View non-flash version
en-US en-US en-USJuly 2012 en-USwww.sname.org/sname/mt en Model simplication For vibration applications, a number of sim -plications of the structure included in the model are typically possible, however a com -mon simplication used for other types of models is not advisable for vibration. As men -tioned in the previous section, small stieners and a number of other features can be repre -sented as beam elements, eliminating the need for a large number of small elements to represent these features. Stress relief notches and other small features can be ignored, although large lightening holes are typically included. However, vibration models will require all elements to be appropriately sized for the vibration frequency required, mean -ing that the practice of using larger elements in areas without small features may not be allowed unless such elements comply with the frequency requirements. e outtting of the vessel is simplied using a number of methods. Large pieces of equipment are typically modeled as mass elements rigidly or flexibly connected to their foundations as appropriate for hard or isolation mounted equipment. Damping treatments and other coatings of signicant mass (such as poured oors) are included by modifying the plating elements in the area with additional damping and/or additional mass. e weight of small auxiliary equip -ment, piping, cabling, joinery, insulation, etc. is either spread over the entire model using non-structural mass properties (for whole body natural frequency analyses) or ignored partially or wholly (for local -ized forced response analyses). e latter is done primarily for those items that are not strongly coupled to the structure. Boundary conditions For a full ship FEA model, the vessel should be treated as unconstrained. This will result in six additional ?phantom? modes being reported for natural frequency analysis, representing the six degrees of freedom of the body that is the ship. ese modes do not represent any motions of the ship, but are simply an artifact of the analysis method. When ship sections are modeled, the boundaries of the model should extend far from the area of interest. Without sophis -ticated sub-structuring models (which are not typically performed for commercial ship vibration analyses), the presence of any articial boundary condition will neg -atively inuence the prediction accuracy. It is generally recommended that the model contain structures from the area of interest out to the next major structural connection (deck, bulkhead, or shell) in each direction. The edges of this structure are typically treated as a pinned connection, as this leads to the best approximation for the con -tinuation of structure, though if the model extends far enough from the area of inter -est, any boundary condition can be used as the inuence of the boundary is reduced the more the model extends away from the area of interest. Keep in mind that the lowest natural frequency (or frequencies) may not be accurate if it depends on the total length or size of the model, as the specic bound -ary condition used (or even the presence of any boundary) will articially inuence the modeling results. Force inputs Force inputs are applied in one of two ways, as distributed pressures for sources such as the propeller or bow thruster, or as nodal forces from other sources. e pressure dis -tributions for propellers and bow thrusters may be obtained from model testing, com -putational uid dynamics predictions, or measurements of existing vessels. e dis -tribution of pressure should be applied over the areas of hull/tunnel plating as indicated from the measurements or prediction. Where possible, the phase distribution of the pressure field should be applied for a more accurate prediction; however, an ?in phase? assumption is more conserva -tive and is often necessary due to lack of available information. Alternating thrust forces are typically obtained from powering reports and are applied as a force in line with the axis of the shaft at the thrust bearing or other structure that takes the thrust (possibilities include the propulsion engine/motor or gearbox). All other machinery items are modeled using forces at the mounting connection. Forces may be obtained from manufacturer?s data, measurements on previous vessels, or factory acceptance testing. If available, free vibration Vibration analysis of a ship typically consists of two analysis components: hull natural frequency and forced response. Approach ASound