Health monitoring of a complex structure using a sparse array of modally selective transducers

We describe a novel method of monitoring the structural health of a lattice-like structure using a permanent sparse array of structural-acoustic modally selective transducers. The transducers are used to measure transfer functions across the structure. Changes in these transfer functions can indicate changes in the structure, including corrosion, cracking and physical damage. Combining information across the set of transfer functions can allow the location of a change in the structure to be pinpointed. The method makes possible low-cost, automatic monitoring of large complex lattice-like structures. In this paper, we report results of trials on a scale model of an offshore jacket (for example, the cross-braced structure that supports an oilfield production platform in the North Sea). Results demonstrate quite accurate location of damage at two well-separated points on the structure, using two different arrangements of acoustic sources and sensors. The first arrangement-in which sources are at the top and sensors at the bottom of the structure-showed limited vertical resolution of damage location. The second arrangement-with both sources and sensors at the top-shows much better vertical resolution. Best results would be obtained by deploying sensors at both top and bottom, but this is probably impractical on jackets. The main limitation was signal drift due to both electronic drift and temperature drift in the structure, which masks small changes in transfer functions caused by structural damage.