TANK ARRANGEMENT

The purpose of a drilling rig surface fluid processing system is to provide a sufficient volume of properly
treated drilling fluid for drilling operations. The active system should have enough volume of properly
conditioned drilling fluid above the suction and equalization lines to keep the well bore full during wet
trips. The surface system needs to have the capability to keep up with the volume-building needs while
drilling; otherwise, advanced planning and premixing of reserve mud should be considered. This should
be planned for the worst case, which would be a bigger-diameter hole in which high penetration rates
are common. For example for a 143 4-inch hole section drilling at an average rate of 200 ft/hr and with a
solids-removal efficiency of 80%, the solids-removal system will be removing approximately 34 barrels of
drilled solids per hour plus the associated drilling fluid coating these solids. More than likely, 2 barrels of
drilling fluid would be discarded per barrel of solids. If this is the case, the volume of drilling fluid in the
active system will decrease by 102 barrels per hour. If the rig cannot mix drilling fluid fast enough to
keep up with these losses, reserve mud and or premixed drilling fluid should be available to blend into
the active system to maintain the proper volume.

The surface system should consist of three clearly identifiable sections

 Suction and testing section

 Additions section
 Removal section

Fig : Surface circulation system

1. ACTIVE SYSTEM

1.1 Suction and Testing Section

The suction and testing section is the last part of the surface system. Most of the usable surface volume
should be available in this section. Processed and treated fluid is available for various evaluation and
analysis procedures just prior to the fluid recirculating downhole. This section should be mixed, blended,
and well stirred. Sufficient residence time should be allowed so that changes in drilling-fluid properties
may be made before the fluid is pumped downhole. Vortex patterns from agitators should be inhibited
to prevent entraining air in the drilling fluid.

In order to prevent the mud pumps from sucking air, vertical baffles can be added in the tank to break
up the possible vortex patterns caused by the agitators. If the suction tank is ever operated at low
volume levels, additional measures should be taken to prevent vortexing, such as adding a flat plate
above the suction line to break up the vortex pattern. Proper agitation is very important, so the drilling
fluid is a homogeneous mixture in both the tank and the well bore. This is important because if a ‘‘kick’’
(entrance of formation fluid into the well bore due to a drop in hydrostatic pressure) occurs, an accurate
bottom-hole pressure can be calculated. The well-control procedures are based on the required bottom-
hole pressure needed to control the formation pressures. If this value is not calculated correctly, the
well bore will see higher than necessary pressures during the well-control operation. With higher than
required pressure, there is always the risk of fracturing the formation. This would bring about additional
problems that would be best avoided whenever possible.

1.2 Additions Section

All commercial solids and chemicals are added to a well-agitated tank upstream from the suction and
testing section. New drilling fluid mixed on location should be added to the system through this tank.
Drilling fluid arriving on location from other sources should be added to the system through the shale
shaker to remove unwanted solids. To assist homogeneous blending, mud guns may be used in the
additions section and the suction and testing section.

1.3 Removal Section

Undesirable drilled solids and gas are removed in this section before new additions are made to the fluid
system. Drilled solids create poor fluid properties and cause many of the costly problems associated
with drilling wells. Excessive drilled solids can cause stuck drill pipe, bad primary cement jobs, or high
surge and swab pressures, which can result in lost circulation and/or well-control problems. Each well
and each type of drilling fluid has a different tolerance for drilled solids.

Each piece of solids-control equipment is designed to remove solids within a certain size range. Solids-
control equipment should be arranged to remove sequentially smaller and smaller solids. A general
range of sizes is presented in Table and in Figure. The tanks should have adequate agitation to minimize
settling of solids and to provide a uniform solids/liquid distribution to the hydrocyclones and
centrifuges. Concerning the importance of proper agitation in the operation of hydrocyclones, efficiency
can be cut in half when the suction tank is not agitated, versus one that is agitated. Unagitated suction
tanks usually result in overloading of the hydrocyclone or plugged apexes. When a hydrocyclone is
overloaded, its removal efficiency is reduced. If the apex becomes plugged, no solids removal occurs and
its efficiency then becomes zero. Agitation will also help in the removal of gas, if any is present, by
moving the gaseous drilling fluid to the surface of the tank, providing an opportunity for the gas to break
out.

Mud guns can be used to stir the tanks in the additions section provided careful attention is paid to the
design and installation of the mud gun system. If mud guns are used in the removal section, each mud
gun should have its own suction and stir only that particular pit. If manifolding is added to connect all
the guns together, there is a high potential for incorrect use, which can result in defeating proper
sequential separation of the drilled solids in an otherwise well-designed solidsremoval setup.
Manifolding should be avoided.
1.4 Piping and Equipment Arrangement

Drilling fluid should be processed through the solids-removal equipment in a sequential manner.
The most common problem on drilling rigs is improper fluid routing, which causes some drilling
fluid to bypass the sequential arrangement of solids-removal equipment. When a substantial
amount of drilling fluid bypasses a piece or pieces of solids-removal equipment, many of the
drilled solids cannot be removed. Factors that contribute to inadequate fluid routing include ill-
advised manifolding of centrifugal pumps for hydrocyclone or mud cleaner operations, leaking
valves, improper setup and use of mud guns in the removal section, and routing of drilling fluid
incorrectly through mud ditches.

Fig: General solids control equipment removal capabilities

Each piece of solids-control equipment should be fed with a dedicated, single-purpose pump, with
no routing options. Hydrocyclones and mud cleaners have only one correct location in tank
arrangements and therefore should have only one suction location. Routing errors should be
corrected and equipment color-coded to eliminate alignment errors. If worry about an inoperable
pump suggests manifolding, it would be cost saving to allow easy access to the pumps and have a
standby pump in storage. A common and oft-heard justification for manifolding the pumps is, ‘‘I
want to manifold my pumps so that when my pump goes down, I can use the desander pump to
run the desilter.’’ What many drilling professionals do not realize is that improper manifolding and
centrifugal-pump operation is what fails the pumps by inducing cavitation. Having a dedicated
pump properly sized and set up with no opportunity for improper operation will give surprisingly
long pump life as well as process the drilling fluid properly.

Suction and discharge lines on drilling rigs should be as short and straight as possible. Sizes should
be such that the flow velocity within the pipe is between 5 and 10 ft/sec. Lower velocities will
cause settling problems, and higher velocities may induce cavitation on the suction side or cause
erosion on the discharge side where the pipe changes direction. The flow velocity may be
calculated with the equation:

Velocity, ft/sec = Flow rate, gpm / 2.48(insided diameter in) 2 (1)

Pump cavitation may result from improper suction line design, such as inadequate suction line
diameter, lines that are too long, or too many bends in the pipe. The suction line should have no
elbows or tees within three pipe diameters of the pump section flange, and their total number
should be kept to a minimum. It is important to realize that an 8-inch, 90 welded ell has the same
frictional pressure loss as 55 feet of straight 8-inch pipe. So, keep the plumbing fixtures to a
minimum.

1.5 Equalization

Most compartments should have an equalizing line, or opening, at the bottom. Only the first
compartment, if it is used as a settling pit (sand trap), and the degasser suction tank (typically the
second compartment) should have a high overflow (weir) to the compartment downstream. The
size of the equalizing pipes can be determined by the following formula:

Pipe diameter =  Max. Circulation Rate; gpm / 15 (2)

A pipe of larger diameter can be used, since solids will settle and fill the pipe until the flow velocity
in the pipe is adequate to prevent additional settling (5 ft/sec).

An adjustable equalizer is preferred between the solids-removal and additions sections. The lower
end of an L-shaped, adjustable equalizer, usually field fabricated from 13-inch casing, is connected
to the bottom of the last compartment in the removal section. The upper end discharges fluid into
the additions section and can be moved up or down. This controls the liquid level in the removal
section and still permits most of the fluid in the suction section to be used.

1.6 Surface Tanks

Most steel pits for drilling fluid are square or rectangular with flat bottoms. Each tank should have
adequate agitation except for settling tanks. Additionally, each tank should have enough surface
area to allow entrained air to leave the drilling fluid. A rule of thumb for calculating the minimum
active surface pit area is:

Area, ft2 = Flow rate (gpm) / 40 (3)

For example, if the active circulating rate is 650 gpm, the surface area of each active compartment
should be about 16 square feet. The depth of a tank is a function of the volume needed and ease
of stirring. Tanks that are roughly cubical are most efficient for stirring. If this is not convenient,
the depth should be greater than the length or width. If circular tanks are used, a conical bottom is
recommended and centrifugal pump suction and/or a dump valve should be located there.
Another consideration is that the tanks need to be deep enough to eliminate the possibility of
vortexing at the centrifugal pump suction. The depth required is a function of the velocity of the
drilling fluid entering the suction lines