Time-Variant Reliability Assessment of FPSO Hull Girder With Long Cracks

[+] Author and Article Information
Efrén Ayala-Uraga

Department of Marine Technology,  Norwegian University of Science and Technology, N-7491 Trondheim, Norwayefren.a.uraga@ntnu.no

Torgeir Moan

Centre for Ships and Ocean Structures,  Norwegian University of Science and Technology, N-7491 Trondheim, Norwaytorgeir.moan@ntnu.no

J. Offshore Mech. Arct. Eng 129(2), 81-89 (Apr 07, 2006) (9 pages) doi:10.1115/1.2355513 History: Received April 28, 2005; Revised April 07, 2006

An efficient time-variant reliability formulation for the safety assessment of an aging floating production storage and offloading (FPSO) vessels with the presence of through-thickness cracks (i.e., long cracks), is presented in this paper. Often in ship structures, cracks are detected by means of close visual inspection when they have already propagated through the thickness. The propagation of long cracks in stiffened panels is therefore considered, as they may be present in critical details of the deck and/or bottom plating of the vessel. Although it has been found that stiffened panels are tolerant to fatigue cracking, the safety of such structural components with the presence of long cracks may be threatened when exposed to overload extreme conditions, i.e., brittle or ductile fracture may occur. The probability of brittle fracture of an aging hull structure, i.e., a stiffened panel at the bottom plating with the presence of long cracks is studied in this paper. The mean stress effect due to the continuously varying still-water loading as well as residual stresses is explicitly accounted for in the crack growth calculation procedure presented herein. An analytical model is established for determining the equivalent long-term stress range including the mean stress effect. The continuously varying still-water load effects due to the operational nature of FPSOs introduce additional uncertainties in the estimation of fatigue damage as well as in the likelihood of fracture failure mode. In the present case study it is found that the time-invariant approach is a good approximation when dealing with the time-variant reliability problem. One of the main conclusions drawn from this study is that the still-water mean stress has a significant effect on the failure probabilities of stiffened panels with long cracks under brittle fracture mode.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Case studies of long cracks in main plating of FPSOs

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Figure 2

Associated mean stress level definition based on Weibull distributed SWBM of FPSOs, e.g., in sagging

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Figure 3

Mean stress effect in fatigue

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Figure 4

Stress range reduction factor as a function of the mean stress. Comparison between model of Eqs. 10,11 and design recommendation and one actual measurement.

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Figure 5

Crack growth in a stiffened panel for different mean stress levels, according to Eqs. 10,11. Legend for long-term equivalent stress range approach used: σ range—all stress ranges are fully effective; σm=n—equivalent stress range based on mean stress n=0,5,10,50MPa, respectively; variable stress—equivalent stress range obtained with SW as defined in Table 3.

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Figure 6

Long crack propagation in a stiffened panel, considering residual stresses and mean stress effect. Legend explanation: s range—all stress ranges are fully effective; no residual stress—the residual stresses are disregarded; residual stress—fixed residual stress field as crack propagates.

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Figure 7

Sensitivity of the operational temperature on the failure probability as function of time. Time-variant approach used.



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