Statistical Sampling for In-Service Inspection of Underground Storage Tanks (PVP2011-57011)

Abstract

Savannah River Remediation, LLC (SRR) is implementing a statistical sampling strategy for in-service inspection (ISI) of liquid waste (LW) tanks at the United States Department of Energy’s Savannah River Site (SRS) in Aiken, South Carolina. As a component of SRS’s corrosion control program (CCP), the ISI program assesses tank wall structural integrity through the use of ultrasonic testing (UT). The statistical strategy for ISI is based on the random sampling of a number of vertically-oriented unit areas, called “strips,” within each tank. The number of strips to inspect was determined so as to attain, over time, a high probability of observing at least one strip of the worst 5% in terms of pitting and corrosion across all tanks. The hypergeometric distribution was used in the probability estimation to determine the number of strips for inspection. In addition to the strip sampling strategy, a single strip within each tank was identified to serve as the baseline for a longitudinal assessment of the tank safe operational life. Statistical tolerance limits for pit depth and corrosion rates were calculated using existing data and will be updated as additional inspections are made. The statistical tolerance limits provide conservative estimates for use in tank life projections. The statistical sampling strategy enables the ISI program to develop individual profiles of LW tank wall structural integrity that collectively provide a high confidence in storage tank safety and integrity over operational lifetimes.