A novel stepwise method to predict ultimate strength reduction in offshore structures with pitting corrosion

Sajad Rahmdel,Kwanghoon Kim,Sangjin Kim,Seonghun Park

doi:10.1177/1687814015600677

Sajad Rahmdel, Kwanghoon Kim + Show 2 more

Open Access

https://doi.org/10.1177/1687814015600677

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Abstract

Corrosion is a factor that decreases the ultimate strength of a structure, which can lead to structural failure in an aged structure. This study proposes a reliable method to predict the ultimate strength reduction caused by pitting corrosion of a structure at various ages. Hence, the objectives of this study are as follows: (1) finding relationship between the ultimate strength reduction and pit depth and (2) finding relationship between the ultimate strength reduction and the age of structure. The presented method has six steps and was assessed by applying it to the simplified example of a thin-plate panel structure. The ultimate strengths of the panel with different pit depths were calculated using finite element analysis. The effects of material and geometric nonlinearities as well as the initial imperfections and buckling were also considered. A linear regression model, representing the relationship between the ultimate strength reduction and pit depth, was then developed. Furthermore, using the experimental data, the weighted mean value of ultimate strength reduction was acquired for different age groups of the panel. The introduced stepwise approach contemplates the nonlinearity of pitting corrosion with time by considering the experimental data. As corroborated by the successful application of the presented methodology, this method is more accurate, compared to the accepted practices available in the literature.

Highlights

For the offshore structures, such as offshore platforms, ships and land-based structural systems, the decrease in the structural strength and structural integrity engendered by corrosion is a paramount challenge that should be examined in the design phase of a structure
Several deterministic ultimate strength models were developed by the researchers to approximate the ultimate strength reduction (USR) values caused by pitting corrosion, but none of them shows an accurate prediction, because pitting corrosion varies nonlinearly with the age of a structure
The aim of the study was to investigate the effect of pitting corrosion depths and structure ages on the USR and it was successfully achieved by our newly proposed stepwise semi-empirical method to predict the ultimate strength of the offshore structures, to calculate the ultimate strength reduced by the effects of pitting corrosion and structure ages and to find semi-empirical relationships between the structural ages and the USR as well as between the pitting corrosion depths and the USR

Summary

Introduction

For the offshore structures, such as offshore platforms, ships and land-based structural systems (i.e. pipelines), the decrease in the structural strength and structural integrity engendered by corrosion is a paramount challenge that should be examined in the design phase of a structure. As the age of a structure increases, the loss of the structure’s thickness caused by corrosion will cause a decrease in the ultimate strength of the structure and potentially a loss of integrity through pit, whose loss is critical when protective coating systems, such as galvanizing or painting coatings, are ineffectual.[1]. Several probabilistic models have been proposed with the majority of them focused on predicting the amount of pitting corrosion for a structure with different ages.[3,4,5,6,7,8]

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