Abstract
Synthetic mooring lines are becoming a popular alternative to conventional chain mooring systems. For marine renewable energy devices, they have been considered as an enabling technology for this nascent sector, given their reduced costs and ease of deployment. However, the extreme operating environment has led to an increased interest in the ‘in-situ’ condition monitoring of these mooring lines. This paper considers the use of polymer fibre optic technology and the optical time domain reflectometry (OTDR) technique for the condition monitoring of synthetic mooring lines. To establish the operating envelope of the fibres, Polymethylmethacrylate (PMMA) polymer optical fibres are mechanically tested. Additionally, an OTDR is used to monitor fibres whilst under elongation using a tensile test machine, and the sensitivity of the system in monitoring strain is established. At the lowest strain rate, the average proportional limit and yield points of the fibres are found at 1.16% strain and 5.41% strain, respectively. Fatigue exposure of fibres up to 1.25% strain identifies no measurable effect on fibres’ proportional limit or yield point. The occurrence of significant creep is identified for fibres strained beyond 1.5%. The OTDR system is able to identify strains at and above 4%. The study identifies important criteria that should be considered in the integration of polymer optical fibres for mooring applications. Limitations are discussed and suggestions for progressing this technology are provided.
Highlights
The existing non-destructive testing (NDT) methods employed for wire ropes or chain moorings [5] cannot be utilised with fibre ropes and, given the challenging and highly dynamic environment of many marine renewable energy devices (MREs) installations, this has led to an increased interest in novel ‘in-situ’ condition monitoring approaches for mooring ropes
These studies have established the use of the optical time domain reflectometry (OTDR) technique to detect a strained section of the polymer optical fibres (POFs) through a change in the backscatter of the optical signal
Despite demonstrating the principle of strain measurement using the OTDR technique, there has been limited research to date comparing the mechanical operating envelope of PMMA fibres with the sensitivity of the OTDR strain measurement technique and the operational requirements of a synthetic mooring system
Summary
For the bourgeoning marine renewable energy sector, driving down costs and improving reliability are key to establishing a successful market. Whilst the use of synthetic mooring systems is being promoted in the oil and gas sector, the requirements for marine renewable energy devices (MREs) are notably different. The existing non-destructive testing (NDT) methods employed for wire ropes or chain moorings [5] cannot be utilised with fibre ropes and, given the challenging and highly dynamic environment of many MRE installations, this has led to an increased interest in novel ‘in-situ’ condition monitoring approaches for mooring ropes. Some early pioneers of condition monitoring in ropes, [6], set out key requirements that such a condition monitoring system should attain. These include the capability to measure strain up to 10% at a 0.1% resolution and the ability to be integrated into a rope over 300 m long. A review of potential methods investigated for this monitoring application is detailed in [7] and includes magnetic techniques, electrical conductive methods, computed tomography, thermography, acoustic emission monitoring and fibre optics—the focus of the work presented here
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