A Spectral Model for Forces Due to Ice Crushing

[+] Author and Article Information
Tuomo Kärnä

 Karna Research and Consulting, VTT Technical Research Centre of Finland, Lauttasaarentie 31, 00200 Helsinki, Finlandtuomo.karna@gmail.com

Yan Qu, Xiangjun Bi, Qianjin Yue

Dalian University of Technology

Walter Kuehnlein

Hamburg Ship Model Basin

J. Offshore Mech. Arct. Eng 129(2), 138-145 (Sep 14, 2006) (8 pages) doi:10.1115/1.2426997 History: Received March 20, 2006; Revised September 14, 2006

This paper presents a model of dynamic ice forces on vertical offshore structures. The model concerns a loading scenario where a competent ice sheet is drifting and crushing against the structure. Full scale data obtained on two offshore structures were used in the derivation of a method that applies both to narrow and wide structures. A large amount of events with directly measured local forces was used to derive formulas for spectral density functions of the local ice forces. A nondimensional formula that was derived for the autospectral density is independent of ice thickness. Coherence functions were used to define cross-spectral density functions of the local ice forces. The two kind of spectral density functions were used to obtain the spectral density of the total ice force. The method takes into account both the spatial and time correlation between the local forces. Accordingly, the model provides a tool to consider the nonsimultaneous characteristics of the local ice pressures while assessing the total ice force.

Copyright © 2007 by American Society of Mechanical Engineers
Topics: Force , Ice
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Figure 1

Illustration of observed phenomena during continuous ice crushing (19)

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Figure 2

A typical time signal of the local ice force due to continuous crushing of level ice: (a) and the corresponding autospectral density function (b)

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Figure 3

Norströmsgrund lighthouse

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Figure 4

The mooring pole MDP2 at the JZ9-3 oil field

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Figure 5

Ice load panels on the JZ9-3 mooring pole: (a) general location of the load panels; and (b) detailed arrangement of the 12 ice load panels

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Figure 6

Local forces acting on an offshore structure

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Figure 7

Intensity of the time-varying ice force due to ice crushing. Data from the Bohai Bay

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Figure 8

One-sided autospectral density functions (PSD) of local forces measured on all force panels.

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Figure 9

Mean of the nondimensional autospectral densities shown in Fig. 8

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Figure 10

Nondimensional autospectral density based on measurement and using the best fit expressed by Eq. 9

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Figure 11

Coherence as a function of distance and frequency

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Figure 12

Coherence function

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Figure 13

Application of the present theory for the determination of the global ice pressure as a function of the aspect ratio w∕h



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