Trelleborg Marine and Infrastructure

Foam fender behaviors:
a combination of solid and
pneumatic characteristics

Rubber, pneumatic and foam fenders are extensively used in shore to ship berthing operations. Pneumatic fenders are usually preferred in offshore ship-to-ship transfer applications, although foam fenders are a good alternative in these cases too.

Whilst rubber, pneumatic and foam fenders are all used, they have varying energy absorption media and mechanisms. Below, we can see the differences between these three types of fenders.

Rubber fenders are produced using long chain elastomeric (rubber) materials, which exist in a coil form. During compression, the long chain molecules are compressed and stretched. The chains are uncoiled and energy is stored inside the elastomer molecules. Elastomers are viscoelastic in nature. Therefore, some part of the energy will be dissipated as heat inside the rubber molecules. The rest of the energy is used to bring the fender back into its original shape when the load is removed.

As some part of the energy will be lost in each compression cycle, it is impossible to get 100% recovery after compression for a rubber fender. The recovery process is time dependent. A fender needs to be compressed a few times to stabilize the coiling and recoiling process of the rubber molecules, to provide a consistent performance. Therefore, rubber fenders usually need a few “break in” cycles prior to the determination of its performance through the compression test.

Pneumatic fenders are flexible pressure vessels which contain compressed air within a flexible, thin-walled, rubberized fabric cylinder. When pneumatic fenders are compressed, the gap between the air molecules reduces, and therefore the volume of the air inside the fender decreases. The energy is absorbed through the compression of the air molecules.

Air molecules are simple gas molecules, unlike long chain polymeric (rubber) molecules. Therefore, air molecules do not show viscoelastic behavior. Once the load is released the air molecules return to their original volume and original shape instantaneously, provided that the outer layer has high air retention capacity and very low air permeability.

Unlike rubber, pneumatic fenders absorb energy through the contraction of air molecules. Therefore, they do not require “break in” cycles. The difference in performance values between the first and subsequent cycles is negligible. Therefore, the first compression performance value can be taken as the final performance of a pneumatic fender.

Foam fenders are produced from closed-cell PE foams, a combination of gas bubbles embedded into a polymeric matrix. When compressed, most of the energy in the foam fender is absorbed by the polymeric matrix (as described under rubber fender), and compression of gas molecules (as illustrated under pneumatic fender).

A certain percentage of energy is absorbed by the PU skin. The recovery of the fender after compression is expected to be slower than for a pneumatic fender, but faster than solid fenders, depending on the viscoelastic behavior of the polymeric matrix. The embedded air/gas bubbles do not assist in quick recovery. The recovery dynamic and Scanning Electron Microscope studies indicate that the recovery of foam is a result of viscoelastic recovery of bent cell walls.