Abstract
Cook Inlet has great potential for tidal stream energy development. However, the presence of drifting sea ice could create hazardous collision risks for tidal turbine farms. Before turbines can be installed in Cook Inlet, sites must be surveyed to determine how often sea ice is present, how fast it will be moved by the current, and where the trajectories of drifting sea ice will be concentrated. In this study, we assess the collision risk of drifting sea ice for a tidal turbine farm in Cook Inlet using a methodology that combines three elements: 1) remotely sensed data to characterize the seasonal sea ice conditions, 2) a hydrodynamic model to map the water velocities, and 3) a particle tracking model to simulate the drifting sea ice trajectories.
Sea ice conditions in Cook Inlet were mapped with 3.125 km resolution sea ice concentration data from a spaceborne microwave radiometer. Microwaves penetrate through clouds, which enables sea ice concentration measurements regardless of the weather. Monthly maps of average and maximum sea ice concentrations were produced from five years of daily data. The average sea ice concentration maps showed that February is the month with the most sea ice coverage and the maximum sea ice concentration maps showed January, February, and March as all having the possibility of sea ice coverage from upper Cook Inlet south to 60°N. December, April, and May all showed coastal sea ice but less ice in the middle of the inlet. Almost no sea ice was present in Cook Inlet from June until November.
The hydrodynamic model mapped the current speeds in Cook Inlet and was used as input to the particle trajectory tracking model. The highest average current speeds were identified in the Forelands area off the coast of Nikiski, due to the narrow channel. The particle trajectory tracking model showed that many particles transit through the Forelands area closer to the western shoreline than the eastern shoreline. Collision risk maps generated from the particle trajectory maps showed a narrow track near the western shoreline where many particles traveled and the collision risk was high. The collision risk maps were further refined by discarding locations where the sea ice concentration was below 30% in the period of analysis. A final map combines information from the current speed distribution predicted by the hydrodynamic model, the sea ice concentration analysis, and the particle trajectory tracking model. The collision risk map shows there are areas near the eastern shoreline that have high current speeds but lower collision risks than near the western shoreline. The resulting map will enable tidal energy developers to identify the best locations for the deployment of tidal turbines and other offshore platforms in Cook Inlet.