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ENERGIX-Stort program energi

New High Strength- High Conductivity Aluminium Conductor Concept in Subsea Cables for Sustainable Energy Transport

Alternative title: Aluminum med Høy Styrke og -Lednignseven for Nye Leder Konsepter i Sjøkabelkabler for Bærekraftig Energi Transport

Awarded: NOK 8.0 mill.

Project Number:

296508

Project Period:

2019 - 2022

Funding received from:

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. Within the NASCAR project, Nexans has developed an new aluminium alloy- and fabrication method considering the final conductor, which can massively elevate the conductor strength without significantly impacting its conductivity. Project results- including experimental testing and modelling demonstrate that installation of subsea cables to- and beyond 3,000 m is possible. Nexans now attempt to build in this to enable lean cable designs where the conventional steel armoring is omitted- even for larger water depths. This can be an particularly efficient contribution towards cost reduction of cables for floating wind. The project show that the developed material has significantly improved fatigue- and creep strength compared to legacy alloys- even types that is stronger considering monotonic tensile tests. This is critical as even temporary dynamic riser configurations during installation and offshore operations demands severe cyclic loading under high axial loads. The same features that allow the cable to cope with this- that is: Fatigue and creep resistance, also allows deployment of this material for permanently dynamic cables for floating wind. Sustained and cyclic loading during installation and other offshore operations are 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 has in the NASCAR project developed the methodology required to predict fatigue- creep failure over a wide range of cable designs, application and environment. The project has utilized 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. These results was coupled with full-scale validation to ensure that the proposed designs and applications can be realized.

Reduction in material and production costs of subsea power cables: The developed project results are well inline/ exceeds the initial expectations considering the material performance. Ongoing and planned cable qualification work using the developed materials are now embedded in Nexans technology roadmap and qualification of such products is expected within the next years. This will realistically allow cost- and GHG emission reduction by 15- 20 %. New application for subsea cables (e.g. larger installation depths and wider spectrum of installation conditions): Nexans currently qualify deep water installation beyond 3,000 m. Nexans expect this to be extended to 4,000 m water depth using the developed materials. Furthermore; Nexans currently pursue qualification of dynamic cables for floating wind using the developed material. Developed methods for more precise prediction of lifetime and conditions for safe and optimal use and installation of power cable: NASCAR has demonstrated that fatigue of conductors are inherently complicated and entails features that is not captured in prior-art. i.e. fatigue life estimations using such data is inherently inaccurate and has little predictive power for dynamic cables in particular. This is particular critical for dynamic cables in an floating wind context. Furthermore; installation analysis of lean cable designs where the mechanical role of the conductor is maximized is entirely inaccurate using earlier data available in the public domain. The developed fatigue- and material models where both the inherent complexity of conductors, as well as cable environment is appropriately accounted for, in now an invaluable tool going forward.

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.

Funding scheme:

ENERGIX-Stort program energi