Ultimate Strength Of Ship Structures With Corrosion Wastage

Corrosion is one of the main reasons which cause structure failure and damage. Much attention has been given to estimate the ultimate strength of ship structures with corrosion wastage, since 1970s.Recently, the focus on the reliability analysis of ship structures considering marine corrosion has been directed from general corrosion to localized corrosion, especially, the pitting corrosion. Unfortunately, due to the complexity of pitting mechanism and the scarcity of corrosion data, there still have no perfect time-dependent model which can precisely predict the corrosion wastage characteristics of ship structures. In addition, it is essential to analyze the effects of the pits shape on ultimate strength of structures and establish an analysis procedure of pitted structures.This thesis aims at the ultimate strength assessment of corroded structures, mainly consists of the following aspects:(1) A literature review of the related research works, including the corrosion of mild and low-alloy structural steels under marine immersion conditions, the corrosion detection and measurements of ship structures, corrosion models, the ultimate strength analysis of ship structures considering corrosion wastage, is summarized.(2) For ship structures under marine environments, the main influencing factors and the specific process of corrosion are summarized. After that, the theoretical basis and technical background of the related criterion in CSR (Common Structural Rules) is studied here carefully. With this understanding, a three-level corrosion database of ship structure with the initial plan of its definition, performance features, and elements are proposed. These works will be helpful to the improvement of relevant norms and the establishment of new ship corrosion database.(3) The validity of several uniform corrosion models and their application field are discussed here. Then the influences of corrosion models on the corrosion addition are analyzed for different ship lives. The common approach to determine the corrosion addition of ship structures is displayed here by a practical example.(4) Based on the corresponding pitting mechanics and Melchers'pit corrosion model, a simplified maximum pit depth model is proposed, which will be more concisely to predict the statistical characteristics of maximum macro-pit depth. Then the new model is applied to the analysis of various corrosion gauge data from some long term field investigations in China. Comparing with other pitting corrosion models, it is more reasonable to describe the maximum pit-depth curve of mild and low-alloy steels in marine immersion conditions as a Weibull function of exposure periods. Furthermore, the potential influences of some seawater environmental factors on model parameters are discussed herein, including seawater temperature, dissolved oxygen, salinity, PH, etc. Since the variations in the composition contents of mild and low-alloy steels will also make a full impact on its maximum pit depth, comparative analyses of the effect of steel composition contents on pit depth and the several model parameters are also carried out.(5) According to the pitting corrosion mechanism and corrosion data from ship structure investigated, the shape of corrosion pits and its formation on ship structures have been surveyed and summarized in this thesis. It was shown that the main pits shape may be cone or compound-cone. Obviously, the shape of corrosion pits on ship structures was affected by its locations and ship age. The shape difference between those two pits type was resulted from the divergence in their corrosion-control factors and their influencing degree. It will be reasonable to describe the ratio of diameter to depth of corrosion pits as a function of ship age. The time dependent model of pits ratio is also established in this thesis, based on the model of corrosion pits depth and the corrosion data of hold frames in cargo holds from the literature.(6) Combining the maximum pit depth model, the model of pits shape, as well as the statistical characteristics of corrosion depth and the distribution of pits location, the concept of multi geometric parameters pitting model is proposed in this thesis. Based on this integrated model, some advices to the future collection of pitting measurement data are given.(7) The ultimate compressive strength characteristics of the simply supported plates with pitting wastage are investigated in the present thesis. A series of non-linear finite element analyses for steel pitted plates under axial compressive loads are carried out, varying the plate geometric properties, the number of corrosion pits and the shape of pits. It is proved that the ultimate compressive strength of pitted plates is mainly influenced by its volume-loss rate and the slenderness ratio. The corresponding design formulas for pitted plates are derived here by regression analysis of numerical results obtained from non-linear FEM analyses.(8) The ultimate tensile strength characteristics of the simply supported plates with pitting wastage are investigated in this thesis, also. A series of non-linear finite element analyses for steel pitted plates under tensile loads are carried out, varying the plate geometric properties, the number of corrosion pits and the shape of pits. It is pointed out that the ultimate tensile strength of pitted plates is mainly influenced by its smallest cross-sectional area. The design formulas for pitted plates are derived by nonlinear least-squares regression analysis of numerical results in the present study.The main innovations of this thesis include the following aspects:(1) A time-dependent maximum pit depth model of mild and low alloy steels in marine immersion environments is proposed.(2) A time dependent model of the ratio of pit diameter to pit depth is established, based on the pit data of hold frames in cargo holds from literatures. Further more, the concept of multi geometric parameters pitting model is proposed.(3) The effects of the pits shape on ultimate compression/tensile strength of simply supported plates are rationally analyzed here. The ultimate strength formulas for pitted plates are derived by regression analysis individually, which are need for the ultimate limit state-based reliability assessment of corroded structures.