In this paper, we propose and validate an accuracy control system for an Offshore Floating Dock (OFD) developed by Samsung Heavy Industries (SHI). The OFD was developed for constructing offshore structures on floating docks, and has a rectangular shape. Since the offshore structures are very heavy, the erection of offshore blocks causes global and local deformations of the OFD, which in turn adversely affects the alignment of the erection blocks. We propose a ballast plan optimization system for accuracy control. The proposed system was used and validated with data from the Jack & Saint Malo (JSM) offshore plant project that was being constructed on the OFD.