Vertically sided offshore structures subjected to level ice are designed to withstand the effects of ice-induced vibrations. Such structures are, for example, offshore wind turbines on monopile foundations, multi-legged oil-and gas platforms or lighthouses. For the prediction of dynamic interaction between ice and structures, several phenomenological models exist. The main challenge with these models is the limited amount of data available for validation, which makes it difficult to determine their applicability. In this study, an attempt is made to validate one of the existing models. First, the parameters which define the ice in the model were derived from new model-scale experiments with a rigid rectangular structure. The model was subsequently applied to simulate the interaction between ice and two compliant rectangular structures with different structural properties. Finally, model-scale experiments were conducted for the two compliant structures. Results of the experiments and model were compared to assess the capability of the model to predict dynamic ice-structure interaction. Results show that the adopted approach allows for a definition of the input parameters of the model and accurate prediction of frequency lock-in and continuous brittle crushing for compliant structures. Intermittent crushing was not observed in the model-scale experiments due to the model-scale ice bending significantly during low ice speeds. As a consequence, the model could not be validated for this regime of interaction. The approach followed-and challenges encountered during its application-are discussed.

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 deflection 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.

Model tests in ice have been conducted at the Large Ice Basin of HSVA with cylindrical and conical, compliant structures exposed to drifting level ice to investigate the influence of slope and compliance on the ice load and its breaking frequency. Main goal of the test campaign was to study the importance of structural feedback during ice-cone interaction. This is a major issue e.g. for numerical simulation of offshore structures during design phase. Four shapes were tested: 50°, 60°, 80° and 90° slope angle. The cylinder was tested in order to define the worst case scenario regarding magnitude of ice load and severity of ice-induced vibrations. Stiffness and natural frequency of the structure were chosen similar to typical values for offshore wind turbine support structures. All shapes were tested both in a compliant and fixed configuration. The breaking frequency was found to be more pronounced for the lower slope angles where the ice failed in flexural failure only, while a transition to crushing failure as observed on a cylindrical structure takes place at 80° cone angle already. This results in significantly higher ice loads on the 80° cone than on those with lower angles, but a reduced risk of severe ice-structure interaction due to the unsteady nature of the mixed mode breaking process. Although the breaking frequency is rather constant e.g. during ice impact on the 60° cone, it was not possible during the model tests to match the ice drift speed and the dynamics of the structure in a way that causes resonance. However, model test results prove that there is a risk of conical structures with low natural frequencies and low stiffness in ice plane being excited by periodic ice failure in their natural frequency, thus response amplification may take place and pose a risk to the structural integrity of conical offshore structures exposed to sea ice. This paper presents the model test setup, analysis of the results, and general findings.

A compliant cylindrical structure has been built and tested in a series of model tests in ice in the Large Ice Model Basin at HSVA. The structure’s stiffness in ice plane is higher in ice-drift direction than crosswise, enabling the model to vibrate in different oscillation patterns. In total, 4 ice sheets have been used to perform tests in different ice thickness, covering a wide range of ice drift velocities between 0.005 and 0.15 m/s in model scale. Several events of ice-induced vibrations were observed throughout the test campaign. Oscillations are found to reach different types of beginning steady-state, mainly depending on ice drift velocity and ice thickness. Dynamic amplification of structural response in ice plane as well as ratio of static and dynamic forces is highly dependent on the type of vibration. While the dynamic amplification is highest when the ice load’s frequency equals the first natural frequency of the structure, the highest dynamic forces occur when the crushing frequency is an integer fraction of the natural frequency. The paper describes the design of the test set-up, instrumentation and calibration, performance and analysis of conducted tests, and general findings.

The development of HSVA’s current standard model ice dates back to 1993 when Evers and Jochmann introduced a new production process [1]. Since that time, a lot of experience has been gained in numerous ice sheets, regarding both, the process of production itself and the properties of the resulting model ice. Meanwhile, requirements on ice model tests, and also on the model ice itself, have changed during the past decades. This relates to more diversified test types, which do not only concentrate on ice-breaking ship designs with typical flat bows, but also on a variety of structures and hull designs that include different failure modes. Also, the typical property range of model ice regarding strength and thickness has significantly widened. This paper summarizes the changes that have been made over the years, leading to the standard model ice that is available at HSVA today. Its properties are presented and discussed, showing limitations regarding absolute property values and their uncertainty, as well as the ability for correct representation of specific interaction scenarios. Further, methods to modify the model ice for different specific test scenarios are suggested and discussed. This includes for instance global and local crushing against vertical structures.