| Résumé | Although many studies have been carried out using field, laboratory and analytical methods, uncertainty remains in the magnitude of forces generated by ice ridges on offshore structures. Norstro msgrund lighthouse, located in the Bay of Bothnia in the Baltic Sea, is a cylindrical concrete structure with a sand filled caisson foundation and a waterline diameter of about 7.5 m. It was instrumented to measure ice forces from 1999-2000 for the LOLEIF project, and from 2001-2003 for the STRICE project. Instrumentation included load panels around one half of the structure for direct measurement of ice force, a tiltmeter for assessment of lighthouse tilt motion, and devices to record ice thickness at the site. Load panel and tilt meter data have been analysed to assess global force on the lighthouse during several first-year ice ridge or hummock field interaction events in late winter and early spring of the years 2000, 2002 and 2003. Ridges were encountered with maximum consolidated layer thicknesses of 1.1 to 1.5 m, and unconsolidated keels potentially of 8 m or greater draft. Global loads are calculated from the forces recorded by load panels, and are also interpreted from tilt data. The vertical and horizontal distribution of ice pressures is examined. Most previous publications have focused on the load panels, whose measurement area is limited by their height of 1.6 m. In the present analysis, an attempt was made to assess whether any keel load was measured by the tiltmeter. The forces generated by ridged ice are also compared to those from the surrounding level ice. The methods of the ISO 19906 Standard are applied to predict ice force using the geometry and assumed properties of ridges at Norstro msgrund. These force predictions are compared to the actual measurements, and reasons for discrepancies discussed. The variability in force estimates is examined with reference to the range of possible values for the assumed parameters, including thickness and strength of the consolidated layer, and porosity and cohesion of the keel rubble. |
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