Recommended and legal depth capacity of drill rigs 

November 17, 2020

by Colin Rice, Managing Director at Colin Rice Exploration and Training (Pty) Ltd.

All manufacturers of diamond core drilling machines provide specifications that relate to the depth to which a particular machine can drill. Often, these specifications are confusing because the depth capacities claimed are substantially greater than the ‘legal’ depth capacity of the drill. This can have serious legal implications for both the contractor and the manufacturer.

Hard to believe, but this rope was in use on a mineral exploration project

Drill rigs are configured to hoist using either the main pullback cylinder or a hoist and a steel wire rope. Some drill rigs can pull and lower in both ways. Therefore, most manufacturers of diamond core drilling rigs provide three specifications related to depth capacity:

  • ‘pullback capacity’;
  • hoist single line pull capacity;
  • recommended depth capacity in terms of a range of standard wireline drill rod sizes.

Unfortunately, if we do some simple calculations, we will find that these three specifications imply different things – as a result, the actual and, more importantly, the ‘legal’ depth capacity of the drill is not always obvious and clear enough. So, there stands a fair chance that the contractor will use the drill beyond its safe working limit. Let us consider a very common chuck drive drill rig, built by a highly-respected manufacturer. Table 1 summarizes key specifications related to depth capacity.

Table 1

Pullback capacity 14 MT (138 kN)
Main hoist single line capacity 8.2 MT (80 kN)
Wireline rod size Recommended maximum depth (m) Drill rod mass (kg/m) Mass at max depth (kg)
B 1540 (5052.5 ft) 6.0 (4.03 lb/ft) 9240 (20 370.71 lb)
N 1200 (3937 ft) 7.6 (5.11 lb/ft) 9120 (20 106.16 lb)
H 800 (2624.7 ft) 11.45 (7.69 lb/ft) 9160 (20 194.34 lb)
P 475 (1558.4 ft) 15.3 (10.28 lb/ft) 7268 (16 023.20 lb)

The ‘pullback capacity’ is a calculated value based on the dimensions of the main pullback cylinder and the hydraulic system pressure. It indicates the maximum load that the pullback cylinder can safely apply.

Table 1 includes a column showing the mass of the drill rods and a calculation of the drill string mass at the maximum recommended depth.

It is clear that the total drill string mass of B-, N- and H-size drill strings at the maximum recommended depth are very similar. So, it seems that the Original Equipment Manufacturer (OEM) is saying that the drill can safely lift approximately 9.2 MT of drill string. It is also clear that the total drill string mass of P-size drill rod at the maximum recommended depth is substantially less than 9.2 MT. The reasons for the maximum recommended depth in P-size being only 475 m (1400.9 ft) will become clear later.

If the maximum mass that the drill is expected to pull is only 9.2 MT, why is a pullback capacity of 14 MT necessary?

Load at maximum recommended depth

Good drill rig manufacturers recognize several factors that will affect the total load that the drill has to lift.

  1. The buoyancy effect of the drilling fluid reduces the weight of a drill string. When immersed in fresh water the weight of the drill string is only 87.5% of its weight in the air.
  2. When a drill rig pulls using the pullback cylinder it also has to lift the rotation head.
  3. If the drill bit is blocked and the drill string is tripped ‘wet’, the mass of the drilling fluid inside the drill string will add to the force required to lift the drill string.
  4. In-hole friction (drag) caused by angle changes in the boreholes will increase the force required to lift the drill string.

The worst-case scenario would be when the borehole is at maximum recommended depth, the bit is blocked and there is a total circulation loss (no buoyancy effect), in this case, the total load to be lifted can be determined as follows:

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

Table 2 shows the internal volume of standard wireline drill rods and the total ‘wet’ drill string mass at maximum recommended depth.

Table 2 

Wireline rod size  Drill rod mass (kg/m)  Internal volume (L/m)  Wet mass at max depth (kg) 
B  6.0 (4.03 lb/ft)  1.7 (0.14 gal/ft)  11 858 (26 142.41 lb) 
N  7.6 (5.11 lb/ft)  2.9 (0.23 gal/ft)  12 600 (27 778.25 lb) 
H  11.45 (7.69 lb/ft)  4.8 (0.39 gal/ft)  13 040 (28 748.28 lb) 
P  15.3 (10.28 lb/ft)  8.4 (0.67 gal/ft)  11 258 (24 819.64 lb) 

If we assume that the rotation head has a mass of 250 kg (551.16 lb), then the total mass to be lifted at maximum recommended H-size depth would be:

Total load = 9 160 + 250 + (800 x 4.8) + drag

Total load = 13 250 + drag

We can do similar calculations for the other drill rod sizes and we will get a similar result – load at maximum recommended depth in the worst-case scenario will be approximately 13 MT which only leaves about 1 MT of excess capacity to account for any possible drag in the borehole.

This is the worst possible outcome and it would appear that the rig designers have fully accounted for the additional factors that can affect the total load to be lifted.

It is important to recognize that the above arguments assume that we pull using the pullback cylinder, if however, we trip the drill string using the main hoist and steel wire rope, the situation is somewhat different.

The OEM states that the main hoist has a single line capacity of 80 kN (8.2 MT). We earlier saw that the drill string mass at the maximum recommended depth is about 9.2 MT. Why is there a difference? The answer is simple – if we multiply 9.2 MT by 87.5%, we get approximately 8.2 MT – it is clear therefore that the OEM has based the hoist capacity on boreholes that are full of drilling fluid i.e. when there is maximum buoyancy effect.

It follows that if there is a total circulation loss at maximum depth, or if the bit is blocked, the hoist will not have the capacity to pull a drill string off the bottom. While this is a serious problem, the issue it raises is potentially bigger.

When we use a hoist and steel wire rope to hoist, it is the steel wire rope that is doing the lifting and so irrespective of the capacity of the hoist, the maximum load that any hoist can lift is limited to the Safe Working Load (SWL) of the wire rope fitted to the hoist. In most countries, legislation requires that a factor of safety is applied to steel wire ropes – i.e. the Safe Working Load (SWL) of the rope is the Mean Breaking Load (MBL) divided by the factor of safety.

Clearly, the factor of safety is a critically important parameter, but in many countries, it is not clear what this factor should be when applied to exploration drilling operations. It often varies from industry to industry – in some the law is silent, therefore, its application is left to interpretation.

In South Africa for example, the legislated factor of safety is 6 and, according to my research, it is 5.5 in both Australia and the US. Interestingly, ISO 18758 requires a factor of safety of 3 for steel wire ropes, but this ISO standard would be superseded by existing local legislation. The drill in our example is fitted with a 16 mm (⅝ in) steel wire rope – the OEM does not provide any specifications on the rope that they fit to their drill, but if we assume that the best grade of steel wire rope (2160 MPa) is fitted, the Mean Breaking Load of the rope will be approximately 24 MT. If this drill were to be used in South Africa for example, the SWL of the hoist rope would be only 4 MT – this is the ‘legal’ hoisting capacity of the drill – half of the capacity that the OEM specifies.

Since the capacity of the hoist is 8.2 MT, it seems that the manufacturer has applied a factor of safety of a little less than 3 to their specifications. This is absolutely fine if the drill is used in a jurisdiction where the legislated factor of safety is 3. However, if the required factor of safety is greater than 3, then the contractor will be infringing the law, if he drills to a significant depth. It is interesting that the effective factor of safety for some other commonly used drills is sometimes as low as 2.5.

It is clear that manufacturers should provide more detailed information in their specifications and technical literature to enable contractors (and mining companies) to fully assess the depth capability of the drill rig. Manufacturers should provide two further pieces of information:

  1. Clearly specify the pulling mechanism upon which their recommended depth capacities are based.
  2. State the grade of steel wire rope upon which their hoist capacity is based and the factor of safety upon which the specifications are based.

It is important to recognize that in the example used earlier, it was assumed that the manufacturer fitted the strongest grade of rope available, a 2160 MPa. Hoist ropes are consumable items and if the contractor replaces the hoist rope with one of a lesser grade, then both the recommended and the ‘legal’ capacity of the drill will be significantly less than the manufacturer’s specifications.

In summary, both contractors and mining and exploration companies must know local legislation when it comes to the factor of safety and the grade of rope (i.e. the Mean Breaking Load) that they use. If we are not aware of these two pieces of information, there is a possibility that we could be infringing the law.

Hoist rope in use

About the company 

Colin Rice Exploration Drilling Advisory is a leading provider of training and consulting services to the exploration drilling industry. Based on 34 years of experience in many sectors of the industry, they have developed a unique combination of high-quality training, consulting, safety auditing services, and their newest addition – an online safety initiative, DrillSafe Southern Africa (DrillSafe).

DrillSafe aims to improve safety performance on all drill sites by increasing the knowledge and understanding of major drill site hazards. According to DrillSafe, sharing knowledge and experience and then learning from it, is the only way that the industry objectives of ‘no repeats’ and ‘zero harm’ can be achieved. Every month the company publishes a series of technical articles that examine and explain the major hazards associated with exploration drilling operations.

The company has developed a strong reputation for delivering outstanding quality services based on sound technical ability, independent opinions and absolute integrity.

Their consulting services include assisting mining, exploration and contracting companies to improve production, efficiency and safety performance.

The company is uniquely placed to offer independent, objective and thorough safety auditing services to mining and contracting companies based on a full evaluation of the four critical components of drill site safety – equipment, people, environmental issues and operational procedures. Their safety auditing services provide their customers with an in-depth report on their current safety status and recommendations to improve performance.

More recently, the company has become increasingly involved in assisting several mining companies and research organizations in the development of tender specifications and in the evaluation of tender submissions. An extension of this part of their services is assisting in the resolution of disputes between contractors and clients.

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