Velocity factors: its creation and relation to strain rate & chemical composition

By Mishra Kumar, Business Unit Director - R&D, Trelleborg’s marine and infrastructure operation

Currently, fender performance data in catalogs is presented either in constant velocity (CV) with a compression velocity of 0.33-1.33 mm/s, or Rated Performance Data (RPD). CV performance data is multiplied by velocity factor (VF) (calculated at 150 mm/s) to convert to RPD. However, there is rarely advice on the creation of velocity factors and effects of velocity on the CV performance data within the catalog.

Utilizing a manufacturer’s testing equipment, fender testing is conducted at a compression speed of 0.33-1.33 mm/s. During the compression process, a load deflection graph is created. The reaction force of a fender is the maximum load the fender generates during compression, while its energy absorption capacity is calculated by the total area under the load deflection curve.

In reality, vessels berth at a speed of 20-600 mm/s. The difference between slow speed testing and actual real-life high-speed berthing conditions (those used for the design of fender systems and wharf structures) needs to be accounted for in the engineering design.

Fenders are made of natural and synthetic rubbers which are high polymer (a viscoelastic material) in nature. For viscoelastic materials, the strain rate (inverse of compression time) has a significant impact on the load deflection curve. The higher the strain rate, the higher the reaction force and the VF. For the same compression velocity, a larger fender needs more time to compress compared to a smaller fender, hence a larger fender is expected to have lower strain rate and lower VF.

For a given velocity, the type of raw rubber used in compound formulation has the greatest influence on VF. Natural rubber is less sensitive to strain rate compared to synthetic rubber.

PIANC’s 2002 “Guidelines for the Design of Fender Systems” highlighted the importance of VF in the design and selection of fenders. PIANC suggests that velocity factors generated through “Type Approval” are published in the manufacturer’s catalogs. However, despite a general industry-wide understanding as to what VF is by definition, little has been done to educate designers, operators and owners of port infrastructure as to how VF figures are created and calculated.

PIANC advises that VF should be created by compressing various sizes / heights of model fenders at PIANC specified speeds for each published grade to account for the strain rate effect. The impact of the types of rubber on VF should be determined by using model fenders of different rubbers and conducting compression tests at various speeds. The resulting data should then be published in the catalog.

As industry and product experts, it is imperative that we equip consultants with comprehensive guidance and accurate data on marine fenders so that they can make fair and accurate comparisons. To ensure that strain rate and material grade is properly considered and results in the most appropriate grade and type of fender being specified, guidance on the effects of VF on their fenders is imperative.

For more on the importance of VF in the design of fender systems, read ‘Applying the right Correction Factors’, now: