The Effect of Viscoelastic Sea Ice Behavior on Limited Drilling Forces

Abstract

ABSTRACT Measurements of sea ice stress associated with limited driving forces were made during the Coordinated Eastern Arctic Experiment (CEAREX). Accounting for the visco-elastic nature of young sea ice, this data illustrates that the ice pack may be ableto transmit a load greater than the load associated with ridge building. INTRODUCTION The concept of a limited driving force as the controlling ice load in the design of a structure in the Arctic was introduced by Croasdale1 and Croasdale and Marcellus2 "As illustrated in Fig. 1, the governing design ice force will be the lower of the limit-stress load and the limit-force load. The limit-stress load is determined by the force to locally fail the ice in front of the structure and is therefore a function of ice thickness (local), structure width, and ice strength in the relevant mode of failure". Croasdale and Marcellus2 continue: "If we consider a very large ice feature, either a multi-year hummock field or a large ice island colliding with a structure, there are three stages of paper interaction. During stage (1), the load is dominated by an interaction force which slows down the ice feature and destroys its momentum (or kinetic energy). In this phase, relative penetration between ice and structure takes place. For a hard structure the ice is penetrated, whereas, for a soft structure such as an island, the ice penetrates the underwater slopes of the berm. The ultimate force reached during this stage is a function of the relative penetration, which in tum is governed by the work required to dissipate the kinetic energy of the ice feature." "Once the large ice feature has stopped moving, the surrounding pack ice continues to move against it. This is represented by stages 2 and 3 of the interaction. For an extreme ice feature, it is the average ridge building forces in the thinnest ice, over a width of several kilometres, which will govern the pack ice forces." Ridge building forces have been considered by several investigators, including Parmerter and Coon. 3 The model of Parmerter and Coon calculates the average force during ridging from anenergy consideration. In this work the effect of friction was initially considered small. However, studies by other investigators which calculate force consider friction to be an important parameter. Coon et al.4 documented ridging forces and observed sail heights during the Coordinated Eastern Arctic Experiment (CEAREX), from which a friction coefficient for the ridge force in the Parmerter and Coon model was derived. In this paper, the limit-force concept is reviewed based on CEAREX observations. A VISCO-ELASTIC VIEW OF LIMITED DRIVING FORCES Ridge building forces were measured as part of the Coordinated Eastern Arctic Experiment (CEAREX). During the early fall of 1988, three fluid filled, twenty-centimeter diameter flatjack stress sensors were implanted in a rosette pattern in a l60-cm thick multi-year ice floe in the Eastern Arctic north of Svalbard, as reported by Coon et aI.4 On October 7, 1988, a pressure ridge formed from 40-centimeter thick lead ice adjacent to the instrumented floe.