The marine industry is likely overspending on dock fenders – in some cases by only 10 to 20 percent but in many cases by as much as 80 percent or more. And this is not taking into account the missing potential savings in dock construction costs of more than 10 percent, or the missed opportunities for reducing fender maintenance costs, if the design process were different.
Design energy determination
A major cause of this is adherence to practices developed more than 40 years ago that have been enshrined as accepted practice despite all that has been learned since. The theoreticians who developed the original design guidelines for fenders incorporated adjustment factors to account for all the variables they foresaw affecting the calculated energy requirements.
The theory is reasonably sound. But its accuracy is heavily influenced by the values that were postulated for the various adjustment factors: construction, eccentricity, hydrodynamic and softness factors. Two of these, the softness and eccentricity factors, are reasonably accurate, but the other two are simply unproven postulations. Subsequent super-computer simulations indicate that their traditional values and formulas likely overestimate their effects considerably.
The satisfactory performance of fenders over the decades is proof only that designs based on those calculations are conservative enough to survive. Ideally, good practice would provide a design factor of maybe 25 to 40 percent over actual applied loads. My broad, in-depth experience over the last 30 years has convinced me that actual, as-built design factors are likely much higher than this. Otherwise, there would have been a series of fender failures over the years, during berthings, and the fact is there hasn’t. This is not due to brilliant design. Instead, fenders’ success in general is due to actual berthing energies being dramatically lower than what the formulas calculate.
Design energy: the problem and solution
All of these comments about the overestimation of design berthing energy inherent in the traditional energy calculation are based on the general practice in North America, which does not adjust the calculated energy value by a further arbitrary service factor of at least 1.4, as stipulated by the British Standard (BS). Thus, probably 90 percent of installations in North America are empirical proof that the service factors required by the BS are simply serving to drive the cost of fendering much higher than necessary. It necessarily results in larger fenders, greater reactions into, and more wear and tear on dock structures.
It is ‘easy’ to simply drop the BS service factor and make a major savings without going into uncharted waters, since that has been the practice in North America. However, North American practice is also conservative. There is no tested means of reducing the design energy, as is possible in countries using the BS. It is not likely that many designers will be willing to actually reduce the calculated energy value, but it should be standard practice not to use maximum values on every adjustment factor. Where ranges of values are permitted, try to select approximately 90th percentile values.
Shear deflection restraint: the problem
In a similar vein, much design effort and significant cost is expended trying to restrain fenders, especially buckling fender systems, from shear deflecting vertically and/or horizontally under the almost universal belief that shear simultaneous with normal deflection has a deleterious effect on a rubber fenders’ ability to perform its design function. In fact, up to at least 40 percent shear deflection there is no effect on performance or on useful fender life.
Shear restraint: the solution
Adding shear restraints not only adds cost, it is unnecessary, because they are incapable of stopping moving objects without physical damage. Without shear restraints, there is a greatly reduced chance of damage, and the fender will function optimally. If damage does occur, it will be less severe than if restraints are employed, so avoid shear restraints.
Optimal fender design strategy
When designing a dock from scratch, the optimum solution is achieved by designing the fender system in conjunction with the dock design process. This permits the coordination and optimisation of all factors affecting dock and fender life: energy absorption, resisting the reaction force of the fenders and optimizing dock/fender geometry for the most effective operation.
Types of marine fenders
There are five basic types of marine fenders. Three, foam, pneumatic and the rubber buckling type, have either significant niche or broad based application and provide good energy dissipation. A fourth, rubber cylindrical fenders, is not economically competitive as an energy absorber whether end loaded or transversely loaded. This is not the ‘general knowledge’ throughout the marine world, but rubber arch fenders, also called V-fenders, dissipate more energy per unit of cost, and exert a lower reaction in any given envelope. A fifth type, shear fenders, has a small but loyal market niche even though it is not clear they are economically competitive with the first three types.
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