Cable ties

Around the world, there are powerful connections between countries that you wouldn’t know were there. Shallowly buried under the seabed are thousands of kilometres of high voltage cables, transferring power between countries and bringing power from offshore windfarms onto land.

As the requirement for renewable energy grows, so does the role – and cost – of these copper and aluminium cables.

A multidisciplinary team of researchers from Ocean and Earth Science (OES), Electronics and Computer Science (ECS), and Civil and Environmental Engineering (CEE) is running a series of interlinked projects and consultancy activity to optimise the design of these Marine High Voltage Cables (MHVCs) in order to maximise power transfer and save money.

Not only this, the project could lead to a much better understanding of how climate change may be affecting ocean bottom temperatures and, in turn, biological and geochemical processes at the seabed – which are critical to the health of the oceans.

Justin Dix, Professor in Marine Geology and Geophysics in OES, explained: “MHVCs primarily operate two ways. Either transferring power between one country and another, for example, hydroelectric power from Norway to Denmark or nuclear power from France to England, or used to bring power from our proliferation of windfarms to land.

“The north western European shelf has the most windfarms in the world, and there is currently a global expansion in offshore wind in places including North East America, the Mediterranean, South East Asia, the South Chinas seas and Australasia. As we explore other sources of renewable energy like tidal power and wave power, again the power will need to be brought to land in the most efficient manner.”

MHVCs for a typical 1-gigawatt (GW) windfarm cost about £400 million to design and install, with operation and maintenance costs of several millions of pounds every year.

“If you can better understand the environment the cables are in, and therefore effectively model how heat dissipates away from the cables, you can optimise cables and reduce costs significantly,” said Justin.

Launching a multidisciplinary project

The projects came about thanks to an event organised by the Southampton Marine and Maritime Institute (SMMI).

Testing the heat dissipation from cables buried in marine sand in a test tank

Expertise in both areas is key. Tom Gernon, Associate Professor in OES and another member of the team, said: “The seabed is variable and dynamic – it’s not cold, wet, stable and made of sand. That’s important because the environment in which a cable is buried dictates how heat is dissipated away from it. The one thing you cannot afford to do is allow a cable to heat up too much, as this can significantly affect power transfer and, in worst case scenarios, actually lead to cable failure.”

Optimising cable design

“The current approach to cable rating is effectively based on decades of research into land-based cables,” said George Callender, Lecturer in ECS. “How the seabed environment would alter conventional rating approaches was poorly understood.”

The research team has established that, in many cases, cable designs can be changed whilst maintaining a healthy margin of safety. This was determined by a combination of numerical models of heat transfer, lab based experiments and latterly analysing temperature data recorded from active cables.

Tim Henstock, Professor of Geophysics within OES, highlighted that the team was the first to establish that cables under the sea floor disperse heat through a combination of convection and conduction, and that the ratio of one to the other is determined by a combination of the permeability and thermal conductivity of the sea bed. “Properly understanding these processes allows us to modify conventional ratings approaches to be more applicable to the marine environment,” he said.

David White, Professor of Infrastructure Geotechnics, added: “We are developing a better understanding of not only how to measure how these parameters, which vary in space and through time, but also how they are altered by the different cable burial processes – via jetting, trenching, ploughing or even cutting with a huge rock-saw.”

In a historical connection, laser technologies (known as Distributed Temperature Sensing) that were developed by the University’s Optoelectronics Research Centre in the ‘80s and ‘90s are now regularly being used to measure temperatures in the cables in real time.

George explained: “These temperature measurements are hugely under-utilised by the cable operators due to the large data volumes and the lack of understanding of the environment. Cost-effective numerical codes, developed by the group to assist real-time cable rating and burial-depth prediction, are now being translated into commercial products. Also, existing datasets are being reanalysed to better inform cable design.”

A new role for the ‘3D Chirp’

Similar historical research has also been repurposed, with the application of 3D Chirp for post-installation cable burial surveys.

3D Chirp is a high-resolution acoustic system that was developed by some members of the group and colleagues from the Institute of Sound and Vibration Research in a project led by Professor Jon Bull, from OES, in the early 2000s. The system is capable of tracking cable routes in three-dimensions and providing critical data input for cable operation.

The system is now operated commercially through a company called Sand Geophysics, set up by a group of OES alumni, and is not only being used for cable detection but also for seeking out unexploded ordnance and other man-made objects ahead of the construction of offshore infrastructure.

All of this work fits with the University’s Coastal and Offshore Archaeological Research Services (a joint OES and Archaeology initiative), which has been providing heritage assessment services for the offshore renewable sector – including extensive cable route surveys – for the last eight years.

Current and future work

The team’s research is now being used by windfarm developers.

It is currently helping to inform the design of SOFIA Windfarm, being built in the central North Sea by RWE. On completion this will be RWE’s largest windfarm, providing 1.4GW to 1.2 million homes. The team’s expertise is also being employed in the construction of Inch Cape Windfarm, which will be one of Scotland’s largest sources of renewable energy producing 1GW for up to a million households.

“We’re working with a large number of windfarm companies who want to use our approach rather than just the standard landbased method,” said Professor Paul Lewin, from ECS. “We’re also working with AP Sensing, a fibre optics company, integrating our ideas into burial-depth prediction software, and most recently we have entered discussion with cable manufacturers such as Nexans.”

Looking to the future, the research is also taking a different turn – to understand temperatures at the bottom of the ocean.

Justin said: “The cables strongly record the seasonal variation in ocean bottom temperatures. We think we can back-calculate ocean bottom temperatures from the catalogue of data we have from the last few decades. Ocean bottom temperature is one of the least studied ocean parameters on the planet.

“Our primary aim is to optimise power transfer for the renewable sector and for interconnectivity between countries. But I also like the idea that it could become part of a global monitoring system on how ocean temperatures are changing, and to have a better understanding of how the ocean works.”

3D Chirp assessing underwater cables at sea

For further information, visit: www.southampton.ac.uk/coars