Ice load prediction for design of ice-going ships for inland waterways

Abstract

With increasing interest in utilizing the inland waterways (IWW) in European countries, the design of IWW vessels gains attention both from a transport efficiency and an emission control point of view. However, unlike in western and central European countries, in Nordic countries, e.g. Sweden, IWW ships must deal with ice on the fairway during every winter. Usually, IWW ships are designed without ice concerns and are structurally weaker compared to ships designed according to ice class notification from the classification societies. Developing such ships requires particular concerns since there is no strict requirements regarding ice class notifications for IWW ships. A primary challenge is to estimate both the global and local ice loads acting on the ship hull structure. To consolidate the design problems for IWW ice-going ships, Lake Mälaren is selected. Ice conditions, i.e. ice type and concentration, and ice data, e.g. ice thickness and ice flexural strength, are extracted and analysed for the ice load estimation. The ice mechanical properties have great influence on the ice load. Ice characteristics are studied based on empirical formulae and properties are calibrated by reference data.

The deterministic approach is widely used to predict the ice loads. It is suitable when all variables, i.e. ship geometry and ice properties, are known and refers to rule-based design hereby. For first year light ice conditions in Lake Mälaren, the Finnish Swedish Ice Class Rule (FSICR) is widely used. The thesis uses guidelines from the Finnish Swedish Ice Class Rules as a reference and compare the results with other methods.

The probabilistic approach, on contrary, is useful when certain variables are unknow, which are interpreted as random variables, for instance ice breaking pattern. Here the probabilistic method and ice-hull interaction mechanism are studied. The probabilistic method simplifies the ice pressure in relation to the contact area between the ice and the ship hull. It predicts maximum ice pressure acting on the ship hull based on field ice test data and ice exposure conditions. Such semi-empirical method can be used regardless of ship type and size. For this, a numerical model is introduced based on ice-hull collision mechanisms and the essential ice breaking characteristics. The physical mechanism is studied for idealizing ship-ice impact model. The idealization model includes the ice failure process, ice conditions and ship geometry. The ice failure is assumed to be initiated by crushing ice and followed by breaking due to bending failure. Ice properties are set as constant values without any variations. The stochasticity in interact process is represented by randomness in collision location and number of pieces of ice floe formed after breaking. An energy method is used to calculate the ice crushing force, indentation displacement and contact area. The ice bending scenario is simplified as an infinite plate resting on an elastic foundation under a concentrated load. Ice impact load and critical load can be obtained for global and local structural assessment respectively. The structural responses and structural strength of a representative panel at linear and nonlinear contexts are investigated as well. Ship structure is commonly designed with material yield strength as limit. However, the study shows a lighter structure can be achieved if plastic deformation is allowed without causing failure. Therefore, the design can be optimized with regards to ice loading capacity and weight control.

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