TWI Frequently asked questions
by Andrew Low
There are a number of sophisticated analytical and measurement techniques which enable engineers with the relevant knowledge and experience to predict whether the presence of some feature in a weld represents a risk to the integrityof the structure of which it is a part. The results of these techniques can be used to set general acceptance criteria or assess the effect of a defect or flaw contained within the weld.
The British Standard Guide 'Guide to methods for assessing the acceptability of flaws in metallic structures' comprehensively describes some of the techniques required. These require substantial knowledge and experience of thetechniques to be able to select the appropriate assessment level and judge what aspects of the structure, material and its behaviour that must be allowed for. Two simple applications of the procedures are outlined in Assessment of welded joints (TWI Knowledge Summary) which shows what information is required to perform an assessment. It should be noted that before conducting an assessment it is advisable to seekadvice from a specialist engineer e.g. structural_integrity@twi.co.uk.
BS 7910 addresses these scenarios:
- Fracture and plastic collapse
- Fatigue
- Creep and creep fatigue
- Leakage
- Corrosion, erosion and corrosion fatigue
- Stress corrosion and instability as may arise from the presence of flaws represented by cracks
- Lack of fusion or penetration
- Cavities, inclusions and shape imperfections
For the purpose of structural integrity assessments, there are other procedures which exist such as API 579-1/ASME FFS-1 and WES 2805 (Japan Welding Engineering Society). Whilst differing in detail, these procedures can be also usedto carry out integrity assessments.
For an assessment of welding flaws and corrosion damage in a pipeline, more specific procedures exist which can be used as an alternative to the integrity assessment Codes, such as API 1104 (American Petroleum Institute) or EPRG(European Pipeline Research Group).
