TABLE OF CONTENTS

Introduction Trials What is TOFD? Applications Training Products Service AEA Technology Energy's current TOFD product range

Introduction

AEA Technology is delighted to be celebrating 25 years at the forefront of non-invasive inspection with our own invention: the Time-of-Flight Diffraction technique. TOFD is now recognised as the most rapid, versatile and reliable method of ultrasonic NDT available to industry today.

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.

Trials

TOFD has been trialled extensively ever since the PISC I trials in 1977. The 1980 study led by the Welding Institute on behalf of the DTI, used planar and non-planar welding defects. The CEG13 participated using the pulse-echo technique, while UK AEA apphed TOFD. The results demonstrated that TOFD offered the best sizing accuracy in the through-wall direction of all the methods used, with negligible system error and standard deviation of just 1-2 mm.

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
Year	Trial name	Category/ flaw type	Mean error (mm)	Standard deviation (mm)
1980	DTI MEMT Requirement Board (Welding defects in plates 34-94mm)	Planar Non-Planar	+0.5 -0.3	1.8 1.0
1983	UKAEA DDT Trials Plate 1 (Planar flaws in 250mm clad plate weld) Plate 2 (Various flaws in 250mm clad plate weld) Plate 3: Underclad flaws in thick plate Plate 4: Underclad flaws in nozzle I.R	Clad side Unclad side Clad side Unclad side Planar Planar	-1.4 -1.3 +1.6 +2.1 +1.0 +1.9	2.5 2.0 8.6* 6.9* 2.6 1.4
1985	PISC-ll Trials Plate 2: Weld in 250mm clad plate Plate 3: Inset nozzle weld. 250mm plate	Verious Verious	+1 +2.8 -0.4	13* 3.3 1.3
1989	EPRI Trials. Vessel & nozzel samples	Verious	-0.4	1.3
1983-86	Dutch NIL Trials NDO-I 30-150mm plate & welds	Verious	0.1-1.0	1.6-2.1
1990-91	GF.10-38mm plate & welds	Verious	-	1.0-1.5
1991-95	NDP 6-15mm plate & welds	Verious	-0.6-+0.3	1.2-1.6

*Results are affected by the presence of unintended flaws surrounding intended flaws

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.

What is TOFD?

Typical probe configuration TOFD diffraction coefficients Typical application on heavy wall pressure vessels (Kobe Steel, Japan)

TOFD is an effective method for the detection and sizing of flaws, almost irrespective of type or orientation.

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)

Applications

TOFD has been applied to a wide range of components and structures, including ferritic plate, pipe, nozzles, tubular intersections (sub-sea),T-butt welds (see Fig 3), cruciform welds, clad plate, clad nozzles (Fig 4) and even steam generator tubing. Papers have been published on most, if not all, of these. (Please contact Sonomatic@aeat.co.uk for a list of papers available.)

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.

Training

Because TOFD is a specialised technique, AEA Technology has offered training courses in its use since the early 1980s. Since then hundreds of operators from over 25 countries have been trained either at customers' own premises or at our sites in the UK and The Netherlands. These courses are aimed at real life issues and use practical methods to avoid or resolve problems. Often we are able to customise the course contents to suit the trainees or their employers.

Products

Since 1984 TOFD systems have been manufactured and marketed by A-EA Technology (formerly part of UKAEA) or their licensee, mostly under our Sonomatic brand name, enabling companies to purchase their own systems and handle their own inspection regimes with all the advantages of the TOM technology. These have been subject to continuous research, development and improvement, resulting in the availability of systems from the first ZIPSCAN to µ+ (Microplus) and MINUT-TOFD, and a range of compatible scanning systems and suitable ultrasonic probes. A software package, TOFD-AlDS has also been developed and marketed, to enable companies to improve their inspection design systems. Over 100 units have now been sold worldwide.

Services

Through our field services, we also provide a full range of services (including the TOFD technique) to those customers who prefer to outsource their inspection requirements on an as-and-when needed basis.

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.

AEA Technology Energy's current TOFD product range

1. TOFD and pulse-echo inspection services from the UK and via our agents and collaborative partners worldwide
2. µ+ (Microplus): a field-proven portable PC-based system, suitable for TOFD or PE applications. 16-channel system, bandwidth 0.5-30 MHz. Four encoders and scan control features
3. MinuUT-TOFD: a modestly priced single-channel TOFD system, MS Windows environment, bandwidth 0.5-30 MHz
4. µPIPELINE: combined PE and TOFD circumferential weld inspection system
5. Variety of scanners to suit most TOFD applications
6. Standard probes and probes manufactured to order
7. TOFD training courses in UK, The Netherlands or on customer premises anywhere
8. TOFD-AlDs inspection design package: provides beam coverage, error analysis, plate and cylindrical geometry design tools
9. NTPlot data display and interpretation tool, in Windows 95 or NT
10. CGTOFD 2-D complex geometry TOFD design tool in Windows 95 or NT for T-butt weld and simple nozzle designs
11. TOFD inspection design and validation services
12. TOFD technical justification preparation services (ENIQ)