The Naval Architect: May 2018
The 5 February 2018 saw the publication of ISO 21984: 2018: ‘Guidelines for measurement, evaluation and reporting of vibration with regard to habitability on specific ships’, developed by ISO/TC8/SC8 (the Ship design sub-committee of the Ships and Marine Technology technical committee). On the final ballot, draft ISO 21984 was approved by 17 ISO members without any other ISO members with no votes cast against it.
Prior to this, ISO 20283-5: 2016: ‘Guidelines for measurement, evaluation and reporting of vibration with regard to habitability on passenger and merchant ships’ was developed by ISO/TC108/SC2 (Measurement and evaluation of mechanical vibration and shock as applied to machines, vehicles and structures) and published on 15 December 2016, replacing ISO 6954: 2000.
Since there are now two similar international standards for ship vibration, it is necessary to understand the characteristics and background of each standard.
ISO 6954: 2000: ‘Guidelines for measurement, reporting and evaluation with regard to habitability on passenger and merchant ships’ has been referred to frequently in the newbuilding contract specifications for passenger ships and cargo ships without particular complaints since the earlier guidelines for vibration (ISO 6954-1984) were revised.
For each space classification, i.e., A: passenger cabins, B: crew accommodation areas and C: working areas, there are two sets of overall frequency-weighted RMS values: ‘Values above which adverse comments are probable (Upper values)’ and ‘Values below which adverse comments are not probable (Lower values)’. It is noted in ISO 6954: 2000 that: “The zone between Upper and Lower values reflects the shipboard vibration environment commonly experienced and accepted”.
In general, Upper values have been agreed as a ‘Guarantee’, whereas Lower values have been agreed as a ‘Non-binding target’ between the interested parties (e.g., ship owner and shipbuilder) based on the common understanding of the above note.
ISO 20283-5: 2016, however, specifies a single unified limit for both passengers and crew onboard all types of ships without due consideration to technical obstacles to design for protection against vibration on numbers of, in particular, large merchant ships. In other words, there is no distinction between passengers onboard passenger ships and crew on board merchant ships, unlike ISO 6954: 2000, IMO SOLAS requirements and Class voluntary guidelines for ‘Comfort on ships’.
At the latest ISO/TC108/SC2/WG2 meeting held in France in July 2015, in which experts from France, Germany, Japan, South Korea and the USA participated, the Japanese presented the results of statistical study based on actual ships (built in and after 2004, around 1,100 measuring points) Based on these findings, the Japanese requested to keep Upper values of ISO 6954: 2000 as one set of Guideline values in ISO/CD 20283-5, since probability of exceedance of the maximum vibration in various spaces, in particular in the crew accommodation areas and on the navigation bridge, were expected to increase significantly if the original set of Guideline values in ISO/CD 20283-5 were adopted. The reason these areas are vulnerable to large vibration is self-evident: they are located near the top of deck house (superstructure).
At the latest ISO/TC108/SC2/WG2 meeting, South Korea also presented the results of their statistical study based on recent ships delivered successfully and requested either to keep two sets of values (Upper and Lower) as per ISO 6954: 2000 or to raise the values specified for crew accommodation, navigation bridge and offices if one set of guideline values should be adopted for the said spaces in ISO/CD 20283-5. This would mean that all ships studied by Korea would satisfy the values.
Also at the meeting, France presented the results of their own statistical study based on actual ships (around 6,000 measuring points), and requested further to reduce the one set of Guideline values in ISO/CD 20283-5. Averages of actual vibration were incredibly low and almost equivalent to standard deviations as compared with those presented by Japan and Korea. However, the background was that actual ships studied by France did not cover sufficient number of ‘large’ cargo ships and the majority were passenger ships (including cruise ships) to which the voluntary “Vibration notation” was usually granted by Bureau Veritas (BV) upon the request of ship owners. It is natural that passenger ships have numerous numbers of cabins and crew accommodations of the same types, which consequently result in numerous measuring points and therefore the averages of vibration are dominated by the measurements on passenger ships.
According to the statistics presented by Japan and Korea, the common vibration limits specified by ISO 20283-5: 2016 are quite easy to achieve for passenger ships, but hard to achieve for 20-30% of (in particular large) cargo ships because of the significant difference in design conditions.
- Passenger ships (including cruise ships) have various technical advantages over cargo ships in terms of minimisation (control) of vibration:Since the superstructure (deck house) is arranged widely along the length and breadth of the ship, vibration in longitudinal and transverse directions is negligible.
- The propulsion plant usually consists of generators driven by medium-speed diesel engines and electric motors and/or azimuth thrusters including podded thrusters for better manoeuvrability in port. Elastic mounting technique is applicable to such diesel engines, which can effectively cut off (reduce) the transmission of exciting forces into the hull structure. Medium-speed diesel engines of a large number of cylinders arranged in ‘V’ configuration with bank angle have less exciting forces by nature because of the self-cancelling effect. Motor rotors have no exciting forces as compared with reciprocators.
- The total lengths of the propeller shaft, intermediate shaft and crankshaft are relatively short, which can result in less probability of resonance because of higher natural frequencies of axial and torsional vibrations of shafting.
- Displacement and draft are almost unchanging, and hence have almost fixed natural frequencies and vibration modes. This makes it easier to avoid resonance.
- Controllable pitch propellers (CPP) are fitted in most of the cases for better manoeuvrability in port. If resonance of vibration is observed, rotational speed of propellers may be changed by adjusting pitch angles so that the resonance can be avoided.
- Because of business needs, higher priority is given to the minimisation (control) of vibration and better manoeuvrability by the ship owner.
On the contrary, cargo ships have various technical disadvantages over passenger ships in terms of minimisation (control) of vibration:
- Since length and breadth of the deck house (superstructure) are limited as compared with its height, and the deck house (superstructure) is usually arranged just above the engine room and near the propeller, vibrations in longitudinal and/or transverse directions of a cantilever mode are expected in addition to vertical direction. The vibration levels in longitudinal and transverse directions increase with the height levels.
- The propulsion plant usually consists of a low-speed and long-stroke diesel engine. Elastic mounting technique, which is effective to cut off (reduce) the transmission of exciting forces into the hull structure, cannot be applied to such low-speed diesel engines since movement of the diesel engine and shafting in rough seas becomes inevitably large, which leads to mechanical damage to bearings, etc. A low-speed diesel engine with a relatively small number of cylinders arranged in ‘L’ (upright) configuration without bank angle has large exciting forces by nature because of a smaller self-cancelling effect.
- Total length of the propeller shaft, intermediate shaft and crankshaft are relatively long, which can result in a greater probability of resonance because of lower natural frequencies of axial and torsional vibrations of shafting.
- Displacement and draft are drastically changing, and hence significant changes in natural frequencies and vibration modes. This means that avoidance of resonance is rather difficult.
- The fixed pitch propeller (FPP) is fitted in most cases. Even though resonance of vibration is observed under normal output of the diesel engine, the rotational speed of propellers or diesel engine is hard to change.
- Because of business needs, the highest priority is given by the ship owner to the fuel efficiency (EEDI/EEOI) and reduction of NOx/SOx/CO2 emissions.
In consideration of IMO’s Resolution A.947(23) and the International Goal-based Ship Construction Standards for Bulk Carriers and Oil Tankers (GBS), and the ILO’s Maritime Labour Convention (MLC), 2006, at the end of 2013, the International Association of Classification Societies (IACS) developed Recommendation (Rec.) No. 132: ‘Human Element Recommendations for structural design of lighting, ventilation, vibration, noise, access and egress arrangements’, which covers all ship types. Since GBS requires ‘Human Element considerations’ as one of its 15 functional requirements, the conformity of IACS Rec. No. 132 to GBS has been verified by an audit team established by IMO. Neither nonconformity nor observation was found by the team about values of acceptable vibration specified in Rec No. 132, which was duly authorised by IMO at MSC 96 in May 2016.
This means that for bulk carriers and oil tankers (the majority of cargo ships), IACS Rec. No. 132 has effectively mandatory status in real terms, which is stringent for shipbuilders and ship owners.
The 5mm/s for Accommodation Areas and 6mm/s for Workspaces limits are in-between the two sets of values (Upper and Lower) defined by ISO 6954:2000. It is stated by IACS that these values “are generally not considered to be uncomfortable” but “should not be exceeded”.
Likewise, ISO 20283-5: 2016 may also be bound to mandatory SOLAS in the future even though the ISO is of recommendatory status. If the Guideline values of acceptable vibration specified in the ISO should prove impractical for a significant number of cargo ships, that could bring serious confusion and trouble across the industry, as well as IMO.
Considering both the statistics presented by Japan and Korea and Rec. No. 132, ISO 21984: 2018 specifies slightly higher vibration limits for crew accommodation spaces and the wheel house onboard cargo ships, compared with those specified by ISO 20283-5: 2016 as shown in Figure 8. However, no complaints against such vibration limits have been made by the owners and cargo ships conforming to ISO 21984: 2018 and IACS Rec. No. 132 have satisfactorily been in operation.
As described in the scope, ISO 21984: 2018 is applicable to specific ships satisfying one or both of the following conditions:
a) A 2-stroke cycle, long-stroke, low-speed diesel engine directly coupled to the fixed-pitch
propulsion propeller is installed.
b) The length of deck house (L) is limited as compared with its height (H) (i.e., deck house of around
1,0 and above in slenderness ratio of H to L).
ISO 20283-5: 2016 is generally applicable to all ships. Requirements for measurement, evaluation and reporting of vibration with regard to habitability for all persons on board of passenger and merchant ships, including specific ships to which ISO 21984: 2018 may also be applicable can be found in ISO 20283-5: 2016. ISO 21984: 2018 is neither complementary nor additional but supplementary to ISO 20283-5: 2016. The shipbuilder can select either ISO 21984: 2018 or ISO 20283-5: 2016 to apply to any specific ship upon due consideration to individual design conditions of the ship and experience, if any, in building sister or similar ships.
Future study on human response to vibration on cargo ships is needed to provide a more sound basis for specifying acceptable vibration limits, although this requires time and sponsorship and therefore cannot immediately solve the problem for cargo ships caused by ISO 20283-5: 2016 (the revision of ISO 6954: 1984 and ISO 6954: 2000 took 16 years, respectively).
The study is needed since ISO 6954: 1984 was developed based on the vibration measurements on board cargo ships which had been built up to the late 1960s and no new data was considered additionally in the development of ISO 6954: 2000 and ISO 20283-5: 2016. Development of ISO 21984:2018 was itself based on research of the Japan Ship Technology Research Association supported by the Nippon Foundation.
The establishment of a single ISO standard for vibration, which can realistically cope with both passenger and cargo ships, is the best solution in the future, too. The acceptable vibration values for both types of ships need to be decided based on the results of the future study mentioned above.
Shinichi Hirakawa PhD is general manager for the Structural Research Group at the Technical Research Centre of Japan Marine United Corporation.
hirakawa-shinichi@jmuc.co.jp
