Abstract

Modern subsea pipelines are now routinely exposed to high strains. When above yield they receive appropriate attention since Engineering Critical Assessment (ECA) derived flaw acceptance criteria would normally be considered mandatory. For stress based designs, (applied axial strain up to 0. 4%), standard flaw acceptance criteria, that originated with empirically based workmanship levels, will be applied unless an optional ECA has been performed. Some of the best current guidance for ECA on pipeline girth welds can be found in DNV-OS-F01 [1]. DNV also provide guidance for workmanship level flaw sizes, (height and length) to be used with Automated Ultrasonic Testing (AUT).

In this paper the tolerability of the DNV-guided workmanship level flaw sizes has been assessed using ECA. The methodology used in this study is in line with the guidance as per DNV-OS-F101 (2013) [1]. When applying this it can be seen that DNV-guided workmanship flaw sizes may not be considered appropriate when the applied strain approaches yield level. This finding implies that the extra testing and analysis required for ECA may result in more stringent acceptance criteria, which does not make sense and creates confusion in the industry. It may be that the recommended ECA methods are overly conservative or the recommended workmanship flaw sizes are not actually safe, or both.

New initiatives have recently been launched, (e.g. TWI) to standardize ECA for pipeline girth welds and based on the findings here it is recommended that these studies also check workmanship flaw acceptance and if necessary restrict further the domain in which they can be applied, e.g. adjust the maximum applied stress to well below yield. It would be useful if a standard ECA approach could consistently allow less stringent criteria over the domain that ‘workmanship’ acceptance criteria can be applied when overmatching weld strength and good toughness data are ensured by testing and enhanced welding control.

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