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? 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. Reduced blowout frequency With a DHSV Without a DHSV Risk reducing effect Normal production 4.47E-3 1.85E-2 1.14E-2 Accidental event 1.30E-9 2.29E-8 2.16E-8 Workover 1.7E-4 8.5E-5 -8.5E-5 Total 1.13E-2 In total a removal of the DHSV will increase the risk of blowout due to the ‘TOP’-event by a frequency of 1.13E-2 per well year. A sensitivity analysis is performed to illustrate the accidental event contribution in the total blowout frequency. Different frequencies of accidental events are applied to Equation 5-9. The result of the sensitivity analysis is presented in Figure 5-8. The total blowout risk is always higher for a well without a DHSV than for a well with a DHSV. Prior to the calculations the author was assuming that for some values the blowout frequency was lower for a well with a DHSV than for a well without a DHSV. The point where the two lines cross would represent an equal blowout frequency in the sensitivity analysis. Figure 5-8 Sensitivity analysis of the total blowout frequency at different frequencies of accidental events per well year Diploma thesis, NTNU 2002 34
Is it necessary to install a downhole safety valve in a subsea oil/gas well? 5.5.1 Comments to the risk reducing findings The author was expecting to find a blowout frequency for a well with a DHSV similar to ref. [7], 5E-5 per well year. The frequencies in ref.[7] are based on actual events. The blowout frequency found in the CARA-calculations is about 100 times larger. It is reasonable to believe that the calculations in this report are either based on insufficient data, a bad model or calculation errors. Other factors may also have contributed to the high frequency. Reliability data The author has limited access to valid data. The reliability data used in the calculations is gathered for both platform and subsea wells. Valid data only for subsea wells should be applied in the calculations. Another problem is the age of the applied data. All data in this thesis is more than five years old due to the confidentiality of the operating companies. Implementing new up-to-date data may change the outcome of the total blowout frequency and is recommended. Calculation model In the model of this thesis the blowout frequency during production is based on the assumption that all leakage classifies as a blowout. This generates a very conservative value. The leakage may in some cases be very small and not classify as a blowout. The frequency of the ‘TOP’event of the CARA calculations are assuming that a leakage in closed position and fail to close failure of the wing valve leads directly to sea. If the safety barriers in the flow direction fail there are still barriers further up the line that may prevent a blowout, e.g. the manifold, separator or other equipment. The calculation methods used are, however, valid. Alternative calculation method To illustrate the effect of the DHSV a calculation based on the blowout frequency found in ref. [7] will be performed. It is obvious that the assumption that the all occurrences of the ‘TOP’event result in blowout is wrong. If the applied reliability data for the barriers is correct the proportion between the frequency of the ‘TOP’-event with and without a DHSV will be reasonable. The frequency of the ‘TOP’-event in CARA is 3.3 times higher, for a well with than a well without a DHSV (see subsection 5.4.4). It may therefore also be reasonable to operate with a production blowout frequency 3.3 times higher. A blowout frequency of 5E-5 per well year during production [7] is applied for a well with a DHSV. This results in a production blowout frequency of 1.65E-4 per well year for a well without a DHSV. The same values for the accidental event frequency and the well intervention frequency are applied. In total a removal of the DHSV will then increase the blowout frequency of 3.0E-5 per well year. This result sounds more reasonable than the one found in the other calculation. Diploma thesis, NTNU 2002 35
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Page 43: Is it necessary to install a downho
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
Page 77 and 78: Appendix C Fault trees constructed
Page 79 and 80: Production Wing valve ITL WV Produc
Page 81 and 82: Component: Production Packer Failur
Page 83 and 84: Reliability data dossier Component:
Page 85 and 86: Component: Production wing valve Fa
Page 87 and 88: Component: Crossover line Failure r
Page 89: Appendix F The guide-words used in

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

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