Abstract

Main oil line (MOL) pumps are responsible to deliver the partially stabilized crude oil from an offshore oilfield to a processing and storage facility where oil is exported through a loading station. MOL pump is categorized as a critically rated machine due to the fact that the consequences of its failure leads to major business and operational interruption, mainly production loss. This paper studies a failure prevention approach of MOL pump due to excessive vibration using actual site measurements, engineering test runs, and finite element analysis to predict the failure and maximize pump reliability.

This study presents a case of a MOL pump operating with vibration levels, at the drive-end bearing housing of the electric motor driver, beyond the allowable limits. This was initially experienced during the Factory Acceptance Test at the vendor premises. Vendor carried out corrective measures to overcome this unacceptable mechanical condition including, baseplate leveling, soft-foot check, rotor balancing and alignment. However, no significant improvements were achieved with these measures. Nevertheless, the pump package was sent to the construction yard to avoid delays to the project, at which the vibration issue was persisted leading to availability and reliability concerns. A detailed three-dimensional finite element (FE) model of the pump baseplate is built to simulate its dynamic behavior. Actual site vibration survey was carried out and measurements were collected, analyzed and compared to the FE predictions. Mitigation measures to overcome this excessive vibration are provided.

The results of the analysis show that localized resonance of the electric motor supporting beams at the baseplate are the main reason of the excessive vibration, which are transmitted to the motor bearing housing. The FE simulations were used to predict the system’s natural frequencies in the working frequency range. The calculated natural frequencies and mode shapes of the baseplate are verified against vendor calculations and actual site measurements. In order to reduce the vibration to allowable levels, additional steel plates were added with pre-specified mass, determined using simplified spring-mass system model, at a specific location on the resonating beams, mimicking the behavior of a dynamic absorber. The location and mass of this dynamic absorber were fine-tuned during site testing trials, and the vibration amplitude at the initial resonant peak was greatly reduced achieving allowable vibration level. Hence, the risk of equipment failure due to high vibration is eliminated.

The impact of weld residual stresses, pre-stressed structural components or distortions on the dynamic behavior of structures is not well recognized. This study utilizes in-depth advanced engineering analysis techniques to enhance the reliability of critical equipment and prevent its failure due to excessive vibrations.

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