Hurricane Force: 5 Natural Hazards Risks Affecting Offshore Wind Structure Design
ABS Group Power and Global Renewable Energy Division
The U.S. Gulf Coast and Caribbean have experienced an active 2017 hurricane season, with many petrochemical facilities and floating platforms affected by major storms in a region that contributes 46% of U.S. refining capacity. Given the catastrophic risk of natural hazards such as hurricanes, the question has emerged as offshore wind farms enter the U.S. energy mix: how will offshore wind turbines fare in these conditions?
While it is true that the vast majority of offshore wind farms are not currently located in areas prone to hurricane activity, the risk of not being able to withstand hurricane force winds must be addressed if alternative resource developments continue to expand on the U.S. Outer Continental Shelf (OCS).
The following five (5) risks should be considered to prepare offshore wind farms for catastrophic weather:
5 Wind Structure Design Risks
1. Windspeed Effects on Turbines
Current International Electrotechnical Commission (IEC) design guidelines for offshore wind turbines do not address the type of winds seen in Category 3-5 levels, and it is likely that significant modifications and/or redesigns would need to be made in order to confirm the structural reliability of current turbines operating in these conditions for a standard 20-year service lifetime.
2. Waves and Water Levels
Field measurements of sea states, wave conditions and the significant variability of wave heights during extreme events are essential for the design of offshore wind farms. There are a number of parameters used to describe sea states during the design process, however, one important parameter is the significant wave height, or the average of the highest 1/3 of the waves. For offshore wind turbines, it is possible that both wind and waves can drive different aspects of the structural design. Some turbines and towers can experience their extreme load condition during operational conditions where the wind load on rotor is a significant source of load, whereas braces in a jacket foundation may be driven by large wave loads during extreme sea states. Therefore, the extreme value analysis methodology is important in selecting design criteria including wind/wave combinations for extreme loads.
3. Structural Reliability
Research on structural reliability has shown that due to the higher variability of wind and wave conditions associated with hurricanes typically experienced off U.S. coastlines, the implicit safety level in IEC would not be achieved for offshore wind turbine structures. For design of foundations in hurricane-prone areas, the ABS Guide for Building and Classing Bottom-Founded Offshore Wind Turbine Installations (BOWTI) suggests an increase in the return period of extreme environmental conditions from 50 years to 100 years, or to use 50-year environmental conditions with additional safety factors and/or a robustness assessment.
4. Climate and Temperature Effects
Higher and rising average temperatures may also contribute to the increasing frequency and severity of storms in the region. This effect can be mitigated by making sure that temperature predictions are considered during the design phase, and working with wind turbine manufacturers to thoroughly understand how their technology will work in tropical and semi-tropical climates.
5. Backup Power
During hurricanes, failures of the electrical power grid are likely to occur. This situation should be considered in the design, and load cases prepared and related to hurricane conditions should assume the loss of the grid. Extreme wind direction changes are also likely over a short period of time. A back-up power supply should be available to enable the wind turbine to yaw during grid loss, otherwise the extreme wind speeds at a misaligned direction may result in structural damage. Damage of the wind vane and anemometer should also be considered during design as this could lead to control system failure in the event of a storm.
Assessing Natural Hazards Risk During Design
Current wind turbine design classes may not be adequate for hurricane-prone regions and may not provide the same reliability levels seen in areas not subject to tropical weather conditions. The high frequency of major storms in the U.S. Gulf Coast and East Coast regions may result in the reduction of the fatigue life of offshore wind turbine components, and the effects of this will need to be considered during the design phase. This poses not only design challenges, but also may increase operations and maintenance costs as well as downtime.
Based on the severity of the environment and the limited experience of offshore wind facilities in the U.S. and Caribbean waters, a site-specific design risk assessment should be undertaken to identify potential risks and the probability and severity of them. This would allow the developer and designer to identify ways to mitigate those risks through engineering, manufacturing, installation, monitoring and maintenance/inspections programs.
Leading U.S. Offshore Wind Risk Services
ABS Group has more than 11 GW of offshore wind experience worldwide and served as the Certified Verification Agent for the Block Island wind farm, the first commercial offshore wind farm in the U.S. We also provided an offshore wind inspection safety assessment for the U.S. Bureau of Safety and Environmental Enforcement (BSSE) to deliver insights into risks that need to be assessed in order to promote safer, more reliable wind inspection activities.
To assist the offshore wind industry with addressing natural hazards risk, our engineers have experience with site specific design factors in hurricane areas, including extreme value analysis, selection of design criteria, site specific risk assessment and integrated load analysis with hurricane load cases.
With a long history of assessing offshore oil and gas facilities on the U.S. OCS and Gulf of Mexico, including evaluating the impact of storm effects on floating and fixed facilities, we can provide multi-disciplinary technical consulting and independent engineering capabilities in the areas of project certification, verification, quality assurance, inspection and owner's engineering/advisory services.