By Colin Rice
Wire rope will develop defects during use through several different mechanisms that will affect the strength of the rope and therefore limit the working life of the rope.
This is the first article of Part 2 of our Technical Series on Wire Rope Safety. Click here for an outline of the entire Technical Series on Wire Rope.
Wire rope wear
Wire ropes are used as hoist ropes, wireline retrieval ropes and as pulldown ropes on some drills and in all cases these ropes work extremely hard. As the wire rope begins to carry load, the individual wires that make up the rope will settle into position. As progressive loss of the metallic cross-section takes place due to abrasion, corrosion, wire breaks and by other changes in the structure of the rope takes place, the breaking strength decreases rapidly. As breaking strength gradually decreases, the rope gradually approaches the end of it's service life. Our challenge is to determine when a particular rope has reached the end of it's service life and should be discarded.
In the course of doing many safety audits on operational drill rigs over many years, I have come across some terrifying examples of badly worn hoist ropes that were clearly way beyond their safe working life yet they were still in service and the contractor was oblivious to the danger posed by the ropes. Consequently, I began to investigate wire rope discard standards in our industry – I began by asking a number of contractors a simple question; “when do you change a hoist rope?” The responses that I got were very varied – one said when one wire is broken, some said when 3 wires are broken – the best response however was, “I change the hoist rope before it breaks”!!!!
Obviously, contractors would want to maximize the life of the rope and so there may be a tendency to delay discard and so this issue assumes very great importance in drill site safety management.
Essentially, we have to decide what defects we can accept and what defects warrant the rope being discarded – how many broken wires can we tolerate, how much corrosion can we tolerate and what other defects warrant discard?
This is not a simple exercise, I researched many different industries to see what we could borrow and apply to our industry but there was little that was of direct benefit. Consequently, I developed a very simple and easy to apply set of criteria that can be used as hoist rope discard criteria in the exploration drilling industry. These criteria are listed a bit later in this article.
It is obviously important that the person/s entrusted with conducting inspections of wire rope are able to identify common hoist rope defects and are therefore able to make informed and objective decisions on when to discard a rope. This means that all persons who will be entrusted to inspect wire ropes must be trained in what to look for and how to look for defects when doing the inspection.
As outlined above, we have to inspect wire ropes before the start of every shift and we need to fully examine our ropes (and other lifting tackle) every 6 months – generally the service life of a wire rope is less than 6 months and so the daily inspection procedures assumes very great importance in ensuring that wire ropes (or other lifting tackle) are not used beyond their safe working life.
Inspecting a hoist rope is easier said than done - it isn't simple exercise to inspect a hoist rope. Hoist rope reeving systems vary depending upon the design of the drill but single line systems can be relatively easily inspected on a daily basis.
In a typical 6-meter pull mast using a single line system, with the rope at its lowest position in the cycle, we can think of the hoist rope being in 3 sections; the section in the front of the mast, the section behind the mast and the section on the hoist drum. The total length of the hoist rope will be approximately 26 meters and so each of these sections of rope will approximately 11 to12 meters long. It is relatively simple therefore to visually inspect the entire length of rope by slowly spooling the rope off of the hoist drum and then back onto the hoist drum.
During the inspection, special attention must be placed on the following areas:
- Areas with the highest number of stress reversal cycles caused by the rope moving through sheaves and onto and off of hoist drums. It is in these areas where the greatest number of fatigue wire breaks can be expected.
- At or closely adjacent to terminal fittings.
- Areas of maximum wear on the hoist drum, the anchor point of the hoist rope onto the drum and the cross-over points on the drum (the places where a new layer of rope forms) are places where increased abrasive wear can occur.
Attention must be given also to the evenness of the spooling - the first layer of rope on the hoist drum must be tightly wound to provide a firm base for following layers to spool onto. Poor spooling can cause severe abrasive wear.
The next article in this series explores various wire rope defects.