When Modeling Isn't Enough

On June 8, 2004, the 42-inch concrete pressure pipe, which supplies potable surface water to one of the City of Houston's (City) Water Pump Stations, failed flooding several residential properties. After failure of the pipeline, the City authorized Lockwood, Andrews, & Newnam, Inc. (LAN) to further investigate the cause of the failure, review operational procedures, and simulate in a computer model the conditions that led to the failure. The computer model indicated that a transient pressure surge had occurred, which caused the pipe to rupture and potentially damaged over 29,000-lf of pipe. The length of pipe thought to be damaged was later verified in the field. The model results were within 100-feet from where circumferential and longitudinal cracks were observed in the pipe. In a pro-active step, the City of Houston also called for LAN to assess the potential for damage to the refill lines of two other water pump stations with similar operational procedures, and recommend preventative measures against future water refill line failures. The results from the transient surge model indicated that a 36-inch refill line had potentially been subjected to surge pressures almost two times the pipe's rated pressure. However, when a section of the 36-inch refill line was removed and inspected as part of a current construction project, no visible crack pressures or damage was noted. However, it was later discovered that the 36-inch refill line, while also concrete pressure pipe, was bar-wrapped pipe versus the 42-inch that ruptured, which was concrete cylinder pipe. This paper will discuss how condition assessment of pipelines cannot be made solely by modeling, but requires looking at the "big picture" consisting of a combination of modeling, understanding of the system and construction of pipelines, field verification, and experienced engineering.