Sensitivity Considerations for Overturning Moment Calculations in the High Arctic, Deep Water, Offshore Environment
Authors: Blunt J.D. , Garas V.Y. , Matskevitch D.G. , Esenkov O.E. 
Published: December 2011

This paper examines sensitivity to treatment of multi-year sea ice loads against sloped sided, bottom founded structures for use in the high Arctic, deep water environment (up to 100+m water depth). The sensitivities are examined within the context of real data; an example ice feature has been selected and the influence of seasonal variation is considered as well. An established analytical technique - Ralston's (1977) method for sheet ice loads on conical structures, specified in the ISO 19906 (2009) Arctic offshore structures standard - is used to develop the load criteria for the selected ice feature and various structure slope angles. The study demonstrates gap areas where expert assumptions are required in order to completely characterize the sea ice load demand. Resultant locations for horizontal and vertical ice loads on the structure are shifted within a reasonable domain of occurrence and the corresponding overturning moments are calculated. Results showed significant sensitivity of the ice induced overturning moment to structure diameter along the load resultant plane of action, ice ride-up thickness, and the structure diameter at the maximum ride-up height. For the base case values assumed in this study, increasing the assumed contact diameter may increase the overturning moment. Increasing the ride-up thickness also results in larger overturning moments, while a larger top diameter (lower ride-up height) reduces overturning moment. Variations in these three parameters are not explicitly accounted for in the Ralston method.

Introduction

Global energy demand has driven oil and gas exploration to some of the most remote regions of the world. One such region is the Arctic offshore environment where loads from sea ice pose significant challenges for offshore structure design. Historically, hydrocarbon exploration in the Arctic offshore has been conducted in relatively shallow water (&lt; 30m) within the continental shelf. Typical methods consisted of using a large gravity-based structure (GBS) designed to withstand the ice loads related to a specific drilling location. In the case of the Canadian Beaufort, as exploration shifts toward the outer reaches of the continental shelf, both the deeper water environment (up to 100+m water depth) and the dominant ice regime (multi-year ice) will have significant influence on the selection of drilling and production concepts.

Given these considerations, the question arises as to whether current analytical techniques, primarily calibrated for use in the first-year ice sub-Arctic environment, are robust enough to be used in the high Arctic deep water environment. For sloped and conical structures in particular, operational experience and the amount of full scale data for multi-year ice interactions are minimal. This makes the extension of analytical techniques (particularly adept at characterizing model scale behavior) difficult when full scale behavior is considered. Given these limitations, the analytical relationships can be explored through sensitivity studies to highlight the epistemic (more commonly referred to as Type II) uncertainties in the method. This is performed in order to identify variables that should be emphasized in future basin testing and analytical studies in order to gain a more robust understanding of the ice-induced demands on high Arctic structures.