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Study On The Failure Mechanism And Reliability Evaluation Of Buried Pipelines

Posted on:2009-12-23Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y ShaoFull Text:PDF
GTID:1102360242985542Subject:Municipal engineering
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The buried pipelines are important lifelines of modern urban infrastructure. Most of them are exposed to different deleterious reactions; as a result, the failure occurs when the significantly degraded resistance of the structure can not bear the external loads. The problem of the deteriorating pipelines, resulting in leaks, breaks and insufficient supply potentials, affects municipalities worldwide. Therefore, the failure mechanism of buried pipelines should be understood, and the reliability evaluation methods should be developed to predict the failure behavior of pipelines. Then the repair or replacement plans can be arranged suitably to change the passive management to the proactive management. In this research, a systematic method, integrating theoretical analysis, numerical simulation, and verification by case studies is then developed for studying the pipe-soil interaction model, the deterioration behavior of corrosion and crack growth, the stress concentration factors and residual ultimate strength for pipes with local defect, and the methods for the probabilistic risk and fuzzy reliability analysis. The main research findings are summarized as follows:(1)Based on the Winkler elastic foundation beam theory, the uncoupled axial and flexural elastoplastic pipe-soil interaction model was improved to consider the unsupported length, soil nonlinearity and temperature change. This analytical model was verified by the numerical results of finite element method. The sensitivity analyses for the pipe material and size, bedding conditions, soil reaction modulus, and temperature change indicates that: the smaller pipes are prone to circumferential break, while most of the larger pipes fail with longitudinal split; the axial tension stress induced by temperature change and the flexural stress induced by the unsupported length cause the smaller pipes to fail more easily; since the difference of the stress between elastic and elastoplastic model is small, the elastic analyses can be used to substitute for the elastoplastic analyses for simplification.(2)The different pipe materials have their special deteriorating properties. For the metal pipes, the loss of pipe wall thickness due to corrosion could be expressed as corrosion models, including the power model, two-phase model and linear model. For the plastic pipes with inherent cracks, the crack growth model was given. According to the measured corrosion pit depth and the surrounding soil properties, the correlation analyses between corrosion and soil properties was taken on. The calculated pit depth by the corrosion model sometimes deviates from the measured pit depth. So the corrosion model should be modified by measured data or area experience in practice. (3)Corrosion decreases the resist stresses of pipelines. To quantify the impact of the corrosion and defect during the process of pipes' failure, the stress concentration factors and residual ultimate strength of the corroded pipelines were studied individually.The nonlinear finite element model was used to calculate the stress concentration factors of cast iron pipe with sphere corrosion pits. The stress concentration factors curves with corrosion pit depth and suffered stresses were given. For considering the material nonlinear of cast iron pipes, the stress concentration factors were functions of the corrosion pit dimensions and pipes' stresses.The three-dimensional finite element model was established to solve the stress intensity factors of pipes with defect. The stress intensity factors curves along the crack line with different corrosion pit dimensions, under four load cases, which are longitudinal flexure, axial deformation, ring deflection, and hoop stress. The residual ultimate strength formula was obtained by fitting the normalized stress intensity factors with corrosion pit depth.(4)Modelling the failure of pipe involves uncertainties inherent in mechanistic models and their input parameters. Based on the random probabilistic theory, Monte Carlo (MC) simulations and First Order Reliability Methods (FORM) were used to perform the probabilistic analysis for the three typical cases of buried pipelines, including the cast iron (CI) pipe, steel pipe, and UPVC pipe. The sensitivity analyses revealed that unsupported length, temperature change, corrosion parameters, and residual strength parameters were more significant than other random variables for CI pipe; the corrosion parameters, yield strength, internal pressure, wall thickness, and residual stress were more important for steel pipes; the inherent defect size was most significant, and also the internal pressure, residual stress and wall thickness played an important role in the failure of UPVC pipes.(5)A fuzzy reliability analysis method for consideration of fuzzy uncertainties to estimate the failure risk of corroded pipelines was given. The fuzzy arithmetic algorithms were discussed deeply about their basic principles, computational efficiency, and application conditions. Moreover, combining the probability method with fuzzy arithmetic, a hybrid algorithm for consideration of both random and fuzzy uncertainty was offered. The numerical program was coded to solve complex fuzzy or random-fuzzy problems. The analysis of corroded steel pipe indicates that the application of the fuzzy possibilistic theory reduces the difficulty to obtain the data significantly. Using the probability interval to measure the probability of failure, keeps the real existed uncertainty. The given probability interval has provided reference for scientific decision-making.
Keywords/Search Tags:buried pipelines, pipe soil interaction, resistance deterioration, residual strength, probability, fuzzy reliability
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