DRILLING WITH HOLE OPENERS
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OPERATIONAL ASPECT AND ADVANTAGE  OF DRILLING WITH HOLE OPENERS

With a capability of drilling a larger diameter hole in comparison with Rock Roller Bits, drilling with hole openers is becoming more popular in certain parts of the world. Hole opener provides convenient way to drill larger diameter hole for setting surface casing.

The larger the diameter of 3 cone RR Bit, the more difficulty in drilling a straight hole by drilling a small pilot hole with a small tricone or a hammer bit a straight hole can be drilled more easily . After a straight pilot hole is drilled the hole opener will then follow the pilot hole to result in a straight enlarged diameter hole.

Drilling with larger diameter 3 cone RR bits more costly and expensive in comparison to drillers with hole openers. The recommended weight necessary to give good penetration with larger diameter 3 cone RR bits often  prohibits drilling with smaller capacity rigs . However, by using multiple passes with small tricones and hole openers, larger diameter hole can be drilled.

The use of 3 Cone RR bits is limited to only certain sizes and formations. Hole openers can be built to any size with any formation cutters. To reduce the cost the cutters on hole openers are replaceable with the some body and can also be redressed locally.       

OPERATIONAL CONSIDERATION :

RPM

The speed at which a hole opener is turned is considerably slower than the speed of a tricone bit. For example, a 24” hole opener with 12-1/4” cutters would be turned at 12.25/24 x regular tricone speed. In general, a hole opener should be run at about 50% of the speed of a tricone.

WEIGHTS

The optimum weight for hole openers is the same as it would be for the tricone from which the hole opener cutters came. For example a 24” Hole opener with three 12 ¼” cutters will require the same weight as a 12 ¼” tricone. If the hole opener has four cutters, the weight should be increased by 33%.

HOLE CLEANING

An important factor of drilling any hole is good hole cleaning . With larger diameter holes, the volume of chips to be removed increases quickly. For example the  volume of a 24” hole is over four times the volume of a 12” hole. When using a hole opener, it is usually advantageous to plug two of the jets of the pilot bit to force the  bailing medium to the cutters of the hole opener.

VIBRATIONS

In general, the larger the number of cutters, the smoother the operation. However, the greater the number of cutters, the greater the weight needed for optimum penetration. Hole openers are usually available with three to five cutters.