A Modal for the Prediction of Ice-Structure Internation

Abstract

In an effort to predict ice-structure interaction, a simple mechanism is proposed. The structure is represented by a spring-mass system and the ice is replaced by a succession of elastic-brittle elements which impinge on the structure at a rate determined by the relative motion between the ice and the structure. A computer program is used to solve for the dynamic response of the structure. A number of test cases and variations have been solved, and the results compared with limited laboratory and field measurements that are available. Interesting agreement has been obtained with observed behavior at various ice velocities. It is believed that the present approach can be used to determine reasonably well the response of a structure to an impinging ice sheet. Introduction Since the beginning of the exploitation of the oil reserves in Cook Inlet, significant efforts have been devoted to the study of the loads imposed on offshore structures by ice floes. The most comprehensive of the studies are those of Peyton, who has examined the wide variation of sea ice properties and has conducted both laboratory and field investigations of the interaction of ice with structures (Refs 1 thru 4). His reports include a limited number of field measurements made on drilling structures erected in Cook Inlet where extreme tidal variations and velocities are found. Of primary interest in the present study are the force-time recordings in Ref 1. These records indicate that the measured response of the structure is greatly dependent on the velocity of the ice floe. Fig 1 shows a simplified representation given by Peyton (Ref 3). At relatively high velocities the structure exhibits small-amplitude, high frequency vibrations about a constant mean value. At low velocities the amplitude undergoes severe, low-frequency variations between zero and a maximum level that is approximately double the high-velocity mean. The term "racheting" has been applied to this type of oscillation. It is concluded by Peyton that the structure receives the most severe loading and acceleration when the floe is virtually stopped. Added impetus has been given to the study of ice-structure interaction by a proposed crossing of Turnagain Arm. an extension of Cook Inlet (Ref 5). To gather additional information for design the Alaska Department of Highways has developed preliminary plansfor an instrumented test pier to be erected near the intended location of the crossing. The present analytical study is an outgrowth of considerations related to the design and selection of instrumentation for the facility. The number of analytical studies of ice-structure interaction reported in the literature is quite limited. Typical of these is the work by Kivisild (Ref 6), which appears to be based on elastic plate and membrane theory. These theoretical treatments, however, are not sufficiently general to permit predictions of the forces, velocities, and accelerations necessary for the design and cushioning of the instrumentation for the proposed test structure. In an attempt to improve the understanding of the structural response, a simple mechanical analog has been developed and an associated computer program has been used to study a variety of cases.

Modal Analysis of the Ice-Structure Interaction Problem

“…Matlock, Dawkins & Panak [1969, 1971 One of the first mechanical models developed to analyze ice-structure interaction was presented by Panak (1969, 1971). The study was originated in response to a planned Turnagain Arm crossing/bridge.…”

“…(Matlock, et al 1969(Matlock, et al , 1971 Figure 4. Sketch of ice accumulation on pier at impact height adapted from Matlock, et al (1969Matlock, et al ( , 1971 Tables Table 1. Matlock, et al Model Parameters (Matlock, et al 1969, 1971 Chapter 1-Introduction and Literary Survey…”

“…The bars/teeth are represented as linearly elastic-brittle beams, which experience instantaneous brittle fracture at a characteristic deformation. (Matlock, et al 1969(Matlock, et al , 1971 The model provides a simple idealization of the fracturing of ice by crushing up against an offshore structure. The ice force acting on the structure is assumed to vary linearly and elastically with the deformation of the ice until the moment of ice tooth brittle fracture.…”

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AbstractIn the present study, the author builds upon the single degree of freedom ice-structure interaction model initially proposed by Matlock, et al (1969Matlock, et al ( , 1971. The model created by Matlock, et al (1969Matlock, et al ( , 1971, assumed that the primary response of the structure would be in its fundamental mode of vibration.

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“…The model created by Matlock, et al (1969Matlock, et al ( , 1971, assumed that the primary response of the structure would be in its fundamental mode of vibration. In order to glean a greater physical understanding of ice-structure interaction phenomena, it was critical that this study set out to develop a multi-mode forced response for the pier when a moving ice floe makes contact at a specific vertical pier location.…”