This study proposed a design procedure for determining optimal design load and reliability for novel 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, operating expenditure, and risk expenditure, were estimated to determine the optimal reliability. A case study was conducted for the Pile-Guide System (PGS) as a novel offshore installation. The PGS was designed to maintain 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 were considered to study change of optimal reliability with respect to same structural system. Finally, the optimal reliabilities of the PGS at the two target locations were determined with the minimum LCC. The optimal reliability might differ depending on the types of structures, the economic roles of the system, and the environmental conditions at various locations. Thus, in contrast to a prescriptive strategy, the proposed procedure for determining the optimal design load and reliability would be meaningful and applicable to design of novel offshore structures.
본 논문에서는 새롭게 개발되는 해상 구조물의 최적 설계하중 및 목표 신뢰도를 결정하는 방법론을 연구하였다. 경제적인 관점에서의 목표 신뢰도를 결정하기 위해 Life-cycle cost (LCC) 분석을 수행하였다. 초기 투자 비용(CAPEX), 운영 비용(OPEX), 구조물이 극한 환경 하중에 의해 파괴될 때 발생할 수 있는 경제적 손실(RISKEX)이 LCC계산에 고려되었다. 극한 환경 조건 및 하중을 산출하는 확률 모델을 기반으로 구조물의 파괴 확률을 추산할 수 있는 방법을 연구하였다. 정확한 RISKEX를 계산하기 위해 구조물의 파괴 정도를 나누고, 그 정도에 따라 각기 다른 파괴 확률을 고려하였다. Pile Guide System (PGS)이라는 신개념 해상 구조물을 부산 신항과 북항 근처에 설치할 것으로 가정하여 사례 연구를 진행하였고, LCC 분석을 바탕으로 각 지역에서의 목표 신뢰도를 결정하였다. 선급 회사들이 해상 구조물의 목표 신뢰도를 한 값으로 제시하고 있지만, 같은 해상 구조물을 설계할 때에도 설치 지역에 따라 최적 설계하중과 목표 신뢰도는 바뀔 수 있다. 따라서 본 연구에서 제안한 방법론은 새로운 해상 구조물의 목표 신뢰도를 결정하는데 있어서 유용할 것으로 판단된다.
