Extreme response prediction and mooring control of a large outfitting ship under typhoon-induced wind-wave conditions using multidimensional environmental contours

Large unpowered outfitting ships are vulnerable to excessive motions, line breakage, and drift during typhoons because of their light-loading condition, high freeboard, and large wind-exposed area. This study develops a response-driven framework integrating multivariate typhoon characterization, governing sea-state identification, extreme-response prediction, and failure-oriented mooring optimization for a 15,000 TEU outfitting container ship. Weibull marginal distributions and Copula/Pair-Copula models were used to establish the joint distribution of wind speed, significant wave height, and peak wave period, and an IFORM environmental contour was constructed. Representative conditions on the contour were evaluated using a validated AQWA time-domain model. GMM, ACER, and POT produced extreme-response predictions within 5% of each other, while POT achieved accuracy comparable to the 150-run GMM using only 30 simulations. The original 20-line system reached 78.08% of MBL and exhibited susceptibility to progressive failure. Adding four symmetric diagonal anchor chains and strengthening the breast lines reduced the maximum tension to 44.98% of MBL in the intact state and 46.17% after line failure, while markedly suppressing horizontal motions. The framework avoids unrealistic combinations of marginal extremes and provides an efficient basis for robust typhoon mooring design.