Experimental And Theoretical Study Of The Dynamic Response Of The Natural Gas Pipeline Aerial Crossing During Pigging Process

Natural gas pipeline aerial crossing (NGPAC), which is basically fromed as suspension structrure or cable-stay structure, is essential for gas transmission pipelines when their routings come across deep canyons, streams with sensitive aquatics habitat and rivers with shifting or unstable beds. A typical pipeline aerial crossing is composed by pipes, towers, cables and potential stiffening girders. During pigging process, the NGPAC is suffered from complex dynamic loads generated by the pig and the slug. Strong vibration as well as large deformation would be observed on NGPAC due to its highly flexible feature which was introduced by the cable system. On the other hand,the strong vibaration and large deformation occurred on the NGPAC would affect the distribution of the pigging load, which may induced stronger vibration and larger deformation onto the structrue.Aimed on the strong vibration and large deformation issure of NGPACs during pigging process, the thesis selected suspension and cable-stay pipeline aerial crossing as the object of research, and studied the basic theory as well as the vibration law of the dynamic response of the aerial crossing during pigging process based on the hydrodynamics, mechanics of materials, structural mechanics as well as finite element theory. A dynamic response analysis model was also established for estimating the deformation and stress of NGPACs under pigging loads. Finally, an assessment method for evaluating the safety of NGPACs during pigging process was proposed based on the research findings.The specific research achievements are as follows:(1) The NGPAC is always under static loads during operation. The deformation and stress of NGPAC induced by the static load is the initial condition of dynamic response research. Based on the geometry non-linear and static stress balance equation, a static stress analysis FEM model including geometry non-linear behavior was established with carefully consideration of the connections and constraints between different parts of the NGPAC. The initial strain was set as the iteration variable applied to the cables to solve the model. The solution procedures were also proposed. Three different NGPAC which are Nujiang suspension NGPAC, Lancangjiang suspension NGPAC and Nanguanghe cable-stay NGPAC are analyzed by the static stress FEM model.(2) An in-door experiment platform was established, which included one suspension NGPAC scaling model designed based on Nujiang suspension NGPAC of Zhong-Mian natural gas transmission pipelines as well as one cable-stay NGPAC scaling model designed based on Nanguanghe cable-stay NGPAC of Fu-An natural gas transmission pipelines. The experiment platform was aimed to clarify the phenomena of the dynamic response of NGPAC during pigging process, and to select the key parameters which could present the dynamic response beavior. Pigging process under different pigging velocities and liquid injection volumn was conducted on the platform, and the vibration paremeters as well as deformation trends were measured and analyzed. The experiment showed that the large deformation during pigging process is the main feature of the dynamic reponse of NGPAC, and the displacement from the inistial position of the pipeline as well as the deformation angle between the tangent line and the horizontal axis of a specific position on the pipeline are the key parameters for large deformation description.Based on the experiment data as well as the method of non-linear data fitting, non-dimensionalization and theory derivation, a model for sizing the deformation curve of NGPAC is established with consideration of five parameters which are liquid holdup of slug, length of slug, pigging velocity, length of the NGPAC and pipe diameter. The model can calculate the deformation and inclination angle at any location of the pipeline on the NGPAC under different time. So it is the biasis for evaluating the pigging load evaluation and NGPAC dynamic response.(3) With consideration of the pigging operation parameters influenced by the large deformation of the NGPAC, a pigging load calculation model contained gravity load, shock load at the elbow and the load location information was proposed and solved based on the mass conservation equation and momentum conservation equation of multiple phase flow.(4) Based on the static stress analysis FEM model as well as the pigging load calculation model, the model for estimating the NGPAC dynamic response behavior during pigging process is established. In the model, the displacement of the pipe achieves the mutual coupling between the structure deformation and the load calculation. So the displacement of the pipe was chosen as the iteration variable to solve the model, and the deformation curve model obtained by experiment data was used to calculate the initial value of the iteration variable. On this basis, the dynamic response model was solved in accordance with the weak-couple theory and prediction-revision method. Based on the external data modules, the solution method of the NGPAC dynamic response model was accomplished by the software of ANSYS.The deformations obtained by the simulation model were compared with the experiment data, which showed highly consistence of the vary trends. Besides, the relative deviations of Nujiang suspension NGPAC deformations between simulation and experiment data are 1.8% and 2.4% respectively for two different conditions and for Nanguanghe cable-stay NGPAC, the relative deviations are 7% and 0.49%.The applications of dynamic response model also showed, under certern pigging operation conditions, the maximum Von-Mises stress of the Nujiang suspension NGPAC and Lancangjiang suspension NGPAC would exceed the allowable stress but not exceed the specific maximum yield stress (SMYS) of the construction material. On contrary, under certain pigging operation conditions, the maximum deformation of Nanguanghe cable-stay NGPAC would be as large as 1.48m, and the stress of the pipeline would exceed the SMYS of pipe material.(5) On the basis of allowable stress and displacement criteria obtained by "Design Code for Natural gas and Oil pipeline aerial crossing GB50459-2009", an limite strength design factor is proposed for allowable stress calculation based on the deterministic limite State analysis. Combined with the research achievements (1)-(4), an assessment method for evaluating the stress condition safety of NGPAC during pigging process, which can be applied as theory reference for standard revision.