The number of installed subsea electrical transmission cables is increasing rapidly. The growth in installed transmission capacity is driven by changes in the electricity supply, including the building of large offshore wind farms and also of interconnects used to balance electricity demand and supply across national boundaries, in particular under the North Sea.
Subsea cables face a number of hazards, such as trenched sections being uncovered by undersea currents, damage from anchors and fishing gear, in addition to the failures that also exist in onshore cables. Moreover, finding the cable for repairs or to avoid damage when other infrastructure is installed can be challenging because tides and currents sometimes cause the cable to shift significantly away from the position at which it was installed.
Subsea electrical cables are increasingly fitted with optical fibres either for telemetry and control of the offshore wind farm or, in interconnectors, to carry additional telecommunications traffic. The opportunity therefore exists for using spare fibres or spare wavelength-division multiplexing channels for distributed measurements.
In this context, distributed vibration measurements can locate a cable by detecting sounds emitted by a source boat navigating in the region where the cable is expected to be. For systems that are fitted with permanently installed distributed vibration sensing and real-time event detection and analysis, third-party damage such as dragging anchors, fishing gear or possibly deliberate damage can also be detected immediately, thereby providing an opportunity to attribute the damage to vessels in the vicinity appropriately.
Wind and Ice Loading in Overhead Cables
Since the early 1980s, overhead power lines have increasingly included optical fibres integrated in the ground wire (optical ground wires, known as OPGWs). The fibres are usually of the single-mode type; they were primarily installed for telemetry and, with deregulation of the sector, as a telecommunications carrier. However, spare fibres have been used for sensing, mainly for temperature but also for vibration.
One benefit of vibration data in overhead lines is that these provide an understanding of the effects of wind, including resonant effects (sometimes known as the ‘galloping wave’) that are potentially destructive.