Asset condition estimation and depreciation


Courtesy of Innovyze

Vitens is one of three water companies in The Netherlands, with 2.3 million connections, 5.4 million customers, 47,5000km of pipes and 96 treatment works that provide 330M.m3 a year of potable water at a cost of €1.32/m3 ($1.72/m3).

Each day the company receives calls from customers reporting leaks. The maintenance division provides information on the pipe material and diameter, and the business centre manages valve control to divert water around the repair location so that the work can be undertaken. If a large pipe is involved and there is the possibility of an impact on service levels, then water may be brought into the zone from a different treatment works and fed into the local distribution area to compensate.

The company also evaluates the causes of bursts. In the province of Gelderland, for instance, there are 8140km of pipes, details of which are contained in 139,000 records within the company’s InfoNet asset data management system. InfoWorks WS models of the Gelderland province contain 500,000 pipes, 500,000 nodes and 800,000 customer points. There are three models in all, representing the region, to ensure that each is of a manageable size.

Bursts in the region are always related to either the pipe diameter or the year of installation. Pipes can last between 80 and 120 years - the oldest one in Gelderland dates back to 1888 - but many pipes do not last as long as expected because of damage during road or sewer reconstruction. Material can also be critical - for example, an asbestos cement pipe conveying a high-pressure supply is very likely to burst. In general the average pipe age is 40 years. In Gelderland a wide range of pipe materials is in use including PVC, PE, asbestos cement, cast iron and steel. In the Vitens GIS system it is important to ensure that when a pipe is replaced the associated financial information is also correctly handled. For example, if a replaced pipe had a residual value of 20% it would be necessary to ensure that this was also removed from the net present value data to avoid error in financial calculations.

In Gelderland there were two companies before the creation of Vitens - NWG and WG. Vitens only has evaluation data for one of the areas from 2001 to 2006, and for the other areas it holds data for just two years.

The utility decided to import all of this information into InfoNet, with every evaluation being given a code to indicate whether the leak was at a hydrant, a valve or a burst pipe. Every incident is booked to an address connected to a customer point, but not every burst is reported with an exact location. For instance, a burst on the corner of two busy streets would be reported in InfoNet as a general incident.

For this reason, from 10,000 evaluations only 6,000 customer points could be imported. The cause of the remaining bursts was therefore associated with customer points in InfoWorks WS. Other data are also input in the evaluation - unregistered pipe materials, the diameter and year of installation, the ground type, whether any customers were cut off as a result of the incident, and the length of the process between discovery of the burst and the completion of the repair.

Identifying burst pipes
The burst pipe identification process was undertaken by inputting the evaluation data into InfoNet’s ‘general incident’ category. Then ‘burst pipes’ were selected within the ‘pipe repair’ categorization, which brought up on screen details of all of the pipes that had experienced a burst of any type.

Vitens has a policy of replacing pipes that burst four times in one year for several years, and as a result of this identification process it pinpointed 100 pipes that needed to be replaced. The utility had to further prioritise which pipes should be replaced most urgently.

This was done by considering the location of each burst. When four bursts were found in the same street, they were put on the list for priority replacement as one contract. Using InfoNet it was easy to quickly identify problem areas. Vitens’ policy now is to replace pipes when they burst four times in a year or where the effect is unacceptable (such as in proximity to a dyke, railway or highway) or where the resultant level of service is unacceptable.

The company was left with a list of remaining unprioritized pipes, which were sorted by checking the burst data against further information such as total number of bursts, the number of affected customers and metered losses. By selecting among the data and checking the number of customers affected it was possible to choose options for replacement and repair.

Critical link analysis in InfoWorks WS
In InfoWorks WS it is possible to undertake critical link analysis, and in The Netherlands pipe pressure is used as the key critical factor. Bursts in asbestos cement are often pressure related so it is possible to create a spread of burst likelihoods according to the pressure, from zero bursts at zero pressure to 100% bursts at 20m head pressure.

The impact of a burst can be analyzed in InfoWorks WS. With critical link analysis the burst time till valve control is achieved can be calculated for any pipe in a network. Criticality can also be updated from InfoWorks to InfoNet, but not the impact of valve control when a valve bursts. The impact of valve control can only be gauged for single pipes - the company would like to undertake critical valve section analysis, with a report function that would enable the number of affected customers on a valve section to be calculated.

The company focused the search for critical pipes by identifying customers that would be left without water, or only for those that would experience reduced pressure, and calculated the repair time in minutes. The number of affected customers is multiplied by the repair time to give a figure that can be used to prioritize repairs and replacements.

InfoWorks and InfoNet have the ability to update each other, with InfoNet enabling monitoring of associated pipe repairs using its SQL function. Vitens also requires information on the number of bursts per section, and other influences such as groundwater levels, tree proximity, traffic levels and various other external impacts such as highways, railways and sensitive customers such as hospitals. With all this information a complete profile of a pipe would be available and a definitive criticality score could be calculated.

Future plans
Vitens also wants to examine the effect on the network if a valve section has no water, and plans to create a replacement scheme for every main with 500 to 2000 customer points so that it is no longer considering single pipes but replacement of complete sections. Within the five regions that Vitens serves it is possible to score any pipe and prioritize its replacement - a proactive, rather than reactive, strategy.

With both InfoNet and InfoWorks the utility has the ability to direct investment in and facilitate the replacement of pipes. The utility can take into account not only the costs of investment but also operation - for instance it is possible to estimate for a particular diameter of pipe what the additional power costs would be. This means the company is no longer only calculating criticality according to pressure but also using other key factors. Vitens calculates the replacement cost with inflation and depreciation to obtain a daily value, the ‘net present value’, a facility that InfoNet can already provide for sewers.

The complex multiple effects of criticality can be seen in InfoNet - not only the effects on the surrounding network but also the effects of repairing the valve. It is possible to create a replacement plan that incorporates a pipe replacement programme for an entire neighbourhood, thereby enabling Vitens to assess the overall costs for entire areas. The annual costs of a pipe need to be calculated, as well as the treatment, pumping and reservoir costs, all on an annual basis, and these costs provide an overall annual cost of production that can be used to calculate the service cost for an entire network area.

InfoNet and InfoWorks WS are enabling Vitens to actively manage its aging distribution network so that when investment in new pipes becomes necessary the pipes and areas that need to be replaced, as well as which pipe diameters, can be readily identified. This enables the generation of, an effective forward pipe replacement plan.

This article is based on a paper presented at the Wallingford Software International User Conference in September 2008 by Erik Amting (TITLE) of Vitens in The Netherlands.

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