Page 147 - Well Control for Completions and Interventions

Well Control Surface Equipment 139

blind ram to close above the suspended pipe. An inlet is required
between the closed blind ram and the pipe ram where the pipe is sus-
pended to allow pressure to be monitored and circulation to occur
through the suspended pipe.
• Variable bore rams (VBRs) have reduced load-bearing capability, and
can only be used to suspend a limited length of pipe. Stripping
through VBRs is not recommended, since they wear easily. They also

have lower temperature limitations (225 F) than fixed diameter pipe
rams. Nevertheless, they are often used during completion and work-
over operations, since tapered productions strings are common.
• Inlets/outlets below the lowermost ram should only be used to moni-
tor pressure. A leak in the stack or associated lines below the bottom
ram would result in loss of control and the release of fluids. In excep-
tional circumstances, it may be necessary to kill the well through the
line at the base of the stack, i.e., where the well is shut-in on the low-
ermost ram and the kick is migrating.
Whilst having more rams in a BOP stack will improve safety and
operability, it also adds to the weight of the stack. More rams also mean
more expense. A well-designed stack is one that is both functional, safe,
and cost-effective.

4.2.1 Blow out preventer classification
BOPs are classified and configured based on well pressure. The American
1
Petroleum Institute (API) defines six working pressure classifications
with higher pressure systems requiring more complex configurations to
meet the API minimum requirement. These are:
1. 2000 psi (13,800 kPa)
2. 3000 psi (20,700 kPa)
3. 5000 psi (34,500 kPa)
4. 10,000 psi (69,000 kPa)
5. 15,000 psi (103,500 kPa)
6. 20,000 psi (138,000 kPa)
a
In addition to these classifications, NORSOK also include a
25,000 psi (172,369 kPa) rating.
These API pressure classifications are not the same as the working
pressure rating of the BOP components. For example, a stack might be
configured for a 2000 psi maximum surface pressure but be constructed

a
NORSOK