Paper presented at BINDT annual conference 2008, 15-18 September 2008, Macclesfield, Cheshire, UK.
Ultrasonic Guided Waves have been used to assess long lengths of pipe and structural tubulars since 1977 in a process known also as Long Range Ultrasonic Testing (LRUT). In principle, LRUT is an Ultrasonic Testing (UT) technique, however, in practice it employs ultrasonics in a very different way from conventional UT. Until recently, all operators of Guided Wave (GW) test equipment have only been able to obtain training in the use of the equipment and the theory behind its operation from the individual equipment manufacturers. This will usually result in a certificate of satisfactory performance issued by the equipment supplier. Industry is increasingly demanding third party certification of the competency of testing equipment operators. This paper describes a practicable Training and Certification scheme that complies with European Standards (BS ISO 18436 or EN 473), can be easily employed across the EU and the rest of the world, and will accommodate GW equipment from any manufacturer.
The low frequency ultrasonic guided wave technique, also known as LRUT, has been developed for the rapid survey of pipes, for the detection of both internal and external corrosion. The propagation of the so-called guided waves is affected by changes in thickness of the component, so that they are sensitive to metal loss defects, notably corrosion. The principal advantage is that long lengths, 30m (~100ft) or more in each direction, may be examined from a single test point. The benefits are:
- Reduction in the costs of gaining access to the pipes for inspection, avoidance of removal and reinstatement of insulation (where present), except for the area on which the transducers are mounted,
- The ability to inspect inaccessible areas, such as under clamps and sleeved or buried pipes,
- The whole pipe wall is tested, thereby achieving a 100% examination.
Site trials have demonstrated that this method is capable of detecting corrosion <30% wall thickness deep and <25% circumference wide. The technique is now commercially available as an inspection tool for use in hydrocarbons transmission and processing facilities.
The impetus for the use of long range ultrasonics is that ultrasonic thickness checks for metal loss due to corrosion or erosion are highly localised, in that they only measure the thickness of the area under the transducer itself. To survey a large area requires many measurements and access to much of the surface of the component being examined. Where access is difficult or costly a detailed survey becomes unattractive economically, with the result that often limited sampling only is carried out. Similar restrictions also apply to other methods of measuring wall thickness, such as radiography, eddy currents etc. Partial inspection of this type is not likely to be effective in reducing the numbers of significant defects which may cause leaks or failure being present in pipes as the probability of detection of defects in uninspected areas is zero. The benefit of using long range testing to examine 100% of the pipe wall along the length tested is therefore considerable. Evidence for this is provided by a study carried out by the UK Health and Safety Executive [Patel and Rudlin], which reported that over 60% of the reportable hydrocarbon release incidents from offshore platforms in the UK North Sea sector were related to pipework. The adoption of adequate inspection and maintenance practices for pipework therefore has a considerable effect on the incidence of both unscheduled plant down-time and leaks of potentially hazardous materials. The use of long range ultrasonics to ensure that the whole pipe wall volume is tested provides a commercially attractive means of improving coverage. The principle of long range testing is shown in Figure 1.
This paper reviews the current situation regarding the performance of long range testing using ultrasonic guided waves and presents the developments in training and certification of inspection personnel currently being carried out under an EU funded Collective project:- 'Long Range Ultrasonic Condition Monitoring of Engineering Assets' (LRUCM).
2. Current status of guided wave testing
Guided waves propagating in the pipe wall, similar in nature to Lamb waves in plates, can propagate many hundreds of metres in plain pipe. However, unlike bulk waves used for conventional ultrasonic testing, where generally only a single mode such as either compression or shear exists, a large number of guided wave modes are possible. Consequently, the test system, the transducer array and the test parameters, such as frequency, need to be carefully designed to ensure that only the relevant wave modes are generated. One major difference between long range testing and conventional UT is that the test frequency needs to be as low as 20-30 kHz in order to reduce the number of wave modes present and to enable non-dispersive wave propagation. If the appropriate test conditions are chosen, the test results are readily interpretable. An example result is shown in Figure 2.
An important point to note is that the long range techniques currently available are screening tools and do not provide the same kind of resolution as local thickness measurements. The aim is to provide a rapid method of screening so that more appropriate test methods may be directed at areas requiring further attention in an efficient manner. Most importantly, long range UT does not provide a direct measurement of wall thickness, but is sensitive to a combination of the depth and circumferential extent of any metal loss, plus the axial length to some degree. This is due to the transmission of a circular wave along the pipe wall which interacts with the annular cross-section at each point. It is the reduction in this cross-section to which the long range technique is sensitive. This is shown schematically in Figure 3.
Within the past 10 to 15 years a considerable amount of work has been done on the properties and use of ultrasonic guided waves for inspection of both plates and pipes.[Cawley & Alleyne, 1996], [Kwun et al, 2001], [Rose, 2002], [Mudge, 2004] This has led to a high level of understanding of the characteristics of these wave systems and their performance.
It should be noted that all commercially available guided wave test systems transmit axially-symmetric annular waves which sweep along the pipe. The extent to which this wave interacts with an area of metal loss is determined by its depth and the circumferential extent. The detection capability is therefore governed by this cross-sectional area of the defect. From this, it may be seen that these techniques do not give a direct measurement of the remaining wall thickness, so cannot currently be used to provide a replacement for conventional thickness gauging. Equally, whilst there is a relationship between overall defect area and the amplitude of a reflection from it, this relationship is greatly affected by the shape and roughness of the defect itself and cannot be relied upon to predict severity. This limits the current techniques to detection of suspect areas for follow up activity by other methods.
Other factors which impose restrictions on the capability of long range guided wave testing are:
- Complexity of the wave mode system. Under most conditions more than one ultrasonic wave mode exists. These modes each travel at different velocities and many exhibit dispersive behaviour, i.e. the wave velocity varies with frequency, all of which makes interpretation of the resulting test signals difficult,
- Limitations of sensitivity. Where a large volume of material is tested from a single location, as is the case with long range ultrasonic's, it is inevitable that the sensitivity and resolution will not be as good as a local test where the material directly underneath the test head is examined. This limits the size of discontinuity which can be reliably detected,
- Attenuation and scattering of the ultrasonic energy by surrounding material. This is due to losses caused by the ultrasonic energy leaking out of the plate or pipe and may include the influence of coatings and, in the case of items buried in the ground - either pipes or sheet piling - the effect of the soil in contact with the surface,
- Component complexity. This generally causes mode conversions, resulting in useful energy from the controlled transmitting transducer being dissipated into other wave modes and thus generating a noise signal which reduces sensitivity and may mask signals from discontinuities. Such effects include bends in pipes and heat exchanger tubes, the shape of railway rails and the multi-strands in wire ropes,
- Lack of qualified test operators. This technology is new compared with other main NDT methods. Consequently, the uptake of the technology is slowed by the non-availability of a large pool of suitably qualified personnel. This in turn is caused by the lack of standardised procedures and training and certification schemes.
3. The need for training and certification
The field of quality inspection relies heavily on the knowledge and experience of the individual inspectors concerned, and NDT and CM inspectors are no exception. In the past, many organisations employing NDT and CM type inspections relied on their own knowledgeable and experienced staff passing their skills on to new inspectors either by a system of mentoring or by internal training courses and examinations. The organisations confidence in the individual capabilities were ensured by a combination of personal assessment, written and practical examination and continuing satisfactory performance over a period of time (often years). This system provided a very competent and experienced inspection service. However, the advancement in the capabilities of the individuals could be very slow and the introduction of new technologies into the organisation relied on the capabilities of the senior inspection staff to embrace them.
The increasing adoption of outsourcing and subcontracting by many of the larger engineering organisations, and the lack of a large pool of staff to carry out inspection work in smaller organisations has led to the need for contracting in specialist inspection staff. In order to ensure that the contracted inspection staff are qualified and competent to carry out the required inspection work to a satisfactory quality level, a universally recognised and accepted standard of training and certification is required.
4. Standardisation of the training and certification process
All of the established NDT methods and some of the new emerging methods and techniques benefit from the availability of training and certification which complies with national and international standards, most commonly EN473 in Europe and SNT-TC-1A (ASNT) in the USA. A major difference between the EN 473 system and the SNT-TC-1A system is third party impartiality. The ASNT document is entitled 'Recommended Practice SNT-TC-1A' and, as such, is not mandatory inits entirety. It describes a system of training and certification which is wholly employer based and 'in house'. While most organisations which employ this scheme operate it with integrity, there may be occasions where the implementation of correct procedure gives way to commercial expediency. EN 473 (and its international equivalent ISO9712) requires implementation in its entirety; it is a mandatory scheme. In addition to this, it is a third partyscheme, requiring certification to be carried out by an independent Certification Body (established in accordance with EN 45013).
As described above, LRUT is a screening tool which can fall into the category of NDT or into the new category of Condition Monitoring (CM). An international standard, ISO 18436: Condition monitoring and diagnostics of machines -Requirements for training and certification of personnel has been published in 2004. Part 1 describes the general Requirements for certifying bodies and the certification process. The requirements of this standard mirror the requirements of EN 473 without specifying the metrics for individual methods. This has been delegated to other parts of the standard to specify the exact requirements for each method. Currently, subsequent parts have been allocated as follows:
- Part 2: Vibration condition monitoring and diagnostics (published)
- Part 3: Requirements for training bodies (under preparation)
- Part 4: Lubrication management and analysis (under preparation)
- Part 5: Thermography (under preparation)
- Part 6: Diagnostics and prognostics (under preparation)
- Part 7: Condition monitoring specialists (under preparation)
It may prove to be suitable for the LRUT training and certification scheme described here to form a future part of ISO 18436.
For maximum credibility, a LRUT training and examination scheme should be operated independently of the equipment supplier or the operator.
The training and certification format described in this paper is intended to comply with the requirements of EN 473/ISO 9712.
5. Levels of qualification
EN 473/ISO 9712 and SNT-TC-1A describe three levels of qualification, Level 1, 2 and 3. It may be useful to reiterate the responsibilities of each level, as specified in EN 473, in order to help us understand the division of training and certification requirements for LRUT.
Level 1: An individual certified to level 1 has demonstratednstrated competence to carry out NDT according to written instructions and underng>under the supervision of level 2 or 3 personnel. They may be authorised to:
- set up NDT equipment;
- perform the test;
- record and classify the results of the test in terms of written criteria;
- report the results.
Level 1 personnel shallng>shall not be responsible for the choice of test method or techniqueechnique to be used, nor for the assessmentsessment of test results.
Level 2: An individual certified to level 2 has demonstratednstrated competence to perform NDT according to established or recognised procedures. They may be authorised to:
- select the technique for the method to be used;
- define the limitations of application of the method;
- translate standards and specifications into instructions;
- set up and verify equipment settings;
- perform and supervise tests;
- interpret and evaluate results according to applicable standards codes or specifications;
- prepare written instructions;
- carry out and supervise all level 1 duties;
- provide guidance for personnel at or below level 2; and
- organise and report the results of NDT.
Level 3: An individual certified to level 3 has demonstrated competence to perform and direct NDT operations for which he is certificated. They may:
- assume full responsibility for a test facility or examination centre and staff;
- establish and validate NDT instructions and procedures;
- interpret standards, codes, specifications and procedures;
- designate the particular test methods, procedures and instructions to be used;
- carry out and supervise all level 1 and level 2 duties.
6. Training and experience
LRUT is, as the name implies, a technique based on ultrasound and much of the fundamental theory for LRUT is the same as that for conventional UT. However, LRUT employs different wave modes and lower frequencies to conventional UT and the mechanics of application and equipment are very different. Training in the application of guided waves and the correct use of the test equipment needs to be specific to LRUT, but, in order to reduce the training hours and to provide a good background understanding of ultrasonic testing the candidate should already hold a level 1 certification in UT testing to EN 473 or an acceptable equivalent (a candidate who holds a bachelors degree in an appropriate science or engineering discipline will be acceptable).
Training to level 1 should give the candidate sufficient theoretical knowledge to have an understanding of the process of LRUT without necessarily the detailed knowledge needed to devise or modify a test instruction. The emphasis for level 1 training should be placed on the practical application and use of the test equipment to ensure that the candidate is confident to collect data under the direct supervision of a level 2 technician. The level 1 candidate will need sufficient knowledge of the test system to recognise that he has gathered data of sufficiently good quality for interpretation and if there is a need to carry out further tests to achieve this. He is not required to be able to fully interpret this data. The duration of the level 1 training should be 40 hours (1 week).
Training to level 2 should build on the level 1 training by covering more of the theory of guided wave testing and to a more thorough degree. This should then enable the candidate to make informed decisions on technique selection and equipment settings modification to accommodate changes in the test requirements. The practical training will concentrate on the interpretation of gathered data. During this training the candidate will be exposed to as many different scenarios of test conditions as possible, e.g. differences in types of coating, types of content, types of defect and features. The duration of the level 2 training should be 40 hours (1 week). For a candidate wishing to certify directly to level 2 this is in addition to the level 1 training requirement.
The amount of experience of LRUT operations prior to certification should be 3 months for level 1 and 9 months for level 2. EN 473 allows for this experience to be gained after the training. This is acceptable, even desirable, for level 1 candidates. However, it is strongly recommended that level 2 candidates do not present themselves for training before gaining the majority of the 9 months required experience. As with all methods of NDT, gaining the minimum amount of experience prior to certification is essential to becoming an adequately qualified inspection technician who is able to carry out a competent inspection in all circumstances. Any circumvention of the experience requirements will only prove to be a false economy and may lead to inadequate inspections.
7. Outline of proposed LRUT certification exam format
The format of the examination at each level should follow the format laid down in EN 473.
7.1 General theory
The general theory examination should address the general theory of guided wave ultrasonic testing as well as the relevant parts of the basic theory of ultrasonic's, to the appropriate level. The questions may be either the narrative type or multiple choice answer type. As a minimum, the candidate should be required to answer a minimum of 40 multiple choice questions for level 1 and level 2.
7.2 Specific theory
The specific theory exam needs to address the knowledge required in applying LRUT to the specific product. LRUT has the potential to inspect many types of product - railtrack, sheet piles, condenser tubes and lamp posts. However, the product for which LRUT has the most widespread use is pipes and pipelines. The questions may be either the narrative type or multiple choice answer type. As a minimum, the candidate should be required to answer a minimum of 20multiple choice questions for level 1 and level 2. If the specific exam covers more than one product sector the number of questions should be increased to a minimum of 30 multiple choice questions.
7.3 The practical exam
EN 473 and other similar NDT certification standards and procedures were written with the conventional NDT methods in mind. With all of the conventional methods (and most of the new emerging technologies) the practical part of the exam involves the testing of a suitable number and range of samples of the product for which certification is sought. These samples are always relatively small; the largest need not be more than about 500mm in the greatest dimension. The training school and examination centre are required to hold sufficient numbers of these samples to give the students and candidates the opportunity to be trained and examined on a range of product and defect types. LRUT, however, is by definition an inspection technique which covers a large amount of material in one application. It is typical for LRUT to be able to inspect in excess of 30m and the system is usually unable to detect indications in less than 1m.A suitable minimum length for a training or examination sample is 12m. If physical samples were required for LRUT in the same manner as the conventional methods it would result in a large number of long lengths of pipe to be held by the training and exam centres. For example, the training centre would need pipe lengths to give examples of different pipe dimensions, contents, coatings and defects (e.g. 6 lengths). In addition, the exam centre will need a minimum of2 lengths for an initial exam and a further 2 lengths in case of a retest or recertification exam. To hold this number of long lengths of pipe would be onerous on the training and exam centres, and would also restrict the capability to conduct the training and examinations at remote locations. To overcome this problem it is proposed that training and examination of the physical aspect of equipment operation and data collection be carried out on one length of pipe. This length of pipe will require a minimum set of attributes as shown below:
7.4 LRUT training and examination pipe samples. Minimum requirements
||Carbon and Low alloy steel.
||between 100mm (4 inch) and 610mm (24 inch).
||between 5mm and 55mm.
||12 m (incl. 1 bend, min. 2 m after the bend).
|Minimum No. of welds:
||2 on the straight part of the pipe (should be uniform); if 1 weld has a defect, 3 welds are required.
|Minimum No. of bends:
||1 (if only 2 welds present, the bend should not separate these).
|Minimum No. of defects:
||4 (at least 1 at the weld, 1 at the support, the rest along the pipe).
||Industry acceptable type.
In a similar manner to the Radiography requirements in EN 473, the sample need not have any defects as the candidate is only being tested on his ability to gather data. Also, in a similar manner to the Acoustic Emission requirements in EN 473, only one sample is required to be used due to its size. However, to ensure that the candidate can correctly use the test equipment and gather the data, the invigilator must assess each individual candidate with the aid of a check list. In order to test the candidates ability to correctly identify that the correct data has been collected for a given length of pipe, the candidate will then be given data sets taken from a minimum of three different pipes or pipelines, which may or may not have been correctly collected. The candidate will be required to identify if the data has been correctly collected and if the data suggests any further tests which need to be carried out. A candidate for level 2 will, in addition, be required to analyse a further 5 sets of data and correctly report on all the features and defects within those data sets. Each data set will be taken from a range of sample types and contain a range of features and defects. The candidate should be prepared to encounter all of the features and defects in the following list, although he may only encounter a selection in the examination.
7.5 List of minimum features in data sets
|Type of coatings:
||concrete, bitumen, painted surfaces, plastic, epoxy, thermal insulation, foams
|Type of supports:
||simple support, longitudinal welded, circumferential welded, circumferential saddle welded, clamp support
|Type of surrounding:
||air, sand, mud, water, clay, road crossing
|Contents of the pipe:
||gas, liquid (range of viscosities), sediments and deposits
|Types of defects:
||corrosion (concentrated, distributed) or other losses of material in parent material, at supports, welds and bends; Gross weld defects
||bends (45°, 90°, cast, drawn, range of radius), welds, flanges, t-pieces, branches, taps, stainless steel pipes
7.6 General practical
The general practical exam will require the candidate to correctly set up the test equipment, including any calibrations and function tests as appropriate. The candidate will then be required to collect data from a suitable sample(this need NOT be a dedicated examination sample), and to carry out any additional tasks the test equipment may be capable of (e.g. a focus test in the case of Teletest). This part of the exam is to be under the one to one supervision of the invigilator who will report on the candidate's performance using a suitable checklist.
7.7 Specific practical
The candidate will be provided with 3 pre-recorded data sets which have been correctly collected from suitable samples (e.g. 30m to 50m length with at least 2 uniform welds and other features to include possible defects) which have been collected in the manner of the general practical described above. The candidate will also be provided with line drawings and dimensions of features which will normally be determined by visual inspection (e.g. pipe end/flanges, welds, bends etc). The candidate will be required to correctly set up DAC curves as appropriate, identify and mark known features (from the line drawings), and indentify any anomalies for further investigation/interpretation. The candidate will be required to produce a simple report of his findings and/or submit the annotated data sets.
The Level 2 candidate will, in addition, be required to interpret 5 data sets which have been collected from a variety of sample types and saved in the manner above. These data sets may or may not be correctly annotated and any subsequent data collection may or may not have been made. The candidate is expected to correctly identify any incorrect annotation and to identify and make recommendations for any anomalies present. The findings are to be presented in an appropriate report format.
The Level 2 candidate will produce an instruction suitable for a Level 1 operator to follow to inspect a sample chosen by the invigilator.
In the last ten years LRUT has matured into a very creditable method for the screening of pipes and tubular products as well as showing great promise for other product areas. However, the general acceptance and confidence of the technique within industry is being affected by the lack of a universally recognised training and certification scheme.
The EU funded Collective project:- 'Long Range Ultrasonic Condition Monitoring of Engineering Assets' (LRUCM) has addressed this problem and proposes a training and certification scheme which will comply with the format of current NDT training and certification standards while at the same time ensuring that the certified personnel are up to the standard required to perform the inspections adequately.
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- Cawley P & Alleyne D N, 'The use of Lamb Waves for the Long-Range Inspection of Large Structures', Proceedings of Ultrasonics International 95, published in Ultrasonics, Vol. 34, pp287-290, 1996.
- Kwun H, Kim SY and Light GM, 'Long-Range Guided Wave Inspection of Structures Using the Magnetostrictive Sensor', Journal of the Korean Society of NDT, Vol. 21, pp383 - 390, 2001.
- Rose J L, 'A baseline and vision of ultrasonic guided wave inspection potential', Journal of Pressure Vessel Technology, Aug. 2002, 273 - 282
- Mudge P J, 'Practical Enhancements Achievable in Long Range Ultrasonic Testing by Exploiting the Properties of Guided Waves', proceedings of the 16th World Conference on NDT, Montreal, Canada, September 2004.