Sensor qualification by full-scale accelerated fatigue testing

To this end BOPTIC delivers innovative optical fibre sensing techniques to monitor the Balance of Plant (BOP), covering both the structural integrity of offshore constructions and the health of the cables that transport their power. Techniques covered under the BOPTIC project are so-called distributed optical sensing techniques known as Distributed Acoustic Sensing (DAS) and Optical Frequency Domain Reflectometry (OFDR).

OCAS – together with partners Marlinks, Com&Sens, IMDC, Parkwind, 24SEA, Laborelec, UGent and VUB and funded by VLAIO and the Blue Cluster – has joined this project with the specific aim to investigate improved condition and structural health monitoring of offshore wind foundations. The task OCAS has performed is to compare the performance of the OFDR optical sensing technology with normal fibre Bragg-grating strain sensors and classical strain gauges. With a correct application method these strain sensors are a reliable tool for measurement of fatigue loading conditions. Different full-scale pipes with girth welds were equipped with the different strain sensors and submitted to accelerated fatigue tests. Each test was carried out with up to 30 million cycles. With a testing frequency of 26 Hz, a 30 million cycle test can be completed in 2 weeks while a classical hydraulic bending test would take a full year. This has been made possible by one of OCAS’ resonant fatigue test benches whereby we submit full-scale specimens to higher frequencies than classical testing systems. Such technique has been validated – on full-scale offshore wind specimen – to be equivalent to hydraulic testing in the Carbon Trust OWA JaCo project.

The breakthrough surface on the post-mortem analysis has in the meantime revealed that the crack initiation and propagation indicated by the sensors was accurate. As such we have been able to qualify the lifetime performance of the optical fibre towards monitoring of an offshore wind foundation in a record time.

“We have been able to qualify the lifetime performance of the optical fibre towards monitoring of an offshore wind foundation in a record time.”

To this end BOPTIC delivers innovative optical fibre sensing techniques to monitor the Balance of Plant (BOP), covering both the structural integrity of offshore constructions and the health of the cables that transport their power. Techniques covered under the BOPTIC project are so-called distributed optical sensing techniques known as Distributed Acoustic Sensing (DAS) and Optical Frequency Domain Reflectometry (OFDR).

OCAS – together with partners Marlinks, Com&Sens, IMDC, Parkwind, 24SEA, Laborelec, UGent and VUB and funded by VLAIO and the Blue Cluster – has joined this project with the specific aim to investigate improved condition and structural health monitoring of offshore wind foundations. The task OCAS has performed is to compare the performance of the OFDR optical sensing technology with normal fibre Bragg-grating strain sensors and classical strain gauges. With a correct application method these strain sensors are a reliable tool for measurement of fatigue loading conditions. Different full-scale pipes with girth welds were equipped with the different strain sensors and submitted to accelerated fatigue tests. Each test was carried out with up to 30 million cycles. With a testing frequency of 26 Hz, a 30 million cycle test can be completed in 2 weeks while a classical hydraulic bending test would take a full year. This has been made possible by one of OCAS’ resonant fatigue test benches whereby we submit full-scale specimens to higher frequencies than classical testing systems. Such technique has been validated – on full-scale offshore wind specimen – to be equivalent to hydraulic testing in the Carbon Trust OWA JaCo project.

The breakthrough surface on the post-mortem analysis has in the meantime revealed that the crack initiation and propagation indicated by the sensors was accurate. As such we have been able to qualify the lifetime performance of the optical fibre towards monitoring of an offshore wind foundation in a record time.

“We have been able to qualify the lifetime performance of the optical fibre towards monitoring of an offshore wind foundation in a record time.”