Structural condition assessment and health monitoring of offshore structures in service are becoming essentially important. An offshore platform has exhibited unexpected excessive vibration under mildly hostile sea state in Bohai Bay, China. The Vibration behavior and condition assessment of the offshore platform were investigated through analysis of ambient field measurements. Modal analysis demonstrated that the fundamental frequency didn't remain as a stable value and varied from 0.3Hz to 0.80Hz. In addition, the characteristics of impulsive responses (owing to impacts) are observed. Analysis revealed that the impact was most likely resulted from the collision between the jacket and the piles initiated by the wave loading. Engineering control measures were conducted to eliminate the potential unsafety by grouting the gap between the piles and the jacket legs. Treatment results indicated that the excessive vibration had been effectively reduced.
Offshore structures, which have been working in harsh environmental conditions, are continuously exposed to wave action, corrosion, fatigue effect, etc. In general, the occurrence of damage during the lifetime of an offshore structure is inevitable. A feasible way to avoid or reduce the loss of capital, equipments or possible lives is to monitor the safety state of the structures and carry out remedies before disaster occurs. Among the commonly used inspection method such as manual, visual inspection, ultrasonic method, etc., structural condition assessment and health monitoring of offshore structures based on ambient vibration measurements in service are becoming extremely important; and have received much researchers' close attention (Doebling, et al, 1996). For one thing, there is no easy way to measure the input excitations or alternatively, to externally excite the structures artificially, which is just critical for large-scale structures such as offshore platforms; and the interference of the normal operation process can be minimized. Furthermore, this process under ambient measurement makes it possible for a long term and online structural health monitoring.