ROTARY DRILLING
In the rotary method, the hole is drilled by a rotating bit to
which a downward force is applied. The bit is fastened to,
and rotated by, a drill string, composed of high quality drill
pipe and drill collars, with new sections or joints being
added as drilling progresses.
The cuttings are lifted from the hole by the drilling fluid
which continuously circulated down the inside of the drill
string through water courses or nozzles in the bit, and
upward in annular space between the drill pipe and bore
hole.
At the surface, the returning fluid (mud) is diverted
through shale shakers, desilters, desanders and series of
tanks or pits which treat the fluid. In the last of these pits
the mud is picked up by the pump suction and repeats the
cycle.
Figure 1 shows the basic components of a rotary
drilling rig.
1
2
Making a connection, the process of adding a new joint of
pipe to the drill string is shown in Figure 2. Periodically the
pipe must be removed from the hole in order to replace the
bit. The operation is illustrated in Figure 3.
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The basic drilling components are:
1.
2.
3.
4.
5.
6.
7.
8.
Derrick, mast and substructures
Drawworks
Mud pumps
Prime movers
The drill string
Bits
Drilling line
Miscellaneous and auxiliary equipment
Derrick, mast and substructures
The function of a derrick is to provide the vertical
clearance necessary to the raising and lowering of the drill
string into and out of the hole during the drilling operations.
Derricks are rated according to their ability to withstand
two types of loading:
1.
Compressive loads
2.
Wind loads
The allowable compressive load of a derrick is computed as
the sum of the strengths of the four legs. Derricks with
load capacities from approximately 86,000 to 1,400,000 lb,
depending on steel grade and leg size are available.
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Allowable wind loads for API derricks are specified in two
ways, with or without pipe setback.
With pipe setback, the wind may be blowing perpendicular
to it, which is essentially a pipe wall.
This is the worst
possible condition.
Wind loads are calculated by the formula:
P = 0.004V2
where
P = wind load, lb/ft2
V = wind velocity, mph
Calculation of Derrick Loads
The block and tackle arrangement for a rotary rig is shown
in Figure 4. Assuming that the system is frictionless, the
following relationship are apparent:
Fd 
where
n 2
W
n
Fd = total compressive load on the derick
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n = number of lines through the travelling
block (those supporting W).
W = hook load
The derrick load is always greater than the hook load by the
the factor of (n+2)/2 due to the two additional lines
(drawworks and anchor) exerting a downward pull.
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During hoisting:
vL = nvA
where
vL = velocity of line being spooled (or
unspooled) at the drawworks during
hoisting.
vA = hook velocity
Drawworks (hoist)
The drawworks or hoist is the key piece of equipment on a
rotary rig. The functions of the drawworks are:
1.
It is the control centre from which the driller
operates the rig.
It contains the clutches, chains,
sprockets, engine throttles and other controls wich
enable the rig pwer to be diverted to the particular
operation at hand.
2.
It houses the drum which spools the drilling line during
hoisting operations and allows feed-off during drilling.
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Drawworks are commonly designated by a horsepower and
depth rating.
HP 
where
W vh
1
x
33,000 e
W = Hook load, lb
vh = hoisting velocity of travelling block, ft/min
33,000 = ft.lb/min per horsepower
e = Hook to drawworks efficiency
Hook to drawworks efficiencies are commonly between 80
to 90%, depending on the number of lines in use.
Mud Pumps
the function of the mud pumps is to circulate the drilling
fluid at the desired pressure and volume.
The pump
normally used is the reciprocating piston, double acting,
duplex type. The term “double acting” denotes that each
side of the piston does work, while “duplex” refers to the
number of pistons (two).
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The superiority of the piston type pump for drilling service
is due to the following features:
1.
Ability to handle fluids containing high percentages of
solids, many of which are abrasive.
2.
Valve clearance will allow passage of large solid
particles (typically lost circulation materials) without
damage.
3.
Ease and simplicity of operation and maintenance.
Liners, pistons and valves may be replaced in the field
by the rig crew.
4.
Wide range of volume and pressure available by using
different liner and piston sizes.
Prime Movers
The bulk of rig power is consumed by two operations:
1.
Circulation of the drilling fluid
2.
Hoisting
Fortunately these requirements do not occur at the same
time and the same engine perform both jobs.
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The prime movers used are the steam engines, electric
motors and internal combustion engines.
The most
commonly used is the internal combustion engines such as
the automotive type (multicylinder, light flywheel) diesel
and gas engines capable of rapid acceleration.
The Drill String
The rotary drill string includes the components as in Figure
5.
Fig. 5: Schematic diagram of drill
string components and bit. Bit load is
furnished by heavy walled largediameter drill collars.
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Rotary bits
The bit is the part that drill the hole. Basically there are
three types of drill bits, these are the drag type, rolling
cutter type and the diamond bit. The most common is the
rolling cutter type. The diamond bit is commonly used in
hard formations.
Drilling line
The rotary drilling line afford a means of handling the load
suspended from the hook during all drilling operations.
The maximum load occurs when running casing, although
fishing operations frequently require line pulls in excess of
the drill string weight.
Travelling Block, Hook and Swivel
The travelling block is the travelling pulley assembly that
connects the drilling line to the hook and swivel. The swivel
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must suspend the drill string and allow rotation at the same
time.
Blowout Preventer (BOP)
The main function of a blowout preventer is to furnish a
means of closing off the annular space between the drill
pipe and casing.
It is not always possible to predict the exact manitude of
pressures which will be encountered in the drilling of a well.
Consequently it is not uncommon to encounter pressure
greater than those imposed by the drilling fluid, with the
result that formation fluids flow into the bore hole and
eventually to the surface.
This effect is called a blowout, and is one of the most
feared and expensive accidents which can occur in well
drilling.
Most blowout preventers are either hydraulically or
pneumaticaly operated, with manual operation available as a
safetry precaution.
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