An analysis of stiffened plating subject to extreme ice loads

Butler, Trevor Roy (2002) An analysis of stiffened plating subject to extreme ice loads. Masters thesis, Memorial University of Newfoundland.

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Abstract

Discovery of large oil and natural gas deposits in arctic and subarctic regions of Canada has led to vastly increased offshore activity in these areas. This activity has increased the exposure of marine structures to ice loads. The ability of designers to assess accurately the response to ice loading is essential for the efficient design of these structures. In arctic areas, structures regularly encounter various forms of ice such as level ice, pack ice and pressure ridges, which are composed of both first-and multi-year ice. In contrast, in subarctic regions (for example, off the coast of Newfoundland), structures have much less frequent encounters with first-year pack ice and icebergs. In these situations, inadequate consideration of ice loads could make the risk level of operations in ice unacceptably high. -- This study is focussed on gaining an understanding of stiffened plating behaviour when subject to ice loads. Literature in three main areas was studied: ice failure processes during ice-structure interaction, definition of stiffened plate failure modes (or limit states) and reliability analysis, which incorporates these limit states into a probabilistic framework for design. The estimation of the response of a stiffened plate to extreme loads is greatly facilitated through an experimental analysis. To this end, a small-scale stiffened plate panel, subject to a lateral patch load, was tested to failure in the Structures Laboratory at Memorial University of Newfoundland. The patch loading is an idealisation of the two possible ice interaction scenarios in the waters off Newfoundland. The first is when level ice (or pack ice) acts upon, say, a ship's side shell. The second is the impact of a growler or bergy bit on a ship or platform leg. A detailed description of the physical model is given, along with results obtained. The use of finite element (FE) modelling techniques has greatly improved the study of complicated loading scenarios and structural response. A comparison of experimental results with those from a finite element analysis was carried out to assess the accuracy of the developed model. Once verified, the FE model is used in a sensitivity analysis of stiffener size to determine the effect size has on the ultimate strength of a stiffened plate. This will help understand the role the stiffeners play in structural design. Finally, a sample analysis is conducted to show how information regarding stiffened plate behaviour is implemented in reliability-based design.

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