200709 Care of wire ropes

Adapted from Britannia Risk Watch Vol 13, No 2

A three-year old cargo ship suffered a failure of the hoist wire rope in one of her deck cargo cranes. The failure caused severe injury to a stevedore and gave rise to a lengthy off-hire period.

Root cause/contributory factors
Although the report does not specifically identify the reasons for wire rope failure in this particular incident, experts rule that the following typical in-service circumstances can lead to rapid deterioration of wire ropes on ships' cargo cranes:

Ineffective inspection technique;
Inadequate lubrication, especially internally between the strands and in the fibre core;
Inappropriate lubrication, especially a thick external coating of grease, which prevents proper inspection and is likely to trap moisture and accelerate corrosion;
Corrosive or abrasive cargo particles adhering to the wire rope and accelerating wear of the wires as well as other contact surfaces;
Contact damage with cracked or broken sheave;
Chafing damage against ship's structure;
Exposure to detrimental environmental conditions e.g. salt, moisture, airborne sand and dust;
Improper winding under load;
Crushing, chafing damage as a result of wire rope jumping off sheave and jamming between sheave and side plate/cross member.

Other lessons
In all wire ropes, the strands and constituting wires are in motion relative to each other and, primarily, lubrication is meant to reduce this friction.
The ideal lubricant must be a relatively thin oil-based, water-resistant liquid containing appropriate additives.
During manufacture, the lubricant is applied under pressure and is designed not only to coat the exterior but also to penetrate and remain within the core of the rope. Such a degree of lubrication is difficult to achieve on board a ship, particularly in situ, on a rigged lifting appliance.
On a typical shipboard crane, the two main wire ropes are the hoist and the luff. The hoist, especially if attached to grabs, is subject to greater wear, but statistics show that the luff is more prone to part under load.
Wire ropes should be inspected monthly, regardless of use. If they have been exposed to cargo residues, sand or dust, or if the old lubricant has 'dried' or 'caked', the surfaces must be cleaned by brushing.
A suitable penetrative lubricant applied as a thin coat, preferably under warm ambient conditions, taking care that 'idle' turns on the winch drum and around 'static sections'on purchases and sheaves are run out and treated.
When a crane is stowed for sea the jib is put into a cradle and the ropes are slackened off, the wires are constantly on the move due to the motion of the ship. In addition, the same part of the wire is bent around the sheaves and thereby partially opened up, giving access to seawater, airborne sand and dust.
Annually, before class inspections, a rope should be inspected minutely over its entire length and then should be opened up at convenient intervals with special tools to inspect the inside for rust, and adequate residual lubricant on the inner surfaces and the core(s). Due to paucity of time, the surveyor may not be able to do this himself, and in the interest of safety, this task may well fall on the shoulders of ship's staff.
Finally, the end fittings, splices and talurit crimpings on each wire rope should be specially examined for damage. End clamps must be re-tightened.
Wire ropes that have suffered any physical damage, change or corrosion must be put beyond use.