A STATISTICAL ASSESSMENT OF INERTIA LOADS ONTO SHAFT BEARINGS OF THE SHIP DURING LONGITUDINAL SHIP MOTION IN CONFUSED SEA
https://doi.org/10.33815/2313-4763.2019.2.21.067-076
Abstract
Statistical assessment of accelerations in the shaft area, caused by longitudinal ship motions, for a number of head confused sea states has been performed. The assessment of extreme acceleration value has been given considering long-term distribution of waves climate and also according to the recommendations, provided by a number of classification societies. The impact of the shaft inertia forces on the bearing loads during ships motions has been researched. The extreme bearing loads assessment for the whole term of the ship operation has been performed. Ship motions were modelled in terms of theory of linear ship motions, and shaft bending was estimated by the finite element method. All the calculations have been performed for three ships with various length, which enabled drawing a conclusion about the influence of length on the value of additional inertia loads onto shaft bearings.
A statistical assessment of significant levels of acceleration at various irregular wave modes, as well as maximum accelerations for the life of the vessel, which can be used to calculate the inertial loads on the shaft shaft when choosing the technological parameters of its alignment, is performed. The assessment was performed as part of the ship’s linear pitching and the spectral theory of sea waves.
The recommendations of various classification societies regarding the choice of calculated extreme accelerations vary greatly, however, the recommendations closest to the direct calculation of maximum accelerations are the recommendations of DNV, DCS and RMRS.
If the vessel will be operated in areas of the oceans with intense waves, then when designing the shafting and calculating the alignment, it is necessary to take into account the inertial loads on the bearings caused by rolling. To this end, it is necessary to ensure, as far as possible, the most distant values of the loads on bearings in quiet water from the boundary permissible maximum and minimum values.
In addition to inertial loads, one should also take into account the non-stationary hydrodynamic loads on the propeller during rolling, deformation of the hull of the vessel during waves and elasto-hydrodynamic lubrication in the bearings. Since various factors can act with a different phase of shift relative to the ship's rolling process, this problem should not be solved in a frequency-dependent setting, but a simulation system should be developed in time taking into account all important nonlinearities, and then a statistical analysis of the resulting bearing loads should be statistically analyzed.
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