Risk-based design methodology of pile-guide mooring system for an offshore LNG bunkering terminal

Abstract

This study designed a new mooring system for an offshore LNG bunkering terminal (LNG-BT) and proposed a practical procedure to determine safety target of the system through three research topics. First, this study proposes a pile-guide mooring system (PGMS), a new mooring concept, for large offshore floating structures such as an offshore LNG-BT. The economic feasibility of the new mooring system was demonstrated through a cost–benefit analysis. The environmental loads acting on the floaters were computed using wave data at the target location. The mooring system was designed using finite element analysis to estimate the additional investment. An LNG ship-to-ship (STS) bunkering operation that included LNG-BT, LNG carrier (LNGC), LNG bunkering shuttle (LNG-BS), and LNG receiving ship (LNG-RS) was adopted. To estimate the technical feasibility and economic benefit of the proposed mooring system, the availabilities of two types of LNG-BTs were compared considering the acceptance criteria for LNG STS transfers. One LNG-BT was a typical barge-type floater and the other was the pile-guided floater. The relative motion of the terminal with the LNGC and the LNG-BS was analyzed. The limiting wave height was determined from the maximum relative vertical motion between the floaters at the position of the LNG loading arms. The availability of the pile-guided LNG-BT was significantly improved owing to the reduced vertical motion. Finally, a cost–benefit analysis verified that the new mooring concept for an offshore LNG-BT was economically feasible.

Second, this study proposed a decision procedure for determining optimal design load and annual target failure probability for marine structures. The life-cycle cost (LCC) was estimated for a range of characteristic environmental loads. An iterative design optimization procedure was employed to find the safety target 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 annual failure probability 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 annual target probability of failure. A case study was conducted for the PGMS. Two target locations near Busan city were considered to study the change of safety target with respect to the same structural system. Finally, the annual target failure probabilities at the two target locations were determined with the minimum LCC. The safety target 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 would be meaningful and applicable to setting the safety target of marine structures.

Third, this study proposed a decision procedure to determine an optimal set of component reliabilities satisfying 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. The PGMS for an offshore LNG-BT 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.