Effect of Backfill Temperatures on Axial Restraint of Pipelines

One consideration for pipeline design is the Delta-T. There may be a tendency to assume a conservative value for winter construction, the extreme value being the air temperature. In applying a low restraint temperature in winter, the Delta-T is increased. The consequence of an artificially high Delta-T is that the pipeline may notionally experience high stresses; in extreme cases, mitigation is required to counteract these stresses. In winter, the soil at pipeline depth will usually be unfrozen or slightly frozen. When a pipeline nominally at ambient air temperature is backfilled, the warmer backfill soil will quickly warm the pipeline steel from the air temperature to a more moderate temperature. As a result, the pipeline is unlikely to attain full axial restraint at low air temperatures but instead at more temperate temperatures. In contrast, for pipelines constructed in summer and assumed to be backfilled at warm air temperatures, the resulting Delta-T may be nonconservative, particularly for hot pipelines. This paper presents the test results for several pipelines installed in winter and summer. The pipelines were instrumented with thermistors to monitor pipeline and soil temperatures and strain gauges to monitor pipeline strains during the backfilling process. Guidance is provided as to appropriate tie-in temperatures.