This paper presents a probabilistic procedure for determining the maximum loads and pressures on a fixed offshore structure due to multi-year ice floes during, winter loading events. The approach depends on a !computerized simulation procedure which is described 'together with mechanistic models for estimating the floe contact width, peak indentation pressure, pressure vs. displacement, and interaction between the multi-year floe and the surrounding first-year ice.
Several load scenarios involving different assumptions are investigated and the predicted loads compared on the basis of their cumulative probability distributions. The paper also presents a procedure for selecting the design ice pressure as a function of the loaded area size.
Among new types of offshore structures being developed to support hydrocarbon exploration and production in the Beaufort Sea is a mobile drilling structure which can be removed from one drilling site, towed to and reinstalled at another site, perhaps, once a year. This structure type will perform a function similar to that of jack-up barges in more temperate and sheltered areas.
The structure schemes currently under active development include a conical structure and a caisson (vertical-sided) structure made of interconnected vertical cells. The design of these structures is strongly influenced by the possibility of large loads and pressures from collision with thick multi-year ice floes. This paper concentrates on caisson-type structures in water depths up to 60 feet.
Design ice loads and pressures have often been selected by combining a load prediction algorithm (e.g., an equation relating indentation pressure with compressive strength of ice) together with the "design" ice feature dimensions (e.g., thickness and floe diameter), the "design" ice strength parameters (e.g., compressive or flexural strength), as well as other relevant environmental parameters (e.g., temperature, salinity, etc.). This deterministic approach has a major advantage in that the ice feature and environmental Conditions capable of imposing the predicted loads are clearly defined. From the negative side, the risks 'associated with the loads predicted by this approach cannot be clearly defined.
A probabilistic load prediction procedure is based on the premise that the probability of occurrence of a particular load level depends on the probability of occurrence of all possible combinations of ice parameters capable of producing that load level. For a complex problem involving several parameters, this approach requires a simulation procedure by which a large number of "load events" is selected through the use of theappropriate probability distribution functions (PDF). The load magnitude-probability relationship is then estimated from the predicted load "samples".
Section 5 will present a comprehensive procedure for simulating winter ice load scenarios involving multi-year ice floes. Before that, predictive models for three physical phenomena are presented. These include;
Peak indentation pressure (Section 3).
Relation between indentation pressure and ice excursion (Section 4).
First-year ice loads on the multi-year floe (Section 4).
Besides the total ice loads which will be discussed in Section 6, the designer of an arctic structure is concerned also with the manner by which these loads are to be distributed onto the structure.