Oil Tanker Simplified Fatigue Assessment with Inspection and Repair Approach and Parameters

Abstract

The occurrence of cracks in the hull structure of oil tankers is an important concern for the maritime industry because crack propagation will reduce collapse strength of deck-stiffened panels and, consequently, decrease the ultimate hull girder capacity of ship’s structures.  Fatigue is an important design criteria for ships to ensure a sufficiently high safety level. Fatigue life predictions of ship’s structural details have traditionally been carried out using S-N approach and the Palmgren-Miner’s rule. The principal objective of such approach is to estimate the time to failure in order to ensure a satisfactory design lifetime of ship’s structural components. Potential cracks are considered to occur in the side shell, in the connections between longitudinal stiffeners and transverse web frame. The main objectives of the present study are to evaluate the fatigue life of vessel’s amidships using the simplified fatigue method, which is based on DNVGL-CG-0129 “Fatigue Assessment of Ship Structures” in order to determine the main cause of the observed cracks on the single skin oil tanker. Fatigue assessment was based on worldwide trade. Longitudinal stiffeners at transverse frames amidships are considered. The results show that fatigue life is generally above 20 years; however, analysis has revealed that the fatigue life of typical stiffener transitions in the side shell is below 20 years. The fatigue lives of side shell longitudinals are regarded as normal for ships built in the period between 1980 and 1990 with extensive use of high tensile steel in the side shell. Inspection and repair proposals of details with fatigue lives below 20 years are advised accordingly. Findings of fatigue analyses provide remaining life assessment, inspection plan definition, determination of repair and modification solutions, and avoiding integrity issues resulting in production downtime and hot work or dry dock.