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? List of figures Figure 3-1 Levels of risk and the ALARP principle [32] .......................................................... 9 Figure 4-1 Consequence breakdown structure of the installation of a subsea production well11 Figure 4-2 Consequence breakdown structure of normal production on a subsea production well................................................................................................................................... 11 Figure 4-3 Consequence breakdown structure of a workover on a subsea production well.... 12 Figure 5-1 Horizontal X-mas tree from the Åsgard field [24] ................................................. 18 Figure 5-2 A sketch of the production well used as an example in this thesis......................... 20 Figure 5-3 Barrier diagram for an oil/gas producing well with a DHSV................................. 23 Figure 5-4 Barrier diagram for an oil/gas producing well without DHSV .............................. 24 Figure 5-5 A plot comparing the approximation and the general formula of unavailability. .. 27 Figure 5-6 The parallel structure of two tested barriers ........................................................... 29 Figure 5-7 A fault tree illustrating the scenario when the flow has to be shut in due to a crisis .......................................................................................................................................... 32 Figure 5-8 Sensitivity analysis of the total blowout frequency at different frequencies of accidental events per well year......................................................................................... 34 Figure 6-1 A conventional subsea blowout preventer stack .................................................... 38 Figure 6-2 Flow chart of a HAZOP examination procedure (based on [2]) ............................ 44 Figure 6-3 A typical blowout preventer used on subsea wells................................................. 46 List of tables Table 3-1 Experienced overall FAR values for offshore workers in the UK, Norway, and U.S. GoM OCS, January 1980-January 1994 [7]....................................................................... 8 Table 3-2 Safety integrity levels for safety functions operating on demand or in a continuous demand mode from IEC 61508-1, Table 2 and 3)[28]..................................................... 10 Table 3-3 Minimum SIL requirements - global safety functions, an extract [28] ................... 10 Table 4-1 Blowout frequencies as input to risk analysis of offshore installations, both subsea and platform wells [7] ...................................................................................................... 13 Table 5-1 The unavailability of the ‘TOP’-event for a well with and without a DHSV at different points of time t and the annual average calculated in CARA............................ 30 Table 5-2 The unavailability of selected cut sets concerned by the presence of a DHSV....... 30 Table 5-3 Calculation result of the cut sets with an applied lifetime of t=5years, t=10years and t=15years .......................................................................................................................... 31 Table 5-4 Increased unavailability of the different cut sets when the DHSV is removed ....... 31 Table 5-5 The blowout frequency per well year for a well with and without a DHSV, and the risk reducing effect of the DHSV..................................................................................... 34 Table 6-1 Advantages and disadvantages with the HAZOP analysis [9] (with some supplements of the author.) .............................................................................................. 43 Table 6-2 HAZOP work sheet of the BOP handling procedure............................................... 48 Diploma thesis, NTNU 2002 IX
Is it necessary to install a downhole safety valve in a subsea oil/gas well? 1 Introduction Subsea installations are expensive to repair. If a failure occurs a workover rig is needed to perform an intervention. The weather conditions in the area also affect the situation and repairs may take a long time. Accidents have to be avoided, and the production loss kept to a minimum for the situation to be viable. High reliability is therefore essential for all subsea production systems. According to regulations issued by the Norwegian Petroleum Directorate and the U.S. Mineral Management Services (MMS), a Downhole Safety Valve (DHSV) is required in all production and injection wells. The DHSV functions as a safety barrier. If a critical situation occurs the DHSV may shut-in the flow from the reservoir and prevent a disaster. The DHSV protects the surroundings when it functions as intended during production. Studies have shown that 50% of all well interventions are caused by DHSV failure [25]. During well intervention the risk of blowout is increased to an essential degree compared to the production phase. The consequences related to subsea production blowouts are not of the same proportions as for a platform well. Human risk is dramatically reduced when the well is not in direct connection with the platform. All available reliability data is provided mainly from platform wells. Due to the reduced human risk it is therefore reasonable to assume that subsea wells are subject to a lower risk level. Some experts say that the risks related to well intervention caused by DHSV failure is equal or higher than the risk reduction a fully functioning DHSV represents. The need for a DHSV in a subsea well is therefore not considered necessary in a subsea well. A paper was issued in the Society of Petroleum Engineers (SPE) in 1999 [4] concerning this matter. The basis for the statements made here is not well founded [19]. A better and more precise evaluation of the role of the DHSV is needed before a conclusion can be stated. 1.1 Objectives 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. This diploma thesis will focus on the risk related to a DHSV at different phases during its lifetime. The contribution the DHSV represents to the overall risk during installation, production and well intervention is considered. The objectives are as follows: a) Give an overview of the requirements related to safety barriers and downhole safety valves (DHSV), in Norway, the United Kingdom and the United States of America. b) Introduce the reader to the use of acceptance criteria within offshore oil/gas production. Explain the use of the “ALARP”-principle and the Safety Integrity Level (SIL) in solving acceptable risk problems. Diploma thesis, NTNU 2002 1
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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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