The Noise and Vibration management (N&V) for marine vessel has multiple dimensions, right from the identification of vibration sources, transmission paths through the diverse vessel structure and then the coupling with the water medium as noise transmitted to the environment. Multiple stakeholders have their specific application related requirements and thus it is important to understand these varied requirements to satisfy all parties involved.
Firstly, the human resource on-board marine vessels i.e., the passengers and the crew are significantly affected because of the excessive noise and vibration levels produced by the propulsion and auxiliary machinery. On a global scale, 16% of the disabling hearing loss in adults is attributed to occupational noise of which around 5% is due to the shipping industry. According to official statistics, an estimated $242 million is spent annually on workers’ compensation for hearing loss disability. Thus, it is crucial to maintain a safe work environment to ensure proper safety for all. Secondly, fatigue failure is the formation and propagation of cracks due to a repetitive or cyclic load. Propulsion-induced loads and vibrations are among main causes of fatigue on ships. Higher frequent loads caused by engines and propellers result in forced vibrations with high number of load cycles, are main cause of fatigue.
Thirdly, in naval applications, excessive noise can cause a ship to be detected by enemies and unnecessary vibrations may pop up on radar detection systems. Acoustic mines are in place at various locations underwater and they may be triggered by unwanted noise levels. The navies throughout the world give very high priority to acoustic stealth and after World War II, massive progress has been made in this field. Finally, the most critical application of these studies is the Underwater Radiated Noise (URN). A detrimental, low-frequency ambient noise radiated by maritime sub-systems because of different machines and experimental activities carried out to the surrounding aquatic environment is called URN. This anthropogenic ocean noise is the cumulative result of the human maritime activities including seismic exploration by the oil and gas industries, military and commercial use of sonar, recreational boating and shipping traffic. This noise harms the peaceful aquatic eco-system for all living beings underwater, reduces the available dissolved oxygen and creates a plethora of impactful problems.
A brief understanding on how noise and vibration gets generated :
Energy is found in nature in various forms such as mechanical, thermal, electrical etc. Sound is a medium through which energy propagates by means of oscillation of particles. Vibrations are mechanical oscillations or the intermittent motion of a particle or body, resulting when it is moved from its equilibrium condition. The necessary conditions for oscillatory motion are elasticity and inertia. Elasticity is the ability of a body to return its equilibrium position, after it is displaced while inertia is the measure of tendency of a body to resist change in its current state of motion. In order for the particles to fluctuate around their default position, the medium in which sound waves propagate must have both inertia and elasticity. Noise is defined as being unwanted sound and is generally a result of vibrations. By measuring the amplitude of noise, we can identify the precise force or energy of the sound wave and the measure of amplitude indicates the intensity of noise. Noise and vibration (N&V) analysis is a combination of computational and experimental procedures to measure noise and vibration levels for a system, compare the obtained values with a standard reference and monitor these real-time values over a period of time to detect any possible failure within the system.
The marine vessel represents a very complex system of N&V sources determined by the operation of numerous on-board installations and specific activities of crew and passengers.
Classification of vibrations
The classification of vibrations is done on the basis of the components which cause these vibrations. The first classification is machinery vibrations. Engines, propulsion shafts, gearboxes, propellers, pumps, diesel generators etc. have various moving parts that can induce vibration while operating. These machinery vibrations may be further classified into the following three types. Torsional vibration can be defined as the angular vibration of an object along its axis of rotation. The main propulsion system of a ship consists of the main engine and a marine shaft which consists of an intermediate shaft and a propeller shaft, which are connected by means of coupling flanges. The presence of connections, like coupling flanges, thrust block, engine connection flange, and the cylinder-piston system in the main diesel engine creates torsion in the rotating shaft system and thus it creates an excitation.
Axial or longitudinal vibrations of the propulsion system are one of the most interesting cases of machinery vibration and also possibly the likeliest to cause vibration. The velocity of the water incident to the propeller blades determines the thrust generated by the propellers also called as wake. Since the hull has a curvature at the aft, the wake on the top propeller disc is different from that on the bottom propeller disc. This change is repeated with every revolution of the propeller. Thus, this periodic thrust generated by the propeller is known as alternative thrust which acts as the exciting force resulting in the axial vibration of the propulsion system. Lateral or transverse vibration is perpendicular to the axis of the shaft’s rotation. Because of the curvature of the shafts, the ideal centreline of the shaft and its center of gravity do not coincide with each other. Hence, when the shaft rotates its shifts away from the ideal center line because of the centrifugal force on the center of gravity resulting in a vibratory motion called whirling of shafts.
The second main classification is the hull girder vibrations. The overall effect of the excitations discussed previously is also propagated to the hull structure. One of the main sources of vibrations are the low-speed main diesel engine which exerts 2 main types of forces resulting in vibrations of the hull. Gas pressure forces are caused by the transverse reaction forces resulting from firing orders and are also dependent on the number of cylinders. When the frequency of these forces is in range of the natural frequency of the engine foundation, the latter starts resonating. This causes local vibrations in the bottom structure of the engine room. In a low-speed main diesel engine, the rotating parts usually have a high mass. Hence, high inertia forces are generated during the acceleration of the reciprocating engine parts. Propeller excitation is the other significant source of hull vibrations. Propeller cavitation is one of the 2 types of propeller excitations. This causes bubble formation which implode on the propeller blade. These imploding bubbles generate periodic excitation force. Hence, the cavitation at various speeds must be factored into the designing process of a propeller. The other type of propeller excitation is related to the stern of the ship. As the propeller rotates, the stern of the ship experiences vertical pressure forces. The frequency of the pressure forces and the excitation frequency of the propeller are similar. Ships that have long overhanging sterns are more prone to this type of excitation and the vibration is usually felt in the aft section of the vessel.
Finally, the last main classification is the superstructure vibrations. With every new model, the length of cargo ships has been increasing at a rapid rate. To accommodate this change, the engine room of most ships is shifted aft from midships in an attempt to reduce the shafting length. To make this happen, discontinuities have to be shifted away from the midship. Hence, it is necessary to shift most of the superstructures towards the aft. In order to get a proper view, the navigation deckhouse has to be at a certain height from the main deck. Usually, the engine room cavity houses the deckhouse structure. Since the propeller and the superstructure are close to one another and the top structures of the deckhouse are quite light, propeller-induced superstructure vibration has become an important aspect of ship vibrations.
Transmission Path Analysis (TPA) is a mathematical procedure to evaluate noise contributions from the source to the receiver. This methodical approach to vibro-acoustic design helps to identify which components and structures contribute to specific noise issues. The results are used to optimize the design by choosing desirable characteristics for these components. Transmission Path Analysis is crucial for source-transmission-receiver modelling and the use of more precise empirical formulas for error minimizing model is crucial. If transmission losses could be calculated more precisely then errors will be minimized.
Various ways in which sounds gets transmitted. Sound can be transmitted from a source to a receiver in 4 ways. The first path is the airborne path. Airborne noise is when sound propagates through air such as machinery casing noise. When airborne sound propagates in a free field, it gradually attenuates over the distance it propagates. However, when it meets a solid object, such as a steel plate, the attenuation increases significantly. Noise sources producing airborne sound could influence open deck areas and locations close to these openings. Due to the high attenuation of the decks and bulkheads in ships and offshore units, the airborne path is usually a critical factor only within a source space itself and the compartments directly adjacent.
The second path is the structure-borne path. Structure-borne paths which are generated because of contact between elements often carry acoustic energy to almost all the locations on the vessel, including remote spaces and spaces adjacent to the area where the source is located. Structure-borne sound usually attenuates gradually in a continuous structure. However, obstacles or discontinuities, such as deck/bulkhead intersection and frames will cause a significant attenuation. The structure-borne noise generated by a machine depends on the characteristics of the source, along with the characteristics of the coupling elements and the dynamics of the receiving structures.
The third path is the duct-borne path. The duct-borne path is where the heating, ventilation, and air conditioning (HVAC) system transmits sound from air conditioning equipment such as the air handling unit (AHU) to the duct outlets. When the sound transmits via ducts, it attenuates due to some major factors such as Plenum, Silencers, Straight Duct Attenuation, Branches where Flow Divides, Turns where Flow Changes Directions by More than 30 Degrees and End Reflections at Duct Openings.
Finally, fluid-borne path is a result of the hydro-excitation sources such as propeller, thruster, and wave-slap which transmit the hydro-acoustic pressure to the hull by water and result in hull pressure force. The fluid load of ships and offshore units can be divided into two major groups which are, ocean water outside the structure and fluid inside the structure such as ballast water. Normally, the effect of the fluid inside the structure on the on-board noise is slight and can be ignored. Although the water will increase the damping loss factor of the structures by several times, the damping is still too small to affect the structure-borne noise. Ocean water is recommended to be considered. Although its effect on the high frequency range is slight, it may be significant in the low frequency range.
Thus, there are different sources of noise and vibration onboard marine vessels which each have their sources and harmful effects. Vibrations are generated at any particular location on a ship but they are transmitted to all parts of the vessel because of mediums like air, fluid, structures etc. Hence, it is important to understand the various ways in which these vibrations travel and affect the properties of various components of the ship. These fundamentals of N&V studies can be used in various ways to go more in detail on specific aspects of vibrational studies.
Atharva is a B.Tech Final Year Mechanical Engineering student at Vishwakarma Institute of Technology (VIT), Pune. He is currently interning with the Maritime Research Centre, Pune.