Vibration monitoring for damage in offshore platforms

Abstract

A number of vibration‐based techniques have been considered for the detection of underwater structural damage in fixed offshore platforms for oil and gas production. Most platforms are primarily composed of welded steel pipes, and are pile anchored to the sea floor in depths of up to 320 m (1050 ft). The standard inspection technique employs divers to visually survey for severance or cracking of members. This is costly, can be hazardous, and is subject to environment constraints. Various vibration instrumentation approaches are described. All employ accelerometers to measure ambient excited or forced vibration. The crucial issues are (1) the ability to reliably detect damage before there is a significant loss of structural integrity, (2) the avoidance of false alarms due to nondamage changes, and (3) the masking of the desired information by machinery and other noise. A highly promising new technique, soon to be field tested, employs accelerometer placements down special chutes to reach various underwater positions for detection of overall shear stiffness of the individual platform bays. Results are described for a USGS/ONR sponsored blind laboratory test program in which several techniques were evaluated.