Is it necessary to install a downhole safety valve in a subsea ... - NTNU

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Is it necessary to install a downhole safety valve in a subsea oil/gas well? Preface This report presents the results of a diploma thesis by Stud. Techn. Rune Vesterkjær, completed at the Department of Production and Quality Engineering, Norwegian University of Science and Technology. The work has been performed from January 2002 through May 2002. The evaluation, analysis and calculations performed have been subjected to a number of assumptions, limitations and definitions of system boundaries, all of which are stated further in the report. The author will accept no liability for conclusions being deduced by readers of the report. The results derived in this report are based on a limited amount of sources. Caution should be taken when using the results from this report further. A greater amount of reliability data must be gathered to make use of the values calculated in the report. I would like to thank my supervisors Prof. Marvin Rausand, the Norwegian University of Science and Technology and Geir-Ove Strand, ExproSoft AS for valuable comments and contributions to this diploma thesis. I would also like to thank Stein Børre Torp at Statoil Åsgard RESU for providing information concerning well intervention, Ivar Ove Endresen at Statoil for providing HAZOP information, and the rest of the staff at ExproSoft AS for helping me when needed. Trondheim, 2002-06-05 Rune Vesterkjær Diploma thesis, NTNU 2002 I
Is it necessary to install a downhole safety valve in a subsea oil/gas well? Summary and conclusions The overall objective of this diploma thesis is to develop an understanding of the contribution a downhole safety valve (DHSV) represents to the overall risk in a subsea oil/gas well. The contribution the DHSV represents to the overall risk during installation, production and well intervention is considered. Norway and the United States of America have specific requirements of subsurface safety devices like the DHSV. There are no specific requirements in the UK regulations for a DHSV. The Norwegian Petroleum Directorate (NPD) requires that there at all times shall be at least two independent and tested well barriers during well activities. Other countries have similar requirements. Acceptable level of risk in an activity is described by acceptance criteria. Combining acceptance criteria with the “ALARP”-principle solves acceptable risk problems. The Norwegian Oil Industry Association has developed a list of minimum safety integrity levels (SIL). The SIL requirement concerning the shut-in of the flow in a well (Wing valve, Master valve and DHSV) is set to 3. It is therefore reasonable to require the SIL of the well without a DHSV to be the same. There are two main risk factors regarding oil/gas production, delays in time (lost production) and the blowout risk. Lost production occurs when the well is unable to produce as expected due to different problems, and is equal to economic loss in oil production. Risk related to the installation and completion of a subsea oil/gas well is mainly related to blowout risk and time delays. Production related risk comprises economic and environmental risk. The workover risk is represented mainly by time delays and the blowout risk. A case example is included illustrating the effect of a DHSV. A comparison of unavailability calculations for a well with, and without a DHSV proves the effect of a DHSV. Barrier diagrams and fault trees are constructed providing a basis for the calculations and illustrating the leakage paths. Fault trees display the interrelationships between a potential critical event in a system and the reasons for this event. The ‘TOP’ event of the fault tree analysis in this study is formulated, “Sustainable leakage to the surroundings through either the x-mas tree or the wellhead during normal shut-in conditions.” Unavailability calculations are done in regard to the two exampled situations. The Mean Fractional Dead Time (MFDT) model is applied in the calculations. MFDT can be given two different meanings; the percentage of time where we are unprotected by the safety function, or the probability that the safety function will fail on demand. An introduction to well intervention methods and equipment is given in the thesis. There are two types of well interventions, light and heavy (also known as workover). A blowout preventer (BOP) system is a set of valves installed on the wellhead to prevent the escape of pressure either in the annular space between the casing and tubing during drilling, completion and workover operations. HAZOP (Hazard and Operability analysis) is a method used to identify and assess problems that may represent risks to personnel or equipment, or prevent efficient operation. Essentially Diploma thesis, NTNU 2002 II
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APPENDIX A A1 Subsea production sys
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A1.3 Well completion A1.3.1 Tubing
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A1.5.2 Guidelineless system Several
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Appendix B In this appendix a figur
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Appendix C Fault trees constructed
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Production Wing valve ITL WV Produc
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Component: Production Packer Failur
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Reliability data dossier Component:
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Component: Production wing valve Fa
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Component: Crossover line Failure r
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Appendix F The guide-words used in
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Is it necessary to install a downhole safety valve in a subsea ... - NTNU

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