Our nation’s greatest potential for abundant wind energy lies offshore. Locating wind turbines off both coasts, where wind speeds are the highest, could promise high energy yields, akin to the offshore wind production in the North Sea near northern Europe.
But constructing wind turbine platforms in water deeper than about 60 meters presents problems. Turbines in shallow waters, like those in the North Sea, can be mounted on fixed-bottom platforms that are held to the seafloor by a rigid structure such as a monopile or foundation.
The offshore turbines operating in the U.S. are fixed-bottom turbines located in the relatively shallow waters over the continental shelf near New England. But deeper water, like that along the Northeast and the West Coast, which represents two-thirds of U.S. offshore wind resources, renders this type of mount impractical. Engineers are therefore turning to floating offshore platforms and attempting to create economical, innovative floating structures that can harvest wind energy while also capturing energy from wave action.
Floating platforms, which originated with oil and gas drilling, are held in place by mooring lines anchored to the ocean floor in various designs and configurations. But unlike oil and gas platforms, these structures need to carry the taller, unbalanced load of a wind turbine. That presents an additional set of problems—issues that Muhannad Suleiman, a professor of civil and environmental engineering, and his team are working to solve.
The interdisciplinary team of Suleiman, deputy director of Lehigh’s Advanced Technology for Large Structural Systems (ATLSS) Engineering Research Center; James Ricles, Bruce G. Johnson Professor of Structural Engineering and ATLSS director; Richard Sause, Joseph T. Stuart Professor of Structural Engineering; and Keith Moored, an associate professor of mechanical engineering and mechanics, recently received a $1 million National Science Foundation Research Advanced by Interdisciplinary Science and Engineering Clean Energy Technology (NSF RAISE CET) award for their three-year project.
Offshore wind platforms experience extreme forces from just about every direction. The winds that load the turbine can be quite strong, pushing both the turbine blades and the platform. The ocean waves can also cause force against the platform and pull on the mooring lines, which in turn exert a pulling force against the anchors holding them in place. Extreme weather events, such as a hurricane or major storm, can exacerbate all these forces.
Such events “apply significant loads to the structure, including wind and wave loading on the structure,” says Suleiman. Wind and waves are also sources of energy. The research team is trying to combine wind and wave energy generation to make these floating offshore platforms more efficient and resilient.
The project has four goals. The first is finding new ways to reduce the platform’s motions. Pitch (up and down) and roll (side to side) motions are a focus here as they amplify the movements along the length of the turbine, from sea level to the rotor. Limiting these motions improves the resilience of the structure.
The second aim is to evaluate the effects of platform motion reductions on mooring line fatigue response. After establishing a baseline for platform movement, innovations such as ballast placement and damper systems will be studied for their effects on the platform’s movement.
The third goal involves investigating bio-inspired concepts for energy generation and foundation design to determine potential increased power output and improve platform stability. Oscillating hydrofoils, kinetic devices attached beneath the platform, are modeled according to the movements of aquatic animals, which masterfully navigate unsteady and vortical flows. Bio-inspired anchors will also be analyzed. These rough-surfaced anchors—inspired by snakeskin—increase friction between the foundation and surrounding soil to resist pull-out forces.
The fourth goal involves real-time hybrid simulations. The data from the previous research tasks will be used to simulate their combined interactions under extreme loading conditions, allowing the team to address the technical challenges and interactions involved in floating offshore wind energy production.
The wind, water, and mechanical loads on the turbine, platform, and mooring lines will be analyzed and combined with experimental results on the soil and the foundation to produce “the response of the whole floating offshore wind turbine system, which helps us understand the interaction between different parts, or subsystems. interact,” Suleiman says.
This is important, he says, because typically these subsystems are treated separately: A structural engineer will look at the loads on the tower, a mechanical engineer will study the aero and hydrodynamic loads, and so on, but there are few ways to study the entire system as a single operating unit. Bringing all the parts together not only mimics the real interactions of these subsystems, but also allows researchers to better understand their operation and maximize their energy output.
The team will use facilities at ATLSS and Suleiman’s soil-foundation-structure interaction facility, as well as Lehigh’s real-time hybrid simulation facility.