Clipper is one of the gas producing platform in the UK waters. The platform features two low-pressure compressors that have been known to cause difficulties on start-up. Often Operations would report failed starts on gearbox vibration. These compressors are driven via a speed increasing gearbox. The compressor has no axial thrust bearing and as a result thrust force has to be transmitted to the gearbox thrust bearing via a rigid coupling.
Vibration data was collected during several start-ups, showing failed starts present high synchronous vibration and all characteristics of shaft unbalance. During normal operations and cold starts the compressor present very low vibration.
A detailed analysis using nyquist plots allowed the analyst to understand how unbalance was changing at every start. A hot restart profile (vibration severity per start from first failed start) was also built, which helped understanding what conditions would make the compressor move from a failed hot restart to a successful start-up.
After ruling out looseness and rotor sag, attention was given to external factors. These were listed and their influence tested in the field during a test campaign.
It was found that vibration is mainly affected by leaving the rotor idle in hot gas. A secondary factor was the influence of cold nitrogen injection in seal gas cavity. While more intrusive solutions were possible and considered, the problem was solved by a combination of control changes which allowed the platform to save several days of lost production per year.

Charles Cottin graduated from Cranfield University with a Masters in Rotating Machines Engineering and Management. He has worked for Total (France), NAM (The Netherlands) and Shell UK in the selection, maintenance and troubleshooting of various rotating equipment. His specialties include maintenance optimisation, root-cause analyses and troubleshooting of performance and vibration issues. He is a Vibration Analyst level III and an MIMechE chartered engineer.