Large Diameter Couplings for Seismic Conditions

Pipelines experience seismic forces when subjected to earthquake induced acceleration. However, unlike seismic design of above ground structures including building, bridges, and tanks, the science of earthquake engineering related to buried water and waste water pipelines is complex and code accepted design procedures are not readily available to the engineering community and pipeline design practitioners. For example, American Water Works Association, Steel Water Pipe: A Guide for Design and Installation (M11), Fifth Edition., Appendix D-Design of Steel Water Pipelines in Geohazard Areas, and American Society of Civil Engineers, Steel Penstocks, Manual of Practice 79, Second Edition, provide placeholders for future additional information with only several pages of general design and references to other publications. Recognizing that the previous editions of M11 contained no information at all on seismic design of pipelines, the introduction of the subject in the 5^th edition is welcome however, little tangible design information is presented and much work is needed in future editions of M11 to adequately cover the subject of seismic design of pipelines. One of the technical challenges to overcome is obtaining a good understanding of how buried pipelines interact with the surrounding soil (soil/structure interaction) and behave in a seismic event. For example in an earthquake, some soils may actually liquefy, causing local loss of soil strength, resulting in severe demands on buried pipelines. It is generally recognized that welded steel pipelines that are restrained by field-welded joints, flanged joints, or grooved joints perform well in seismic conditions. Estimation of seismic forces and concepts for resistance to seismic forces in addition to design strategies are needed to make pipelines function reliably as a safe, integral part of a conveyance system, as intended by the designer. One strategy to reduce seismic forces is to introduce flexibility into the pipeline structure. Structural engineers performing building design recognize that a flexible structure will adsorb much less energy during a seismic event than a rigid structure. Energy reduction translates into reduced forces on the pipeline, joints, and structural interfaces. It is common to have a pipeline enter/exit an underground structure however the interface acts as a boundary for seismic waves that are exaggerated at this location. This paper will address the concept of flexibility to resist seismically induced pipeline forces by application of multiple self-restrained flexible couplings. Structural pipeline design considerations for future editions of AWWA Manual M11 will be addressed.