Ultimate Strength Research On Ship Hull Structures With Corrosion Wastage

Ship hulls are inevitable to suffer corrosion damage during their lifetime in service.Corrosion damage is one of the main factors leading to the failure of aged ship structures and will decrease the ultimate bearing capacity of hull structures.As a kind of common non-uniform corrosion,the causes and propagations of pitting corrosion are random,irregular and complex.The reduction of plate thickness caused by corrosion has a severer negative effect on the ultimate strength of ship structures made by high-strength steel.Hence,scholars in many countries have paid high attention to this matter.What's more,Classification society around the world,the international associations of classification society and other organizations have also made the selection rules of the corrosion allowance to ensure the safety of hull structures.The influence factors of corroded ship structures ultimate strength include the dimensions of structures and corrosion pits.They are the slenderness of stiffened plates,distribution of corrosion pits,area of corrosion,depth of corrosion and volume of corrosion.Scholars still have many obscures and controversies on the specific influence and effect degree of these factors.Further study is needed.The main content of the present paper is to analyze the ultimate strength of corroded ship structures using nonlinear finite element method.It can be divided into three parts as follows.The ultimate strength of pitting corroded plates simply supported on four edges with initial deflection under longitudinal compression is studied.The shape of pitting is cylinder.Pitting corrosions of ship plates are divided into local massive pitting and overall pitting.The effects of mesh size,plate slenderness,location of corrosions,area of corrosions,depth of corrosion and volume of corrosion on the ultimate strength of ship plates with local massive pitting and overall pitting are investigated.An empirical formula about the ultimate strength reduction of plates with local massive pitting related to plate slenderness,area ratio of corrosions and area ratio of corrosions is obtained after analysis.An empirical formula about the ultimate strength reduction of plates with overall pitting related to plate slenderness andvolume ratio of corrosions is also obtained.The latter is applied to the ultimate strength reduction calculation of plates with local massive pitting.The results are found to basically match with the former one.The ultimate strength of pitting corroded stiffened plates with initial deflection under longitudinal compression is simultaneously studied.The effects of mesh size,plate slenderness,stiffener slenderness,type of pitting,distribution of pitting,area of corrosions,depth of corrosion and location of corrosions on the ultimate strength of pitting corroded stiffened plates are investigated.It's found that the ultimate strength reduction of pitting corroded stiffened plates has close relation with these parameters.An empirical formula about the ultimate strength reduction of stiffened plates with pitting on the opposite side of the stiffener on the plates is derived.In addition,the effects of corrosion on the longitudinal ultimate strength of ship hull girders are studied.The sagging and hogging ultimate strengths of a new bulk cargo ship are analyzed and made comparison validation with other researcher's results.The longitudinal ultimate strength of ship hull girders based on CSRB selection rules of the corrosion allowance and its uniform pitting volume is calculated.It's obtained from comparison that the ultimate strength of ship hull girders with pitting corrosion is smaller than that with uniform corrosion.The relationships between the effects of corrosion on the sagging and hogging ultimate strengths of ship hull girders and the minimum cross section area of ship hull girders as well as the minimum section modulus of deck and bottom plates are also analyzed.