Y. Garbatov

Reliability of maintained, corrosion protected plates subjected to non-linear corrosion and compressive loads

A model is proposed for the non-linear general corrosion wastage in a plate, which is able to describe an initial period without corrosion due to the presence of a corrosion protection system, and an exponential increase in wastage up to a steady-state value. The influence of adopting this model instead of a linear one is demonstrated by studying the reliability of a corrosion-protected plate subjected to compressive loads, and maintenance actions. The paper examines how the plate collapse strength will vary in time as a result of generalised corrosion and plate repair. In the presence of general corrosion, the plate thickness will decrease at a random rate as a function of time and will affect the net plate section resisting the applied load. The collapse strength against compressive loading is described by a function of time and the resulting reliability is quantified. The sensitivity of the reliability estimates with respect to several parameters is also studied. The reliability is predicted by a time-variant formulation and the effects of repair actions in the reliability assessment are shown.

Cost and reliability based strategies for fatigue maintenance planning of floating structures

Abstract Floating structures are designed in such way that the appearance of fatigue failures cannot be avoided, implying the need for inspections during their life. Their maintenance has to be planned from an economic point of view so as to minimize maintenance costs but satisfying a minimum reliability level. A method is proposed to quantify the repair costs resulting of different reliability-based maintenance strategies. As an application of this approach a side shell structure typical of a floating production unit is analysed and the influence of different parameter with respect to the repair cost is also studied here.

Fatigue reliability of the ship hull girder accounting for inspection and repair

A formulation is presented for the assessment of the reliability of a ship hull with respect to fatigue failure of the longitudinal members. The model allows for the existence of multiple cracks both in the stiffeners and in the plating and it models the crack growth process. The effect of the cracks on the midship section modulus is modelled as a function of time and the resulting reliability is quantified. It is assumed that the inspection method is perfect and detects all cracks larger than a detection limit, which in turn is method dependent. Detected cracks are subjected to a perfect repair being restored to the initial crack size. The effect of hull maintenance is seen in the step changes of the reliability function. The sensitivity of the reliability estimates with respect to several parameters is also studied.

Nonlinear time dependent corrosion wastage of deck plates of ballast and cargo tanks of tankers

The corrosion wastage of deck plates of ballast and cargo tanks is analyzed based on a nonlinear corrosion model. This model is able to describe an initial period without corrosion due to the presence of a corrosion protection system, a transition period with a nonlinear increase of wastage up to a steady state of long-term corrosion wastage. This model is applied to corrosion wastage data of deck plates collected by the American Bureau of Shipping. The objective of this work is to fit this corrosion wastage model to the service measured data, determining the values of the model parameters that represent the best fit to the data so as to describe how corrosion wastage varies in time as a result of generalized corrosion.

Reliability of maintained ship hulls subjected to corrosion and fatigue under combined loading

Abstract A formulation is presented for the assessment of the reliability of longitudinal members of a ship hull as a result of fatigue and corrosion under combined loading. The model allows for the existence of multiple cracks and it accounts for the crack growth process. The time dependent degrading effect of corrosion and crack growth on the midship section modulus are modelled as random processes. The reliability is predicted by a time variant formulation and, using a two-failure criterion, the effects of maintenance actions in increasing the reliability are shown. The sensitivity of the reliability estimates with respect to different loading combinations is also studied. The formulation has been applied to a tanker.

Reliability of maintained ship hull girders subjected to corrosion and fatigue

A formulation is presented to account for the effect of corrosion and fatigue on the reliability of ship hulls. A time variant formulation is presented in which the effect of these degradation phenomena on the hull section modulus is quantified. The effect of maintenance actions is accounted for by considering that the repaired elements are restored to a state as new. Different repair policies can be studied and the approach can be used as a tool to plan the maintenance actions based on reliability results.

Fatigue reliability of the ship hull girder

A formulation is presented for the assessment of the reliability of a ship hull with respect to fatigue failure of the longitudinal members. The model allows for the existence of multiple cracks both in the stiffeners and in the plating and it models the crack growth process. The effect of the cracks in decreasing the midship section modulus is quantified as a function of time and the resulting reliability is quantified. The sensitivity of the reliability estimates with respect to several parameters is also studied.

Effect of environmental factors on steel plate corrosion under marine immersion conditions

Abstract This study considers different environmental factors affecting corrosion under marine immersion conditions and proposes a mathematical model incorporating their effects on corrosion degradation through the ship life. The study adopts a nonlinear function of time as the reference corrosion model calibrated with operational corrosion data at long term average conditions to represent the corrosion behaviour under standard environmental conditions. Based on previously obtained field data, a corrosion deterioration model is developed, in which a correction factor is proposed to account for the effect of each environmental factor. These multiplicative factors correct the standard prediction of corrosion degradation to obtain short term estimates under any specific environmental condition. Then, a long term estimate of corrosion degradation is determined by summing the short term corrosion contributions during the various discrete time periods in which the ship life can be decomposed. A numerical example of a representative application of the new empirical based corrosion model is presented, to demonstrate how to apply the model.

Fatigue Reliability of Maintained Welded Joints in the Side Shell of Tankers

A formulation is presented for the assessment of the reliability of a local ship hull structure regarding fatigue failure. The potential cracks are considered to occur in the side shell, in the connections between longitudinal stiffeners and transverse web frame. The analysis is performed for T-shaped stiffeners. The model accounts for the crack growth process applying linear elastic fracture mechanics. The long-term stress range acting on the elements is defined as a function of the local transverse pressure of internal cargo and outside water, combined with the stresses resulting from the longitudinal bending of the hull. The global hull loading is a combination of horizontal and vertical bending moments. The present paper analyzes the importance of including the transverse pressure loading for the process of the crack growth and also the distribution of the net section area of midship and location of stiffeners. The formulation has been applied to a tanker. The fatigue reliability is predicted by a time variant formulation and the effects of maintenance actions in updating the reliability assessment are shown.

Reliability assessment of maintained ship hulls with correlated corroded elements

Abstract This paper reviews the factors that affect the corrosion rate of the ship hull girder. It is shown that corrosion rates vary as a function of the location of the element in the structure, of the ocean area and type of ballast tank and steel. However, the corrosion rate in elements that are subjected to similar conditions is highly correlated. A time-variant formulation is proposed including the correlation between the corrosion rates in neighbouring elements in modelling the degrading effect that corrosion has on the reliability of ship hulls. The effect of corrosion is represented as a time-dependent decrease of plate thickness that affects the midship section modulus. One repair policy was considered and the example results show the effect of plate replacement when its thickness reaches 75% of the original value. The results also illustrate how the limit value of the thickness in the repair criteria influences the reliability and the decision about repair actions. This is also shown for the effect of different corrosion rates, inspection intervals, and allowable stresses.