API TR 17TR3-2004 pdf download.An Evaluation of the Risks and Benefits of Penetrations in Subsea Wellheads below the BOP Stack.
1 Purpose The purpose of this study is to provide an evaluation of the risks and benefits of allowing penetrations in subsea wellheads below the blowout preventer (BOP) stack so annuli other than the production tubing (commonly referred to as the “A” annulus) could be monitored. Current industry standards (API Spec 17D and ISO 13628-4) for the design of subsea wellheads prohibit penetrations below the (BOP) stack. In contrast, Minerals Management Service (MMS) regulations (30 CFR 250.517) require that all annuli be monitored for sustained casing pressure and that every occurrence of sustained casing pressure be reported immediately to the District Supervisor. 2 Scope 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).
3 Background 3.1 Regulatory Requirements The current industry-standard design philosophy of subsea wellheads prohibits penetrations below the BOP stack. This is codified in the current standards (API Spec 17D and ISO 13628-4). The “no penetrations” language was instituted by the authors of the first edition of API Spec 17D based on intuition and industry practice. No formal reliability analysis (qualitative or quantitative) was performed to ascertain the risks and benefits of allowing penetrations in the wellhead housing below the BOP stack. At a high level, the authors of the standard were concerned with protecting the integrity of the well during drilling operations by preventing the likelihood of a leak below the BOP through a penetration. The apparent concern was that introducing a penetration below the BOP could lead either to a well-control incident caused by loss of the hydrostatic head of drilling fluid in the riser above the BOP or the exacerbation of an existing well-control incident by loss of integrity below the stack. Standard subsea wellhead designs have always provided for monitoring of the “A” annulus for pressure by means of an annulus monitor line in the tree’s production control umbilical and/or an electronic pressure sensor in the tree’s annular flowpath.
However, historically, subsea wellheads have not provided any annular access to outer casing string cavities once a wellhead packoff has been installed after cementing operations. In 1989, the MMS established regulations, described in 30 CFR 250.517, for sustained casing pressure. These regulations required that all annuli be monitored for sustained casing pressure and that every occurrence of sustained casing pressure be reported immediately to the District Supervisor. In 1991, in an effort to streamline government and reduce burdensome paperwork, the MMS issued a letter that dictated changes in the sustained casing pressure policy. The revised policy allowed for continued operation if: • The sustained casing pressure is less than 20% of the minimum internal yield pressure (MIYP), and • The casing pressure bleeds to zero in 24 hours or less when bled through a ½-inch needle valve. If both of these requirements were met, the lessee was not required to submit the diagnostic test results to the MMS for review and approval for continued operation. Wells meeting both of these criteria were placed into a separate category and referred to as Self-Approved. Records of each diagnostic test must be maintained for each casing annulus with SCP. The records must contain: • Identification of the casing annulus, • SCP value at beginning of test, • Pressure chart or time required to bleed pressure down to zero shown on the gauge, • Type of fluids bled, • Volume(s) of liquid(s) recovered, • Pressure build-up chart or pressure recorded at least once per hour, • Shut-in and flowing tubing pressure, • Producing rates of gas, oil, and water, and • Well status.