Session: 04-02-01 Stress Analysis & Data Analytics - Part I

Paper Number: 86098

86098 - Experimental and Numerical Analysis of Model Scale Buried Gas Pipeline Under Patch Surface Loading 

Roads, railways or embankments create additional loading on buried steel transmission pipelines throught pipe/soil interaction (a surface loading is diffused into the ground and transmitted to the buried pipe). When the loading is applied after the pipeline construction (sometimes decades after) the original design of the pipe does not take into account this secondary loading.

Depending on the specific situation, it is required to protect the pipe from the effect of surface loading. Various technics can be used; such as placing reinforced concrete slabs between the surface loading and the pipeline, or increasing the pipe depth thanks to a down-lifting during operation. Both are expensive works, and should therefore be avoided when the pipeline integrity and safety is guaranteed. In order to confirm or invalidate the need for this works, the transmission operator can use advanced modelling in order to predict the internal stress in the pipe wall. GRTgaz has used such modelling techniques for decades with its in-house software RAMCES (500 computations each year and several millions euros of avoided works).

This paper aims at providing data from both experiments and numerical analysis in order to better understand the soil/pipe interactions under surface loading in a sand tank, and try to improve the current engineering practices by reducing the conservatism of the models.

The originality of this experimental device consists in representing a typical pipeline of a network operator via similarity laws (ratio between thicknesses, modulus of elasticity / plasticity, etc.) in order to reduce its size. The result is a tank filled with sand with metric dimensions, allowing lab testing of model pipes (100mm diameter for up to 2mm thickness, made of aluminium at a scale of 1/3 of the prototype pipe). The soil used for the experiment is the Fontainebleau NE34 sand, a poorly graded silicious sand, in medium dense state. Various configurations and pipes can be tested, based on real-life situations; such as impact of pipe wall thickness, distributing slab or eccentricity of the load. Furthermore, a system is currently under developement to replicate the gas pressure, in order to study the effect of simultaneous action of the inner pressure and the external loading.

The experimental procedure includes an extensive instrumentation with strain gauges and inner displacement sensors, while the soil response is assessed through the use of miniaturized stress transducer. This combination of instrumentation and complex loading capability allows extensive analysis of the soil/pipe system behavior. Results including stresses, deformations, and the global deformed shape of the pipe in different testing conditions, as well as soil pressure distribution surrounding the pipe will be presented.

Experimental data will be compared with numerical simulations, using FEM models. The aim of those simulations is to assess the prediction capabilities of commercial softwares such as Plaxis when it comes to gas transportation pipeline’s behaviour under complex loading. Modeling is conducted using the hardening-soil model with consideration of small-strain stiffness. The sand used being well described in the litterature, experimental data are available for parameters calibrations.

Numerical and experimental data are to be confronted to actual design guideline and verification methods, such as RAMCES software. The comparison will relate to the ability to predict pipe ovalization, and the maximum stress to determine the safety coefficient of Ramces models mainly for low to zero internal pressures, when the impact of surface loads is maximum.

Presenting Author: Antoine Mertz Laboratoire 3SR

Presenting Author Biography: I&#39;m a PhD Candidate in Civil And Geotechnical Engineering, located in Grenoble, France. <br/>My project is a collaboration between the laboratory 3SR and GRTgaz, the french company in charge of high-pressure gas transportation. <br/><br/>My research activities are oriented toward soil structural interractions, using both experimental testing and numerical analysis; with the goal of assessing current engineering and design practices, as well of improving them.