Ice rubbling and ice interaction with offshore facilities

In the context of this paper, ice rubbling is the term given to the process of ice interacting with itself and creating ice rubble. In nature, ice rubbling is most common in ridge-building in pack ice, but also occurs in the creation of stamukhi and ice pileups at shorelines. In ice interaction with platforms, the process of ice rubbling can have a significant influence on ice loads and other issues such as ice encroachment and platform access and egress. It is also well known that ridge-building in pack ice represents a limit to ice pressure within an ice field and has a significant influence on ice circulation models, ice forecasting and ship transits. In this paper, knowledge of ridge-building and pack ice pressures is briefly reviewed together with methods for applying this knowledge to ice interaction with platforms. The early work in investigating ridge-building forces by Parmerter and Coon is one of the starting points. The way rubbling limits can be used to assess Limit Force ice loads is well known, but uncertainties are implicit in the methodology which is largely based on empirical treatment of very limited measured data. Because of the limited measured data and ignorance of associated ice conditions, a factor of 5 (between lower and upper bounds) is included in the methodology for Limit Force ice loads in ISO 19906. This paper reports on new work to narrow this uncertainty by using ice mechanics theories originally developed for ice interacting with sloping platforms; and which (to a large extent) have been verified by field observations and measurements of ice loads on platforms.

► New model developed for ridging forces based on algorithms for sloping structures.
► It considers; ice breaking upwards; ice breaking downwards; slip line failure.
► The results are compared to the ISO 19906 equation for rubbling forces.
► The model leads to reduced uncertainties for ice rubbling and ridging forces.