Efficient Combination of Numerical Simulations and Ice Basin Testing in the Design Process of Moored Structures in Ice

Abstract

The use of moored structures in ice infested waters is of relevance for the exploration or the exploitation of hydrocarbon fields in arctic waters. The assessment of the impact of ice actions in the design process is challenging as the ice actions and the structural response will differ for a moored structure. Further, design criteria may be given by severe and complex scenarios resulting from the interaction with intact level ice and ice ridges in variable ice drift.Ice model testing is an important part in the design process of a moored structure designed to operate in ice infested waters. The ice basin tests allow the measurement of the structure response to complex ice interactions. However, the conditions achieved in the ice tank can deviate from the target full scale scenario. Some structural parameters may not be replicated fully (for instance mooring forces on the structure), and the achieved ice conditions at the instant of the testing may deviate partially from the target ones (it is challenging to scale several ice parameters at the same time resulting in distorted ice properties).A numerical model was developed to simulate the response in real time of moored structures to drifting level and ridged ice. The engine simulates ice actions of the structure including sub-surface ice transport of broken ice around and under the structure. The response of the structure is computed accounting for non-linear effects in the time domain.The most promising use of these modeling capabilities is the possibility to correct and update ice basin measurements for the deviations between the achieved and target interaction scenarios: The tests performed in the ice basin are simulated numerically. A numerical model of the test setup needs to be built (of the structure, the mooring and the achieved ice conditions). By comparing the simulated and measured response, the numerical model can be calibrated and validated.  Numerical simulation of the target scenarios: A numerical model of the target design setup is built, and the response of the structure to full-scale target ice interactions is then estimated. The numerical simulation of the target scenarios represents then the corrected ice basin measurements accounting for all deviations. The simulated response will be used further in the design process, with a confidence based on the quality of the calibration exercise.Examples from two ice basin campaigns are presented where numerical simulations are compared with ice basin measurements, and where full-scale responses to target scenarios are then simulated numerically: Turret moored Offloading IceBreaker (OIB) vaning in level ice (Breivik et al., 2007),  Turret moored Arctic Shuttle Barge (ASB) interacting with drifting ice ridges (Jensen et al., 2000a,b). BackgroundMoored structures are a promising alternative for the development of fields in areas which present sea ice intrusions. Ice actions and the corresponding structural response have to be assessed in the design process. Both extreme and operational aspects are of...