Page 133 - Well Control for Completions and Interventions

Completion Equipment 125

valve located in the side pocket. The gas mixes with the produced
fluids, enhancing production. In some systems a series of side pocket
mounted unloading valves are needed to unload fluid from the well
before flow can begin.

3.19 SUBSURFACE SAFETY VALVES

The API defines a subsurface safety valve as a “device whose design
function is to prevent uncontrolled well flow when closed.” It can protect
against collision, fire, equipment failure, unforeseen catastrophic events,
and sabotage. The need to run a safety valve will in part be determined
by local regulations, in part by company policy. Nowadays many wells are
equipped with some form of downhole protection.
Although subsurface safety valves are intended for emergency use
only, they have been used to provide an additional barrier when working
on the Christmas tree and wellhead. Some operating companies now
accept the subsurface valve as a barrier for well intervention work, but
this is by no means universal.
Modern tubing retrievable SC-SSSVs have evolved from simple pop-
pet type valves developed in the 1930s. Those valves were subsurface con-
trolled. They relied upon an increase in flow velocity to close a tapered
valve against a valve seat. A catastrophic failure of the wellhead would
cause an increase in flow. Subsurface controlled valves were often placed
in Gulf Coast wells to afford protection during the hurricane season, so
became known as storm chokes. This term is still used today by some of
the older generation!
Early valves had a number of disadvantages. Flow area was restricted
and tortuous, they could not withstand high differential pressure, and cali-
bration (flow velocity to close) was problematic. Nevertheless, they were
used extensively, and led ultimately to the modern surface controlled high
pressure ball and flapper valve systems in use today.
The modern SC-SSSV is fail-safe. A ball, or more usually a flapper, is
held in the open position for as long as pressure is maintained in a
hydraulic control line running from the surface to the valve operating pis-
ton. If, for any reason, pressure is lost, the valve will close and can only
be reopened by once more applying pressure to the line. Clearly, the
integrity and function of the valve and control line is of great importance