With the recent international economic downturn, most engineering, procurement, and construction (EPC) contractors are incurring deficits in their floating, production, storage, and offloading unit (FPSO) projects. Numerous reasons underpin these situations. One of the most important reasons is the cost-estimation failure. The cost estimation is the key contractual point and mainly depends on a weight estimation of the FPSO topsides. Because the topsides contain a lot of equipment and complex structures, it is very difficult to make an estimation at the contractual stage. To overcome this problem, many methods have been proposed to estimate the weight of offshore topsides; however, most of the methods involve the top–down approach, making it difficult to obtain a sufficiently accurate prediction for field-work usage in terms of the weight estimation. Therefore, a work breakdown structure (WBS) for the performance of the weight-estimation process is proposed in this study. Using the WBS of the FPSO topsides, the corresponding presentation of the weight-estimation process makes the process usable in the field work regarding the WBS-item estimations. Accordingly, estimates of the detailed units (disciplines, modules, and areas) inside the topside that were previously not possible were performed. In addition, a prototype program was developed using the proposed method, and the applicability of the proposed method was evaluated through the application of three projects.