The effect of high fluctuating pressures close to or below the waterline on the side shell stiffener in the way of web frame and bulkhead.

It exist a number of hull areas in oil tankers which are well known to be sensitive to fatigue cracking. Depending on the severity of the loading sequences, these fatigue sensitive areas of structural integrity concern would need to be taken into consideration to prevent unexpected failures.

Side shell longitudinal fractures in oil tankers (Witmer et al 1994)

Side shell longitudinal fractures in oil tankers (Witmer et al 1994)

With a complete ship model it will be possible to assess the fatigue resistance of these structural details for the specific load cases of the installation considered.

From coarse to fine mesh FE ship model

side shell fine mesh

side shell fine mesh

The fine mesh model is necessary to carry out deterministic or spectral fatigue analysis of details showing high stress concentrations and therefore with expected short fatigue life. By increasing the fatigue life of these fatigue sensitive details the structural integrity of the entire structure can be maintained during the extended service life of the installation. Therefore it is important to localize these sensitive locations and assess their fatigue strength with enough degree of accuracy.

Fatigue sensitive area localized and analized

bulkhead longitudinal stress concentration

bulkhead longitudinal stress concentration

Locations showing early signs of degradation, as coating damage in weld connections with high stresses, would need to be analyzed in detail with the help of FE sub-modeling techniques. With the information obtained from the spectral fatigue analysis it would be possible to assess the expected crack free fatigue life for the specific connection. Furthermore the potential fatigue life extension which could be achieved by the ultrasonic peening treatment could be estimated.

Areas of hot-spot stresses treated by ultrasonic peening

bulkhead longidudinal connection ultrasonic peening treated

bulkhead longidudinal connection ultrasonic peening treated

If the analyzed detail shows too short fatigue life in consideration to the targeted service life of the installation, the areas prone to fatigue cracking in the weld connection could be treated by ultrasonic peening. If the weld connection showed a fatigue crack free life of eight years without any treatment and taken into account a four times life extension the fatigue life of the connection will be 32 years after the ultrasonic peening treatment.