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?
c) Find the risk reducing effect of a DHSV in a subsea oil/gas well. A case example will
be used to illustrate the effect. Barrier diagrams, fault trees and unavailability
calculations are used in the assessment.
d) Comment the consequences related to an unwanted event in the different life phases of
a well. Illustrate the economic and environmental effect in form of blowouts and lost
production.
e) Identify and describe risk related to the installation, production and workover phases of
a well.
f) Describe the different types of intervention and the most important intervention
equipment. This is done will be order to provide a basis for a HAZOP of the BOP
handling procedure in a workover situation.
g) Introduce the reader to the use of HAZOP as a risk assessment tool. Use the HAZOP
method to perform a detailed risk analysis on a part of a well intervention.
h) Give a recommendation to whether or not a DHSV should be installed in a subsea
oil/gas well from a risk point of view. The recommendation is based on the findings in
the thesis.
1.2 Limitations
The contents of System Reliability Theory [8] are assumed known to the readers of this report.
This includes the principles of fault tree analysis and reliability definitions. A limited
understanding of oil production is also required, although a brief introduction to subsea oil
production is given in appendix A.
The included rules and regulations concerning the presence of a Downhole Safety Valve
(DHSV) are limited to those of Norway, the United Kingdom and the United States of
America.
Due to lack of provided cost information from the operation companies, part four of the
diploma task is removed in agreement with Prof. Marvin Rausand, supervisor.
When identifying the risk reducing effect of a downhole safety valve (DHSV) is the case
example only the barrier situation “to prevent leakage to the surroundings during a normal
shut-in” will be modelled. External factors affecting the barrier situation like sabotage,
earthquakes and environmental influence on the well will not be applied to the barrier scenario.
The barriers analysed in the unavailability calculations are considered to be independent and
common failures are not included.
A HAZOP analysis of a well intervention is too extensive to be fully performed in this thesis.
Therefore the HAZOP is restricted to the BOP handling procedure, in understanding with Prof.
Marvin Rausand.
Diploma thesis, NTNU 2002
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