No matter how good the rubber fender unit might be, the whole integrity of a fender system is compromised by poor quality and/or badly designed accessories.
This trend has reached dangerous levels with inadequate or weak specifications and pressures to cut prices. Those who specify fenders hope to get the best value for money, but exploitable loopholes mean they can end up with fenders that don’t perform, need excessive maintenance and risk the safety of port staff, ships and structures.
This article will focus mainly on fender accessories and the specifications needed to reduce unnecessary risks and costs.
Rubber fender unit
If correctly selected and positioned, the rubber fender unit(s) will absorb the kinetic energy of the largest, intermediate and smallest ships to use the berth.
The rubber units need to cope with different compression speeds, high and low temperatures, berthing and bow flare angles, and occasional or frequent use. They may need to resist shear or tensile forces. They must be reliable and work to full capacity when they are needed the most – during an abnormal impact.
Steel fender panels (frames)
These are complex steel fabrications and their design should only be entrusted to qualified structural engineers. Surprisingly, most rubber companies offering fenders do not use qualified people to do this work.
Panels need to resist combinations of bending, shear and local buckling. There are many ‘limit state’ design codes (BS5950, etc) and finite element software packages able to determine these loads and stresses, but very often unqualified persons use simplistic methods or even guesswork that lead to dangerously weak and under-designed fabrications.
It has becoming worryingly common to see steel plate sections as thin as 5mm, yet this is far below the minimum needed on any berth. In contrast, International Navigation Association (PIANC) recommends 12mm as the absolute minimum when exposed to seawater on both faces, 10mm for exposure to one face and 8mm for internal sections not exposed to corrosion. This means a panel should be a bare minimum of 160-180mm thick if standard internal steel channels are used to stiffen it. Bigger systems often need panels 250-400mm thick, yet panels as thin as 120mm are being promoted by some manufacturers. Are they cheap? Possibly.
Fit for purpose? Definitely not!
Paint coatings also vary in quality and no paint lasts forever; 10-15 years being typical. After this, the steel will corrode and weaken unless corrosion allowances are added. If corrosion allowances are not specified, they will invariably be ignored and the life expectancy of the panel will be drastically reduced. For cold water climates, a corrosion allowance of 3mm per exposed face might be suitable, much more where temperatures are higher and corrosion is greater.
Connections of the rubber fender and polyethylene (PE) face pads to the panel also need close scrutiny. Rubber fender fixing points should be locally reinforced and sealed to prevent water ingress if closed box panels are used. Anchors and fixings Any fender system is only as good as the weakest component. It is often assumed that whatever is shown in the manufacturer’s catalogue is suitable for every application. Some fender suppliers offer anchors made of mixed materials, including stainless inserts and galvanised bolts – a risky cocktail which saves money, but is likely to fail early.
Calculations for loads should be presented, and fixings selected accordingly. In cooler waters, a galvanised fixing might be appropriate, but in warmer places, stainless steels are the only solution. Independent specialists like the British Stainless Steel Association suggest Pitting Corrosion Equivalent Numbers (PREN) of around 40 for highly corrosive environments like the Middle East and Asia. As a guide, 316 grade has PREN of 25-26 whereas 304 grades are below 19. Despite very poor resistance to pitting corrosion, 304 grades are still used by less scrupulous suppliers due to their low price. They get away with it too because most specifications don’t stipulate a minimum PREN or grade.
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