The engineering characteristics of first year ice ridges are generally poorly known. Yet they represent the ultimate threat to offshore structures in many subarctic and arctic sea areas and are important considerations in ice navigation. This paper describes a programme including field studies, small scale tests and computational developments to improve the knowledge on ridge failure mechanisms and associated loads. An extensive model test project has been performed to study the mechanisms connected with the with first year ridge failures against different types of offshore structures. The results have been used to test and develop computational methods for ridge load analysis. The frictional and cohesion properties of unconsolidated and partly consolidated ice mass has been studied using shear box tests. Field studies of ridges have been performed in the Baltic Sea, Barents Sea and Sea of Okhotsk. These have included mapping of ridge profiles and consolidation using drilling and underwater investigations of ridge structure. In addition, ridge strength characteristics have been tested in full scale with "break-through" tests using icebreakers or heavy weights. Results from the field studies indicate, that ridge characteristics vary significantly in different sea areas. Based on the results of this programme and experience on first year ridge loads on offshore structures in the Baltic Sea new estimates for ridge loads against oil platforms have been computed. The conservative and unconservative assumptions connected with traditional computational methods are addressed as well as uncertainties still to be evaluated.
First year ice ridges are typically long strips of piled ice that form at the contact area of two ice fields being forced against each other. They consist of a keel of loose or partly consolidated ice blocks, a solid ice layer at the surface and a sail of broken ice blocks (Figure 1).
