Vibrations in High Pressure Centrifugal Compressors of Offshore Platforms

Abstract

During the startup of one of the compression trains of an offshore platform, in August/2006, excessively high vibrations were observed at the high pressure compressor. The same problem was also verified one week later, during the preliminary tests of the String Test for the compressors of other two offshore platforms, which were supplied by the same vendor. The vibration records showed that the high vibration was due to the synchronous vibration component (1X vibration) and that it increased with the gas temperature. The Vendor’s previous experience with this problem, which was designated as Thermal Induced Unbalance Vibration, indicated that this could happen when there were mounted components secured to the shaft through two interference fits. This was true for the balance piston of the high pressure compressor of P-50. The design of this component, with a cylindrical shape, was not usual, but was employed because of the small diameter of the component. The same design was repeated on the HP compressors of P-53 and P-54. After modifying the balance piston of the HP compressor at the test bed, a high level of vibration has been again observed in the following pre-test. The analysis of the vibration records showed that now the problem was more related to pressurization, rather than to a thermal effect. Rotor dynamical simulations confirmed that the high vibration was now caused by the design of the balance piston seal. The objective of this work is to present a detailed description of the causes of these vibration problems and discuss the ways of avoiding them for future equipment.