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.
본 연구는 해상에서 액화천연가스(Liquefied natural gas, LNG)를 선박에 주유하는 부유식 LNG 벙커링 터미널(LNG Bunkering Terminal, LNG-BT)의 안정성과 가용도를 향상시키기 위하여, 세가지 연구 주제를 통해 새로운 부유체 계류 방식을 제안하고 설계 초기 단계에서 활용할 수 있는 위험도(Risk) 기반 구조시스템 설계 방법론을 연구한다. 첫째, 구조설계 및 운동해석을 바탕으로 해상 LNG-BT를 위한 파일 가이드식 계류시스템(Pile-guide mooring system, PGMS)을 제안한다. 부유체 운동해석 결과를 가지고 해상 LNG 주유시스템의 가용도(Operational availability)를 평가한다. 설계된 구조물과 시스템 가용도를 활용하여 비용 편익 분석을 함으로써 제안된 부유체 계류 방식의 경제적 타당성을 검토한다. 둘째, 새롭게 설계된 해상 구조물이 가지는 위험도(Risk)를 경제적 관점에서 정량화 하고, 위험도가 반영된 생애주기비용(Life-cycle cost)을 최소로 하는 안전 목표(Safety target)를 결정하는 방법론을 연구한다. 제안된 방법론을 사용하여 PGMS의 안전 목표를 결정한다. 셋째, 결정된 구조시스템 안전 목표를 최소비용으로 만족시킬 수 있는 하부 구조물들의 신뢰도 조합, 구성 요소 및 여분 개수를 결정하는 방법론을 연구한다. 제안한 방법을 PGMS에 적용하여, 최적 파일 개수, 각 구성 요소들(가이드파일, 트러스 구조물, 말뚝 파일)의 목표 신뢰도(Target reliability) 조합을 결정한다.
