Study On Fatigue Test Scheme And Life Prediction Method Of Welded Joints In Marine Pipelines

Marine pipeline is the aorta of sea oil and gas resources transportation,thus its strength and safety directly affect the normal operation of marine oil and gas production system.Once the fatigue fracture occurs,it will cause significant economic losses and environmental pollution problems.Welded joints are the parts which are easy to experience fatigue failure of the marine pipe,and its fatigue life become the key factor in the overall life expectancy.According to the fatigue failure characteristics of domestic X65 pipeline steel,we design two fatigue test programs,and derive the theoretical calculation expression.In this thesis,a full-scale and a small-scale marine pipe welded joint theoretical calculation methods are proposed,then the methods are proved applicability,which improve the accuracy of ocean pipeline life prediction results.The above issues were carried out in the following aspects:(1)A full-scale and a small-scale fatigue test scheme of X65 steel marine pipe is developed,and the feasibility of the test scheme is verified by actual test operation,which is aimed at the overall characteristics of the pipeline and the fatigue failure of the local welded joint.A total of 10 different stress amplitude of the full-scale and 24 small-scale specimens fatigue tests are conducted,and the fatigue tests life values are similar to the actual working conditions fatigue life.(2)As the traditional fatigue life calculation model is assumed to be distorted and the result is not accurate enough,the theoretical calculation method of full-scale marine pipeline fatigue life is proposed.The fatigue failure law and life calculation method of the welded joints of marine pipelines were studied by fracture mechanics method,and the theoretical calculation expressions were compared with the experimental life data.The full-scale fatigue life of the welded joints of the X65 steel with the specifications of ?323.9mm × 11.7mm is calculated accurately by using the fitting expression,helping the evaluation personnel to develop a cycle of the evaluation period when they can not obtain the test fatigue life and lack of theoretical basis.(3)Considering the lack of fatigue life calculation of small-scale specimens,a three-parameter Weibull distribution model is proposed to predict the fatigue life of small-scale fatigue.The distribution function parameters are estimated by linear regression method,and the calculation method of small-scale fatigue life is obtained.The fatigue life of the small-scale of the marine pipe with the size of ?323.9mm × 11.7mm is calculated and the reliability of the method is verified,which can provide judgments basis for the pipeline quality inspection and evaluation personnel.