Ice Loads On Offshore Structures: A Software Development
Authors: Derradji-Aouat Ahmed 
Published: October 2008

A finite element computer program for the calculations of ice loads on offshore structures is developed. A brief description of the program is given and the solution techniques used to model general ice-structure interaction problems are discussed. The program is used to calculate maximum ice loads on various structures (vertical and inclined), and the calculations are compared to some laboratory and field measurements. Conclusions and recommendations for future work are suggested.

INTRODUCTION

During ice-structure interactions, the relative motion between the ice and the structure results in the development of forces known as "ice loads". Fixed offshore structures constructed in an ice infested environment must be strong enough to withstand the forces generated by ice. The last two-three decades have seen a considerable amount of ice related research work. The efforts have been directed towards understanding the mechanisms by which ice loads are developed, and subsequently formulate predictive methods for ice loads.

Many empirical and analytical methods have been proposed for the calculation of ice loads on offshore structures. However, their application to actual ice-structure interaction problems requires considerable engineering judgements. First, empirical methods are valid only for the conditions and the geometries of the experiments. Extrapolation of empirical equations to compute ice loads on actual structures is underlined by a number of uncertainties [1]. Second, most existing analytical methods are developed for uniaxial problems (such as beam solutions). Field observations of ice-structure interactions indicate that ice fails in a complex state of stress and the use of uniaxial analytical methods results in unrealistic ice load estimations [2].

Proper application of analytical methods to calculate ice loads on actual structures is very complicated [3]. The complexity results from the high variability of ice conditions in the field. In addition, the intensity of ice loads depends to a large extent on the mechanical behaviour of ice and its mode of failure, geometry of the structure, and the relative motion between ice and the structure. When all these factors are taken into account, analytical solutions become very complicated.

The complexities of ice-structure interaction problems dictate that the method of calculating ice loads on offshore structures should be numerical (finite element, discrete element, ...etc.). The universality and versatility of these methods allow for comprehensive analyses of ice-structure interaction problems, and subsequently realistic estimations of ice loads. However, numerical methods require constitutive models for the description of the mechanical behaviour of the materials involved in the interaction problem. Therefore, in order to carry out a reliable analysis, and subsequently a safe design of the structure, realistic mathematical models for the behaviour of ice, structural material, and soils of the foundation should be used. These models can be implemented into finite element computer programs (or discrete element programs), then used to analyze and design the structure against ice loads.