pipelines, bridges, and offshore structures. However, these procedures have seldom been applied to ship structures due to the complexity of the ship details, the uncertainty of operational loads and of welding residual stresses and redundancy of ship structures. VTT’s study concentrated on the
development and utilisation of fracture mechanics procedures in fatigue life assessment of ship structures with main focus on ship structures like large openings, e.g., window and door corners (see Figure above). Te analysis was made for a ship’s door corner, with the door placed in the area of maximum shear forces of the ship’s hull. Te loading of the door corner was generated by applying measured nominal shear stresses of the associated bulkhead. Te measured stresses corresponded to 1500 hours of data of a passenger ship in normal operation in different wave conditions.
Fracture mechanics calculations
were applied in order to estimate the growth of cracks at a passengership’s door corner for two cases. First, fatigue assessment was made for the case where no visible cracks have been detected in structures. Te second assessment was conducted for the case where cracks have been detected during inspections. Te main purpose of this study was to get an overview of critical crack size and growth, and their influence on the remaining fatigue life of the door corner and the associated bulkhead. In general, the results obtained in the calculations of initial crack formation were in line with the observations made onboard the target ship aſter 10 years of operation. Using the design material data, fatigue assessment for the growth of the detected cracks gave the time period that the ship could operate before maintenance was required for the cracked door and
bulkhead. Te graph above shows the relationship between the number of hours and predicted crack length (c). It was estimated that a critical crack size of 1000mm was relevant for actions to adjust the remaining fatigue life of the bulkhead, before the crack propagated to the deck. Tus, in this case, the cracked door and bulkhead should be repaired in 2.1E4 / 24 / 365 = 2.4 years aſter the first visible cracks have been detected. Overall, the study showed that the
application of fracture mechanics in estimating the fatigue life of welded ship structures seems promising. In particular, the study concluded that it was important to find new methodologies to analyse remaining fatigue life of structural components, such as bulkheads, after the detection of the first cracks. Fracture mechanics is a useful tool which can improve the planning of maintenance and drydocking schedules. NA
The Naval Architect February 2009
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