Factors Governing the Failure of Subsea Critical Connector Bolts

Abstract

Impressed current cathodic protection (ICCP) systems are commonly used to shield offshore drilling rigs, pipelines, and subsea equipment in the oil and gas industry. In underwater service conditions, water temperature, salinity and velocity play a major role in the longevity of subsea applications. Interactions between the preceding factors can induce catastrophic failure to critical systems while the underlying cause is unclear. This paper proposes an approach for elucidating the corrosion process accompanying underwater applications. The service conditions of underwater application are simulated in a multidisciplinary system that records various parameters such as water temperature, reference-electrode potential, and electric current at five-minute intervals during the 21 d of the experiment. This novel, experimental, and inexpensive ICCP system was developed on an “Arduino” microcontroller and applied to an actual ASTM A193 B7 bolt tightened on an ASTM A105 flange at different torque levels. Experimental results indicate a direct relationship between the water day-night temperature profile and the cathodic protection performance. Specifically, the ICCP performance declines with increasing temperature. When the ICCP system was activated, gas bubbles are generated on the metal surface. Presumably these bubbles could induce hydrogen embrittlement cracks which were observed in scanning electron microscope images of the bolt cross-sections.