Ice-structure Interactionsome Recent Veritas Approaches

Abstract

Introduction The major areas of uncertainty relating to ice loading on structures are identified. These include the arrival rate, the mass and velocity (for discrete features) and the movement rate (for landfast ice). In addition, the morphology and thickness of the ice feature and the ice properties are variable and have a strong influence on assessments of risk. Probabilistic models are proposed and an integrated risk analysis approach is advocated. Working models for ice-structure interaction are described and some future directions of research are indicated. Introduction Challenges unique to the Arctic and neighbouring environment must be met, and one of the most important of these is the assessment of the loading imposed by ice on structures. Large amounts of data have been collected in other areas concerning loads imposed by waves on structures. Sophisticated probabilistic models have also been developed. However, in the Arctic, technology of ice-structure interaction has not yet been brought along to the same degree. Further development is necessary, particularly in assessing levels of risk. One of the chief aims of Veritas research in the area of ice-structure interaction is to develop rules for structures in ice, with one of the main objectives being safety. It is essential to consider, in so far as possible, the levels of risk associated with various forces. The underlying ‘’aim of research is risk assessment which in turn guides rule formulation. Risk analysis involves coming to terms with two aspects:Sufficient description of the environmental conditions to provide input into the ice-structure interaction computation, andComputation of ice-structure interaction using principles of mechanics. The input under (a) is of necessity probabilistic in nature; probabilities of various masses and velocities of icebergs are important in deducing levels of risk associated with impact. The constitutive equations for ice are not precisely known in (b) and most existing methods use a semi-empirical approach to account for such aspects. Heterogeneity and randomness of ice features is a subject for future development, and work in this direction is underway in the Calgary Veritas offices. Risk Analysis In a nutshell, risk analysis must deal with undesirable events such as structural failure, accidents, collisions and blowouts. The chance element is dealt with by estimating probabilities, whereas the magnitude of the undesirable event is measured in terms of consequences: deaths, injuries and volume of pollution, The analysis can be illustrated by means of decision trees; two examples are given in Figures I and 2, from reference (1)*, In the trees, possible decisions are indicated thus (ð), and chance events thus (o). Environmental Aspects The methods used in assessing the probabilities of the events involving ice-structure interaction are reviewed. These are;Rates of movement of land-fast ice which transmit forces to structures such as gravel islands andthe impact of a moving feature, such as a multi-year ice floe or iceberg. It is necessary to distinguish the two cases because the statistics required for risk analysis are quite different.