A NEW METHOD FOR DEVELOPING THE MOST COST- EFFECTIVE REHABILITATION PROGRAMS FOR OUR AGEING SEWER NETWORKS Introduction Toby Bourke (MWHSoft), Graham McGonigal (MWH) There is currently widespread concern regarding the condition of our aging sewer collection networks. Many of our existing sewer systems are under considerable stress due to rapid urban growth, or due to the deterioration of aging infrastructure, with areas of some sewer networks dating back nearly 100 years. Renewed interest in urban development and gradual land use changes have resulted in a drastic increase in loads on networks that were not designed to sustain these additional flows. These systems can experience blockages and surcharges which can directly impact sensitive catchments, and ultimately a population s health. The traditional Find and Fix approach is not cost effective or reliable and may not even improve the overall performance of the sewer network. Sewer system rehabilitation is a sound solution for aging networks; however rehabilitation can necessitate large expenditure which can often exceed available funds. Good engineering decisions based on sound analysis procedures will be required if the alterations and improvements to these networks are to be effective and economical. The effective management and rehabilitation of these networks involves the collation of large amounts of digital information with sound engineering analysis to accurately identify problem areas and restore needed capacity with the least amount of system improvement expenditure. MWH Soft s CapPlan Sewer software is a new decision support tool that helps wastewater utilities effectively determine sewer rehabilitation needs and prioritise projects for short and long term benefits. By centralising all data sources into one location, CapPlan Sewer allows the engineer to perform a thorough risk assessment on every single pipe, then develop a customised decision model to generate a capital or operational expenditure decision for pipes with the highest risk ratings.leveraging GIS data, CCTV footage, hydraulic modelling results and various other data sources, CapPlan Sewer allows asset managers to easily implement cost-effective sewer network rehabilitation programs. This paper gives an insight into CapPlan Sewer s methodology to improve the management of the risk of asset failure and to help utilities move from reactive to proactive management of their sewer pipe network. The process for pipe renewals planning is represented diagrammatically in Figure 1, beginning with data and ending with the output of an initial Rehabilitation Plan.
Figure 1 - Pipe Renewal Planning Process Data Required During the renewals planning process, data has to be collected from a variety of sources throughout the business. Typical datasets collected as part of the process include: Hydraulic Models current day and master planning scenarios Infrastructure Data GIS Data & Critical Facilities CMMS & Work Order Data Inspection Data Cost Data A key requirement of the renewal planning process is transparency. This enables all levels of the business to understand the process that the organisation has undertaken to reach the funding requirements for rehabilitation. Any data translation is fully documented and stored at every stage so that it can be easily tracked. The collation of data across many different systems (corporate and stand alone) can be the catalyst for the organisation to think about enhancing its data systems and is one of the most valuable bi-products of this process. It is important to note that it is not a requirement to have all of the datasets suggested to perform the renewal planning process; the pipe rehabilitation plan can be derived from as much or as little data as the organisation has available. This allows the rehabilitation plan to utilise a larger quantity and quality of data as it becomes available. Moreover, the ODBC mapping functionality (Figure 2) allows the simple mapping of external data fields to standard or user-defined data fields, ensuring that virtually any data source can be utilised in the development of the renewal plan.
Hydraulic Model Data Figure 2 - ODBC Exchange Mapping Data Fields GIS-based hydraulic modelling data can be leveraged in a number of ways in CapPlan Sewer: To identify current and future hydraulically insufficient pipes To allow hydraulically driven consequences and likelihoods of failure to be considered To supplement any physical asset data To identify the volume and effect of sewer overflows To ascertain the appropriate diameter pipe for an upsize decision Infrastructure Data To use this kind of prioritisation approach it is important that as much information is known about each asset as possible e.g. construction material, age, diameter. It is accepted that many of the large databases held by water utilities or councils have fields that are incomplete or contain little information. The renewal planning process uses only data that has a high confidence rating to derive likelihood or consequence of pipe failure. If required, procedures can however be implemented to make the best possible renewal planning decisions based on poor or limited data. GIS Data & Critical Facilities GIS data, including the soil type and proximity to transit/fault lines, is utilised to help determine a pipe s consequence and likelihood of failure, Factors that affect consequences of failure include: the impact that a failure is likely to have on customers o proximity to critical facilities o types of customers that will be affected o level of disruption to transit the impact on the physical environment o proximity to river bank (scouring/fouling) o proximity to wildlife habitat o terrain cost of repairing/replacement
o o surface type (road/footpath/grassland) pipe replacement/rehabilitation cost Performance history data derived from customer databases and historical blockage data can be used to identify frequent service failure in pipes that may have an acceptable degree is structural integrity. These pipes must be given appropriately high likelihood of failure scores during the network risk assessment. Unfortunately, a link between sub-standard pipeline installation contractors and structural pipe failure is often found in many networks. CapPlan Sewer allows the development of constructionrelated likelihood of failure factors so that pipes installed by certain organisations can be flagged as having a higher likelihood of failure. Inspection Data Inspection data can be imported into the tool for use to determine the probability of failure for each pipe. This can be either CCTV data or physical inspection data. Both summary CCTV data (Max Score, Peak Score, and Mean Score) and detailed CCTV data (defect codes and positions) can be used to help drive the preferred rehabilitation techniques and timings. Video or picture files can also be linked to the pipe so that the most recent CCTV inspection data can be considered. The WSAA standard or Australian standard can be stored in the tool and used to allocate point scores to each pipe. Cost Data Cost data is utilised heavily when constructing prioritisation models and is commonly one of the most sensitive parameters in the entire process. Traditionally water companies have had little success in associating the entire cost of an activity with a particular asset. This is because maintenance staff perform multiple jobs in their daily routine and utilise materials and equipment from the central storage depot. The advent of more sophisticated maintenance management systems and the increase in the outsourcing of contractors have allowed the implementation of more sophisticated cost allocation methods akin to activity-based costing. To enable the running of a renewals model, cost information is collected for the cost of proactive interventions i.e. replacements, repairing assets if they fail prior to replacement, and the consequence of failure should they fail. The data for the latest financial year is collected from the organisation to calculate the cost, as prices for labour, materials and equipment can vary substantially from year to year in the current financial climate.
Calculation of Risk There are two methods of calculating risk in CapPlan Sewer; 1. Simple Method 2. Matrix Method The Simple Method is a multiplication of Likelihood of Failure (LoF) and Consequence Scores to calculate risk. In this method the LoF scores and consequence scores are not converted to a 1 to 5 score, but are left in their pure form. The Matrix Method follows the Australian and New Zealand standard (AS/NZS 4360:2004 Risk Management). The LoF scores and the consequence scores are converted to a 1 to 5 score and then via a matrix assigned a risk level from Extreme to Negligible. The matrix method will be considered for the remainder of this document. Likelihood of Failure The likelihood of failure or overall condition grade (OCG) is an indication of how often the asset is likely to fail and is an assignment of a score between 1 and 5. To help determine the OCG, three condition grades are assigned; coarse condition grade, performance history grade, observed condition grade. The coarse condition grade is the best guess at the useful life without any information other than pipe attributes and can bring into effect any local knowledge of the network. The performance history grade uses failure information to grade the assets. The observed condition grade uses physical assessments (CCTV or reactive inspection programmes) to understand the condition of the asset. A decision tree is utilised to determine the overall condition grade from the three condition grades. Consequence Modelling The Australian and New Zealand Standard on Risk Management AS/NZS 4360:2004 provides the definition of consequence as: the outcome of an event expressed qualitatively or quantitatively, being a loss, injury, disadvantage or gain. There may be a range of possible outcomes associated with an event An example of some of the criteria applied to assess the consequence of failure is summarised in Table 1 and follows the Triple Bottom Line approach. Consequence assessment area Social Impact Environmental Impact Economic Impact Sewerage Assets Immediate threat to public health and safety Type of customer affected Number of customers affected by loss of service Impact on overall KPIs Quantity of sewage spilt to the environment Impact to receiving environment Cost of asset repair Insurance claims from property damage or business loss Likelihood of fines by EPA Table 1 - Consequence of Failure Criteria
Each of the assets is assessed in terms of the consequence by applying a score and a weight for each individual criterion. The criteria and weightings are usually built up from historical information collected on previous projects and also via consultation throughout the organisation. It is important to realise that the probability of failure and consequence models that are employed by the software can be as complex or simplistic as the datasets allow. For example, if an organisation has developed its own consequence model, the model can be imported into the software and used immediately. Rehabilitation Engine The rehabilitation engine has the following three functions: Prioritises rehabilitation actions using the output of the risk assessment Derives a specific cost of each rehabilitation action for each and every pipe Identifies whether a capital or operational rehabilitation action should performed to avoid pipe failure. These areas are elucidated in the following sections. Prioritisation Once the likelihood of failure models, consequence models, and cost models have been created, the remaining ingredients are required to enable the development of the prioritisation plan: the asset list with the attributes used in the aforementioned models a definition of intervention triggers The likelihood of failure model and consequence model can be utilised to determine the risk of each individual asset at the point in time that the snapshot is taken. The actions that should occur at each level of risk are defined in Table 2. Level of Evaluated Risk Extreme High Medium Low Sewerage Assets Opex Actions Capex Actions CCTV or Inspect Renew or replace immediately to within 12 months of assess LOF, and evaluation determine if repair is warranted CCTV or Inspect annually or Repair within 12 months CCTV or Inspect every 3 years to assess LOF CCTV or Inspect every 6 years to assess LOF Renew or replace within 5 years Renew or replace within 10 years Renew within the lesser of life of asset or 20 yrs Negligible Do nothing Do nothing Table 2 - Inspection and Renewals Decision Rules To enable decisions to be made, a risk decision matrix (Table 3) is used in conjunction with the decision rules in Table 2.
Likelihood of Failure Insignificant 1 Minor 2 Consequence Moderate 3 Major 4 Catastrophic 5 5 L M H E E 4 L L M H E 3 N N L M H 2 N N N L M 1 N N N N L Table 3 - Preliminary Risk Evaluation This methodology provides the following outputs at an individual pipe level: A prioritised list of assets for renewal, including the cost of renewal A prioritised list of assets for further inspection The remaining useful life of each asset The ability to link to asset valuation requirements and depreciation This information can be viewed at pipe level in either a table format or as a GIS layer (Figure 3). Figure 3 - CapPlan Sewer Outputs in GIS Environment
Cost Derivation Costs are included in the renewal planning process to allow the consequence of failure and investment required for renewals / inspections to be considered in the decision making process. These costs are typically identified as reactive and proactive costs respectively. Reactive Costs (OPEX) Reactive costs associated with bursts should be calculated. Ideally the direct costs are collected from the works management system, allowing the cost of an individual job to be assigned to an individual asset. From the sample cost data, relationships can be developed that allow the prediction of repair costs for assets that have not yet failed. Common parameters that are predictors for this cost include area, diameter, surface type and depth. Proactive Costs (CAPEX/OPEX) Proactive costs are those associated with the rehabilitation of the pipeline assets e.g. replacement, patch repair and root cutting. Many different methods of renewals or rehabilitation are possible; CapPlan Sewer can help to identify the most likely rehabilitation technique using a combination of CCTV, GIS and cost data. A site survey will however be required to verify the correct choice of technique as conditions may have changed since the last survey. Inspection/CCTV costs should be derived for each asset so that the total cost of the suggested inspection programme can be calculated. Identification of Rehabilitation Technique At this stage the timing of action has been identified and the cost of each action assigned to each pipe. A more detailed study can now be undertaken using the CCTV data as a driving component. Any assets that have been identified in the risk study for rehabilitation within the next year, but have not yet been CCTV surveyed, are then scheduled for immediate inspection. The results of these inspections are imported into CapPlan Sewer and the rehabilitation engine is re-run in an effort to specify the course of action and also clarify whether or not immediate rehabilitation is required. The CCTV data imported into the system is utilised to identify the intervention that is required to rehabilitate each of the defects in the system (Figure 4). Figure 4 - Rehabilitation Actions Associated With Defect Codes
As a single pipe can contain multiple and various defects, a user defined decision tree (Figure 5) is utilised within CapPlan Sewer to determine the most likely rehabilitation technique (Figure 6). Figure 5 - User Defined Rehabilitation Decision Tree Figure 6 - Pipe Rehabilitation Summary Post Rationalisation Rehabilitation Decision Refinement Pipe defects are not the only consideration when determining the required rehabilitation action. Many other factors, including changing legislation, capacity problems and future demands also need to be taken into consideration to determine the most appropriate action. CapPlan Sewer utilises the results of hydraulic modelling of the sewer network, allowing the identification of pipes that lack capacity in the current day or future planning horizons. CapPlan Sewer is able to consider when a pipe is likely to be upsized, and if the timing of the upsize should be advanced to ensure the lowest possible future capital and operational expenditures in relation to the pipe.
Rehabilitation Plan It is important to remember that the system being used to prioritise the pipe renewal decisions is a decision support system (DSS), and not a decision making system. The outputs of the DSS are considered prioritised based on the best information provided to the system. After a draft pipe renewal plan has been developed, some rationalisation and minor rescheduling of the renewal plan may be require to account for factors such as the packaging / consolidation of rehabilitation works in a particular geographic location.. Following this process, the detailed design process and implementation can be performed. Once implementation has been completed, updated asset data, as well as new performance, condition and cost information data, can be utilised to develop the next rehabilitation plan (of which the timing is dictated by reporting, legislative or financial requirements). The regular development of pipe renewal plans using CapPlan Sewer allows organisations to see how their total system risk varies over time. This is extremely valuable, as it allows organisations to identify if they are spending the appropriate amount of money on pipe rehabilitation, thus ensuring that total system risk is kept at a desirable level. Conclusions It is critical that an organisation s pipe renewals planning process is transparent, defendable and auditable, thus allowing utilities to demonstrate to boards and stakeholders a clear and robust case for funding requirements. The methodology detailed in this paper enables wastewater utilities to develop a cost effective and prioritised rehabilitation plan that incorporates both capital and operational expenditure solutions. The methodology outlined in this paper is unique in the fact that it uses both historical and predictive (hydraulic modelling) data to determine the optimum sewer pipe rehabilitation plan. References AS/NZS 4360:2004 Risk Management