This study proposed a design procedure for determining optimal design load and reliability for offshore installations. The life-cycle cost (LCC) was estimated for a range of characteristic environmental loads. An iterative design optimization procedure was employed to find the target reliability at which the LCC was minimized. The structural system was designed for a given set of environmental loads caused by waves, currents, and winds. Extreme environmental conditions were estimated by a probabilistic model. The relationship between the characteristic load and the structural reliability was considered on the basis of the selected probabilistic model to study the variation of the LCC for the given set of environmental loads. The set of LCCs, which were the sum of the capital expenditure (CAPEX), operating expenditure (OPEX), and risk expenditure (RISKEX), were estimated to determine the optimal reliability. A case study was conducted for a pile-guide system (PGS) as a novel offshore installation. The PGS was designed to keep the position of large-scale floating installations. The system consists of guide piles supporting the floating installation, a subsea truss structure cross-linking the piles, and a seabed base platform fixed to the seabed. Two target locations near Busan city were considered to study the change of optimal reliability with respect to the same structural system. Finally, the optimal reliabilities at the two target locations were determined with the minimum LCC. The optimal reliability could vary depending on the types of structures, the economic roles of the system, and the environmental conditions at various locations. Thus, in contrast to the prescriptive strategy, the proposed procedure for determining the optimal design load and reliability would be meaningful and applicable to design of offshore structures.