1. n. [Drilling]
A system of identifying and categorizing the strength of casing materials. Since most oilfield casing is of approximately the same chemistry (typically steel) and differs only in the heat treatment applied, the grading system provides for standardized strengths of casing to be manufactured and used in wellbores. The first part of the nomenclature, a letter, refers to the tensile strength. The second part of the designation, a number, refers to the minimum yield strength of the metal (after heat treatment) at 1,000 psi [6895 KPa]. For example, the casing grade J-55 has minimum yield strength of 55,000 psi [379,211 KPa]. The casing grade P-110 designates a higher strength pipe with minimum yield strength of 110,000 psi [758,422 KPa]. The appropriate casing grade for any application typically is based on pressure and corrosion requirements. Since the well designer is concerned about the pipe yielding under various loading conditions, the casing grade is the number that is used in most calculations. High-strength casing materials are more expensive, so a casing string may incorporate two or more casing grades to optimize costs while maintaining adequate mechanical performance over the length of the string. It is also important to note that, in general, the higher the yield strength, the more susceptible the casing is to sulfide stress cracking (H2S-induced cracking). Therefore, if H2S is anticipated, the well designer may not be able to use tubulars with strength as high as he or she would like.
