A large-scale test was undertaken to investigate the soil-pipeline interactions associated with the operation of large-diameter natural gas pipelines at sub-zero temperatures. The soil-pipeline interactions under consideration were, frost/pipe heave, ground cracking and thaw/pipe settlement and all had been observed during the winter and spring months along a 900 mm natural gas pipeline which was located downstream of a pressure regulating station (PRS). The objective of the monitoring was to fully investigate the engineering implications of the soil-pipeline interactions which result from sub-zero pipeline temperature operation with respect to the integrity of the pipeline system.

1. Introduction

This paper reports on a long-term study which is being undertaken to assess the feasibility of operating large-diameter natural gas pipelines at sub-zero temperatures in the United Kingdom. As a result of both environmental and operational conditions. a frozen annulus forms in the Winter and dissipates during the Summer. The size and extent of this frozen annulus downstream of the Pressure Regulating Station (PRS) is dependent on the pipe operating conditions together with the spatially dependent soil thermal, hydraulic and stress regimes. Unlike Arctic gas pipelines, only a frozen annulus forms since the average ambient air temperatures are above O°C. Pipe heave and ground cracking have been previously observed during the Winter and Spring along the length of pipeline operating below O°C (Archer et ai, 1984). but pipe settlement has rarely been observed; The results reported in this paper represent those obtained from a large-scale test and emphasis is placed on assessing the impact of frost heave and ground cracking on the safe and economical operation of large-diameter natural gas pipelines. The investigation of thaw settlement is the subject of further on-going studies and as a consequence this paper will concentrate on frost/pipe heave and ground cracking.

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