System Effects in Fatigue Design of Long Pipes and Pipelines

In fatigue design of steel structures the hot spot area is normally limited in size. In pipelines and cylinders used for transportation and storage of compressed natural gas the hot spot area can extend to several kilometers along a seam weld subjected much to the same stress range. The fatigue capacity for large diameter pipes is reduced as the weld length is increased. The seam weld in pipes is subjected to a significant stress range normal to its weld toes when it is subjected to varying internal pressure from start and stop of gas transportation through pipelines and from filling and emptying of cylinders. A reliable methodology is required for fatigue design of long pipes in order to meet target safety level at an acceptable cost. This includes description of physical models that represents actual long term fatigue capacity of the pipes subjected to varying internal pressure. The design methodology includes length of welds, fabrication tolerances, fabrication methodology and non destructive testing. It also involves definition of characteristic values for loading and capacity in addition to a recommended Design Fatigue Factor to be used in fatigue design. Alternative fatigue design procedures require different Design Fatigue Factors to achieve a required target safety level for pipelines and cylinders used for transportation of gas. These issues are further considered in this paper.Copyright © 2010 by ASME