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?
analysis and a useful indicator in an overall risk approach. It should be noted that FAR values
for specific installations could never be verified through experienced fatality statistics. Low
probability incidents with a high number of fatalities will have high influence on the estimated
FAR value. The Piper Alpha and the Alexander Kielland accidents completely changed the
FAR values for the entire North Sea. The FAR represents an average for all offshore workers;
it is obvious that the value will vary from the drilling crew to the catering personnel on the
platform. Table 3-1 presents the experienced FAR value for offshore workers.
Table 3-1 Experienced overall FAR values for offshore workers in the UK, Norway, and U.S. GoM OCS,
January 1980-January 1994 [7]
Area Conditions for the FAR calculations FAR (fatalities per
10 8 working hours)
UK
Total FAR (incl. Piper Alpha) 36.5
Total FAR (disregarding Piper Alpha) 14.2
Norway
Total FAR (incl. Alexander Kielland) 47.3
Total FAR (disregarding Alexander
Kielland)
8.5
US GoM OCS Total FAR 9.0
Environmental risk
The environmental risk is mainly expressed by a spill rate. The spill rate is denoted by
discharges per kg or m 3 per ton produced material. A blowout in Nigeria (1980) is the most
severe incident reported of spills from an installation. 30,000 tons of crude oil polluted the
islands and channels of the Niger delta. In the North Sea or the U.S. GoM OCS there has been
no severe pollution caused by oil spills to sea.
Environmental acceptance criteria must consider the overall risk through the lifetime of a well.
An evaluation of the surrounding environment, the effect of other installations in the area and
the possibility for immediate help in case of emergency must be performed. The use of risk
matrixes and superior criteria, e.g. maximum blowout frequency, provide a basis in a
quantification of the environmental risk.
Financial risk
In a risk assessment the economic aspect will mainly be expressed by cost-efficient
evaluations. The evaluations include a series of important parameters like material loss, loss of
production and costs related to environmental damages. Compilation of acceptance criteria
concerning financial risk is problematic. The risk analysis should ensure that the installation
does not involve an unacceptable high financial risk as a result of an accident.
3.1 The ALARP principle
The ALARP-principle implying that the risk should be reduced to a level “as low as reasonably
practicable” is widely used. ALARP is normally demonstrated using cost/benefit evaluations
with risk reducing measures being implemented when e.g. the cost of averting a fatality are not
prohibitively high.
Combining the acceptance criteria with the ALARP-principle solves acceptable risk problems
[32]. It is compulsory for operating companies to define probability values for certain
Diploma thesis, NTNU 2002
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