CASE BASED SCALING: RECENT DEVELOPMENTS IN ICE MODEL TESTING TECHNOLOGY

摘要

Model ice testing is the state of the art validation and testing method for ships and structures interacting with ice. Its initial design objective was the prediction of resistance forces of ice breaking ships by using Froude and Cauchy similitude to account to the most significant force ratios. In the ice breaking process the forces due to downward bending are considered most significant and therewith much emphasis was spent on the correct scaling of the bending strength or flexural strength of the model ice. Recent research on the mechanical behavior of model ice shows a significantly higher compliance in downward bending than targeted when following the applied scaling laws. This can lead to scale effects in the resistance force also when testing ice breaking ships. The too compliant ice facilitates an additional ride-up of the ship onto the ice and the vertical motions manifest as additional resistance contribution. The low compliance of model ice also imposes uncertainties on wave-ice interaction tests, which gain increasing significance due to the climatic changes in Polar regions. The modeling of ice break-up due to waves, with the current standard model ice, requires much steeper waves than in full scale as the ice surface needs to experience a much higher de- flection to reach the critical failure stress. A similar issue arises for vertical structures exposed to drifting ice. In full-scale a pile-up of ice around the structure is observed and in the contact area so called high-pressure zones may form. Such effects cannot be modeled with classic model ice as it easily bends downwards and produces a failure pattern and failure process very different from full-scale as well as high-pressure zones do not form which is due to the string property gradient in model ice. The mentioned three scenarios are considered being highly relevant in marine research and for the marine industry and therefore this paper introduces two new model ice types with which those scenarios can be modeled. The 'model ice of virtual equivalent thickness' uses a different modeling approach to reach a scaled stiffness for improved modeling of waves in ice and ships' resistance in thicker ice. The 'wave model ice is modeled by using waves in the formation process and can resemble high-pressure-zones acting on a vertical structure. Both methods are considered as an extension to the existing standard model ice for dedicated scenarios by scaling or putting emphasis on different ice properties by altering the production process. The presented approach also emphasizes case-based-scaling, which means that the scaling or the model ice type needs is defined by the modeled scenario as the standard model ice is obviously not fully capable to reflect all properties of sea ice in scale.