船舶在波浪中反應之長期分布預估比較

A Comparison Study on Long-Term Prediction of Ship Responses in Waves

The long-term prediction of ship response is the key criterion when applying the dynamic loading approach (DLA) for ship travelling in waves. The long-term modellings will strongly affect the estimations of critical ship motions and structural damages at seas. In this study, two different methods have been introduced to investigate the numerical calculations of long-term predictions for a low speed bulk carrier and a high-speed monohull. The first technique is the ABS/SCORES method with the H-family wave spectra. Another is the Walden’s scatter diagram method with the Bretschneider and the Pierson-Moskowitz spectra. It has been concluded that the long-term prediction results obtained from the H-family wave spectra are similar to those obtained from the Walden’s scatter diagram method with the low Froude’s number. With the increase of Froude’s number, the extreme values predicted by the Pierson-Moskowitz spectrum are much closer to the results calculated by the H-family wave spectra than those obtained from the Bretschneider spectrum. 船舶在波浪中反應的長期分布預估，為進行全船結構直接動態外力分析（DLA）時最重要的環節，同時也是結構件疲勞強度估算及船舶在波浪中運動適航性評估之基礎；不同的長期分布計算方法及海況、設計波譜的選擇會導致長期分布預估值有極大的出入。本研究使用波浪頻度表（scatter diagram）及設計波譜之長期分布預估模式，與美國驗船協會（ABS/SCORES）配合其實測海況之H-family波譜所提出之長期分布計算方法進行船舶在波浪中之長期分布預估比較，並分別以高速排水型船及較低速之散裝貨船為例，比較兩種長期分布預估方法在運動及船體波浪負荷之長期反應預估值。結果顯示，當Froude數低時，兩種預估模式之極值十分接近；Froude數增高，當波浪資料採用北大西洋海況之Walden’s 波浪頻度表統計模式，但設計波譜應用考慮有義波高之單參數Pierson-Moskowitz波譜為設計波譜之長期分布預估極值與SCORES程式的計算值較接近，而考慮雙參數（波高及週期）之Bretschnider波譜的長期分布預估極值則與ABS/SCORES預估值有較大的差異。