A NOVEL IN-SITU TEST SETUP FOR MEASURING THE INTERFACE RESISTANCE BETWEEN PIPELINES AND SOILS

Abstract

Offshore hydrocarbon transport pipelines operate at relatively high pressures and temperatures. These operation conditions lead to their expansion and/or contraction and may ultimately result in pipeline buckling or “walking” after multiple cycles of operation. Such movements are typically resisted by the pipe-soil axial resistance at the interface, which controls the compressive forces within the pipeline itself. Any errors in establishing the interface resistance will lead to either an over-estimation of pipeline extension or of high compressive stresses possibly leading to buckling. Such erroneous assumptions/outcomes will have significant impacts as to the necessary costs to mitigate their effects. The actual axial interface response whether in the short or longer terms is a function of many factors such as the pipe laying process, the consolidation periods, the shearing rate, the interface roughness, and the weight of the pipeline itself. Reliable and efficient design methodologies are thus needed to optimize the engineering performance of the pipelines while minimizing testing and construction costs. To date, different testing techniques have been adopted in the quest for an accurate estimation of the axial pipe-soil resistance throughout the operational life of the pipeline. These include laboratory soil element testing, laboratory model testing, and in-situ testing using specialized, complex and costly apparatuses. Given their nature and the fact they involve seabed soils in their actual/real conditions, in-situ tests provide the most reliable results. However, they are limited by the very small number of available specialized field equipment, e.g. the Fugro SMARTPIPE, and recently developed “pipe-like” penetrometers, both of which suffer from many draw backs related to their high cost, practicality and testing conditions. The work presented in this thesis presents an attempt at addressing most of the limitations that were identified in the currently available methods, leading up to the development of a new insitu, cost-effective apparatus for measuring the axial pipeline resistance. The new proposed, designed, built, and lab-validated setup directly targets the limitations of currently available systems. The new apparatus was conceived with a particular focus on eliminating the problem of passive stresses generated at the pipe ends and delivering a cost-effective and reliable solution for conducting in-situ interface tests. A laboratory proof of concept experimental setup that could be adapted/automated in future work to become an autonomous field apparatus was thus designed, produced, and tested on a clay bed and under different testing conditions. The tested prototype reliably captured the effect of drainage conditions, normal stress, and rate of loading on the interface resistance. It produced accurate drained interface friction factors that are comparable to the ones obtained from the direct shear tests on the same soil and interface. Under undrained conditions, the measured interface response was realistic but the test section exhibited slight rotations in all directions that affected the pore pressure readings. The results obtained are very promising and confirmed the practicality and functionality of the proposed setup/prototype. Furthermore, these results revealed the need for some improvements that we intended to apply in future work and that would enhance the testing effectiveness and reliability. Part of the background validation work that was done in this thesis was dedicated to compare the interface test results obtained using the two most common laboratory testing methods: the tilt table and the interface direct shear test apparatus. Both sets of laboratory equipment were used to test the drained clay-solid interface response for different soil compositions (high and low plasticity clay), interface roughness (smooth and rough), and the applied normal stress. The comparison suggested that using the Interface Direct Shear machine for determining the drained residual pipe-soil interface resistance is a practical and reliable testing alternative, provided that the conventional direct shear setup is properly modified to reduce mechanical friction and make it amenable to low-pressure testing.