Knowledge of fatigue damage is relevant for lifetime extension and assurance issues of ships and offshore structures, but also for rational planning of inspection, maintenance and repair.


Lifetime prediction moves a step closer with fatigue damage sensors


To measure fatigue damage Kawasaki de- veloped the Fatigue Damage Sensor (FDS). This sensor consists of a base and sensing foil. In the centre of the sensing foil a groove is formed to amplify strain and an initial notch is placed in the centre of this groove. An estimate of fatigue damage is found by measuring the length of the crack in the sensor. The main advantage of the FDS is that fatigue life can be monitored without the necessity of pulling cables or installing a data acquisitioning system. In addition, the sensor is very compact which means it can be installed closer to stress concentrations.


MARIN and Kawasaki have tested the FDS as part of the Monitas Joint Industry Project. For this project, 16 FDS’ were installed on the Glas Dowr FPSO. The fatigue lifetime consumption obtained using these sensors was compared with that determined using conventional Rainflow counting of the measured stress signals. Promising results were obtained. Currently testing is also be- ing done within the Valid JIP for one of the USCG Cutters. Here 28 FDS’ were installed.


Ingo Drummen i.drummen@marin.nl


Ultimate validation On the USCG Cutter the possible occurrence of fatigue cracks in the steel is being monitored using the Ad- vanced Acoustic Emission method. Acoustic emissions are the elastic stress waves that result from the sudden release of strain energy due to micro-fracture events. By re- cording these emissions the instrumentation can detect stable crack growth. The possible


occurrence of a fatigue crack will provide ultimate validation material for fatigue life prediction methodologies.


Knowledge of fatigue damage enables MARIN to give advice regarding lifetime extension and assurance issues but also for rational planning of inspections, maintenance and repair. With the FDS this knowledge can be obtained accurately in a cost efficient man- ner, and without the need for significant intervention on the ship or structure.


Fatigue Damage Sensor. Courtesy Kawasaki


Fatigue Damage Sensor: very small indeed report 15


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