| Summary: | Visible mechanical vibrations were noticed on two topsides piping systems on a producing FPSO, in the Gulf of Guinea following increased production in 2008 and 2009. A field survey was undertaken on the piping of the FPSO to investigate the actual pulsation and vibration levels followed by a computer analysis to investigate if fatigue failure, caused by the actual vibrations as measured, would occur. Vibration data was collected from subsea jumpers, risers and topsides piping to ensure full coverage of the flow line systems from the wellhead to production manifolds on the topsides. The objectives of the study were to: determine the root cause of the vibrations, ascertain the vibration levels, predict fatigue occurrence and reduce vibration to acceptable levels. The study revealed that the maximum vibrations levels were caused as a result of an excitation of several mechanical natural frequencies by a broadband excitation as a result of the multiphase flow. The measured vibration levels exceeded the allowable levels in some points of the pipe system which was caused by a very flexible supporting layout. The results of the analysis showed that fatigue failure of the main piping would not occur. However, to avoid failure of small branch connections (e.g. pressure and temperature transmitters) and valves, modifications e.g. additional supports and stiffening of structural steel members, have been advised to achieve acceptable vibration levels to ensure a safe and reliable system in the long term. The measurements, data processing, analysis results and recommendations are discussed in detail.
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