Evaluation of Global Ice Strength for Design Iceberg Impact Loads

Abstract

AbstractIn designing an oil and gas platform for offshore arctic and subarctic regions, operators may need to consider potential iceberg impacts when determining optimal structure configuration and ice strengthening requirements. Ice strengthening requirements will depend on the frequency of impacts, the sizes and shapes of icebergs impacting, the impact velocities as determined by the response of icebergs to currents and waves and the strength of the ice. Global ice strength will influence overall design, and local ice strength will influence local structural design. Failure of ice in crushing is a complex process involving mechanisms such as spalling, pressure melting and recrystallization, which are very difficult to model. As a practical approach, global force is often modeled as the product of nominal contact area times global crushing pressure, with global crushing pressure estimated based on full scale measurements. During iceberg impacts, contact area increases with penetration, with the maximum area influenced strongly by the initial kinetic energy of the iceberg, and to a lesser extent by driving forces during the impact. Ice strength, as observed during field measurements, has a significant random variance, both in time during an interaction, and from interaction to interaction. This variance is especially important when designing for iceberg impact loads in regions such the Grand Banks off Canada's east coast where load events are very infrequent, on the order of once every 10 years given ice management. While ice strength data for sea ice loads is often presented in terms of upper limit strengths based on the assumption that there are large numbers of interactions per year, a probabilistic approach that explicitly considers the frequency of events is more appropriate.In this paper, emphasis is given to global ice strength as relates to the total force on a structure, rather than local ice pressure as relates to local design for fixed structural areas on the platform. A strong scaling effect is observed in which the average global strength of ice decreases as the nominal area of contact increases. There is a lack of observed ice strength data for interactions involving failure of iceberg ice at large contact areas; a consequence of which is that there is not consensus in industry regarding the most appropriate strength model to use. While ISO 19906 presents a probabilistic model that accounts for variance in ice strength as contact area increases, with random coefficients to account for the variance between impacts, use of a minimum pressure cut-off for large areas is suggested due to the lack of ice strength data for large contact areas. ISO 19906 does not give guidance on the selection of the cut-off. A review of relevant data is presented here and different models for the minimum pressure cut-off considered, with example calculations presented.