In late 1999, British Petroleum (BP) experienced a well failure in the Marlin development in deepwater Gulf of Mexico (GOM). Within hours of starting production, the production tieback casing collapsed, causing failure of the production tubing. Pressurization of outer annuli due to production thermal effects was identified as one of two possible root causes of the failure. The oil and gas industry has been observing, documenting, and reporting cases of annular pressure buildup (APB) in annuli for several years.1 On land, platform, and spar-type wells having access to annuli, the problem can usually be dealt with by bleeding off the 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 temperatures and flowing-production temperatures can exceed 125° to 200°F. A key technique developed during the Marlin project to help prevent APB was the use of nitrified cementing spacers. These spacers provide a compressible cushion that can absorb the pressurization effects caused by thermal heating in the annulus. In addition to carrying nitrogen, the spacers can also provide adequate mud removal and remain stable for at least 72 hr. These features help prevent the migration of nitrogen into the riser prior to setting the wellhead seals. Based on a series of pressure/volume/temperature (PVT) tests and well design simulations, the minimum foam quality of nitrogen was determined. Service company and operator engineering staffs worked proactively and integrated their efforts to identify, evaluate, plan, and implement multiple options for resolving this substantial well-integrity issue. This paper reviews the problems associated with APB, details the large-scale testing that was conducted, and the resulting best practices that help prevent APB from affecting the casing design. These best practices were successfully implemented on the Marlin subsea development, and other projects are also using these techniques.