ABSTRACT

The Arctic seabed in shallow water is repeatedly deeply gouged by drifting ice. If the force that the ice applies to the seabed were transmitted to a pipeline, it would almost inevitably create severe damage. The conventional approach to the design problem is to estimate the extreme maximum gouging depth, add the estimated extent of subgouge deformation, add a safety margin, and require that the pipeline be trenched to that depth. The result is to make the trench very deep and very expensive, and to increase the risk of delay and dispute. This paper explores different ways of reducing the cost of protection, among them the choice of route, combinations of horizontal drilling and conventional construction, and protecting the pipeline in situ.

INTRODUCTION

Seabed gouging by ice keels has long been recognized as a threat to the security of Arctic seabed pipelines. Investigation showed that some of the observed gouges are deep and wide, and that they are formed under present-day environmental conditions and are not just relic gouges from different environmental conditions in the past (Lewis, 1977; Weeks, 2010). Ice fragments weakly attached to the lowest point of a keel will break loose when the keel begins to cut into the seabed, but as it cuts deeper the surviving keel must be strong enough to exert a large force. A gouge can be 5 m deep and 80 m wide (though many are much smaller than that). A simple calculation confirms that the force needed to cut such a large gouge can easily exceed several tens of MN (Palmer et al., 1990), and independent ice strength calculations (IS0, 2010; Palmer and Croasdale, 2012) confirm that the ice can be strong enough to apply that force.

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