Determination of the creep function of viscoelastic pipelines using system resonant frequencies with hydraulic transient analysis

© 2016 American Society of Civil Engineers. The determination of the creep (compliance) function of viscoelastic pipelines is essential for modeling their hydraulic behavior and accurately predicting pressure responses under transient events. This paper proposes a novel frequency-domain technique for the determination of the creep function of viscoelastic pipelines using hydraulic transients. A viscoelastic pipeline system, when compared with a frictionless elastic pipeline under the same system configuration, has nonuniformly shifted resonant frequencies. Analytical analysis shows that the shift in the resonant frequencies of a viscoelastic pipeline system is related to both the pipe wall viscoelastic compliance effects and the unsteady wall shear stress effects. A technique is developed to determine the elastic wave speed and the viscoelastic creep compliances based on the shifted system resonant frequencies. To improve the accuracy of the calibration for the viscoelastic parameters, an approach is proposed to correct the shifting in the resonant frequencies induced by the unsteady friction before the calibration. Numerical simulations conducted on a high-density polyethylene (HDPE) pipeline verify that the elastic wave speed and viscoelastic compliance can be determined with relatively high accuracy.