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C T J U N E 2 0 1 4 the tower would see, the stresses along its length and helped us size the tower section to withstand the loads," explains Dagher. With noteworthy foresight, Dagher's group modeled not only the 1:8 scale VolturnUS system, but future full-scale 4.5-, 6- and 8-MW foating turbine designs as well. Modeling also exposed the relative merits of mass above and be- low the water line. "We worked on reducing the system mass above the water as much as possible to reduce the loads that ... would oc- cur on both the tower and the foating foundation," says Dagher, noting that this strategy would "minimize the cost of a platform that could survive." A 6-MW turbine-rotor assembly, for example, can weigh nearly 800,000 lb (364 metric tonnes). To support that at the design hub height of 300 f/92m, Dagher explains, would require a steel tower weighing a whopping 901,690 to 1.2 million lb (409 to 545.4 metric tonnes), with a base about 30 f/9.2m in diameter, and an exceedingly large hull. But the composite tower proposed by UMaine and Ershigs would support the modeled loads at a little more than half the weight of steel. Te resulting weight reduction in the foating hull is two to three times the tower weight savings. Further, Dagher believes, based on lab fatigue testing, that its corro- sion resistance makes a 60-year design life feasible, compared to the typical 20 to 25 years for a conventional steel tower. "Even though composites are more expensive than steel, the value they bring to the entire foating turbine project over its lifetime, and especially the cost savings in the hull design, can pay for the added costs." "Composite materials ofer compelling advantages, particularly for ofshore foating wind power," notes Ershigs' VP Steve Hettick. TOWERING ADVANTAGES A patent is pending on the consortium's tower design and, there- fore, its fber architecture and fabrication process are deemed proprietary. But Dagher stresses that Ershigs is using "well-estab- lished methods and well-known materials" to keep the project's costs as low as possible for the 1:8-scale tower and future full-scale towers. Material suppliers included Ashland Performance Mate- rials (Columbus, Ohio), PPG Industries (Cheswick, Pa.) and 4 7 Illustration | Karl Reque ENGINEERING CHALLENGE: Design a foating foundation and tower that will minimize the costs of materials, construction and installation of a 1:8-scale prototype wind turbine in a deepwater offshore environment. DESIGN SOLUTION: Coupled aeroelastic/hydrodynamic modeling yields a composite tower half the weight of steel, reducing hull foundation weight by two to three times the tower's weight savings. UMAINE/DEEPCWIND VOLTURNUS 1:8-SCALE FLOATING WIND TURBINE 1:8-scale prototype: 20-kW turbine with 30-ft/9m diameter rotor 1:8-scale prototype features one-piece glass laminate tower Vertical and horizontal hollow-concrete "columns" SEMISUBMERSIBLE HULL (composite tower reduces size/mass/cost) COMPOSITE TOWER (half the weight of steel, 60-year design life) Full-scale towers will feature bolted E-glass laminate "can" sections formed from curved infused laminates Commercial scale-up will include 4.5-, 6- and 8-MW turbines — proposed 6-MW turbine tower will rise 280 ft/86m 0614CT Engineering Insights-OK.indd 47 5/20/2014 9:46:14 AM