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

Is it necessary to install a downhole safety valve in a subsea oil/gas well?
7 Conclusions and recommendations for further
work
7.1 Conclusions
The overall objective has been to determine whether or not a DHSV should be installed in
a subsea oil/gas well from a risk point of view. A conclusion is presented in this section.
Based on the quantitative findings in this study the DHSV reduces the risk of blowout
with approximately 50%. A removal of the DHSV represents an increased failure
probability, and two independent and tested well barriers are not present in all cut sets.
None of the cut sets do, however, violate a required SIL3 level when the DHSV is
removed from the completion.
The blowout frequency caused by the DHSV during workover is 1.7E-4 per well year,
and 8.5E-5 per well year for a well without a DHSV. The blowout frequency during
production is based on the assumption that all occurrences of the ‘TOP’-event, “leakage
to the surroundings”, classify as a blowout. During production the blowout frequency is
found to be 4.47E-3 per well year for a well with a DHSV, and 1.85E-2 per well year
when the DHSV is removed. In addition to the blowout frequency during production and
workover a blowout frequency caused by accidental events should be included. The total
blowout frequency is found adding up the different contributions:
ftotal = fproduction + fintervention + (faccidential event * MFDTaccidential event)
The DHSV blowout frequency contribution during installation is not included in this
study. There is no data revealing the changes in blowout frequency when the DHSV is
removed for the installation phase.
The blowout frequencies found in the calculations are of very high. In total a removal of
the DHSV increases the blowout frequency by 1.13E-2 per well year. It is reasonable to
believe that the calculations in this report are either based on insufficient data, a bad
model or calculated errors. Other factors may also have contributed to the high frequency.
An alternative calculation method based on the proportion of the frequencies of the
‘TOP’-events, and the experience data found in ref.[7] is applied. In the new calculations
a removal of the DHSV will increase the blowout frequency of 3.0E-5 per well year. The
author finds this result more reasonable.
50% of the shut-ins of a well leading to a workover are caused by a DHSV failure. A
DHSV failure requires a workover generating a loss of oil production of up to $5,6
million. In addition there will also be expenses concerning the rental of a workover rig. A
total cost of 20 million dollars per intervention is therefore not unrealistic.
The author cannot recommend removing the Downhole safety valve (DHSV) from a
subsea oil/gas production well based on the findings in this thesis. Although there may be
some economic advantages in removing the DHSV the risk of blowout should be given
the greatest attention. In addition to causing pollution, the occurrence of a blowout may in
severe cases lead to a bad reputation among consumers and environmental organisations.
Diploma thesis, NTNU 2002
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