ON PAGES 197-212, authors from Canada’s Dalhousie University and C-CORE present their findings on the subject of local buckling response of pipelines under combined-loading conditions. The exploration and development of new hydrocarbon resources has been pushed to frontier regions that may encounter permafrost terrain and ice environments, and pipelines are obviously a practical and cost-effective transportation system to bring oil and gas resources from these remote locations. A major design consideration, however, is the effect of large-deformation ground-movements such as those caused by frost heave, thaw settlement, and ice gouging on the mechanical integrity of buried pipelines.
The effects of internal pressure and geotechnical loads, due to thermal expansion and relative ground deformation, impose complex combined loading conditions on buried pipelines, and large-deformation combined-loading events may result in local buckling and wrinkling mechanisms that may impair pipeline operational requirements or strength criteria. Establishing design criteria that define serviceability or ultimate limit states provides an engineering framework to assess mechanical integrity for unique design conditions.
The authors have undertaken a comprehensive parametric study using the finite-element method to assess the effects of diameter-to-wall-thickness ratio, internal pressure, axial load, end moment, and geometric imperfections on the local buckling response of pipelines subject to combined loads. The limit point from the global pipeline moment curvature response was evaluated, and the key factors influencing the peak moment amplitude and curvature to peak moment were examined.
IN A SENSE, continuing this theme, the paper on pages 213-222 considers a case study of pipe-in-pipe application to a landslide site. Typically, episodic and seasonal slope movements due to rainfall generate tensile stresses in the pipe in the upper portion of the slope and compressive stresses towards the slope toe, and in some cases this naturally leads to pipeline failure. A combination of monitoring and periodic in-line inspection has, up to now, usually been considered as the only practical approach to prevent these failures.
It has been suggested that a buried pipe-in-pipe (PIP) system can overcome most of the difficulties encountered by pipelines installed on moving slopes, and such systems can provide a number of advantages over single buried pipelines. In a PIP system, the soil loads are taken by the casing pipe and the carrier pipe does not experience any direct external loads. Since it does not normally carry any pressure loads, the casing pipe can be designed to accept considerable axial movements while allowing the carrier pipe to deform relatively freely within the casing pipe. The response of the carrier pipe can be accurately predicted using a finite-element (FE) model, thus providing a reliable way to predict the design life of a PIP system installed in a moving slope.
Although many PIP systems or pipeline bundles have been installed offshore, these applications were mostly motivated by other considerations such as thermal insulation and ease of construction. However, Snam Rete Gas recognized the benefits of a casing pipe against landslide hazards and has been utilizing a variation of the PIP concept since 1998: its concept was called Tubo-Cunicolo, and involves a series of casings made up of corrugated galvanized pipe installed at periodic intervals.
Based on past successful application of the concept to reduce the long-term pipeline stress accumulation, Snam Rete Gas recently decided to convert a small-diameter pipeline to a PIP design. From the monitoring information obtained from slope inclinometers and strain gauges, it was determined that the pipe was experiencing increased stress. As is common in these kinds of site, it was observed that the pipeline stresses were mostly tensile in the upper portion of the slope and compressive towards the toe. Based on the slope-movement data gathered in the past, a decision was made to convert about 975ft (320m) of the pipe to the PIP configuration, which uses a contiguous casing with periodic piston-like segments that allow for differential axial movement.
The paper describes an FE model of the system that was developed to predict the response of the PIP system and to assess its design capacity. The new model, based on a previously-developed model, is being developed in three stages, in the first of which the casing pipe was assumed to be rigid. The second and third stages, yet to be completed, consider the casing pipe when placed on a Winkler foundation with slope movements applied to the ground nodes of the Winkler springs. In the third phase, a continuum model of the slope will be developed, and the PIP system will be integrated into the continuum slope model.
The authors point out that the main objectives of the project are firstly to demonstrate that the PIP approach can provide a cost-effective solution to prevent pipeline failures in unstable slopes, and secondly to calibrate the FE PIP model. The paper describes the PIP design developed and installed by Snam Rete Gas, and a detailed description of the FE PIP model is then presented. The results from the FE model are discussed, and the last section of the paper considers the behavior of the PIP system and provides the authors’ the conclusions.
THE FINAL paper in this issue, starting on page 223, is the second part of major report on the role of energy pipelines and research in the United States. Prepared on behalf of the US Steering Committee on Energy Pipelines and Research, the report’s subtitle emphasizes the importance of the way it considers the country’s pipeline network: Sustaining the viability and productivity of a national asset. The report was published in May, and provides a comprehensive survey of the contributions of oil and gas pipelines towards meeting the US’ energy needs, emphasizing the critical role that research played in making those contributions possible up until now, and how research will be necessary in the future to meet the challenges facing pipelines. Continued, even increased, dependence on pipelines is clear, not only in the US but world-wide: not only will demand for oil and natural gas grow, requiring greater capacity for distribution across the nation and into communities, but regional patterns of supply and demand will shift, requiring reconfigured pipeline movements. Research is essential to improve pipeline safety, supply reliability, environmental performance, security and efficiency as the system encounters higher capacity utilization and higher bars for performance.
This landmark report eloquently argues the case for the continuing importance of research in the oil and gas pipeline industries in the US and, by extension, in other countries where the pipeline network is both aging and becoming strategically of greater importance. In passing, it also provides a useful overview of the US pipeline network and its current capacities and future requirements.
FINALLY, readers are reminded that from the first issue of 2007, the focus and editorial content of the Journal of Pipeline Integrity (JPI) are being broadened to embrace more comprehensively the oil and gas pipeline industry, and the Journal’s title – in consequence – is being changed to the Journal of Pipeline Engineering (JPE). Co-publishers UK-based Scientific Surveys and US-based Clarion Technical Publishers hope that this change will not only provide a means by which a wider range of important technical and scientific papers can be made available to the industry, but also that the JPE will appeal to a wider range of readers and thereby help to promote pipeline engineering as an important and independent branch of engineering as a whole.
In the widest sense, all pipeline activities are aimed at maintaining and prolonging a pipeline’s integrity, whether they are concerned with establishing a route for a new pipeline to avoid particular hazards or undertaking a fitness-for-purpose assessment for an aging line to allow it to remain operational as safely and cost-effectively as possible. The new Journal of Pipeline Engineering will have this concept as its underlying theme, while increasing its editorial scope to encompass as many areas as possible of the engineering sciences that lie behind successful pipeline operations.