Early Detection of External Damage Incidents in Long-Distance Direct Current Submarine Cables

Early detection of such incidents is critical— for rapid restoration to minimize disruptions and losses, and for predictive alerts to prevent accidents and identify root causes quickly.

Traditionally, fault location after an incident relied on electrical methods such as the Murray loop. Cause identification often involved matching the damage location with Automatic

Identification System (AIS) data—a maritime system that broadcasts vessel position and navigation information. However, electrical methods are not continuous monitoring techniques.

If a fault appears long after the external impact, identifying the responsible vessel becomes difficult, and if AIS was disabled, it is impossible.

Fiber-optic sensing technologies such as Optical Time Domain Reflectometry (OTDR) and

Distributed Acoustic Sensing (DAS) have enabled continuous monitoring of cable conditions.

However, conventional DAS systems are limited to approximately 100 km from one end or 200 km from both ends. Although proximity optical amplifiers, such as those used in telecom systems, could theoretically be integrated to extend the sensing range, their deployment in subsea power cable infrastructures introduces significant complexity. Besides electromagnetic and thermal constraints, complex mechanical, electrical and installation requirements introduce additional risks. To address these challenges, we adopted Remote Optically Pumped Amplifiers

(ROPA) using erbium-doped fiber (EDF), enabling amplification without the need for an electrical power supply at the amplification points.

This paper reports laboratory validation of DAS range extension using ROPA and field trials on a submarine power cable with integrated optical fibers. Offshore tests included simulated anchor impacts using a weight and vessel movement detection, while onshore tests evaluated bottom-trawl gear approach and anchor-drop scenarios. Detection capability and location accuracy were assessed.

Laboratory tests achieved single-ended sensing over 270 km, with potential extension to 540 km using dual-end monitoring. Offshore tests demonstrated reliable detection of a simulated anchor impact using a 40 kg weight, with a positional deviation of only 68.4 m at 262.2 km— significantly better than conventional electrical fault detection methods such as the Murray loop, which typically localize faults within about 1% of cable length (e.g., 5 km for a 500 km cable).

Vessel movement detection was also successful for relatively small vessels of 19 GT and 14

GT. Frequency-domain analysis of DAS signals indicated the potential for vessel classification based on acoustic signatures.

Onshore tests confirmed DAS detection of a 600 kg bottom-trawl gear and a 200 kg anchor, both smaller than typical commercial equipment. DAS detected gear approach approximately 15 seconds before contact and continued monitoring even when optical fibers remained undamaged during anchor-drop scenarios.

Overall, DAS combined with ROPA enables ultra-long-distance continuous monitoring of the entire cable length and provides high sensitivity for detecting external mechanical impacts, supporting early damage detection.