Monitoring Submarine Power T/M Cable Cond. with Optical Fiber Sensing Tech. for OWPG.: NEC Technical Journal

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Offshore wind power generation is one vital measure helping us work toward achieving carbon neutrality, and the submarine power transmission cables that carry electricity ashore from offshore turbines are an essential aspect. Therefore, constant monitoring of the cables is required to mitigate potential damage through early detection. NEC is engaged in monitoring the state of submarine power transmission cables using optical fiber sensing technology that leverages optical fiber cable already integrated within submarine cables. Leveraging the strengths of optical fiber sensing technology, including long-range sensing capabilities, high spatial resolution, and strong environmental robustness, we enable cost-effective continuous monitoring. This initiative aims to address the challenges associated with proliferating offshore wind power generation and contribute to the realization of carbon neutrality by 2050.

1. Introduction

Decarbonization and hydrogenation of power sources are among the various initiatives being implemented as we strive to achieve carbon neutrality by 2050. In order to decarbonize power sources, it is essential that we use renewable energy (hereafter referred to as RE). Offshore wind power generation is increasingly attracting attention as a valuable resource in the much-anticipated energy mix of RE source. The further dissemination and expansion of offshore wind power generation is needed.

Offshore wind power generation uses submarine power transmission cables buried in the seabed to deliver electricity generated by offshore wind turbines to onshore power grids. The submarine cables face various risks, including damage from vessel anchors, fishing nets, and friction or collisions caused by ocean currents. As a result, regular maintenance of these cables is crucial to ensure a stable electricity supply. Focusing on the optical fiber cables embedded in the submarine power transmission cables used to communicate with and control wind turbines, NEC is leveraging optical fiber sensing technology to monitor the condition of these cables. With our capability to provide continuous monitoring at low cost through our optical fiber sensing technology, we aim to contribute to the prevention of damage to the cables through early detection of potential risks. This paper outlines the challenges in providing a stable electricity supply with offshore wind power generation in section 2. Section 3 discusses monitoring the condition of submarine power transmission cables with optical fiber sensing technology, and section 4 elucidates the expected benefits. Section 5 provides a summary of the discussion.

2. Challenges for Stable Electricity Supply with Offshore Wind Power Generation

Offshore wind power generation facilities (wind farms) are expected to operate for approximately 20 years after construction. However, because they operate in harsh offshore environments, there is a constant risk of facility accidents. Most of these accidents involve submarine power transmission cables laid on the seabed. As shown in Fig. 1, 58% of all accidents overseas involving wind farms are related to submarine power transmission cables. Offshore wind power operators use sonar survey vessels and underwater drones to conduct underwater inspections to prevent these cable accidents and verify the condition of the submarine power transmission cables. However, each individual inspection is costly, and marine conditions dictate when inspections can be carried out limiting the feasible time frame. As a result, the reality is that inspections are only conducted about once a year. However, this can cause delays in detecting potential damage to the submarine power transmission cables, rendering a strong need for continuous monitoring.

Fig. 1 Breakdown of wind farm parts involved in offshore wind power generation accidents overseas.

zoom — Fig. 1 Breakdown of wind farm parts involved in offshore wind power generation accidents overseas.

Broadly speaking, there are two sections of cable in submarine power transmission cable installations for offshore wind power generation. As shown in Fig. 2, there are buried sections and non-buried sections. The following is an explanation of what potential damage each section of cable is subject to and what varies between the sections regarding monitoring requirements.

Fig. 2 An installation diagram of submarine power transmission cables for offshore wind power generation.

Buried sections: Estimation of burial depth
To prevent damage from the anchors of large vessels or nets and other fishing gear, most sections of the submarine power transmission cables are buried several meters below the sea floor. However, currents can erode the seabed making the burial depth shallower with a possibility that the cables will eventually be exposed underwater. In the event of such an occurrence, the cables would then be vulnerable to the aforementioned potential damage. Therefore, it is paramount to monitor the burial depth of submarine power transmission cables.
Non-buried sections (near turbines): Submarine power transmission cable disturbances
It is structurally difficult to bury the submarine power transmission cables in the immediate area around turbines, which means there are exposed sections of the cable in the water. Tidal currents can move the cable around causing friction between the cable and the seabed that can eventually lead to damaging the submarine cables. To mitigate this, submarine power transmission cables are armored with protection systems, and the cables are secured in place with large boulders to prevent displacement. However, strong currents can dislodge the boulders, and the protection systems can be damaged or fall off with disturbances in the water. In such cases, the submarine power transmission cables are subsequently exposed to the potential damage mentioned earlier, which makes it essential to monitor submarine power transmission cables for any disturbances.

3. Monitoring submarine power transmission cable conditions with optical fiber sensing technology

As a solution to the challenges mentioned in section 2, NEC is applying optical fiber sensing technology²⁾ to the optical fiber cables embedded in the submarine power transmission cables used for communication and control of the wind turbines. This technology facilitates continuous monitoring of the submarine power transmission cables, including estimating burial depth and disturbances in non-buried sections.

NEC’s optical fiber sensing technology³⁾ enables low-cost monitoring of submarine power transmission cables with the following two features:

Long distance/high spatial resolution sensing: the sensing technology utilizes the optical signal amplification technology developed through submarine optical cable communications. The optical transmit/receive technology, enhanced for greater sensitivity, enables detection over long distances of tens of kilometers with a high spatial resolution of a few meters. As submarine power transmission cables are increasingly deployed offshore, the long-range coverage provided by a single sensing device becomes an even more significant advantage.
Strong environmental robustness: Using artificial intelligence (AI) to analyze and learn from the data acquired with the optical fiber sensing device enables separating out environmental noise and offers the ability to identify specific events. As a result, it is not always necessary to lay new optical fiber cables exclusively for sensing as the communication optical fiber cables embedded within power transmission submarine cables can also be utilized as sensors.

Fig. 3 illustrates the basic principle of optical fiber sensing technology and its system configuration. To enable an optical fiber cable, originally intended for information transmission, to function as a sensor, a sensing device is connected to one end of the optical fiber cable. A light beam with a short duration is sent from the sensing device through the optical fiber cable, generating a backscattered light from every point along the cable. The sensing device observes this backscattered light in a time series. If environmental changes such as vibration, stress, or temperature fluctuations occur along the optical fiber cable, they alter the structure and characteristic parameters of the quartz glass that constitutes the optical fiber. The quality of the backscattered light, such as its phase state and light intensity, will in turn change depending on the environmental changes that have taken place. The location where the environmental change occurred is identified and calculated based on the time it takes for the round-trip of the input light to the observation of the backscattered light with altered quality. Since the observation locations are separated by time, it is possible to simultaneously detect multiple points along the optical fiber cable. Furthermore, by injecting the light at a fixed repetitive frequency to prevent mixing the backscattered light from the far end of the optical fiber cable with the subsequent input lights, the progression of environmental changes at the same location can be observed.

Fig. 3 The fundamental principle and system configuration of optical fiber sensing.

Analyzing data, such as the burial depth of power transmission submarine cables and the disturbances in non-buried sections obtained through optical fiber sensing technology, requires a position calibration process, involving preprocessing to align real-world positions with those on the optical fiber cable. An illustration of the position calibration process is shown in Fig. 4. This calibration process is necessary because the optical fiber cable embedded within the submarine power transmission cable is not originally laid for sensing purposes, leading to positional discrepancies between the real-world positions and positions on the optical fiber cable due to factors such as slack in the optical fiber cable. To address these positional differences, multiple wind turbines demonstrating characteristic vibration patterns are used as reference points, and by scaling the positional differences between each turbine, positions on the optical fiber cable are calibrated to reflect their real-world positions.

Fig. 4 Image of the position calibration process.

Estimates of burial depth and disturbances in non-buried sections of submarine power transmission cables focus primarily on uniform external forces, such as waves, exerted upon the submarine cable. The analysis focuses on the changes in the wave vibrations observed by the optical fiber sensing devices. Fig. 5 illustrates the flow of the analysis process. First, low-frequency signals attributable to waves are extracted from the data obtained by the sensing device. Then, based on extensive vibration information along the distance of the optical fiber cable, the temporal variations at individual points and the similarities among observation points along the cable are calculated. This allows the estimation of burial depth from subtle differences in the transmission of wave-induced vibrations and the estimation of disturbances from the temporal changes in the vibrations of the unburied sections of the submarine power transmission cables beneath the wind turbines.

Fig. 5 Illustration of the flow of the analysis process.

4. Expected benefits

Constantly monitoring the state of submarine power transmission cables with optical fiber sensing technology enables the early detection and handling of potential damage to submarine power transmission cables. We can therefore anticipate being able to effectively prevent and avoid damage to these cables. Offshore wind power operators are forced to shoulder considerable losses when submarine power transmission cables are damaged which can amount to billions of yen in lost sales opportunities due to prolonged operational downtime coupled with the costs associated with repairing the cables, which means that preventing such damage before it happens will yield substantial benefits. Furthermore, monitoring the state of submarine power transmission cables would allow for optimizing the frequency and scope of underwater inspections performed by the sonar survey vessels and underwater drones mentioned earlier. Currently, submarine power transmission cable conditions are not comprehensively monitored, meaning regular inspections of the entire length of the cable are needed. However, an accurate awareness of the current condition of the cables would allow for optimal decision-making regarding the scope of inspections and their frequency based on need, thus potentially reducing underwater inspection costs.

5. Conclusion

In this paper, we have discussed the issues surrounding stable power supply in relation to offshore wind power generation and the effort to monitor the condition of submarine power transmission cables using NEC’s optical fiber sensing technology. After verifying the technology through experiments at their facilities, NEC is now verifying its monitoring feasibility through on-site testing at multiple domestic offshore wind power plants. Currently, the primary focus is on monitoring vibration, but in the future, the target is to improve monitoring accuracy by incorporating multimodal analysis including temperature. In the long term, NEC aims not only to monitor the condition of submarine power transmission cables but also to estimate wave conditions in the area around the cables and to detect vessel navigation routes to offer expanded services such as support for the safe navigation of inspection vessels and the detection of unidentified vessels.

NEC is committed to solving the challenges in proliferating offshore wind power generation by leveraging submarine power transmission cable condition monitoring and contributing to achieving carbon neutrality by 2050.