Risk-based corrosion allowance of oil tankers
Introduction
Corrosion can cause structural degradation of metal structures, and recently, many studies focused on this issue. A profound analysis of the factors influencing the corrosion wastage of metal structures was presented in (Melchers, 2003), for morphology in (Montero-Ocampo and Veleva, 2002), plate surface conditions and material properties in (Garbatov et al., 2014), maintenance in (Garbatov et al., 2016a, 2018a) and reliability assessment in (Garbatov and Guedes Soares, 2009b).
A very important study of the environmental factors governing marine corrosion led to the identification of the governing corrosion factors for corrosion of ballast and cargo oil tanks was reported in (Guedes Soares et al., 2008, 2013). Many real measurements of corrosion thickness have been analysed, and several corrosion models were developed in (Hart et al., 1986; TSCF, 1992, 1997; Guedes Soares and Garbatov, 1998; Paik et al., 1998; Yamamoto and Ikagaki, 1998; Guedes Soares and Garbatov, 1999; Wang et al., 2003; Jurišić et al., 2017).
Nowadays special attention is paid to the fact that corrosion is not only reducing the thickness of structural components but also the mechanical properties are changed as observed in (Garbatov et al., 2014, 2016a, 2018a) leading to a severe strength reduction (Saad-Eldeen et al., 2011a, b; Garbatov et al., 2016b; Woloszyk and Garbatov, 2020).
Zayed et al. (2013a, 2013b) analysed the reliability of ship hulls subjected to corrosion and maintenance, considering the uncertainties related to the loading conditions and inspection events.
A risk-based framework for ship and structural design accounting for maintenance planning was developed by Garbatov et al. (2018b) determining the maintenance plan of a ship hull structural system which can be used in the early stage of design, accounting for different hazard scenarios, specific economic environment and degradation severity along with the service life.
Structural reliability assessment of a real tanker ship was performed based on experimentally estimated ultimate strength by Woloszyk and Garbatov (2019), where the strength estimate of the scaled specimen translation to a real scale tanker ship hull structure was made using the dimensional theory.
Neumann et al. (2019) performed conditional failure assessment to describe the required increasing maintenance with age, where a rule-based failure definition is applied trough the corrosion tolerance levels together with a linear corrosion degradation model.
A very recent study presented by Gong et al. (2020) investigated the economically optimal corrosion addition from a life-cycle perspective by developing a probabilistic growth model consistent with corrosion additions as stipulated by IACS concluding that in some cases they may be less than that the defined ones initially.
The International Maritime Organization, IMO developed Formal Safety Assessment, FSA (IMO, 2005, 2006a; b, 2007, 2008, 2013, 2015) to enhance the maritime safety and was employed in the development of new rules (Psaraftis, 2012; Montewka et al., 2014), designing of ships in the degradation condition (Papanikolaou et al., 2009), performing a sensitivity analysis on the hull girder safety level of a tanker ship (Guia et al., 2016) and for development of a risk-based framework for ship and structural design accounting for maintenance planning (Garbatov et al., 2018b).
FSA considers the limit state concerning the corrosion deterioration mechanism as an ultimate limit state, and its violation may lead to loss of ship, cargo etc. The corrosion degradation mechanism is a highly uncertain process accompanied with a large scatter leading to a substantial statistical deviation as a function of time. As a result of that, when the average corrosion thickness arrives at the level of the corrosion allowance as stipulated by IACS on some places, the plate thickness reduction can be much severe.
The main objective in the present study is to define the corrosion allowance of redundant ship structures derived based on the probabilistic closed-form solution and risk-based direct approach, which are only dependent on ship corrosion environments or spaces according to their physical, chemical and operational characteristics. The concern related to the structural capacity (e.g. bending and yielding) is covered by the net thickness plate design of ship structures, which is not an objective of this study. The present study analyses the structural corrosion degradation of six sets of recently collected corrosion thickness measurements. The data sets were collected during various inspection campaigns of tankers in operation and reflected the ages of the ships and their level of degradation. The developed closed-form solution for the corrosion thickness allowances can be directly used in the ship structural design, avoiding the complexity of the risk analysis. Collecting more data related to the corrosion degradation and information about the economic conditions during the ship life-cycle can be easily incorporated, and the output of the analysis can be updated.
Section snippets
Time-dependent degradation
The time-dependent structural degradation is an important issue that needs to be accounted for when metal structures are designed. Its behaviour needs to be identified with respect to how quickly the structure will degrade when the structural deterioration reaches the non-acceptable level.
In general, three important approaches are employed in corrosion deterioration modelling. The first one considers the corrosion growth with a linear dependence of the time, and this is a very crude model. The
Probabilistic corrosion thickness estimate
The time-dependent corrosion degradation of structural components may reduce the strength of the ship hull structure and can initiate buckling, fatigue damage, rupture and transformation of the structure into a mechanism. In this respect, the deterioration mechanism needs to be considered in the ultimate limit state analysis.
The probabilistic design methods are based on the comparison of the reliability beta index with its target value, . Moreover, it is applied to design so that the
Design corrosion thickness allowance of tanker ships
IACS (2012) defined the corrosion wastage allowances to be added to the structural component net thickness in the different ship corrosion environment spaces. These wastage allowances are defined based on the previous corrosion degradation history of the structural component under certain environmental conditions. The defined corrosion wastage allowances reflect the experience and resources of all the classification societies to achieve a 25-year design life, and they are applied to the design
Conclusion
This study presented here two approaches, one fully probabilistic (closed-form solution) and the second one is a risk-based (direct) one to define the corrosion thickness allowance of different ship corrosion environment spaces accounting for the two-side corrosion degradation. Both approaches are based on calibrating a non-linear time-variant corrosion model to the newly collected data and reliability analysis employing the log-normal formulation. The structural corrosion degradation is
CRediT authorship contribution statement
Yordan Garbatov: Writing - original draft.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was performed within the scope of the Strategic Research Plan of the Centre for Marine Technology and Ocean Engineering (CENTEC), which is financed by the Portuguese Foundation for Science and Technology (Fundação para a Ciência e Tecnologia - FCT) under contract UIDB/UIDP/00134/2020.
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