So You Think You Want to Cut Hard Rock?

In the rock reaming world, size matters.

Cone size, bearing size, seal size, carbide size — all of these have a direct influence on hole opener performance and thereby the overall cost and success of your reaming jobs. In order to make the proper selection of tooling for a hard rock bore, it is important to have a good understanding of the differences the size of these components can make.

Cone and Bearing Size

Let’s begin with the size of the individual cones on your selected hole opener. Larger cones designed specifically for the reaming industry offer two distinct advantages.

1. Larger diameter cones increase the cone length allowing you to take larger cuts thereby eliminating passes. This saves trips, tools and improves the overall safety of the job.

2. Larger diameter cones allow manufacturers to provide larger bearings and seals. This translates into fewer cone revolutions per hole opener RPM thereby giving you longer bearing and seal life. The longer you can stay on bottom drilling, the more cost-effective your tool becomes.

As you can see in Figure 1, increasing the cone size has a direct impact on the cone revolution per hole opener revolution. By utilizing larger cones you reduce the number of cone revolutions while taking a larger cut in the rock, thus reducing the number of reaming passes necessary. The larger bearing also allows for greater load capacity.

If you were to look at a job requiring a 24-in. product pipe, a typical run option with smaller size cones would be a 9 7/8-in. pilot hole, first ream pass of 20 in., second ream pass of 28 or 30 in. and a final ream pass of 36 in.

By using larger cones, a much better option would be a 9 7/8-in. or 10 5/8-in. pilot, first ream pass of 24 in. and final ream pass of 36 in. The clear advantage is the elimination of one reaming pass saving time and money.

When it comes to bearings, they must be specifically designed to allow for greater load capacity and to handle the higher weights required by the HDD industry. In oilfield applications, seldom is more than 80,000 lbs of force loaded on the drill bit. In the HDD industry, it is not uncommon to see in excess of 180,000 lbs used to fail the rock effectively.

Most bearing designs consist of a main roller bearing, ball race, thrust bearing and nose bearing (roller or sleeve). By applying force across the entire cone face or at least the majority of it, we utilize all the bearings properly to distribute the load. Smaller cuts lead to point loading and possible premature bearing failure. In the end, it will cost more time, labor and tools to take smaller cuts than it would to ream more efficiently with hole openers that have larger cones and require fewer reaming passes.

Seal Size

Seal size is also a critical component in the design features of cones. By having a larger effective seal area, it gives an advantage of longer seal life.

Larger Inserts

In a comprehensive study and through varied experience in many drilling applications, it was concluded that fewer bottom inserts were necessary to effectively fail harder rock formations. The study also revealed that conventional designs utilized a different cutting structure that did not allow for enough weight to be placed on hole openers to effectively fail the rock. To determine the best insert configuration, it is important to look at the maximum weight that can be put on a bit due to the torsion limits of the drill string and also the amount of weight per insert necessary to fail the rock.

The study also found that to fail hard rock, a full cone cutting structure, with  five to six very large carbide inserts on the bottom at a time, is needed. This results in a cone that can cut hard rock effectively even though it has the look of a soft formation tool. This offers users greater flexibility for boring in areas where the lithology is not well known with a tool that can cut both soft and hard formations efficiently even though it is designed to cut hard applications without twisting off.

Cutting Zone Illustration

The following case study illustrates the advantage of using larger carbides.

In this situation, the 30-in. hole opener with five cones had a conical cutting structure for medium hard formations. The cone had a “cutting window” of approximately 2 in. that is the section of the cone that is inline with the face of the rock. This cone profile had eight carbides contacting the rock at any given time. In this case, the hole opener had 180,000 lbs of weight being applied to it. The carbide insert weight is shown in Figure 2.

The rock was being failed at 4,500 lbs per insert with a 30-in. hole opener. For the final ream pass, a 42-in. hole opener with six cones utilizing the same conical cutting structure was used. Due to the extra cone, it was necessary to increase the weight on the hole opener to achieve the same 4,500 lbs per insert.

In order to calculate the amount of weight that needed to be applied to the hole opener to effectively fail the rock: 4,500 lbs per insert multiplied by eight inserts per cone equals 36,000 lbs. Multiply 36,000 lbs. by six cones on a 42-in. hole opener and you get 216,000 lbs of weight that needs to be applied to the hole opener. This amount of weight is very hard to achieve without stalling the hole opener due to torque.

Let’s now look at how larger carbides can help us in a case like this. Cones with larger carbides will invariably have less carbide inserts due to their size.

So using the same formula as above, but with cones that have larger carbides, we now have five carbides touching the rock within our cutting window at any given time. In this case, the hole opener would have 135,000 lbs of weight being applied to it as shown in Figure 3.

The larger cones allow you to take the same size cut, but reduces the amount of weight needed on the hole opener to fail the rock, thereby also reducing added torque to your drill string.


Before you bid on a hard rock bore make sure you consider the following questions. Do you have the equipment to handle the compressive strength of the rock? Is the overall bore design compatible? Do you have the proper personnel to run a successful hard rock bore? If you answer yes to these questions and you win the bid for the next hard rock bore, then make sure you use the latest technology in cone, bearing, seal and insert design. Failure is easy, but success will set you apart.

Todd Bielawa is president Century Products Inc., Sussex, Wis.

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