By Colin Rice
Colin Rice Exploration Drilling Advisory - www.colinrice.co.za
We are all familiar with nylon ropes, these ropes are commonly known as ski ropes and are used in many everyday applications. Nylon ropes stretch a great deal and so they store a great deal of energy when under load. In addition, nylon absorbs water and is 10–15% weaker when wet than when dry, these types of ropes are therefore unsuitable for a drilling application. There are other synthetic ropes however that are very suitable for drilling applications and in fact offer some significant advantages over steel ropes.
Synthetic ropes are ropes manufactured by braiding together one or more different types of synthetic fibre. Synthetic fibres are “man-made” fibres such as, nylon, polyester,polypropyleneor high performance fibres such as high modulus polyethylene (HMPE).
Synthetic ropes are manufactured in a number of different designs to suit different applications, for example, mining, construction, fishing, and as mooring ropes for ocean liners.
These ropes are called braided ropes and some of the designs are shown in the illustration below.
A number of extreme high strength (high modulus) synthetic fibres are now available and a number of different synthetic ropes are available that incorporate these high strength fibres. One of these high modulus fibres is Dyneema, a super strong and lightweight man-made fibre that is many times stronger than steel and has very little stretch. Synthetic ropes manufactured from Dyneema are called High Molecular Weight Polyethylene (HMPE) ropes.
Other high strength fibres are available but Dyneema has properties best suited to drilling applications.
Properties of HMPE ropes
HMPE ropes offer some remarkable properties that make them very suitable for use as hoist ropes in drilling applications; they are stronger than a steel rope of the same size, they have very low stretch and so do not store energy when under a tensile load, they are much more flexible than steel ropes and they do not suffer fatigue and wear in the same way that steel ropes do. In addition, these ropes are very lightweight and they have a density less than one and so float on water.
Strength of synthetic ropes
HMPE ropes are substantially stronger then steel ropes of the same diameter. The table below summarises the Mean Breaking Loads for a range of steel wire ropes used in the drilling industry and also the breaking strength of an equal size HMPE rope. Data for the two most common grades of steel rope are included.
In 16mm size both high grade steel rope and the HMPE rope have similar strength characteristics and so there is little apparently little advantage in switching to the synthetic rope. As the rope size increases however, there is an increasing advantage when using the synthetic rope - a 22 mm HMPE rope is 26% stronger than the highest commonly available grade of steel rope and so it will significantly increase the hoisting capacity.
It is important to recognise that in the table above, the percentage increase in MBL shown is based on the highest grade of steel wire rope, obviously, if a contractor uses a weaker grade of rope, the percentage increase in hoisting capacity will be dramatically greater.
It seems therefore that a correctly selected synthetic rope will significantly increase the hoisting capacity and therefore depth capacity of drill rigs that trip the drillstring using a hoist and wire rope.
Flexibility and fatigue resistance
By their very nature, steel wire ropes are subjected to cyclic stress reversals every time that they are spooled off of the hoist drum and every time that the rope runs through a sheave. We know also that the magnitude of the stress reversal depends upon the load that the rope is carrying and the PD ratio, this is the sheave wheel diameter divided by the rope diameter. If the PD ratio is very small, then the rate of fatigue will be high. Fatigue causes both internal and external wire breaks and this is one of the main reasons for discarding steel wire ropes.
A second major problem with steel wire ropes is abrasive wear on the external wires. This occurs in two places – at the sheave wheel and on the hoist drum due to haphazard spooling of the rope.
To minimise wear at the sheave wheel, it is necessary that the sheave wheel groove is sized for the diameter of rope in use. A drill rig that is designed to use a 16mm hoist rope will have a sheave wheel specifically grooved to accept a 16mm rope. This means that if a contractor wishes to change from a 16 to a 19 mm rope to increase pulling capacity, he will have to change the crown sheave to suit the larger diameter rope. If this is not done then very rapid abrasive wear will result. HMPE ropes are more infinitely more flexible that steel ropes and they do not fatigue as do steel ropes. They are able to tolerate a greater level of distortion through a sheave wheel than steel ropes can and so rates of abrasive wear on HMPE ropes will be substantially less than in steel ropes.
Irregular, haphazard spooling of steel rope onto the hoist drum is a major source of abrasive wear and damage to ropes. Some hoist designs are better than others but on almost all drill rig hoists, steel ropes spool erratically. Because of their flexibility and design, HMPE ropes do not suffer the same degree of abrasive wear on the hoist drum that steel ropes do – in fact, it is recommended that synthetic ropes are spooled haphazardly. This is a major advantage of using synthetic ropes in a drilling application.
Why use synthetic ropes?
From the above, it is clear that HMPE ropes potentially provide some very great advantages over steel wire ropes in drilling applications - greater strength on a size for size basis, better flexibility and better wear resistance.
Other advantages are that HMPE ropes do not require lubrication, they are “non-spin”, they will not rust and they will not kink, or “bird’s nest” as wire ropes do. HMPE ropes however offer one further significant advantages over steel ropes.
As an HMPE rope is used it will “bed in”, it will stretch slightly and the diameter of the rope will reduce slightly. The reduction in diameter is due only to the individual fibres “bedding down” it does not reduce the strength of the rope. The reduction in diameter will depend upon the design of the rope but it could be expected that a 24 mm rope, for example, would become a 22 mm rope as the rope is used.
This offers a significant advantage for the contractor because it would be possible now to run a 24mm HMPE rope in a 22mm sheave wheel. The increase in hoisting capacity that this would offer is substantial. Assume that a drill rig is running a 22 mm steel rope of the highest quality, assuming a factor of safety of 6, the drill will have a legal lifting capacity of 5,4 MT (475 metres of HQ drill rod); if a 24 mm HMPE rope is run instead, then the legal lifting capacity of the drill increases to 8,2 MT (720 metres of HQ drill rod). This is a very significant difference and provides a strong argument to consider HMPE ropes.