4 rotary drilling
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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 dierted
through shale sha!ers, desilters, desanders and series oftan!s or pits which treat the fluid. In the last of these pits
the mud is pic!ed up by the pump suction and repeats the
cycle. "igure # shows the basic components of a rotary
drilling rig.
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$a!ing a connection, the process of adding a new joint of
pipe to the drill string is shown in "igure %. &eriodically the
pipe must be remoed from the hole in order to replace the
bit. The operation is illustrated in "igure '.
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The basic drilling components are
#. erric!, mast and substructures
%. rawwor!s'. $ud pumps
*. &rime moers
+. The drill string
. -its
. rilling line
/. $iscellaneous and au0iliary equipment
erric!, mast and substructures
The function of a derric! is to proide the ertical
clearance necessary to the raising and lowering of the drill
string into and out of the hole during the drilling operations.
erric!s are rated according to their ability to withstand
two types of loading
#. 1ompressie loads%. 2ind loads
The allowable compressie load of a derric! is computed as
the sum of the strengths of the four legs. erric!s with
load capacities from appro0imately /,333 to #,*33,333 lb,
depending on steel grade and leg size are aailable.
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Allowable wind loads for A&I derric!s are specified in two
ways, with or without pipe setbac!.
2ith pipe setbac!, the wind may be blowing perpendicular
to it, which is essentially a pipe wall. This is the worst
possible condition.
2ind loads are calculated by the formula
& 4 3.33*5%
where & 4 wind load, lb6ft%
5 4 wind elocity, mph
1alculation of )erric! 7oads
The bloc! and tac!le arrangement for a rotary rig is shown
in "igure *. Assuming that the system is frictionless, thefollowing relationship are apparent
2n
%n"d
+=
where "d 4 total compressie load on the deric!
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n 4 number of lines through the traelling
bloc! (those supporting 2).
2 4 hoo! load
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The derric! load is always greater than the hoo! load by
the the factor of (n8%)6% due to the two additional lines
(drawwor!s and anchor) e0erting a downward pull.
uring hoisting
74 nA
where 74 elocity of line being spooled (or
unspooled) at the drawwor!s during
hoisting.
A4 hoo! elocity
rawwor!s (hoist)
The drawwor!s or hoist is the !ey piece of equipment on a
rotary rig. The functions of the drawwor!s are
#. It is the control centre from which the drilleroperates the rig. It contains the clutches, chains,
sproc!ets, engine throttles and other controls wich
enable the rig pwer to be dierted to the particular
operation at hand.
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%. It houses the drum which spools the drilling line during
hoisting operations and allows feed9off during drilling.
rawwor!s are commonly designated by a horsepower and
depth rating.
e
#0
'',333
2:& h=
where 2 4 :oo! load, lb
h4 hoisting elocity of traelling bloc!, ft6min
'',333 4 ft.lb6min per horsepower
e 4 :oo! to drawwor!s efficiency
:oo! to drawwor!s efficiencies are commonly between /3
to ;3 denotes that each
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side of the piston does wor!, while =duple0> refers to the
number of pistons (two).
The superiority of the piston type pump for drilling serice
is due to the following features
#. Ability to handle fluids containing high percentages of
solids, many of which are abrasie.
%. 5ale clearance will allow passage of large solid
particles (typically lost circulation materials) without
damage.
'. ?ase and simplicity of operation and maintenance.
7iners, pistons and ales may be replaced in the field
by the rig crew.
*. 2ide range of olume and pressure aailable by using
different liner and piston sizes.
&rime $oers
The bul! of rig power is consumed by two operations
#. 1irculation of the drilling fluid
%. :oisting
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"ortunately these requirements do not occur at the same
time and the same engine perform both jobs.
The prime moers used are the steam engines, electric
motors and internal combustion engines. The most
commonly used is the internal combustion engines such as
the automotie type (multicylinder, light flywheel) diesel
and gas engines capable of rapid acceleration.
The rill @tring
The rotary drill string includes the components as in "igure
+.
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"ig. + @chematic diagram of drill
string components and bit. -it load isfurnished by heay walled large9
diameter drill collars.
otary bits
The bit is the part that drill the hole. -asically there are
three types of drill bits, these are the drag type, rolling
cutter type and the diamond bit. The most common is therolling cutter type. The diamond bit is commonly used in
hard formations.
rilling line
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The rotary drilling line afford a means of handling the load
suspended from the hoo! during all drilling operations.
The ma0imum load occurs when running casing, although
fishing operations frequently require line pulls in e0cess of
the drill string weight.
Traelling -loc!, :oo! and @wiel
The traelling bloc! is the traelling pulley assembly that
connects the drilling line to the hoo! and swiel. The swiel
must suspend the drill string and allow rotation at the same
time.
-lowout &reenter (-B&)
The main function of a blowout preenter is to furnish a
means of closing off the annular space between the drill
pipe and casing.
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It is not always possible to predict the e0act manitude of
pressures which will be encountered in the drilling of a well.
1onsequently 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
eentually to the surface.
This effect is called a blowout, and is one of the most
feared and e0pensie accidents which can occur in well
drilling.
$ost blowout preenters are either hydraulically or
pneumaticaly operated, with manual operation aailable as asafetry precaution.
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