A Discussion on new ISO criteria for oil well casing strength calculation

Comparative analysis on the methods for oil well casing strength based on ISO/TR 10400:2007 and API Bul 5C3:1994 is conducted. It is regarded that the classiication of strength calculation methods based on the ISO/TR 10400:2007 is comparatively it for the actual situation of casing load and strength performance. The reliability project method is introduced into strength performance calculation by using the new method. Factors including the casing manufacturing tolerance, straightening manner, thermal treatment mode, inspection threshold value are taken into account, so the calculation results get more accurate. The formulae of ultimate collapse strength and design collapse strength are presented, meeting the requirements under various conditions. Comparative analysis shows that the calculated value of the internal pressure toughness collapse strength of oil well casing by using the new method is bigger than that based on the old criteria. As for the casing of which the diameter-thickness ratio is smaller than 20, the calculated result of the new collapse strength formula is smaller than the old criteria, while the calculated result is bigger as for the casing of which the diameter- thickness ratio is bigger than 20. The recognition change of oil well casing strength based on the ISO/TR 10400:2007 will exert effect on the current pipe string strength design method.