Link Foundation Ocean Engineering and Instrumentation Fellowship Reports
Ice conditions present a significant challenge in the design of ships and structures for the development of offshore energy resources in cold ocean environments. Many regions throughout the world including Alaska, Canada, Russia, Norway, Kazakhstan and China are developing offshore energy resources in ice-prone environments. The growth and stability of the global economy is closely linked to that of our energy supply, and offshore industrial activity in Northern regions has increased significantly in recent years. One of the most significant engineering challenges when designing structures for ice environments is the estimation of design ice loads. The design load scenario for rigid, vertical-walled structures is often taken as the compressive failure of ice. During compressive failure fracture plays an important role in the redistribution of pressure, leading to the localization of contact into zones of high pressure. Within these high pressure zones (hpzs) pressure-dependent microstructural damage leads to localized softening due to microfracture, localized pressure melting and dynamic recrystallization. The aim of the present work is to use a combination of experimental analysis and numerical modeling to enhance our understanding of various aspects of the compressive ice failure process in the context of ice load estimation. The presence of a scale effect, whereby average pressure on a structure decreases with increasing contact area further complicates the calculation of ice loads. This scale effect has important implications for design. The selection of strengthening for full-scale structures based on laboratory-scale ice pressure data would result in highly conservative (and more expensive) designs. Another consequence of the scale effect is that local design areas (order of 1 m2) must be designed to withstand significantly higher pressures than are required for global design (areas order of 100 m2).The primary causes of this effect are fracture and the statistical averaging of high pressure zones across the face of the structure. The study of detailed pressure distributions made available through the use of novel pressure mapping instrumentation (albeit at much smaller than full-scale) is an important aspect of the present work. Since pressure decreases for increasing area, for a region of constant width, it follows that a thickness effect should also exist. The research carried out during the tenure of this fellowship included an examination of the thickness effect and explored possible explanations for it. Statistical fracture processes, associated with distributed flaws inherent to natural ice were indentified as a significant contributor to the scale effect. Since larger samples of ice contain greater numbers of flaws, large specimens have a higher probability of experiencing a local fracture (spall) event at a given stress level. Spalling fracture serves as a load limiting mechanism which plays an important role in the birth, evolution and death of high pressure zones. Results of the present research are summarized below.
Taylor, Rocky S., "Investigation and Modeling of Ice Fracture in Cold Ocean Engineering Applications" (2010). Link Foundation Ocean Engineering and Instrumentation Fellowship Reports. 23.