Abstract: An apparatus is provided for modifying the geometry of at least one part of a turbine, which can include a shell assembly that includes an outer shell that is shaped to modify the shape of a pre-existing element of a turbine. The outer shell of the shell assembly can be composed of a fiber-reinforced polymeric material and can at least partially define an inner cavity. The outer shell can be bonded to a structure to modify the geometrical shape of that structure. Thereafter, a polymer casting can be injected into the inner cavity via at least one injection port attached to the shell assembly. In some embodiments, one or more stiffeners and/or a core can be positioned within the inner cavity to help improve the bonding of the polymer casting to the shell and/or improve a structural property of the apparatus.