Page 224 - Well Control for Completions and Interventions

218 Well Control for Completions and Interventions

For example, a slickline stuffing box is often described as the primary
well control barrier when wireline is in the hole. Although this is not
incorrect, it is not a comprehensive answer. During slickline operations
on live wells the well barrier, as defined by NORSOK, would include
not just the stuffing box, but all the other “barrier elements” that, when
combined, prevent the release of well fluids.

6.2 BARRIER CLASSIFICATION

There are two broad categories of barriers, or to be precise, barrier
elements.
Normally open. These are barriers that are normally open, but can be
closed to contain well fluids. For example, BOP rams, a downhole safety
valve (SCSSSV), or Christmas tree valves.
Normally closed. A permanent barrier in the well that prevents fluid
flow. For example, cemented casing, production packer, liner top seals.
Barriers may be further classified as:
Independent barrier. This is not reliant on another for integrity. A
mechanical plug, properly tested, would constitute a single, independent
mechanical barrier.
Dependent barrier. Relies on another barrier for integrity. For example,
a check valve that requires a full column of kill weight fluid above it to
remain closed.
Primary barrier. The first object that prevents flow from the well.
Secondary barrier. The second object that contains flow, the object that
will contain flow from the failed primary barrier.
Tertiary barrier. The third object that prevents flow from a well. Only
used if both primary and secondary barriers fail.
A properly implemented barrier philosophy will be based on the pre-
sumption that the primary barrier should remain intact. If it fails, opera-
tional priority will be to reinstate the primary barrier before work
continues. For example, during the running of a completion string, the
primary barrier is a column of kill weight fluid. Fluid loss to the forma-
tion results in a loss of overbalance and a kick. Well integrity is restored
by closing the BOP annular preventer (secondary barrier), and installing a
full opening safety valve on the tubing (secondary barrier). Restoration of