Marine engine foundation and study of vibration

An unwanted side effect of building faster and lighter ships is the increasing noise and vibration in ships. In order to retain the full benefit of building faster ships without compromising the ride comfort and safety, effective noise and vibration control needs to be implemented to ship structures. Most severe damage to ship structures, however, is caused by large deformation and high dynamic stress concentration from low frequency vibration. The low frequency noise and vibrations also contribute most to discomfort onboard ships. In cases where there is a demand for high structure-borne noise attenuation a twostage mounting system (also called as raft mounting) is employed. The solution to reduce the structure-borne sound due to diesel excitations is usually to attempt to isolate the engine or the generating set from the surrounding structure by interposing elastic elements. The simplest isolation mounting arrangement normally used is to interpose a spring, often in the form of a rubber mount, between the vibrating diesel and the underlying hull structure. Another conventional but a more attractive arrangement for isolation is the so-called two-stage mounting system where an intermediate mass is attached to both the diesel and hull structure by springs. It has been proven that, in nearly all cases, a two-stage mounting system affords superior vibration isolation at high frequency to a simpler single stage mounting. The aim is to reduce vibration levels from machinery to foundation, and thereby to reduce radiation noise levels from ship hull. Theories for vibration isolation and the attenuation of vibration using resilient mounts have been investigated by many researchers. Comprehensive reviews of the literature concerning many aspects of vibration isolation can be found. The design criteria and guidelines for vibration isolation are available in many design handbooks for the car, ship, and airplane industries. In the prediction of vibration transmission from a diesel via the mounting system to the ship's hull, one important issue is the determination of the diesel excitation level and property. This is also true in the design of a resilient mounting system because the selection of appropriate mounting elements is highly constrained by excitations. The exciting sources of a marine diesel are due to rotational imbalance and reciprocating masses. Combustion, inertia of a reciprocating piston, rotational inertia of the connecting rod and crankshaft, and the impact between the piston and cylindrical liner result in the shaking force and moments on the engine block. In the traditional isolation design of a diesel-mount system, only piston-crank inertia loads are taken into account in determining the excitation level and source frequency, while piston-slap induced exciting components of force and moment are of much less concern in order to simplify the analysis and design. It has been demonstrated both analytically and experimentally that piston-slap is a major excitation source of airborne noise from an internal combustion piston engine, especially from turboxx charged diesels. The role that piston-slap plays in the excitation of a vibratory diesel is still questionable when analyzing the engine vibration transmission via resilient mounts to the ship's hull and consequent underwater radiated noise. Exploring the answer to this question is one part of this work. Furthermore, initial studies in design of diesel-mount systems have focused on keeping the natural frequencies of the system away from the undesirable engine operation frequency range. Often, when choosing isolators only the mass and operational frequency of the engine to be isolated are considered, i.e. chosen only for a force acting on the isolation in one direction. This classical only one DOF of motion or only the translational DOFs of motion vibration isolation prediction will not suffice for diesel-mount systems where there exist a combined force and moment excitation and a multiple-mounts system between engine and foundation. Previous researchers have also studied vibration isolation including more than one DOF of motion, often including only the translational stiffness, or they are considered only for rigid body motion. The shortcoming of modeling the engine source and foundation as a rigid body can be improved by including their dynamic characteristics which can be represented by their mobility or blocked impedance . One more important aspect is devoted to the effects of rotational stiffness of resilient mounts on vibration transmission. Analysis of vibration transmission from a diesel engine to the ship's hull via a resilient mounting system is very complicated, since the transmitted vibration is characterized by a large number of parameters that in some cases cannot be directly compared. Including both force and moment excitations and considering coupled multiple-DOF transmission largely increases the complexity of the problem. On the analysis of vibration transmission from a combined force and moment excited source to a flexible receiver via a coupled multiple-DOF mounting system, a number of publications can be referred to in open literature. From the standpoint of ease of interpreting the results, an analytical method of studying the isolation problem is more advantageous than a numerical method by finite element analysis (FEA). However, to perform the vibration transmission calculation, solutions of mobilities or impedances of exciting source, resilient mounts and receiving structure must be known in advance. Owing to the complexity of construction, it is nearly impossible to obtain analytical solution of mobilities or impedances of such a complex vibration receiver as a ship's hull unless some biased assumptions are made. FEA is an efficient numerical tool to achieve the target of vibration transmission prediction of a complex source-mount-receive system. This is done by placing the engine on anti-vibration mounts which are mounted on a foundation. This foundation is again supported on the inner bottom shell through another spring (inner bottom shell acts like a naturel spring) which act as the second layer mounts. The inner bottom shell may be treated as the fixed support. Thus the engine-mount-foundation system can be modelled as a two degree freedom system with certain assumptions. This change from single stage mounting to double stage mounting, results in reducing the transmissibility of forces to the foundation. Resilient mounting systems of engines also provide a powerful means of isolating structure borne sound on its path from the engine to the foundation. Improvement of the mounting system may be achieved by changing from a conventional single stage to a double stage mounting system. This study consists of four main steps. At first, the engine foundation is focused, and its general characteristics, dimensions, sectional views, concrete properties, gravity centers, ground characteristics and shell properties are evaluated. So a statical endurance capability and strength calculations are overcome. Secondly, the dynamical forces (maximum gas pressure inside the combustion bowl and acting onto the piston crown area) and dynamical loads (fixed and moveable parts of the engine, their masses, inertia, rotating and linear moving mass forces) are evaluated. Vibrations model is utilized by two degrees of freedom. Solution of the mathematical model about the system (engine + mount +foundation+inner bottom shell) is discussed and concluded at final step. At further step, analytical model was formed. The all system is then modelled by a pair of spring and mass system. The behaviour of inner bottom shell is represented as a spring. The vertical , rocking or transverse type of vibrations of the system are assumed to be most predominant. We will analyze effects of changes in various dimension of the foundation. It is aimed to observe the behavious of the system by analyzing foundation. Displacement and rotations are calculated and checked if they are in the permissible limits or not according to the frequency – amplitude charts. Results are shown in graphics and discussion, comments and conclusions are made supported by charts. Suggestions for any academic studies in future are also noted at the end of the thesis.