The seasonal presence of icebergs on the Canadian east coast is a potential threat to any possible production of offshore oil or gas. With the promising results of exploratory drilling, a speedy solution to the iceberg problem is to be found before production could commence. Atlantic icebergs appear in different shapes and usually weigh millions of tons. Drafts as much as 150 m are common for such bergs. The threat, therefore, could be in the form of direct hit of the production platform or scouring of the ocean floor in the process of iceberg grounding.
Bottom scours as deep as 6 m and 3-km length have been measured in the iceberg-infested waters. Thus, buried-offshore installations such as foundations and pipes are as much susceptible to damage as those exposed.
This paper describes an analytical model for the iceberg scouring in which an idealized berg ploughs into a gentle slope of soft ocean-floor soil. Theoretical equations were obtained for the scour size. An instrumented laboratory model was pushed into an artificial slope of soft sediment in a glass-sided tank, thus reproducing the scouring phenomenon. These experiments verified the assumptions of the analytical model. Results obtained for the above model compare well with the scour sizes reported for typical weak surficial sediments. It is hoped that prediction of maximum scour depths would help placing bottom installations well below the zone of possible damage.
Description of the analytical model, the details of laboratory tests, as well as the correlation of the measured and predicted values are given in this paper. Extension of this analysis for possible protection of production platforms from a direct iceberg hit also is indicated.
Iceberg threat is the most critical factor associated with offshore hydrocarbon activities on the Labrador Shelf. Although exploration in this area still is considered to be at its infancy, results so far are quite promising. Solutions to the iceberg problems have to be developed before any production could commence. Improvements to the existing technologies have to be made or new techniques devised, for operations in these iceberg infested waters. Conventional gravity or jacket structures are not likely to withstand a direct hit from a drifting iceberg unless protected by auxiliary structures or an artificial island. Similarly, pipelines laid by routine methods are likely to be torn out by bottom dragging and scouring icebergs unless buried sufficiently deep. Alternative production systems such as a drillship and various forms of subsea methods currently are being examined by the oil industry. A drillship likely to be in the path of a drifting iceberg can be moved off location by disconnecting the riser pipe. If the iceberg is large, the BOP stack itself could be knocked off or scoured out by the berg. Such a situation was reported to have occurred during the drilling season of 1973. Bottom scouring by icebergs still would be a problem even if methods avoiding a direct collision were to be available.