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?
The risk of terror attacks on an oil installation is always present and represents an increasing
threat today. We have seen multiple terrorist-attacks on different target the latter years. The oil
and gas industry is very profitable and may very well be subject to a striking terror attack. The
fatal consequences related to such an attack might not be the major concern of the terrorist.
4.2 Installation related risk
Risk related to the installation and completion of a subsea oil/gas well is mainly related to
blowout risk and time delays. During the installation phase equipment may be broken or
dropped objects causing a delay. Human risk in form of injured personnel or fatalities may
occur. Normal focus on safety in operations and at the site will prevent this from happening.
Completion blowouts occur during installation of downhole and subsea equipment. There have
been seven blowouts concerning the completion phase of the wells during the study in ref. [7].
Most of the blowouts resulted in flow through the tubing or drill string. Holand points out that
for several of these blowouts the BOP-stack (Blowout preventer) did not include a blind-shear
ram. The presence of a functional shear ram would have stopped many of these blowouts at an
early stage and thus prevented the blowout. None of the completion blowouts have caused any
casualties. One blowout caused severe damage when ignited; otherwise there have only been
minor spillage.
During installation the presence of a DHSV makes the procedure more complex. The DHSV
may be scratched or deformed by the pressure and not function. A hydraulic control line is
strapped to the tubing, which increase the installation time. This will not result in any
significant change of blowout risk. Other than the increased installation time the author cannot
se any other inconveniences with the installation of a well with the DHSV.
4.3 Production related risk
Production related risk comprises economic and environmental risk. There are no personnel
present at the site during production and therefore the human risk is neglected.
Production blowouts occur in producing or mechanically closed in production or injection
wells. The SINTEF offshore blowout database includes 12 production blowouts. External
forces like ships collision, storm and fire, caused half the blowouts. The other half was caused
by mechanical failure in the primary and secondary barriers. Five of the six blowouts blew in
the x-mas tree and the last came outside the casing. Only one of the production blowouts has
caused small pollution. There have been no casualties. The blowout outside the casing caused a
subsea crater that tilted the platform and was the only severe damage. A $220 million
insurance claim was the result of this incident.
During normal production the DHSV may fail. The failures occurring are categorised into two
groups, critical and non-critical failures. Critical failures are fail to close (FTC), leakage in
closed position (LCP) or internal leakage (ITL). These failures will, however, not cause the
well to stop the production, but are significant if the DHSV is supposed to fulfil its task.
Premature closure (PC) and fail to open (FTO) will stop the production. If a failure occurs in
between the workover intervals the operating company must force open the DHSV by wireline,
proceed with a full workover or close in the well. The severity of this incident varies according
to when it occurs in the well lifetime. In some cases if failure occurs at the end of the lifetime.
The well pressure may be low and the well may be producing e.g. 80% water. An application
may than be sent to the governmental authorities obtaining a dispensation to produce without a
Diploma thesis, NTNU 2002
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