Inspectioneering Journal

Joint Industry Project Announcement - Non-intrusive Thickness Mapping of AST Tank Bottoms using Lamb Wave Tomography

This article appears in the January/February 2001 issue of Inspectioneering Journal.

The objective of this joint industry project is to develop and commercialize a non-intrusive system for mapping the thickness of aboveground storage tank (AST) bottoms using Lamb wave tomography. Equilon Enterprises, LLC plans to initiate this three-year project in April 2001.


Global on-stream inspection of ASTs could save storage tank owners millions of dollars each year. The industry currently spends millions of dollars each year directly on the maintenance and inspection of ASTs that are otherwise fit-for-service without internal inspection and maintenance. The indirect cost impact of loss-profit opportunities

that arise when these ASTs are taken out of service for inspection is even greater. Equilon has a proposal for technology that will premit ASTs to be examined periodically using ultrasonic computer-aided- tomography, while the AST remains in service without entering the tank.


Advanced sensor and computing technology make this technology possible. An array of passive ultrasonic transducers would be mounted on the chime rim of an AST. A pulsing transducer would scan around the chime rim. Each time the active transducer is pulsed, the ultrasonic energy that is transmitted into the floor would radiate in all directions to each passive transducer. The data acquisition and analysis system would collect time-of-flight and amplitude data for every pulse for every passive transducer and would reconstruct the data using computer-aided tomography (CATSCAN) algorithms. The product of this analysis would be a high-resolution image that would map the thickness of the AST bottom plates. 

Equilon Enterprises, LLC has completed the fundamental theoretical work and has basic proof- of-concept experiments using an eight-foot diameter mockup, a thirty- eight-foot diameter AST, and a one- hundred-fifty-foot diameter AST. This study was performed jointly by The Ohio State University (OSU) and Equilon Enterprises. This study indicates the technical feasibility of mapping the thickness and detecting topside and bottomside corrosion on AST floors. This joint industry project will conduct the next steps in developing and commercializing this innovative global, on-stream inspection technique. 

Project Team

The project will be administered by Equilon Enterprises, LLC. Dr. W. David Wang, Staff Research Engineer, at Shell’s Westhollow Technology Center will be the technical director of the project. Dr. Stanislov I. Rokhlin, Professor of Engineering, The Ohio State University, will perform the theoretical modeling, develop the software algorithms, and jointly conduct the lab/field testing with Equilon. Other NDT development contractors and service providers will be invited to join the project as the technology moves towards commercialization.

Project Plan

This multiyear project will consist of three phases.

Phase 1 - Proof-of-Concept. Phase 1 will be divided into two distinct workscopes to reduce the technical risk to project participants.

Phase 1A - Laboratory Experimentation. In Phase 1A, the work will involve the selection and optimization of the desired wave modes and the minimization of undesirable wave modes. Testing will be performed under ideal conditions on a subscale mockup using flat plate without any lap welds, annular rings, shell to bottom welds, or other structural anomalies. Through this modeling and testing, we will determine the absolute limits of the technology for mapping the thickness of flat plate.

Phase 1B - Field Experimentation. In Phase 1B, testing will be performed on a subscale mockup that includes lap welds, annular rings, and shell to bottom welds. Through this modeling and testing, we will determine the practical limits of the technology for mapping the thickness of tank bottoms. Testing will also be performed in the field on three ASTs, which are in service, to determine the impact of field conditions on the viability of the technology.

Phase 2 - Prototype System Development and Field Trials. In Phase 2, the work will involve the development of a prototype system including the active or source scanning transducer, the passive transducers, and the prototype instrumentation. The data analysis software will be automated. The prototype system will be tested in the field on five or more ASTs.

Phase 3 - System Optimization and Commercialization. In Phase 3, the work will be focused on optimization of the system and the software for ease of use by field technicians. A technology transfer program will be initiated for sponsors and prospective NDT contractors.

Quarterly reports and trimester meetings will be held to review progress in each phase of the project. Interim reports will be issued at the completion of each phase of the project and a final report will be issued upon the completion of Phase III.

Project Participation

The joint industry project is open to all operating, instrumentation, and service companies. Participants in the project will be granted a worldwide, non-exclusive, license to use the technology developed in this joint industry project at the participating company’s facilities. 

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