Limit State Design Methodology for Offshore Pipelines against Ice Gouging - Industry Guidelines from the ICEPIPE JIP

Abstract

Abstract Ice loads on buried pipelines through ice-soil-pipeline interaction is a field with limited industry experience. As exploration in Arctic and cold climate regions increases it is of great importance to understand and to design for the challenges offshore pipelines must be able to withstand in these harsh areas with a suitable level of confidence. Together with thirteen industry partners, DNV has carried out a joint industry project in order to better define and therefore understand the Arctic specific challenges to pipelines. This project has developed initial industry guidelines for offshore pipelines in cold climate regions, in general and for ice scour in particular. A major design issue for Arctic offshore pipelines in shallow water is ice keels gouging the seabed and imposing loads on the pipeline through ice-soil-pipeline interaction. The industry has somewhat limited experience with deep ice gouges and ice keels interacting with buried pipelines. A common practice has been to bury the pipeline well below the deepest registered gouge mark in the relevant geographical area with the objective to limit the pipeline response. In order to render future projects in regions with severe ice gouges economically feasible whilst still being sufficiently safe there is a need to utilise modern design principles based upon statistical methods, advanced FE models calibrated to ice gouging tests, limit state design principles and strain based design. The ICE PIPE JIP provided the introduction of a more systematic approach for the identification, quantification and ranking of design uncertainties, and how this could be adopted to existing pipeline design methodologies and criteria. This paper presents some pertinent results and recommendations from the ICEPIPE JIP. The paper presents a sample case study for how the assessment of some elements of statistical uncertainty can be used in the definition of the 100 year return period ice gouge depth. This paper highlights one methodology for approaching this and it is emphasised that not all the steps that would be required in design are included in this brief paper. A discussion on the use of advanced FE tools in the ICE PIPE JIP is also presented.