二階可靠度逼近法於船舶結構可靠度之研究

Second-Order Reliability Approximation－Applications to Ship Structural Reliability

本研究主要目的是在探討二階可靠度（SORM）逼近法於船體結構可靠度之適用性與精確度評估。對於造船與海洋工程上的大型且複雜結構物而言，其所面臨的海洋環境具有高度的不確定性與複雜性，即使船體所受的外在負荷與結構強度參數呈現非線性狀態，可靠度中之統御函數也為非線性，當破壞面之曲率較大或曲率半徑較小時，以線性方程式來近似破壞曲面中之最可能破壞點的一階可靠度方法就變的較不精確。利用二階可靠度逼近法可改善一階可靠度法的精確性，使可靠度分析不再受困於小曲率和隨機變數少時才可求解的限制，進而提高可靠度的適用範圍。本文是針對一實船利用經驗公式來求取船舶之極限強度，再以二階經驗可靠度方法與多設計點可靠度方法，來進行一階與二階可靠度分析，並且交叉比較以期在進行可靠度分析時能夠獲得更加準確及可信的結果。 The purpose of this study is to examine the feasibility of second order reliability method (SORM) used in ship structural reliability analysis. In the past, the first order reliability method (FORM) is considered to be one of the most acceptable reliability methods and the performance function assumed in the method can be approximated by a linear function at the design point. However, the wave load acting on a ship is nonlinear, when the curvature of the failure surface is large or the curvature radius is small, the results obtained from the FORM may not be as accurate as we thought especially when the performance function is strongly nonlinear. It is believe that utilizing the SORM can improve the accuracy of the results for the case having large curvature radius and large number of random numbers. In this study, a simplified SORM and a multiple design point SORM are used to analyze ship longitudinal structures. To ensure that the ship structures are safe during their service lifetime, the results obtained by SORM are compared with the results obtained by the FORM to check the feasibility of these two methods used in ship structural reliability analysis.