Applying the right correction factors
Trelleborg Marine Systems' Richard Hepworth explains why accurate rubber compound tests are more important than ever for fender specifications.
First introduced by PIANC in 2002, Velocity Factor (VF) and Temperature Factor (TF) should be applied to rubber fenders during testing, to ascertain performance in the field under varying compression times and temperatures.
In recent years, there has been a significant evolution in rubber manufacturing industries: we’ve seen the emergence of contract mixers, locations such as China and India becoming mainstream hubs for rubber compounding, as well as the availability of a wider range of ingredients.
These changes represent a break from tradition, and best practice in rubber compounding needs to keep pace.
Changing rubber compound ingredients have a direct effect on the characteristics of marine fenders and VF and TF are greatly affected by the type of rubber used and further still by its compound composition.
We recently released a new suite of materials – a whitepaper, webinar, short film and rubber specification tool – which highlight just how critical it is to apply VF and TF on a case by case basis, as the value and application of the two factors will vary from manufacturer to manufacturer, based on the rubber formulation of the fender.
Exploring the issue
While steel design and fabrication is a topic that most engineers readily understand, rubber is a very different proposition. Generally speaking, there is limited understanding about compound composition and its impact on performance and longevity in marine fenders.
The changing properties of rubber compounds used in fender systems and the move towards non-traditional fillers mean that VF and TF are now even more likely to have a material impact on engineering design and fender selection, as well as further implications for the design of other wharf infrastructure.
Typically, normal berthing velocity of vessels is between 20mm/sec and 500mm/sec. In a perfect world, fender manufacturers would test at actual berthing velocities to determine the performance of the fenders. However, in practice this is exceptionally difficult given the size of investment in equipment and range of fenders to be tested.
For a given velocity, there are two factors that have the greatest influence on VF: strain Rate (compression time) and the type of rubber used in the fender.
Reaction force and energy absorption are directly proportional to rubber stiffness, which changes dramatically with temperature, and has a tremendous effect on fender performance. Therefore, rubber elements for fender systems must be tested to the temperatures they will be subjected to in the field.
Similar to VF, TF is highly sensitive to the type of rubber used – natural rubber or synthetic based rubber, or a blend of the two, as well as the inclusion of recycled rubber. It therefore varies with the type and grade of fenders and from manufacturer to manufacturer.
We have recently undertaken an extensive testing program, investigating the rubber composition of fenders at many ports around the world. Our findings indicated a wide range of compound variability as well as differences in the physical appearance of the fenders.
It was clear from the results that fenders that had deteriorated had done so primarily as a result of the rubber composition rather than actual age. Higher temperatures also reduced the predicted lifecycle of both superior and inferior compounds. However, predicted life was reduced six to 10 times more for the inferior rubber compounds.
Therefore compound composition is the key to guaranteeing fender performance at extreme temperatures, and should feed directly into the Temperature Factor applied to any batch of fenders.
I believe that PIANC’s recommendations for applying VF and TF are really only the beginning. Suppliers need to make the appropriate investments in R&D to be able to underpin and substantiate their claims. Anecdotally, we’ve found that many actually copy factors which are not relevant to their products.
If suppliers aren’t able to take the necessary steps to understand and guarantee the quality of their products, then it’s essential that specifiers are able to, to avoid further costs on heavy maintenance and early replacements.
Richard Hepworth is a chartered mechanical engineer, having studied for his degree at the University of Manchester Institute of Science and Technology and now holds the position of business unit president for Trelleborg Marine Systems, based in Dubai. Richard has over 20 years’ experience working in the offshore and marine construction industry and has held a number of roles both within Trelleborg and other large engineering companies in this sector, covering engineering, project management, sales, business development and general management.
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