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1	Cost Effective Structural Monitoring of North Sea Jackets Using Acoustic Excitation to Detect and Locate Damage in a Multiply-Connected Structure Dr Mark Harper Dr Martin Thompson Mecon Limited, Cambridge
2	Current Approaches to Damage Detection/Location Modal response: Simple in principle but limited sensitivity. ROV-intensive & diver-intensive methods. e.g.: FMD MPI ultrasound (conventional contact method or Lamb wave). Generally expensive & time consuming.
3	Proposed Alternative: Continuous Active Acoustic Monitoring Used fixed transmitters and receivers to monitor acoustic transfer functions Identify time delays of changes and use to flag up & locate changes in acoustic response of structure Test structure as frequently as required (e.g. every 10sec) Distinguish gradual from sudden changes Reject periodic/temporary (e.g. tidal) changes
4	Can it work in a complex structure?
5	Solution: Ignore all but direct-path compression wave signals
6	Computer Simulation: Forward Modelling Model transit times of signals between all nodes using modified time step method Gain knowledge of propagation characteristics Compare with physical measurements Use as basis for data inversion (identifying & locating damage events)
7	Experiments Plastic 1/100 scale model of Claymore Jacket
8	Signal Characteristics 1: Damage on Direct Path
9	Signal Characteristics 2: Damage not on Direct Path
10	Signal Characteristics 3: Damage far from Direct Path
11	Transmitters & Receivers Transmitters: four piezoceramic cylinder segments wrapped around each vertical leg Receivers: piezoplastic wire helix wrapped around each vertical leg Sensitivity steered towards longitudinal waves “Tomographic” geometry: transmitters at top, receivers at bottom “Reflection” geometry: transmitters and receivers at top
12	Damage Location Algorithms: Inverting Damage Event Arrival Times “Exact” inversion: for each transmitter/receiver/event combination, derive set of possible damage sites. Take intersection of all of these sets to find damage site. Can use more than one damage event per trace. CPU-time intensive (millions of possible routes). “LMS” (least mean square) inversion: use earliest damage event arrival only. Calculate mean square difference between modelled and observed arrival times for each node. Minimum value points to most likely damage location. Fast.
13	Calibrating the numerical model
14	LMS Inversion of scale-model “tomographic” data: Error Spheres correctly locate damage but with poor vertical resolution
15	LMS Inversion of scale-model “reflection” data: Error Spheres show vertical resolution is better
16	Conclusions Both numerical simulations and scale model experiments show active acoustic monitoring to be feasible “Reflection” geometry is: - potentially more accurate - more practical - most sensitive where damage most likely Next step: demonstration on a steel structure: - effect of water movement - effect of topsides
17	Acknowledgement Mecon Ltd gratefully acknowledges the support of the UK Health & Safety Executive