Abstract: A jacket of an offshore platform has a special internal structure of which one side coincides with one side of the jacket but an opposite side is spaced from the corresponding opposite outer side of the jacket. The jacket is made by a rolling-up process while lying recumbent, the special internal structure being made first and then being used to provide the reaction points for the rolling-up of outer-side-forming trusses of the jacket, which are of greater height, when erected, than the special internal structure and carry angle parts which are to form the said outer opposite outer side of the jacket.