A Modeling Approach for Pipeline Condition Assessment

Situated along the Kentucky-Tennessee border, the Fort Campbell Military Reservation is home to the 101st Airborne Division (Air Assault) and several tenant units. Built in 1941, its primary mission is to support training, mobilization, and deployment of mission-ready forces. While focused on these day-to-day core requirements, the base is also transforming for the future.

In 2003, Fort Campbell’s water and sewer systems were privatized through a contract to Jacobs. However, funding has been challenging, and as a result, Jacobs and the Fort Campbell Directorate of Public Works sought a risk-based assessment approach to target assets needing replacement. This included two separate utility systems: approximately 183-mi of sewer pipe ranging in diameter from 1.25- to 30-in; and 258-mi of water distribution pipe ranging from 0.25- to 20-in in diameter. Using KANEW, a geographic information system-based modeling tool, an asset management program and capital investment plan for the water and sewer pipelines was developed. Incorporating model results into the capital investment plan will help to maintain mission readiness and resiliency by renewing or replacing pipes before catastrophic failure occurs.

ASSESSING EXISTING CONDITIONS

Developing a cost-effective and proactive water or sewer main renewal program is critical to long-term operation and maintenance of water and sewer pipe networks. However, the condition of underground pipes is difficult to assess, and forecasting capital rehabilitation and renewal needs can be a significant challenge.

Mission readiness and resiliency require support systems that are well maintained and renewed or replaced before catastrophic failure occurs. System operators and installation leadership are faced with the challenge of making the right decisions regarding which pipe to replace and when.

Pipes in a network are usually installed at different rates over an extended period of time—with the material selected based on various factors such as the external environment, age, pipe material, pipe thickness, and quality of delivered water. As the pipes age, they exhibit an increased frequency of failure and decreased hydraulic capacity. The mains that reach their useful lifespans and no longer meet serviceability requirements are considered candidates for renewal and replacement.

DETERMINING REQUIRED NEEDS

The first step in a pipe renewal program is a desktop analysis where the factors that impact service lives, such as age, material, external and internal stresses, and installation practices, are considered in a condition assessment. The main objective is to determine future rehabilitation needs and investments to keep the condition of the network in a state that complies with required service-level standards. When the condition assessment is combined with computer modeling, it significantly aids in the development and maintenance of a more effective long-term renewal and replacement program.

A computer model is helpful in identifying which pipes in a network need attention and when is the optimum time to rehabilitate them. A modeling tool also can assist in prioritizing rehabilitation projects in order to avoid untimely investment and catastrophic events like a major pipe burst. Without a proactive pipe replacement plan, a system incurs the cost of service failures due to degraded system performance in addition to the cost of rehabilitation. The challenge for decision-makers is to determine the most cost-effective plan in terms of which pipes to rehabilitate, which rehabilitation alternative to use, and when in the planning horizon to carry out rehabilitation (subject to the constraints of service requirements).

At Fort Campbell, the desktop analysis focused on levels of service, lifecycle asset management planning, and the resulting long-term cash flow requirements. A condition assessment for the pipe network was performed to support the analysis.

To accomplish this, the data to be input into the statistical model was collected and prepared. The model was then used to assess the data by providing estimates of pipe survivability. The output was analyzed with regard to inventory and investment to generate potential pipe network renewal and replacement plans and develop a capital investment plan.

Fort Campbell’s pipe inventory was summarized in terms of pipe type, pipe material, and length of installation in 10-year increments. The pipe inventory was also applied to maps of the water and sewer mains. Using this data, the remaining service life of the water and sewer mains was predicted for each year in a 50-year planning period under optimistic, medium, and pessimistic service life assumptions. Following this, an investment scenario analysis with two possibilities was performed.

• Baseline Scenario (Investment Needs). This analysis identified, without budgetary constraints, the annual renewal and replacement investment that would be required to maintain the required level of service.

• Scenario 1 (Revenue-level). Under this scenario, a constant renewal and replacement budget for every year of the planning period was used to determine the length of pipe to be renewed every year over the planning period. It included an annual budget of $1.2 million per year (water) and $1.7 million per year (sewer) for 35 years, followed by unrestricted/unrestrained renewal until the end of the projection period.

The unit costs applied to the two scenarios were based on the cost of pipe material under different size classes and weighted according to the distribution of pipe sizes throughout the system. For the long-range cost analysis, a flat 2 percent inflation rate was assumed.

The annual length and rate of replacement using baseline optimistic assumptions was predicted for the sewer and water mains. Once that had been determined, the costs for both investment scenarios were then compared using multiple service life assumptions. The comparison found that the less-than-required investment in the initial 35 years under Scenario 1 would result in a pronounced jump in the investment need at the 36th year of the planning period. This finding highlighted the drawback of deferred investments.

The pipe inventory and investment scenario analyses yielded a renewal and replacement plan that included a schedule of the funds that would be needed to meet the pipeline service requirements. The analyses indicated that the investment need in a year (for example, 2030) would require $4 million and $5 million, respectively, for the water and sewer systems. The long-term budget needs were determined to be a flat $2.5 million over the 35-year planning horizon.

BETTER PLANNING DECISIONS

The top-down modeling method for condition assessment and capital investment plan development is a cost-effective approach to manage pipe renewal and replacement. In addition, the Fort Campbell study illustrates the consequences when anticipated investment is not sufficient to meet needs.

This cost analysis method will benefit the planning of other buried pipelines. It is recommended that risk analyses be performed based on the probability and consequence of failure. Spatial display of prioritized pipes for renewal and replacement per service life assumptions can help identify the preventative actions required to maintain the serviceability of pipes. Comparing budget scenarios can identify the funding needed to complete preventative actions.

Performing the renewal and replacement activities identified by a condition assessment based on risk analysis and prioritization can preempt catastrophic failure by proactively repairing pipes that are likely to fail. This assessment can lead to better and more defensible planning decisions and risk reduction.

Rajat Chakraborti, Ph.D., M.SAME, is Engineering Technologist, Jagjit Kaur, Ph.D., M.SAME, is Senior Project Technologist, Jonathan Green, CQA, M.SAME, is Deputy Program Manager, Utility Privatization, Phil Facer, M.SAME, is Fort Campbell Utility Privatization Manager, and Chris Williams, M.SAME, is Fort Campbell Utility Privatization Project Manager, Jacobs. They can be reached at rajat.chakraborti@jacobs.com; jagjit.kaur@jacobs.com; jon.green@ jacobs.com; phil.facer@jacobs.com; and chris.williams1@jacobs.com.

[This article first published in the September-October 2021 issue of The Military Engineer.]