Combined distribution of ice thickness and speed based on local measurements at the Norströmsgrund lighthouse 2000-2003

Abstract

Abstract: Fatigue caused by ice-induced vibrations (IIV) is an important design consideration for offshore structures exposed to drifting sea ice. The occurrence of IIV is promoted by, but not limited to, certain combinations of ice thickness and ice drift speed, which makes them fundamental input parameters for structure fatigue life estimation. To that end, this work identifies and analyses the frequency of combined ice thicknesses and ice drift speeds during all recorded ice-structure interaction events at the Norströmsgrund lighthouse during 2000 – 2003. The ice drift speed measurements were performed manually at the lighthouse, leading to bias towards multiples of 0.05 m/s. The ice thickness measurements by EM antenna underestimates ridge keel depth. The current approach gave a total of above 25 days of ice condition measurements. The cumulative distributions for ice thickness and speed were estimated. Given drifting ice conditions, the probability of encountering ice thicker than 0.8 m was 0.34, i.e. the probability of encountering thicker ice than most of the level ice in the area. A relatively frequent combination of ice conditions was ice thickness between 0.1 and 0.6 m and ice drift speed between 0.05 and 0.15 m/s, occurring with a probability of 0.29.