The rotating separator process is based on a different principle than the shale shaker process. The mud line flow is fed into the slowly rotating separator. The drilling fluid contaminated with drilled cuttings is fed into the centre of a screen coated drum as shown schematically in Fig. 1. The outside of the drum is connected to an under-pressurised ventilation system.
The dryer shaker, or dryer, is a linear motion shaker used to minimize
the volume of liquid associated with drilled cuttings discharged from the
main rig shakers and hydrocyclones. The liquid removed by the dryers is
returned to the active system. Dryers were introduced with the closed
loop mud systems and environmental efforts to reduce liquid-waste
haul-off. Two methods, chemical and mechanical, are available to minimize liquid discharge. The chemical method uses a system called a dewatering unit, while the mechanical method takes place through linear
motion shakers. These systems may be used separately or together.
The dryer shaker deliquifies drilled cuttings initially separated by another
piece of solids-separation equipment. These drilled solids can be the discharge from a main shaker or a bank of hydrocyclones. Dryers recover
liquid discharged with solids in normal liquid/solids separation that
would have been previously discarded from the mud system. This
liquid contains colloidal solids, and the effect on drilling-fluid properties
must be considered, since dewatering systems are frequently needed to
flocculate, coagulate, and remove these solids.
The dryer family incorporates pieces of equipment long used as independent units: the main linear motion shaker, the desander, and the
desilter, which are combined in several configurations to discharge
their discard across the fine screens (e.g., API 200) of a linear motion
shaker to capture the associated liquid. These units, formerly used as mud
cleaners, are mounted on the mud tanks, usually in line with the main
linear motion shaker. They can be tied into the flowline to assist with fine screening when not being used as dryers. Their pumps take suction
from the same compartments as desanders and desilters and discharge
their overflow (effluent) into the proper downstream compartments.
A linear motion dryer may be used to remove the excess liquid from
the main shaker discharge. The flow rate across a linear motion dryer
is substantially smaller than the flow rate across the main shaker. The
lower flow rate permits removal of the excess fluid by the linear
motion dryer by using a finer screen. The dryer is usually mounted at
a lower level than the other solids-separation equipment to use gravity
to transport solids to it. Whether by slide or by conveyor, the cuttings
dump into a large hopper, located above the screen, that replaces the
back tank, or possum belly. As the cuttings convey along the screen,
they are again deliquified. This excess fluid, with the fine solids that
passed through the screens, is collected in a shallow tank that takes
the place of a normal sump. The liquid is pumped to a catch tank that
acts as the feed for a centrifuge or back to the active system.
A dryer unit can be used to remove the excess fluid from the underflow
of a bank of hydrocyclones (desanders or desilters). This arrangement
resembles a mud cleaner system. In this configuration, the dryer unit
may be used on either a weighted or an unweighted mud system. The
liquid recovered by the linear motion shaker under the hydrocyclones
can be processed by a centrifuge, as previously described.
The perfection of the linear motion shaker for drilling-fluid use,
coupled with advanced fine-screen manufacturing technology, has made
these dryers very efficient. In most configurations, the dryers use the
same style of screens, motors, and/or motor/vibration combinations as
do other linear motion shakers by the same manufacturer.
Depending on the fluid, saving previously discarded liquid may be
financially advantageous. The dryer discard is relatively dry and can be
handled by backhoe and dump truck rather than by vacuum truck.
Drilling-fluid properties must be monitored properly when the recovered
liquid is returned to the active system. Large quantities of colloidal
solids may be recovered with the liquid. This could affect the PV, YP,
and gel strengths of a drilling fluid.