Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011 FATIGUE RELIABILITY ASSESSMENT OF WELDED JOINTS OF VERY FAST FERRY
ACCOUNTING FOR VEHICLE LOADS (DOI No: 10.3940/rina.ijme.2011.a4.214) Y Garbatov & C Guedes Soares, Instituto Superior Técnico, Technical University of Lisbon, Portugal SUMMARY
This work deals with the fatigue reliability assessment of a welded joint in a longitudinal stiffener of trapezoidal shape in a very fast ferry. Based on the analysis of wave and cargo induced loads the ship hull structure is evaluated. The local structure is represented by a longitudinal stiffener with a trapezoidal transverse section. The critical hot-spots and the stress distributions are defined by FEM. The fatigue damage assessment of considered hot spots is analysed accounting for the combination of wave induced and car-breaking transient loadings. The formulation for the assessment of the welded steel joint is based on the S-N approach and FORM/SORM techniques are applied to evaluate the reliability against fatigue failure accounting for corrosion deterioration. The structural system composed by several hot spots is evaluated as a series system based on second order reliability bounds.
1. INTRODUCTION
Fast ferries are high-speed ships, capable of carrying both cars and passengers at a minimum speed of 25 knots. Increased speed has drastically reduced journey time, greatly improving passenger comfort level. High service speeds of these vessels are possible due to their specific lightweight hull
design. By adopting higher strength steel, instead of the traditional mild steel, the structures can be subjected to higher loads, resulting from either more extreme operational conditions or structures, [1].
from lighter thin plated
This paper addresses the fatigue reliability assessment of welded joints of a fast ferry’s car deck for trucks with very high tensile steel trapezoidal stiffeners. Special trapezoidal shaped longitudinals are designed as vehicle deck stiffeners.
Two different welding connections, between trapezoid longitudinals and deck plate, are examined: spot-weld and all-weld connections. The spot-weld connection refers to welding of the stiffeners using a spot-weld technique of welding of several points with appropriate spacing while the all-weld connection refers to classical, continuous welding along the longitudinal length.
The finite element analysis is performed on two levels: global and detailed local analysis. The global finite element model of the mid ship part of the very fast ferry is subjected to two different loading conditions, one for the ship in hogging and another one for the ship in sagging. In both cases additional design pressure loads are applied on the corresponding decks. The structural detail of concern is located at the middle of the span between two transverse frames, where the longitudinal trapezoidal stiffener is connected by backing strip plate. Detailed finite element analysis was performed to obtain critical hot spot stresses and overall stress distribution. More details were reported by Garbatov et al. [2].
The stiffener is considered to be supported by the constructions and structural
transverse frames, and loaded axially by the vertical hull girder bending moment and by additional transverse force due to the presence of truck-breaking load. The hot spots are analyzed accounting for the combination of transversal and axial loads. As a result of the performed analysis, stress concentration factors are defined and subsequently used for fatigue damage and reliability calculation taking into account the combination of low frequency wave induced loads and transient vehicle loads, accounting also for corrosion deterioration with time.
This
fatigue damage assessment is accomplished by several steps of calculation. The considered
fatigue
loading comprises wave and a truck breaking load effect, which has been already used by Garbatov et al. [3] when performing a fatigue analysis of the joint. Here a detailed reliability analysis of fatigue damage will be presented.
Fatigue damage assessment of this kind of welded joints is based on the hot spot stress approach, being one of the most practical methods in combination with detailed finite element analysis as has been demonstrated by Fricke and Petershagen [4], Niemi [5] and Niemi et al. [6].
However, Xiao and Yamada [7] proposed a method for evaluating the structural stress approach based on the stress at a location 1 [mm] below the weld toe surface in the direction corresponding to the expected crack path, where the finite element analysis uses a mesh size having an element size of 1 mm or less.
The effective notch stress approach was proposed by Radaj [8]. Further research about this approach has been conducted by Radaj et al. [9] and the effective notch stress approach has been included in the IIW fatigue design recommendations, with the fatigue strength design curve of FAT 225 for welded steel joints.
©2011: The Royal Institution of Naval Architects
A-231
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64