Robert Traxl looks at the contribution of rope design on the safety and efficiency of offshore lifting activities.
Crane operation is an inherent part of offshore oil and gas operations. Lifting and mechanical handling operations can pose a major accident hazard. Choosing the right wire rope can minimise the risk of accidents and increase the performance considerably.
When using wire ropes in lifting appliances, the basic needs are to raise or lower objects economically and safely. The fatigue strength of wire ropes in turn depends on the operating conditions, which are demanding for a cost-effective use.
Unfortunately, these conditions arouse unwanted side effects that have to be considered in order to meet the basic needs mentioned before. In particular - to name the most important ones - there is fatigue, wear and corrosion that mainly determine the lifetime of a wire rope. As those influences are inherently linked to wire ropes, appropriate measures are less a question of whether they are necessary and more about who takes them.
Apart from rope inspection, there are limited possibilities to maintain wire ropes in service. Thus the application of preventive measures is primarily a task of the rope manufacturer and is generally referred to as 'product quality'.
It is important to consider that, unlike any other machine, preventive maintenance of wire ropes is mainly determined by the quality of the product. There is a wide range of different wire ropes available on the market, which comes along with a variety of properties. Apart from basic features that the product must have, there are a great deal of attributes that are associated with the job performance of a wire rope. The measures mentioned before are part of the performance attributes, thus they vary considerably from one product to another.
The following example will show how measures taken by the rope manufacturer will influence the occurrence of related phenomena later in the field.
One of the most pressing problems with regard to the failure mode of rotation-resistant ropes is core deterioration. There are extreme high-contact stresses at the crossover points of the outer strands to the steel core, which lead to severe wear and abrasion. It is important to protect the core by reducing these stresses. One effective measure is plastic impregnation.
Plastification has a positive impact on a rope's lifespan in two different ways. Firstly, the plastic cover acts like a cushion to prevent the stresses mentioned before. Secondly, it effectively preserves the lubrication of the core as it prevents the ingress of water.
The lubrication of the core underneath the plastic jacket is a crucial aspect of its own. Temperature is an important influence in this context, because heat is induced either during the extrusion process of the plasticised core and also generated in the field, especially in active heave compensation due to inner friction. Due to the latter effect, the lubricant usually drops out of the rope.
On the outer strands the lubrication can be renewed, for example by pressure lubrication, but the possibilities in regard to the core are limited. This leads to fretting corrosion at the core, which leads to additional friction and heat generation and starts a doom loop. Hence rope deterioration starts from inside. A special heat-resistant lubricant with a higher drop point in combination with plasticised cores prevents core deterioration. The lubrication of the core remains better over the service life and deterioration of the core is reduced.
A frequently asked question in regard to the usage of plasticised ropes in subsea lifting operations is 'what is the behaviour of the plastic jacket under high pressure, such as in a water depth of 3,000m. To find answers to this particular question, tests in a pressure chamber at 300bar water pressure have been conducted. The rope that was tested was a 70mm compacted, rotation-resistant rope with a plastic-coated steel core. The test sample was sealed with resin at both sides of its cut surface.
After the test the sample was opened to investigate the impacts of the pressurisation. In brief, the findings of the pressure test were that high water pressure won't have negative effects on the plastic jacket of a plasticised wire rope. These findings are also supported by experiences from the field, where such ropes are successfully used in subsea lifting on a deep water geophysical research vessel down to water depths of 6,400m.
When it comes to the selection of wire ropes for offshore lifting operation it is important to get down to the nitty-gritty of the different performance attributes of the products currently on the market. Attributes such as plasticised steel cores and heat-resistant core lubrications are not visible at first glance but they have a crucial contribution toward the cost-effective and safe performance of such activities.
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Robert Traxl is head of R&D for Wire Rope at Teufelberger in Austria.