Cascade systems use one set of shakers to scalp large solids and/or gumbo from the drilling fluid and another set of shakers to remove fine solids. The first cascade system was introduced in the mid-1970s. A scalper shaker received fluid from the flowline and removed gumbo or large drilled solids before the fluid passed through the main shaker with a fine screen. The first unit combined a single-deck, elliptical motion
shaker mounted directly over a double-deck, circular motion shaker (Figure 7.17). This combination was especially successful offshore, where space is at a premium. It was, however, subject to the technology limitations of that time period, which made API 80 to API 120 screens the practical limit.
One advantage of multiple-deck shale shakers is their ability to reduce solids loading on the lower, fine-screen deck. This increases both shaker capacity and screen life. However, capacity may still be exceeded under
Figure 7.17. First cascade shaker system.
many drilling conditions. The screen opening size, and thus the size that
solids returned to the active system, is often increased to prevent loss of
whole drilling fluid over the end of the shaker screens.
Processing drilling fluid through shale shaker screens, centrifugal
pumps, hydrocyclones, and drill-bit nozzles can cause degradation of
solids and aggravate problems associated with fine solids in the drilling
fluid. To remove drilled solids as soon as possible, additional shakers
are installed at the flowline so that the finest screen may be used.
Sometimes as many as 6 to 10 parallel shakers are used. Downstream
equipment is often erroneously eliminated. The improved shale shaker
still remains only one component (though a very important one) of the
drilled-solids removal system.
A system of cascading shale shakers—using one set of screens (or
shakers) to scalp large solids and gumbo from the drilling fluid and
another set of screens (or shakers) to receive the fluid for removal of
fine solids—increases the solids-removal efficiency of high-performance
shakers, especially during fast, top-hole drilling or in gumbo-producing
formations, which is its primary application. The cascade system is used
where solids loading exceeds the capacity of the fine screens, that is,
it has been designed to handle high solids loading. High solids loading
occurs during rapid drilling of a large-diameter hole or when gumbo
arrives at the surface.
The advantages of the cascade arrangement are:
1. Higher overall solids loading on the system
2. Reduced solids loading on fine mesh screens
3. Finer screen separations
4. Longer screen life
5. Lower fluid well costs
There are three basic designs of cascade shaker systems:
. Separate unit concept
. Integral unit with multiple vibratory motions
. Integral unit with a single vibratory motion
The choice of which design to use depends on many factors, including
space and height limitations, performance objectives, and overall cost.
1 Separate Unit
The separate unit system mounts usable rig shakers (elliptical or
circular motion) on stands above newly installed linear motion shakers
(Figure 7.18). Fluid from the rig shakers (or scalping shakers) is
routed to the back tank of a linear motion shaker. Line size and potential
head losses must be considered with this arrangement to avoid overflow
and loss of drilling fluid. This design may reduce overall cost by utilizing
existing equipment and, where space is available, has the advantages of
highly visible screening surfaces and ease of access for repairs.
Figure 7.18. Separate unit cascade system.
2 Integral Unit with Multiple Vibratory Motions
This design type combines the two units of the separate system into
a single, integral unit mounted on a single skid. Commonly, a circular,
elliptical, or linear motion shaker is mounted above a linear motion
shaker on a common skid (Figure 7.19). The main advantages of this
design are reduced installation costs and space requirements. The internal
flowline eliminates the manifold and piping needed for the two separate
units. This design reduces screen visibility and accessibility to the drive
components.
Figure 7.19. Integral cascade unit with multiple vibratory motions.
3 Integral Unit with a Single Vibratory Motion
This design is shown in Figure 7.20. Typically, this device uses a linear
motion shaker and incorporates a scalping screen in the upper part of
the basket. The lower bed consists of a fine-screen, flow line shaker
unit, and the upper scalper section is designed with a smaller-width
bed using a coarser screen. Compared with the other cascade shaker
units, this design significantly lowers the weir height of the drilling fluid
inlet to the upper screening area. Visibility of and access to the
fine-screen deck can be limited by the slope of the upper scalping deck.
4 Cascade Systems Summary
Cascade systems use two sets of shakers: one to scalp large solids gumbo
and another to remove fine solids. Their application is primarily during
fast, top-hole drilling or in gumbo formations. This system was designed
to handle high solids loading. High solids loading occurs during rapid
drilling of a large-diameter hole or when gumbo arrives at the surface.
The introduction of high-performance linear motion and balanced
elliptical shale shakers has allowed development of fine-screen cascade
systems capable of API 200 separations at the flowline. This is particularly
important in areas where high circulating rates and large amounts of drilled solids are encountered. After either the flow rate or solids loading
is reduced in deeper parts of the borehole, the scalping shaker should
be used only as an insurance device. Screens as coarse as API 10 may be
used to avoid dispersing solids before they arrive at the linear motion
shaker. When the linear motion shaker, with the finest screen available,
can handle all of the flow and the solids arriving at the surface, the need
for the cascade system disappears, and the inclination may be to discontinue
the use of the scalping screen unit. Even when the fine screen can
process all of the fluid, screens should be maintained on the scalper
shaker. These screens can be a relatively coarse mesh (API 10 to API
12), but they will protect the finer-screen mesh on the main shaker.
The use of finer screens on the scalping shaker will result in fewer drilled
solids being removed by the scalping and main shakers.
Figure 7.20. Integral cascade unit with single vibratory motions.
