As offshore wind energy is expanding into seasonally ice-covered regions, accurate estimation of sea ice strength becomes critical for safe and cost-effective design. ISO 19906 provides an empirical method to estimate the global ice force based on the width of the structure, thickness of the ice, and an ice strength coefficient, CR. It suggests predefined reference values for this coefficient depending on the region of interest. However, if the wind farm is located outside one of these predefined refer- ence regions, the ISO standard gives limited guidance on how to adjust the ice strength coefficient to the local ice conditions. This study investigates the method proposed in ISO 19906 to estimate a site-specific ice strength coefficient, CR,s. The method uses the ratio of the ice strength index obtained from strength measurements at a reference site and at a new site.
In this study, two sites were selected to determine this strength index ratio. Hjellbotn, a temperate brackish ice zone near Trondheim, was selected as a potential offshore wind site and falls outside ISO’s predefined CR regions. Svea, in the Svalbard archipelago, was used as a proxy for the Arctic region for which the ice strength coefficient has been determined by ISO. The ice strength index at both locations was estimated using two ISO-recommended approaches: a direct mechanical measurement with the BHJ and an indirect estimate based on brine volume derived from temperature and salinity.
ISO 19906 provides a predefined ice strength index for the Arctic region when using the brine volume method. However, the standard does not offer a similar reference value for measurements taken with the BHJ. BHJ tests from Svea were used as the Arctic BHJ reference. The same test procedure was used at Hjellbotn for comparison. The BHJ strength ratio suggested lowering the ice strength coefficient for Hjellbotn. Brine-based strength ratios used either the ISO Arctic reference or Svea data. The ISO-based approach also indicated a lower strength coefficient for Hjellbotn, as expected for more temperate ice. Using Svea as the proxy for the Arctic gave a higher strength coefficient for Hjellbotn, which was unexpected. It showed that the choice of method and reference value affects the outcome of the strength estimation. This difference indicated a limitation of the brine volume strength method when trying to scale the ice strength coefficient from the Arctic to warm ice conditions. For warm ice, small temperature changes caused large strength variations, which showed the brine-based method’s high sensitivity.
The findings of this work suggest that while the ISO framework provides a basis for estimating an ice strength coefficient for a new area, careful interpretation is required when scaling the ice strength to temperate and marginal ice regimes. The brine volume method is sensitive to measurement uncertainty at high ice temperatures and may not always reflect the full mechanical strength of the ice.