Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Time-variant reliability of floating structures subjected to corrosion deterioration in South China Sea
Xizhao Wang, Xuekang Gu, Jun Ding, Nan Zhao, Yuchao Chen ,Qibin Wang, Zilin Li, Zhengwei Zhang, Jun Yu
China Ship Scientific Research Center
Wuxi, Jiangsu province, China

deterioration; ultimate strength;

ABSTRACTTime-variant reliability attracts attention from industry when it comeswith corrosion and fatigue in offshore floating structures. With thelong-term serviceability of large floating structures, the general or pit
corrosion may occur in stiffened plates and cause degradation of
longitudinal or local strength. Therefore the reliability assessment for
those floating structures with corrosion deterioration is essential during
the long term serviceability. This paper presents an approach forcalculating time-variant reliability of floating structures under corrosion
deterioration. Firstly, the two-years in-situ corrosion continuousmeasurement in South China Sea is conducted. Based on statisticalanalysis and those corrosion models from preceding research papers, acorrosion prediction model is established and the annual corrosion rateis achieved. Secondly, ultimate strength under discrete stochastic
extreme load events for the large floating structures is chosen as timedependent limit state. The ultimate strength is calculated by non-linear
finite element methods. A time-dependent limit state function isestablished by the load and resistance. And the ultimate strength
degradation effect is considered in the limit state function. Thirdly, amethod for Time-variant reliability assessment subjected to corrosion
deterioration is established based on conditional probability theory. The
estimation of failure probability for a typical floating structure is
calculated by FORM. The time-variant reliability is calculated for thewhole serviceability of large floating structure. The results show thatsevere corrosion deterioration makes the ultimate strength degradation.
Therefore the instantaneous reliability and the time-variant reliability
decrease with time growth.

INTRODUCTION
The progressive strength degradation has a detrimental impact on longterm safety and reliability of floating marine structures(Abbas andShafiee, 2020). Some literatures point out that corrosion is identified as
one of main failure modes, which can cause loss of reliability.,(Guo,Wang, Ivanov and Perakis, 2008), especially for Very Large FloatingStructures (VLFS) which consists of several modules, located far awayfrom harbors. In this scenario, VLFS can not be maintained on scheduleand the corrosion degradation needs to be considered in the service
lifetime of VLFS.
In general, Corrosion is sorted into several categories, i.e. general,pitting, crevice and galvanic. The general and pitting corrosioncommonly occur in marine structures. The corrosion rate measurement
is used as a method to calculate corrosion depth when the corrosion isuniform. The corrosion mainly leads to thickness reduction of plates
and results in reduction of hull girder section modulus of the floatingstructures (Soares and Garbatov, 1999).
Guo, Wang, Ivanov and Perakis (2008) studied the time-varying
ultimate strength considering corrosion wastage effect and considered
three levels of corrosion severity to the cross-section module
degradation. Han, Yang and Frangopol (2019) applied cell automation
method to study Time-variant reliability analysis of steel platesconsidering pitting and general corrosion. Moan and Ayala-Uraga(2008) also studied the combination of fatigue crack growth andcorrosion and established the corresponding reliability assessment
model. Dong and Frangopol (2015) conducted some research on theKEY WORDS: Time-variant reliability; floating structures; corrosion

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