corroded bolt inspection uhi

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Corroded Bolt Inspection Using Phased Arrays

Robert Ginzel1, Darryl Johnston1 and Michael Moles2

1Eclipse Scientific60 Baffin Place, Unit 4, Waterloo, Ontario, Canada, N2V 1Z7

+1 519 886 6717, e-mail [email protected]

2Olympus NDT48 Woerd Avenue, Waltham, MA, USA 02453

+1 416 831 4428, e-mail [email protected]

ABSTRACTPhased arrays can detect cracking in bolts and shafts. In addition, loss of threads – indicating corrosion or gouging – can also be detected. The array is mounted on the end of the shaft affixed in a jig, rotated, and all data collected and displayed. Early work performed on corroded bolts showed promise as a detection mechanism for corroded areas, and also for estimating corrosion depth. This early work showed some limitations of the system, particularly with respect to the length-diameter ratio of the bolts, and data interpretation. Short fat bolts were readily inspected, while long thin bolts could present issues.

A recent blind trial was performed on a series of lab-corroded bolts using a phased array system. Predictably, the phased array system performed well on the shorter, fatter parts of the bolt, though not on the longer, thinner parts of the bolts. On longer, thinner bolt regions, the phased arrays acted primarily as a “go-no go” test. Due to mode conversions, multiple reflections, etc. locating the depth and angle of the corrosion were shown to have inaccurate results in some cases.

This paper shows sample cracked bolt examples, corroded bolt trial inspection results, and a Schlieren system image of modeled corrosion. Overall, a commercial system is now available, and the limitations known.

INTRODUCTIONInspecting bolts has been a challenge for some time. Straight beam conventional ultrasonic inspection or even angled longitudinal inspection, although capable of crack detection, has practical limitations. Alternative techniques for measuring corrosion, e.g. the centrifugally guided wave technique, were not impressive (1). Standard techniques, e.g. normal beam inspection as required by ASME (2), presented many interpretation problems. In practice, there was no suitable technique for detecting corrosion in bolts.

Bolt inspection, shaft inspection and numerous other applications that benefit from beam control, encoded travel and full data collection methods, are ideal for phased array ultrasonic inspection (3). Essentially, the array is scanned rotationally around the bolt top, and “fingerprints” the bolt. Any anomalies show clearly, particularly radial planar defects. This approach has been used before for cracks (4, 5), and is now a standard procedure. New display options and the varied forms of beams using phased arrays can now be combined to provide comprehensive easy-to-interpret data display.

Eclipse and Olympus NDT were first approached to resolve bolt issues for the North Sea oil sector through HOIS (6), and the first requirements were resolution of advanced severe corrosion attack. The requirements for this application included stating the percentage of wall loss with 10% as an early threshold and 20% requiring some extent information. With severe aggressive corrosion the angles of the reflecting face from the corrosion site were easy to discern.

One problem that has been experienced is the lack of established standards on accept-reject criteria. Some acceptance criteria will allow over 50% wall loss, while others require much less. Specifically, the Engineering Institute in the UK only permits 10% loss, which is roughly equivalent to the threads (7). To satisfy all these varied criteria, both thread loss and measurement of wall loss are required, if only as an approximate range.

More recently, the relevant parties developed techniques and procedures for corroded bolts. Corrosion is more of a challenge than cracking as corroded areas do not necessarily present well-oriented, sharp defects. Initial procedures were supplied to HOIS, with some limits on bolt length-diameter ratios defined. Subsequent tests on four moderately long and thin bolts were generally successful.

ASNT Fall Conference and Quality Testing Show 2009 [Columbus, OH, October 2009]: pp 61-67. © Copyright 2009, 2011, American Society for Nondestructive Testing, Columbus, OH.

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A second, blind trial on 34 bolts followed. These 34 bolts were relatively long and thin, thereby presenting a challenge to the phased array technique. This paper will show the advances and significant promise of the phased array ultrasonic inspection methods applied to recent bolt inspection trials. The paper presents material demonstrating the technology benefits, capitalizing on the beam control, the excellent display options, and presents the easily interpreted encoded-for-position scans. With full encoded data collection, the technique is now able to record and archive entire waveform data with detailed positional information for future review and post processing.

The technique developed here is applicable to almost any long shaft, for both corrosion and cracking. This applies to the nuclear industry (mostly cracking), transmission towers, structural bolts etc. The technique can be adapted to the bolt geometry and defects by modeling and the use of suitable arrays and mechanics.

INSPECTION TECHNIQUEA portable phased array instrument with encoding capabilities is used. (See Figure 1.)

Figure 1: OmniScan MX phased array instrument.

A suitable array is mounted in a jig on the top of the bolt. (See Figure 2.) The array is either in direct contact with the bolt, or uses a normal beam wedge. If in direct contact, a protective layer is required as bolt heads are relatively rough (8).

Figure 2: Left, ring and jig fixture. Right, fixture, probe and encoder on bolt.

As always, calibration bolts are used which are representative of the component to be inspected. Figure 3 shows a sample corroded bolt and an appropriate calibration bolt.

More recently, the relevant parties developed techniques and procedures for corroded bolts. Corrosion is

more of a challenge than cracking as corroded areas do not necessarily present well-oriented, sharp defects.

Initial procedures were supplied to HOIS, with some limits on bolt length-diameter ratios defined.

Subsequent tests on four moderately long and thin bolts were generally successful.

A second, blind trial on 34 bolts followed. These 34 bolts were relatively long and thin, thereby presenting

a challenge to the phased array technique. This paper will show the advances and significant promise of

the phased array ultrasonic inspection methods applied to recent bolt inspection trials. The paper presents

material demonstrating the technology benefits, capitalizing on the beam control, the excellent display

options, and presents the easily interpreted encoded-for-position scans. With full encoded data collection,

the technique is now able to record and archive entire waveform data with detailed positional information

for future review and post processing.

The technique developed here is applicable to almost any long shaft, for both corrosion and cracking. This

applies to the nuclear industry (mostly cracking), transmission towers, structural bolts etc. The technique

can be adapted to the bolt geometry and defects by modeling and the use of suitable arrays and mechanics.

INSPECTION TECHNIQUE A portable phased array instrument with encoding capabilities is used (see Figure 1).


As always, calibration bolts are used which are representative of the component to be inspected. Figure 3

shows a sample corroded bolt and an appropriate calibration bolt.

More recently, the relevant parties developed techniques and procedures for corroded bolts. Corrosion is

more of a challenge than cracking as corroded areas do not necessarily present well-oriented, sharp defects.

Initial procedures were supplied to HOIS, with some limits on bolt length-diameter ratios defined.

Subsequent tests on four moderately long and thin bolts were generally successful.

A second, blind trial on 34 bolts followed. These 34 bolts were relatively long and thin, thereby presenting

a challenge to the phased array technique. This paper will show the advances and significant promise of

the phased array ultrasonic inspection methods applied to recent bolt inspection trials. The paper presents

material demonstrating the technology benefits, capitalizing on the beam control, the excellent display

options, and presents the easily interpreted encoded-for-position scans. With full encoded data collection,

the technique is now able to record and archive entire waveform data with detailed positional information

for future review and post processing.

The technique developed here is applicable to almost any long shaft, for both corrosion and cracking. This

applies to the nuclear industry (mostly cracking), transmission towers, structural bolts etc. The technique

can be adapted to the bolt geometry and defects by modeling and the use of suitable arrays and mechanics.

INSPECTION TECHNIQUE A portable phased array instrument with encoding capabilities is used (see Figure 1).


As always, calibration bolts are used which are representative of the component to be inspected. Figure 3

shows a sample corroded bolt and an appropriate calibration bolt.


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Figure 3: Corroded bolt at left. Calibration bolt at right.

The phased array instrument is set up to perform a small angle longitudinal wave S-scan (also called a sectorial scan or sector scan). The beam sweeps through the vertical plane of the bolt, covering the threads, end, and body as shown in Figure 4 for a calibration bolt (with a machined notch, not corrosion in this image). Depending on the bolt length, different scan patterns and angle sweep ranges are used.

Figure 4: Left, scan of notched calibration bolt with threads, notches and end of bolt labeled. Right, photo of notched calibration bolt.

The array is scanned around the head of the bolt, and full data is collected and stored. In effect, the phased array instrument “fingerprints” the bolt for defects around the 360o circumference – for both cracking and corrosion in effect – and the data is stored and interpreted.

In contrast to both conventional bolt inspections and the CGWT, data interpretation is comparatively straightforward as the operator looks for anomalies in the data and predicted images.

INITIAL RESULTSFigure 5 shows a single S-scan from bolt labeled #2. The scan shows the threads clearly, where they still exist. The area of extensive corrosion is labeled.

Figure 4: Left, scan of notched calibration bolt with threads, notches and end of bolt labeled. Right, photo

of notched calibration bolt.

The array is scanned around the head of the bolt, and full data is collected and stored. In effect, the phased

array instrument “fingerprints” the bolt for defects around the 360o circumference – for both cracking and

corrosion in effect – and the data is stored and interpreted.

In contrast to both conventional bolt inspections and the CGWT, data interpretation is comparatively

straightforward as the operator looks for anomalies in the data and predicted images.

Screw threads

Two cal notches

End of bolt

Figure 4: Left, scan of notched calibration bolt with threads, notches and end of bolt labeled. Right, photo

of notched calibration bolt.

The array is scanned around the head of the bolt, and full data is collected and stored. In effect, the phased

array instrument “fingerprints” the bolt for defects around the 360o circumference – for both cracking and

corrosion in effect – and the data is stored and interpreted.

In contrast to both conventional bolt inspections and the CGWT, data interpretation is comparatively

straightforward as the operator looks for anomalies in the data and predicted images.

Screw threads

Two cal notches

End of bolt


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Figure 5: S-scan from bolt 2 showing remaining threads in lower region.

Figure 6 shows a similar single S-scan from bolt #1. In contrast to bolt 2, bolt 1 showed corrosion fully round the bolt, i.e. there were no threads left in the centre region. This is clearly visible in the S-scan in Figure 7, and on the animation replay of the data. The contoured shape of the corroded area is also clearly visible.

Figure 6: Single S-scan from bolt 1 showing corroded area 360o on both sides of the bolt, and the corroded area contour.

Figure 7 shows an alternative display for the bolts. The advanced TomoView software allows viewing of 360° scans for full mapping and selective cross-sections.

In general, these were short, fat bolts, and were readily inspectable as predicted. The initial bolt samples were relatively short with good effective area ranging from 19 to 35 mm diameter with typical bolt lengths of 50 mm to 150 mm.

INITIAL RESULTS Figure 5 shows a single S-scan from Bolt labeled #2. The scan shows the threads clearly, where they still

exist. The area of extensive corrosion is labeled.

S-scan in

area contour.

Corroded area on

one side only.

Note threads on

uncorroded side

Corroded area

all around cuts

off thread

signals on both

sides

INITIAL RESULTS Figure 5 shows a single S-scan from Bolt labeled #2. The scan shows the threads clearly, where they still

exist. The area of extensive corrosion is labeled.

S-scan in

area contour.

Corroded area on

one side only.

Note threads on

uncorroded side

Corroded area

all around cuts

off thread

signals on both

sides


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Figure 7: Alternative images of bolt. Top left, S-scan; bottom left, gated C-scan, right, cursor-selected A-scan.

TRIAL RESULTSA blind trial on 34 bolts organized by HOIS used thinner, longer bolts – which were more of a challenge to this technique, as predicted. This was identified in earlier work as one of the major practical limitations of the technique. These blind trial bolts were typical of offshore applications, and were 200 mm long with an effective inspection area or diameter of 19 mm. Inspections were made from both ends of the bolts, and analyzed in terms of “hit or miss,” and location. Simulated corrosion was made in the ranges of ~10-30%, and different locations along the bolts. There were two uncorroded specimens in the trial, unknown to the inspectors.

The results on the 34 trial bolts are shown in the scatter plot in Figure 8.

Figure 8: Scatter plot showing hits, probable and possible hits, and misses on trial bolts as a function of depth of corrosion and distance down bolts. (Courtesy of HOIS.)

Figure 7 shows an alternative display for the bolts. The advanced TomoView software allows viewing of

360o scans for full mapping and selective cross-sections.

Figure 7: Alternative images of bolt. Top left, S-scan; bottom left, gated C-scan, right, cursor-selected A-

scan

In general, these were short, fat bolts, and were readily inspectable as predicted. The initial bolt samples

were relatively short with good effective area ranging from 19 to 35mm diameter with typical bolt lengths

of 50mm to 150mm.

TRIAL RESULTS A blind trial on 34 bolts organized by HOIS used thinner, longer bolts – which were more of a challenge to

this technique, as predicted. This was identified in earlier work as one of the major practical limitations of

the technique. These blind trial bolts were typical of offshore applications, and were 200 mm long with an

effective inspection area or diameter of 19 mm. Inspections were made from both ends of the bolts, and

analyzed in terms of “hit or miss”, and location. Simulated corrosion was made in the ranges of ~10-30%,

and different locations along the bolts. There were two uncorroded specimens in the trial, unknown to the

inspectors.

The results on the 34 trial bolts are shown in the scatter plot in Figure 8.

the

“go-

was

Research Institute).


66

The results from the trial bolts were pretty much as predicted, and are summarized below. • Defects at 10% to 15% wall loss at the close to array end of the bolt and extending up to 100 mm are detectable in most

cases. • Defects at 20% to 30% wall loss are detectable at distances up to 100 mm and the potential of volume sizing appears to be

viable. • Defects at 20% to 30% are detectable at greater distances, up to 200 mm.

In effect, near corrosion is detectable and reasonably characterizable, while corrosion well down the bolts is “go-no go” only. Perhaps more interesting, many of the far, deep corrosion areas were apparently detected, but located further down the bolt, and sometimes at the incorrect angular position. This inconsistency was analyzed further using Schlieren imaging.

ULTRASONIC ANALYSISWith limited funding, modeling was not a realistic approach to interpreting the ultrasonic interactions at long ranges. Some simple glass specimens were manufactured for a Schlieren system to visualize the ultrasound beam propagation (9). Figure 9 shows an example of a glass specimen with simulated corrosion. While Schlieren systems are designed for interactive viewing, even a static image shows the complex interactions taking place from the notches and corrosion.

Figure 9: Complex interaction of beams in corroded bolts with threads. (Courtesy of Ed Ginzel, Materials Research Institute.)

The movie images showed that the beams could be reflected back and forth from the corroded areas as they travelled back up the bolts. This would explain both the overestimated defect depths, and the incorrect angular positions.

DISCUSSIONFrom this current 34 bolt trial, the initial review reinforced the earlier findings that the detection of 15 to 20% wall loss was a boundary (early detection point) and the 20% to 30% corrosion ingress was the current threshold point for high confidence for detection, and to some level a point at which some extent could potentially be expressed. However, the detection threshold depends closely on the distance down the bolt. The refinements and advancement of the technology will now require some very specific steps to push the technology and our awareness/ability to the next level. The following represent some steps that should be considered if the current state of the approach is to be advanced. These steps are specific to corroded bolts. For cracked bolts, a separate program and trial would be required. • Step 1 – redefine the requirements including awareness of the indicative defects along with clear accept-reject criteria. • Step 2 – perform modeling to examine and further understand the wave mechanics occurring in the bolt/studs targeted

for inspection as defined by Step 1. • Step 3 – generate samples to enable operators to explore and account for anticipated modeling responses and those

unforeseen responses, and to develop effective interpretation procedures. • Step 4 – perform trials to evaluate the refinements achieved and explore any deficiencies in the developed techniques and

procedures. • Step 5 – perform field trials with validation opportunities for feedback of the developed material, enabling additions or

modifications to the established practices.If the statistics and current state of the technology are acceptable at this point of the development then implementation of the current fixture, procedure, techniques and users guide would be appropriate.

the

“go-

was

Research Institute).


67

In addition, a commercial bolt inspection system is now available (8), and is being used in a number of other industries besides offshore oil and gas. Procedures have been developed, as required by HOIS.

CONCLUSIONS1. Phased array inspection of bolts is a viable technology, particularly for cracking, but also for corrosion.2. The four-bolt HOIS trial was encouraging, but deviated from initial development work by using shorter, fatter bolts. 3. The current 34 bolt blind trial was more challenging as thin bolts are more difficult to inspect, particularly past certain

lengths. 4. The tests showed that: a. Defects at 10% to 15% wall loss at the close to array end of the bolt and extending up to 100 mm are detectable in

most cases. b. Defects at 20% to 30% wall loss are detectable at distances up to 100 mm and the potential of volume sizing appears to

be viable. c. Defects at 20% to 30% are detectable at greater distances, up to 200 mm.5. Thread detection is good down to ~60 mm, and would be significantly further for larger diameter shafts.

REFERENCES1. Light, G.M. and R.L. Spinks. “Use of the Cylindrically Guided Wave Technique to Detect Corrosion in All-Thread

Bolts,” ASM. 2. ASME 2004, B&PV Code, Section V, Article 5, T-571.3. R/D Tech, “Introduction to Phased Array Technology Applications,” published by Olympus NDT. 2004.4. Moles, M. and R.G. Ginzel, “Phased Arrays For Detecting Cracking in Bolts,” 6th International Conference on NDE for

Structural Integrity for Nuclear and Pressurized Components, Budapest, Hungary, October 8-10, 2007.5. Moles, M. and R.G. Ginzel, “Phased Array Inspection of Corroded Bolts,” International Chemical and Petroleum

Industry Inspection Technology Conference, sponsored by ASNT, Houston, Texas, June 20-23, 20076. HOIS – Industry forum for discussion of NDT of oil and gas offshore. http://www.esrtechnology.com/page_view2.

asp?InfoID=253.7. Energy Institute, Guidelines for the management of the integrity of bolted joints for pressurized systems, 2nd Edition,

ISBN 978 0 85293 461 6, May 2007, section 7.1, p. 38. 8. Eclipse Scientific Products, WEBSCAN Wheel Encoded Bolt Scanner, http://www.eclipsescientific.com/products/

Scanning/BoltScanner/info.html.9. Ginzel, E.A., F. Honarvar and A. Yaghootian, “A Study of Time-of-Flight Diffraction Technique using Photoelastic

Visualization,” The 2nd International Conference on Inspection and NDT (TINDT2008), Tehran, Iran, October 2008.

ACKNOWLEDGMENTSHOIS partly funded this project, supplied bolts and administered the blind trial. Particular thanks are due to Richard Lee of HOIS. Ed Ginzel of Materials Research Institute supplied the Schlieren system and the glass samples.


corroded bolt inspection uhi

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