NDTnet 1998 April, Vol.3 No.4

How to develop acceptance criteria for pipeline girth weld defects?

O. Forli, Det Norske Veritas, Oslo (N)
European-American Workshop Determination of Reliability and Validation Methods of NDE, Berlin - June 18-20, 1997
TABLE OF CONTENTS
Summary
Introduction
The Task
The Route
The Results
Fitness-for-Purpose Acceptance Criteria
References

Summary

Introduction

The Task

The Route

The Results

Comments
Figure 1a - 1i: Examples of POD curves for different techniques and acceptance criteria (see comments). The data have been taken from the Nordtest NDE Programme (9).
Figs. la. and lb. show grouped POD observation data with fitted curves. Fig la. contains all the radiographic data contained in the Nordtest Programme, whereas Fig. lb. only contains approximately 150 randomly selected observations. In the latter case, the lower 95% confidence limit is just above the fitted mean curve minus 0.1, and the NordtestElementary Detection Criteria still fulfilled for the Reference Technique. Fig. lb. thus gives an indication of the number of observations required to fulfil the criteria. Figure 1a
Different defect types
Radiography
Minus 0.1 R Curve
Number of Defects: 601
Number of Observations: 3414
Figure 1b
Different defect types
Radiography
Minus 0.1 R Curve
Number of Defects: 153
Number of Observations: 3414
Figs. 1c. and 1d. give comparative curves for a mixture of all encountered defect types and, the pipeline girth weld important defect type, lack of fusion. The echo amplitude criteria can be regarded satisfactory for all defect heights, whereas the TOFD acceptance criteria are satisfied according to the Nordtest requirements above defect height 2 mm, corresponding to the radiography 0.5 POD. This must also be regarded satisfactory. In addition, it can be noted that the TOFD curves are steeper than the echo amplitude based curves. This is of course due to the better correlation between TOFD measured defect height and true defect height, than between echo amplitude and height, and implies a better quality examination work with TOFD, as less small and more large defects are revealed (please keep though in mind that the TOFD POD data are a simulation). The total (small and large defects) detection / rejection rate for TOFD is 45% compared to 46% for radiography. Figure 1c
Different defect types
R: Radiography U: Ultrasonics
TOFD: Time of flight diffraction
Minus 0.1 R Curve
Figure 1d
Lack of fusion defects
R: Radiography U: Ultrasonics
TOFD: Time of flight diffraction
Minus 0.1 R Curve
Fig. 1e. shows the inadequacy of the set acceptance criteria for ultrasonics for porosity. In order to reveal pores with ultrasound higher sensitivities than those used must be applied: At 30 mm distance a spherical cavity of 4 mm 0 gives an echo 11 dB below that of a 3 mm 0 side drilled hole, etc. One way to handle this problem is to do an evaluation of the severity of pores (and similar for slag inclusions), and possibly accept relaxed acceptance criteria compared to those for radiography, or use special ultrasonic pattern recognition techniques to map porosity. Figure 1e
Porosity
R: Radiography U: Ultrasonics
TOFD: Time of flight diffraction
Minus 0.1 R Curve
Figs. 1f. and1g. compare POD curves for lack of fusion in different thickness groups (average wall thicknesses 13 and 28 mm), and show that the ultrasonic echo amplitude criteria are not adequate for the thinner material. Figure 1f
Lack of fusion defects
Plate thickness < 15mm
R: Radiography U: Ultrasonics
TOFD: Time of flight diffraction
Minus 0.1 R Curve
Figure 1g
Lack of fusion defects
Plate thickness > 15mm
R: Radiography U: Ultrasonics
TOFD: Time of flight diffraction
Minus 0.1 R Curve
Fig. 1h. show the failure of the set ultrasonic echo amplitude acceptance criteria for lack of fusion of less length than wall thickness. Some caution is, however, required, when making this observation: The used acceptance criteria for radiography allow lack of fusion of length below wall thickness, and the detections made are due either to misinterpretation of defect type or length, or the evaluation as IIW degrees Red or Brown incorporated in the acceptance criteria. There is, however, as further analysis shows, a POD defect length dependency, and an evaluation of this length dependency is required, when lengths are not 'naturally' distributed, or defect significance is also length dependent, as when plastic collapse is the most relevant defect mechanism related to the girth weld defects. Further, a distinction may have to be made between surface and embedded defects. Figure 1h
Lack of fusion defects
Defect lenght < Wall thickness
R: Radiography U: Ultrasonics
TOFD: Time of flight diffraction
Minus 0.1 R Curve
Figure 1i
Lack of fusion defects
Defect lenght > Wall thickness
R: Radiography U: Ultrasonics
TOFD: Time of flight diffraction
Minus 0.1 R Curve

Fitness-for-Purpose Acceptance Criteria

References

  1. F.H. Dijkstra, J. de Raad: Why develop acceptance criteria for pipeline girth weld defects? European-American Workshop Determination of Reliability and Validation Methods of NDE. Berlin. 18 - 20 June 1997.
  2. P. Verhaeghe: Practical application of automated ultrasonic inspection on pipeline welds. 2nd International Pipeline Technology Conference. Ostend, Belgium. 11 - 14 September 1995.
  3. J.A. de Raad, F.H. Dijkstra: Mechanized UT now can replace RT on girth welds. 2nd International PiPeline Technology Conference. Ostend, Belgium. 11 - 14 September 1995.
  4. D. Hodgkinson, D.V. Dorling, A.G. Glover: Mechanized ultrasonic testing of pipeline girth welds produced by mechanized gas metal arc welding. 2nd International Pipeline Technology Conference. Ostend, Belgium. 11 - 14 September 1995.
  5. H.A.M. van Merrienboer: Field application of mechanized US inspection of large-diameter pipelines: 2nd International Pipeline Technology Conference. Ostend, Belgium. 11 - 14 September 1995.
  6. Rules for Submarine Pipeline Systems. Det Norske Veritas. OSIO, Norway. December 1996.
  7. Guidelines for Replacing NDE Techniques with One Another. NT Techn Report 300. Nordtest. Espoo, Finland. October 1995.
  8. Non destructive testing of thin plate. Doc.no. NDP 93-40. Nederlands Instituut voor Lastechniek. Voorschoten, The Netherlands. March 1995.
  9. O. Forli: Reliability of radiography and ultrasonic testing. 5:e Nordiska NDT Symposiet. Espoo, Finland. 26 - 28 August 1990.
  10. Collection of Reference Radiographs of Welds in Steel. International Institute of Welding. 1952.
  11. R. Denys: A plastic collapse based procedure for girth weld defect acceptance. Proc. Int. Conf. on Pipeline Reliability, Vol. II, Paper VIII. Canmet, Pergamon, Calgary, Canada. Pergamon Press. 1992.
  12. F.J. Horsley, A.G. Glover, R. Denys: An assessment technique for defects in under and over matched pipeline girth welds. Conf. Proceedings l 1th PRCI-EPRG Pipeline Research Conference. Paper 30. Arlington, Virginia. 7 - 11 April 1997.
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