AVT Reliability have installed a number of permanent systems to monitor the long-term integrity of sub-sea 'jacket' structures on offshore oil or gas platforms. These are extremely powerful and cost effective systems, globally monitoring the condition of the entire jacket structure using just a few topside mounted sensors.
Every structure with mass and stiffness will have multiple natural frequencies and mode shapes at which it will respond if suitably excited. These natural frequencies are fixed and will only change if there is a change in mass or stiffness.
The Jacket Integrity Monitoring systems installed by AVT use sensitive servo accelerometers (as used in missile guidance systems) to monitor the fundamental sway and torsion natural frequency modes of the structure, excited under ambient wind and wave loadings. Even though structures are not designed to be excited close to their natural frequencies, the instrumentation used is so sensitive it can detect these natural frequencies even during very calm weather.
By trending this natural frequency data for the 3 fundamental modes (North-South sway, East-West sway and torsion) and being aware of any known mass changes (e.g. adding a new module) we can detect any sudden unexpected and inexplicable reductions in natural frequencies, indicating a sudden change of stiffness of the jacket or foundation, which may indicate a serious structural defect such as a major crack.
Actually pinpointing the defect will then involve more conventional inspection work using divers and ROVs and using Finite Element Analysis to try to estimate the likely location of a defect by simulating and correlating the changes to the 3 fundamental natural frequencies.