Post-simulations of Ice Basin Tests of a Moored Structure in Broken Ice - Challenges and Solutions

Abstract

Interaction between a moored structure and drifting broken ice is a complex process. To document the expected structure response, ice basin tests of the interaction are common practice. The outcomes of ice basin tests need to be carefully analyzed before extrapolation to expected full-scale target responses. The preferred strategy is to use numerical simulations to correct the measurements. The numerical model needs to be qualified by successful post-simulations of the achieved ice basin interactions.Post-simulations of interactions between drifting broken ice and a moored floating structure are of high complexity. The response of both the structure and the ice field needs to be replicated. This requires a good modeling of the ice field properties that matter (such as the floe size distributions and concentrations) and the boundary conditions affecting the interactions (such as the effect of the ice basin walls).Statoil's SIBIS numerical model is used to post-simulate ice basin tests of the moored Cat-I drillship. The present paper discusses the challenges with such post-simulations and presents the philosophy chosen for achieving successful postsimulations. BackgroundThere is a limited experience with design and operation of moored structures in ice infested waters. Per today, the screening, feasibility or detailed design phases of such concept rely greatly on ice basin tests. This is inline with ISO 19906 (2010) normative requirements which states: "Appropriately scaled physical models and mathematical models may also be used to determine the response of structures to ice actions, in combination with current, wind and wave actions".The outcome of ice basin tests has to be interpreted and corrected to be exploited in the design process. Different correction methods can be applied, and the use of empirical formulations is a common practice (see e.g. Tatinclaux, 1988). The interaction between a moored structure and drifting ice is a complex process, as changes in the action will affect the structure response and vice-versa. It can be challenging to only use empirical formulations to correct the measurements due to the complex interdependency between the ice action and the structure response. The preferred strategy to correct ice basin measurements is thus to combine ice basin tests with numerical modeling (see also Jensen et al., 2011, Bonnemaire et al., 2014:1. Simulate numerically the ice basin tests, under achieved conditions, 2. Compare measurements and simulation outcome and qualify the numerical model for the considered interactions, 3. Use the qualified numerical model to simulate the response of the structure to the relevant ice interactions, under target conditions. This methodology results in a correction of the ice basin outcome for the effect of all deviations in the achieved conditions under testing. In addition, the numerical model is qualified and can be used further for simulating additional similar interactions.This procedure applied to moored floating structures in drifting ice is presented and d...