Rock Bit Evaluation
As each bit is pulled from the hole its physical appearance is inspected and graded according to the wear it has sustained.
WhyThe evaluation of bits is useful for the following reasons:
- To improve bit type selection
- To identify the effects of WOB, RPM, etc., which may be altered to improve the performance of the next bit
- To allow drilling personnel to improve their ability to recognize when a bit should be pulled (i.e. to correlate the performance of a bit downhole with its physical appearance on surface)
- To evaluate bit performance and help to improve their design
Bit RecordA bit record will always be kept by the operating company, drilling contractor and/or bit vendor. This bit record is used to store the following information about the bit after it has completed its run:
- The bit size type and classification
- The operating parameters
- The condition of the bit when pulled
- The performance of the bit
The IADC Dull Grading
Column 1 - Cutting Structure Inner Row (I) :
- Report the condition of the cutting structure on the inner 2/3 rds of the bit for roller cone bits and inner 2/3 rds radius of a fixed cutter bit (Figure 1)
Column 2 - Cutting Structure Outer Row (O)
- Report the condition of the cutting structure on the outer 1/3 rd of the bit for roller cone bits and outer 1/3 rds radius of a fixed cutter bit.
In column 1 and 2 a linear scale from 0 to 8 is used to describe the condition of the cutting structure
- STEEL TOOTH BITS : a measure of the lost tooth height.
0 - Indicates no loss of tooth height due to wear or breakage
8 - indicates total loss of tooth height due to wear or breakage
- INSERT BITS : a measure of total cutting structure reduction due to lost, worn and/or broken inserts
0 - Indicates no lost, worn and/or broken inserts
8 - Indicates total loss of cutting structure due to lost, worn and/or broken inserts
- FIXED CUTTER : a measure of the cutting structure wear
0 - Indicates no loss of cutter or diamond height due to wear or breakage
8 - Indicates total loss of cutter or diamond height due to wear or breakage
The tooth wear of insert bits is a measure of the combined cutting structure reduction due to lost, worn and broken inserts. For example, if 50% of the inner inserts are broken and/or lost and the remaining inner inserts have no reduction in height, the bit is graded T-4 in column 1. If 50% of the outer inserts are lost and the remaining inserts on the outer rows have 50% reduction in height, the bit is graded T-6 in column 2.
Column 3 - Cutting Structure Dull Characteristics (D)Report the major dull characteristics of the bit cutting structure based on the table below
|bit dull characteristic|
* Show Cone # or #’s under column 4.
Some of the dull characteristics listed above are self explanatory. More detailed description of the two-letter codes can be obtained from bit companies. Examples of some of the dull characteristics are shown below.
Column 4 - Cutting Structure Location (L)Report the location on the face of the bit where the major cutting structure dulling characteristic occurs. This may be reported in the form of a letter or number code The location of dull characteristics for four fixed bit profiles is shown in below table
Location is defined as follows:
- Gage - Those cutting elements which touch the hole wall.
- Nose - The centermost cutting element(s) of the bit.
- Middle - Cutting elements between the nose and the gage
- All - All Rows
Cone numbers are identified as follows:
- The number one cone contains the centermost cutting element.
- Cones two and three follow in a clockwise orientation as viewed looking down at thecutting structure with the bit sitting on the pin.
Column 5 - Bearing Condition (B)Report the bearing condition of roller cone bits. The grading will depend on the type of bit. This space will always be occupied by an ‘X’ for fixed cutter bits.
- NON - SEALED BEARING BITS : a linear scale from 0-8 to indicate the amount of bearing life that has been used :
0 - Indicates that no bearing life has been used ( new bearing )
8 - Indicates that all of the bearing life has been used ( locked or lost )
- SEALED BEARING BITS : a letter scale to indicate the condition of the seal :
E - Indicates an effective seal
F - Indicates a failed seal
N - Indicates “not able to grade”
Column 6 - Gauge (G) :Report on the gauge of the bit. The letter “I” is used if the bit has no gauge reduction. If the bit does have a reduction in gage it is to be recorded in 1/16th of an inch.
The Two Thirds Rule, as used for three-cone bits, requires that the gage ring be pulled so that it contacts two of the cones at their outermost points. Then the distance between the outermost point of the third cone and the gage ring is multiplied by 2/3’s and rounded to the nearest 1/16th of an inch to give the correct diameter reduction.
|The Two Thirds Rule|
Column 7 - Remarks (O) :Report any dulling characteristic of the bit in addition to that reported for the cutting structure in column 3. Note that this is not restricted to only the cutting structure dull characteristic. The two letter codes to be used in this column are shown below.
|bit dull characteristic|
Column 8 - Reason for Pulling (R) :Report the reason for pulling the bit out of the hole. This may be a two or three letter code as shown in below
|Reason for POOH|