows that while the Swedish rehabilitation rate is stable, investments in monetary terms need to double in the next 60 years. The rehabilitation rate is also dependent on the extent to which the network is expanded. This method can be used to calculate national investment needs, and the results can also provide a basis for estimates for Swedish utilities with data scarcity.
In the future, the more frequent occurrence of severe heat waves and long dry periods due to climate change can cause lowering of the ground water level and therefore consolidation of the soil. Consequently, increased differential settlements are expected that may damage underground water infrastructure. Models were developed to assess the impact of differential settlements on pipe failure. The main concept of these models is that the pipe-soil system is schematized as a beam on an elastic foundation using Winkler type springs. For climate change induced settling, a parametric function of the soil settlement is proposed. A Monte-Carlo analysis has been applied to predict pipe failure probabilities.
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