Hull Monitoring and Assessment of Hatch Corners and Hatch Opening Distortion
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- Institutt for marin teknikk 
Torsion is highly important for large container carriers. In order to maximize thecontainer capacity, a large deck area is essential, which often leads to a big boware. To simplify loading and ooading, each cargo hold is equipped with hugehatch openings. These large deck openings, with slender longitudinal and transversedeck strips, will deform due to torsion in oblique seas, referred to as hatchopening distortions. This is regarded a vital strength aspect for container carriers.A master project is performed, in order to investigate how torsion contributes toextreme loading. Full scale measurements of a 8600 TEU container vessel, conductedby DNV GL, theoretical methods, and a global and a local Finite Element(FE) model have been used for this purpose. This was done, to get a greaterknowledge of torsional behaviour of container carriers, which again can be used todocument whether torsion should be included in ship design rules. The ship has been operating on two dierent trades. On Route 1, the vessel followsa relatively sheltered passage from Hamburg to Singapore, whereas the vesselcrosses the North Pacic Ocean from Singapore to USA on Route 2. Resultsfrom a hydrodynamic analysis, and environmental data for each route, was usedto calculate the long term distribution of longitudinal stresses in deck. Long termdistribution of deck stresses veries that Route 2, which is known for more roughseas, gives the worst prediction of stresses. However, the measured stresses provedierent. In order to nd out why measured stresses are higher for Route 1, althoughRoute 2 has more rough seas, the vessel's operating speed was investigated.It was found that the vessel operates at higher speeds on Route 1, than on Route2. With speed reduction, the ship resistance, and the risk for occurrence of highstresses due to wave induced vibrations, such as whipping, springing and torsionalvibrations, is lowered. Additionally, no signicant routing has been observed onthe rst route. On the second route, on the other hand, routing was found, i.e.the roughest seas have been avoided. This will again inuence and lower measuredstresses. Speed reduction and routing are therefore important reasons why measuredstresses are lower on Route 2, than Route 1, despite the fact that the waveenvironment is more rough on Route 2. Measurement data have been investigated to evaluate the degree of torsional vibrations present. During the day where most torsion is measured, the ratio ofdynamic and wave stresses ~ 1.10, which indicates that the ship does not experiencesignicant torsional vibrations. Thus, torsional vibrations are found to be ofminor concern for the 8600 TEU vessel. Global FE-analyses of the container vessel have been performed, with respect to the yielding criteria for Ultimate Limit State, ULS, due to torsion. Additionally,since many modications were done in order to analyse the model, self-checks ofthe model with vertical and horizontal bending moments were carried out. Acoarse mesh was used to obtain nominal stresses. Rule stresses with respect toULS were found by applying torsional moments, calculated from ship rules. Theresults show that maximum measured stresses in the deck strip are approximately0.69-0.74 of ULS rule stresses for Route 1 and 0.63-0.68 on Route 2, i.e. measuredstresses are within the acceptance criteria for ULS, on both routes. Since local stresses may be included in the measurements, the same analysis ofthe transverse deck strip was performed with ne mesh. Local stresses, and thusmeasured stresses, were found to be about 10 - 15 % higher than nominal stresses.The local stresses in the measurements are most likely due to the fact that thesensors are located close to hatch corners and close to a weld between two deckplates, which are source to stress concentrations, i.e. hot spot stresses. The magnitude of the hatch opening distortion has been calculated from the globalFE-model. It was found that hatch opening distortion, due to the maximum measuredstress of 131 MPa (measured on Route 1), has a magnitude of 226 mm. Incomparison, the expected hatch opening distortion due to estimated ULS stressesis 335 mm at the deck strip where the sensors are located. The largest ULS hatchopening distortion obtained is located further forward, at cargo hold 3, where thedeformations reaches 411 mm. The calculations were also done simplied, with use of hand calculations, 3D-Beam,and a deck strip model. This, to nd out if simplied and less time consumingmethods can be used in later studies of hatch opening distortions, or if a globalmodel must be used for this purpose. Values for hatch opening distortion for thesimplied calculation methods are between 7 - 21 % lower than calculated fromthe global model. If further studies of other container ships gives the same relationbetween the simplied methods and the global model, simplied methods mightbe used, introducing a scaling factor.