ABSTRACT An incident with a sour oil pipeline some 15 years ago was responsible for defining the measures necessary to give protection from hydrogen induced and sulphide stress cracking (HIC and SSCC) for pipelines in simmilar duties. These principles are still appropriate today and the considerations necessary when specifying and installing pipelines for aggressive gas duty are discussed for two current field development schemes. INTRODUCTION - A SUBMARINE PIPELINE FAILURE In 1972 a failure occurred in a recently commissioned 30 inch, x60 grade submarine oil pipeline in the Arabian Gulf. Oil on the water led divers to locate a 3½" longitudinal crack in the pipe 9" from a girth weld and almost diametrically opposite the longitudinal weld (Fig. 1). The 40 ft. length of pipe containing the crack was cut out and replaced with a spare length and the pipeline was satisfactorily recommissioned. A 3 ft. length of pipe containing the crack was received in U.K. for examination and a section through the crack is shown in Figs. 2 and 3 from which the "stepped" nature of the crack can be seen. An ultrasonic examination of the pipe section around the crack showed significant areas of laminar defects as illustrated in Fig. 4. Micro-sections in Figs. 5 to 7 illustrate the "stepped" nature of the cracking, the "banded" microstructure of ferrite and pearlite and the cracks between individual steps. Elongated type II manganese sulphide inclusions were in evidence throughout the microstructure (Fig. 8). This is believed to have been one of the first failures of a controlled rolled steel due to what is now described as "hydrogen pressure induced cracking" (HPIC or HIC) or "stepwise cracking". Factors which contributed to the failure of this susceptible steel are related to the commissioning of the pipeline. Hydrotest water was being displaced by a pig propelled by sour crude oil when the pig became stuck. During the period when attempts were being made to re-start the pig, H2S from the crude was able to pass the pig and dissolve in the hydrotest water (seawater) and produce an environment capable of cracking this steel. After the pig had been moved and the water displaced, oil leaked through the crack and was spotted on the surface of the sea shortly afterwards. The pipeline has operated satisfactorily since replacement of the cracked section. However, it must be anticipated that there are other laminar cracks present which have not propagated through the pipewall because only dry crude oil has been transported. During the failure investigations, a test was developed which, within 96 hours, would reveal the susceptibility of a steel to this type of cracking. The test was based on the conditions which led to failure of the pipeline, exposure to seawater saturated with H2S, and became known as the "BP" or "Cotton" test. The test procedure has now been developed and specified sufficiently to become an internationally recognised standard; NACE STANDARD TM-02-84-''Test Method for Evaluation of Pipeline Steels for Resistance to Stepwise Cracking".