Additive Manufacturing of Maritime Structural Components – A Classification Perspective

Abstract In recent years classification societies have developed requirements for the certification of additive manufacturing processes, however the acceptance of additively manufactured components has been on a case-by-case basis and largely limited to machinery and piping components as well as non-safety critical items. Additive manufacturing is now being considered by shipyards, manufacturers and operators for load bearing structural components as a means of saving weight, reducing material wastage and manufacturing complex components, as well as facilitating repairs. There is also a potential for improving load transfer and overcoming stress concentrations. This presents a challenge to classification societies as additive manufacturing is not supported by the existing structural rules due to their dependence on key assumptions concerning the materials. The conventional finite element procedures used as an alternative to Rules are also inappropriate when the material is additively manufactured as well as the requirement to achieve equivalence with conventional structural components and the demonstration of this. The assumptions surrounding the failure modes may also be invalid as the dominant failure mode may differ to that of conventional materials and is influenced by the particular method of manufacture. The method of manufacture also affects the reliability of the materials, and the understanding and mitigation of the flaws is critical, particularly for type approved components where repeatability is essential. This can be in part addressed by a robust materials qualification and certification scheme. However, this is also difficult due to the increased complexity and the need for both the feedstock and the manufacturing process to be approved. Other challenges include the need to ensure that the material is appropriate for the marine environment as the rate of corrosion may be affected by the microstructure of the material. The integration of the additively manufactured structural component into the surrounding structure and in particular the weldability of the material and the effect of thermal stresses when the component is being welded are also important considerations. Finally, there is a lack of service history to provide evidence of successful use of additively manufactured structural components. This paper presents a new classification framework to bridge the gap between the established ship classification rules and the ultimate goal of classing ship structures which have been additively manufactured. The framework describes the steps which need to be taken for an additively manufactured structural component to be classed where the scope of the structural design appraisal is dependent on the criticality of the component and the degree of optimisation/deviation from standard designs. Five levels of structural design appraisal ranging from a component where the only difference is the material, which is prototyped strength tested, and where failure has limited consequences, to a component which is highly optimised (mass, shape, topology), is exposed to sea water and is considered to be highly critical i.e. failure of the component could lead to lead to loss of life and/or pollution or loss of the ship. The classification requirements associated with each level including the information to be submitted, studies to be carried out, feedstock and facility qualification and certification, as well as survey both for new construction and in-service and testing requirements will also be described.