Phased Development and Implementation of a Force Main Condition Assessment and Management Program

INTRODUCTION The City of Omaha Public Works Department owns and operates approximately 26 miles of sanitary and combined force mains. Force main failure in many cases can cause significant impacts to public health, existing infrastructure, and the environment. Because of these impacts, understanding the current condition of these assets is critical. It is understood, based on the results of the City's Capacity Management, Operations, and Maintenance (CMOM) Lift Station Gap Analysis Summary and Action Plan completed in 2019 that 'most force mains are not inspected routinely and are not in the City's Computerized Maintenance Management System (CMMS). Force main inspections that do occur are not formally documented or tracked. Based on these recommendations, the City worked with HDR to develop a Force Main Condition Assessment and Management Program Plan (Plan) for these critical assets that is scalable over time to progressively adopt a more proactive mindset when it comes to planning and executing condition assessments of this critical asset. PURPOSE AND BENEFITS As demonstrated in Figure 1, force main assets in the United States are being replaced or rehabilitated at a rate of just 0.1-0.2% per year. Force mains currently installed in the United States are expected to last well beyond their design life of 50-60 years; and in many cases, the replacement of these assets can be cost prohibitive and may be significantly disruptive for many communities. This underscores the importance of establishing a force main condition assessment and management plan to better understand the assets that currently exist in a given collection system, and also when and where condition assessment activities are prudent. More specifically, the benefits of completing such a plan include: Creating a comprehensive listing of all force main assets, organized by originating lift station. Determination of data gaps, such as location of isolation and air release valves. Establishment of a risk-based prioritization of all assets based on available data that can be enhanced over time as new data becomes available. Recommended suite of condition assessment technologies Recommended sequence of condition assessments Recommended yearly budget for condition assessments A better understanding of how limited funds should be allocated throughout the collection system. STATUS OF COMPLETION The Force Main Condition Assessment and Management Program Plan was completed February 2021, and early action items are currently being implemented by the City of Omaha. The plan is arranged such that the initial low cost, 'low hanging fruit' are scheduled for Year 1. FORCE MAIN CONDITION ASSESSMENT RISK PRIORITIZATION A force main prioritization model was developed using a risk-based approach involving Likelihood of Failure (LoF) and Consequence of Failure (CoF) factors. As shown in Equation 1, a risk score is calculated for each force main segment by weighting and adding its LoF and CoF scores. The risk score is a relative score used for comparing the relative priority of force mains for inspection. Risk Score=[33%* Likelihood of Failure+67%*Consequence of Failure] (1) Table 1 lists the LOF criteria, weight, sub-categories, and sub-criteria scores utilized as part of this plan. Table 2 lists the COF criteria, weight, sub-categories, and sub-criteria scores utilized as part of this plan. MANAGEMENT TIERS Condition assessment methods will vary by material, size, and access requirements. The force main inventory was reviewed and evaluated to identify force main management tiers based on a risk analysis using available desktop information. The management tier definitions are a tool to help the City organize the many different force mains into groups with similar inspection requirements. The Plan utilizes a risk-based condition assessment approach that implements condition assessment strategies commensurate with risk. Higher risk force mains receive higher resolution, more comprehensive assessments. Conversely, low risk force mains receive easier to deploy and, in most cases, less expensive assessments. This maximizes the impact the City will receive for their condition assessment budget. In general, the management tiers help define 'what' assessments are recommended, whereas the risk prioritization defines 'when' and in what order assessments are recommended. The primary criteria used in the development of management tiers included diameter, material, age, system criticality, and ability to access or inspect the asset. Management Tiers as defined by this plan are shown in Table 3. PROGRAM IMPLEMENTATION The condition assessment program is proposed to be carried out in four levels: Desktop Analysis (Level A), Surface Inspection (Level B), External Inspection (Level C), and Internal Inspection (Level D). The level of assessment should be commensurate with pipeline risk and not all levels can be or should be performed for all force mains. The cost of the inspection is proportional to the level of effort required to obtain the information needed to assess the condition of a particular force main. Levels A and B are less costly than Levels C and D, both of which require mobilization of expensive equipment. Levels A and B primarily consist of tasks that could be performed by City staff (or consultants) and approximate person-hours were identified to determine staffing needs to perform those tasks. Person-hours were converted to equivalent FTEs (Full Time Equivalent) assuming one (1) FTE = 1,880 hours/year. This plan recommends Level A and B inspections are performed on all force mains in the first three (3) years of the program. Level A and B inspections are appropriate to perform on all force main materials and sizes except for the soil resistivity tests which are appropriate for alignments along metallic pipes only. Data collected as part of a Level A and/or Level B inspection may lead to a reassessment of the Levels of Inspection recommended in subsequent years or may lead to a decision to move to replacement of the force main. Level A and B inspections should also endeavor to identify locations for access to perform internal inspections or identify needed improvements and costs for creating such access. Level C and D inspections were assigned based on the applicability of available technologies when considering pipe diameter, material, and length each force main. In cases where both Levels C and D are identified in the same year for a particular force main, it is recommended that internal inspections (Level D) occur first to help better identify where is makes the most sense to perform external inspections (Level C). CONCLUSION With the completion of this Plan, the City of Omaha now possesses a manageable plan to progressively plan and execute condition assessment activities for their force main asset. The Plan provides necessary justification for increases in O&M budget and staffing which is required to carry out the recommended and prioritized condition assessment activities in a timely and cost-effective manner.