The purpose of the study was to assess the relative degree of severity of the test methods that are available to determine the Sulfide Stress Cracking (SSC) resistance of oilfield tubulars. Three different steel grades were used: a Sour Service Line Pipe grade (X65), a Sour Service OCTG grade (T95) and a non sour service OCTG grade (P110). The SSC threshold stresses of these steels were obtained in the NACE TM0177 solution A through Tensile Tests, C-Ring Tests, and Four Point Bent Beam Tests. The two Sour Service grades exhibited high threshold stress values ( ³ 90% YS) whatever the test method that was used. Not unsurprisingly the non-Sour Service grade P110 gave much lower values (<40% YS). The exact threshold stress values depended on the test method, the tensile test being the most conservative of all with a threshold stress below 10% YS. This first part of the study allowed us to rank the test methods in terms of severity.
However, for a non Sour service grade the exact value of the threshold stress itself is of little interest for the user. It is not practical nor even possible to reduce the stress level in service, and therefore the only conclusion that can be drawn from the P110 test results is that the material is not fit for service in the conditions of NACE TM0177 solution A (PH2S 1bar, pH 2.7). More meaningful is the information regarding under which PH2S -pH conditions the P110 grade becomes fully resistant to SSC (threshold stress ³ 90% YS). The remaining part of the study was devoted to obtaining this information through the use of the EFC 16 guidelines.
For decades the Oil and Gas Exploration & Production industry has been faced with the necessity of selecting proper materials to avoid the catastrophic consequences of Sulfide Stress Cracking (SSC). Metals that are SSC resistant, the so-called "Sour Service" materials, are listed in the NACE standard MR0175 1. Although field history remains a major source of experience, a considerable amount of laboratory testing has also been devoted to the effort of building this standard. Various test methods are in fact available to determine the SSC resistance of oilfield equipment such as OCTG and Line-Pipe. These methods which are described in the standard NACE TM0177 2 and EFC publication number 16 3 are not always interchangeable. It is therefore important for the suppliers and the users to be able to compare the SSC resistance of materials tested with different tests methods.