A novel wave protection system replaces chain and rope mooring, which pulls taut, rattles and wears quickly as it moves against the sea. The new Dynamic Tethers system is the result of a collaboration between Ireland’s Technology from Ideas (TFI) and DuPont Performance Polymers, Wilmington, Del. It is said to allow moored devices to move with the sea, which dramatically reduces peak loads up to 70%, boosts durability of mooring lines that tether and protects high-value marine devices like wave energy devices and fish farms.
Central to this system’s design is the incorporation of an energy-damping bellow that acts similar to a shock absorber, which delivers high load and fatigue resistance to protect devices from harsh sea wave conditions and is designed to stay slightly taut and respond smoothly when stretched to its limit, while dramatically reducing wear and tear, improving stability, reliability and positioning and lowering cost.
The bellow is extruded by Ireland’s Radius Systems Ltd. using DuPont’s Hytrel TPC-ET polyester and a tensile elastomer element made of a soft material with low stiffness which deliver the low-force response of the system with elongations of up to 250% of the original length, while withstanding repeated low-force events (>3.5 million cycles/yr). Nearly 4 ft. in length, 10 in. in diameter, the 44-lb energy-dampening bellow is engaged as the tether reaches its maximum extension and is designed to deliver a specific targeted smooth response as it compresses.
According to DuPont technical specialist Mark Hazel, the tethers’ 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.” TFI’s CEO Noel Halloran says Hytrel was selected over competitive materials due to its excellent load recovery performance.
In the development process, TFI provided the load deflection curve for a quarter-scale model and DuPont’s 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. The new tethers are being monitored at a number of installation in Europe and will be made commercially available globally in early 2015.
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