Section 8 Vibration problems and solutions

8.1.1 As mentioned in Ch 1, 6.2 Machinery 6.2.1, slow speed Diesel engines often feature significant dynamic forces at several orders. Hence, comments in this section relate to this type of engine. Engine manufacturers normally make recommendations regarding vibration countermeasures for fitting to their engines, if appropriate.

8.1.2 Engine excitation moments at first and second order can be resonant with the lower hull natural frequencies of large cargo ships, which can be a problem if the excitation magnitude is large and the engine is located around a node point of a corresponding vibration mode shape. First order moments feature in the horizontal and vertical sense, and static balance weights can be used to reduce moments in one direction at the expense of the other, depending on the potential problem. Second order moments are in the vertical sense only, and these can be neutralised by balancers fitted at the aft and forward end of the engine that consist of a system of rotating weights driven by the engine.

8.1.3 Some engines have a significant H-moment (sideways rocking) at firing order, for which the engine manufacturers may recommend transverse top bracing. These stays may be of the friction pad or hydraulic type, and they should be attached to substantial ship structure in order to be effective. They essentially alter the dynamic behaviour of the engine together with the ship structure in way of the engine room. Transverse top bracing may also be recommended for engines with large X-moments (twisting).

8.1.4 Troublesome H- and X-moments may also be reduced by changed the firing order of the engine, though this may not always be possible due to other restrictions, such as torsional stress limits on the shaft and certification of the engine for emissions etc.

8.1.5 In ships with more than one engine, phasing of the engines’ firing has been used effectively to reduce certain orders that were problematic. The fundamental concept is the same as that of using a compensator. The systems typically require relatively inexpensive additional instrumentation to be fitted to allow the ship’s engineers to read the engines’ relative phasing.

8.2.1 As mentioned in Ch 1, 6.2 Machinery 6.2.5, dynamic forces in the propulsion shafting system can be transmitted into the ship structure via the thrust block, primarily at engine firing frequency.

8.2.2 The magnitude of the dynamic forces depends upon the design of the shafting system; most notably the shaft diameter in relation to the torsional natural frequency, together with the engine characteristics. The ‘thin shaft’ design is usually employed, where the fundamental torsional natural frequency of the shaft is below engine firing frequency at operational RPM, and this leads to a ‘barred speed range’ of RPM in way of this resonance, whereby the ship is not to operate continuously within this range in order to avoid excessive vibration. The ‘thick shaft’ design has sometimes been used in the past, where the fundamental torsional natural frequency of the shaft is above engine firing frequency at operational RPM, in order to avoid a barred speed range. However, it can mean operating on the rising flank of this resonance, leading to unfavourable vibration responses, so it is not usually recommended.

8.2.3 Axial and torsional dampers are often a standard fitting for slow speed Diesel engines, at the forward end of the engine crankshaft.

8.3.1 Vibration problems resulting from propeller excitation are, in some cases, the result of excessive excitation levels; and, in others, a result of resonances of structural natural frequencies with propeller excitation frequencies. A reduction in propeller excitation will be beneficial in both cases.

8.3.2 Propeller dynamic forces depend upon propeller design, loading and flow conditions into the propeller (wake).

8.3.3 The characteristics of the wake are primarily determined by the after-body design. Further, in some cases, the wake has been adversely affected by disturbances, such as cooling water discharges on the hull forward of the propeller. The wake is usually assessed by model measurements, though CFD is expected to have increasing application in this regard, and also leads to prediction of cavitation extent on propeller blades.

8.3.4 As mentioned in Ch 1, 6.3 Propulsors 6.3.1, propeller excitation may initially be evaluated by calculations or model measurements. As an approximate guideline for cargo ships, at propeller blade frequency, hull surface pressures above the propeller in excess of 8 kilopascals (kPa) would have a high probability of producing vibration problems, irrespective of notable structural resonances. Levels in the region of 4 to 8 kPa would probably lead to problems if there are significant structural resonances. Pressures of less than 4 kPa would be a favourable indication in most cases. In relation to twice the propeller blade frequency, the guideline would be half of the pressures stated for blade frequency. A further reduction of amplitudes is required for higher harmonics.

8.3.5 For passenger ships, which have more demanding vibration criteria, these guideline values typically could be halved, or even further reduced, for some mega yachts.

8.3.6 As may be discerned from the hull surface pressures mentioned above for blade and for twice the blade frequency (that is, first and second blade harmonics), it is expected that magnitudes should decrease in a similar manner for increasing harmonics. If that is not the case, problems may be indicated for the higher orders in relation to propeller noise and excitation of local panels of structure close to the propeller. In relation to the latter, it is generally not feasible to deal with such problems by alteration of local structure to avoid resonances, in view of the spread of panel natural frequencies and the broad frequency band of the excitation.

8.3.7 High propeller excitation levels usually imply high incidence of cavitation. Improvements can be effected by alterations to the propeller design or modification of the wake. The latter is more easily effected and can be investigated using CFD, which for example may lead to a recommendation to fit a flow alteration device such as a vortex generator, usually of elongated pyramid shape, to the hull forward of the propeller for modifying the wake pattern. A typical example is shown in Figure 1.8.1 An example of a vortex generator fitted to the hull, forward of the propeller, where the right hand side of the picture is forward.

Figure 1.8.1 An example of a vortex generator fitted to the hull, forward of the propeller

8.3.8 In a few past cases of problematical propeller excitation in service, air injection into the top blade region has usefully been employed. However, such a system is expensive to fit and maintain.

8.4.1 In general, it is not feasible to address overall hull vibration problems by structural alterations. Hence, remedial measures would comprise reduction in excitation levels, such as through balancing of the main engine. The lowest hull natural frequency is normally the 2-node vertical mode (as shown in Figure 1.8.2 FE analysis result showing the 2-node vertical vibration mode) or 1-node torsional mode. For the super long ships, this is often in the region of 0.5 Hz.

Figure 1.8.2 FE analysis result showing the 2-node vertical vibration mode

8.4.2 As mentioned in Ch 1, 8.1 Main Engine, slow speed Diesel main engines can cause significant hull vibration, normally occurring at a resonance with the fundamental natural frequencies of the hull. If such a problem were indicated, then the usual solution would be the prescribed balancing for the engine. This requirement is shown as an example in Figure 1.8.2 FE analysis result showing the 2-node vertical vibration mode (4-node vertical mode). The engine is below the superstructure, and thereby unfavourably located for vertical moments, as it is at a node point.

Figure 1.8.3 FE analysis result showing the 4-node vertical vibration mode

8.4.3 Unfortunately, cases have occurred where both a vertical and a horizontal hull mode were in the operational speed range and were excessively excited by the first order moment of the engine. In these cases, balancing to reduce either the horizontal or vertical engine first order moment may be undertaken, and a compensator fitted to counter the remaining out-of-balance engine first order moment.

8.4.4 In relation to high propeller excitation, this can sometimes bring about significant vertical vibration at the aft end of the hull, particularly when there is resonance between propeller blade frequency and a vertical aft end vibration mode – often known as a ‘fan-tail’ mode (see Figure 1.8.3 FE analysis result showing the 4-node vertical vibration mode). Local structural alterations, particularly regarding longitudinal bulkheads in the aft end of the hull may be feasible at an early design stage; otherwise attention can be given to propeller excitation or the number of blades. It should be noted that the aft end of the hull in cargo ships does not contain permanently occupied spaces that are subject to habitability standards.

Figure 1.8.4 FE analysis result showing ‘Fan-Tail’ effect

8.4.5 In Ch 1, 6.4 Sea Waves 6.4.2, it was stated that sea waves can cause uncomfortable vibration by way of slamming impacts at the ends of a ship. Slamming forward can, of course, be addressed by a reduction in ship speed and/or a change of heading. For a few passenger vessels with flat counter sterns at a particular height above the sea surface, slamming impacts can cause unpleasant vibration in accommodation spaces aft. However, this tends to be at low speed whilst manoeuvring, or stationary in response to wave trains caused by passing vessels in port, since at higher speed a stern wave usually immerses the flat counter stern. If such a relatively unusual phenomenon is experienced in service, a known solution is to create an air cushion under the flat counter stern by incorporating a ‘skirt’ arrangement around the area, including internal diaphragms for strength purposes and air pressure relief apertures.

8.5.1 Most cargo ships have crew accommodation and operating spaces placed in tower block deckhouses at the aft end of the hull, for reasons of payload maximisation and cargo handling. However, this is the least favourable location from the vibration point of view, since it is above machinery compartments and close to the propeller. Furthermore, the tower block is relatively high for visibility from the navigation bridge.

8.5.2 Hull and superstructure vibration modes are invariably coupled, such that the superstructure will follow hull vibration mode shapes. Notwithstanding, it has been indicated many times by past practical experience that the fundamental longitudinal natural frequency of a superstructure accommodation block is particularly important to consider in relation to resonance avoidance. Relevant excitations in this respect are firing frequency of slow speed Diesel engines and propeller blade frequency.

8.5.3 Excitation at engine firing frequency can arise from an H-moment and, although this is a transverse moment, it can sometimes lead to some longitudinal vibration in the superstructure. Further, as previously discussed for some cases, there can be a longitudinal dynamic force imposed on the thrust block by the propulsion shafting system.

8.5.4 The primary excitation at propeller blade frequency is usually the hull surface force on the aft end of the hull above the propeller, as mentioned in Ch 1, 8.4 Hull, whereby vertical movement of the aft end in turn brings about longitudinal displacement, by way of rotation for a tower block superstructure that is located just forward of the aft end (see Figure 1.8.5 Effect of propeller hull surface force on aft end and superstructure).

Figure 1.8.5 Effect of propeller hull surface force on aft end and superstructure

8.5.5 The fundamental longitudinal natural frequency of a tower block superstructure is primarily determined by its height and length, followed by the configuration and continuity of the longitudinal bulkheads, the support below the superstructure, and the total mass including outfit. This natural frequency is often within the range 6 – 10 Hz.

8.5.6 Transverse vibration and torsional vibration of the superstructure, which are mostly induced by the main engine H-type moment and X-type moment through transverse top bracings and double bottom structure, may also occur during vessel sea trials and in normal operation. Quite often, the natural frequencies of transverse and torsional vibration modes are higher than those of longitudinal modes, but they may vary in different cases.

8.5.7 If a problem of resonance with the fundamental longitudinal natural frequency of a superstructure accommodation block is indicated, significant structural alterations are not usually feasible or very effective, except for a connection of the accommodation block to a separate funnel casing block for applicable cases (at the expense of increased noise transmission). Normally, structural alterations are also not feasible if resonance happens with transverse and torsional vibration modes of the superstructure block.

8.5.8 The excitation source may be addressed to effect a solution. If it is the propeller, attention can be given to excitation level and/or number of blades. If it is the engine, attention can be given to the H-moment / X-moment by way of top bracing, or damping devices for the propulsion shafting system, as appropriate.

8.5.9 Another possible solution that has been employed in some cases, usually post-trials, is the fitting of a vibration compensator. This is a compact, electrically controlled device incorporating rotating weights that can be installed on or near to the navigation bridge. It can be adjusted for force magnitude, phase and frequency to neutralise or reduce vibration. It tends to be more effective as a countermeasure for machinery excited vibration, rather than from the propeller, as the former is of a more consistent nature. Overall dimensions are within about one metre.

8.6.1 Cantilever bridge wings are often incorporated in cargo ships, for the purpose of visibility along the ship sides.

8.6.2 Undesirable vibration levels can occur if there is resonance between the fundamental longitudinal or vertical vibration modes of the bridge wings, with slow speed Diesel engine firing frequency or propeller blade frequency in way of operating RPM and, to a lesser extent, at twice the propeller blade frequency. Further, if there is resonance between engine firing frequency or propeller blade frequency, together with the fundamental longitudinal natural frequency of the tower block superstructure and, simultaneously, the fundamental longitudinal natural frequency of the bridge wings, the vibration responses can be very high.

8.6.3 Consequent to the above, it is recommended to calculate the fundamental longitudinal and vertical vibration modes of the bridge wings at the design stage. This can be part of a global ship vibration analysis, or a bridge wing local model with suitable boundary conditions at the bridge side, to evaluate natural frequencies. An example of such a model is shown in Figure 1.8.6 FE analysis result showing deflection of a cantilever bridge wing, indicating the fundamental longitudinal mode.

Figure 1.8.6 FE analysis result showing deflection of a cantilever bridge wing

8.6.4 If a problem is identified, it can be addressed by structural modifications, such as struts or large brackets. An addition of mass at the ends of the bridge wings, in order to lower their natural frequency, may also be feasible.

8.7.1 These are of primary concern for accommodation areas in passenger ships, which are subject to habitability standards. Vehicle decks in Ro Ro ships also feature, but these would only be described as ‘occasionally occupied spaces’.

8.7.2 From the resonance avoidance point of view, it is possible to study large span deck panels by using local models, subject to suitable boundary conditions being arranged. However, they would normally be incorporated in a global vibration analysis that would evaluate natural frequencies and vibration responses. Typical large deck panel vibration modes are shown in Figure 1.8.7 FE analysis result showing a possible vibration mode of a large deck panel.

8.7.3 The relevant excitation frequency would be the propeller blade frequency and the main engine firing order frequency; thereby essentially being relevant for panels aft of amidships. In order to avert possible undesirable vibration while occasionally using manoeuvring devices, such as tunnel thrusters, resonance avoidance can be implemented for large deck panels close to their locations.

8.7.4 Problems that are identified can be attended to by increasing numbers and/or scantlings of primary girders/stiffeners or by introducing additional vertical supports such as pillars or bulkheads with structural continuity below, subject to arrangement considerations.

Figure 1.8.7 FE analysis result showing a possible vibration mode of a large deck panel

8.8.1 A resonance avoidance procedure should be applied for these, using analytical or local finite element analysis to evaluate natural frequencies. For naval ships, this is a LR class requirement.

8.8.2 Resonance avoidance should be implemented for aft peak panels, including the effect of the added mass of fluid where appropriate, in order to avoid potential structural damage. The relevant excitation frequencies for comparison are at propeller blade and at twice the blade frequencies. The most common problem experienced in this regard is cracking of fresh water tank bulkheads that are located in the aft peak.

8.8.3 Resonance avoidance should be implemented for local panels in accommodation decks close to propellers such as in tower block superstructures aft, including outfit mass, where appropriate, in order to avoid potential infringement of habitability standards. The relevant excitation frequency for comparison is propeller blade frequency.

8.8.4 Problems are usually addressed by an increase to stiffening of the panels.