Study On Reliability And Risk Assessment Of Casing In Complex Well Status

Reliability analysis and risk assessment of casing in complex well status are the important research of applied theory and technology used for evaluating reliability, safety and prospective life of casing in complex well status including many risk factors, furthermore, preventing effectively casing failure, increasing casing reliability, lengthening casing life. Firstly, based on the obtained research achievement, the mechanisms of casing failure, the influences of various typical loads on casing failure, and the influences of multifold factors on casing strength were attained by means of the data collection and academic deduction. The formations and causations of different typical loads were studied. Especially, the axial and lateral loads of casing string under the earthquake action were analyzed in virtue of the spectral modal analysis theory and the method of equivalent static load. Secondly, because casing strengths and loads are random, according to the probability and reliability theory, the randomnesses and their influencing factors of casing strengths were analyzed by means of random numerical simulation and academic calculation. The regularities and parameters of distribution of various casing strengths, and the influences of the randomness of casing properties on the randomness of casing strengths were achieved. The reliabilities of API casing strengths were worked out. The distributions of typical loads were obtained through academic deduction, relational empirical formulas, and analysis of probability models of typical loads. The result shows that, the more random casing properties are, the lower the reliability of casing strengths is. The reliability of API collapse strength and tensile strength is reasonable to engineering, but the reliability of API internal yield pressure is much higher. Casing loads often follow the normal distribution, and the randomness of loads lies on the evaluation precision. Thirdly, the ultimate state equations of casing string were established in accordance with the basic failure forms, casing strengths and loads. The computing method of failure probability of casing string was confirmed. The influence of randomness of casing strengths and loads on casing reliability, and the relationship between casing reliability and safety factor were obtained. The reliability model of casing was founded depending on analysis of fault tree of casing string. The method and process of reliability design of casing were perfected, and the advices of design and construction of casing string in complex well status were brought forward. The fuzzy compositive quantitative risk assessment method and model of casing based on Borda number were established. The influence grade and occurrence probability of risk factors of casing string were analyzed synthetically, and these risk factors include human error and others which are difficult to be quantified or short of data. The reliabilities and risk assessments of some practical casings were completed in order to verify the reliability model and risk assessment model. The result shows that the reliability model and risk assessment model are both reasonable and practical, and they are reciprocally complementary. Fourthly, the method and model of learning and prediction of casing life were founded based on support vector machine with the result of risk factor analysis of casing string. The life of some practical casings was predicted, and the basic parameters of life prediction model were amended by virtue of sample training of casing failure. Lastly, according to the aforementioned achievement, the software"Reliability and Safety Analysis of Casing in Complex Well Status"was developed and applied to evaluate the security of casing successfully based on dynamic database, optimizing filtration technology and computer graphics. What's more, because casing inspection at downhole is very important for safety assessment of casing, and it is difficult to transfer instrument for casing inspection in the horizontal well, the self-adjusting tractor with wheels at downhole in horizontal well was developed by way of academic analysis, computer simulation, and experiment. The tractor that is simple, compact, reliable and powerful can be fit for wellbore of variational diameter, so it can transport the instrument for casing inspection at downhole in the horizontal and directional well.