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C O M P O S I T E S W O R L D . C O M Engineering Insights lthough the business case for ofshore wind farms is compel- ling (see "Learn More"), installing ofshore turbines is no easy task. Even in shallow coastal waters, it's expensive. Large turbine installation vessels (TIVs), with jackup technology and massive cranes, transport tower and turbine components to the site, then hammer steel piles into the seabed, assemble the steel tower, and lif and install the turbine and rotor. Deep water along many coastlines, however, precludes the relative simplicity of a steel-pile foundation. As developers confront regions where shallow water is scarce, a diferent approach is necessary — specifcally, foating foundations. Floating turbine concepts, in fact, are abundant, says Main(e) International Consulting LLC's (Bremen, Maine) ofshore wind ex- pert Annette Bosler. "Tis year may become a boom year for various European launches, if projects stay on track," she points out. "Who- ever likes to refer to foating ofshore wind technology as 'niche' and a long way of may want to reconsider that statement." In anticipation of this boom, the University of Maine's (Orono, Maine) Advanced Structures and Composites Center (ASCE) head- ed a consortium of companies called DeepCWind and began to develop the concept four years ago. "When we started work, noth- ing like this … had been done before," says consortium leader and UMaine professor Dr. Habib Dagher. DeepCWind plans to feld a grid-connected, pilot foating wind farm in coastal Maine by 2017. In its frst step toward that goal, ASCE launched a 1:8-scale foating turbine research prototype last year, funded by a 2012 U.S. Department of Energy (DoE) grant and featuring a composite tower built by consortium partner Ershigs (Bellingham, Wash.). COUPLED MODELS Floating foundations presented DeepCWind a complex problem: how to understand, and then design for, the interaction between aeroelastic loads — caused by the wind and rotor movement — and the hydrodynamic loads imposed by the water. Together, wind and water impose diferent loads on foundations, towers and turbines than those experienced by onshore turbines. And, says Dagher, "Te physics of a foating, rotating turbine afected by wave motion are harder to work out than for land-based systems." A computer analysis was undertaken in partnership with the National Renewable Energy Laboratory (NREL, Golden, Colo.), using NREL's open-source coupled model, called FAST (see Learn More). FAST simultaneously modeled the coupled aerodynamic, hydrodynamic, control system and structural response of ofshore wind systems. Te consortium also used experimental data collected from test- ing the impacts of wind and waves on small-scale (1:50) foating turbines in the experimental wind and wave basin at the Maritime Research Institute Netherlands' (MARIN, Wageningen, Te Neth- erlands). Dagher's group analyzed wave amplitudes/frequencies and passage rates, asking how quickly the waves move past the founda- tion, how they interact with the foating foundation, and whether those frequencies would mirror the vibration frequencies experi- enced by the tower as the rotor turns. Te tower design had to account for a variety of loads, includ- ing buckling, bending, wind shear and torsional loads. "Te FAST model, together with our MARIN data, told us what accelerations GFRP enables frst grid- connected The VolturnUS, shown here at its station off the coast of Maine, is a 1:8 scale prototype with a composite tower on a concrete hull. 4 6 A Composite tower on 1:8-scale system reduces hull size/weight and helps mitigate the overall cost of electric power generation. U.S. FLOATING WIND TURBINE Source | UMaine 0614CT Engineering Insights-OK.indd 46 5/20/2014 9:45:08 AM