In-situ acoustic imaging applied on ship engine noise
The acoustic imaging system Scan&Paint 2D is a measuring tool that enables the detection of anomalous behavior of complex machinery by assessing their vibro-acoustic signature. The investigation presented is focused on analyzing the visualization of acoustic quantities such as particle velocity and sound pressure radiated by a ship's engine. It is demonstrated that acoustic imaging reveals the location of the primary sound sources as well as possible anomalous behavior. This case study demonstrates the feasibility of utilising Scan & Paint 2D for noise identification and troubleshooting of ships' engines.
- Rodriguez Rodriguez, F. J. et. al., "Usage of the advanced acoustic equipment Scan&Paint for the maintenance of the Spanish Navy" in Proceeding of VII Congreso Nacional de I+D en Defensa y Seguridad, 2019.
- Implementation of Reliability-Centered Maintenance (RCM) strategies in boat engines. Published by Centro Universitario de la Defensa en la Escuela Naval Militar 2019, Francisco José Asensio Viseras, BSc Thesis.
- Predictive maintenance protocol for the Spanish Navy fleet using advanced acoustic software. Published by Centro Universitario de la Defensa en la Escuela Naval Militar 2019, Mariano Ramis Pasqual de Riquelme, BSc Thesis.
- Identify noisy elements in a reverberant environment
- Characterize multiple operating conditions, in-situ
- Perform a fast, precise and reliable analysis
- Localize low frequency sound sources
Characterize the acoustic radiation of a ship's engine
Maintenance comprises all the actions required to keep a machine or device permanently in the best condition for optimum efficiency and performance. There is a growing tendency to invest in maintenance activities, achieving a reduction of long term losses resulting from potential malfunctions or failures of the system. Within the three maintenance philosophies implemented in the industry, predictive maintenance is used for analyzing the system performance trends based on experimental tests, and then predict potential failures associated with the acquired data. The study evaluates the radiated sound of multiple engine component under different operational conditions to get a better understanding of the main mechanical elements involved in changes of a ship engine vibro-acoustic signature.
Sound Imaging: Scan & Paint 2D
The sound field was measured with Scan&Paint 2D by manually moving a P-U probe near a ship's engine whilst filming the event with a camera. The sensor position is extracted by applying automatic color detection to the video. Acoustic variations throughout space can be determined by segmenting the signals acquired. The resulting color maps of sound pressure, acoustic particle velocity and sound intensity are then displayed over a background image, obtaining a visual representation of the sound spatial distribution.
Setup on the engine room
All measurements were captured in an Instruction Boat of the Spanish Naval Military School. The analysis focused on the main engine: Caterpillar C-18. Two different conditions were measured: docked and cruising. The 6 cylinder engine was kept running at 600 RPM in both cases in order to compare the results. Furthermore, additional measurements were performed in idle conditions while docked, for further analysis on low frequency noise.
Cruising Noise Versus Docked Noise
While cruising, the combustion and the forces generated during the combustion process increase the engine temperature, causing the expansion of metal components, reducing gaps and mechanical clearances. As a result, the acoustic radiation significantly increases.
The spatially averaged power spectra shows a significantly higher broadband noise in mid and high frequencies. Furthermore, low frequency noise is mainly caused by the dominant engine orders. It is observed that the first engine component (3rd order, 30 Hz) is similar in both measurements, whereas the 6th order (60 Hz) is much louder while cruising. This may indicate that the noise generated by each movement cycle increases, probably caused by the higher frictional forces.
The sound imaging results of particle velocity reveal that the valve cover and camshaft are the primary sources of noise radiation below 100 Hz, with different elements highlighted, depending on the operating condition (front and side view). On the other hand, the air intake turbine (bottom right picture) becomes the dominant source of excitation while docked, for low frequencies.
Low frequency sound visualization
Sound visualization based on direct sound mapping enables the identification of the noisiest elements of the engine at different operational conditions, even for very low frequencies.
The results present here identify the main excitation areas for multiple low frequency bands when the engine is operated in idle conditions on the dock. In this case the air collector system is again the dominant source of excitation in the first audible band (20 Hz to 45 Hz).
The valve cover is highlighted as the main excitation source between 50 Hz and 70 Hz. In addition, between 110 Hz and 200 Hz, both the valve cover as well as the cylinder area where several crankcase cooling elements are mounted, have similar excitation levels.
Acoustic imaging results obtained with Scan & Paint 2D enable the identification of the components of a ship's engine with the highest acoustic excitation. Despite the challenging environment (complex machinery in a room with highly reflective surfaces), it was possible to localize the dominant sources of excitation even down to 20 Hz. Furthermore, the operational vibro-acoustic behavior of the motor was captured in detail, information that can be used to select key areas to monitor and apply predictive maintenance procedures, both while cruising or docked. Future work could be focused on further analysis of the main noise sources inside the ship with the objective of gaining a tactical advantage for military purposes.