Comparative life cycle assessments of greenhouse gas emissions associated with directed energy deposition repairing and sand-casting replacement of a ship propeller

Extending the lifetime of large components by repairing using additive manufacturing (AM) methods is foreseen as an environmentally preferred option. However, the decision of AM repair instead of replacement can be arduous due to the lack of knowledge on greenhouse gas (GHG) emissions of AM repair methods fulfilling the requirements. There is also a need for comparative life cycle assessments of AM repair and replacement to support decision-making. The current study contributes to knowledge building through a case study of a ship propeller, which requires two advanced repair processes based on Directed Energy Deposition (DED) to extend the lifetime. These DED repair processes differ by material feedstock (powder and wire), DED system (laser and arc), and repairing procedure combining the DED process with conventional preprocessing and postprocessing. To understand their contributions to GHG emissions, comprehensive life cycle assessments (LCA) over a realistic usage period were performed considering both repair and replacement scenarios. The results demonstrate that the contribution from both repairs during the studied period does not exceed 62% of the GHG emissions of propeller’s replacement. Due to the propellers large size, material feedstock was found to have a major contribution, which is equivalent to the contribution of secondary materials and energies consumed in all subprocesses. Considering that material production and manufacturing subprocesses are powered by electricity, the comparative LCA demonstrated significant dependency on the electricity power mix of the country where they are performed. Repairing is beneficial in all cases evaluated, except one: when production of the original propeller and its replacement is performed in Norway compared to the production of the original propeller and repair in China. The electricity profile of the country where feedstock is produced demonstrated a significant impact on GHG emissions. This needs to be considered in the planning of ship maintenance to reduce GHG emissions by prioritizing renewable sources of electricity.