A method for constructing an FPSO comprises (a) assembling and integrating a plurality of modules to form a module assembly for installation on the FPSO. In addition, the method comprises (b) supporting the module assembly with one or more ballast adjustable pontoons. Further, the method comprises (c) positioning the module assembly over a deck of a vessel after (a) and (b). Still further the method comprises (d) de-ballasting the vessel and/or ballasting the one or more pontoons to load the module assembly onto the deck of the vessel after (c).