Pumps and Systems, June 2009

In response to the substantial flood damage that New Orleans, La., sustained from Hurricane Katrina, the U.S. Army Corps of Engineers Hurricane Protection Office awarded a $85 million design and build contract to Weston Solutions, Inc. in February 2007.

The goal of the project was to increase the pumping capacity at two outfall canals, allowing for additional storm drainage to be pumped out of the city and into Lake Pontchartrain. Specifically, the 17th Street Canal's total pumping station capacity was to be increased to 7,600 cubic feet per second (cfs), and the London Avenue Canal's total station capacity to 5,000 cfs.

The design and construction of two additional pumping stations to achieve these objectives involved massive pumping platforms, vertical turbine pumps, diesel engines, gearboxes and the associated piping and support equipment. Figure 1 shows the London Avenue Canal pumping station platform under construction.

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Weston's design team included Gresham Smith Partners, BBG&S Engineering Consultants and Mechanical Solutions, Inc (MSI). MSI was charged with evaluating the platform designs from a vibration standpoint and identifying and mitigating potentially disastrous resonant conditions. The entire design and construction project was completed on-time and prior to the 2007 hurricane season.

Using computer-aided engineering techniques, MSI was able to predict the worst-case vibration responses of the structures that resulted from the operation of the mechanical equipment. Complex, 3-D finite element analysis (FEA) models were created and analyzed to assess any problems in a timely manner and, if necessary, address them prior to completion of the construction.

The Modeling Effort and Analytical Findings

The starting point of the analysis of the larger of the two new pumping stations, the 11-bay 17th Street Canal Pumping Station, was a set of detailed two-dimensional drawings of the pumping platforms, and of all of the associated mechanical equipment and piping. Weston also provided data on the geotechnical properties of the soil strata, since MSI had expected that the soil conditions would play a significant role in the vibration responses of the platforms. Initially, solid modeling was performed using a CAD software system. An FEA software package was then used to create the extensive finite element model and conduct the vibration analyses.

Figures 2 through 4 depict various views of the 3-D finite element model of the 17th Street Canal Pumping Station.

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Figure 2. Complete finite element model of the 11-Bay 17th Street Canal pumping station

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Figure 3. 17th St. Canal pumping station FEA model with the soil removed to expose the pipes

 

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Figure 4. Cutaway view of one bay of the 17th Street Canal pumping station FEA Model

The FEA model, which included a combination of solid, shell, beam and mass-point elements, consisted of approximately 5 million degrees of freedom. Of particular interest were the structural natural frequencies and the vibration levels at various locations that resulted from the machinery operation. The goal was to avoid resonant conditions that could lead to structural failure and/or the improper function of the pump system.

Conservative estimates were made for the forcing functions imparted to the platform by the diesel engines, gearboxes and pumps. The frequency content of the 1,200 rpm diesel engine loads was considered up to 120 Hz, which corresponded to the frequency of the cylinder firings. Rotating imbalance loads for the gears and pump impeller were also applied. Various phase conditions of the pumps were considered. The model predicted that the worst case loading would occur when the imbalance loads of all 11 pumps of the platform were in phase.

Analysis of this truss-like model through the full frequency range of interest presented considerable difficulty in terms of computer resources, even with multiprocessor capability available. The large number of plate and beam elements resulted in thousands of natural frequencies and intractable computer runtimes. To alleviate this problem, substructuring techniques available in the FEA software were employed, which allowed for the significant reduction of the number of degrees of freedom without a loss of accuracy. Since the 11 bays of the platform were identical, one superelement was created for a single bay and replicated to represent the other bays. The bay superelements were then attached to the discharge piping and to the soil models.

Virtual sensors were subsequently placed at various locations throughout the model, such as on the gearboxes, pump columns, platform corners and engines. The forced response FEA yielded the vibration frequency response functions at the sensor locations. These vibration levels were compared to industry standards and the vendors' requirements. Typical predicted vibration spectra are illustrated in Figure 5. The peaks within the spectra revealed excitable structural natural frequencies. Armed with this information, we made assessments of the vibration behavior of the platform.

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Figure 5. The predicted vibration frequency spectra at various locations of the 17th Street Canal pumping platform

Had excessive vibration levels been predicted, the FEA model would have provided an analytical tool for the redesign of the structure. Many different design modifications could be simulated rapidly and thecorresponding effects on vibration assessed. For example, resonant natural frequencies could have been de-tuned from the frequencies of the forcing functions by the addition or subtraction of platform cross members. Such modifications would have been costly to implement after the completion of construction, since the substructure of the platform is submerged. In addition to the structural vibration analysis, we performed an acoustic evaluation of the discharge piping to assess the potential for acoustic resonance within the 9-ft diameter manifolds that led to the lake. No resonance problems in the discharge manifolds were identified by the analysis.

The results of our analytical evaluations verified that the 17th Street Canal pump platform was designed to withstand the loadings of the mechanical equipment without excessive vibration at any key location throughout the system.

After the successful completion of the construction phase of the project, the pumps were operated to simulate a range of conditions, and no vibration problems were identified. However, it was not until the 2008 hurricane season that Gustav, a strong Category 2 hurricane, gave the U.S. Army Corps of Engineers the first opportunity to operate the gates and pumps at the 17th Street and London Avenue outfall canals in an actual storm event. Under those severe operating conditions, the pumping stations functioned in a superb manner as designed, keeping the potential flood waters from Hurricane Gustav safely in check.

"The gates and pumps worked as designed." said Col. Al Lee, commander of the U.S. Army Corps of Engineers, New Orleans District, after the hurricane.