Professor Sanjay R. Arwade of the Civil and Environmental Engineering Department at the University of Massachusetts Amherst is a principal investigator on the research team that has been awarded a $325,000 grant by the National Science Foundation to tackle a key gap in the international engineering protocols for building offshore wind turbines. The problem is that those protocols fail to guide wind-turbine designers on how to account for the risk of hurricanes. Arwade’s NSF project is entitled “Reliability-based Hurricane Risk Assessment for Offshore Wind Farms.” A 2011 report from the National Academies listed this problem as one of the “major deficiencies” in the international protocols.
“Certainly the current international engineering specifications and protocols do not have a robust requirement for addressing hurricanes,” notes Arwade about the clear need for his study. “The requirements do not include a robust design for hurricane resistance.”
Special Report 305, entitled “Structural Integrity of Offshore Wind Turbines: Oversight of Design, Fabrication, and Installation” as produced by the Transportation Research Board of the National Academies, is very specific about the problem. As it states, “The current standard does not specifically address hurricanes in the estimation of peak wind and wave heights, duration of sustained high winds, or extreme directional wind changes.”
Arwade is the principal investigator at UMass Amherst, and he is collaborating with two researchers from the Northeastern University Department of Civil and Environmental Engineering: Professor Jerome F. Hajjar, the department head; and Assistant Professor Andrew T. Myers.
“The realization that Andy and Jerry and I came to as we were talking about doing some work together,” says Arwade, “is that, if we’re going to start putting hundreds of wind turbines in the waters of the coastal Atlantic, we should really have a better understanding of how these very expensive, very large industrial installations are going to be affected by hurricanes.”
Meeting the national goal of generating 20 percent of total energy needs from renewable sources by 2030 should, according to the National Renewable Energy Laboratory, include the installation of 54 gigawatts of electricity generated from offshore wind. That translates to potentially thousands of offshore wind turbines.
Among the offshore installations currently under consideration along the Atlantic coast, says Arwade, are the controversial Cape Wind project off Cape Cod, a small project off Rhode Island, an industry-academic partnership in the Gulf of Maine, and prospective projects on the coasts of Delaware and Maryland. All these projects follow in the wake of well-established offshore projects in Germany, Denmark, Holland, and the UK. But the Atlantic coast of the U.S. has something special which those countries have never encountered.
“Developing offshore wind turbines is a well-known model in Europe, where they have many hundreds of turbines in the water,” explains Arwade, “but one barrier we have here that they don’t have in Europe is hurricanes.” He adds that severe weather in the North Sea, where most of those wind turbines are located, does not approach the intensity of hurricanes and may have different characteristics associated with the wave and wind loads.
The project includes four integrated research tasks: (1) the development of a probabilistic hurricane hazard modeling framework for the Atlantic coast; (2) a fragility analysis framework for offshore wind turbine support structures, including case studies of prototype support structures; (3) relationships between individual turbine fragility and wind farm reliability; and (4) novel design details for improving the robustness of turbine support structures to combined, hurricane-induced, wind and wave loading.
Giving offshore wind farms the strength, durability, and stability to withstand hurricane conditions is no easy task. “These structures include everything from the bottom of the foundation, which might be tens of meters below the sea floor, to the top of the tower, where the turbine is sitting, hundreds of meters above the ocean surface,” says Arwade. “So we’re looking at the structural height of a 30- or 40-story building. These are big structures!”
The $64,000 question is how these structures will respond to howling hurricane conditions. “They will be loaded in dramatic ways by the wind,” explains Arwade. “But also by the waves generated by hurricanes.”
One major emphasis for the UMass-Northeastern research team will be a concerted effort to have its research applied to international engineering specifications. But their findings will also be very valuable for all the groups proposing to build offshore wind installations along the Atlantic coast. That’s because the research will establish that fine line between building offshore wind turbines which are sturdy enough to withstand hurricane forces but economically sound enough to be feasible.
“In the long run, we want to rationally design for the real risk of hurricanes as they could impact offshore wind turbines,” explains Arwade. “Not to be blindly over-conservative, thus making offshore wind turbines overly expensive, nor equally blindly under-conservative, thus putting the structures at high risk for hurricane damage.” (September 2012)