Engineering Solutions to Acoustic Problems
Example model of the engine room
on U.S. Navy's Fast Sea Frame (FSF-1)
NCE uses Finite Element Analysis (FEA) software to model ship structures ranging from localized ship sections such as masts and machinery spaces to full ship models. FEA models are used to assist with the identification of potential low frequency (< 100 Hz) vibration problems as well as identifying optimal solutions. NCE uses FEA to understand and solve vibration issues on new designs as well as on existing vessels.
NCE typically uses modal and forced response techniques for vibration problems. Modal analyses are used to identify natural frequencies that may coincide with known forcing frequencies from machinery or propeller sources. Forced response analyses are used to predict the actual vibration levels of ship structures to ensure compliance with vessel specifications. NCE uses standard guidelines (IMO, DNV, SNAME, ABS) for model creation and vibration assessment, particularly when the vessel specification only calls for "low vibration".
NCE also uses FEA to predict and analyze acoustic phenomena, including structural-acoustic interaction, such as the underwater radiated noise created by an underwater diesel exhaust, the insertion loss of novel underwater noise treatments, and the effectiveness of specialized silencers used in gas turbine exhausts. NCE has also performed stress analyses on specialized structures ranging from composite deck plating to tow bitts.
Lastly, when diesel engines are resiliently mounted it is necessary to also isolation mount the exhaust piping. The design of the mounting system can be complicated, and care needs to be taken to account for thermal growth and motion of the piping while maintaining high transmission loss. Improper design of these systems can lead to noise problems. NCE uses specialized piping based, finite element software (CAEPipe) to investigate appropriate resilient mounts and mounting locations to allow for a robust system that will keep vibration induced nose from diesel engines low.
NCE primarily uses NEi Nastran Finite Element Analysis Software for ship vibration and stress analysis. NCE also uses COMSOL Multiphysics for acoustics and structural-acoustic interaction modeling. Other FEA software used at NCE includes CAEPipe and MAESTRO.
First Bending Mode for a Patrol Vessel.
Analysis results for machinery foundation
and surrounding structures (cutaway).