Extreme Hydrodynamic Loading on Horizontal Pipelines

Abstract

Extreme events such as tsunamis and floods have caused massive damaging consequences to nearshore infrastructure. This has been more significant recently due to a changing climate. Transmission pipelines are among such infrastructures and need to be protected against potential extreme events. Design of pipelines requires a comprehensive understanding of the exerting hydrodynamic forces. Understanding the hydrodynamic forces acting on pipelines is vital in ensuring their safe operation and avoiding potential damage to the environment. The American Society of Civil Engineers (ASCE), in its ASCE7 Chapter 6: Tsunami Loads and Effects, the new standard for tsunami impacts and loading, stresses the necessity to study tsunami loads on pipelines. To address these issues, the following study is the first of its kind to investigate loading on horizontal pipelines due to tsunami-like bores. Research experiments are in line with the American Society of Civil Engineers, ASCE7 recommendation for studying tsunami loading on pipelines. The primary objective of this study was to measure and analyze the forces induced by extreme hydrodynamic events on submerged and above-ground pipelines. This was achieved by combining a comprehensive experimental study and detailed CFD numerical simulations. The study aimed to provide guidelines for the design of pipelines in tsunami-prone areas. At the same time, this study investigates the flow hydrodynamics in the case of dam-break waves impacting the horizontal pipelines and proposes specific values for resistance and lift coefficients in the case of a transient tsunami-like coastal flow for different relative gap ratios and submergence conditions. This study presents results from investigating the loading on pipelines located on flat and sloped beds induced by transient dam break wave. Different pipe configurations were investigated by changing the distance of the pipe to flume bed as well as the initial pipe submergence level and different flow and initial bed conditions. Different bed slopes were also tested. It was found that under dry bed condition, the initial horizontal impulse force and the lift force were considerably larger for smaller e/D. However, altering the gap ratio (e/D) under wet bed conditions did not considerably change exerted force components. Investigations over the impact of pipe submergence level showed a decreased impulse force with an increasing initial level of submergence (S/D) due to the decreased effective contact area of the pipe exposed to surge. Increasing the bed slope demonstrated to increase the induced horizontal force. This study suggests force coefficient values for various Froude numbers and several pipe configurations through proposed force coefficient versus Froude number graphs. Suggested resistance and lift coefficients for the experimental conditions investigated are in the range of 1 <CR<3.6 and -2.8 ≤ CL<2.8 respectively. This wide range of suggested force coefficients for various flow and pipe characteristics are essential for design purposes. Results of this work were presented as force coefficient graphs for different flow regimes. These graphs were approved to be included in the ASCE 7 Chapter 6. The CFD numerical model was developed and validated against the data obtained from the experimental study. Various turbulence models were tested and the Renormalization Group (RNG) k-ɛ model was defined as the most accurate Reynolds Averaged Navier-Stokes (RANS) turbulence model in predicting the induced forces. The numerical model allowed for testing the effect of a wider variety of parameters such as bed roughness and pipe diameter. Pipe diameter showed little influence on the impulse force magnitude at the time of bore impact in the tested diameters range. However, after the initial bore impact, force magnitude increased with increased pipe diameter. Numerical modelling results showed that increasing the bed roughness results in decreasing the induced forces on pipelines while pipe roughness showed no significant impact on the force magnitude. Findings from this study characterized the effect of the tested parameters on the induced extreme hydrodynamic loadings on horizontal pipelines. Results of this study will assist in a better understanding of the phenomenon and the improvement of the currently available design guidelines.