Discussion Of Dulling Characteristics
Following is a discussion and photographs where possible of the dulling characteristics common to roller cone bits. While the possible causes listed and possible solutions for the problem wear modes are not presumed to be exclusive, they do represent situations commonly encountered in the field.
Broken Cones (BC) -
This describes a bit with one or more cones that have been broken into two or more pieces, but with most of the cone still attached to the bit. Broken cones can be caused in several ways. Some of the causes of BC are:
- Cone interference where the cones run on each other after a bearing failure and break one or more of the cones.
- Dropped drill string.
Broken Teeth (BT) -
In some formations BT is a normal wear characteristic of tungsten carbide insert bits and is not necessarily an indicator of any problem in bit selection or operating practices. However if the bit run was uncommonly short, broken teeth could indicate one or more of the following: the need for a shock sub, too much WOB and/or RPM, or improper bit applications. Some causes of BT are:
- Bit run on junk
- Bit hitting a ledge or hitting bottom suddenly.
- Excessive WOB for application. Indicated by broken teeth on the heel or gauge row.
- Formation too hard for bit type. Indicated by broken teeth on inner rows.
Balled Up (BU) -
A balled up bit will show tooth wear caused by a cone, or cones, not turning due to formation being packed between the cones. The bit will look as if a bearing had locked up even though the bearings are still good. Some causes of balling up are:
- Inadequate cleaning of the bottom of the hole.
- Forcing the bit into formation cuttings with the air off.
- Drilling a sticky formation (wet mudstone/shale)
Cracked Cone (CC) -
A cracked cone is the starting of a broken or lost cone and has many of the same possible causes. Some of these causes are:
- Junk on the bottom of the hole
- Bit hitting a ledge or bottom
- Dropped drill string.
- Overheating of the bit. (poor air through the bearings)
- Cone shell erosion.
Cone Dragged (CD) -
This dull characteristic indicates that one or more of the cones did not turn during part of the bit run, indicated by one or more flat wear spots. Some of the possible causes are:
- Bearing failure on one or more of the cones.
- Junk lodging between the cones.
- Pinched bit causing cone interference
- Bit balling up.
- Plugged air courses.
Cone Interference (CI) -
Cone interference often leads to cone grooving and broken teeth and is sometimes mistaken for formation damage. Broken teeth caused by cone interference are not an indicator of improper bit selection. Some of the causes of cone interference are:
- Bit being pinched
- Bearing failure on one or more cones.
Cored (CR) -
A bit is cored when its center most teeth are worn and/or broken off. A bit can also be cored when the nose part of one or more cones is broken. Some things that can cause bits to become cored are:
- Abrasiveness of formation exceeds the wear resistance of the center of the cutters.
- Cone shell erosion resulting in lost cutters.
- Junk in the hole causing breakage of the center cutters.
- Low air volume causing cuttings to pile up in center of the hole.
Chipped Teeth (CT) -
On tungsten carbide insert bits, chipped teeth often become broken teeth. A tooth is considered chipped, as opposed to broken, if a substantial part of the tooth remains above the cone shell. Chipped teeth are not usually a sign of any problems in bit application or operating practices. Some causes of chipped teeth are:
- Impact loading due to rough drilling.
- Slight cone interference.
Erosion (ER) -
Abrasion or “sandblast” erosion leads to a cutter reduction and/or loss of cone shell material. The loss of cone shell material on tungsten carbide insert bits can lead to a loss of inserts due to the reduced support and grip of the cone shell material. Erosion can be caused by:
- Abrasive formation contacting the cone shell between the cutters, caused by tracking, off center wear, or excessive WOB.
- Abrasive formation cuttings eroding the cone shell due to inadequate air. (running on cuttings)
- Excessive air resulting in high velocity erosion.
Flat Crested Wear (FC) -
Flat crested wear is an even reduction in height of all teeth. Flat crested wear occurs when the not enough weight is applied for the tooth to overcome the compressive strength of the rock. The causes of flat crested wear are:
- Low WOB (and high RPM) for the formation
- Bit selected is too soft for the formation being drilled.
Heat Checking (HC) -
Heat Checking happens when an insert is repeatedly heated due to working against, twisting, or dragging on the formation, and is then rapidly cooled either by ground water or by water used for dust suppression over many cycles. HC usually occurs in hard, tough, and abrasive formations.
It is not always a cause of tooth failure, although tooth failure can result from extensive heat checking. HC may also result from inserts being dragged.
Junk Damage (JD) -
Junk damage can be detected by marks on any part of the bit. Junk damage can lead to broken teeth and shortened bit runs and therefore can become a problem. It is sometimes necessary to clear the junk out of the hole before continuing to drill. Some common sources of junk, and therefore causes of junk damage are:
- Junk dropped in the hole from the surface.
- Junk from the drill string (reamer pins, stabilizer blades, etc.)
- Junk from the bit itself (tungsten carbide inserts. etc.)
Lost Cone (LC) -
It is possible to lose one or more cones in many ways. With few exceptions, the lost cone must be cleared from the hole before drilling can resume. Some of the causes of lost cones are:
- Bit hitting hole bottom or a ledge.
- Bearing failure (causing the cone retention system to fail).
Lost Nozzle (LN) -
While LN is not a cutting structure dulling characteristic it is an important “Other Dulling Characteristic” that can help explain a bit run. A lost nozzle causes a pressure decrease which decreases the air to the bearings. A lost nozzle is also source of junk in the hole. Some causes of lost nozzles are:
- Improper nozzle installation.
- Improper nozzle.
- Mechanical or erosion damage to nozzle and/or nozzle retaining system.
Lost Teeth (LT) -
This dulling characteristic leaves entire tungsten carbide inserts in the hole which are far more detrimental to the rest of the bit than are broken inserts. Lost teeth often cause junk damage. Lost teeth are sometimes preceded by rotated inserts. Lost teeth can be caused by:
- Cone shell erosion.
- A crack in the cone that loosens the grip on the insert.
Pinched Bit (PB) -
Bits become pinched when they are mechanically forced to a less than original gauge. Pinched bits can lead to broken teeth, chipped teeth, cone interference, dragged cones and many other cutting structure dulling characteristics. Some possible causes of pinched bits are:
- Bit being forced into under sized hole.
- Bit being pinched in the bit breaker.
- Wrong size bit breaker.
Plugged Nozzle (PN) -
This characteristic does not describe the cutting structure but is useful in providing information about a bit run. A plugged nozzle can lead to reduced hole bailing air and an improperly cleaned hole. Plugged nozzle can be caused by:
- Jamming the bit into cuttings with the air off.
- Solid material entering the drill, string through the bit on a connection and lodging in a nozzle when circulation is resumed.
- Rubber, rust, or other material breaking loose from inside the air hoses or air piping.
- Junk rocks, dirt, paper, soda cans, etc. - left in a new drill pipe just installed on the drill.
Rounded Gauge (RG) -
This describes a bit that experiences gauge wear that rounds the gauge inserts over. This can be caused by:
- Drilling an abrasive formation
- Excessive RPM
- Reaming an under sized hole.
- Bit is too soft for the formation being drilled
Shirttail Damage (SD) -
Shirttail damage may be different thank junk damage and is not a cutting structure dulling characteristic, Shirttail wear can lead to bearing failures. Some causes of shirttail damage are:
- Junk in the hole
- A pinched bit causing the shirttails to be the outermost part of the bit
Self-Sharpening Wear (SS) -
A dull characteristic which occurs when teeth wear in a manner such that they retain somewhat of a cutting edge. This characteristic is often considered an indication of proper bit selection and operating parameters.
Tracking (TR) -
This dulling characteristic occurs when the teeth mesh like a gear into the bottom hole pattern. The tooth wear on a bit that has been tracking will resemble a “tear drop” whereas the cone shell will wear between the rows. Tracking can sometimes be alleviated by using a softer bit to drill the formation or using a different cutting structure pattern.
Tracking can be caused by:
- Formation changes from brittle to plastic
- Hard formations
Worn Teeth (WT) -
This is a normal dulling characteristic of the tungsten carbide insert bits as well as for the steel tooth bits. When WT is noted for steel tooth bits, it is also often appropriate to note self-sharpening (SS) or flat-crested (FC) wear.
No Dull / No Other Wear (NO) -
This code is used to indicate that the dull shows no sign of the other dulling characteristics described. This is often used when a bit is pulled after a short run for a reason not related to the bit, as drill string wash out.