Imagine that the wires to your house not only have to withstand high electrical current flow, weather and wind, but also salt water, ocean currents, temperature changes and large movements. This is the big challenge in connecting large, electrical structures at sea to the power grid.

Dynamic underwater cables are the solution for this challenging task. They are large, robust and flexible cables that have to be able to withstand the forces found in the ocean and in structures like floating solar power plants, offshore fish cages, oil and gas platforms and offshore wind turbines.

The dynamic cables act as an umbilical cord to the static underwater cables that carry electricity to or from shore.

“They’re a bit like the power cables we have for all the electrical gadgets that we connect via a plug to the fixed cables in the wall,” says Naiquan Ye, a SINTEF research manager.

He is working to ensure the robustness of the dynamic cables, which has a major impact on the cost of many projects.

“According to the current plans, Europe will need 6000 km of underwater power cables annually. That’s as far as the distance from Norway to Bermuda,” says the researcher.

It is rare for us to have cable breaks in the wires in our walls – but how often have we had to toss a charging cable or replace an extension cord because it was coiled or mishandled a few too many times?

According to Lloyd Warwick, a company that specializes in claim settlements for the insurance industry, 83 percent of offshore wind insurance claims are due to cable faults. The cables become vulnerable when they heat up due to the current flowing through them and are moved by ocean currents, waves, and the constantly shifting distance between the floating structure and the stationary seabed.

The cables are also multi-layered to ensure reliable electricity transmission. These layers need to be waterproof, carry control signals, be unaffected by magnetism, not leak electricity, and also withstand the stresses of constant movement in both ice-cold and warmer seawater.

• Dynamic cables are mobile and carry energy, and often information as well, between a floating installation and the static submarine cable.
• Static submarine power cables are cables that lie stationary on the seabed and carry energy between installations at sea and on land, or between countries. These are not exposed to the same stresses as dynamic cables.
• Communication cables are like static submarine power cables, but contain fibre optics and other information technology.

Lowering costs

So it is not so surprising that these cables are expensive to produce. Demand for them has been low until now, with the main purchaser being the oil and gas industry – which has had the budget to pay a little extra for safety.

This is changing with the transition to renewable energy.

Budgets are usually tighter here, and unlike an oil platform where one cable suffices, an offshore wind farm needs a dynamic, underwater cable for every single wind turbine.

The design and production must be optimized in terms of cost, but also so that the cables last as long as possible.

This is where Ye’s team comes in. Through many years of simulating and testing cables, the group has learned a lot about how the different components of these cables behave and how they handle internal and external stresses.

“Since the 1980s, SINTEF researchers have developed advanced models to simulate the properties of cables in complex marine environments. These numerical tools are world-leading, and the industry uses them to ensure safe and sustainable production of ocean-based energy, both in the oil and gas and offshore wind industries,” says Ye.

The biggest threat to a cable’s lifespan is fatigue. Simply put, the materials wear out. As current flows through the cable, it behaves a bit like a garden hose when you turn on the water – it starts to move and bend.

In a water hose this is not a problem because the water flows through easily, but in an electric cable you have multiple metal wires – just like the cross-section of a charging cable at home. If you twist and turn them too much, they will eventually crack and then break.

Then there are the insulating materials, like a data cable that transmits control signals, and the cable itself. All of them consist of varied materials that can withstand different amounts of movement over time.

On top of all this, each of the materials will move differently. Just as the rubber or plastic around the outside of a regular household cable has a completely different mobility if you remove what is inside.

“However, the properties and movements of cables are much more complicated than numerical models can predict. Impurities in the material, production, installation and the environment they are in, like temperature, can affect the lifespan of cables,” says Ye.

Laboratory testing is a critical method for determining the effects of these influences. In the design laboratory, we monitor how the cables behave in real life, and the results are fed back into the numerical tools to find more precise methods for estimating the service life of the cables.

“If lab testing is done early in an offshore wind project, it can significantly reduce the cost of the power cables while optimizing the design,” Ye says.

By twisting and turning the cable in a test rig, researchers can see how the different components of the cable move relative to each other. These components can often move inside the cable when they are in operation. The movements are usually non-linear due to friction between the components – that is, they are not directly proportional to the movements around the cable. That is why it is so difficult to calculate how a cable and its contents move.

Testing full-scale cables makes it possible to see how much the cable can withstand, how strong it is, and what will ultimately be the weak point that causes a cable to break.

• At SINTEF’s Design Laboratory, which is part of the new Norwegian Ocean Technology Centre, cables are tested for an average lifespan of 30 years. Structures, structural components and materials are tested here.
• Typical issues include fatigue testing, tensile strength and fracture testing. The experimental work is often combined with analytical or numerical analysis.
• The cables are also tested in the Ocean Laboratory to see how they behave in water. The Norwegian Ocean Technology Centre will consist of laboratories at Tyholt in Trondheim combined with installations in the Trondheimsfjord, in the sea off the islands of Hitra and Frøya, and Ålesund municipality, and will play a central role in testing future offshore installations.

It’s one thing when you work with cables on dry land, but what happens when you put the cable in water and add to that the natural movements of the water above it?

These are not forces that fit into a simple formula. Currents in the water are variable at different depths, locations and temperature, and wind and waves affect everything – including how the wind turbine moves and thus moves the cable. The cables may also differ in design and material choice.

By pulling up to 30 metres of cable through an ocean basin, we can mimic many of these forces and collect data. This data is compared with the data from the tests on land and goes into computer models that are becoming more and more accurate.

Although each cable and each ocean area is unique, it is still possible to create simulation tools to improve the design process of cables before they are tested. Every time cables are tested, data is generated, which is fed back into the tools to constantly improve them.

The goal is for the cable design to be done so well from the start that the cables pass all tests with flying colours, without being oversized or unnecessarily expensive.