Chap-2 Rotary Drilling

8/13/2019 Chap-2 Rotary Drilling

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Rotary Drilling

In rotary drilling the rock is fragmented bycrushing and shearing action of the bit.

The bit is pressed down by compressiveforce and rotated. The compressive forceprovides crushing and rotation providesshearing action. The cuttings produced arebrought to surface by the drilling fluid whichis circulated in the bore hole.


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Operations required for

rotary drilling Hoisting

Rotation

Drilling fluid circulation

Feed


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Hoisting operation

Hoisting operation is done for raising andlowering the drill string.

The drill string is raised and lowered downfor

Adding dill pipes for advancing the bore.

Changing the bit. Any other operation.


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Hoisting operation

The hoisting operationrequires the followingequipment:

Draw worksCrown pulley bockMast/DerrickTraveling pulley blockHookWire rope/line


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Derrick/mast

Derrick is a steel structure used forsupporting the load of drill string. It alsoprovides necessary vertical clearance for

adding the pipe or making trip forchanging the bit. For making trip a standof two or three pipes is unscrewed/screwed for reducing trip time and wear

on the threads. In order to minimise andcause uniform wear, the alternate jointsare screwed/unscrewed.


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Height of the derrick

The height of the derrick is either 41 m or 52 mdepending whether two pipe or three pipes stand isused.

Parts of the derrick:LegsBraceGirt

LadderPlatformWater tableCrow nest


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Types of the derrick

Four legged derrick: are normally used onoff shore and heavy duty drills on shore

Two legged derrick- A shaped known asmast used on shore


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Load on derrickTwo types of loads are subjected on derricks:

Wind load Load of drill string/ casings/pull during fishing.

Wind load: The wind exerts horizontal force on the derrick .p = 0.04 V2Wherep- pressure in psiV- velocity of air in miles per hourThe force due to wind is maximum when the pipes arewithdrawn during trip and stacked vertically along thederrick and works as wall.

Force ( F ) = p*A where A is the area of wall made by pipes.A = dop*n*lpdop is out side dia of drill pipen is no of pipes stackedlp is length of stand of pipes


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Load on derrick due to drill string

The load on derrick may be considered With out friction in pulleys

With friction

Load with out friction:W = HL( n+2 )/n

where:

W = load on derrickHL = hook load

n = no of lines which is twice the no of

pulleys in traveling block


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Load on derrick with friction

n = no. of working sheaves or no of lines strung toTraveling block

Fl = fast line tensionDl = Dead line tension

K = sheave and line efficiency per sheaveEF = block and tackle efficiency factorp1, p2, p3, ..pnare pull in lines 1, 2, 3,np

1= Fl* K,

p2= Fl*k2,

p3 = Fl*k3,

pn = Fl*kn


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Hook load (W) = p1+ p2+p3+.+pnW = Fl*k+ Fl*k2+Fl*k3+.+Fl*kn

W = Fl (1- Kn ) /(1K)Fl = W (1- K) /K (1- Kn)

Since last pulley is not rotating Dl = pn

Load on derrick = Fl+ W + DlIn absence of friction Fl= p1=p2= pnpav = W/N

Efficiency factor EF = Pav/FlEF = K (!Kn)/N (1K)


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Hook loadHook load comprises of loads due bit, drill collars,

weight of drill pipes and weight of swivel etcDrill collars are thick wall pipes used to provide theweight on bit. They also help in reducing thevibrations of the drill string and keep the hole straight.Weight on bit depends on the diameter of the bit andtype of rock being drilled.WOB = Diameter of bit in mm * wob / mm diameterof bitwob /mm depends on rock hardness as given below:

very hard rock - 90 to 120 kg/ mm diameter of bithard rock - 70 to 90medium hard - 60 to 70soft rock - 40 to 60very soft rock - 20 t0 40


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Size and length of drill

collarsThe diameter of the drill collar should about15mm less than the diameter of hole being

drilled. Since the well is telescopic it may notbe possible to select drill collar for every bit toused fo drilling a complete well. How ever drillcollar may be selected based on smallestdiameter of the bit to uses. The followingdetails have to found from drill data book.Out side dia of drill collarIn side dia of drill collar

Weight of drill collar/meter length


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Length of drill collar string (lc)= WOB/ wt of drillcollar/ meter

Actual length of drill collar string should be

Lc = 1.3* lcBy keeping the drill collar length longer thanrequired, null point on string will be ensured on drillcollars.

Null point is the point on the string where the stressis zero. The string above null point will be in tensionblow it will be in compression.If the drill string is raised off the ground, the string

will be in tension and its weight will be indicated onthe weight indicator. To apply load on bit the string islowered and allowed to touch the ground. The loadwill now be transferred to the bit. Continuetransfering the load till required wob is achieved .


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Length of the drill string made upof pipes

The string can be made in any of the followingmanner:

Uniform diameter

TelescopicLarger dia pipes on top and smallerdia pipes for remaining length.

Uniform diameter but different thickness ordifferent grades or different class of pipes

The criteria is load on pipes. The maximum loadcomes on top of the string.


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Hook load

Hook load can now be calculated as given below

HL = (wp1*lp1+wp2*lp2 +wc*LC )(1-m/ s)+weight of bit and swivel etc

wp1= weight/m of pipe 1 in meterlp1 = length drill pipe 1

wp2= weight/m of pipe 2

lp2 = length drill pipe 2 in meter

m = specific gravity of muds = specific gravity of steel

weight of bit and swivel etc may added lump sum


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Drill string designThe forces to which drill pipes are subjected to are:Tension: due to self weight of pipes and is maximum at

top.Collapse this is due the pressure exerted by mudcolumn out side the pipe. This maximum at thebottom of the pipe. Normally this pr is zero because

of equal height of mud in side out side of the pipe. Insome cases, as in drill stem testing, the drill pipe isrun partially full, to reduce hydrostatic pr exertedagainst the formation. This is done to permit theformation fluid to flow in the well bore

Torsion: This is due to the twisting action of rotatingdrill pipes caused by resistance offered by rock to bit.This can be calculated by torsion theory.Complex stresses will develop due to tension and

torsion combined.


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Drill string design

Pa=Pt*0.9 - MOP

Pt is drill pipe yield strength to be taken from drill databook as per API

MOP is margin of pull normally range from 25000-50000 Kgand is provided to take care for extra pull during fishingetc

Now length of smaller dia pipe is calculatedPa =( wp1*lp1 +wc*Lc) BFLength of upper pipe lp2=Well depth-(Lc+lp1)Pa for upper pipe= Pt*0.9- MOP .

Pt is for upper pipe and to be taken from drill data bookPa =( wp1*lp1+wp2*lc2 +wc*Lc) BFPa calculated should be smaller than the permissible load

on drill pipe


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Power for hoisting

Hoisting power=( HLin Kg*Vel of hoisting inm/min)/4500*


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Multiple pulley system

Advantages:Tension in wire line is reducedLoad on derrick is reduced

Velocity of hoisting is reducedWe can drill deeper for a given power by reducingvel.wn= HL/n

Wherewn = tension in wire linen = no of lines


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Power of winding

Hp of winding = wn*vel of winding/4500*

vel of winding = n* vel of hoisting


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Draw works

Draw works is central unit and is important to theextent that rig is known by the name of drawworks, even though the other units may be ofdifferent makes. The main function of the drawwork is to perform hoisting operation. It consists

of the following sub units.Hoisting drumbrakeclutch

control panelcooling system for braking drum.cat head

The power for rotary table is also taken from

draw works.


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Design of drumT= wn*R

Hp = 2 NT/4500*

The diameter and length of the drum should be

designed in manner that the length of the wire ropefor hoisting should be accommodated in two layersonly. The drum is grooved for wrapping the wirerope. The length of the drum should be such that itshould be transportable in commercial vehicles.


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Control panel

All the controls and gauges of the rig areprovided on the panel so that operatormay control all the functions fron one

location. Controls include speed ofhoisting, wob, stroke per minute, pumppr, mud details etc.


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Tackle block

Tackle block consist of crown pulley blocktraveling pulley, hook and wire line.

Crown pulley block: Crown pulley block consisting

a set of pulleys is mounted on top of derrick.Pulleys are mounted on the shaft with double ballbearing in steel frame which is filled withlubricant partially. It has one number of pulleyhigher than no. of pulleys in traveling block.


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T li ll bl k T li ll bl k i ti


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Traveling pulley block: Traveling pulley block consisting aset of pulleys is suspended with the wire rope. The hookis attached to the frame of TB.


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Wire rope: A wire rope is reeved over the pulleys of CB & TB.One end of wire rope is attached to hoist drum and is calledfast line. The other end is connected to dead end is called deadline. The pulley on CB connected to dead line remains

stationary and does not rotate.A wire rope is made of strands and strand is made of wires.Wire rope is specified by X*Y where X is no. of strands in wirerope and Y is no. of wires in strand.

Nominal diameter: The diameter is measured over top ofwires and not on top of strands.

Lay of wire rope: Lay of wire rope is the direction of layingwires in strand and strands in wires in strands.There are two types of lay: Lang lay and regular lay.

Lang lay: in lang lay wires in strands and strands in wire ropeare twisted in same direction. This can be right lay or left layRegular lay: The wires in strands and the strands in wire ropeare twisted in opposite directions.Core The central part of rope is called core which may be

made of fiber or steel.


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swivel

Swivel is installed above kelly, and its mainfunction is to prevent the rotary motion of thekelly being transferred to the traveling block.Swivel is suspended to hook with link. Gooseneck is connected to spindle though neck bush.The other end of the goose neck is connected toflexible hose pipe through which the mud fromthe pump enters the swivel and drill string whichflows down to bit. The neck bush does not rotate

while spindle rotates. The packing between bushand spindle prevents the leakage of mud. Thespindle is supported on heavy duty roller bearingsto take the load of drill string.


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K ll


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KellyKelly is upper most pipe of drill string. It is either square or

hexagonal shape. Its main function is to transmit rotary

motion of rotary table to bit through drill string. Kelly alsoserves as medium for flow of mud to drill pipes and bit.During tipping the kelly with swivel is put in side hole calledrat hole.

Kelly is made of high grade chrome molybdenum

Steel heat treated, quenched and tempered over the length,normalised and tempered. Hardness is Bhn = 281 to 341,length is 12.2m and 16.5m

Diameter is 2.5 inch to 6 inch.Kelly bushing and master bushing are provided in rotary table

for transfer of motion from rotary table to kelly.Kelly savers and kelly cocks are provided at both ends of kelly

to save wear on spindle of swivel and wear on threads ofkelly itself due to screwing and unscrewing of pipes foradding pipe or during tripping.

Kelly cocks are used for closing the passage through kelly incase of kick of over ressure


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Drill StemThe drill stem consists of drill pipes, drill collars,

accessories and bit.

Drill Pipe: The main function of drill pipe is to transmitrotary motion and mud under high pressure to drill bit.

Classification of drill pipes:

Nominal diameter range 63.3mm to 168.3 mmLength 18-22 ft, 27-30 ft, 38-45 ft

Grade of materials E75, X95, G105, S135

End condition Internal upset, External upset and internal

external upset.Class of pipe

New pipe, premium wall thickness 80%, class two wallthick ness 65%, class three wall thickness 55%.


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Drag bits

Drag Bits: Drag bits cuts due to shearing andcrushing action of blades on the formation. Theyare two types: Two blade type and three blade

type. The flushing holes are so placed that themud strikes at 2/3 radius of bit for keepingblades clean.

The front side of the blades are made wear

resistant by welding TC tips. These are used fordrilling in soft and sticky formations.


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Rock roller cone bits

Tri cone rock roller bit: These bits employs rollercones which rotates about their axis.The bit has three main partsLegs, cone and bearings

Legs are made of forged steel by forgingseparately. Each cone is mounted on bearingswhich run on a pin that forms an integral part ofthe bit leg. Three legs are welded together andform the cylindrical section, Which is threaded to

make a pin at top to make connection with drillstring. Each leg is provided with a hole in whicha nozzle is fitted to provide a constriction inorder to obtain a high jetting velocity of mud foreffective cleaning of bore well.


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Design features of roller bit

Design features of roller bit depends on type andhardness of formation and size of hole to bedrilled.

Design features include (a) journal angle

(b) Amount of offset (c) teeth (d) bearings and (e)interrelationship between (c) & (d)

Journal angle: angle between a line perpendicularto axis of journal and axis of bit. The smaller the

journal angle the greater the gouging andscraping action by three cones. The optimumjournal angles for soft and hard rock roller bitsare 330& 360, respectively.


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Offset

Offset angle: The offset is defined as thehorizontal distance between the axis of the bitand vertical plane passing through the axis of

journal. It is the angle between the journal axisand geometrical axis of the cones if they areplaced at 1200.

For hard rocks the offset angle is zero and coneshave pure rolling action. For soft rock rockscones should have offset angle. With offsetcones have rolling and sliding action. To dislodgethe chip cut by bit sliding action is required. Forrocks of medium hardness the skew angle can beupto 20.


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h


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TeethTeeth: The are two types- Integral, milled or cut teeth and

other are inserted teethMilled teeth are normally for comparatively for softer rock andinserted teeth are for harder rocks.

Spacing and interfitting of teeth large spacing is used for softerformation and permits meshing of teeth for cleaning.

Shape and length of teeth: Long, slender and widely spacedteeth are used for drilling soft formation rocks. The includedangle for soft bit tooth ranges from 390 to 420.

For hard formations the teeth are manufactured shorter,heavier and more closely spaced, to with stand the high

compressive loads required to break the rock.For medium hard formation would have moderate no of teeth.


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Bearings

Bearings are used for supporting radialloads, thrust and secure the cones on thelegs. Radial forces are born by roller

bearings and thrust is taken by ballbearing. Ball bearing also works asretainer for the cone.


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Forces on drill pipes and properties

The drill pipes are subjected to followingforces:

Tensile force: Due to self weight,maximum at top

Collapse Pressure: Due to mud pressuremaximum at bottom

Shear force: due to twist of drill pipe

cyclic stresses due to bending

PROPERTIES


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PROPERTIES

The drill pipes should be seamless, straight,

should have uniform thickness. Screws at endsshould not be damaged and Pipes should bestrong to bear all types of forces.

TOOL JOINTSThere are two tool joints, box and pin jointswhich are threaded and welded at both ends ofeach pipe.The joint of two pipes is made between box end

of one pipe and pin end of other pipe. Thethreads on tool joints are tapered to save thetime of screwing and unscrewing. These jointsare replaceable on wear of screws. Thus life ofpipes is increased.


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Drill Collars as discussed before.

Other components of drill string

areReamers

Stabilizers

Tool joints


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Bit selection

Bit selection depends on the followingCost per meter

C = B + ( T + t ) R/L

B = bit cost

T = trip time

T = rotating time

R = Rig cost per hour

L = Length drilled by bitSpecific energy

Bit dullness

Offset well bit records

Chap-2 Rotary Drilling

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