The American Petroleum Institute (API) contracted with Stress Engineering Services (SES) to analyze the risks and benefits inherent in continuing to prohibit penetrations in subsea wellheads and compare them to those introduced by allowing the practice. Special attention was paid to the risk and benefits introduced by monitoring annuli other than the 'A' annulus (the annulus between the production tubing and the production casing strings). The 'risk-based' portion of this specific study did not need to be fully quantitative, but the analysis was done in a way that will easily lead to a fully quantitative analysis as more complete data become available. The scope of this study is limited to completed conventional subsea wells in the Gulf of Mexico (GOM). This report documents the results of this study of the risks and benefits of additional penetrations in subsea wellheads below the BOP stack for the purpose of monitoring additional casing annuli for sustained casing pressure (SCP).