Surface Ice Effects on the Extreme and Fatigue Loading of Bottom Fixed Offshore Wind Turbines

Abstract

As interest and investment in offshore wind projects increases worldwide, some turbines will be installed in locations where ice of significant thickness forms on the water’s surface. This ice moves under the driving forces of wind, water, and thermal effects and may result in substantial forces on bottom-fixed structures. The North and Baltic Seas in Europe have begun to see significant wind project development and the Great Lakes of the United States and Canada, all regions that experience significant floating ice, will likely see wind projects in the near future. Because the forces imparted by surface ice are dynamic in nature, design of the support structures for these projects will require the calculation of the simultaneous effects of turbine operational, wind, and ice forces. The IEC standard for offshore wind turbine design and the ISO standard for offshore structures provide requirements and algorithms for the calculation of ice-induced forces; however, currently none of the widely used wind turbine dynamic simulation codes provide the ability to calculate and apply dynamic ice loads. A new suite of subroutines has been developed by the authors, collectively called IceFloe, which meets the requirements of these standards for design of support structures in ice prone waters, and has been coupled and tested with four wind turbine simulation codes. The IceFloe routines have been linked and tested with FAST, a tool developed under the management of the National Renewable Energy Laboratory. This integrated tool has been run with a 5 MW example turbine and ice conditions from selected areas of the Great Lakes with a range of ice thickness and velocity. Extreme and fatigue load calculations have been made and compared with and without the effects of ice loading. Example results from these calculations are presented. Results indicate that surface ice loading can impact the design of offshore support structures.