Page 432 - Marine Structural Design

408 Part III Fatigue and Fracture

ultrasonic testing (UT) (sizing internal defects using ultrasonic signals)
Radiography is most sensitive to volumetric defects, such as porosity and slag. Any detectable
crack is rejected because of the difficulty to detect and size crack-like defects.
Among of the above inspection methods, the ultrasonic inspection is the most reliable way of
detecting and sizing internal defects. UT works very much like radar. Probes can be moved
over the surface in the region to be inspected, in which piezo-electric crystals generate
ultrasonic signals. The waves are reflected by the surface of the examined body, and also by
any defects that might come in their way. The probe that generates the signal also detects these
echoes. By measuring the time delay between the emission signal and the reception of each
reflection, the source of reflection can be located and the position of defects identified. The
basic features of UT are:
UT is more sensitive to the more serious types of defects because it depends on the signals
reflected. In decreasing order, the severity of defects is: cracks, incomplete fusion,
inadequate penetration, slag and porosity.
UT can locate defects in three dimension.
UT can be conducted quickly and simply, without radiation hazards.
UT can handle complex geometry of welded connections though use of the transducer.
Over 70% of the defects may be detected by UT, and the false alarm is less than 30%.
Where radiographic or ultrasonic inspection is required, the extent and location of inspection
and choice of inspection methods are to be in accordance with AWS (1997) and ABS (1986),
the materials and welding procedures involved, the quality control procedures employed and
the results of visual inspection. In AWS (1997) and ABS (1986), criteria are defined to
determine whether the inspection results (signals) are to be non-conforming or to be
disregarded or to be evaluated against defect acceptance criteria.

22.6 References
1. ABS (1986), “Rules for Non-destructive Inspection of Hull Welds”, American Bureau of
Shipping.
2. ABS (2002), “Rule Requirement for Materials and Welding”, American Bureau of
Shipping.
3. Almar-Naess, A. (1 985), “Fatigue Handbook, Ofl’sshore Sled Structures”, Tapir, Norway.
4. API 1104 (1994), “Alternate Standards for Acceptability for Girth Welds. Appendix A,
Standards for Welding Pipelines and Related Facilities”, 1 sth edition, American
Petroleum Institute.
5. AWS (1997), “AWS Structural Welding Code - Steel, AWS Dl.1-96”, American
Welding Society.
6. Bai, Y. (2001), “Pipelines and Risers”, Elsevier Science Ocean Engineering Book Series.
7. BSI (1993) “BS 7608 - Code of Practice for Fatigue Design and Assessment of Steel
Structures”.
8. BSI (1999) “BS7910 - Guidance on Methods for Assessing the Acceptability of Flows in
Fusion Welded Structures”.