TWI Industrial Member Report Summary 921/2009
By M J Cheaitani and M R Goldthorpe
The use of engineering critical assessment (ECA) to derive flaw acceptance criteria for pipeline girth welds allows the maximum tolerable size of surface and embedded circumferential planar flaws to be determined on a fitness-for-purpose basis. A typical ECA involves assessing the significance of such flaws with regard to failure mechanisms, including fracture, that the pipeline may experience during construction, commissioning and service. The most commonly used approach to assess pipeline girth weld flaws for fracture is the failure assessment diagram (FAD), which involves the calculation of a plastic collapse parameter, Lr, and a fracture parameter (Kr, ?r or Jr). The plastic collapse parameter characterises the proximity to failure by yielding mechanisms and is defined as either the ratio of applied load to the limit load or, equivalently, the ratio of the reference stress to the yield strength. The fracture parameter characterises the proximity to brittle or ductile fracture of the flaw under linear elastic conditions.
The work described in this report concerns the development of a reference stress (or limit load) model for use in FAD-based fracture assessments of circumferential embedded girth weld flaws. This work is considered necessary since existing reference stress solutions for embedded flaws are not consistent with reference stress solutions that are typically used to assess surface flaws because the approaches used in their derivation are different. Whereas there are several reference stress models for surface flaws, including some intended specifically for circumferential flaws in pipe sections, there are fewer reference stress models for embedded flaws and these are intended for flaws in flat plates.
One of the most widely used reference stress solutions for embedded flaws is that of BS 7910:2005 (BSI, 2005), which assumes that plastic collapse occurs locally when the net section stress on a small area surrounding the embedded flaw reaches the yield or flow strength of the material. It also assumes that tensile loads are applied through pin-jointed coupling, ie that there is negligible bending restraint. The use of this solution to assess embedded flaws can lead to overly conservative results which may in some cases be counter-intuitive, eg for a given applied loading, the maximum tolerable length for an embedded flaw (whose ligament is greater than or equal to the height of one weld pass) is smaller than that for a surface breaking flaw of the same height.
The objective of this work is to develop a limit load (and equivalent reference stress) model appropriate for assessing circumferential embedded flaws in pipe girth welds.
Existing standard solutions and methods for determining limit load (and/or reference stress) for embedded flaws within the context of FAD-based assessments of J, including solutions recommended in BS 7910 (BSI, 2005) and R6 (BEGL, 2001) for flaws in flat plates, have been reviewed and evaluated against the results obtained from the finite element analyses.
The J-based limit loads were found to vary with applied load/strain level. A parameter was derived based on MJ2B E and the ratio of the elastic-plastic pipe stress to the elastic pipe stress (?M1/?M) that was largely independent of load level, and which could act as a benchmark to determine MJ2B E at any load level (up to about 1% strain). A new general equation was derived for estimating MJ2B E at a strain of 0.5% (MJ2B E 0.5%) for pipes containing circumferential embedded flaws that is consistent with MJ2B E determined from the finite element analyses. With a simple correction factor, the new equation can also be used to estimate MJ2B EP at a strain of 0.5% (MJ2B EP 0.5%).
Data on the plastic strain concentration in the region of the smaller of the two ligaments adjacent to an embedded flaw have been obtained as this can be relevant to failure by excessive straining.