Challenges Implementing ISO 19906 for Probabilistic Assessment of Multi-year Sea Ice Loads on Sloping Structures

Abstract

The ISO standard ISO 19906:2010 provides guidance for determining design loads for offshore structures in arctic and sub-arctic regions, using both deterministic and probabilistic approaches. References are provided for models of both first and multi-year ice loads on both vertically-faced and sloping structures. ISO 19906 indicates that consideration should be given to limit stresses at the ice structure interface based on the dominating ice failure mechanisms, limit forces associated with the available driving force that can act on the interacting ice floe due to wind, currents and surrounding ice, and the kinetic energy of the impacting feature. Analyses show that the kinetic energy of an impacting multi-year floe driven by surrounding ice will result in larger loads than those from driving forces alone. In order to determine penetrations and maximum loads, it is then necessary to consider the variation in load with penetration during an impact. In the case of multi-year ice loads on sloping structures, full-scale data to properly validate models is lacking. Several models are available that estimate maximum loads for level ice interactions with sloping structures. These models can partially account for rubble forming on the structure during the interaction thereby increasing loads, though the ability to estimate rubble heights for different structure shapes and ice conditions at present is limited. In the case of multi-year ridges, several models are available that estimate the maximum load during the interaction of a prismatic beam shaped ridge contacting a conical structure. Data and model for loads from more realistic multi-year ridge shapes are lacking. Additionally, the influence of rubble existing on the structure at the time the ridge impacts is not explicitly considered. This paper examines issues in determining probabilistic design loads for multi-year interactions with sloping structures and presents sensitivity results for key assumptions and parameters. The analyses were carried out using the Sea Ice Loads Software (SILS) developed by C-CORE as part of a Joint Industry Project (JIP) to implement ice load models referenced in ISO 19906 within a probabilistic framework for the purpose of determining design loads. Interpretation was required where ISO 19906 does not provide explicit details on the models and probabilistic implementation. Monte-Carlo simulation was selected for carrying out probabilistic calculations of design loads as this approach is robust.