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?
Summary and conclusions
The overall objective of this diploma thesis is to develop an understanding of the contribution
a downhole safety valve (DHSV) represents to the overall risk in a subsea oil/gas well. The
contribution the DHSV represents to the overall risk during installation, production and well
intervention is considered.
Norway and the United States of America have specific requirements of subsurface safety
devices like the DHSV. There are no specific requirements in the UK regulations for a DHSV.
The Norwegian Petroleum Directorate (NPD) requires that there at all times shall be at least
two independent and tested well barriers during well activities. Other countries have similar
requirements.
Acceptable level of risk in an activity is described by acceptance criteria. Combining
acceptance criteria with the “ALARP”-principle solves acceptable risk problems. The
Norwegian Oil Industry Association has developed a list of minimum safety integrity levels
(SIL). The SIL requirement concerning the shut-in of the flow in a well (Wing valve, Master
valve and DHSV) is set to 3. It is therefore reasonable to require the SIL of the well without a
DHSV to be the same.
There are two main risk factors regarding oil/gas production, delays in time (lost production)
and the blowout risk. Lost production occurs when the well is unable to produce as expected
due to different problems, and is equal to economic loss in oil production. Risk related to the
installation and completion of a subsea oil/gas well is mainly related to blowout risk and time
delays. Production related risk comprises economic and environmental risk. The workover
risk is represented mainly by time delays and the blowout risk.
A case example is included illustrating the effect of a DHSV. A comparison of unavailability
calculations for a well with, and without a DHSV proves the effect of a DHSV. Barrier
diagrams and fault trees are constructed providing a basis for the calculations and illustrating
the leakage paths.
Fault trees display the interrelationships between a potential critical event in a system and the
reasons for this event. The ‘TOP’ event of the fault tree analysis in this study is formulated,
“Sustainable leakage to the surroundings through either the x-mas tree or the wellhead during
normal shut-in conditions.”
Unavailability calculations are done in regard to the two exampled situations. The Mean
Fractional Dead Time (MFDT) model is applied in the calculations. MFDT can be given two
different meanings; the percentage of time where we are unprotected by the safety function, or
the probability that the safety function will fail on demand.
An introduction to well intervention methods and equipment is given in the thesis. There are
two types of well interventions, light and heavy (also known as workover). A blowout
preventer (BOP) system is a set of valves installed on the wellhead to prevent the escape of
pressure either in the annular space between the casing and tubing during drilling, completion
and workover operations.
HAZOP (Hazard and Operability analysis) is a method used to identify and assess problems
that may represent risks to personnel or equipment, or prevent efficient operation. Essentially
Diploma thesis, NTNU 2002 II