The Fatigue Assessments Research Based On CSR And HCSR For Double Hull Oil Tanker

International Association of Classification Societies draw upCommon structural rules for bulk carriers and oil tankers (HCSR),which can be used for the design of both oil tankers and bulk carriers.This rule ensures the consistency of design requirements for oil tankersand bulk carriers. HCSR was released for a short time from its first draft,and fatigue strength is an important aspect of the structural strength.There are less researches about the fatigue strength under HCSR,therefore it’s necessary to study it.This paper studies the requirements of oil tankers’ fatigue strengthunder HCSR, and compares with the requirements under commonstructural rules for oil tankers (CSR-OT). It focuses on the impact of theselection of S-N curves and long term distribution of stress range on theresult of S-N curves computational method. The calculation formulasof fatigue life and fatigue damage which has considered the differentcircumstances are also mentioned. The differences of two rules indynamic fatigue loads based on the envelop value have been analyzed.The two methods for hot spot stress calculation have been analyzed: very fine mesh finite element stress analysis and fatigue screening assessment.Details are as follows:(1) Three aspects which have an great impact on the fatigue strengthcalculation has been analyzed, namely long term distribution of stressrange, the S-N curves under different circumstances and the method offatigue life and damage calculation corresponding to differentcircumstances. The difference on these three aspects between two ruleshas been summarized and the reason for HCSR’s probability change hasbeen given.(2) This paper also analyses the difference of requirements andmethods about dynamic loads calculation. Based on the calculationmethod of envelop load, the dynamic loads in the middle cargo holds often oil tanker have been calculated under two rules, then compared.Finally some instructive conclusions have been got.(3) The difference on the hot spot stress calculation method of veryfine mesh finite element stress analysis between HCSR and CSR-OT hasbeen studied. The paper gives details about the hot spot stress calculationmethod for web-stiffened cruciform joint, and compares HCSR’scalculation of hot spot stress in the web for web-stiffened cruciform jointwith CSR’s. The effects of these differences have beenstudied throughreal ships. (4) In the paper, a new method for hot spot stress calculation namedfatigue screening method has been recommended. Here analyze theproduce necessity of this method and expound the process of fatigueassessment. The bracket toe of transverse web frame and toe of stinger inthe middle cargo hold of159000tons of oil tanker have been evaluated.