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Taking plastics offshore

Taking plastics offshore
In an increasing number of mechanically and environmentally challenging applications, steel is being displaced by plastic, not least because modern plastics are superior to steel in terms of performance, life and cost-effectiveness. Justin Leonard of igus UK examines this trend in the offshore industry, with particular reference to energy chains and moving cable systems.

For applications involving tough environments, steel has been the constructional material of choice for decades and it's not hard to understand why. Steel is strong, relatively inexpensive, reasonably durable and readily available. But it also has shortcomings, particular for use offshore. 

When steel is exposed to weather extremes and salt spray, it is susceptible to corrosion that greatly reduces its life. It is also heavy and, while strong, it can be bent and distorted if exposed to impact or unusual loads. Finally, when systems with moving parts are fabricated from steel, regular lubrication is essential, or the reliability and operational life of the system will be dramatically reduced. Stainless steel solves many of these problems but comes at a premium, is heavy and still requires regular lubrication.
But what's the alternative to steel? For many applications, there really wasn't one, but that situation has changed dramatically with the introduction of modern high-performance engineering plastics. Many of these are stronger than steel for a given size and are, of course, much lighter. They are also inherently resistant to corrosion and weathering, and the best types are unaffected by exposure to virtually every type of chemical and petrochemical, as well as by exposure to UV light from the sun. One subgroup of these new engineering plastics, the triboplastic polymers, also provides a complete answer to the lubrication problem. 

In the development of these polymers, special attention has been given to optimising their frictional properties without, of course, detracting from their other desirable characteristics. As a result, it is now possible to construct systems and products from triboplastics that require no lubrication throughout their working lives, even in the presence of high levels of soiling and contaminants. Not only does this dramatically reduce or even eliminate the need for routine maintenance, it also means that the life of the product can be accurately predicted, as it no longer depends on how well it is maintained.

Electrostatic discharge
The properties of these new plastics clearly make them well-suited to use in offshore applications, but there is one further factor that needs to be addressed. Plastics are inherently non-conductive. This makes them, under certain conditions, susceptible to the build up of electrostatic charges, which can lead to electrostatic discharge (ESD). In areas where explosive vapours or gases may be present - that is, those areas where the ATEX Directive applies, as it does to many areas of offshore petrochemical installations - this is potentially a very dangerous situation as it can provide an ignition source.

To provide a solution, plastics such as igumid ESD from Igus have been developed. These plastics are made permanently conductive by the incorporation of special additives and are thus able to dissipate electrostatic in a controlled manner, thereby eliminating the risk of sparking. These materials should not be confused with plastics that use applied layers of material to achieve antistatic properties, as such layers are susceptible to damage and in any case have limited effective lives. As we have seen, the latest plastics outperform steel in all important respects, but in what types of offshore applications are they being most widely used? One important area is in energy chains, which are articulated support systems for the cables that provide power and communication to the moving elements of systems, such as extending boom conveyors. 

Energy chains essentially consist of multiple elements that are linked with moving joints. Fabricating energy chains from high performance plastics brings all of the benefits that have been discussed - they're tough, resistant to corrosion and impact, they are lubricant-free so require no maintenance and have very long working lives, making them a very cost-effective choice. Energy chains from Igus are also available in ATEX compliant versions, and have been proven by test to function reliably even at extreme temperatures and in adverse weather conditions. 

Durable cables
Of course, it is essential to use energy chains with cables that equally well suited for operation in demanding conditions and to satisfy this need, special cables, such as those in the Igus Chainflex range are now available. These offer excellent flexibility of both their copper cores and their insulation over a very wide temperature range, and are also exceptionally resistant to abrasion and other forms of mechanical damage.

While ideally suited for use with energy chains, these durable cables can also be used alone, as is demonstrated by a recent example on an underwater boring machine, the Vibrocorer VKG-6 produced by Wissenschaftlicher Gerätebau Thomas Schmidt of Rostock in Germany. This drill operates in the most demanding conditions in water depths of up to 200m. For energy supply and to deliver control signals to the drill head, as well as for the transmission of data relating to the penetration depth of the drill, Igus Chainflex servo cables are used.

Of course, the benefits provided by Igus energy chains means that they are being widely adopted in offshore applications. Since plastic energy chains are much lighter than steel, their size is not self-limited by weight. On offshore drilling platforms, for example, the Igus E4.350, which is the largest plastic energy chain in the world, is used to carry large cables and hoses 15m up and down during the drilling head stroke, or lateral rig-skidding. 

The Igus energy chain protects the Chainflex motor and signal cables against mechanical stresses during the lifting and lowering movements of the turbine and also protects them against the aggressive ambient conditions. The travel distance amounts to approximately 20m in vertical direction above and under water. The plastic chain makes any kind of maintenance on the open sea unnecessary and was the only practical solution because a conventional metal chain would not have withstood the combination of the environmental and mechanical demands involved in this application.

As we have seen, there are many applications where the latest engineering plastics deliver decisive advantages compared to steel. As a result, products like energy chains that are manufactured from these plastics are rapidly displacing their stainless steel counterparts, and are already proving their worth in some of the world's toughest offshore applications. For your next project, therefore, why not shun steel and instead take plastic offshore?
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