Evaluation of Weld Metal Strength Mismatch in X100 Pipeline Girth Welds Available to Purchase

A relatively simple method based on standard fracture mechanics flaw assessment procedures, such as BS 7910, but modified using published mismatch limit load solutions is described. It is used to illustrate the effects of weld width and strength mismatch on CTOD requirements for girth welds in Grade X100 strength pipeline material subjected to axial stress. It is shown that fracture toughness requirements based on standard analyses not allowing for mismatch effects can be unnecessarily conservative when either undermatched or overmatched welds are present. Adverse effects of undermatching, in reducing the allowable stress, can be mitigated by reducing weld width. It is shown that even small amounts of overmatching (e.g. 10%) can be beneficial by allowing axial stress to exceed the SMYS of the parent pipe and reducing CTOD requirements.