Development of an acoustic sensor to monitor synthetic mooring lines

Fiber ropes (polyester, nylon, HMPE, etc.) are being considered for Floating Offshore Wind Turbines mooring lines, as they can reduce mooring weight and cost. Measuring the strain of these synthetic mooring lines could help to understand their behavior and long term monitoring of these systems could lead to their design and maintenance optimization. A new strain sensor for synthetic mooring lines is proposed in this paper. It uses an ultrasonic altimeter to measure the distance between the transducer and a target fixed on a synthetic mooring line. This instrument is non-intrusive and can be fixed on anchor lines after their installation at sea. The measurement principle and prototype design are presented. Then, the validation of the concept in a water tank is described, demonstrating the feasibility of measuring length using acoustic waves with a high degree of accuracy. The acoustic strain sensor was deployed in operational condition on a demonstrator buoy mooring line in the Mediterranean sea, together with a wire displacement sensor and a load sensor. The measurements from these three instruments are compared, showing the ability of the acoustic sensor to monitor the behavior of a mooring line at sea. • First presentation of a non-intrusive, autonomous, acoustic strain sensor using an ultrasonic altimeter. • Field validation of the acoustic strain sensor by comparison with wire transducer and load cell measurements. • Field assessment of the dynamical stiffness of a demonstrator buoy nylon mooring line. • Comparison of the field measurement with testing bench results.