ODonnell Consulting

Mitigating Catastrophic Failures in Structures Used in Offshore Oil & Gas Production

“Mitigating Catastrophic Failures in Structures Used in Offshore Oil & Gas Production” January, 2011 Carl E. Spaeder, William J. O’Donnell The United States Department Of The Interior Bureau Of Ocean Energy Management Regulation And Enforcement, Boemre Contract No. M10PC00061

Keywords: yield strength; steel; offshore platforms; safety margins; structural integrity; plastic limit load; structure failures

This project was launched in early 2010 by the Bureau of Ocean Energy Management Regulation and Enforcement (BOEMRE) of the United States Department of the Interior, formerly the Minerals Management Service. The initial intent of this work was to evaluate the acceptability of the industry’s use of the mean yield strength of steel instead of the minimum specified yield strength in the structural evaluation of offshore platforms. This practice was evaluated herein and was found to reduce the safety margins against plastic collapse during overload by about 25 percent. Such safety margin reduction is unacceptable. Further work herein focused on efforts to assure the structural integrity of offshore structures using recent developments in materials, design and inspection technologies.

Structures, including oil and gas offshore structures, are designed to resist loads with a safety factor against the plastic limit load. Codes and Standards safety margins are based on the assumption that the yield strength of the steel is the manufacture’s specified minimum yield strength. Studies of mean properties vs. minimum specified properties show that the mean properties are about 25 percent higher. In fact the ASME Code uses minimum specified yield strength values equal to 80 percent of the mean values. Accordingly, use of the mean yield strengths in lieu of the minimum specified values would reduce the safety margins for overloads by 25 percent.

Between 2005 and 2010, approximately 300 out of 3,000 oil and gas related structures in the Gulf of Mexico failed. Some of these failures disrupted energy production, impacted the environment, and resulted in costly clean up and removal efforts. BOEMRE has undertaken several studies to identify and evaluate methods to help prevent future offshore structure failures.