Stress Combination for Fatigue Analysis of Ship Structures

[+] Author and Article Information
Yung S. Shin

 American Bureau of Shipping, Houston, TX 77060, USAyshin@edgle.org

Booki Kim

 American Bureau of Shipping, Houston, TX 77060, USAbkim@edgle.org

Alexander J. Fyfe

 PAFA Consulting Engineers, Hampton, Middlesex, TW12 1BN, UKajfyfe@pafa.co.uk

J. Offshore Mech. Arct. Eng 127(2), 175-181 (Dec 22, 2004) (7 pages) doi:10.1115/1.1924399 History: Received June 04, 2004; Revised December 22, 2004

A methodology for calculating the correlation factors to combine the long-term dynamic stress components of ship structure from various loads in seas is presented. The proposed methodology is valid for a stationary ergodic narrow-banded Gaussian process. The total combined stress in short-term sea states is expressed by linear summation of the component stresses with the corresponding combination factors. This expression is proven to be mathematically exact when applied to a single random sea. The long-term total stress is similarly expressed by linear summation of component stresses with appropriate combination factors. The stress components considered here are due to wave-induced vertical bending moment, wave-induced horizontal bending moment, external wave pressure, and internal tank pressure. For application, the stress combination factors are calculated for longitudinal stiffeners in midship cargo and ballast tanks of a crude oil tanker. It is found that the combination factors strongly depend on wave heading and period in the short-term sea states. It is also found that the combination factors are not sensitive to the selected probability of exceedance level of the stress in the long-term sense.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

Sketch of midship section with longitudinal locations for 298,300 DWT crude oil tanker

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Figure 2

Combination factor C3 over wave scatter entry

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Figure 3

Combination factor C3 over wave heading and period

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Figure 4

Combination factor C1 at probability levels 10−4 and 10−8

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Figure 5

Combination factor C1 with different weighting methods

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Figure 6

Sketch for demonstration of equivalence of the two expressions for combining transfer functions




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