Environmentally Assisted Cracking of Ultra Strength Low Alloy Steel in Sour Environments

摘要

Catastrophic failures of drill pipes are a major concern in HPHT drilling environments. The National Energy Technology Laboratory, U.S. Department of Energy and DNV, Columbus are investigating mechanisms of crack propagation in ultra-high strength low alloy (UHSLA) martensitic steel drill pipe under cyclic stress as functions of pH, H_2S concentration and temperature in simulated production fluids. These fluids are deaerated 5% NaCl solution buffered with NaHCO_3/Na_2CO_3 or CO_2 in contact with 0.83 kPa H_2S. Microscopic investigations of the pre-cracked sample surfaces after corrosion fatigue tests revealed the presence of additional cracks that had propagated in the bulk material in neutral pH solution at 20°C. This paper discusses effects of the production fluid temperature on susceptibility of the UHSLA steel to environmentally assisted cracking. Post-corrosion fatigue surface characterization methods, such as optical and scanning electron microscopy, were used in this study.