Ambient Vibration Survey of Offshore Platform

Natural frequencies from 0.65 Hz to 4.5 Hz and associated mode shape parameters for vibration modes of a 390-ft (119-m) tall steel platform were determined from measured ambient accelerations. The most accurate modes identified were the fundamental laterals and torsion, the second torsion, the third end-on, and a higher complex mode involving considerable vertical motion. Five other modes were identified by frequency and qualitatively by shape; 13 other modes, by frequency only. The measurements were made in lateral, vertical, and angular directions 20 ft (6 m) above sea level and between decks. Data acquired in both calm and rough seas were found to be equally useful. The sub mg random accelerations were analyzed by standard spectral techniques. This is the first phase of a pilot study for the USGS on the feasibility of ambient vibration monitoring as an inspection technique for underwater failure. Changes in vibration modes are the basis for the assessment. If successful, reliance on costly and hazardous underwater inspections can be lessened.