Abstract

To facilitate development of future deeper water assets, riser and pipeline designs using current X65 carbon steel requires increased wall thicknesses to manage the demand from large tensile loads. This increase in wall thickness is due to the weight of the pipe body and the fatigue performance at the hang-off and the touch-down point for steel catenary risers (SCR) and steel lazy wave risers (SLWR). The increased wall thickness poses challenges for fabrication, design, installation and project execution due to the single side welding of thick pipe. Increasing the steel grade from the current X65 to X80 or higher strength grade could reduce the pipe wall thickness and provide potential cost savings from total tonnage and is therefore considered as a potential alternative for deeper water steel riser applications. However, it is challenging to meet the hardness acceptance criteria (i.e. 250 HV10) as per DNV-ST-F101 for the higher strength steel weldment, particularly for sour service.

The objective of this study was to understand the effect of hardness on sulfide stress cracking (SSC) resistance of X80 line pipe base material and weldment in sour service environments. A 323.9 mm OD × 25.4 mm WT seamless quenched and tempered X80 line pipe was used for this study. Coupons were extracted from the pipe and then heat-treated by welding simulator (Gleeble™). Implementing controlled cooling, hardness values ranging from 270 to 350 HV10 were achieved, and representing the heat affected zone (HAZ) hardness. Four point bend SSC test were performed on coupon halves as per NACE TM-0316, loaded to 80% actual yield strength (AYS) and tested for 30 days in selected NACE Region 2 and 3 environments as specified in NACE MR0175/ISO 15156-2. Coupons showed no indication of SSC cracking for the samples with measured hardness values of ∼320 HV10 in the selected NACE Region 2 and 3 environments.

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