DYNAMIC INTERACTIONS BETWEEN FLOATING ICE AND OFFSHORE STRUCTURES (ARCTIC, SUBARCTIC, OIL, GAS, RESOURCES).

摘要

Development of oil and gas resources at Arctic and Subarctic latitudes requires engineers to carefully consider the action of ice in designing offshore facilities for these areas. In this study, the interactions between an offshore structure and floating ice are investigated. Attention is directed to the case where ice fails by crushing at the face of the structure. A formulation suitable for investigating the dynamic response of platform systems with surrounding floating ice sheets or ice features is developed and applied.;An unidirectional finite element for modeling dynamic ice-structure interactions is developed. The element can simulate elastic loading or unloading and the treatment for crushing may include the effect of strain rate on ice strength. This "ice" element is incorporated into a general nonlinear dynamic analysis formulation which may include the offshore structure, the surrounding seawater in terms of added masses, the foundation in terms of frequency-independent springs, dashpots and virtual soil masses, and the mas, actual and virtual, of the floating ice feature.;Three types of problems are studied in order to demonstrate the effectiveness of the formulation and to gain some insights into the physical phenomena: (1) the impact of large ice floes against fixed gravity-type offshore drilling platforms: it is found that a static approach may not be sufficient to properly describe the interaction and a dynamic analysis is shown to be essential in analysing for extreme design events when a limited permanent deformation of the foundation may take place; (2) the response of slender structures subjected to continuous crushing by a uniform ice sheet: the model is shown to be effective in describing the large amplitude ice force variations observed during sustained slow velocity crushing; and (3) the earthquake response of an offshore structure in the presence of floating ice: a typical Condeep gravity structure is analyzed under a severe ground motion, with and without an ice cover at the surface of the sea. Under the assumed conditions, the ice is found to have a favorable effect.;Material properties of freshwater- and sea-ice relevant to the problem are first reviewed and summarized. Korzhavin's crushing formula, which is the basis of the proposed analytical model, is then studied in detail. The review discusses plasticity theory, yield criteria for ice, strain rate, the contact factor and the shape factor.