Mooring equipment gets shock absorbing capability

A mooring device that acts as a shock absorber against powerful waves has been developed in a joint project by DuPont and Ireland-based Technology From Ideas (TFI). The pair collaborated to develop a novel way to stop moored devices being damaged from waves jarring the attaching lines.

At present, high-value marine equipment is attached using chains and ropes that are regularly pulled taut and potentially violently rattled, causing wear on the chains and damage to attached equipment.

As a result the Dynamic Tether Wave Protection System was developed to provide a level of shock absorption against the peak loads generated from harsh seas. The system is designed to keep chains slightly taut and respond smoothly as they are stretched to the limit. This has been shown to dramatically reduce wear and tear, as well as improve stability.

The energy-damping bellow is extruded using a DuPont Hytrel TPC-ET polyester thermoplastic elastomer, with a tensile elastomer element made of a soft material with lower stiffness to deliver a low force response to the system. This enables elongations up to 250% of the original length while withstanding repeated low force events in excess of 3.5 million cycles per year.

DuPont used its design knowledge from automotive shock-absorbing bellows to create precise corrugations so it would compress at different rates to give increasing stiffness. The energy-dampening bellow is 1.2m long, 250mm in diameter and weighs 20kg. It is engaged as the tether reaches its maximum extension and is designed to deliver a specific targeted smooth response.

The new tethers are being monitored at a number of installations in Europe and are expected to be commercially available in early 2015.

Mark Hazel, technical specialist for DuPont, said: "The design takes advantage of the elasticity and flex fatigue of Hytrel. We adapted our technology for constant velocity joint (CVJ) boots, which are a mainstay in the automotive industry, and developed a highly functional bellow to handle the extreme load conditions and meet the demanding overall performance requirements of the marine industry."

Justin Cunningham

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Doing 3.5 million cycles a year would be great for wave energy but for how many years? Please tell us about cost per unit force for the top end of the force range.

Comment Stephen Salter, 10/01/2015

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