Multi-Year Ice Loads: Alignment of Experts Workshop

Abstract

Abstract A Joint Industry Project (JIP) was conducted in 2007 to determine the degreeof consensus of leading ice mechanics experts on the loads exerted bymulti-year ice on offshore platforms. Seven international experts on multi-yearice loads were asked to predict loads for three different ice loading scenariosinvolving multi-year ice floes: isolated floe, a multi-year floe in pack ice, and a multi-year hummock field in a sheet of first-year ice. The Experts wereasked to calculate the loads from these three ice conditions interacting with a150 m wide vertical caisson structure and a 45 degree conical-shapedstructure. There were significant differences in the methodologies usedand the assumptions made to estimate the loads. Load predictions variedconsiderably for each scenario with estimates differing by a factor of 4.6 forthe vertical caisson and 3.5 for the conical structure. In spite of the lowerratio of predicted loads for the conical structure, the Experts were moreconfident with loads on the vertical caisson. The key areas for furtherresearch were identified and these include improved knowledge of the icethickness and its variation for Old Ice, new and innovative techniques forobtaining ice loads, improved knowledge of pack ice driving forces, and betterunderstanding of the failure behavior of multi-year ice. This paper provides anoverview of the loading scenarios, details of the load predictions, andoutlines the areas identified for future research to help to provide morereliable load predictions. Introduction The presence of ice in the frontier regions of Canada, USA, Greenland, Norway and Russia presents a unique set of obstacles to the safe and economicproduction of offshore oil and gas. Ice interaction with structuresraises design, operational and regulatory issues, and if not understoodproperly, can lead to exceedingly high design and operational costs foroffshore production. Global ice loads are needed for overall stabilityconsiderations of gravity-based structures, caisson structures and floatingproduction systems. In addition, ice impacts may generate high local icepressures, causing damage or failure of small but critical areas of offshorestructures and/or petroleum tankers. Research on ice loads over the pastseveral years has led to significant understanding of these loads especiallyfor level ice conditions. Recently, Timco and Croasdale (2006) invited twentyinternational ice mechanics specialists to predict loads for differentfull-scale scenarios. The results show general agreement in predicted loadsgenerated by level ice on a vertical-sided structure. However, predictions of loads for multi-year ice differed by a factor of seven. Clearlythis large uncertainty would lead to conservative assumptions regarding iceloads and significantly higher construction and operational costs in regionswhere multi-year ice is present. A better understanding of multi-year iceloads, and the factors that affect ice loads is the key to reducing thisuncertainty.