|TABLE OF CONTENTS|
TOFD was originally developed as a method of accurately sizing and monitoring the through-wall extent of welding defects and in-service flaws, primarily in steel components. Maurice Silk, its inventor, released the first report for publication in 1974, stating the main principles of the technique (see text box). These original principles have not needed revision since that time a clear indication of the integrity of the technique. Maurice and his co-workers have since presented clear explanations at international conferences in the UK, France and Germany.
AEA Technology's willingness to share this technology and expose it to industry scrutiny has continued throughout the intervening years, with regular publication of comprehensive theory papers about the TOFD technique, eg Brian Lidington et a], BJNDT Nov 1976,Jill Ogilvy and Andrew Temple, Ultrasonics, Nov 1983).
Maurice has drafted the British Standard BS-7706 and the first draft of the European Standard ENV-583 Part 6.
Since then, AEA Technology has participated using TOFD in every round-robin trial to which we have been invited, and also organised our own Defect Detection Trials (DDT) before the Sizewell B Public Inquiry. The results of these are shown above (Fig 1): in every trial TOFD was proved to offer the best sizing accuracy.
TOFD has also proved to be fast: in the DDT trial all the scanning required to detect and size all 29 defects in the first plate (1400 nun of clad buttwelded plate 250 mm thick) was completed in under 24 hours.
|Summary of TOFD sizing errors from round-robin trials
||Trial name ||Category/|
DTI MEMT Requirement Board|
(Welding defects in plates 34-94mm)
|1983||UKAEA DDT Trials
(Planar flaws in 250mm clad plate weld)
(Various flaws in 250mm clad plate weld)
Plate 3: Underclad flaws in thick plate
Plate 4: Underclad flaws in nozzle I.R
Plate 2: Weld in 250mm clad plate
Plate 3: Inset nozzle weld. 250mm plate
|1989||EPRI Trials. Vessel & nozzel samples
|Dutch NIL Trials|
NDO-I 30-150mm plate & welds
||1990-91||GF.10-38mm plate & welds
||1991-95||NDP 6-15mm plate & welds
TOFD has been compared to pulse-echo and radiographic techniques for defect detection and length sizing performance. On all but the thinnest plate, TOFD performed at a similar level to these systems; however, it operates much faster than the pulse-echo technique, and of course avoids the access restrictions associated with radiography.
Typical probe configuration
TOFD diffraction coefficients
Typical application on heavy wall pressure vessels (Kobe Steel, Japan)
In the TOFD technique the transmitting and receiving probes are located equidistant over the weld centre and scanned parallel with the weld. Normally a single pass is sufficient to attain the required inspection coverage.
In the TOFD technique, to inspect a butt weld, the transmitting and receiving probes are located one each side of the weld. For some applications, such as the detection and measurement of weld root erosion or to measure defects found by other methods, a single scan parallel to the weld is sufficient. For other applications two or more passes, or a scan across the weld, may be needed.
A transmitting probe emits a short burst of sound into a material, and this energy spreads out and propagates in an angular beam. If the beam is obstructed by a defect, some of the energy is diffracted at its edges and travels towards a receiving probe, the signals are recorded, graphically displayed in a greyscale form and analysed using simple geometry calculations. A lateral wave, which propagates just below the surface, or a reflection at the other side of the plate (the backwall), is used in conjunction with the known ultrasound velocity in the calculations to allow for hidden delays in the system.
The data is collected using a simple scanning frame or scanner with optical encoders for positional information. if required the data can be enhanced by special software routines and analysed on a PCbased ultrasonic imaging system such as the Sonomatic µ+ (Microplus).
TOFD has now been demonstrated on a thick section (350 mm) qualification block, to meet the requirements of ASME Code Case 2235, Use of ultrasonic examination in lieu of radiography, Section 1/711, Divisions 1 and 2 which became effective in 1996.
TOFD has been shown to offer higher Probability of Detection (POD), lower False Call Rates (FCRs) and now offers a real alternative to radiography during construction (see Fig 2).
|Results oft a recent survey for the Dutch Welding Institute (NIL)|
Figure 3. TOFD data from a deep crack in a T-butt weld in an offshore platform
Figure 4. Interpretation of TOFD data from a clad nozzle test specimen using CGTOFD and NTPlot
TOFD is not, however, suited to all applications, and is particularly sensitive when used on coarse grain materials such as austenitic stainless, where the low energy diffracted signals, can be obscured by grain scatter noise.
We are never slow to become involved in joint projects when the need arises. Sometimes the solution is not to buy a product or a service but to develop a tool for a particular application. One such example is the BRUCIE project which was a collaborative project with our Australian partners, PNDT, and Woodside Energy Ltd working on the Goodwyn Alpha platform off the coast of Western Australia. Over a period of two years, we worked together to develop a bespoke inspection tool to inspect the steel caissons of the platform. Full details are given in this issue 65 of our newsletter, Inspection Solutions.
In addition, we have been working with the Royal Navy in the development of a high pressure accumulator bottle inspection system for use in the confined space of submarines.