By Colin Rice · Colin Rice Exploration and Training (Pty) Ltd. · www.colinrice.co.za

Manufacturers of hydraulic longstroke drills typically provide three different specifications that give an indication of the depth capacity of their rigs. Since these drills can pull drill rods in two ways, these specifications can be confusing. In this article I explain what the specifications mean and I address the important issues; the safe depth capacity of a drill and the "legal" depth capacity of drill - these are not the same thing!

This is the second article of our Technical Series on Drill Rig Capacity. Click here for an outline of the entire Drill Rig Capacity Series.

Examples of two common Hydraulic Longstroke Drill Rigs

In the first article in this series I explained that hydraulic long stroke drills can trip drill rods in two different ways, using the main feed cylinder and using a hoist and wire rope. Manufacturers typically quote three specifications in their sales literature; the main cylinder pullback capacity, the hoist pull capacity and also the recommended depth capacity in terms of standard wireline drill rod sizes.

These specifications do not tell you the same thing and so they can be confusing and even mis-leading. The main cylinder pullback capacity and the capacity of the hoist are always very different but no manufacturer specifies the method of hoisting to which the specifications apply. Hydraulic longstroke drills are therefore frequently used beyond their legal capacity and this exposes both the contractor and the mining company to significant risk.

To explain the meaning of the drill rig specifications, I have selected two very common hydraulic longstroke drill rigs manufactured by two highly respected manufacturers; the Atlas Copco (Epiroc) CS14 and the Longyear LF90D, both drills are extremely popular in many parts of the world. Tables 1 and 2 below summarise information provided in the published specifications for the drills.

Both manufacturers provide three different specifications:

the main cylinder pullback capacity,
the main hoist single line capacity and,
recommended depth capacities by drill rod size.

I have converted kilonewtons to kilograms and vice versa where necessary so that we can compare the specifications.

Table 1: Summary of specifications for an Atlas Copco CS14 drill (based on vertical boreholes)

Table 2: Summary of specifications for a Longyear LF90D drill (based on fluid filled boreholes)

It is important to note that, neither manufacturer specifies anywhere in their literature whether the recommended depth capacity is based on pulling the drillstring with the main feed cylinder or with the hoist.

In each table I have included a column showing the drill rod mass (in kilograms per meter) and I have used these values to calculate the "total drillstring mass" at the recommended depth capacity. The tables show that, for each drill, the total drillstring mass of BQ, NQ and HQ drillstrings at the maximum recommended depth are very similar and so it seems that Epiroc are saying that their drill has a safe pullback capacity of approximately 9,2 MT and Longyear are saying that the safe pullback capacity of their drill is 8,2 MT. I cannot explain why, for both drills, the PQ recommendation is so much less than it is for the other drill rod sizes.

It is very important to note that, both manufacturers quote a pullback capacity that is much greater than the force required to hoist the drillstring at maximum recommended depth – in the case of the CS14, total drillstring mass at maximum depth is 9,2 MT but the “pullback” is quoted as 14 MT and in the case of the LF90D, total drillstring mass at maximum depth is 8,2 MT but the “pullback” is quoted as 11,4 MT.

Why is this? To understand why manufacturers do this we need to delve a bit deeper into the force required to pull a drillstring at maximum depth.

Load required to pull at maximum recommended depth

The "pullback capacity" is a calculated value based on the dimensions of the main feed cylinder and the maximum hydraulic system pressure and so it indicates the maximum load that the drill rig can safely lift. Good drill rig manufacturers recognise that in-hole conditions can dramatically increase the total load placed on a drill and so they build in a safety factor to prevent a catastrophic failure of the drill. Several factors can affect the total load that a drill has to be able to lift:

1. When a drill rig pulls drill rods it also has to lift the rotation head, and so the total load that the drill has to lift will be greater than merely the total mass of the drillstring.

2. Boreholes are never straight and angle changes in a borehole (doglegs) will result in added in-hole friction (drag) when the drillstring is tripped – the more tortuous the borehole, the greater the drag will be.

3. In some ground conditions it is possible that the drill bit could become blocked by a piece of core stuck in the face of the drill bit. If this happens then the drillstring will be tripped “wet” i.e. full of drilling fluid and the mass of the drilling fluid inside the drillstring will add significantly increase the force required to lift the drillstring.

To illustrate how significant the additional load can be, I have shown the internal volume of standard wireline drill rods in Table 3. The table clearly shows that the increase in load when tripping "wet" is significant; 38% in the case of NQ drill rod and 42% in the case of HQ drill rod.

Table 3: Internal volume of standard wireline drill rods

4. The last factor that we have to take into account when we calculate total load is the buoyancy effect of the drilling fluid. We all know from Archimedes that when a body is immersed in a liquid, it experiences an up-thrust and this up-thrust causes an apparent loss in weight of the body - the weight of a body immersed in a fluid is therefore less than its weight in air.

It is beyond the scope of this article to prove this but, it can be shown that the weight of a body is approximately 12,5% less in fresh water than it is in air. This means that in all situations when we have full return, i.e. the borehole is full of drilling fluid, then the drillstring will weigh only 87,5% of what it weighs in air.

Bearing all of the above in mind we can therefore say that the total load that a drill rig has to lift off bottom can be determines as follows:

Total load = (drillstring mass x 0,875) + mass of rotation head + drag

Similarly, we can say that if the bit is blocked and the drillstring is full of drilling fluid then total load that a drill rig has to exert as it lifts the drillstring off bottom can be determines as follows:

Total load = (drillstring mass x 0,875) + mass of fluid inside drill string + mass of rotation head + drag

The worst possible situation would be when the bit is blocked and there is a total circulation loss (i.e. there is no buoyancy effect). In this case the calculation would be as follows:

Total load = drill string mass + mass of fluid inside drill string + mass of rotation head + drag

Let’s play with some assumptions and calculate the total possible load that a CS14 drill could be asked to lift when at maximum NQ depth, i.e. we assume a total circulation loss and a blocked drill bit. In this case:

Drillstring mass = 1 200 x 7,6 = 9 120kg

Mass of fluid inside the drillstring = 1 200 x 2,9 = 1 203 kg

Mass of rotation head (estimated) = 250 kg

Total drillstring mass = 10 573 kg

Obviously, this is the maximum load that the drill would have to lift, as the drillstring is tripped off bottom, the load on the drill will gradually reduce as drill rods are removed from the drillstring.

In the calculation above, I have not made an allowance for drag (in-hole friction), but given the specified pullback capacity of 14 000 kg for the CS14 drill, we have more than 3 000 kg of spare pullback capacity before the drill reaches its pullback limit. We can do further examples in other drill rod sizes but these will show that in the worst case scenario, the maximum pullback capacity of the drill is unlikely to be reached, even in a very deviated borehole.

This is great news! If we purchase a drill rig from a reputable manufacturer then we can be satisfied that the drill will be able to safely drill to the depths recommended by the manufacturer. Please bear in mind that I have only considered pullback capacity in this discussion, we have to also ensure that the drill has sufficient torque to turn the drillstring at the required rotational speed - this is a different discussion.

It is worth noting that Longyear quote two different depth capacities in their specifications, one is based on the hole being fluid filled and the other is based on a dry hole. If you care to do a quick calculation you will see that Longyear have taken into account the buoyancy effect of fresh water and so the depths recommended in a fluid filled borehole are approximately 12,5% greater than in a dry borehole.

The calculations and arguments above are all based on the drill rig tripping the drillstring using the main feed cylinder and we can be comfortable that if we pull with using the main feed cylinder, with either drill, that we will not exceed the rated pullback capacity of the drill. However, if we trip the drillstring using the main hoist and wire rope however we could be in serious trouble! Let's examine this option.

Depth capacity using the main hoist and wire rope

Both manufacturers fit their drills with a hoist that has a capacity much less than that required to trip a drillstring at maximum recommended depth. The Epiroc CS14 hoist, for example, has a single line capacity of 80kN (approximately 8 200 kg) and the Longyear LF90 hoist has a capacity of 7 258 kg. Provided the borehole is full of drilling fluid (we have full buoyancy effect of the drilling fluid), both drills will (theoretically) just manage to trip the drillstring at maximum recommended depth. It is very important to recognise that should the borehole be deviated, the bit is blocked and there is a total circulation loss, neither hoist has the capacity to pull a drillstring at maximum depth.

This is of huge concern because many contractors to use the hoist despite adverse hole conditions. This is a serious issue for two reasons, firstly, is is not recommended to use a hoist to its maximum rated capacity and secondly, we must remember that, irrespective of the capacity of the hoist, the maximum load that any hoist can lift is limited by the Safe Working Load (SWL) of the wire rope fitted to the hoist. The capacity of a hoist is limited by the Safe Working Load of the rope NOT the capacity of the hoist.

In South Africa (and most other countries), legislation requires that steel wire ropes have to be used with a factor of safety of 6. This means that the Safe Working Load (SWL), the maximum load that we can legally lift with a wire rope, is the Mean Breaking Load (MBL) divided by 6.

The Epiroc CS14 drill is fitted with a 16mm steel wire rope - Epiroc do not provide any specifications on the rope that they fit to their drill and so it is not possible to determine the legal capacity of the CS14 hoist. If we assume however that the drill is fitted with the best grade of steel wire rope available (MBL = 23 MT), then the legal capacity of the hoist is a little less than 4 MT !!

The Longyear LF90 is fitted with a 15mm (actually a 16mm) steel wire rope and Longyear actually quote that the minimum breaking strength of the rope is 23 042 kg (23 MT). Again, the legal capacity of the hoist is only a little less than 4 MT.

Obviously the "legal" capacity of the hoist is determined by the legislated factor of safety in the country in which the drills are being used. For example, I am told that in the USA the factor of safety that must be applied to a steel wire rope is 5,5. In many countries legislation does not specify a factor of safety and so contractors and mining companies should be cautious when operating in these countries. .

I believe that Longyear potentially expose themselves by stating the Mean Breaking Load of the wire rope that they fit to their drill. Wire rope is a consumable and there is no guarantee that a contractor will fit a rope of equal quality when it comes time to replace the hoist rope. If a lesser grade rope is fitted, the capacity of the hoist will also be reduced. In my experience, Buying Departments frequently purchase the least expensive (lowest grade) rope and not the best grade rope. For example, if the lowest grade of 16mm wire rope available (MBL = 12 MT) is fitted to the drill then, the capacity of the hoist will only be 2 MT!!

Why have I highlighted this issue - is it a safety issue? Of course it is, many contractors use hoists to pull loads much greater than the legal capacity of the hoist rope and so expose themselves and their customers to risk. Often this is done because neither the contractor nor the mining company are familiar with the legal requirements. It is obviously important that contractors and mining companies are aware of the risk that they are exposing themselves to if they do so.

The problem really is that manufacturers do not properly specify the capacity of their drills. They provide a list of specifications and then leave it up to the contractor to use the rig. In my opinion, this is wrong and most certainly in South Africa, could conceivably land a manufacturer on the wrong side of a legal dispute. So then, how do we solve the problem? I believe that it is very simple, the solution to the problem rests with the manufacturers and I recommend that:

Manufacturers consider fitting hoists with lesser capacity so that the capacity of the hoist is less than or equal to the safe working load of the wire rope used.
Manufacturers change their sales literature to clearly state that the recommended depth capacities are based on tripping the drillstring with the main feed cylinder and not the hoist.
Manufacturers insert a clause in their specifications along these lines - "the hoisting capacity of the main hoist is limited to the safe working load of the steel wire rope used".

In the Legal Section I examine in some detail, the responsibilities of manufacturers and suppliers in terms of the Mine Health and Safety Act.

The next article (to be published at the end of May 2018) discusses the depth capacity of top drive drill rigs.