Published in the October 2016 edition of GeoDrilling International
DOWN TO THE WIRE
Colin Rice of Colin Rice Exploration and Training (Pty) Ltd. presents some observations on steel wire rope safety
Drill-site safety has become an extremely important part of all exploration operations, and many companies have developed and implemented extensive safety management systems to identify hazards and manage risk. Despite this, many drilling operations operate illegally thanks to ignorance of some basic physical principles. One example, and a constantly recurring issue, is the use of wire ropes. Steel wire ropes are widely used on drill rigs for a number of functions, including drill string hoisting, wireline overshot deployment and retrieval and on some drills as pulldown/pullup ropes. Whatever the function of the rope, it will run over one or more sheaves (pulleys). If it is a hoist or wireline rope, it will be wound onto a hoist or winch drum. In every application, the rope can be considered a critical element of the lifting system on a drill rig.
Wire rope is a very complex mechanical device and ropes are available in a great number of diameters, constructions and grades, and because wire rope represents a significant hazard, a factor of safety has to be applied to its application. In South Africa and many other countries, for example, the legislated factor of safety for steel wire ropes is six. This means that in order to determine the Safe Working Load (SWL) of the rope one will divide the mean breaking load (MBL) or proof breaking load by six. In other words, if a rope has a MBL of 15 metric tonnes (t) then the maximum load that can legally be lifted with the rope is 2.5t. In terms of an NQ borehole for example, this means that we can legally pull about 315m of NQ drill rod using a single line. If the borehole is shallower than 315m, then there is no problem, but if the borehole is to be drilled deeper, then the drill string must be pulled using a double line system or using the main cylinder of the drill rig, both of which have time and cost implications for the contractor. Contractors, therefore, frequently ignore the legal requirement and pull rods below the legal limit. Adding to the comlexity, in most countries steel wire ropes are available in four different grades. Consider, for example, a 16mm IWRC hoist rope, the lowest grade will have a MBL of approximately 15t and the highest grade will have an MBL of approximately 24t. The SWL of a 16mm wire rope can therefore vary. It is very important that the contractor knows the grade of rope in use – all wire ropes must therefore be supplied with a valid test certificate that details, among other information, the MBL of the rope. It is then a simple exercise for the mining or exploration company to determine the SWL and the depth limit of the rope.
KNOW WHEN TO DISCARD
Wire ropes are used in drilling operations because they have the ability to change the direction of an applied tensile load and this is achieved by running the rope over a sheave or pulley. However, this adds another dimension of complication to the use of a wire rope. As a rope moves over a sheave it is subjected to a cyclic stress reversal and this leads to fatigue in the affected section of rope. It is clear that the smaller the sheave wheel, the greater the amplitude of the stress reversal, and so the greater the rate of fatigue in the rope. This means that we need to ‘de-rate’ the SWL of a rope by a factor depending upon what is called the PD ratio. This is the ratio of the diameter of the sheave wheel to the rope diameter, and the smaller the PD ratio the greater the de-rating factor that has to be applied to the SWL of the rope. It is beyond the scope of this brief article to delve too deeply into how the de-rating factor is determined, suffice to say that at a PD ratio of about 30, the de-rating factor is approximately 4%, but at a PD ratio of 16 it may be as much as 15%. This effect is very well illustrated if we look at the crown sheave on an Atlas Copco CS14 drill. The sheave wheel and wire rope combination yields a very high PD ratio and so a very low de-rating factor. This large diameter sheave wheel is not, therefore, cosmetic: it is there for a very particular reason. Wire ropes deteriorate over time due to the work that the rope does, but also due to the fact that ropes spool poorly on most drill rigs. As a rope deteriorates, its ability to safely lift a load will diminish and so the critical question becomes: at what point do we discard a wire rope? The decision is affected to some extent by the construction of the rope and also on its application. Many different industries that use wire ropes have developed their own discard standards but none of these are universally applicable to the drilling industry and so we must borrow from some of these. The following criteria are suggested as workable discard criteria for hoist ropes used in drilling applications:
- If three or more broken wires are found in the close proximity to a rope termination;
- If three or more broken wires are found in one strand;
- If rope diameter anywhere is reduced to 90% of the nominal diameter; or
- If a wave, birdcage, knot, loop, kink, localised flattening or any other defect is detected.
Colin Rice Exploration and Training, based in South Africa, is a provider of training courses for the exploration drilling industry