Effect Of Pitting Damage On Mechanical Properties Of Offshore Platform Steel

The jacket platform has been in the Marine environment for a long time.The main material used to construct the jacket platform is low carbon steel,which is prone to pitting corrosion in seawater containing chloride ions.The weakening of mechanical properties of steel by pitting will reduce the bearing capacity of pitting pipe or plate,and indirectly affect the bearing capacity of jacket platform.In engineering,the cross-section reduction method is usually used to reduce the thickness of the plate or the wall thickness of the pipe to simulate the corrosion situation,but the mechanical properties of the plate or pipe treated by the cross-section reduction method are usually different from the mechanical properties of the real corroded plate with the same degree of corrosion,which will directly affect the accuracy of structural analysis.Based on the above discussion,the following aspects of research work are carried out in this paper:A modified constitutive model is proposed.The mechanical parameters obtained from the stress-strain relationship curves of nonlinear materials include ultimate strength,yield strength,young's modulus or elongation.At present,most of the work focuses on the study of the change law of single or several mechanical parameters with the corrosion degree,but ignores the change of stress and strain curve with the corrosion degree.At the same time,in the actual engineering analysis,the stress-strain relationship obtained by the ideal elastic-plastic isoplastic model based on the mechanical parameters is far from the actual stress-strain relationship.So based on the single point corrosion specimen uniaxial tensile test data as the foundation,analyzes the ultimate strength,yield strength and young's modulus or elongation and other mechanical parameters changing with the corrosion mass loss,and put forward a little bit of corrosion on the basis of the modified constitutive model,is used to calculate the degree of corrosion of stress-strain relationship curve of the material.The accuracy of the model is verified.The uniaxial tensile test of a single pitting specimen was simulated by finite element method,and the finite element simulation results were compared with the experimental results to verify the accuracy of the pitting correction constitutive model.In addition,the accuracy of extending the pitting correction constitutive model to simulate multi-point corrosion is verified by uniaxial tensile test of simulated multipoint corrosion specimen.The pitting correction constitutive model is applied to the safety reserve analysis of jacket platform.The zonal corrosion model is introduced and the pitting correction constitutive model and section reduction method are used to simulate the corrosion of jacket platform.By the wave force and earthquake force calculated two lateral load distribution,the two lateral load on jacket platforms respectively,the static elastoplastic analysis,through calculating the reserve strength ratio RSR,analysis,the constitutive model for pitting and cross section reduction of two kinds of method to simulate the corrosion influence on the bearing capacity of the jacket platform.The effect of corrosion on mechanical properties of welded specimens was studied.The components of Marine structures are usually connected by welds.In this paper,the electrochemical corrosion of welding specimen was studied.The uniaxial tensile test was carried out on the specimens after corrosion,and the variation law of mechanical parameters such as ultimate strength,yield strength,young's modulus or elongation with corrosion and welding was analyzed.The results show that corrosion can lead to the degradation of mechanical properties of steel.Compared with the traditional section reduction method,the pitting correction constitutive model proposed in this paper based on the uniaxial tensile test results of single pitting specimen can simulate the corrosion situation more accurately.