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Research Papers: Structures and Safety Reliability

# Time-Variant Ultimate Reliability Analysis of Jacket Platforms Considering a New Probabilistic Corrosion Model for the Persian Gulf

[+] Author and Article Information
Hossein Gholami

Civil Engineering Faculty,
K.N. Toosi University of Technology,
No. 1346, Vali Asr Street, Mirdamad Intersection,
Tehran 1996715433, Iran
e-mail: Hosein.Gholami@Gmail.com

Behrouz Asgarian

Professor
Civil Engineering Faculty,
K.N. Toosi University of Technology,
No. 1346, Vali Asr Street, Mirdamad Intersection,
Tehran 1996715433, Iran
e-mail: Asgarian@kntu.ac.ir

Saeed Asil Gharebaghi

K.N. Toosi University of Technology,
Civil Engineering Faculty,
No. 1346, Vali Asr Street, Mirdamad Intersection,
Tehran 1996715433, Iran
e-mail: Asil@kntu.ac.ir

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received November 17, 2017; final manuscript received May 31, 2018; published online July 24, 2018. Assoc. Editor: Nianzhong Chen.

J. Offshore Mech. Arct. Eng 140(6), 061601 (Jul 24, 2018) (10 pages) Paper No: OMAE-17-1204; doi: 10.1115/1.4040505 History: Received November 17, 2017; Revised May 31, 2018

## Abstract

Corrosion is identified as one of the most important deterioration factors for structural integrity of offshore platforms. For reliability analysis of these platforms, a probabilistic model for prediction of long-term corrosion loss as a function of time is essential. The purpose of this study is to propose a novel model for steel corrosion of jacket platforms in the Persian Gulf region. Field measurements for members in seawater are collected and statistically analyzed to identify the probability function for corrosion loss at different times. A new probabilistic model with time-dependent parameters is suggested, based on the statistical analysis results. Application of above-mentioned model in the reliability analysis of jacket platforms is investigated by introducing a new reliability analysis framework. This framework is a general solution for probabilistic analysis of jacket platforms with several stochastic variables which can be used for the platforms with different configuration and loads. In this framework, direct analysis is performed in each stage of first-order reliability method (FORM) instead of using the response surface method which is a common approach to obtaining the required response. This framework is applied to three jackets and the annual probability of failure $(Pf)$ over the platforms service life is computed. Comparison of results revealed that among the years beyond the platform design life, the amount of annual $Pf$ is increased in parabolic function. Also, studying the results is illustrated that in the case of ignoring the corrosion loss as a stochastic variable, $Pf$ is estimated 7% lower than values obtained in actual condition.

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## Figures

Fig. 1

Variation of corrosion loss mean value in time

Fig. 2

Variation of variance in time

Fig. 3

Probabilistic diagram for 35 years of the platform lifetime at the 95% confidence interval

Fig. 4

The log-logistic PDF obtained from measurement data of different years of the platform lifetime

Fig. 5

Approximation of the shape parameter in the log-logistic model

Fig. 6

Approximation of the scale parameter in the log-logistic model

Fig. 7

Comparison of the scale parameter resulted from the probabilistic model and measurement data

Fig. 8

Comparison of the shape parameter resulted from the probabilistic model and measurement data

Fig. 9

Overview of reliability analysis

Fig. 10

Fig. 11

Verification of wave force generation

Fig. 12

Overview of capacity determination

Fig. 14

Variations of the probability of failure over time: (a) jacket 1, (b) jacket 2, and (c) jacket 3

Fig. 13

Illustration of finite element model of jacket 1, 2, and 3: (a) jacket 1, (b) jacket 2, and (c) jacket 3

## Errata

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