This paper describes a failure analysis carried out in fuel tanks of a lightweight ship that was designed to operate not far from shore and weights 95 tons. Its main dimensions are 28.4x5.95x2.8 meters (length x width x draft). The ship under study is a lightweight structure that was entirely fabricated with two types of aluminium series alloys, namely the 5083-H111 and the 6082-T6 alloys, which are commonly used in shipbuilding. The former alloy can be strain hardened by cold work and is used in the form of rolled plates and sheets for the vessel hull, the superstructure, and the bulkheads. The 6082-T6 series alloy is a high-strength alloy that can be heat treated and softens near the heat affected zone during welding. It is used in the high-strength structural members of the ship, namely in the stiffeners of the panels. The two fuel tanks under study are located in the rear of the ship, one at starboard and the other at portside. Cracks have propagated from the weld toe of some welded joints possessing low fatigue strength resistance and high stresses applied on. The welded joints were made using TIG and MIG processes and weld quality was assessed by radiography. The fuel tanks were modelled and analysed using the finite element method in order to determine its critical regions and compare them with the locals where cracks have been detected. Then, several effective and economic design improvements were proposed and studied from the numerical point of view, in order to lower the stress level applied on the critical details of the fuel tanks. It was concluded that an increment of 40kg in the ship’s weight would result in a reduction of about 65% in the maximum Von Mises stress value induced in the fuel tanks under study.
|Publication status||Published - 1 Jan 2012|