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Physics-based Probabilistic Failure Modeling of Non-metallic Pipelines in Oil and Gas Applications
- Stewart, Theresa Mae
- Advisor(s): Mosleh, Ali
Abstract
The use of composite pipelines has grown in recent years as an alternative to steel in the oil and gas industry, due to their excellent corrosion resistance and high specific strength. Among these developments, thermoplastic composite pipes have seen a recent rise in research and use due to their greater flexibility and impact resistance than traditional thermoset-matrix composites. To understand and predict the performance of these materials in the field, studies have derived experimental, analytical, and theoretical estimates of the mechanical and thermal properties of thermoplastic composite pipes (TCP) and reinforced thermoplastic pipe (RTP) in dry conditions, but studies in wet or corrosive environments are currently restricted to the empirical domain. A better understanding of the performance of thermoplastic-matrix composites in marine and water-saturated environments is needed, because 30% of the world’s current oil and gas supply is produced offshore; the offshore portion of the upstream segment of the oil and gas industry has risen dramatically in recent years, and this trend is expected to continue [1], [2].To address part of this gap in modeling capability, this research used a probabilistic approach to predict degradation rates of the constituent polymer matrix and reinforcement fibers within an individual composite ply given the local temperature and the amounts of absorbed fluids. This method was applied by developing a TCP simulation code, which consists of three major time-dependent processes: 1) absorption of oil- and water-based fluids for each layer of the pipe given spatial and temporal variations in the diffusivity parameters, 2) evaluation of the local composite properties given the amount of degradation to the matrix and fibers during each time window, and 3) prediction of the stresses at each ply under defined thermomechanical loading conditions given the current state of each ply’s mechanical properties and subsequent determination of ply failures. The ply failures determined from the third process incur further reduction in the stiffness of the failed ply, resulting in greater amounts of stress applied to the remaining un-failed plies until all plies in the laminate have failed, thus constituting a progressive failure mechanism. In order to model these processes while accounting for various uncertainties in material and environmental inputs, new methods for estimating certain process factors, such as diffusivity and the extent of composite stiffness and strength reduction following degradation and ply failure, are developed using comparisons to real-world data. Similarly, to verify the validity of each of the three major physical processes in this simulation, the results of each given set of controlled inputs are compared to experimental data on TCP and RTP from various lab and field experiments. It was found that in cases where the observed degradation was low, agreement with the model was generally good, but the model tended to underestimate the amount of degradation in the more severe cases. Future work will aim to improve the prediction capabilities of this method by adjusting the existing processes in the model and incorporating factors that the model currently does not consider, such as fillers and the pH of absorbed fluids. Finally, the model presented herein has been implemented as a new module in a pre-existing online application initially developed to predict the reliability of metallic pipes in a real world environment, in order to provide insights to the oil and gas industry on the risks of certain locations. The current implementation for the non-metallic pipes includes a prediction of the failure rates, mechanical stiffness degradation, and fluid absorption for segments of pipe, with plans to add maintenance and sensor placement features in the future.
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