A constitutive model for iceberg ice is implemented into an explicit finite element analysis. The focus of this effort was to establish a validated model for high-speed impact events in which the ice strength undergoes a transition from a ductile to brittle mode of failure at strain rates around 10-3 s-1. The proposed iceberg ice response is linear elastic prior to the stress state reaching the ice failure envelope. The ice failure envelope is represented by a set of concentric ellipses in the octahedral shear stress (t) and hydrostatic pressure (P) plane, where the size of each envelope reflects a specific strain rate and temperature; increasing strain rate and decreasing temperature contribute to increased strength in the proposed failure envelope. Upon reaching the specified envelope, a brittle failure mode is simulated using exponential degradation of material stiffness.

To demonstrate the predictive capabilities of the proposed iceberg ice model, small-scale finite element models of a variety of experimental ice tests were used for validation. Published experimental parameters were used to develop constants for the material model. The work examined finite element predictions for the following laboratory tests:

  • Triaxial compression

  • Uniaxial compression

  • Uniaxial tension

  • Flexure

  • Indentation

The natural variability inherent in iceberg ice influences several ice properties, including strength and associated failure modes, with a particular sensitivity to temperature and strain rate. Overall, the proposed constitutive model for iceberg ice response predicts failure with reasonable agreement based on triaxial compression tests but tends to over-predict the strength of the ice in uniaxial tension and compression. When implemented with material damage and exponential degradation of material stiffness after the stress state exceeds the ice failure envelope, the proposed model is believed to conservatively capture linear elastic response followed by a brittle failure mode and can be used to simulate an iceberg impacting a structure at strain rates > 10-3 s-1.

Keywords:
upstream oil & gas, failure mode, experiment, iceberg ice, material model, failure criterion, failure envelope, mpa, constitutive model, strength
Subjects:
Information Management and Systems
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2015. Offshore Technology Conference
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