Dynamic Analysis and Design of Large Compressor Foundations in High Seismic Zone

Abstract

Very large heavy centrifugal compressors are required in the design process for petrochemical, liquefied natural gas (LNG) plants, and power plant facilities. In a LNG plant, it requires multiple compressor units that contain methane, ethylene and propane compressors for converting natural gas to LNG by cooling the liquid temperature to –270°F. The heavy compressor vibrating machines are supported on concrete table top pedestal that consists of concrete slabs, beams, columns and mat foundation. For plant site located in a high seismic area, the dynamic analysis using the response spectrum method is applied to calculate the seismic force for design of the machine anchorage and the tabletop pedestal/ foundation. Also, the steady-state vibration analyses of the table top pedestal are performed to ensure that during the normal operating condition, the resultant maximum dynamic displacement (or velocity) amplitudes are within the limiting amplitudes set by machine manufactures and industrial standards. Both concrete cracked and un-cracked properties are considered in the analysis. Concrete members and connections are designed to comply with the seismic provisions in Chapter 21 of ACI 318-08 to ensure that the concrete pedestal will behave in a ductile manner during severe seismic events. This paper presents the design of a typical concrete table top structure supporting two compressor units. The plant is located in high seismic area. The presentation includes foundation sizing criteria and design loads, finite element modeling of the concrete table top structure, dynamic steady state vibration analysis due to machine imbalances, vibration response limit criteria, seismic response spectra analysis, and evaluating seismic design forces based on the static and dynamic analysis results. Due to heavy weight machines and deck, the P- Δ effect is considered in the static design. The reinforced concrete structural structural members are designed in accordance with ACI 318-08 seismic provisions.