Page 173 - Well Control for Completions and Interventions

Well Control Surface Equipment 165

Ram shaft
Closing pressure
area
Piston area
Well pressure

Figure 4.18 Closing ratio.

Resultant
force on Seals Resultant
ram block force on
shaft

Figure 4.19 Opening forces.

ratio of piston size to shaft size will give the closing ratio, which can be
defined as “a dimensionless factor equal to the wellbore pressure divided by the
operating pressure necessary to close the BOP ram against wellbore pressure”
(Fig. 4.18).
Closing ratios are generally in the range from 6:1 to 9:1, meaning 1
unit of force of closing pressure is needed for every 6 9 wellbore pressure
units of force to close the preventer. For example, if a preventer has a
closing ratio of 6:1, and wellbore pressure is 3000 psi, then 500 psi
hydraulic pressure will close the preventer (ignoring friction). Maximum
ram operating pressure is the working pressure rating of the BOP divided
by the closing ratio.
It is highly unlikely there will ever be a need to open rams when pres-
surized, and opening rams against well pressure is certainly not recom-
mended. If an attempt is made to open the rams, hydraulic pressure needs
to overcome the wellbore pressure that acts against the large surface area
at the back of the ram (Fig. 4.19).