This study proposed a decision procedure to determine an optimal set of component reliabilities to satisfy the system target reliability with a minimum investment. The relationships between the initial costs and reliability were studied for each structural component to establish an objective function. Finite element analysis and Monte Carlo simulations were performed in order to set the relationships. The system configuration and target reliability of the structural system were used as the inequality constraint of the optimization process to maintain its safety level. A pile-guide mooring system (PGMS), a new mooring concept for an offshore liquefied natural gas bunkering terminal, was considered as a case study. The PGMS was modeled as a series system combining k-out-of-n components to consider the redundant parts. Finally, the proposed method determined the optimal number of guide-piles, redundant parts, and an optimal combination of component target reliabilities for the PGMS.