Strength of Offshore Gravel Islands To Resist Ice Loads
Authors: Kotras T.V. , Arnold C.L. , Bergman K.M. 
Published: October 2008

One of the prime concerns in designing offshore gravel islands in ice-covered waters such as in the U.S. and Canadian Beaufort Seas is the ability of the island to resist the imposed design ice loads. In presenting this paper, the goal is to describe a methodology to aid in the preliminary assessment of the overall strength of a gravel island to resist a design ice load for a set of prescribed design and environmental parameters. Conversely, the described methodology can be used to aid in developing a preliminary island design in terms of its size, height and geometry to ensure the island had sufficient strength to resist the design ice loads.

More specifically, the purpose of this paper is twofold:to describe a methodology for establishing the strength of offshore gravel islands to resist imposed ice design loads; andto use this methodology to illustrate how the performance of gravel islands to resist the imposed ice loads varies with the major design and environmental parameters.

In summary the results of the analysis presented in this paper indicate that, from the standpoint of resisting ice loads, gravel islands will continue to be technically feasible offshore drilling platforms for both explorations and production in the U.S. and Canadian Beaufort Seas as the oil industry moves into deeper and more harsh areas.

INTRODUCTION

With the recent offshore lease sales in the Diaper Field and the future scheduled lease sales, interest in investigating various drilling platform concepts for exploration and production in the U.S. and Canadian Beaufort Sea areas has continued. Proposed concepts have ranged from gravel islands, caisson retained islands, and mobile structures to moored floating platforms. To date gravel islands have been used as the primary man-made exploration drilling platform in the U.S. and Canadian Beaufort Seas. As interest has expanded to areas farther from shore, deeper water and less protected areas, the question arises as to whether or not gravel islands will continue to be viable and will they have sufficient strength to resist the imposed design ice loads.

In designing gravel islands for the Beaufort Sea it is important to note that other major concerns must be considered in addition to the island strength to resist ice loads. These other concerns include: ice ride-up/ pile-up potential; wave run-up and overtopping; seabed scour due to ice, waves and currents around the structure; and slope protection to prevent island erosion by waves and ice. Of these, however, ice loads tends to be one of the major concerns and is the focus of this paper.

GRAVEL ISLAND STRENGTH DETERMINATION

Overview

In determining the overall gravel island strength to resist the design ice load, three failure modes were considered: failure at the freeze front, failure at the ice loading plane, and failure at the gravel fill/ silt interface which may be at the seafloor or lower if surface silts are dredged before construction. For the range of soil material properties investigated (see Table 1) the governing failure mode was primarily a function of the depth of freeze as depicted in Figure 1.