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


for the all-weld model is denoted as hotspot 4, 5 and 6 respectively. The fatigue damage of the welded joint due to the contribution of different loading is shown in Figure 6.


It can be seen from Figure 6 that in general, fatigue damage is lower for the all-weld model than the spot- weld model. The location of highest fatigue damage for the all-weld longitudinal


and for the spot-weld


longitudinal is the hotspot 4. 5.


GENERAL CORROSION


Corrosion of interior spaces in ship structures has an important role in the long-term structural integrity. Under conditions of high temperature, inappropriate ventilation, high stress concentration, high stress cycling, high rates of corrosion can be achieved at specific structural details such as horizontal stringers or longitudinal and web frames.


The conventional models for general corrosion wastage presented for example by Guedes Soares [22] assumed a constant corrosion rate, leading to a linear relationship between the corrosion thickness and time. Experimental evidence of


shows that a non-linear model is more appropriate.


Guedes Soares and Garbatov, [23, 24] proposed a model for the non-linear time-dependent function of general corrosion wastage. This time-dependent model separates corrosion degradation into three phases. In the first one there is no corrosion because the protection of the metal surface works properly. The second phase is initiated when the corrosion protection is damaged and corresponds to the start of corrosion, which decreases the thickness of the plate. The third phase corresponds to a stop in the corrosion process and the corrosion rate becomes zero.


The model used to define the corrosion deterioration here is based on the solution of a differential equation of the corrosion wastage proposed by Guedes Soares and Garbatov [24]:


dt de t t


 1, 0,


   


   


 c t


 c   t (9) c where d is the long-term corrosion wastage ( d is less


or equal of as a build thickness), d(t) is the corrosion wastage at time t ,


 c is the time without corrosion


which corresponds to the start of failure of the corrosion protection coating (when there is one), t is the transition time (see Figure 7).


where


fX (.) is the joint probability density function of the n basic variables and fP denotes probability of


failure. The n-dimensional integral is defined over the failure region.


In practical applications, the reliability cannot be evaluated in the exact manner as given by Eqn 11. This is A-236 ©2011: The Royal Institution of Naval Architects


P Pg x 0() 


fX gx


    0  f x dx (11)


dt


 c t A


d


O O


B


C Figure 7 Thickness of corrosion wastage, [23, 24]


This model has been validated and calibrated with corroded plate data from tankers, allowing representative values of the parameters to be determined [25].


The long-term wastage is defined as an extreme value in the service time interval for deck. The descriptors of the regression equation of the corrosion depth for the long- term corrosion wastage for truck deck plates is d


=2.3 corrosion, reported by various authors,


mm. The coating life for deck plates is c =3 years and the transition period of deck plates is t=17.24 years are assumed here.


6.


RELIABILITY ASSESSMENT The reliability analysis presented


here is using


FORM/SORM techniques to identify a set of basic random variables, which influence the failure mode or the limit-state under consideration. The


limit-state


function is formulated in terms of the n basic variables given as:


gX g X X X     12  , ,..., n (10)


This function defines a failure surface when equals to 0. It defines an (n-1) dimensional surface in the space of n basic variables. This surface divides the basic variable


space into a safe region, where  g x0 g  x0


region where  and an unsafe  . The failure probability of a


structural component with respect to a single failure mode can formally be written as:


t


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