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

Den elektriske lederen i høyspente sjøkabler består vanligvis av rent kopper eller aluminum. Aluminium blir i dag mer og mer populær på grunn av lav tetthet og pris sammenlignet med kopper. I praksis betyr det at lederen kan lages opp til 50 % billigere og lettere for samme elektrisk ledningsevne. På den andre siden er aluminium av elektrisk grad vesentlig svakere enn kopper og avhengig av stålarmering til å bære kabelvekten under installasjon, selv på grunt vanndyp. Innenfor NASCAR prosjektet har Nexans utviklet en aluminiumslegering- samt fabrikasjonsmetode i lys av elektriske ledere for sjøkabel, som kan mangedoble styrken uten å påvirke ledningsevnen. Innen prosjektet viser tester og modelleringsarbeid at installasjon av sjøkabler til- og forbi 3,000 m er mulig. Nexans forsøker nå å bygge på dette for å muliggjøre ny kabeldesign uten stålarmering- også for store havdyp. Dette kan være et kostbesparende bidrag mot (statiske) eksportkabler for flytende vindparker. I tillegg viser det seg at dette materialet har betydelig bedre utmattings- og sigeegenskaper en legeringer- selv med høyere styrke. Det er kritisk da kabelinstallasjon krever midlertidige dynamiske konfigurasjon under deler av levetiden for kabelen- herunder installasjon og skjøting. De samme egenskapene som gjør at kabelen tolerere dette- Dvs. utmatting- og siging, muliggjør også permanente dynamiske kabler med aluminium for å spare kost for fremtidig flytende havvindsparker. Syklisk- og vedvarende last under installasjon og andre operasjoner til havs vil kunne forårsake utmatting og siging av ledematerialet. Å ta høyde for begge fenomenene, samt vekselvirkingen derimellom krever sofistikert modellerings- og eksperimentelt arbeid. Sintef og Nexans har i denne konteksten utviklet metodikk for å predikere når en lastbærende leder vil feile på grunn av utmatting og siging for en rekke kabeldesign, applikasjoner og miljø. For å komme hit, har prosjektet benyttet avanserte eksperimentelle teknikker, inkludert nano-mekanisk testing. Resultatene har blitt validert under full-skala testing for å sikre at foreslåtte design og anvendelser kan realiseres.

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.