·Table of Contents
·Methods and Instrumentation
The Development of Alternating Current Field Measurement (ACFM) Technology as a Technique for the Detection of Surface Breaking Defects in Conducting Materials and its Use in Commercial and Industrial ApplicationsAlan Raine
TSC Inspection Systems Limited
The inspection applications were then extended to the inspection of pressurised systems and process plant i.e. pressure vessels and pipe work. Evaluation trials have been carried out to prove the detection and sizing capabilities of the technique compared to the results obtained using more conventional techniques such as magnetic particle inspection, eddy current and ultrasonic creep wave. The technique was then used on the inspection of non-magnetic materials such as stainless steel and titanium and proved equally successful. Since then ACFM has played a useful role in the inspection for non-fatigue cracks at normal and elevated temperatures. Procedures have been produced so that transverse defects can be differentiated from longitudinal defects and cracks detected on the inner surface from the outer surface can be easily identified (only on certain materials and within a specific thickness range).
Because the technique has proven so versatile ACFM is now being used on a wide number of applications. The technique has been used to inspect the steel structures of roller coasters and the people carrying components with great cost benefits to the theme park owners. The ACFM technique has also been used in the inspection of bridges with the use of abseilling techniques to inspect the more difficult access areas as well as through coatings using conventional inspectors. The combination of rope access (abseilling) and the ACFM technique has also been adapted to the inspection of dockside and other large cranes, once again with great economic benefits. A major crane being inspected at its most critical areas in 6 hours.
The oil and gas industry has not been ignored and the technique is still being used in petrochemical applications. In one recent application $500,000 was saved by changing from magnetic particle inspection to ACFM inspection.
The ACFM technique is taking its place in the inspection world as a technique, which can reliably detect and size surface defects with additional economic and time saving benefits.
A good correlation was produced between the theoretically predicted magnetic field disturbances and those measured and thus showed that it was possible to make quantitative measurements of the magnetic field disturbances and relate them to the size of the defects which produced them. Special techniques are used to induce these electric currents and the components used are built into the ACFM probes Small detectors or sensors are also built into the probe, which measure the magnetic field disturbances. The probe is scanned longitudinally along the weld with the front of the probe parallel and adjacent to the weld toe. Two components of the magnetic field are measured, the Bx along the length of the defect which responds to changes in surface current density and gives an indication of depth when the reduction is the greatest and Bz which gives a negative and positive response at either end of the defect caused by current generated poles. This gives an indication of length. Figure 1.
A physical measurement of defect length indicated by the probe position is then used together with a software program to determine the accurate length and depth of the defect.
In order to aid interpretation the Bx and Bz components are plotted against each other and when a complete loop indication is produced this confirms the presence of a crack. This is called the Butterfly plot Figure 2 and because it is not sensitive to probe speed aids in the interpretation of the data collected and confirms defect indications. During the application of the ACFM technique actual values of the magnetic field are being measured in real time. These are used together with mathematical model look-up tables so that there is no need for calibration of the ACFM instrument using a calibration piece with artificial defects such as slots.
The ACFM technique was originally developed for the inspection of carbon steel welds on subsea structures, which were usually nodal welds. A number of probes were developed, a general purpose weld inspection probe, a 30 degree angle probe for examining tight angle geometry's and a pencil probe specially designed to examine welds that had been subjected to grinding. This was used to inspect the bottom of the ground toe of the weld to determine if defects were present and then determine their length and depth or to confirm that the defect had been removed. During a trial organised by University College London where samples were produced to reproduce some of the difficult geometry's and access problems located in process plants, it was found that additional probes were required to gain access and detect and size the defects located within the samples. A range of mini and micro pencil probes has now been produced with straight and 90 degree access with increased sensitivity. In addition to this it was realised that the inspection of short lengths of weld also created problems in that the communication rate was too slow to produce a good representation of the weld result on the VDU screen. New software has now been produced that eliminates this problem including communication rates, which allows scanning speeds seven times faster than before.
This allows greater presentation on the screen for shorter lengths of welds and faster scanning speeds for the inspection of long lengths of weld.
The technique was also used to inspect structures that had been coated with protective or anti fouling coatings so that the expensively applied coatings did not have to be removed and reapplied thus avoiding costly preparation and reinstatement. The topside inspection engineers also adopted the technique for the inspection of process and pressurised plant, structural steelwork and crane pedestals. The system was used in conjunction with rope access teams allowing inspection without scaffolding and proving the usefulness of two man operations and the Butterfly plot. Inspections could be carried out up to 50 metres between the ACFM operator and the probe pusher.
The technique has also been applied to the inspection of drill threads on casing and drill tools. A special transportable system has been produced to automatically inspect the drill thread ends and classify them. This provides Go-NoGo reporting. The system is based on new ACFM array technology. A hand held probe has also been produced to inspect drill threads with the portable ACFM system.
New materials are being used for components and coatings on offshore structures but the ACFM system has now been successfully applied to ferritic steels, austenitic stainless steels, aluminium, duplex, super duplex, monel and inconel. It has also been used to inspect through the following coatings, flame sprayed aluminium, epoxy coating, standard paints, ferrite based paints and copper coated threads.
Some inspections have to be carried out when the plant is operational and ACFM has been used during inspections at -20°C and up to 500°C.
Because of the above advantages the ACFM technique has been used to inspect coated flare booms, epoxy coated pig traps, painted nozzle welds, pipe butt welds, pipe and saddle support welds and pressure vessel seam welds as well as the above mentioned inspections.
The communications rate between the ACFM instrument and the computer needs to be fast to obtain a meaningful length of weld on the screen of the computer. The alternative is to scan slowly.
Technical Software Consultants have addressed these problems with the introduction of the mini and micro pencil probes. Both of these probes have either straight or 90-degree access and have stainless steel probe faces. The mini and micro probes have slightly different sensitivity in that one can detect defects 0.04" deep and the other 0.02" These probes are particularly suited for the detection of shallow defects in tight access areas.
A new range of control software QFM 2 has also been produced which has additional features such as a faster communications rate allowing scanning speeds of up to 2"/second.This can be used for scanning long welds faster or producing longer images on the computer screen for short weld inspection. This software also allows automatic centralisation of the data display and the ability to select and print single scans of data. Different values of lift off can also be selected in order to inspect through different thicknesses of coating.
The combination of these developments will allow the experience gained from critical offshore inspection to be applied to the inspection of the theme park components so that they can be carried out more efficiently and reliably.
In one theme park the track of one of the rides is made up of 300 ties each one having 70 welds of varying length and geometry. During the annual shutdown of this ride a number of these ties are cleaned, inspected using magnetic particle inspection techniques and then the ties are repainted. This normally takes three weeks, one for the cleaning, one for the inspection and one for refurbishment and repainting. This is one of the major problems, as the paint has to be matched as closely as possible with the original colours. During one inspection 30 ties were inspected with magnetic particle inspection. During the next inspection the ACFM technique was used. No prior cleaning was required and 64 ties were inspected in four days and one day was used for repairs and re-inspection. No additional painting was required except for the localised painting where the repairs had taken place. In an industry where the customer expects all of the rides to be available when they visit the theme park the reduction in down time is very important. The ACFM technique has now being used in a number of theme parks and a mechanised system has been installed in one park where more than 1000 supports require inspection. Using the new system the inspection time/ support has been reduced from 8 minutes to 3minutes.
There are a number of road bridges produced from box girder construction, which have longitudinal as well as transverse cracking. Unfortunately the majority of these welds are coated and to clean and inspect would be very expensive and labour intensive. These box sections are about 40' long with both horizontal and vertical welds present. The problem of inspecting for and detecting fatigue cracks through coatings has been well known in the offshore industry for over thirty years and is now being tackled with the use of electromagnetic techniques such as the ACFM technique. The results obtained following inspection are a major factor in calculating the structural integrity of these welds and determining the valid life of the welded joint in terms of Probability of Failure and Reliability Index.
The problems of inspection of road bridges are not unrelated to that of the inspection of offshore structures in that the material is steel which is coated, the welds have difficult access and geometry and the inspection has to be reliable and repeatable. These were the same problems and background with which ACFM technique was presented. The ACFM technique has since successfully overcome these problems using the portable unit, two man rope access, specially developed probes and communication techniques and has been used to carry out inspection of coated steel structures such as offshore structures and bridge sections.
One other problem which has arisen is the failure of overhead signs and light supports which has been subject to high cycle fatigue. This has caused fatalities in one county in the UK and also in one of the Northern States of the USA and has caused the inspection and design of these structures to be re-examined. Some of these designs such as the flagpole design where the weld is on the elbow may have to be changed. Because of their location it is not easy to remove the protective coating inspect and re-coat without causing some disruption to the traffic flow. Comparative trials has shown that there can be a 60% saving in time and cost when changing from magnetic particle inspection to a non contacting technique such as ACFM.
The complex metallurgical structure of duplex steels produces problems with most NDT techniques. The complex structure is produced because duplex can occur as islands of ferrite in an austenite matrix or vice versa. If it is the former the material will perform as if it was an austenitic material and the current flow would penetrate several millimetres. If the structure were of austenite islands in a ferrite matrix then the penetration would be only a fraction of a millimetre. The ACFM technique performs equally as well under both conditions but has the added advantage that in the austenitic situation the technique has detected internal defects and defects occurring on the inner surface.
When inspecting ferrite based duplex it was found that the depth-sizing model was quite accurate but with the austenite based duplex a 10% inaccuracy was detected. This was overcome by multiplying the value determined by a compensation factor. So far over 15000 welds have been examined using the ACFM technique with a good detection rate and a low number of false calls. New procedures are being developed using higher frequencies to allow discrimination between surface and subsurface defects.
Two more indications were also noted, one was located at 45° between defects 5 and 6 and this was located during the longitudinal weld toe scan and gave a weak indication. A second indication was noted between defects 4 and 5 also during the toe scan but this was only 4mm long (also transverse) and thus was not covered by the weld cap centre line scan. The defects were then sized using the normal ACFM procedure to produce length and depth information.
One total inspection of a drum took eight hours with the ACFM technique and the results were compared with the other techniques and gave a good correlation. The oil company has specified the ACFM technique for the inspection of these drums and it is estimated that the total inspection will take four days. The ACFM technique is going to be used to detect and then monitor the crack growth during the life cycle of the coke drums.
The same probes can also be used to inspect plant with surface temperatures of -20 °C. A gas process plant in Scotland was coming to the end of its first year in service and re-certification was required. Because a major part of the plant required inspection the only way that it would be possible to carry out this inspection would be to do some of the inspection prior to and after the shutdown period. This would mean inspecting the plant live at high and low temperatures.
A ten-week inspection programme was organised to include the two-week shutdown period. Pressure vessels, separators, saddle welds, girth welds, pipe work and fractionating columns were inspected over this period using the same probe with a 25 metre cable length and a U9 crack microgauge instrument. Six defects were detected and these ranged from 10mm-70mm long x 4mm deep. The operating temperatures of the plant ranged from 225°C to -20°C.
Because of access problems it was not possible always to locate the crack microgauge near to the worksite. This problem was overcome by using a 25-metre probe cable from the unit to the probe and a 30-metre cable between the unit and the laptop. The probe operator communicated with the crack microgauge operator using a head-up display system and two-way audio communications. This enabled the inspection of fractionating columns and other high parts of the plant to be inspected without the use of scaffolding. The ability of the technique to be used to inspect through coatings as well as at high and low temperatures meant that critical areas which could not be inspected on line were inspected during the shutdown period and the remainder was inspected on line.
During the first inspection ACFM was used as the front line inspection tool with verification of defects being carried out by local coating removal and MPI followed by grinding to remove the defects. Good correlation was found between the techniques and verification is not now carried out and the ACFM technique has been used during every maintenance shutdown period.
A new 347 stainless steel pipe line was been fabricated and in the past the process has been to produce the root weld allow the weld to cool down then inspect with dye penetrant. Any repairs were then carried out and the next pass laid down after re-heating. At each inter-pass stage the weld was allowed to cool, then inspected and then reheated to allow welding to continue. The ACFM system together with a pencil probe was used instead of the dye penetrant inspection as a quality control tool. No cooling below the re-heating temperature was required and the weld production increased. The specialised welding time had been reduced from 12 hours /weld to 2 hours /weld because of the reduced inspection time and heat cycle time. No repairs were necessary during the fabrication.
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