Fitness for purpose Sulfide Stress Cracking (SCC) testing has been undertaken in a mildly sour environment on thick walled (47.2 mm) linepipe using full ring tests, full thickness 4-point bend (4PB) specimens, standard size and large-scale double-cantilever beam (DCB) specimens. The full ring test method and full thickness 4PB specimens were selected to retain the original surface condition of the linepipe, since an intermittent shallow ‘hard’ skin was present. The KISSC of the material was determined using ‘modified’ NACE TM0177 – Method D DCB specimens with an increased width and length in order to prevent arm bending, which was a concern due to the relatively low strength of the DNV GR450SD steel and comparative tests were also conducted using standard size specimens.
In addition, the test duration for the DCB specimens was increased to 21 days to take into account the mildly sour environment. The arm displacement for the large scale DCB specimens was determined using the KIAPPLIED value and load vs. displacement data. Fitness for purpose testing using full ring and 4PB tests demonstrated the suitability of the linepipe for the set of operating conditions evaluated in this case. Similarly, valid large scale DCB testing results were obtained which met project specific requirements. The paper describes the development of the methodology for the full ring, full thickness 4PB and large scale DCB specimens and the results obtained from these tests.
Key words: Full ring, 4-point bend, 4PB, double-cantilever beam, DCB, sulfide stress cracking, SSC, stress-orientated hydrogen-induced cracking, SOHIC, hydrogen-induced cracking, HIC, soft-zone cracking, SZC, fitness-for-purpose, OTI 95 635, BS 8701, sour service, test method.