Leakage Test Criteria for Subsea Christmas Tree Valves
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This Master`s Thesis deal with integrity and barrier philosophy. A special focus has been on internal leakage through barrier elements. There is an increasing discussion about what a primary and secondary barrier is, how many barriers there should be, their permitted internal leakage acceptance criteria and their integrity. Subsea Christmas tree (XMT) is considered as a pressure barrier element in subsea production systems. Rules and regulations require frequent leakage testing of subsea XMT valves. Applicable standards include NORSOK D-010, ISO 10423 and 13628-4 and API RP 14B. Due to the remoteness, of the subsea well head, testing is more complicated compared to testing of platform trees (dry trees). Applicable standards specify a maximum permitted leakage rate. However, when considering testing of valves using small fluid test volume the model has limitations. A challenge is to determine if the leakage test criteria are fulfilled or not. Leak testing of subsea XMT valves is an engineering challenge. Engineers must meet leak rate standards as well as they have to understand all aspects of the leak testing procedure. Operators address a question of whether or not their correlation tool of pressure to leak rate is accurate enough. Another question regards the permitted internal leakage acceptance criteria governed by API RP14B; how to apply the leakage criteria to small test volumes – typical XMT volumes. By this Master`s Thesis, an issue that does not have an ideal solution today is highlighted. The issue that operating companies faces is: In questionable situations whether a valve has fulfilled the criteria or not – how to apply the leakage criteria to small test volumes, how to decide whether a XMT have to be pulled, how to increase planning security, how to save money, time and estimate the lifetime of different valves on the XMT. The problems that the Operating Company meet regarding leakage testing of XMT valves may be that the differential pressure across the valve is lost in a couple of minutes. This questions the leak test procedure; is the permitted leak rate acceptance criteria or the test interval too large? To be on the safe side when evaluating leakage tests, the Operating Company has included new permitted internal leakage acceptance criteria into their work procedures. Service personnel fear that such a strict acceptance criteria can result in an unmanageable situation with a numerous of exceptions. Another problem that may be faced is that there is no procedure of when to initiate the leakage test. This results in difficulties in comparing historical and recent leakage tests. Also, this can result in a leakage test that shows a smaller leakage than it actually do. Data from previous leakage tests of production Master – and –Wing valve on the XMT are collected. The data are used to evaluate today`s procedure of leakage testing and for correlating pressure to leak rate. Both, the new internal leakage acceptance criteria that the Operating Company uses and the permitted internal leakage acceptance criteria governed by API RP14B are evaluated. Today, a basic tool for correlating pressure to acceptable leak rate exists. From the technical advisors point of view it is desirable with a more accurate software tool that can correlate pressure to leak rate. To evaluate today`s practice of correlating pressure to leak rate the MultiProScale simulation tool is used. It is assumed that the simulation tool correlates the most correct leak rate due to the complexity of the simulation tool. However, this Master`s Thesis shows that such a powerful tool is not needed for correlating pressure to leak rate. Based on this Master`s Thesis, the contribution to the leakage testing procedure is an improved calculation tool of correlating pressure, temperature and compressibility changes to actual leak rate. A presentation of this Master`s Thesis is presented for the Technical advisor of Well Technology, Hilde Brandanger Haga at Statoil. She agreed with the challenges and solutions that were presented. The improved calculation tool was also introduced. A proposal was presented that not only could the calculation tool be used onshore, but it could be used to show the actual leak rate during leakage testing of the PMV and PWV offshore. The advantage with this tool is that it can ensure the competence, it helps the decision-making process, it increases planning, it decreases cost and safeguard barriers. Other contributions are recommended changes both to the leak testing procedure and the data documentation. In relation to both of the permitted internal leakage acceptance criteria, the main conclusions are: To apply the internal leakage acceptance criteria governed by API 14B to the subsea XMT, the leakage time interval have to be reduced from 10 – to 2 minutes. This will not result in an increased number of valves that does not fulfill the acceptance criteria. Or to apply the acceptance criteria governed by API RP14B, the test volume has to be > 50 liters. And, the additional internal leakage acceptance criteria that are implemented by the Operating Company are stricter than the one governed by API RP14B. A decrease in time interval from 10 – to 2 minutes gives an increased number of valves that do not fulfill the permitted internal leakage acceptance criteria.