Iceberg scour risk analysis for pipelines on the Labrador Shelf

King, Anthony D. (2002) Iceberg scour risk analysis for pipelines on the Labrador Shelf. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (25MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

An iceberg risk analysis was performed for a network of 3 pipelines proposed by Petro-Canada (1983) to allow the transport of natural gas on the Makkovik Bank to a landfall at Cape Harrison on the Labrador Coast. The risk analysis originally performed indicated that scouring icebergs would make direct contact with these three pipelines an average of thirteen times per year (total for all three) if the pipelines were trenched to provide a cover depth of 2.5 m. The required burial depth to reduce failure rates to one every second year for the pipeline network was estimated to be approximately 5 m, which was not considered technically feasible. -- Since the original Petro-Canada (1983) analysis, a significant amount of work has been done on iceberg scour and the associated risk to subsea facilities. Given increasing interest in the development of offshore natural gas reserves, it was considered worthwhile to perform a review of the original work and, if warranted, perform another risk analysis. Certain elements of the original risk analysis were identified as being extremely conservative: the mean scour depth, the iceberg draft distribution, and the method used to determine the proportion of icebergs scouring over the pipelines. -- In order to perform a risk analysis for the pipeline network, a model was developed to estimate iceberg grounding rates on the seabed. The model was tested using data from the Grand Banks, where iceberg parameters are fairly-well established. The model was verified using scour rates estimated at the Hibernia and White Rose sites from seabed surveys and was found to provide reasonable estimates of iceberg scour rates. -- Data for the Makkovik Bank was reviewed for use in the grounding model, to allow calculation of pipeline scour crossing rates and to determine the scour depth distribution. The failure rates depend on the criterion used to define pipeline failure. If direct contact between a scouring iceberg keel and a trenched pipeline was defined as failure (which was the criterion used for the Petro-Canada (1983) analysis) then the mean time between failures for the three pipelines with 2.5 m cover varied from 18 to 23 years, with a resulting mean time between failures of 7.5 years for the entire network. While the direct-contact criterion was commonly used at the time of the original analysis, modern analyses require a clearance between a scouring iceberg keel and the top of the pipeline. If a conservative criterion of 1 scour depth clearance between the scouring pipeline keel and the crown of the pipeline (i.e. 2 m cover for 1 m scour) is used to define pipeline failure, the mean time between failures for the three pipelines (2.5 m cover) varies from 3.4 to 5.3 years, with a mean time between failure events for the pipeline network of 1.4 years. A detailed analysis of pipeline response would yield more favourable results. -- Additional work is recommended to allow better definition of scour parameters and iceberg frequency. A high-quality seabed survey would likely indicate a shallower mean scour depth than was used in the analysis (0.75 m), potentially in the range currently used for pipeline risk analyses for the Grand Banks (< 0.5 m).

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