Retrofit of 60-Inch-Diameter Stainless Steel Tank-Inlet Riser Pipe with Stiffener Rings

The Agua Vista Station, which is the cornerstone of the San Antonio Water System’s Vista Ridge Integration Project in San Antonio, Texas, includes one of the tallest 10 million gallon (MG) prestressed concrete ground storage tanks ever constructed. The tank was placed into service in early 2020 before the 45 million gallons per day (MGD) water supply project went online. The tank has a 60-in. diameter stainless steel inlet pipe that enters through the tank’s floor slab and extends vertically 82 ft to the high-water level. After construction, it was determined that if the inlet riser was empty while the tank was full, it would be subject to an external hydrostatic pressure of 36 psi that could result in a collapse of the pipe. The riser pipe will be retrofitted with stainless steel stiffener rings to increase its external pressure capacity to withstand the potential external pressure condition. Replacing the riser pipe with a new thicker pipe was not feasible. The 60-in. steel riser pipe has a 5/16-in. thick wall with only two flanged joints and is connected to the concrete tank wall by 3/16-in. × 4-in. wide stainless steel straps at 10 ft center to center spacing. The contractor is planning to perform the retrofit activities in-place since the window for the retrofit work is very short and must coincide with the annual maintenance window for the entire water supply project; the tank must be drained before retrofit work can start. In preparation for the retrofit, the contractor submitted the designs of the stiffener rings, including dimensions of the rings and required spacing, which had to be approved by the owner’s engineer. The contractor also submitted the installation and passivation after welding procedures. A total of nine, 1/2-in. thick × 4-in. high stainless steel stiffener rings are required. The retrofit work is scheduled to be performed by the end of 2022. The stiffener ring designs, installation, and passivation procedures will be presented and discussed.