In my previous article covering NDT in the sixties, I left the narrative at the point where I had become an independent sole technician in the early seventies. My NDT activities were associated with ships and shipyards, as well as piping and pressure vessels in refineries.
The shipyard work involved both thickness gauging of hull plating and internal structures and weld examination on commercial vessels and US Navy ships. Access to certain areas of Navy ships required a security clearance. I was classified as a ‘resident alien’ at that time and, as I later found out, the security clearance review process included a visit by two FBI gentlemen all the way to Edinburgh to investigate my former life as a student and later as a school teacher, which included interviews at my favourite pub! Fortunately, I was given the necessary security clearance.
Some of the ship inspection duties involved travelling with the ships, usually oil tankers, when they were under ballast and conducting the thickness measurements at sea. Trips involved journeys to Hawaii, Alaska, Panama, Aruba, Curacao, Puerto Rico and Singapore. Access to the necessary locations in the tanks usually involved climbing on the existing support structure, although a so-called refinement of this required an inflatable boat, which the technician paddled to the required locations as the ship’s crew gradually raised the level of the ballast water in the tank. Heavy weather produced waves in the tanks and posed a significant danger to the occupant! Classification societies evaluated testing contractors like me and provided certification.
A later ‘improvement’ that came as ships and tanks grew larger was to fill the tanks and use divers equipped with video cameras and ultrasonic transducers on long cables attached to the umbilical, with the technician topside recording the data.
Speaking of divers, in the early 1980s I was engaged to teach NDT at the College of Oceaneering in the Port of Los Angeles. I taught the theory and practical courses in ultrasonic testing (UT) and magnetic testing (MT) and the underwater applications. I had previously conducted underwater hull thickness gauging and ultrasonic examination of welds and propellers using local commercial
divers.
I had expanded my small NDT service company with additional personnel, many of whom I had trained at the College of Oceaneering and who had chosen NDT over commercial diving. The next boost to my small company came in the unfortunate guise of a large refinery fire in Los Angeles:
‘Five Feared Dead, 45 Injured in Carson Oil Refinery Fire: Mushroom Cloud Forms After Blast’
5 December 1985
Typically, the refinery’s reaction to the conclusion that the fire was caused by a failure of corroded piping was to inspect every section of piping in all vulnerable areas. This required recruiting as many ultrasonic technicians as were available in the Greater Los Angeles area. I was able to supply five crews using students who had recently graduated from the College of Oceaneering.
There was considerable confusion due to the large infusion of technicians with a variety of training and experience. Analysis of early measurements showed a wide disparity of results and inaccuracies. Sections of material from the damaged and ruptured piping were used to qualify the technicians. This resulted in a reduction from the original thirty NDT crews to about twenty, based on technicians who were deemed qualified using the performance demonstration testing. My five crews passed the demonstration qualification with flying colours.
The equipment being used for thickness measurements at that time was portable and competent for accurate thickness measurement, although measurements were limited to two decimal places.
The refinery operator made the decision to require the use of the EPOCH 2002, the world’s first field-portable all-digital flaw detector. This was ill suited for field thickness measurement, incapable of echo-to-echo measurement and slowed down the process. After first requiring the inspection contractors to purchase this equipment, the refinery conceded and permitted the use of the small portable units once again.
The introduction of battery-powered portable ultrasonic instruments affected the ultrasonic process, especially for thickness measurements. The two-person crew now comprised the technician with the portable instrument strapped to their body and the assistant recording the results.
The early instruments used analogue instrumentation with cathode ray tubes and valves (or tubes in the USA). Digital signal processing (DSP) concepts were introduced and emerged as the primary means of detecting, conditioning and automatically classifying a variety of signal types. Compared to analogue signal processing systems, digital systems have greater signal transmission fidelity, large non-volatile storage capabilities, processing/calculation/classification capabilities and more sophisticated filtering and signal analysis methods.
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