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? Annulus swab valve Annulus wing valve Control panel Annulus master valve Tubing hanger seal Annulusbore Casing hanger seals Hydraulic control line Tree cap Swab valve Figure 5-2 A sketch of the production well used as an example in this thesis Diploma thesis, NTNU 2002 Crossover valve Wing valve Production master valve Tubing hanger seal (productionbore) SEA BED Wellhead seal Tubing hanger Casing hanger seals DHSV Seal assembly Liner hanger seal Production packer 20
Is it necessary to install a downhole safety valve in a subsea oil/gas well? 5.2 Barrier analysis The theory presented in this section is based on reference [7]. A well barrier is defined as: “An item that, by itself, prevents flow of the well reservoir fluids from the reservoir to the atmosphere” [26] A well barrier system will vary depending on the operational phases of the well. The barrier analyses are carried out to identify potential leakage paths and are based on barrier diagrams. Components provide safety barriers when working as supposed to. There are two main types of barriers, dynamic barriers and static barriers. A static barrier is a barrier available over a “long” period of time. A dynamic barrier is a barrier that varies over time. This will apply for drilling, workover, and completion operations. The barriers are normally denoted as primary, secondary and third barrier. It can be stated in general that the primary barrier is the one in contact with the reservoir. For completion and workover, in a shut-in well, the hydrostatic pressure is regarded as the primary barrier, and the subsea equipment, usually a BOP, is regarded as the secondary barrier. During production the downhole safety valve (DHSV) among others acts as the primary barrier and the x-mas tree as the secondary. Barrier diagrams are easy to understand and provide a good view of the barrier situation. Possible leakage paths between the reservoir and the environment have to be identified to establish a barrier diagram. The diagrams are read from the reservoir at the bottom and up through to the surroundings. In the construction of barrier diagrams the safety barriers are represented with rounded rectangles and connected with lines. To make the diagram easier to understand triangles and colour codes are added representing different areas of the well. Barrier ratings are also presented with a colour code. This is not standard, but gives the reader a better overview of the diagram. The leakage probability depends on each barrier and the structural relationship between the barriers. If reliability data for the various components are added, a total leakage probability is possible to assess from the diagram. The complexity in calculation increases with the complexity of the situation. Therefore barrier diagrams are often transferred to fault trees; these are commented in section 5.3. Chapter two of this thesis deals with regulations regarding well barriers. The Norwegian Petroleum Directorate states: “During drilling and well activities there shall at all times be at least two independent and tested barriers after the surface casing is in place “[29]. Other countries have similar requirements. 5.2.1 Case example In the case example the barrier situation, “to prevent leakage to the surroundings during temporary shut-in of a production well”, is modelled. The downhole safety valve (DHSV), the Diploma thesis, NTNU 2002 21
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Page 69 and 70: APPENDIX A A1 Subsea production sys
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Page 73 and 74: A1.5.2 Guidelineless system Several
Page 75 and 76: Appendix B In this appendix a figur
Page 77 and 78: Appendix C Fault trees constructed
Page 79 and 80: 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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