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INNOVATION IN COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping, G. Montero, R. Montenegro
Pipelines as Lifelines: Computational Problems and Methods
EMPA, Dübendorf, Switzerland
M. Farshad, "Pipelines as Lifelines: Computational Problems and Methods", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Innovation in Computational Structures Technology", Saxe-Coburg Publications, Stirlingshire, UK, Chapter 25, pp 541-567, 2006. doi:10.4203/csets.14.25
Keywords: pipelines, plastic pipes, composite pipes, computational technology, structural analysis, ADAP, standards, long-term extrapolation, failure analysis, lifecycle analysis, expert system, smart pipelines.
Piping systems for gas and water distribution, sewer, and drainage systems, cable protection, communication, and industrial installations constitute the lifelines of various industries and communities. Pipipelines may be subjected to various types of environmental conditions. The active environment may include mechanical loads, thermal effects, radiation, chemical influences, and biological factors. These conditions may individually or collectively cause a failure mode in the pipeline. Piping systems and pipe-liners may be subjected to a variety of effects such as external pressure, thermal gradiants, and internal negative pressure that may cause large deformations and instability in the pipe. Buried pipe systems are subjected to a variety of environmental actions, which may not be treated properly by the avialable computational schemes in general-purpose structural analysis technology. The computational technologies of pipelines should take into account international standards. This may exclude the use of many of the computational softwares, which may otherwise be used for other structural analysis purposes.
In this contribution, computational aspects of piping systems, and in particular plastic piping systems, as lifelines, and related strategies and methodologies are reviewed. The computational technology of pipelines includes methodologies for structural analysis and design of the whole piping system under various loading conditions, determination of long-term behaviour, prediction of failure modes, standardisation, service life assessment and expert systems for failure analysis of pipelines. The computational methods of pipelines are based on mechanical theories, results of failure investigations, international standards, and computational software. The salient features of pipelines, and in particular plastic and composite pipes, are discussed in the context of: time and temperature dependence, multiplicity of environmental actions, specific structural behaviour, and a variety of pipeline-specific failure modes. One of the phenomena related to the pipe systems dealt with in this contribution is rapid crack propagation (RCP) along the pipe. The mathematical explanation of RCP poses challenging computational problems some of which have been outlined in this contribution. There are, however, some open computational questions, which have been outlined as challenges for future research. Another phenomena, which may occurs in the piping system is buckling of the pipe and the pipe-liners. The stability consideration of this event needs to take into account the interaction between the environment and the material properties. For this purpose, a computational methodology including the reduction factors is proposed.
Design and analysis of piping systems in real environmental conditions require appropriate computational technology that can take into account installation conditions, interaction with the environment, load cases, and available international standards. In this connection, computational software for Automated Design and Analysis of Pipelines (ADAP) is introduced. ADAP is analytical-based software which takes into account bedding conditions, international standards, length effects in the piping systems, and the long-term response of plastic pipes.
Plastic pipes are normally designed for fifty years of service life. Hence, determination of the long-term resistance of plastic pipes is one of the major issues that requires appropriate computational technology. The available international standards propose statistical schemes, which may fall short of certain predictions. In particular, the issue of the long-term resistance of multiplayer pipes requires appropriate theoretical modelling and statistical analysis. In this connection, a new regression analysis method for prediction of the long-term behaviour of single-layer and multi-layer pipes under internal hydrostatic pressure is presented. This is achieved by an innovative hybrid linear and second order regression analysis. For regression analysis one can specify up to sixty data points. An example showing the structural analysis and the long-term extrapolation of pipes using ADAP is presented. The creep rupture curve obtained from ADAP does not depict a sharp knee as prescribed by the existing international standard. Instead, it gives a relatively "smooth transition" from the first part to the second part of the creep rupture lines. This computational result is physically plausible in the sense that the transition from the ductile to the brittle behaviour, that is, the aging of the pipe, takes place in a period of time and not in an infinitely small time period.
This review contains a short discussion of some non-classical finite element simulations of pipes. Examples are presented of nonlinear finite element stability analysis of plastic pipes. It is shown that by a slight variation of the boundary conditions two different stability behaviours and the corresponding critical loads and post-buckling modes can be obtained. It has been concluded that some of the buckling modes calculated by the finite element method may not be realisable in certain practical situations. An example of this is the ocurrance of the snap-through type of buckling in pipe-liners and concrete embedded pipes.
In another part of this contribution, aspects of the service life parameters of plastic pipes and methods for predicting their effective lifetime are reviewed. In the final part of this contribution, aspects such as the computational assessment of aging pipelines and the remaining service life are outlined. In this respect a mathematical model of the so-called aging systems has been used. This model predicts the service life behaviour of piping systems according to the bath-tub theory. An example using this model is presented. Further, part of this contribution deals with an automated expert system developed for the failure analysis of pipes. Finally, the emerging technology of intelligent pipelines is briefly reviewed.
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