Page 287 - Well Control for Completions and Interventions

280 Well Control for Completions and Interventions

contraction and ballooning. If the tubing is free to move (seal
assembly and PBR) the combination of cooling, ballooning, and
piston force will result in significant upwards movement of the tub-
ing end, and there is a risk the end of the tubing will pull out of
the seal bore. If the tubing remains within the seal bore, the
combination of compressive piston force and high internal pressure
will promote buckling.
• Wellhead and tree pressure working pressure.
• Safety valve opening pressure: the control line pressure required to
keep a downhole safety valve open is normally about
1500 2500 psi (10,000 17,000 kPa) more than tubing pressure.
Tubing pressure at the beginning of a bullhead may be significantly
higher than the normal wellhead closed-in pressure. If the well is
equipped with a tubing retrievable safety valve, control line pres-
sure will have to be increased to compensate. Failure to do so risks
damaging the downhole valve. If the well is equipped with a wire-
line retrievable valve, this can be pulled before the bullhead starts.
• Condition of completion:
• Tubing burst limits: most operators recommend using 80% of API
(American Petroleum Institute) burst as an upper limit during a
bullhead kill. Burst pressures for API pipe grades are readily avail-
able from tables. If no tables are available, burst can be calculated
using Barlow’s formula (API values are calculated from Barlow’s
formula).
2Y p t
P b 5 Tol (7.7)
OD
where: P b : burst pressure, psi.
Tol: API wall thickness tolerance (0.875 for most API grades;
0.9 for some cold worked corrosion resistant alloys).
Y p : tubing yield, psi.
T: tubing wall thickness, in.
OD: external diameter of tubing, in.
API burst value assumes new undamaged pipe. A lower burst
value should be used if the pipe has been corroded or worn. API
burst values should be treated with caution, as they will be altered
by tri-axial loading during the kill. If the tubing is anchored to the
packer injection of cold fluid under pressure, this will increase ten-
sion. Tubing under tension can have a tri-axial burst limit higher