Eighteen ice experts from nine countries were actively involved in the development of new guidelines for ice actions that are an essential part of the new standard ISO 19906. This paper provides an overview of the methods that are recommended for ice action determination.
This standard is based on the limit states design method. Therefore, guidelines are provided for ice actions in four different categories of limit states. Characteristic values of ice actions depend on the limit state concerned and also on the life-safety and consequence categories that are applicable to a structure.
As a general principle, design shall be carried out separately for global ice actions, local ice actions and for dynamic ice actions. Operational procedures may be used to reduce ice actions. Ice action scenarios need to be selected by taking account of the specific conditions in the ice regime for which a structure is designed.
Data from full scale measurements on instrumented structures were the basis for developing methods for calculating ice actions. These methods are recommended for the practical determination of design ice actions on offshore structures. Both probabilistic and deterministic methods can be used. For many action scenarios, it is useful to consider limit-stress, limit-energy and limit-force mechanisms as well as specific ice failure modes. ISO 19906 provides a variety of methods and formulas that consider these limiting mechanisms and ice failure modes for the determination of global ice actions. Three methods for determining local ice actions in different conditions are provided. Basic modes of dynamic ice actions are described and simple models are provided for vertical and conical structures.
Several surveys of predicted ice actions have been carried out in recent years (Shkhinek et al. (1994); Croasdale (1994) and Timco and Croasdale (2006)). International experts were invited in these surveys to determine ice actions for a few basic scenarios. These studies have revealed little consensus and a wide spread in the predicted values of ice actions. This situation has been a good reason for the development of the new standard ISO 19906, which should significantly reduce the discrepancies in the prediction of ice actions.
The new guidelines of ice action determination are provided in the sub-clauses 8.2 and A.8.2 of the new standard. These clauses were written by an international group of eighteen experts from nine countries, as shown in Table 1. The resulting clause is the consensus of these experts.
It is well known that formulas on ice action can not be derived directly from first principles. Empirical factors that are obtained from experiments play a very central role in all ice load prediction. The new standard emphasizes the importance of full-scale data.