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Roller Cone Dull Grading

Roller Cone Dull Grading

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Roller Cone Dull Grading

Roller Cone Wear Scale

• All the cutters are averaged on a zero to eight scale

• Zero is no wear & eight is flush with cone steel

Measured Distance

Amount outof Gage

MeasuredDistance

= X 2/3

Broken Cone (BC)

• Hydrogen embrittlement - Photo

• Hitting a ledge during a trip

• Cone interference after bearing failure

Broken Teeth (BT)

• Can be normal dulling characteristic

• Caused by rock that is harder that carbide

• Excessive inner row BT - Maybe too much WOB

• Excessive outer row BT - Maybe too much RPM

• Run on junk, hit a ledge, and poor break in practices

Worn Teeth (WT)

• Most common characteristic

• Normal in most applications

Balled Bit (BU)

• Inadequate hydraulics

• Re-drilling cuttings without flow

• Sticky formations

Cracked Cone (CC)

• Cone shell wear

• Junk left on bottom

• Hitting a ledge or bottom

• Dropped drill string

• Hydrogen sulfide embrittlement

• Overheating

Cone Dragged (CD)

• Bearing failure

• Junk lodged between cones

• Pinched bit that caused cone interference

• Balled up bit

Cone Interference (CI)

• Pinched bit

• Reaming undergage hole with too much WOB

• Seal failure

Cored Bit (CR)

• Preceding cored bit

• Improper break in procedure

• Junk

• Lost teeth & cone shell erosion

Cored Bottom Hole Pattern

• If care is not taken to establish a new bottom hole pattern, there is a high probability that the next bit will core

Standard bit

Cored bit

Chipped Teeth (CT)

• Chipped is substantial part remains above the cone shell

• Usually leads to breakage

• Usually not indicative of any application or operating problems

Erosion (ER)

• Abrasive cuttings

• Abrasive mud

• Can lead to lost teeth and cracked cones

• Poor bottom hole cleaning

• Excessive jet velocity

• Running HX bits in abrasive formations

Flat Crested Wear (FC)

• Normal wear pattern in most applications

• Can be caused by decreased WOB and increased RPM to improve deviation

Heat Checking (HC)

• Repeated heating and cooling of compacts

• In the right application, HC can lead to broken teeth

Junk Damage (JD)

• Common sources of junk, thereby junk damage are: junk dropped in the hole from the surface, drill string junk, junk from the previous bit or junk from the bit itself

Lost Cone (LC)

• Bearing failure

• Hitting a ledge or bottom

• Dropped drill string

• H2S embrittlement

Lost Nozzle (LN)

• Improper installation

• Mechanical or erosive damage to nozzle or nozzle retention system

Lost Teeth (LT)

• Caused by loss of grip around the compact– Cone shell erosion– Corrosion– Cracked cone– Oversize hole & undersize compact– Compact pressing problems

Off Center Wear (OC)

• Formation change from brittle to plastic

• Inadequate stabilization in deviated hole

• Improper bit type

• Motor runs with AKO settings

• Overbalanced drilling

Off Center Wear cont.

True running bits leave little or no

uncut bottom.

3 2

B E A R IN GB IT

1

Off Center Wear cont.

Off center

running bits

leave uncut ridges.

Pinched Bit (PB)

• Mechanically forced to less than original gage– Forcing the bit into an under gage hole– Forcing the bit through casing that does not

drift to the bit size– Pinching in the bit breaker– Reaming

Pinched Bit (PB) cont.

• Signs of cone interference and under gage with effective bearings

Plugged Nozzle (PN)

• Jamming the bit into bottom hole fill with the pump off

• Solid material pumped through the drill string• Cuttings U-tubing into bit during connection• Asymmetrical nozzles

– Pebbles entering through large nozzle and then plugging the smaller ones

Rounded Gage (RG)

• Heel teeth out of gage but the cone back face is still in gage

• Common in abrasive rocks

• Excessive rotary speeds in abrasive rock

Rounded Gage - Step Gage

Rounded Gage cont.

Rounded Gage cont.

Shirttail Damage (SD)

• Different than junk damage and is not a cutting structure dull characteristic– Junk in the hole– Reaming an under gage hole in faulted or

broken formations– Pinched bit that makes the shirttail the OD

Self Sharpening Wear (SS)

• Commonly used in the past, but not recommended for dull grading today - usually something else to describe the dull

• Not prevalent today because hard facing is placed all over the tooth

• SS on new designs can be a sign of tracking

Tracking (TR)

• Teeth meshing like gears with the hole bottom– Formation change from brittle to plastic– Hydrostatic pressure significantly exceeds the

formation pressure (Overbalanced)– Change bit type to change bottom hole pattern

Weld Wash Out (WO)

• Poor welding - Very rare!

Practice

• 5-8-WT-A-E-FC-HRS– Even wear indicates this was the proper

application for this bit type

Practice

• 7-1-BT-M-E-I-WT-PR– Inner row tooth breakage indicates the bit failed

in a formation harder than was intended by the design

– Excessive WOB also indicative of inner row breakage

– Only an experienced field technician can say for certain