API RP 90: FIRST EDITION
DOD Adopted ANSI Approved Approved
This Recommended Practice is intended to serve as a guide for managing annular casing pressure in offshore wells. Although the prevention of annular casing pressure is very important, it is beyond the scope of this document. Prevention of annular casing pressure is planned to be addressed in API RP 65, Part 2. The remediation of wells because of sustained casing pressure is also beyond the scope of this document and is planned to be included in API RP 65, Part 3. This guide is meant to be used for offshore wells that exhibit annular casing pressure, including thermal casing pressure, sustained casing pressure (SCP) and operatorimposed pressure.
This Recommended Practice covers monitoring, diagnostic testing, the establishment of a maximum allowable wellhead operating pressure (MAWOP) and documentation of annular casing pressure for the various types of wells that occur offshore. Included also is a discussion of risk assessment methodologies that can be used for the evaluation of individual well situations where the annular casing pressure is not within the MAWOP guidelines.
This Recommended Practice recognizes that annular casing pressure results in various levels of risk to the safety of personnel, property and the environment. The level of risk presented by annular casing pressure depends on many factors, including the design of the well and the source of the annular casing pressure. This Recommended Practice provides guidelines in which a broad range of casing annuli that exhibit annular pressure can be managed in a routine fashion while maintaining an acceptable level of risk. Annular pressures that do not conform to the guidelines in this Recommended Practice may still have an acceptable level of risk, but they need to be evaluated on a case-by-case basis.
This Recommended Practices establishes an acceptable level of risk for annular casing pressure using three parameters. First, annuli that exhibit annular casing pressure of 100 psig or less present little risk and should be monitored. Second, annular casing pressure that is greater than 100 psig and that has been diagnosed as sustained casing pressure (SCP) must bleed to zero psig. Third, a Maximum Allowable Wellhead Operating Pressure (MAWOP) is established for each non-structural casing annulus that exhibits annular casing pressure, including thermal casing pressure, sustained casing pressure or operator-imposed pressure. If the annular casing pressure does not meet the criteria established in this Recommended Practice, this does not mean that the risk presented by the annular pressure is unacceptable. Rather, it indicates that the annular casing pressure needs to be managed on a caseby- case basis that goes beyond the scope of this Recommended Practice. The case-by-case management of annular casing pressure may include the use of risk assessment techniques. Techniques that may be used for case-by-case risk assessment are discussed in Section 10 of this Recommended Practice. In some cases, the annular casing pressure may need to be reduced or eliminated by well work. In other cases, the risk may be mitigated by other methods. Procedures for eliminating annular casing pressure or mitigating the risk are beyond the scope of this Recommended Practice.
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