© Omnisens 2013 Omnisens, Morges, Switzerland. www.omnisens.com -1-

Condition Monitoring and Optimization of Subsea Energy Cables Using Optical Fiber Sensing.

Jane Rowsell, Marc Niklès, Etienne Rochat Omnisens SA, Riond-Bosson 3, CH-1110 Morges, Switzerland

1. Introduction  To provide an uninterrupted electricity supply to a growing market is the challenge Power Engineers face. All to be done at minimal financial and environmental cost, while obeying the regulatory requirements of this dynamic industry, where the life expectancy of an asset is usually in excess of 30 years. HV, EHV and UHV cables are costly, and, when buried on land, installed subsea or in tunnels, no routine visual inspection is possible. So dramatic are the effects of a failure of these assets that many governments have introduced tough penalties for a power failure or reduction. How then do you know how this valuable asset is behaving over time and in relation to changes in its operation and environment? The answer is: by monitoring its temperature continuously.

The temperature of a power cable provides at a minimum condition monitoring information, but when the emphasis is on asset performance, temperature monitoring shows how the cable is responding to load and, using a dynamic cable rating system, allows the load to be managed according to the actual temperature of the cable.

Since 1997 the energy industry has recognized the benefits of distributed temperature sensing (DTS) using optical fiber, which is ideal for buried cables, including those ffshore, because:

• long distances can be measured along the entire cable length • fiber sensor is resistant to humidity and corrosion and insensitive to electromagnetic

interference (EMI) • telecommunications fibers nearby or incorporated in the cable can be employed as the

sensor • sensing fibers have a life expectancy similar to energy cables • sensing fiber placed close to the energy cable monitors the thermal resistivity of the

backfill • temperature data supplies real time input for a dynamic cable rating system, enabling

the cable operator to optimize the cable’s operation.

2. Monitoring  subsea  energy  cables    ‘Out of sight’ is definitely not ‘out of mind’ where cables are concerned. These vital assets carry energy around the world, and although ‘concealed’ below ground or water, they are affected by load variation and changes in their environment. When installed subsea, cables risk becoming exposed or being covered with mud/sand and they are subject to threats such as dropped objects, anchor drag, etc. which may damage the cables and dramatically change their performance.

© Omnisens 2013 Omnisens, Morges, Switzerland. www.omnisens.com -2-

When monitoring the temperature of energy cables using DTS, the fiber sensing cable is either integrated within the energy cable, attached to it, or placed close by. A nearby communications cable can also be employed. The fiber optic sensing cable monitors temperature, and so requires a strain-free fiber in the cable, usually in a stainless steel tube.

Fiber optic sensing cable integrated into power cable

3. Buried  cables  The fiber optic sensing cable can either be integrated into the power cable or installed close to the power cable.

An integrated fiber optic sensing cable is required for monitoring temperature increases due to load when a dynamic cable rating system is considered, whereas an external one monitors the ground ambient gives vital information for an understanding of the cable environment.

4. Submarine  cables-­windfarms    One result of the quest for affordable renewable energy is the construction of onshore and offshore windfarms. Many offshore windfarms are constructed at significant distances from the onshore substation, requiring long export cables. Tides, storms, moving sea and river beds, sea temperature changes, fishing, shipping and submarine activities all present challenges for the cable operator. Existing pipelines, cables and other installations can also affect cable temperatures in unexpected ways. The energy produced by the wind farm can vary dramatically, so monitoring the temperature of the cable, all along its length, as accurately as possible is the best way to monitor and therefore manage the cable’s operation. As soon as an unplanned change in the temperature is logged (a range of alerts can be used for this) action can be taken; the load can be adjusted and/or that part of the cable can be investigated.

Due to distance limitations of earlier fiber optic temperature monitoring techniques many submarine cables are only monitored at the shore ends, or with limited accuracy. Shore ends are vulnerable (e.g. from direct sunlight and tidal seabed migration), so monitoring

them is useful.

© Omnisens 2013 Omnisens, Morges, Switzerland. www.omnisens.com -3-

But the risks to the cable are not limited to shore ends. In fact many export cable routes lie in shallow water (less than 100 m), and in areas of dramatic soil migration. A cable may be surrounded by cooling water one day and covered with several meters of mud the next, which will dramatically increase its temperature under the same load. In shallow water areas, the risk of the cable being damaged by fishing and shipping activities is also significantly higher. Only by monitoring the entire cable length can such changes in the cable’s environment and condition be managed. Distances of over 65 km can be monitored from a single Omnisens DITEST (depending on optical loss in the fiber), with accurate temperature information available every meter (depending on the precision and speed of measurement required) along the whole cable length.

Balancing cost with performance, while ensuring a secure supply of electricity is another challenge and here the Omnisens DITEST ability to monitor long distances can reduce the total cost of monitoring. How? To reduce operating costs, many wind farm operators prefer to operate off-shore substations remotely, housing all ‘active’ operations in the on-shore substation. The power of the Omnisens DITEST to measure over 65 km from one interrogator unit using several measurement channels for each of several cables gives operators the flexibility to achieve this. It also ensures flexibility to add cables and distance as the wind farm expands.

Monitoring with this accuracy along the length of the cable, with the flexibility to house the Omnisens DITEST onshore offers flexibility, cost effectiveness and greater peace of mind

5. Field  results  In autumn 2012 the following data was obtained during a survey carried out on a wind farm export cable in the North Sea, installed about 12 months earlier in shallow water. Using a DITEST interrogator, temperature measurements were made using the optical fibers (FIMT- fiber loose in a metal tube) integrated within the power cable. During a 14 day period and at different levels of generated energy (Power 1-4) the temperature profile along the power cable was recorded. The temperature profile at different power outputs along a 6 km section of the cable was consistent, as shown in Figure 1 below. An unusual peak in the temperature was seen at 2 500 m, and a trough at 4 750 m from the measurement source. These were correlated to the burial depth and to the profile of the seabed, showing a direct relationship.

The peak (2 500) coincided with a dramatic reduction in the burial depth of the cable, from its normal depth of 2 m to almost zero. In this almost ‘unburied’ state the cable temperature is significantly higher than elsewhere along the cable route.

The seawater surrounding this wind farm is often only a few meters deep. At 4 750 m the depth drops suddenly from 4 m to 8 m. The effect of this on the temperature of the cable over a distance of about 100 m is clear. The deeper water offers a cooler environment for the power cable.

© Omnisens 2013 Omnisens, Morges, Switzerland. www.omnisens.com -4-

Figure 1: the effects of cable exposure and water depth on wind farm export cable temperature, (offshore North Sea).

6. Interconnectors  Interconnectors are highly engineered, prized assets, frequently crossing national borders, and often critical to international electricity trading. Built to withstand the test of time and their environment, they suffer the same challenges as the submarine cables deployed for windfarms.

The Omnisens DITEST provides uninterrupted temperature monitoring for these long cables, and there are a wealth of options to ensure that the needs of each project are met.

7. Detecting  strain  in  power  cables  Subsea cables may be subjected to strain (e.g.: during installation, when brought to the surface for repair, dragged, or when a span is exposed and unsupported. This strain may be detected in the integrated optical fibers within the power cable.

Omnisens DITEST has been used to investigate strain in subsea power cables. Unlike an optical fiber sensor used for monitoring temperature, a tight buffered optical fiber, with no over-length would ideally be integrated in the power cable to detect strain. However, useful results detecting strain have been obtained by Omnisens specialists using the standard Fiber in a Metal Tube (FIMT) integrated into the power cable for communications and temperature monitoring. These results were used by the operator as a basis for further investigation of the affected section of power cable.

Promising results were obtained from trials to detect bending strain and elongation in long power umbilicals for use in offshore Oil and Gas production.

© Omnisens 2013 Omnisens, Morges, Switzerland. www.omnisens.com -5-

8. Conclusions  As windfarms become larger the boundaries for what is known about the long term condition of the assets are exceeded.

“Export cable and HVDC technologies represent a lot of risk for the currently proposed projects over 500 MW. There is little or no track record for the submarine cable and converter equipment although many major manufacturers are focusing on bringing these technologies to market.” (Brook Knodel, Electrical Group Manager, Mott MacDonald LLC, 2010).

Expanding existing wind farms usually requires the installation of further export cables. The Omnisens system can easily be extended to ensure further cables can be monitored. Where the cables are laid in proximity to each other, the resulting thermal effects can thus be monitored.

Fiber optic distributed sensing of power cable provides continuous temperature monitoring along a power cable and is widely regarded as a ‘must-have’ condition monitor. The stimulated Brillouin fiber optic technology found in the Omnisens DITEST system provides monitoring of up to 65 km from a single interrogator. With an optical budget of 14 dB the system can monitor using sensing fibers which are lossy or have a large number of splices.

Performance parameters available with Omnisens DITEST range.

© Omnisens 2013 Omnisens, Morges, Switzerland. www.omnisens.com -6-

The benefits of using Omnisens systems on wind farm export cables and interconnectors can be summarised as follows:

• Provide continuous temperature monitoring along the length of the cable, up to 65 km from a single interrogator, giving the operator flexibility as to where to locate the interrogator.

• Provide cable load optimization via a software which continuously compares operating temperature with load.

• Detect the development of hot spots which are signs of abnormal cable operation before these cause possible failure.

• Detect and alarm should the cable become exposed thus no longer correctly protected, which may result in lower temperature due to external perturbations.

Further  reading:  

Malcolm Sharples, P.E.,”Offshore Electrical Cable Burial for Wind Farms: State of the Art, Standards and Guidance & Acceptable Burial Depths, Separation Distances and Sand Wave Effect.” prepared for Bureau of Ocean Energy Management, Regulation & Enforcement - Department of the Interior, November 2011

Scottish Enterprise Insight “Offshore Wind Power: Priorities for Research and Development (R&D) and Innovation, www.scottish-enterprise.com

Brook Knodel, “A High-Level Overview of Offshore Wind Project Development Identifying Current Technologies, Challenges, Risks and Costs. Offshore Wind.” IEEE Boston PES- November 16 2010.

Thomas March, Ralf Wasserthal “Assessment of the Environmental Impacts of Cables” OSPAR 2009 www.ospar.org

G. Gerdes, A. Tiedemann, S. Zeelenberg “Case Study: European Offshore Wind Farms- A Survey for the Analysis of the Experiences and Lessons Learnt by the Developers of Offshore Wind Farms“ Deutsche WindGuard GmbH, Deutsche Energie-Agentur GmbH (dena) University of Groningen www.offshore-power.net, 2007.

C. Zucco, W. Wende, T. Merck, I. Köchling and J. Köppel, “Ecological Research on Offshore Wind Farms: International Exchange of Experiences PART B: Literature Review of Ecological Impacts” BfN-Skripten 186 2006.

D. Dutoit, M. Nikles, E. Rochat, P. Willemoes, H. Little “Distributed Fiber Optic Strain and Temperature Sensor for Subsea Umbilical” Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering Conference, Rhodes, Greece, June 17–22, 2012.

A Garrow, S Myren, “Condition and Performance Monitoring of the 32 KM Guangdong Hainan Island Subsea Interconnector, China, using Distributed Temperature Sensing and Dynamic Rating System” CEPSI 2012, 19th Conference on Electric Power Supply, Bali 15-19 October 2012.

G. J. Anders, “Rating of Electric Power Cables in Unfavorable Thermal Environment”, IEEE Press Wiley Interscience, Piscataway, NJ, 2005.

DITEST Energy Cable Monitoring Solution (Omnisens Application Note AN003). www.omnisens.com

Subsea Cable Monitoring: The Walney Offshore Wind Farm (Omnisens Case Study CS-004).

Monitoring the temperature of 154 km Export Cables for Greater Gabbard Offshore Wind Farm, UK (Omnisens Case Study CS-007).

Power Umbilical Strain and Temperature Monitoring (Omnisens Case Study CS-013).