In late 1999, BP plc experienced a well failure in the Marlin development in deepwater Gulf of Mexico. Within hours of starting production, the production tieback casing collapsed, causing failure of the production tubing. Pressurization of outer annuli because of production thermal effects was identified as the most likely cause of the failure.
IntroductionThe industry has reported cases of annular pressure buildup (APB) in annuli for several years. 1 In land, platform, and spar-type wells with access to annuli, APB is usually handled by bleeding off annular pressure as needed. Subsea completions, however, do not allow this capability, and the technology to provide access is still being studied. Especially susceptible to APB are deepwater developments in which the differential between mudline and flowingproduction temperatures can exceed 125°to 200°F.Three primary mitigation techniques were employed to help reduce the impact of APB on the Marlin project. The first used an enhanced casing design capable of withstanding higher pressure conditions. In the second method, a burst disk was installed in the outer casings to provide a controlled leak path. The third technique involved the application of nitrified cementing spacers in the annulus to provide a compressible cushion and to help reduce the effects of temperature expansion.Service company and operator engineering staffs worked proactively and integrated their efforts to identify, evaluate, plan, and implement multiple options for resolving this substantial wellintegrity issue. This article reviews the problems associated with APB, describes the large-scale testing conducted in the Marlin project, and discusses the best practices developed to help prevent APB from affecting casing design. These best practices were successfully implemented on the Marlin subsea development, and other projects are also using these techniques.