Noise from ships in ports. Possibilities for noise reduction
4 Possibilities for noise reduction
- 4.1 Diesel generator exhaust
- 4.2 Ventilation
- 4.3 Secondary noise sources
- 4.4 Other noise reducing measures
- 4.5 Summary – noise reduction
4.1 Diesel generator exhaust
4.1.1 Silencers
On the majority of larger ships silencers should be included in the exhaust stack of the diesel engines to fulfil the external IMO noise limits. The needed noise attenuation of a silencer should be determined by thorough calculations taking the type of engine, design of the exhaust stack, external noise limits and other relevant factors into consideration. In Figure 5 three different types of silencers are shown:
Figure 5 Different silencer principles applicable for reducing engine exhaust noise.
Conventional engine exhaust silencers are of the reflection type, absorption type or a combination of these two. The reflection type silencer is used for attenuating low frequencies. The absorption type attenuates the intermediate to the higher frequencies. The complete frequency range can be attenuated by a combination of the two.
Figure 6 Examples of the expected noise attenuation in dB of an absorption type silencer and a reflection type silencer. The data is supplied by a silencer manufacturer.
In order to attenuate the very low frequencies the reflection type silencer should be sufficiently long. The magnitude of attenuation in the low frequency region depends on the volume of the silencer. Space is often limited in the exhaust casing and therefore the silencer should be selected early in the design phase. The criteria for selecting the silencer should include which frequency range needs to be attenuated and how much the noise should be attenuated.
If noise problems are apparent after the exhaust stack has been installed it can be very cumbersome and expensive to solve because of space limitations in the casing. Depending on the specific noise problem e.g. low broadband frequency noise, a large reflection type silencer should be installed. A structural modification on this scale should be carried out at a shipyard which is costly. The new silencer design should be carried out in close cooperation between the yard, engine manufacturer, silencer manufacturer and a noise consultant.
In special cases an exhaust noise problem can be solved with simpler means. Diesel generators run at fixed speed and the frequency content in the exhaust noise does not change. If a single tone is significant compared to the rest of frequency spectrum of the exhaust noise then a quarter-wave side branch or a Helmholtz resonator, which attenuate discrete frequencies, can be designed and installed in the exhaust stack. As an example a solution using a quarter wave resonator has previously been designed and successfully installed in the exhaust stack on the advice of Lloyd's Register ODS to solve a discrete frequency noise problem on a Danish fast ferry, [15]. A significant noise reduction of approximately 20 dB was achieved. As another example an unfortunate combination of a frequency component in the exhaust noise of an engine and the exhaust pipe length before and after the silencer can also cause significant pure tone problems, [11]. In this special case the noise may be reduced by changing the length of exhaust piping before or after the silencer.
In addition to the conventional silencers a number of different engine exhaust silencers exist. The Helmholtz- and the quarter wave resonator attenuation principle are used in some of silencer designs. These silencers consist of several Helmholtz-resonators with different volume and quarter wave resonators with different lengths, see the figure below:
![Figure 7 A silencer as described in [16] with different size of resonators.](images/s24.gif)
Figure 7 A silencer as described in [16] with different size of resonators.
As a result of the multiple resonators many discrete frequencies are attenuated and the silencer attenuates over a broader frequency region. The main advantage of this type of silencer is the broadband low frequency attenuation combined with a low pressure loss. If the pressure loss of a complete exhaust stack is too high the diesel engine does not function properly. An advantage is therefore that it is possible to post-install this type of silencer without increasing the total pressure loss of the exhaust stack too much. However, the installation should be performed at a shipyard. The silencers are described in e.g. [16] and [17].
4.1.2 Example
A cruise vessel faced the problem of excessive harbour noise when the vessel was planned to have regular calling at Stockholm Port. The noise level from the auxiliary exhaust was too high, and the harbour authorities rejected the application from the owner of the vessel. Furthermore, using a less noise sensitive location out of the town centre was not attractive to the cruise operator. The operating conditions and the design of the ship were studied. Initially, conventional solutions were considered, i.e. redesigned auxiliary engine exhaust systems with significantly improved silencers. However, space limitations and limited time available for modifying the systems meant that a conventional solution was difficult to find.
Finally, an elegant solution was found by Lloyd's Register ODS: because the main engines were out of operation in the port condition, it was possible by means of valves to redirect the auxiliary exhaust gas flow, and utilize the main engine exhaust silencers for the auxiliary engines. Modification of the pipe routing of the auxiliary engine exhaust was uncomplicated, since only the introduction of an extra valve and limited additional pipe work were required. The achieved noise attenuation was significantly high so the cruise vessel could easily get the environmental permission.
Figure 8 Rerouting of the diesel generator exhaust by utilising the main engine silencer during berth.
4.1.3 On shore power
Onshore power supply for the vessels during berth is another option that eliminates the need for power generation onboard. The noise from the diesel generator is thereby eliminated and the need for engine room ventilation is reduced. Engine room ventilation noise is therefore also expected to decrease. According to [20] ports are not normally prepared for supplying power to vessels and many vessels are not prepared for onshore power supply. Shore power has been established in a number of ports e.g. Göteborg, Helsingborg, Stockholm and Helsinki.
There may be technical difficulties in establishing on shore power and there are no standard for the power used on board ships i.e. voltage and frequency, but standardization is in progress. Ships that are obvious to consider for on shore power supply are ships frequently calling ports (such as ferries), with long port stays (such as bulk carriers), with high noise levels or with tonal content in the noise. Establishing on shore power is considered as a large investment. The noise reduction potential is however significant.
4.1.4 Calculation example 2: Reducing noise from the engine room ventilation
A 144m tanker is at port. A single diesel generator and the engine room ventilation are in operation. No noise reducing measures have been included in the engine room ventilation system. Silencers have been included in the diesel generator exhaust stacks. Hence, during normal sailing operation the ship fulfils the IMO noise limit at the listening post of 70dB(A).
A model of the ship including the noise sources has been implemented in the environmental noise calculation software SoundPLAN, see Figure 9. It is assumed that the sound propagates over a flat hard reflective ground. The sound power levels of the ventilation and the diesel generator are shown in the table below:
| 1/3-Octave frequency bands, Hz | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| 31.5 | 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 | Total | |
| Diesel generator exhaust | 94 | 98 | 95 | 89 | 83 | 82 | 82 | 80 | 73 | 101 |
| Engine room ventilation | - | 52 | 68 | 81 | 96 | 96 | 96 | 93 | 87 | 102 |
Figure 9. Tanker without / with noise reducing measure in the engine room ventilation system.
The noise maps in Figure 9 shows that the noise at the portside of the ship to a large extend is influenced by the engine room ventilation. In order to decrease the noise, mineral wool is installed in the fan room. As a result the noise decreases dramatically with up to 10dB. The external noise after the modification is mainly influenced by the diesel generator exhaust.
4.2 Ventilation
4.2.1 Standard silencers
Noise reducing measures for ventilation systems are less costly compared to decreasing noise from diesel generators.
Various ways of attenuating noise from different ventilation systems exists. Figure 10 shows a hold ventilation fan room with some of the different possibilities for attenuating the noise from the ventilation fan.
The most inexpensive way of reducing ventilation noise is to install mineral wool in the fan room or the ventilation ducts. The mineral wool increases the sound absorption in the room and in the ducts. The mineral wool is normally covered with thin perforated steel plates, glass cloth or similar. The covering should be porous. Depending on installation details and properties of the mineral wool 10-15dB noise reduction can at best be achieved, [18].
Figure 10 Principle sketch of different noise reducing measures to decrease the noise at a ventilation intake/outlet.
The fan noise can also be attenuated with silencers. Many of the silencers are based on the noise absorption principle. These silencers include circular silencers, baffle silencers and noise reducing louvers.
Figure 11 The principle of baffle silencers and cylindrical silencers
The noise attenuation of absorption silencers is proportional to the ratio between the circumference of the acoustic material and the free flow area. The more the flow is restricted the higher the attenuation. The noise attenuation increases with increasing baffle and cylindrical silencer length. The drawback of increasing the attenuation is that the free flow is restricted i.e. the pressure loss of the silencers increases and a too high restricted flow may cause additional flow noise. The restricted flow should not exceed approximately 15 m/s. It is difficult to attenuate lower frequencies i.e. 63Hz and 125Hz with absorption silencers. The attenuation in these bands can however be increased by increasing the width of the baffles or installing an absorption core in the cylindrical silencer.
4.3 Secondary noise sources
In some cases secondary noise sources such as compressors, pumps etc. can be a significant contributor to the noise emitted from the ship to the surroundings. Different solutions for decreasing the noise exist depending on the specific noise problem. E.g. if a pump is rigidly mounted on a ship the large ship-structure helps radiating the structure-borne noise introduced by the pump. By resiliently mounting the pump the acoustic coupling between the pump and the ship-structure is minimised and the noise caused by the pump is reduced. If the problem is mainly airborne from the external machinery an acoustic enclosure can be engineered.
4.4 Other noise reducing measures
The noise emitted from a ship is in some cases asymmetric, similar to the case shown in Figure 9. The ship can therefore advantageously be berthed with the less noisy side facing the noise sensitive areas in the harbour. Another similar option is to require that ships provide data on measurements of the noise radiated to the surrounding before calling at a port. In this way the noisiest ships can be berthed furthest away from residential and noise sensitive areas. This could further be implemented by using an economical incentive for reducing the external noise from the ships. E.g., ships calling at a port without information on the noise emitted to the surroundings, or with high noise source levels, could be charged with an increased fee for berthing.
Another option is to enter a dialogue with the ships in order to explore if the operational time of different ventilation systems and other machinery could be decreased.
In some container terminals it may also be possible to rearrange containers and similar moveable equipment to screen noise sensitive areas from the ships.
4.5 Summary – noise reduction
Figure 12 shows a diagram of different solutions to different external noise problems and a ranking of the cost of the solutions. The diagram is based on the major sources of external noise on board a ship during port stay. Each of the solutions is assigned with a colour a letter defining the approximated cost. Red colour and the letter C indicates that the implementation of the solution is evaluated as being cumbersome, time-consuming and expensive. Yellow colour and the letter B indicates that the cost of the solution is estimated to be in the intermediate range i.e. that the solution can be implemented with less structural changes to the ship, that it is less time-consuming and is less expensive. Green colour and the letter A indicate that the solution is the least cumbersome to implement, the least time-consuming and estimated as the least costly of the presented solutions. Reducing noise from diesel generators will often require major structural changes to the ship e.g. installing new silencers. Structural changes of this magnitude are performed at a shipyard which is considered costly. Whereas, installing mineral wool in a fan room is simple and can be carried out by the ship’s crew or a contractor during port stay. A noise reducing measure on this scale is therefore considered less costly.
Figure 12 Diagram of different solutions to different external noise problems and the approximated cost of the solutions. Red colour and the letter C indicates estimated highest cost, yellow colour and the letter B indicates estimated less cost and green colour and the letter A indicates estimated least cost.
Version 1.0 June 2010, © Danish Environmental Protection Agency