ROLLING CUTTER BITS

• Rolling Cutter Bits

– Steel Tooth (milled tooth)

– Carbide Tooth (tungsten carbide insert)

• Introduced in 1909 by Howard Hughes

– 2 - cone bit

– Not self-cleaning

Tungsten Carbide Insert Bit

Milled ToothBit

ROLLING CUTTER BITS

• The three-cone rolling cutter bit is by far the most

common bit

• Large variety of tooth design and bearing types

• Maximum use is made of limited space

• Cone offset to stop rotating periodically to scrape

the hole like (PCD) bits

• It increases drilling speed but tooth wears faster. (4

for soft, 0 for hard)

ROLLING CUTTER BITS

• Shape of teeth: long widely spaced steel teeth are

used for drilling soft formations

• As the rock type gets harder the tooth length and

cone offset must be reduced to prevent tooth

breakage

• Tooth action = Scraping and twisting

• Zero offset cones action = Crushing

• Smaller tooth allows more room for the

construction of stronger bearings

Classification of Tricone Bits(a)(a) Milled tooth cuttersMilled tooth cutters

(b)(b) Tungsten carbide insert cuttersTungsten carbide insert cutters

• Hard facing on one side of the tooth allows self

sharpening

• Chipping tends to keep tooth sharp

• Intermeshing is advantageous.

• Heel teeth = outer-raw very difficult job it wears it

leads to out of gauge bit (hole)

Example tungsten carbide insert cutter Example tungsten carbide insert cutter used in rolling cutter Bitsused in rolling cutter Bits

ROLLING CUTTER BITS• Improvements

– 3 - cone bit (straighter hole)

– Intermeshing teeth (better cleaning)

– Hard-facing on teeth and body

– Steel Tooth (milled tooth)

– Carbide Tooth (tungsten carbide insert)

– Change from water courses to jets

– Tungsten carbide inserts

– Sealed bearings

– Journal bearings

ROLLING CUTTER BITS

• Advantages

– For any type of formation there is a

suitable design of rock bit

– Can handle changes in formation

– Acceptable life and drilling rate

– Reasonable cost

Fluid flow through water courses in bit

Proper bottomhole cleaning is

very important

Fluid flow through jets in the bit (nozzles)

ROLLING CUTTER BITS• Milled Tooth Bit (Steel Tooth)

– Long teeth for soft formations

– Shorter teeth for harder formations

– Cone off-set in soft-formation bit results in

scraping gouging action

– Self-sharpening teeth by using hard-facing

on one side

– High drilling rates - specially in softer rocks

Milled Tooth Bit

(Steel Tooth)

ROLLING CUTTER BITS

• Tungsten Carbide Insert Bits

– Long life cutting structure in hard rocks

– Hemispherical inserts for very hard rocks

– Larger and more pointed inserts for softer rock

– Can handle high bit weights and high RPM

– Inserts fail through breakage rather than wear

– Tungsten carbide: very hard, brittle material

Tungsten

Carbide Insert Bits

Roller Cone Bearings

Bearings

• Ball Bearings (point contact)

• Roller Bearings (line contact)

• Journal bearing (area contact)

• Lubrication by drilling fluid . . . or . . .

Bearings

Sealed Bearings (since 1959) Grease lubricant (much longer life)

Pressure surges can cause seal to leak!

Compensate?

Journal Bearings (area contact) Wear-resistant hard surface on journal

Solid lubricant inside cone journal race

O - ring seal

Grease

Grading of Dull Bits How do bits wear out?

• Tooth wear or loss

• Worn bearings

• Gauge wear

Grading of Dull BitsHow do bits wear out?

• Steel teeth - graded in eights of original tooth height that has worn away

e.g. T3 means that 3/8 of the original tooth height is worn away

Grading of Dull Bits Broken or Lost Teeth

• Tungsten Carbide Insert bit

e.g. T3 means that 3/8 of the inserts are broken or lost

Grading of Dull Bits How do bits fail?

• Bearings: B3 means that an estimated 3/8 of the bearing life is gone

Balled up Bit Cracked Cone

Grading of Dull Bits How do bits fail?

Washed out Bit Lost Cone

Grading of Dull Bits How do bits wear out?

Examples:

• T3 – B3 - I

• T5 – B4 - 0 1/2

• Gauge Wear:• Bit is either in-Gauge or out-of-Gauge• Measure wear on diameter (in inches),

using a gauge ring

BIT

GAUGE RING