Subsea power cables use high conductivity materials such as copper- or aluminium alloys to transfer electricity. The most popular approach involves high purity copper.
Aluminium becomes increasingly popular due to its low cost and density. This means that the subsea conductor can become up to 50 % cheaper and 50 % lighter for the same conductor resistance. The strength of electrical grade aluminium is on the other side low and the cable is fully dependent on high strength steel armoring for installation in even shallow waters.
Increasing the mechanical load on the conductor will make sustained and cyclic loading during installation and other offshore operations challenging. Cyclic loading and sustained load cause the material to fatigue and creep. Accounting for both of them- and the interaction between them requires sophisticated models and experimental background. Sintef and Nexans will develop the methodology required to predict fatigue- creep failure over a wide range of cable designs, application and environment. The project will utilize state-of-the art experimental techniques, including nano-mechanical testing, in order to characterize how the material is damaged when subjected to cyclic and sustained load. The results will be coupled with full-scale validation and in turn ensure that proposed designs and applications can be realized. Results thus far show that one of the developed alloys is particularly suitable when takin into account cyclic and sustain loads, and can provide Nexans an unique competitive edge.
The available results exceed expectations: This is specifically due to the elevated creep resistance. By co-development of the necessary supply-chain, use of this alloy rapidly becomes feasible. Today this material is an key enabler in ongoing tendering processes towards deep water interconnectors to reinforce the intercontinental grid. Corresponding full-scale testing is ongoing. The NASCAR project has also improved jointing technology which is the limiting factor for designing and installation cheaper subsea cables.
Safe, robust, cheap and efficient transmission of electrical energy through subsea power cables will be of increasing importance in the future energy market; offshore wind energy and trans-national energy transport are examples that require this technology.
While Copper (Cu) is the most commonly used conductor material in subsea power cables, Aluminium (Al) has been used for more than 30 years and is today used as conductor in the deepest existing subsea power cable in the world. Use of Al as conductor offers several great advantages over Cu, such as 40% lower weight and 30-40% lower price for a given electrical conduction capability, and – for some Al-alloys – higher strength. The conductivity and strength of aluminium alloys usually offset each other as strengthening elements tend to also increase the electrical resistivity. Nexans has over the past years developed novel compositions and thermo-mechanical treatments to improve the conductivity of high strength Al without compromising the material strength. This opens up the possibility to re-think and innovate new designs of subsea power cables. Through further understanding and methods for exploiting the mechanical properties of these Al-alloys, NASCAR will result in important steps towards the design of more robust, lighter and cheaper subsea power cables, and thereby extending the range of subsea power cables to larger depths and challenging dynamic loading situations.
The primary research objectives of NASCAR revolves around the technology- and knowledge gaps related to fatigue and creep of Al and in particular High strength- High conductivity Al conductors. Nexans Norway and Sintef Industri will together investigate these concepts by means of advanced nano- and macro scale characterization, modelling and full scale testing in order to predict the limitations due to fatigue and creep. The results are expected to provide the technology basis for groundbreaking new subsea power cable concepts.