Ice crushing forces on offshore structures: Global effective pressures and the ISO 19906 design equation

Abstract

Failures of offshore structures in ice covered waters carry high consequences for onboard personnel, the environment, and the economy. To avoid these consequences designs must address one of the most severe loading scenarios: the crushing of level ice sheets against a structure. Unfortunately, the ability to accurately predict global effective pressures is impeded by the complexity of the ice crushing process. Ice crushing, and the resulting effective pressure, is influenced by a number of parameters. Currently, ISO 19906 provides an equation for engineers to use in predicting these pressures. This equation incorporates the consideration of the aspect ratio, region specific ice properties and the ice thickness. This paper uses the format of the ISO 19906 equation to provide an exploration into how ice-structure interaction parameters influence global effective pressures. Full-scale loading data, empirical studies and theory are utilized. Additionally, the scope of this study is limited to level ice interactions with vertical or near-vertical faced structures. The ISO 19906 equation is determined to provide a reasonable empirical consideration of aspect ratio effects. However, an improved ability to quantify the aspect ratio effect is thought to be achievable in the future. Freezing degree days (FDD) are shown to support variations in the relative severity of ice pressures across different regions and ice seasons. Still, further statistical support is required in order to associate FDD based ice strength coefficients with specific return periods. Lastly, a multivariate analysis of the ice thickness and temperature effect is conducted. The validity of a linear relationship with the inverse square root of the ice thickness (h−0.5) is strongly supported. Furthermore, it is strengthened when a linear relationship with measured temperatures is also considered.