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Proposed Method for Calculation of Spectral Response to Random Wave Loading Using Top-Down Finite Element Modelling.

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Technical memo.

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Ship structural design and analysis is moving away from empirical static design wave balance and towards more rational methods involving computer modeling of the sea load and structural response. A ship load history is usually only known or predicted in terms of an operational profile defined by the time spent in many combinations of different ship speeds, headings and sea environments. Each combination of speed, heading the wave frequency spectrum defining the sea environment can be used to derive a frequency spectrum of the local structural response such as stress, strain or stress intensity factor at a location or region of the ship. The resulting response spectra can then be applied in a fatigue or ultimate strength assessment. This report proposes a method for the calculation of the frequency spectrum of the structural response based on the use of regular wave hull pressure loads and rigid body accelerations provided by PRECAL a linear frequency domain hydrodynamics code based on 3D potential flow and a top-down quasi-static structural analysis procedure to be implemented in the DND suite of finite element codes called VAST. Static finite element analyses are conducted for unit hydrodynamic facet pressure and rigid body acceleration load cases. This should reduce tremendously the computational effort required compared to directly applying a set of wave pressure loads for each combination of regular wave frequency, ship speed and heading needed to represent a ship operational profile. Transfer functions relating a regular unit amplitude wave directly to the structural response are calculated before computing the response spectra, eliminating the need to explicitly apply large cross spectral density matrices of hull pressure loads to the finite element model as is often done in classical random response methods.

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  • Marine Engineering

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