Application of Fatigue Damage Sensors in the Monitas System

This paper is one of a series of papers presenting research results from the Monitas Joint Industry Project (JIP). The name Monitas stands for Monitoring Advisory System. This paper focuses on the application of the Fatigue Damage Sensor (FDS) for monitoring of fatigue life consumption of FPSOs. The sensor has been developed by Kawasaki. It is a passive sensor similar in size and installation to a conventional strain gauge. After the installation the sensor feels strains of the structure.These strains develop a crack in the sensor. The crack length is a measure of fatigue lifetime consumption. This paper describes the sensor and compares its readings with conventional fatigue assessment methods based on the rain-flow counting of measured strains. This paper focuses on the first successful application of FDS to an FPSO. In general, the FDS is applicable to any structure. However, for the FPSO application the sensor has to be calibrated in order to take into account the characteristic stochastic nature of fatigue loading of marine structures subjected to action of waves. The beauty of the sensor is its simplicity and low cost. The disadvantage is that the crack length has to be read periodically. The paper demonstrates that the FDS gives reliable results and is very attractive for direct assessment of fatigue lifetime consumption of vital structural elements of FPSOs. The paper gives guidance on application of FDS including preferred locations, installation procedure, number of sensors per location and reading procedure.

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