Influence of Structural Geometry on the Ice Pressure Distribution on Vertical Structures
Authors: Sand B. , Fransson L. 
Published: December 2011

The interaction of ice sheets with rigid vertical structures is an important problem in the design of arctic offshore structures. Offshore structures often experience an enormous ice load since the ice sheet breaks by crushing into the structure. The finite element method is adopted to calculate ice forces on vertical structures of various shapes. The effect of material nonlinearities and friction between the ice and structure is taken into account. The ice is treated as a transversely isotropic, nonlinear material and formation of crushing or cracking is treated as a transformation of state. The contact interaction between the ice sheet and the structure is simulated with a contact formulation based on finite sliding interaction between a deformable body and a rigid body with Coulomb friction sliding. To verify the applicability of the proposed constitutive models, the numerical results obtained during the present study are compared with data from field measurements.

Introduction

Design of offshore structures in Arctic waters is strongly dependent on local and global ice loads. These loadings are, in general, contact forces transmitted to the structures during interaction with ice floes, ice ridges or icebergs. The prediction of ice forces on structures relies heavily on a thorough understanding of ice mechanics as well as on in-depth knowledge of interaction between ice features and structures. The complexities of the constitutive behavior of ice and the variations in ice failure modes are major obstacles in the development of efficient and reliable methods for the estimation of ice loads on fixed or compliant structures. A great number of geometric and material parameters will affect the magnitude of ice sheet loads on structures; moreover, the displacement boundary conditions as well as the application of the external loading also influence the results. A special intricate problem is the pile up of rubble. To consider all these effects is beyond the scope of this study, which restricts attention to the effects of various geometric shapes of vertical structures and the friction between the ice sheet and the surface of the structure. The use of computerized spatial discretization techniques, such as the finite element method, in simulation of ice-structure interaction phenomena may relieve some of problems inherent in the existing empirical procedures. In particular, the finite element method can effectively be utilized to study the effects of different geometry and boundary conditions, varying velocity, shape and mass of ice features, and so forth. On the other hand, the practical value of results from numerical simulations depends on several factors of which the accuracy of the constitutive model the most important. Thus, a close collaboration between scientists involved in experimental ice research and specialists in numerical analysis is a prerequisite for the successful application of nonlinear finite element analysis to ice-structure interaction problems. In this study, the finite element method is adopted to calculate ice forces on vertical structures. The effects of material nonlinearities and friction between ice sheet and structure are taken into account. The ice is treated as a transversely isotropic, nonlinear material capable of cracking and crushing and the contact interaction between the ice sheet and the structure is simulated with a contact algorithm that permits surface-to-surface contact with Coulomb friction sliding. The numerical results presented in the present study were obtained by use of the general finite element code ANSYS.