From the remark database of 52 ships where sorted out all cases which contained word crack and fracture. 10 of these 51 ships, had one or more remarks documented regarding fatigue damages in their hull structures. Addition to this, one ship was added to the list which was informed from the personnel from ship operations. The complete research data contains 11 ships, which represents 22% of all fleet’s ships. These 11 ships which was noted to have fatigue damages, where build in 6 different building sites.
It can be noted that all fatigue damages from these ships are not documented, as they are repaired during the operation in part of the normal maintenance of these ships, only serious findings are documented (Mestrovic 2019). Also, there might be damages hidden behind the outfitting linings which are not visible and has not yet caused any repair actions. In any case the available data gives comprehensive view for fatigue damages found in cruise ships.
Figure 10. Fleet’s cruise ships age distribution
The age distribution of fleet’s ships is presented in Figure 10. This distribution is almost linear, and there are 25 ships (48%) which has been in operation less than fifteen years. This group was noted to have no documented remarks regarding fatigue damages, and damages were found in ships which had been in operation for fifteen years or more. There are also
0
ships that has been less than year in operation, these ships are on Figure 10 at the level of zero.
3.1 Timeline of fatigue damage appearance
In first, the data was shorted between date the ship was build, and the date that written document was made regarding fatigue damage findings. Because there is time between crack initiation and fully developed crack, it was chosen to handle this timeline in years.
As can be seen from Figure 11, first written documents from cracks was made in written for the observed group, from 4 years after ship has taken into use. This is not good indication for that ship, taken into account the time life cruise ships are designed for. After that, the number of cracks on the observed group was noted to be increasing almost steadily during the years in operation. This trend is logical and supports the theoretical nature of fatigue phenomena. At the time of 15 years, there is small group of cracks observed, which can be understood to presents the highest possibility for appearance of cracks. Further, this group is small and as taken into account the limitation of observed cases, this can be seen only indicative. Only one case was noted to appear after design life of 25 years, at the time of 27 years. This case is at its right place, and the ships design regarding fatigue resistance adequate.
From the research data is also noticed that after appearance of one crack, there is more to come when ship operation continues.
Figure 11. Cracks documented in cruise ships compared to time when ship has taken into use.
3.2 Locations and amount of crack findings
Second phase of the research data analysis was to categorize all the documented cases by their locations, to see the most common places for fatigue failures. Cases in main data was documented in written and sorted up one by one for different remark categories. When no other information was available regarding the number of cracks, and remark was made using wording “several”, in such cases, the number of cracks documented was 3, to be on the conservative side of the study. Data was categorized by the location points of findings in general, coarse level.
Listed in Figure 12, are all the observed cases categorized by their location on the ships.
Most documented crack findings are located to the door openings. Opening corners are the location where stress concentrates, and therefore outcome of these findings is logical. Next highest number of cracks were found in tanks. Tanks are filled up when starting operation, and during it, they lose their content, which causes stress level variation on tank boundaries.
Third highest amount of remarks was found to be placed on aluminium superstructure / deck category. In chapter 2.5.4, where aluminium and steel fatigue resistance properties were compared in form of S-N curves, was noticed that there is high risk of failure if aluminium is used for load bearing structures, this also shows in the research data. There are also
aluminium-steel transition joints which has failed during the ship operation, and these two groups should be handled as one, but for documented here for research purposes, as transition joints are only used when aluminium constructions is used within the intersection point of steel and aluminium. When observing the data from the base of their locations, cracks on the ship shell are also worrying, from the cause of the serious nature from this, which can cause losing the longitudinal strength of the ship.
Figure 12. Locations of crack finding from observed cases
At the same time these findings were documented, in some of these cases also buckling was noted. Buckling was noted in one tank, aluminium superstructure / deck, and door opening categories. This means that in these spots, the local strength of the structures has been lost.
One observation in this data is that there are transverse frames and bulkheads in this list.
Cruise ships has high transversal rigidity cause of vertical bulkheads which divides the ship in watertight zones from bottom to top, however these cases were also found from remark database. All fatigue failures are serious, when they are noted during the ship operation.
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Balcony Transversal bulkheadinboard longitudinalBracket toeSteel pillar StabilizerStiffener Aluminium superstructure / deckDoor openingTank
The amount of remarks
3.3 Number of remarks per ship
In third phase of the data analysis, total amount of remarks was counted for each ship. In Figure 13 are listed total amount of remarks for each ship, and time they have been in operation. From this can be noticed that remarks are not evenly spreader among all ships.
Three highest amount of remarks having ships had remarks as follows, ship 2 had 42%, ship 3, 21% and ship 1, 12% of all remarks found from database.
Also, both of ships which has the highest amount of remarks, should still have service life on them. Third ship instead, has been already 27 years in operation, and can be understood to be on her last years in operation. Least amount of remarks was spotted on ship which has been 29 years in operation.
From this data in Figure 13 can be seen, that there is no linear increasing amount of fatigue damages in all ships, when ship age goes higher. This trend is ship specific. Also, that in this list are no ships that have been in operation for less than 15 years.
Figure 13. Total amount of documented remarks per ship, and their years in operation
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