Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011


It has to be pointed out that the calculated local stress, around the structural singularities, depends very much on the structural idealization, the element type used and the mesh subdivision.


Marine structures operate in a complex environment, which is defined by water properties such as salinity, temperature, oxygen content, pH level and chemical composition that can vary and influence the corrosion deterioration.


The effect of the different factors on the behaviour of corrosion have been analysed by Guedes Soares, et al. [10] in marine atmosphere and for immersion corrosion by Guedes Soares, et al. [11] all over the ship’s service life.


Reliability based methods have gained acceptance as being proper tools to support design decisions and for assessing the level of safety in structures. The inspection and repair work performed during the ship lifetime never allows a very dramatic spreading of cracks to be developed and this effect was incorporated in the time variant formulation of ship hull reliability by Guedes Soares and Garbatov [12]. That formulation and the corresponding results yield the required information to assess the effect of inspections and repairs at different points in time on the reliability of the hull girder.


A similar formulation can be made for the effect of corrosion on ship reliability as shown by Guedes Soares and Garbatov [13], but normally both fatigue and corrosion are present and their combined effect needs to be considered in that the decrease net section due to corrosion will increase the stress levels, which in turn increase fatigue damage. This effect has been recognised by Guedes Soares and Garbatov[14].


Fatigue damage of structural joints accounting for


nonlinear corrosion has been analysed by Garbatov, et al. [15] and fatigue reliability of maintained welded joints in the side shell of tankers by Garbatov and Guedes Soares [16].


The study presented here covers a complete stochastic fatigue damage analysis of a 25 year service life for a very fast ferry. The fatigue analysis of the vessel is conducted in a way that environment, operational conditions and structural are taken into account. The ferry is expected to operate in a zone with particular sea- state conditions.


The assessment of fatigue damage of welded steel joints is based on the S-N approach and thus this assessment accounts for the whole ship lifetime and is time independent.


The effect of corrosion deterioration leads to a decrease of plate thickness with time and a consequent increase of the stress levels and thus of the fatigue damage predicted


Figure 1- Global model deformation, sagging A-232 ©2011: The Royal Institution of Naval Architects


by the S-N approach. FORM/SORM techniques are then applied to evaluate the reliability of structural


joints


against fatigue failure. It must be noted however that this approach is different from the one of Guedes Soares and Garbatov [12] in which fatigue damage was calculated by the crack growth model of Paris Erdogan.


2. The global


FINITE ELEMENT MODELING finite


generated containing all


element model of the longitudinal


elements


ferry was that


contribute to the longitudinal strength. All existing longitudinals are introduced in the model using beam finite elements. The length of the fast-ferry global model in longitudinal, i.e. x-axis, direction is 36 meters and comparing to the ship breadth


of 24.70m is being


considered to be sufficient in size for global deformation analysis.


The deck No 5 of the fast ferry was designed for trucks that are supposed to be driven in, parked and driven out. Trucks are loaded with heavy cargo. The fast ferry is carrying cars and trucks parked on several decks. The main particulars of the ferry analysed here are length, L=205.00 m, breadth, B=24.70 m, depth, D=9.00 m, draught, T=5.42 m, draught, speed, v=50.00 kn, light ship


weight, LW=6932.13 t and deadweight, DW=2769.87 t.


To properly take into account both global and local loads, two-step sub model


finite element analysis is performed based on Guedes Soares, et al. [17].


The mid ship part of the ferry that forms the global finite element model was generated using 2-node beam and 3- node and 4-node shell elements. The global model finite element mesh is presented in Figure 1.


The global model consists of 10,034 2-node beam elements, 9,802 4-node shell elements and 6,626 3-node shell elements. It has a total of 74,640 degrees freedom.


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