Eddy Current-Caused Platform Vibration and Correction

Abstract

The fixed-base steel platform experienced unusual deck vibrations soon after installation during Eddy current episodes. The approximate 70 cycles/minute oscillation of the deck resulted in the temporary evacuation of personnel. The platform is a skirt-piled structure in 430 ft. of water. The paper will present analyses performed, current measurement methods employed, motion detection devices utilized and corrective measures investigated and selected.The identification of the cause of the vibrations will help in understanding and preventing future occurrences. The corrective measures investigated and utilized will assist operators encountering similar behavior on existing facilities.Eddy currents are capable of producing vibrations on fixed-based platforms when the platform geometry is prone to excitation from steady high currents due to the shedding of vortices from submerged elements. Current-created vortices shed from platform members and appurtenances created a resonant condition on the platform J-tubes. The J-tubes fundamental period of vibration matched one of the deck's vibration modes, causing the 70 cycles/minute motion. This type of platform behavior resulting from the influence of high Eddy currents was essentially new to the industry and is not found in the literature. Field data, results of computer analyses, field measurements and details of corrective measures investigated and selected are included in the paper. Dynamic, fatigue and vortex shedding analyses were performed utilizing commercial and proprietary software. Computational Fluid Dynamics simulations were also employed. Mass dampers, although not employed, were investigated and devices designed for the platform.The paper provides critical new technical knowledge to resolve an important problem: Identification of platform geometry prone to Eddy current excitation.  Analytical methods best suited to solving the problem.  Corrective methods utilized to arrest the motion.  Other corrective methods investigated but not utilized on this platform.  Current measuring and motion detection devices utilized to obtain applicable data.