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? Probability of dropping an object and hitting the x-mas tree Prior to the workover operation the workover rig is located at the site above the well. During the period of preparation until the BOP is connected to the wellhead, there is a possibility of dropping objects. This may result in a destruction of the x-mas tree and result in a blowout. If this happens the downhole safety valve (DHSV) will play an important role. The DHSV will seal off the well and prevent leakage. This scenario is, however, reasonably unlikely to happen. According to ref. [3] the probability of dropping any crane load and hit something within 25 meter radius at 1000meters depth is 9,17E-3, for a derrick 7,84E-4. In addition there is a probability of dropping a crane load of 2.0E-5. The probability of dropping a BOP or a compact object from the derrick is 5.75E-4. The x-mas tree is covering an area of about 4 square meters on the seabed. This is about 0.2 percent of the area for the crane load and 5 percent of the area for the derrick. In total the frequency of dropping something from a crane or a derrick and hitting the x-mas tree at the seabed is 2.29E-8 per year. These calculations have not considered the impact the hit will have and does not include the effect of water currents. Currents contribute significantly to the total spreading. [3]. Although the wellhead is hit, the impact may also vary. Not all hits end with a broken x-mas tree. If the use of safety integrity level (SIL) (see section 3.2) for a low demand system is required, the probability of dropping an object and hitting the something on the seabed is fulfilling the requirements of SIL4. A dropped object damaging the x-mas tree and providing a need for the DHSV to prevent a blowout is therefore by any means within the acceptance criteria. Diploma thesis, NTNU 2002 16
Is it necessary to install a downhole safety valve in a subsea oil/gas well? 5 Risk reducing effect of a DHSV The presence of a downhole safety valve (DHSV) shall reduce the risk of blowout in a subsea production well. In this chapter, the risk reducing effect of a DHSV is found. A case study of a subsea production well is included as an illustrative example. A comparison of unavailability calculations for a well with, and without a DHSV proves the positive effect of the DHSV during production. Barrier diagrams and fault trees are constructed to provide a basis for the calculations and illustrate the leakage paths. 5.1 Case example The studied well is composed out of standard well components. A technical description of the different components is given in appendix A. The study is limited to the downhole and x-mas tree configurations; topside equipment, manifolds and riser systems are not included. To be able to calculate the risk with and without a DHSV in the well, the well lifetime is assumed to be 15 years. This generates in most cases a conservative value for the non-tested barriers. Periodic testing is carried out to reveal hidden failures in the system. For safety reasons the most critical functions, including the DHSV, are tested every 6 months. Other parts of the system are not tested. The tested functions are: Closing, opening and leak tight function of the DHSV Closing, opening and leak tight function of the master valve Closing, opening and leak tight function of the production wing valve 5.1.1 Production tree The production tree in the example is a horizontal tree, as illustrated in Figure 5-1. A production tree is an assembly of valves. It provides control of the well flow during production and is capable, among other things, of cutting of the flow from the reservoir. In the risk assessment the failure modes, external leakage (EXL), fail to close (FTC), internal leakage (ITL) and leakage in closed position (LCP) are considered for the different valves. Diploma thesis, NTNU 2002 17
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Page 69 and 70: APPENDIX A A1 Subsea production sys
Page 71 and 72: A1.3 Well completion A1.3.1 Tubing
Page 73 and 74: A1.5.2 Guidelineless system Several
Page 75 and 76: 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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