Risk-Based Inspection Interval for Storage Tanks
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Solution Overview
Problem
Current methods for determining the time interval between out-of-service inspections of aboveground and underground storage tanks are often costly and inefficient, relying on scheduled intervals rather than accurate assessments of tank condition, leading to potential over-inspection and environmental impact.
Innovation Solution
A risk-based inspection approach using Bayesian analysis and leak detection tests, combined with in-service measurements of tank floor thickness and corrosion rates, allows for more accurate and reliable estimation of the time until the next out-of-service inspection, enabling longer inspection intervals without increasing the risk of tank failure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If scheduled inspection intervals are used, then inspection frequency is high and safety is ensured, but inspection cost and operational disruption increase
Solution Approach 1:
The patent changes the inspection approach from fixed time intervals to condition-based intervals by monitoring corrosion rates and tank floor thickness. This allows inspection intervals to be dynamically adjusted based on actual tank condition parameters, extending intervals when conditions are good and shortening them when degradation is detected, thus resolving the contradiction between safety and operational downtime
Solution Approach 2:
The patent implements a feedback mechanism where corrosion rates and thickness measurements are continuously monitored and used to update inspection interval calculations. This closed-loop system ensures safety by detecting when conditions require immediate inspection while avoiding unnecessary inspections when conditions are stable, thereby reducing operational disruption
2Measurement precision
If frequent out-of-service inspections are performed, then tank integrity is thoroughly assessed, but inspection cost and environmental pollution increase
Solution Approach 1:
The patent enables tanks to essentially self-monitor their own condition through corrosion rate measurements and thickness monitoring. This self-service approach provides accurate condition data without requiring frequent disruptive out-of-service inspections, reducing the need to empty and clean tanks and thereby minimizing environmental pollution from inspection activities
Solution Approach 2:
The patent performs preliminary condition assessment through in-service corrosion monitoring and thickness measurements before scheduling out-of-service inspections. This preliminary action identifies tanks that truly need detailed inspection, reducing the number of unnecessary out-of-service inspections and associated environmental impacts
3Duration of action of moving object
If in-service measurements are used, then inspection intervals can be extended, but measurement precision and reliability may be compromised
Solution Approach 1:
The patent uses corrosion coupons and thickness gauges as intermediary devices that can be deployed in-service to measure corrosion rates and floor thickness. These intermediaries provide accurate condition data without requiring the tank to be taken out of service, enabling extended inspection intervals while maintaining measurement precision through standardized measurement protocols
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method reduces the frequency and cost of inspections, minimizes environmental pollution, and keeps tanks in service, while ensuring safety by accurately assessing tank integrity and extending inspection intervals based on actual corrosion rates and tank condition.
Implementation Method 1
an in-service acoustic emission (AE) corrosion activity test where the results indicate little or no corrosion
Implementation Method 2
in-service measurements of tank floor thickness and corrosion rate
Data Source
AI summary
The method and apparatus of the present invention will provide a means for a tank owner to determine the time until the next out-of-service API 653 internal inspection of an aboveground storage tank (AST) or an underground storage tank (UST) should be considered or performed based on in-service measurements of the tank filled with fuel or another liquid. The inspection interval is determined from a leak detection test with a pass and a probability distribution of the survival rate of the tank bottom that is representative of the tank being inspected. More accurate estimates are obtained by using measurements of the thickness and corrosion rate of the tank bottom at one or a few locations in the tank and the results of an acoustic emission (AE) corrosion activity test and/or previous out-of-service inspection measurements of the tank floor bottom thickness of the floor at many locations. The method is based on the concept of equivalent risk.