Risk-Based Calibration of Action Factors in the new ISO 19906 Arctic Offshore Structures Standard
Authors: Maes M.A. , Thomas G.A.N. 
Published: December 2011

This paper describes the results of a calibration of partial action factors in the new ISO 19906 Arctic Offshore Structures Standard. The work was scoped and directed by a panel of world experts in offshore Arctic engineering, constituted as ISO Technical Panel TP10 "Case studies and calibration". The calibration focuses on environmental action factors both for principal actions and for various environmental companion actions. The calibration is essentially a weighted risk-based optimization which targets three risk or exposure levels L1, L2 and L3 and which considers two limit states (ULS and ALS) as well as two basic sets of design check equations: a set I based on ISO 19902:2007 and a set II based on earlier suggestions in ISO/DIS 19906 and CAN/CSA-S471–04. It accounts for weighted combinations of all action effects in various proportions, for different resistance models, for different levels of action effect model uncertainties, for different levels of uncertainty, and for different mean action event occurrence rates. Weights used in the optimization were validated by the ISO panel TP10.

Ice actions are regionalized on the basis of uncertainty characterization of extreme action effects (tails and extreme values). This is done with respect to both geographical region and type of structure. Regions with different ice action magnitudes can be considered together if their probabilistic description is similar. Four different calibration scenarios were considered for ice actions: a calibration of separate principal ice action factors for each combination of region and type of structure, a calibration of region-wide ice action factors, a calibration of one overall principal ice action factor, and the inclusion of ice actions in the larger group of all environmental actions. All four scenarios lead to reasonable action factors that are consistent with ice type and structure type. The multitude of environmental/ice companion factors in various environmental action combinations is reduced to just 4 companion action factors each of which is subsequently calibrated.

Introduction

The DIS version of ISO 19906 published early in 2009 contained normative clauses with respect to reliability, risk and consequences, but it did not provide definitive partial action factors for its limit states design framework. Accordingly, during 2009 a calibration was performed of the environmental action factors for exposure level L1 in order to ensure that the specified target reliability for each ULS and ALS action combination was achieved for circumstances that could reasonably be foreseen. A secondary objective was to also recommend calibrated environmental action factors for L2 and L3. A detailed report was published about the calibration in October 2009 (Maes, 2009). The present paper provides an overview of the analysis and the results of the calibration study.

In addition to the above objectives, the report also provides detailed guidance on a recommended calibration method should the user of ISO 19906 wish to perform a site-specific or structure-specific calibration using a fully probabilistic approach. This objective is in itself quite important as ISO 19906 permits such an approach when the potential economic benefits of performing a site-specific calibration are likely to be considerable for large arctic offshore installations.