112 research outputs found
Assessing mechanical vulnerability in water distribution networks under multiple failures
No full textUnderstanding mechanical vulnerability of water distribution networks (WDN) is of direct relevance for water utilities since it entails two different purposes. On the one hand, it might support the identification of severe failure scenarios due to external causes (e.g., natural or intentional events) which result into the most critical consequences on WDN supply capacity. On the other hand, it aims at figure out the WDN portions which are more prone to be affected by asset disruptions. The complexity of such analysis stems from the number of possible scenarios with single and multiple simultaneous shutdowns of asset elements leading to modifications of network topology and insufficient water supply to customers. In this work, the search for the most disruptive combinations of multiple asset failure events is formulated and solved as a multiobjective optimization problem. The higher vulnerability failure scenarios are detected as those causing the lower supplied demand due to the lower number of simultaneous failures. The automatic detection of WDN topology, subsequent to the detachments of failed elements, is combined with pressure-driven analysis. The methodology is demonstrated on a real water distribution network. Results show that, besides the failures causing the detachment of reservoirs, tanks, or pumps, there are other different topological modifications which may cause severe WDN service disruptions. Such information is of direct relevance to support planning asset enhancement works and improve the preparedness to extreme events
Reliability analysis of the water distribution network of Oslo using WDNetXL
No full textOne of the most important objectives for water distribution networks is to provide safe and reliable water supply also changing system boundary conditions. This can be compromised by inadequate configuration of the system, unforeseen high water consumption and high leakages. Large water networks are complex and analysis of hydraulic performance requires computer models.
In Oslo municipality, Water and Wastewater Department, is currently estimating the reliability of the water distribution system including the level of robustness, redundancy and preparedness of the system under today and future demand. First analyses indicate that closure of certain pipes may have relevant consequences as loss of water supply and reduction of pressure for a significant number of customers. Therefore, a major ongoing project on revision and safeguarding of Oslo water supply system also includes further analysis of the hydraulic system reliability. WDNetXL model system has been applied to this purpose
Letter to the Editor: Comments on the Paper “Inertia Effects in High-Rate Flow Through Heterogeneous Porous Media” by M. Fourar, R. Lenormand, M. Karimi-Fard, and R. Horne, Transport in Porous Media, DOI 10.1007/s11242-004-6800-6, 2005
No full textIn a recent article, Fourar et al. (Transp Porous Med, 2005, doi:10.1007/
s11242-004-6800-6) analyzed the effect of heterogeneity in the permeability distribution on Forchheimer flow in porous media. They derived expressions to calculate the effective inertial coefficient in serial layers, parallel layers, and two-dimensional correlated media. Here, we highlight an inconsistency in their first-order expression for serial layers and extend their findings by providing closed-form expressions for the effective inertial coefficient in the case of a lognormal permeability distribution
Parametri effettivi per moto di Forchheimer in mezzi porosi eterogenei: il caso monodimensionale
No full textLa comprensione della interazione, nel moto in mezzi porosi, tra effetti inerziali e eterogeneità a scala di campo riveste notevole importanza per diverse applicazioni ingegneristiche. Nella presente nota, per domini monodimensionali caratterizzati da eterogeneità statistica, viene investigato il flusso non lineare, governato localmente dalla equazione di Forchheimer, dovuto ad un gradiente di pressione esterno uniforme. Si considera una assegnata distribuzione di probabilità della permeabilità, mentre il coefficiente di Forchheimer locale è legato al valore locale di permeabilità da una relazione empirica di proporzionalità inversa secondo una legge di potenza. Nell’ipotesi ergodica, i parametri effettivi sono ricavati in funzione dei parametri della distribuzione di probabilità della permeabilità, per una variazione della stessa in direzione parallela od ortogonale al moto (strati in serie od in parallelo). Per entrambe le geometrie, il coefficiente di Forchheimer effettivo risulta funzione crescente dell’eterogeneità del dominio, mentre è funzione crescente o decrescente dell’esponente della legge di potenza inversa, a seconda che gli strati siano in serie od in parallelo. A parità di eterogeneità, la forma della distribuzione di probabilità adottata influenza il valore del coefficiente di Forchheimer effettivo
Estimation of service life of pipes: alternative measures to prevent blockages and flooding events in the wastewater network in Oslo VAV
No full textOslo VAV (Vann og Avlopsetaten) – the water and wastewater department of the Municipality of the Norwegian capital of Oslo is working on reining in its expenses and optimising the levels of service it provides to the inhabitants of Oslo. Blockages of pipelines leading to basement flooding are by far the dominant challenges which Oslo VAV faces and counters every year. Pipelines which get blocked and lead to basement flooding and excessive phosphorus discharges into the receiving fjord, may either be maintained without rehabilitation at lower cost, or rehabilitated at a higher cost. In the former case, the chance of recurrence of flooding would be higher. When flooding occurs, in addition to the additional post-damage maintenance expenses, compensation would have to be paid. In the latter case, one could eliminate the risk of recurrence, by expending a little more. However, Oslo VAV has to operate within some limits and would do well to rein in the costs and strike the perfect balance between service levels and expenditures. This paper, using the blockage and basement flooding records of the last 16 years (1991-2006) and the financials associated therewith, and using suitable assumptions, attempts to chart a picture for the future (till 2027) to enable intelligent decision-making. With the assumptions made, rehabilitation emerges as an economically-wiser option. One however, has to look beyond just the numbers born out of the calculations based on the assumptions made. It is difficult to absorb numerous factors belying the simple mathematical inequalities obtained in this paper, into the model adopted for the study.
The economic analysis conducted in this paper is a logical sequel to the analysis of blockages carried out in Ugarelli et al. 2010
Displacement of non-Newtonian compressible fluids in plane porous media flow
No full textDisplacement of non-Newtonian fluid in porous media is of paramount importance in the flow modeling of oil reservoirs. Although numerical solutions are available, there exists a need for closed-form solutions in simple geometries. Here we revisit and expand the work of Pascal and Pascal (1988), who analyzed the dynamics of a moving stable interface in a semi-infinite porous domain saturated by two fluids, displacing and displaced, both non-Newtonian of power-law behavior, assuming continuity of pressure and velocity at the interface, and constant initial pressure. The flow law for both fluids is a modified Darcy’s law. Coupling the nonlinear flow law with the continuity equation considering the fluids compressibility, yields a set of nonlinear second-order PDEs. If the fluids have the same consistency index n, the equations can be transformed via a self-similar variable; incorporation of the conditions at the interface shows the existence of a compression front ahead of the moving interface. After some algebra, one obtains a set of nonlinear equations, whose solution yields the location of the moving interface and compression front, and the pressure distributions. The previous equations include integrals which can be expressed by analytical functions if n is of the form k/(k+1) or (2k-1)/(2k+1), with k a positive integer. Explicit expressions are provided for k = 1, 2; for other values, results are easily obtained via recursive formulae. All results are presented in dimensionless form; the pressure distribution and interface positions are studied and discussed as a function of the self-similar variable for different values of the mobility and compressibility ratios
Comment on ‘Yamada H, Nakamura F, Watanabe Y, Murakami M and Nogami T. 2005. Measuring hydraulic permeability in a streambed using the packer test. Hydrological Processes 19: 2507–2524’
No full textIn a recent paper, Yamada et al. (2005) derived an expression to calculate hydraulic permeability under non-Darcy flow conditions using the packer test; their results were obtained via numerical integration of the derived expression. Their findings are extended by providing a closed-form solution to the problem, and its dependence upon key parameters is illustrated
Optimal scheduling of replacement and rehabilitation in wastewater pipeline networks
No full textTo fulfil the objective of providing acceptable level of service to customers, the water managers have to plan how to operate, maintain and rehabilitate the system under budget constraints.
The model presented in this paper uses risk cost as an appropriate framework to define the optimal replacement time prediction based on the balance between investment for replacing and expenditures for maintaining the asset. An economic analysis compares the costs associated with maintaining an existing pipe in service, being completely depreciated or not, to the cost of replacing or rehabilitating the pipe. On this basis, the right time in the future to rehabilitate the pipeline can be determined. The costs associated with an existing pipe include direct operational and maintenance costs and indirect costs, such as those associated with risk of failure. The optimal replacement time is identified as the year in which the cost to maintain the existing stock of pipes exceed the investment to replace it.
A dynamic programming tool was developed to search the vast combinatorial solution space of the problem.
The model was applied, with the aim of supporting management decisions, to the wastewater network of Oslo in Norway, managed by Oslo Vann og Avløpsetaten (Oslo VAV), using available real-world information to estimate expected costs of maintenance and rehabilitation.
The results show that a constant value for lifetime should not be applied to all the pipelines in the stock, as currently done by the utility for long-term investment; rather it is wiser to define different values for different cohorts of pipelines, to reduce the uncertainties associated with generalisations for simplification.
The model has been applied to wastewater pipes, but is in principle valid for any aging infrastructure
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