How to work our the strength of a piece of rope

Knowing how rope is categorised in terms of strength is essential for safe management of your rig and shore lines

Rope strength is measured in a variety of ways but it is important that the end user understands what that strength is in order to decide whether the rope is suitable for its intended use.

A rope’s breaking strain, tensile strength or minimum breaking strength refers to the minimum amount of force required to break it. In terms of rope this is often expressed in tonnes.

The safe working load (SWL) (also known as normal working load – NWL) is the maximum load that can be applied to the rope without fear of it breaking. This will be anywhere from one-tenth to a quarter of the minimum breaking strength of the rope and will be determined by the manufacturer.

Some rope manufacturers also specify the working load limit (WLL). This is often used interchangeably with SWL, but it should be remembered that it is not the same thing, as it is not necessarily the maximum load in a specific application.

In simple terms, when selecting a rope you will need one with a breaking strain that is many times greater than the maximum load the rope is likely to encounter.

However, the following factors can all have a negative impact on both the breaking strain and the safe working load of any given rope.

Knots and splices

Knotting or splicing a rope can reduce its strength, in some cases by more than 50 per cent. Therefore, in selecting a rope, it is essential to ensure that the need to knot or splice is taken into account when calculating the necessary strength that will be required. As a general rule, a properly constructed splice will not reduce the strength of a rope to the same extent as a knot might, but it will still have the effect of weakening the rope.

Ultraviolet light

Ultraviolet light will, over time, reduce the strength of a rope. Ensuring there is a protective covering on lines such as HMPE will certainly delay the impact of UV but it will not prevent its weakening effect completely. There is no foolproof way of calculating the extent of the impact of the UV on the strength of a rope.

Shock loading

Subjecting a rope to shock loading (for example, a preventer line that is subjected to the boom slamming during an uncontrolled gybe) will result in weakening of that line. A static line (one that has little stretch capability), will be impacted more severely than a dynamic rope (one that can stretch).

Ageing

The simple fact that a rope has been used, has encountered some loading, and has been exposed to the elements will have the effect of weakening it from the manufacturer’s original recommended breaking strain when it was new.

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