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Seattle Public Utilities

Risk-Based Economic Decision-Support Models
Seattle Public Utilities

Seattle, Washington

BACKGROUND

BIS worked with Seattle Public Utilities to develop risk-based economic decision-support models for their major wastewater collection system assets. The asset groups included force mains, pump stations, control structures, and sewer pipes. The outputs of the models include projections of long-range capital spending requirements, spending prioritization, and expected repair cost projections.

SEWER PIPE MODEL

The sewer pipe model determines the optimal spending strategy for each pipe in the system, considering defect repair (i.e., point repairs), lining, and replacement. The cost due to risk of pipe failure is assessed for each pipe based on its unique characteristics, including installation date, location (e.g., depth, steep slope, and high groundwater areas), diameter, material, and NASSCO condition. The model optimizes costs from risk against the cost of repairs or re-placement to minimize life-cycle costs.

The model also includes a module for considering the benefit of point repair or pipe replacement in conjunction with other projects, such as road work or other utility work. Concurrent work may mitigate the cost for road repair, for example, which increases the incentive for intervention.

PUMP STATION MODEL

The pump station model identifies the optimal replacement timing for each major pump station asset, including pumps, motors, valves, and electrical power and control equipment. Additional modules include:

CONTROL STRUCTURES MODEL

The control structures model assesses the cost-effectiveness of a proposed control structures upgrade or repair project. The assets comprising the structure (e.g., outfalls, manholes, weirs, hydrobrakes, gates, pipes, etc.) are selected from a pre-defined asset class list, allowing the user to build a particular structure and apply the model to any configuration of control structure assets. Justifications for control structure upgrades are based on avoided overflows and minimizing life-cycle costs. This model also includes real-time repair or replace decision-support for failed assets, similar to the one for pump station assets.

FORCE MAIN MODEL

Determines the optimal replacement timing for each force main in the collection system. Annual repair costs are also calculated based on projected failures.

Highlights
Developed risk-based planning and prioritization tools. Projected short and long-range spending requirements. Support for prioritization of spending for replacement, repair, and refurbishment.
Reference
Frank McDonald
Wasterwater Conveyance
Business Area Manager
(206) 386-1838
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