REFACE - Next generation damage based fatigue design of cable sheathing

Nexans and SINTEF have studied how cyclic mechanical loads impact subsea cables with particular emphasis on the water barrier of the cable: A metal sheath which excludes water penetrating into the electrical insulation of the cable. The insulation must be kept dry for cables used at voltage levels above 66 kV. Such cables therefore use a metallic sheath around the insulation. If the sheath cracks, the system can fail. Conventional cable predominantly uses an extruded lead alloy sheath as water barrier. Its advantage is high formability which facilitates extrusion, cable manufacturing and installation. Its disadvantage is low resistance to fatigue fracture; Cracks which initiate and grow throughout the life time of the cable. The understanding and analytical data on fatigue of the water barrier have been limited. Available predictive techniques and associated test methods have previously not been able to accurately predict the fatigue life of the component in a full- scale cable cross section. This have been made increasingly clear in the context of REFACE test results which clearly contradicts pre-existing literature on the topic. The REFACE project have developed new methodology for calculation of fatigue in ductile materials in full-scale multi- material components. Experimental and analytical work have been directed against optimization of lab-scale specimen geometries with respect to the final product. The project has also developed material- and damage models with takes into account the interaction between creep and fatigue. In the other end of the scale, the project has focused on full- scale tests to validate the smaller scale results. Over the course of the project, multiple new materials and processes have been developed and characterized. The results show significant improvement over legacy materials in terms of manufacturing and mechanical properties. This allows Nexans to pursue qualification of dynamic dry design power cable systems above 145 kV- particularly relevant for offshore wind, and static power cable design with significant reduction of material usage and extended installation capasity.

As of today, the following values have been realized as a direct consequence of the REFACE project results: i) Updated subsea cable designs uses on average 20 % less lead. This amounts to approximately 1,500 Tn lead per year. ii) Experimental results from testing of a novel lead alloy demonstrate that the above lead use can be further reduced by approximately 30 %. iii) Testing of non- lead cable sheathing have been used to select appropriate material and process to realize a high- voltage dry design dynamic subsea cable for large depths. This represents a first-to- market opportunity. The achievements given above positively affects not only development of the global electrical grid, but exploitation of offshore installations and in particular the prospect of dynamic cables at higher voltages is a significant contribution to realization of cost efficient offshore wind.

Subsea power cables are today widely used all over the Norwegian Continental Shelf toward Norway and Europe. Static and dynamic voltage cables with power above 36 kV for subsea applications require a radial water barrier and, for the expected operational life time up to 50 years, the only accepted solution is a metallic sheathing. In both cases, fatigue is one of the limiting design factors for the cable realization. However, the available design rules and methodologies adopted by the conventional subsea industry do not adequately apply to components and materials subject to major cyclic plasticity as in the case of the lead sheathing. This marks a clear need for a systematic and research-based verification of the current design methodologies toward metallic sheathing fatigue life determination. The REFACE project aims to fill this knowledge gap and, with the support of the latest technologies in the field of computational mechanics and material testing, provide the proper bases for the design methodologies and the production processes parameters of next generation metallic sheathing for electric cables. The project is divided into 5 work packages: (1) Fatigue full scale testing of lead-alloy sheathing for cables, (2) Small scale fatigue testing of lead-based alloy used for cable sheathing, (3) Numerical modelling of lead-based sheathing material definition and (4) Damage-based fatigue model development, in addition to one project management and dissemination work package. The project mobilizes Nexans, one of the main supplier in both petroleum and energy sectors, to develop new solutions and technologies for the renewable energy sector and the restructuring of the energy system. The project evidently aims to substantial cost reductions, and therefore higher level of affordability, access as well as safety, not only for offshore wind application but also in oil and gas offshore plants by further easing process of electrification of offshore installations.