Hi Dave,
Your response
"You say in a) "a stress multiplier of 2". Is this to address your condition (I don't know how to better put this) that the "real SIF" for a girth butt weld is 2, not 1? If so, then, yes, that doubling effect can be applied with this multiplier. BUT, I cannot say the number you type in is "2". Note that for US units, the FAT file contains the value "0.5" is used so in that case, a value of "2" would quadruple."

has me confused a little as you seem to be agreeing initially that a stress multiplier of 2 will address the shortfall in the SIF's for butt welds( and more importantly the components in a piping system) but then you say the number is "2".

If we have a fatigue curve which is in US units and is based on Stress Range values vs cycles what would you recommend the Stress multiplier should be to perform a "detailed" Fatigue analysis?