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Operations, Applications & Components

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

[+] Author and Article Information
Lindsay R. Baxter, Stephen P. Harris

 Computational Sciences,  Savannah River National Laboratory, Aiken, SC 29808

J. Pressure Vessel Technol 134(5), 051601 (Sep 10, 2012) (9 pages) doi:10.1115/1.4005935 History: Received April 28, 2011; Revised October 13, 2011; Published September 10, 2012; Online September 10, 2012

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.

Copyright © 2012 by by ASME
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References

Figures

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Figure 14

Dot plots with mean diamonds for corrosion rates by plate

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Figure 15

Three parameter lognormal distribution fit to overall corrosion rates (mil/ year) using the Distribution Analyzer software. [5]

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Figure 16

Percent relative error in the mean corrosion rate versus number of strips inspected

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Figure 13

Wall thickness (mil) by month/year for the lower plate

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Figure 12

Normal distribution fit to maximum pit depth on the log10 scale for the lower plate only

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Figure 11

Confidence curve for the probability of inspecting at least one strip from the worst 5% within any tank

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Figure 10

Maximum pit depth (mil) for the lower plate

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Figure 9

Maximum pit depth (mil) per 1 ft segment by plate

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Figure 8

Pit depth distribution (mil) across all tanks

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Figure 7

Confidence curve for the probability of inspecting at least one strip from the worst 5% across all tanks

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Figure 6

Left: profile of a LW tank wall; right: profile of a typical plate

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Figure 5

(a) In-use UT wall crawler with P-scan device and (b) top view of the wall crawler with P-scan device

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Figure 4

Riser access port

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Figure 3

Cut-away drawing of an underground Type III LW tank

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Figure 2

Top view of an underground Type III LW tank

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Figure 1

Type III LW tank under construction during the 1960s

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