Unbalance Response Assessment of a Subsea Compressor

Robustness to unbalance degradation is examined for a subsea compression application in-volving a hermetically sealed, integrated motor-compressor supported on active magnetic bear-ings. The unique rotordynamic analysis methodology employs induced infinity norm principles to determine whether various limits, such as probe vibration, seal clearances and AMB force capacities, are exceeded for worst case unbalance distributions. The compressor’s acceptability with respect to API unbalance response design criteria is examined. Additional results indicate that, by introducing speed-scheduled synchronous filter control, the machine should be able to handle significant balance degradation (approximately 7.5x the API allowable residual) before reaching the ISO recommended shutdown vibration levels. For this particular design/application, it was determined that, with the nominal feedback con-troller, the unbalance capacity is dictated by the AMB dynamic capacity, not by probe vibration or seal clearance limits. Such knowledge can be vital to the end-user and manufacturer where, if unbalance degradation capacity is less than expected for the particular service, design changes can be made to help ensure that the machine’s desired robustness and service life are achieved.