A modular mold system for manufacturing a shell part of an oblong composite structure having a longitudinal direction from a fiber reinforced matrix material. The modular mould system includes a number of mold sections, which are adapted to being assembled to an assembled mold part. The number of mold sections includes a first mold section having a first molding surface with a contour that defines a surface of a first longitudinal part of the shell part and a first end section; and a second mold section having a second molding surface with a contour that defines a surface of a second longitudinal part of the shell part and a second end section; which, when the modular mold system is assembled to the assembled mold, the second end section of the second mold section abuts the first end section of the first mold section.