The Limiting Driving Force Approach to Ice Loads

ABSTRACT This paper reviews the overall logic to ice loads on structures and discusses the applicability of the limiting driving force approach to ice loads. It is suggested that "limit-force" loads may control ice forces in the winter on future production platforms placed beyond the land fast ice in the Arctic Ocean. "limit-force" loads are controlled mainly by average ridge-building forces in the thinner ice between large thick floes. A review of ridge-building forces and pack ice strengths has been conducted and typical "limit-force" loads are calculated. Uncertainties in "limit-force" estimates are discussed and topics for future research are proposed. INTRODUCTION The classical approach to ice forces on structures is to assume that the ice moves under the action of infinite driving forces. Then the global load is governed by ice deformation immediately in front of the structure. Under these circumstances with full contact .across the structure ' width, the ice force can be defined as simply the product of the ice thickness, the structure width and the ice stress or pressure in the relevant mode of deformation. The author 1,2 coined the term "limit-stress" to describe this approach to ice forces because of its direct dependence on the local ice stress. Most research into ice forces has addressed the "limit-stress" condition, the major focus being to understand the stress/ deformation characteristics of ice for a variety of failure modes, boundary conditions, ice types, strain rates, ice temperatures, etc. For relatively narrow structures and ice thickness appropriate to normal first year sheet ice, or less, the "limit-stress" approach seemed to be intuitively correct and indeed several investigators3,4 have' demonstrated that there is generally sufficient wind drag over the ice to create typical "limit-stress" loads on narrow structures. With the move towards platforms for the offshore arctic, structures became wider than had been previously built, and much thicker ice features became part of the ice interaction scenarios for design. Loads calculated using the "limit-stress" approach became very large and engineers began to question whether such loads could be generated by the natural forces of wind and current. In land fast ice zones with the ice sheet generally being competent and continuous over hundreds of square miles, the answer appeared to be yes, typical "limit-stress" loads based on first-year ice thicknesses in front of the structure could be generated under storm conditions4 On the other hand, when ice interaction scenarios extended to extreme ice features colliding with large diameter production structures, whether these features had enough momentum to fully envelop the structure and generate the full "limit-stress" loads became an important design check. Furthermore, during the winter, with the surrounding pack ice acting on the large floe, the 'driving-forces' from the pack ice became a potential limiting condition for design. The limit to ice forces based on limiting driving forces was discussed by Vivitrat and Kreider5 and by the author in recent papers. l,2.