Fracture Mechanics - ISO 10400 Annex D

We have selected a 110ksi SMYS full fledged sour service grade of casing to meet NACE Region 3 (geothermal temperature is 4degC). The casing has been successfully tested with NACE Method A Solution A at 85% SMYS.

Annex D of ISO 10400 was recently brought to our attention. We have calculated the VME stress for the 100% gas kick load and it is below the threshold stress (i.e. less than crack initiation pressure), so fracture should not occur as a result of initiation of new crack.

However, Annex D also suggests calculation of the fracture burst capacity under the assumption of a pre-existing crack. Based on SR2 pipe specifications our maximum flaw depth is 5% of nominal wall thickness. Using the equations in Section D2 and an assumed value of K1ssc we have determined the maximum internal load the casing may be subjected to before fracture propagation. This number is approximately 50% of the burst capacity and is not sufficient for a 100% gas kick load.

My question for the community is; are you designing wells based on fracture burst capacity in sour environments? I am also keen to hear of other interpretations of the Annex as it makes vague references to 'sharp cracks' and 'large cracks' when calculating fracture burst capacity (which seems to suggest that a 5% flaw depth is not a crack).

Thanks, Eric.

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