Superhigh Pressure Dense Phase Arctic Pipelines Increase Reliability and Reduce Costs

Differential movement in permafrost terrain due to ground freezing or thawing challenges the reliability of buried pipelines proposed for transporting natural gas from Prudhoe Bay and the Mackenzie Delta. Arctic pipelines designed to operate at conventional pressures (that is, below 10 MPa) are susceptible to wrinkling, bulging, and ovalling due to the differential movements they cause at interfaces between frozen and unfrozen ground and between different types of soil. Arctic pipelines designed to operate at superhigh pressures—defined here as pressures above 25 MPa—can accommodate the differential movements. A fair comparison between large diameter artic pipelines with operating pressures in the range from 10 to 42 MPa was made by accurately simulating flow performance with Greenpipe’s PipeCraft™ software. For any given design flow, superhigh pressure dense phase pipelines have smaller diameters and thicker walls, making them more flexible and better able to handle differential movements. And at superhigh pressures, Joule-Thomson cooling is negligible so that flowing gas stays close to ground temperature, reducing potential for frost heave or thaw settlement in the first place. Although weight per meter of superhigh pressure pipelines is similar to conventional pressure pipelines of similar flow capacity, increased flexibility means they are easier to lift and handle during construction. They also conform more easily to the terrain, resulting in less excavation and less pipe bending to make them fit the contours of the trench. The net result is reduced construction costs. When construction, maintenance and reliability are factored into the selection process, superhigh pressure dense phase pipelines provide a cost effective option for handling the challenges of arctic environments.