Feature 2 | CAD/CAM UPDATE Efficient hull girder FEA
Finite element analysis is an integral part of the ship structure design process at Vuyk Engineering*, Rotterdam. Senior structural engineer, Raymond Marchee, reports on how it was applied to a recent trailer suction hopper dredger project.
N
owadays, using finite element analysis (FEA) on a ship hull structure during the design process
of a vessel has become quite common. Also commonplace, however, remains the misunderstanding that ‘entering your ship in a finite element programme will solve all your structural problems’. In reality, though, it is oſten remarked that FEA is lagging behind the design process as a whole. In fact, FEA can be effectively integrated
in the design process, without delay. What follows is a presentation of a recent FEA involving a 190m long 32,000m3
trailing
suction hopper dredger design performed at Vuyk Engineering, which also showed some interesting particulars of the overall structural behaviour of such vessels. The analysis considered is 3D rule based, meaning that rule design wave loads are reproduced in a (3D) FEA, using a (3D) diffraction analysis. At the end, a short description is given of the hull girder analysis of a ship which is outside the scope of application of the ship rules.
Setting analysis goals Te basic structural design of a sea-going offshore work vessel or dredger typically takes two or three months. The key to practical application of FEA in this process is not to wait for the structural drawings before starting the FE work. Despite tremendous advances in computer
capabilities, it is still not possible to model complete ship structures in FE. It would take too much time and the model would be impractically large. Fortunately, it is not necessary to model the complete structure.
*Vuyk Engineering Rotterdam designs ships and equipment from concept phase up to class approval mainly for the dredging, offshore and heavy transport industry. Also marine operations engineering and general consultancy is included in the scope.
www.vuykrotterdam.com
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Efficient modelling can be achieved by determining in advance what is being looked for: which stresses are required, occurring in which members, due to which loads? In an analysis of the governing load–effect,
combinations can be made even before the basic design phase. Such an analysis identifies the parts of the structure which have to be modelled. Te modelling process can then be started, requiring only a (preliminary) arrangement and a line plan, which are both usually available within the first two weeks of the basic design process. Figure 1 shows the area model (i.e. without an FE mesh) of the 32,000m3
190m long
Trailing Suction Hopper Dredger. The model shown has the following
• To check the efficiency in longitudinal bending of the mid ship region;
purposes:
• To calculate the overall hull girder effects at the aſt and fore hopper ends;
• To provide a basis for detailed stress analysis for fatigue check using sub modelling.
Although the model may look rather
detailed, many structural parts are either not present or modelled in a simplified way (e.g. the deckhouses). On the other hand, the total ship is modelled, including a fairly accurate representation of the hull shape. Tis
is done for reasons of accurate loading and to ensure proper load transfer to, and boundary conditions for the regions of interest. Te model is built using semi-automated
batch files, facilitating easy modification at any stage of the process where the modelling should follow the progress of the structural design.
Still water loads Modelling takes at least a month (depending on the complexity of the hull and the number of load effects sought). By this time, a preliminary loading conditions report is available, which serves as input for the hydrostatic load definition in both the FE and diffraction soſtware. An FEA still water load case should always
be based on a loading condition defined in a hydrostatic programme, in order to provide a clear reference. In theory, the hydrostatic loading condition can be reproduced in FEM with high accuracy, but this requires a lot of effort and is seldom necessary. Practically, the buoyancy in FEA is determined by the shape of the FE model which is inherently different from the shape of the hydrostatic model. Consequently, the LCB will always differ (where the total displacement can be tuned with a coefficient). Te reference weight distribution will have
The Naval Architect July/August 2009
Figure 1 Area model (i.e. without an FE mesh) of the 32,000m3 Suction Hopper Dredger.
190m long Trailing
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