DuPont Performance Polymers and Ireland-based Technology from Ideas (TFI) collaborated to develop a novel way to allow moored devices to move with the sea, dramatically reducing peak loads up to 70% and improving durability of mooring lines that tether and protect high-value marine devices, such as wave energy devices and fish farms.
The Dynamic Tethers wave protection system replaces chain and rope mooring, which pulls taut, rattles and wears quickly as it moves against the sea. This wave protection system incorporates an energy damping bellow that acts similar to a shock absorber. It handles high load and resists fatigue, protecting devices from harsh sea conditions. It is designed to stay slightly taut and respond smoothly when stretched to its limit, while reducing wear and tear, improving stability, reliability and positioning and lowering cost.
The bellow, extruded by Radius Systems, is made of DuPont Hytrel TPC-ET polyester thermoplastic elastomer and a tensile elastomer element made of a soft material with low stiffness. This material delivers the low-force response needed by the wave protection system with elongations up to 250% of the original length. Simultaneously, it withstands repeated low-force events (>3.5 million cycles per year). The energy dampening bellow is 1.2 m (nearly 4 ft) long, 250 mm (nearly 10 in.) in diameter and weighs 20 kg (44 lb) and is engaged as the tether reaches its maximum extension. It delivers a specific targeted smooth response as it compresses.
The tethers are being monitored at a number of installations in Europe and are expected to be commercially available in early 2015. “The design takes advantage of the elasticity and flex fatigue of Hytrel,” said Mark Hazel, technical specialist for DuPont Performance Polymers. “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.”
“The key to developing such wave protection systems is to ensure excellent fatigue resistance coupled with high performance over time in sea water. Hytrel thermoplastic elastomer was selected over other materials due to its excellent load recovery performance,” added Noel Halloran, TFI’s CEO.
In the development process, TFI provided the load deflection curve for a quarter-scale model and DuPont’s computer-aided engineering (CAE) experts used their design knowledge from automotive shock-absorbing bellows to create a Hytrel bellow with corrugations of different pitch that compress at different rates to give increasing stiffness as the compression progresses. Advanced finite-element analysis (FEA) techniques were used to ensure the design met the performance specification.
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