Page 190 - Well Control for Completions and Interventions

182 Well Control for Completions and Interventions

For both land and offshore fluid systems, the pits are normally catego-
rized as “active” and “reserve.”
Active pits: During most operations that require fluid circulation, fluid
from the active pit is returned to the active pit in a closed loop system.
Consequently, if the active pit capacity is too large, it becomes more diffi-
cult to spot the pit gains or losses that are an early indication of a well
control problem. The active pits should ideally be equipped with a pit
volume totalizer (PVT) that accurately monitors the level (volume) in the
pit. Modern PVT systems normally feature visible and audible alarms set
to warn the rig crew if there are pit gains.
Reserve pits: Where possible, the reserve pits should be large enough to
store all surface volumes required for the completion of the well. Ideally
there should, as an absolute minimum, be enough excess to deal with
1
losses equivalent to 1 1/2 times the hole volume.
Trip tank: A trip tank is a low-volume (100 barrels or less), calibrated
tank that can be isolated from the remainder of the surface fluid system. It
is used to accurately monitor the amount of fluid going into or coming
out of the well whilst pipe is being tripped. Properly configured, main-
tained, and monitored, small volume fluid gains or losses can be detected.
Trip tanks are also used to monitor fluid level during static conditions, for
example when logging operations are taking place. If pipe is being
stripped into the well, a second, stripping tank is used to measure fluid
transferred from the trip tank (Fig. 4.31).

4.12.1 Fluid pumps
Most rigs have at least two mud pumps in case of breakdown. They must
be able to pump the maximum anticipated weight of kill fluid at the rate
required to kill the well. During completion and workover operations, a
bullhead kill is common and normally requires a higher pump rate than a
circulation kill (to overcome gas migration). Pumps need to be equipped
with a functional stroke counter and the number of strokes per barrel
recorded on any kill sheets. The maximum discharge pressure that can be
obtained from mud pumps is typically 7500 psi, if higher pressure is
required to initiate a well kill normally the cement unit would be used.
Cement pump: Most offshore drilling units and offshore platforms with
drilling derricks have a cement pump as part of the permanent facilities.
Most are diesel driven, and are therefore independent of the rig power
supply. On land locations, a trailer-mounted cement unit is brought to
the location as and when required. Cement pumps provide a back-up to