Page 182 - Well Control for Completions and Interventions

174 Well Control for Completions and Interventions

The compressibility (Z factor) of nitrogen at (80F) 1000 psi is 1.01, at
1200 psi it is 1.02, and at 3000 psi it is 1.06. Since these values are rela-
tively small, temperature and compressibility are normally ignored, as is
the difference between gauge pressure and absolute pressure (14.7 psi).
Including nitrogen compressibility into the calculation gives a slightly
smaller result, 4.91 gals.
In subsea wells the precharge pressure in subsea accumulator bottles
needs to be increased to account for the hydrostatic pressure of the
hydraulic fluid in the power fluid supply umbilical. A conservative
approach is to use the gradient of seawater, 0.445 psi/ft, rather than the
gradient of the power fluid.
Having calculated the useable volume in each accumulator bottle, the
number of accumulator bottles needed to supply the BOP can be deter-
mined. For example, a surface BOP stack has been configured with:

Blow out preventer stack component Volume to close (gallons) Volume to open
1 3 annular BOP 24.1 24.1
3 3 ram BOP 11.8 11.8
2 3 HCR valves 0.46 0.46

The total fluid volume required to close from a full open position at
zero wellbore pressure all of the BOPs in the BOP stack plus 50% reserve
is 90.63 gallons. The number of 11-gallon accumulator bottles is therefore:
90:63=5 5 18:126-19 bottles:

4.10 IN PIPE SHUT-OFF DEVICES

If it becomes necessary to shut-in a well, flow through the annulus
is stopped by closing either the annular preventer or the pipe rams. It is
also necessary to prevent flow to the surface through the pipe in the well.
There are several “in-pipe” shut-off devices that can be used.

4.10.1 Kelly valves
On rigs where a kelly is still in use, an upper kelly valve (sometimes called
a kelly cock) is positioned between the swivel and the kelly. A second,
lower kelly valve, is placed on the bottom of the kelly. Kelly valves are
normally manually operated full opening ball valves. They are closed by