Table of Contents ECNDT '98 Session: Chemical, Petrochemical

CHIME- A New Ultrasonic Method for Rapid Screening of Pipe, Plate and Inaccessible Geometries Fiona Ravenscroft, Roger Hill, Colin Duffill & David Buttle AEA Technology plc, National NDT Centre, Culham, Abingdon, UK. Corresponding Author Contact: Wilbert Martens, AEA Sonomatic BV, Krombraak 15, 4906 CR, Oosterhout (Netherlands), Phone: +31 (0)162 42 55 88, Fax: +31 (0) 162 42 43 43, Wilbert.Martens@aea-technology.nl, http://www.aeat.co.uk

Abstract

Global screening techniques, for the inspection of pipes and vessels at installations or plant, are a major requirement for many operators. A new ultrasonic technique called CHIME has been developed to meet this need and is described in this paper. Ultrasound is directed into the specimen at the critical angle and two probes act as transmitter and receiver. CHIME is a medium range screening technique which provides coverage of the full volume between the two probes separated by a distance of up to one metre or more. The technique has demonstrated sensitivity to corrosion or cracking over a range of wall thickness and has been successfully demonstrated on corroded pipe samples, in validation and field trials.

Introduction

Conventional ultrasonic methods for the inspection of large areas, such as vessels or long pipelines, are generally time intensive and costly. Grid thickness readings or point by point inspection is carried out to form a C-scan image of the inspected region. In addition, inaccessible regions such as pipes on pipe supports or under clamps generally cannot be inspected by these conventional methods without significant effort and cost. For these applications long range global screening techniques have significant advantages over conventional techniques especially if rapid scanning of large areas and into inaccessible regions is possible in a single measurement.

Guided wave systems using, for example Lamb waves, are being developed for long range inspection [1,2]. The most useful modes for inspection have wavelengths of the size of the plate thickness. These relatively low frequencies give rise to long inspection ranges but inevitably couple with low resolution to defects. At high frequencies many harmonics are produced and the sound is dispersive making interpretation of defect signals complicated.

A new ultrasonic inspection technique is described that has been developed within the Harwell Offshore Inspection R&D Service (HOIS) [3]. The technique utilises the properties Creeping (or Lateral waves) and Head waves in parallel or near-parallel walled metal. This patented technique has been named CHIME (Creeping/Head wave Inspection MEthod) [4]. Creeping waves are alternatively known as Lateral or, more precisely, surface skimming compression waves. As the surface compression wave generated by a probe at the critical angle is generally referred to as the Creeping wave, the authors will use this name.

Concept

Creeping waves can be generated at the surface of a specimen using a transducer angled at the critical angle, but this alone does not satisfy the boundary conditions at a stress-free surface. The Creeping wave continually generates a shear wave propagating into the bulk of the object that is generally called the Head wave (Figure 1). In a wall or plate with two opposed surfaces the Head wave will generate another Creeping wave at the opposite surface (secondary Creeping wave), so that Creeping waves propagate along each surface each continually generating Head waves. The Head wave travels as a plane wavefront at a specific angle, the critical angle for shear waves in the material.

Fig 1: Schematic snapshot of Creeping and Head wave generation from a probe at the ultrasonic critical angle. Illustrated are the Creeping wave (C), Head wave (H), direct compression waves (P) and shear waves (S) generated due to the finite size of the probe.

The generation and theoretical description of these compression waves and associated transverse Head waves have been described in previous papers in connection with the detection of near surface cracks [5,6,7,8]. Creeping wave probes have been used, for example, for short range butt weld inspection. The region of the weld fillet can be monitored by the Creeping wave while the root region can be monitored by the Head wave and the secondary Creeping wave [6]. They are extremely useful for the detection of shallow surface-breaking flaws [7] as they follow the surface but are not affected by surface spatter, liquid or couplant. They have been used, for example, to detect radial fatigue cracks in weep holes of airframe stiffeners [8]. Erhard [5], amongst others, carried out theoretical work and showed that Creeping waves do not exist by themselves on the surface and that the Head waves also formed during the generation of the Creeping wave. This constant generation of the Head wave infers that the first generation Creeping wave does not have a great range since energy is being removed from it continuously. Consequently applications have generally been for short range inspection for surface breaking defects.

The CHIME Technique

The Creeping and Head waves can be generated using a piezoelectric transducer mounted on an angled shoe. The shoe is angled to produce shear waves in the parallel walled material at the critical angle (Fig 2). The transmitter fires a pulse of ultrasound which gives rise to the Creeping waves propagating along both the surfaces, and the Head waves are generated at all parts along these surfaces. If the receiver is placed on the sample some distance away (up to 1m typically) then a series of signals is observed at the receiver and the entire volume of the sample between the transducers is inspected (Fig 2). The signal is a series of peaks made up of a direct, Creeping wave, arrival followed by a signal path that has carried out one full skip across the plate as a Head wave and travelled the remaining distance as a Creeping wave. Then a signal arrives which has carried out two full skips and travelled the remaining distance as a Creeping wave and so on up to a maximum number of skips. For each successive signal peak, the path length of the Creeping wave is reduced. Note that each skip across the pipe takes place anywhere between the transmitter and the receiver so each signal is made up of a continuum of ray paths. In addition to the Creeping and Head waves there will also be bulk waves generated below or above the critical angle due to the beam divergence of a finite width transducer (indicated in Fig 1). The first Creeping/Head wave combinations are generally weak however the bulk waves, which travel in more direct paths to the receiver, combine with the first few Head wave/Creeping wave signals to produce a close pattern of peaks over the first half of the signal (Fig 2). The Head waves are non-divergent plane waves therefore they have little decay and show strong peaks in the latter half of the signal.

Fig 2a: CHIME Probe arrangement

Fig 2b: A scan of a CHIME signal typical in clean plate

The unique way in which the waves propagate provides complete isonification of plate or pipe with little attenuation allowing the transmitting and receiving probes to be well separated compared to traditional Creeping wave inspection. Complete wall coverage is achieved for typical wall thickness up to 40mm at standard operational frequencies and inspection widths (distance between the two probes) of over 1m have been established.

The receiver will display a characteristic signal that can provide information about the thickness of the sample or about any defect which locally changes the thickness. Defects due to corrosion or cracking affect the signal magnitude and arrival time of the signal peaks (Fig 2) independent of their location between the transmitter and receiver. Defects may also reflect the signal train and these reflections can be monitored in pulse echo. This is especially useful for the detection of cracking.

Theoretical Modelling

Fig 3: Model of the peak amplitudes of Head waves generated in a plate. The frequency thickness product is 50mm.MHz.

Theoretical modelling of the CHIME signal has given insight into the formation of the complex signal patterns. The general approach has been to obtain a rigorous solution to the wave equation for a plate using software originally developed at NIST in the USA [9] and modified for this application. Theoretical A-scans could then be derived as a function of various operational conditions. Demonstration of the coverage of the Head waves in the plate has been achieved by solving the propagation function for an imaginary receiver placed at all points within a plate. The total Head wave peak amplitudes were predicted and summarised using a contour representation over a cross section of a plate.

Fig 3 shows the peak amplitude of Head waves in a plate. At a frequency thickness product of fifty (mm.MHz) full coverage of the plate with Head wave energy is achieved after only one skip. If the frequency-thickness is then doubled the coverage is shown to become less uniform because the distance between skips becomes significant compared to the decay of the Creeping wave. The modelling has also demonstrated that a small range of ultrasonic frequencies will provide full coverage in walls of thickness beyond the typical range used in plant.

Defect Detection

The CHIME technique can be effectively used to screen for any defects in the sample as the presence of a defect will affect the transmission of the Creeping/Head waves. In order to monitor for these changes the data collection is been carried out using an ultrasonic digital data acquisition system [10] and the data is displayed as a B-scan (Fig 4).

Fig 4: CHIME B-scans over a machined notch a) probes arranged in pitch catch, separation 260mm and b) single probe in pulse echo. Defect start (D1) and Defect end (D2) is indicated on the scans.

To detect a defect the signal transmission (and, in some cases, reflections) is monitored whilst coupling monitors check the integrity of the coupling. When the transmission signal changes significantly (over and above random fluctuations) then a defect is flagged. Total transmitted signal loss will occur on large defects, making them easily and reliably detected with this technique.

Fig 4 shows an example of the CHIME signal response over a machined notch of width 50mm, length 20mm and depth 4mm, in a 305mm diameter pipe of wall thickness 11mm. The pitch catch and pulse echo responses show the effect of the notch on the signal. In pitch catch the probes are either side of the defect moving perpendicular to the line between the probes. The flat base of the defect allows the signal to continue to the receiver but the arrivals are displaced in time due to the shortened path length. The pulse echo signals from one probe show reflections from the steep side of the notch.

Fig 5: CHIME B-scan over three isolated pits P1, P2 and P3 in pitch-catch.

Fig 5 and 6 show the pitch-catch responses due to real defects. Fig 5 is an example of a CHIME B-scan on a sample containing isolated corrosion pitting. The regions of signal change in the CHIME signal are indicated and the pits identified. In this sample the probe separation is around 350mm and the isolated pits are located somewhere between the probes.

The sizes of the pits are shown in Table 1. It is significant to note that the amount of signal loss in the CHIME signal relates to the area and depth of the pit.

Pit ID	Full Diameter at inner surface (mm)	Depth at centre(mm)	Depth at centre (% of wall thicknesst)
P1	11	2.4	31
P2	62	7.8	100
P3	30	4.8	61

Table 1. Diameter and depths of three corrosion pits inspected and detected using CHIME.

Fig 6: CHIME B-scan over general corrosion

Large area corrosion is also detected and an example of this is shown in Fig 6. The sample is a large diameter pipe section taken out of service due to an area of corrosion developing at the base of the pipe. The wall thickness is nominally 13mm and a 50mm wide line of corrosion is present that penetrates from 3 up to 6.8mm into the wall (between 23 to 52% of wall thickness). The sample is inspected from the external surface which is the opposite surface to the corrosion. As the beam of sound moves over the corroded region the signal transmission is disrupted.

Experimental work has also been carried out to determine the sensitivity of CHIME to non-parallel walls and curved surfaces such as pipe. For CHIME signals to be established in a specimen, it is necessary for Head waves to be generated in the material to intersect with the walls of the sample at the critical angle. If the Head waves do not strike at the critical angle then the conversion to a Creeping wave does not take place. It is important therefore to investigate this effect on the detectability of defects, especially small volume changes such as cracks

. In curved plate or pipe the outer and inner surfaces are necessarily not parallel and experiments were carried out on a range of 6" tubes of different wall thickness. The results show that complete ultrasonic coverage of the wall is dependant on the angle of reflection at the inner surface, which in turn is related to the ratio of the outer to inner wall radii. The estimates of the maximum wall thickness to pipe diameter ratio for sustaining the CHIME signals is 12mm in 6" pipe. The conclusion from these results is that CHIME inspections and wall thickness measurements can be carried out on any tube whose outer/inner ratio does not exceed a value of 1.19. For example a one metre diameter pipe with a wall thickness up to 80mm could be inspected. For larger outer/inner ratios, CHIME inspection can still be carried out however full volume coverage of the pipe is not guaranteed.

A second set of experiments was carried out on a sample with varying levels of wall divergence to test when the CHIME signal breaks down. The regular peaks shown in a typical CHIME signal on clean plate (Fig 2) break up when the wall thickness changes by 1mm over a distance of 75mm (13%).

These examples illustrate the potential of CHIME as a fast screening inspection technique reducing the inspection time significantly compared to a detailed wall thickness survey. For example, isolated pitting can be detected using only two probes up to one metre apart. The probes could be placed on the top surface of a pipe to provide complete circumferential coverage. A schematic of a pipe inspection system is shown in Fig 7. CHIME has been tested on samples containing general and isolated corrosion, stress corrosion cracking, hydrogen cracking and has demonstrated tolerance to surface condition such as general roughness or thin layers of coating. Cracks are not necessarily likely to disrupt transmission but they can be reliable reflectors and CHIME has potential for the detection of even shallow cracks.

Defect Characterisation

Once the presence of corrosion has been identified, the operator then requires to know the extent and the remaining wall at the corrosion site. The CHIME signal response to defects shows no direct relation to the remaining wall but the loss of signal is some function of the cross section and depth of the defect. For large areas of accessible plate or pipe CHIME acts as an ideal technique to carry out initial screening to locate regions and then to examine the flagged areas in more detail with a thickness monitor.

Fig 7: Schematic of a CHIME pipe inspection system

However for inaccessible areas such as corrosion under pipe supports, the operator needs information on the defect severity and a yes/no decision as to whether to take any action over any corrosion detected. Some recent validation trials have shown a strong correlation between signal loss and defect severity. The CHIME signal can be divided into the first arrivals, mainly due to bulk wave arrivals, and the latter arrivals due to the Creeping/Head waves combinations. The latter peak arrivals are the first signals to disappear when the uniform nature of a plate/pipe profile is disrupted, such as in the presence of corrosion. However the bulk waves have well-defined paths that can survive in the presence of shallow corrosion. Their transmission is only disrupted in the presence of deeper defects usually greater than 40 to 60% penetration into the sample wall. Recent work has shown this correlation and some validation trials are ongoing for the use of CHIME as an operator tool for monitoring pipe supports and other inaccessible regions where corrosion can be a major problem.

Discussion and Conclusions

CHIME is a novel technique principally for the rapid screening for, and flagging of, defects in pipe or plate. CHIME is the transmission of ultrasound between two probes placed a distance apart (up to 1m) in parallel (or near parallel) walled material. The sound travels as a Creeping wave along the surfaces and as a Head wave between the surfaces. The unique way in which the waves propagate provides complete isonification of plate or pipe with little attenuation allowing the transmitting and receiving probes to be well separated compared to traditional Creeping wave inspection.

The main features of the technique are:

• 100% wall coverage between two probes up to more than one metre apart.
• Large probe separation allows for rapid large area screening and circumferential pipe inspection.
• Complete coverage in pipe for outer/inner ratio not exceeding approximately 1.2.
• Measurement of wall thickness in areas between corrosion.
• Wall thickness up to around 40mm.
  
• The signal change is not directly proportional to the remaining wall thickness but the signal loss can provide some information on defect severity.
  
• Tolerance to surface conditions and thin coating.

Some applications that have been identified to date are: Inaccessible regions such as pipe supports, saddles or sleepers, riser clamps, vessels, long pipelines, tank floors and pipes with limited access, e.g. half buried.

Acknowledgements

The authors would like to thank the members of the Harwell Offshore Inspection R&D Service who have sponsored this development. The current HOIS members are Amerada Hess, British Gas, BP, DNV, Norsk Hydro, Phillips Norway, Phillips UK, RTD, Saga, Saudi Aramco, Shell, Statoil, Texaco Britain & HSE.

References

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