Section 6 Marine vibration excitation sources

6.1.1 A term that is often used in relation to vibration excitations is ‘order’. First order is invariably defined as propeller shaft frequency. For example, if operating shaft Revolutions per minute (RPM) is 90, then first order is 90 cycles per minute (CPM), or 1.5 cycles per second (Hz). Other orders may then be defined in relation to this reference order. For the example of a four-bladed propeller, blade frequency would then be 4 x 1.5 = 6 Hz, which could be termed propeller blade order, or more specifically, fourth order.

6.2.1 Slow Speed Diesel Main Engines

Slow speed Diesel engines are usually selected as the main engine in large cargo ships for reasons of power and economy: however, they usually have significant imbalances at several frequencies, and as they have to be bolted directly on to ship structure due to their invariably large size and mass, vibration excitations can be fully transmitted into the ship.

This type of engine is direct drive to the propeller shaft, that is, there is no gearbox. Hence, engine crankshaft frequency and propeller shaft frequency are the same. ‘Slow speed’ usually implies an operating RPM that is somewhere in the region of 70 to 130 RPM.

Excitations typically include first order moments in the horizontal sense (engine rotation about a vertical axis) and vertical sense (engine rotation about a transverse axis), and also second order moment in the vertical sense. Sometimes, a fourth order moment in the vertical sense may also be present.

At engine firing frequency (the order equals the number of engine cylinders for this type of engine), invariably there is a transverse rocking moment (engine rotation about a longitudinal axis) that is termed the ‘H-moment’. Sometimes, there can also be a longitudinal force of significance that is generated in the crankshaft at engine firing frequency and transmitted into the ship structure via the thrust block.

Other slow speed Diesel engine excitations generally comprise engine twisting moments, which are termed ‘X-moments’. These normally feature at several orders, but because they are internal moments whereby they sum to zero in the external sense, usually they only need to be considered if they are of large magnitude.

Because of the significant size, mass and vibration excitation characteristics of a slow speed Diesel engine, it is recommended to include a coarse representation of the engine in a ship finite element model; this also facilitates application of engine excitation moments, and transverse top bracing, if fitted. Engine dimensions, mass and vibration excitation details may be obtained from the engine builders or the engine designers’ websites.

Figure 1.6.1 H-mode and X-mode vibration of slow speed diesel engines shows the typical vibration excitation moments described above.

6.2.2 Medium and High Speed Diesel Main Engines

Medium speed Diesel engines operate in the region of 500 RPM and high speed Diesel engines in the region of 1000 RPM, through reduction gearboxes to the propeller shaft.

They are significantly smaller and lighter than slow speed Diesel engines. Engine manufacturers normally state for these types of engines that there is no significant external imbalance that could affect global ship vibration, and they normally specify resilient mounts together with suitable system connections. This should always be arranged for passenger ships and ferries where low vibration is required.

6.2.3 Diesel Electric and Auxiliary Machinery

Electric motors do not constitute a significant source of vibration within the frequency range applicable to global ship vibration.

Power generation for supplying electric motors or auxiliary services is usually provided by medium speed Diesel engines. Thereby, comments in Ch 1, 6.2 Machinery 6.2.2 apply.

6.2.4 Turbines

Steam turbines and gas turbines are in balance and do not emit vibration. If they do, it is an indication of failure, such as a broken turbine blade.

6.2.5 Propeller Shafts

As mentioned in Ch 1, 1.1 General description of document the shafting system is subject to mandatory classification rules, in relation to torsion, lateral and, in some cases also axial, vibration.

Dynamic characteristics of shafting systems can affect vibration behaviour of ship structure. Axial vibration of the shafting system can be transmitted into ship structure via the thrust block. Torsion vibration of the shafting system would be translated into axial vibration at the propeller, and could then also be transmitted into the ship structure via the thrust block.

A non-uniform wake will cause fluctuating forces on a propeller which will be transmitted to the shaft. Lateral modes of shafts may be excited by transverse and vertical forces and moments which are developed in this way. Vibration may be transmitted to the ship structure via the bearing housings. Avoidance of the coincidence of a lateral mode of the shaft within 20 per cent of an excitation source is typically required, though lateral modes may be sufficiently damped that excessive vibration does not occur despite coincidence. The propeller blade passing order is typically one of the more important sources of excitation for this phenomenon.

Propulsion shafting system vibration analysis, including the engine crankshaft, is often carried out by engine manufacturers and submitted via the Shipbuilder to classification societies for approval. Alternatively, the calculations can be carried out by consultants and submitted for approval, or performed by the classification society.

In some cases therefore, it may be relevant to consider longitudinal dynamic forces emanating from the propulsion shafting system in relation to ship vibration analysis, which would be primarily with respect to engine firing frequency of slow speed Diesel engines.

For certain passenger ships which have relatively stringent vibration criteria, in a few cases, excitation from the propulsion system at first order (propeller shaft frequency) has been noticed in measurements. In theory, this should not exist unless there is an imbalance in the system due to a fault. Where no fault is evident, the behaviour essentially remains unexplained. Another feature that has sometimes been noticed during measurements for multiple screw passenger ships is gradual alteration in phase between the propulsion systems, causing variation in measurements over time. This ‘propeller beating’ effect can be subjectively annoying. It can often be controlled by phase-locking the propeller shafts, preferably at the most favourable phase relationship for minimising vibration response.

6.3.1 Open Propellers

Propellers gain in terms of propulsive efficiency by being located behind the ship, in water that is moving at a velocity relative to the ship, which is lower than the ship speed: the so-called ‘wake’. However, there is a penalty in terms of vibration, because the water flow speed varies around the propeller disk. Hence, when the propeller rotates, dynamic forces are generated at the propeller shaft bearings, and through the water on to the hull surface above the propeller. Where propeller cavitation exists, the latter effect dominates propeller excited vibration.

The degree of non-uniformity of the wake, that is, irregularity of flow into the propeller disc, depends upon the position of the propeller and the after-body form of the ship. For a full form ship with a single propeller, such as a typical tanker, the wake may be quite irregular, with low velocity flow at the top of the propeller disk, and high at the bottom: in such conditions, significant cavitation may be generated around the top blade position. For a ship of finer form with a single propeller, the wake would tend to be more uniform, leading to lower dynamic forces. For twin screw ships and pod arrangements, propellers would usually be positioned in relatively uniform flow, which is favourable from the vibration point of view.

Propeller excitation of the hull can be thought of as consisting of a non-cavitating part, resulting from blade loading and thickness, and a cavitating part. For ships fitted with fixed pitch propellers, the non-cavitating part will dominate at low ship speeds where cavitation is not very pronounced. The non-cavitating pressure excitation resembles a sine wave with the blade passing frequency. At the ship’s normal continuous rating, hull pressure is, typically, dominated by pressure radiated by cavitation dynamics. Cavitation is relatively repeatable per blade passage; however, temporal variations in the inflow can produce differences in practice.

Open propellers normally have four, five or six blades. During the design phase, propeller dynamic forces for input to a ship vibration analysis can be obtained from specialist calculations or model measurements. Reliable predictions from calculations are essentially limited to blade frequency and, somewhat less accurately, at twice the blade frequency; however, these are usually adequate to cover the frequency range that is relevant for global ship vibration. Calculations to predict excitation levels at higher orders are generally not reliable: these orders can be relevant in relation to noise and vibration of local panels close to the propeller. Model measurements can provide propeller excitation forces at blade and at twice the blade frequencies: also at higher orders, however, these have not always proven to be reliable.

Propeller dynamic forces at shaft bearings input to a ship vibration analysis would typically include longitudinal force (thrust variation) at the thrust block position, and transverse and vertical forces at the stern tube bearing position (shaft bracket locations for a twin screw arrangement).

Hull surface forces are applied to the hull above the propeller. LR uses total integrated hull surface force and applies it to a single substantial structural point above the propeller. In the past, forces distributed over an area of points above the propeller, together with phase differences between the points, were investigated, but the increased complication was generally not justified by the improved correlation with full-scale measurements for global ship vibration responses.

Recommended minimum propeller clearances are stated in LR Rules and Regulations for the Classification of Ships, Part 3, Chapter 3, 3.15.

6.3.2 Ducted Propellers

Generally, it is not possible to obtain reliable predictions of dynamic forces from ducted propellers by calculation or model tests. Hence, predictive analysis is usually confined to resonance avoidance for large deck panels local to these devices. Thrusters can be of the ‘fixed RPM, variable pitch’ or ‘variable RPM, fixed pitch’ types. The former type is easier to deal with from the resonance avoidance point of view.

6.4.1 Springing

Sea waves can induce ship vibration of a continuous nature, mostly at the lowest vertical natural frequency of the hull (2-node vertical) in way of the resonance with wave encounter frequency, which is termed ‘springing’. Generally, this is not significant with regard to acceptable levels of ship vibration from the habitability point of view, but may be of relevance in relation to structural fatigue for certain types of vessel.

6.4.2 Slamming and Whipping

Slamming impacts at the ends of a ship, that depend upon hull form, draught, sea conditions, ship speed and heading, can cause transient global vibration in the form of whipping, as well as local response. In most cases, structural strength issues are more relevant to consider than aspects of vibration, since comfort issues can usually be addressed by a change of ship speed and/or heading.