Pressure Ridge Keel Shapes in the Chukchi and Beaufort Seas
Monday, 15 December 2014
Abstract: Recent aspirations for offshore development in the Chukchi and Beaufort seas, off the North Slope of Alaska, have precipitated the need of additional information for various sea-ice parameters. Of specific interest to engineers and regulatory agencies are the magnitudes of parameters representative of major hazards with a low expectation of occurring over some timeframe. The thickness and width of large pressure ridges that may impact offshore structures is of particular interest. Here, we analyze an extensive dataset from moored ice profiling sonars (IPSs) and acoustic Doppler current profilers (ADCPs) deployed in the vicinity of offshore oil-and-gas lease areas in the US portions of the Chukchi and Beaufort Seas. In all, we examine 16 annual time series of ice draft and velocity from six different mooring locations over five years. Using an automated algorithm, we identify individual pressure ridge keels based on deviations in ice draft from surrounding level ice and derive measurements of the maximum depth and shape of each. Of particular note, and contrary to many idealized ridges portrayed in the literature, we find that flat-bottomed keels are rare in this region. Since the orientation of each ridge as it passes over the mooring is unknown it is not possible to directly measure the precise width. Instead we bin the keels by maximum depth and find the modal width value for each depth bin. Finding that most keels have broadly the same triangular cross-section, we assume the modal width value corresponds to cases when the ridge passes over the mooring approximately perpendicular to its axis. Using this approach, we find a constant ratio between maximum keel depth and width, corresponding to a mean keel slope of around 33 degrees. This is somewhat steeper than is commonly found in literature. We also analyze the relationship between keel depth and drift speed, but find no statistically significant dependence.