Page 174 - Well Control for Completions and Interventions

166 Well Control for Completions and Interventions

Wellbore pressure is, in fact, helping keep the ram in the closed posi-
tion. Since the area of the ram block is large, the opening ratio is much
lower than the closing ratio. Whilst there are preventers that have an
opening ratio of less than 1:1, requiring an opening pressure greater than
well pressure, most are in the 1:1 1:4 range.
Rams should not be closed on pipe tool joints or tubing upset areas,
otherwise seal damage will result. Most ram preventers can be locked in
the closed position, desirable during well control operations or when
shutting in for the night (daylight-only operations).
Nearly all ram preventers are based on designs from three leading
equipment providers:
1. Cameron (Schlumberger).
2. Hydril (GE).
3. Shaffer (NOV).
Many smaller independent manufacturers make copies of the original
Cameron, Hydril, and Shaffer BOP designs, as well as a range of spares.
Not all of them conform to the exacting standards of the mainstream
manufacturers.

4.8.1 Pipe rams
Pipe ram are constructed with a semi-circular cut-out matched to a spe-
cific pipe size. Most have a front packer that seals around the pipe and an
upper seal that prevents pressure by-passing the ram block. Pipe rams
should never be closed on an open hole, as this risks damaging the
elastomer.
During a completion or workover, it is normal to have at least one
pipe ram for each size of tubing that is being run. However, if completion
complexity or space prevents this, VBRs can be used. If control lines,
chemical injection lines, instrument, or ESP cables are being run, closing
the pipe ram is ineffective. In these circumstances, the control lines would
have to be cut and then lowered below the stack. This is normally done
using a kick stand (drill pipe), and the rams should be sized accordingly;
i.e., matched to the kick stand diameter.
Most pipe rams can support the weight of the string when a tool joint
is lowered onto the top of a closed pipe ram (Fig. 4.20).

4.8.2 Variable bore rams
VBR can close on a range of pipe sizes, and are particularly useful when
running a tapered completion string. All VBRs use a similar method of