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?
7.2 Recommendations for further work
This thesis is carried out within a limited period of time and with limited access to data. It
is recommended that the conclusion of the thesis be explored further. A range of tasks
may be carried out to provide an even better basis in deciding whether or not a downhole
safety valve (DHSV) should be present in a subsea oil/gas well
Improved calculation model
In the calculation 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 model used in the fault tree calculations assume 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. A better and improved
model could generate a better and more realistic blowout frequency result.
Data accuracy
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. The presented approach is, however, valid and applicable
when new data is found.
Including the consequence
The calculations done in this thesis do not consider the blowout consequences. Although
a blowout occurs it may not always cause severe pollution. None of the blowouts in the
production, workover or installation phase, recorded in ref. [7], has caused severe
pollution. During the production phase only one of seven blowouts caused small
pollution; the other six caused no pollution. A workover blowout is more likely to cause
severe pollution than the other stages of the well. An evaluation of the consequences of a
blowout at the different well life phases should be included in a further evaluation of the
risk reducing effect of the DHSV.
Economic cost evaluation
In this thesis some of the economic aspects related to the DHSV are discussed. Due to a
lack of cost information from operating companies an evaluation of the reduced cost a
removal of the DHSV would represent is not included. A total economic evaluation of the
costs related to the DHSV for the operating company should be performed. The findings
in this thesis have revealed that a substantial amount of money may be saved.
Alternative solutions
In some cases alternative solutions may be more reliable than a DHSV. A further study
developing new solutions and examining other existing solutions is recommended. The
economic cost and the environmental risk related to a frequent workover rate caused by
the DHSV may be reduced.
Diploma thesis, NTNU 2002
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