Extreme keel drafts in the Fram Strait 2006-2011

Abstract

Knowledge about extreme keel drafts is needed for appropriate design of offshore installations in ice ridge infested waters. Ice drafts were measured with upward looking sonar by the Norwegian Polar Institute in the western part of the Fram Strait along 79°N in the period 2006 to 2011. This is where the Transpolar Drift exits the Arctic Ocean, and the ice consists of a mixture of first-year and old ice originating from most parts of the Arctic Ocean. In total, 8 year-long deployments at 4 locations were analyzed. A generalized Pareto distribution was fitted to all ridges deeper than 17 m. This only amounts to a small fraction of all the ridges, but follows the methodology common for the calculation of extremes. All ridges deeper than 25 m were investigated prior to the analysis to ensure that no icebergs or other misidentified features were included. In total, 5 identified ridges were removed. The deepest ridge observed in the period was 35 m deep and 5 more ridges were deeper than 30 m in these 8 deployment seasons. Since the shape parameter in the generalized Pareto Distribution was close to zero the distribution could be simplified into an exponential distribution. Assuming an exponential distribution gave an estimated 100-year return value in the range 37 to 41 m.