Main engine
Section 6. Accommodation and workspace vibration
6.1 Scope
6.1.1 The scope of this section is the evaluation of hull and superstructure vibration with regard to crew habitability in all normally occupied spaces. The vibration levels given in this section do not apply to passenger cabins or other passenger spaces except in so far as they are working areas.
6.1.2 The evaluation applies to normal service operating conditions with the main propulsion machinery developing at least 85% of the maximum continuous rated power. The evaluation is not intended to apply to manoeuvring conditions or operation in heavy weather.
6.2 Measurements
6.2.1 Measurements should be taken on the deck in three mutually orthogonal directions: vertical, athwartships and longitudinal.
6.2.2 Where normal transducer attachment methods are impractical, due to the presence of deck coverings such as carpets, an inertia block with a mass of at least 1kg with spiked supporting legs may be used to locate the transducers.
6.2.3 In cabins, vibration readings are to be taken in the centre of the floor area. For larger spaces, such as mess rooms and the navigating bridge, sufficient measurements should be taken to define the vibration characteristics.
6.3 Test conditions
6.3.1 Measurements should be taken with the vessel proceeding ahead at a constant speed and straight course in a depth of water not less than five times the draught.
6.3.2 Tests should be conducted in sea conditions not greater than sea state 3 (slight).
6.3.3 The vessel should be at a displacement and trim corresponding to a normal operating condition.
6.3.4 Rudder angle variations should be limited to ± 2° of the amidships position.
6.4 ISO 6954:1984 and ISO 6965:2000
6.4.1 ISO 6954:1984 Guidelines for the overall evaluation of vibration in merchant ships have been superseded by ISO 6954:2000. At the time of writing (2006) the latter version remains to be widely adopted. This reflects the considerable experience which exists among shipowners, shipbuilders, operators and Classification Societies with regard to the application and interpretation of the earlier version. This suggests that ISO 6954:1984 will remain in general use as a specification requirement until confidence has been established in ISO 6954:2000.
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Ship Vibration and Noise Guidance Notes Section 6
6.4.2 Assessment criteria and limits for both standards are given below. The measurement parameters, frequency range, analysis methods, and severity criteria are different for the two versions and care must be taken to define which one is being used.
6.5 Lloyd’s Register’s assessment of accommodation and workspaces based on ISO 6954:1984
6.5.1 The guidance values are based on ISO 6954:1984 Mechanical Vibration and shock -Guidelines for the overall evaluation of vibration in Merchant Ships (1984). The limits of this Standard have been adopted as Figure 6.1 of the Guidance Notes.
The amendments and interpretation are intended to facilitate: simpler analysis of routine survey measurements; evaluation of the sum of two or more frequency components; and differentiate between rest and working areas.
6.5.2 The severity criteria are intended to apply to the position of maximum vibration within the space under consideration. Vibrations in each of the three orthogonal directions should be assessed independently. Analysis of measurements should be in accordance with
Section 3.2.
6.5.3 Broadband measurements.
The following severity classification applies to broadband measurements of the maximum overall vibration amplitude (peak) for vibrations within the frequency range 1 to 100 Hz.
The values are the maximum repetitive amplitudes which occur during a minimum measuring period of 60 seconds.
Vibration levels are normal if:
• Velocity is below ± 6 mm/s peak,
or
• Acceleration is below ± 0.15 m/s2 peak.
Vibration levels are excessive if:
• Velocity exceeds ± 15 mm/s peak,
and
• Acceleration exceeds ± 0.30 m/s2 peak.
For intermediate vibration levels severity should be assessed from a narrowband frequency analysis as described in Paragraph 6.5.3.
6.5.4 Narrowband measurements.
Broadband measurements of vibration having levels which fall between the normal and excessive levels given in Paragraph 6.5.2 should be assessed using a narrowband frequency analysis in relation to the guidance values shown in Figure 6.1. Values are the maximum amplitudes of individual narrowband components which occur during a minimum measuring period of 60 seconds.
No single component should exceed the “upper boundary”. For sleeping and rest areas only one component may be within the stippled zone.
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SECTION 6 Ship Vibration and Noise Guidance Notes
Figure 6.1 Accommodation and workplace vibration levels.
Adverse comments not probable Adverse comments probable
1 1 mm/s
0.01 m /s² 10 mm/s
Velocity (peak), ± mm/s
Frequency, Hz 10 0.
1 m /s² 100 mm/s
0.1 m m
1 m /s²
100 0.01 m
m 10 m
/s² Displacement, ± mm Acceler
ation, ± m/
s² 100 m
/s² 1 m
m 10 m
m
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Ship Vibration and Noise Guidance Notes Section 6
6.6 Assessment accommodation and workspaces based on ISO 6954:2000
6.6.1 Guidance values are based on ISO 6954:2000 Guidelines for the overall evaluation of vibration in merchant ships.
6.6.2 The severity criteria are intended to apply to the position of maximum vibration within the space under consideration. Vibrations in each of the three orthogonal directions should be assessed independently. Analysis of measurements should be in accordance with
Section 3.3.
6.6.3 The recommended maximum acceptable values of frequency weighted r.m.s. vibration velocity are given in Table 6.1. for vibration within the frequency range 1 to 80 Hz.
Table 6.1 Accommodation and workplace vibration levels.
Normal Excessive Frequency weighted r.m.s. mm/s.
Crew accommodation areas 4.5 6
Working areas 6 8
6.7 Other ship types, PCAC
6.7.1 Vibration habitability criteria for passenger ships, high speed craft and yachts are given in LR’s Provisional Rules for Passenger and Crew Accommodation Comfort, (PCAC).
6.7.2 These Rules define the vibration severities in all relevant areas of the vessel for classifications of passenger comfort. Additionally, criteria are given for crew accommodation and working spaces which can be applied to conventional merchant ships where an enhanced comfort standard is required in relation to the basic acceptability limits given above in Sections 6.4, 6.5 and 6.6.
6.7.3 The PCAC Rules also stipulate more comprehensive test conditions and survey procedures that must be followed if a Class comfort notation is assigned to the vessel.
6.8 Motion sickness
Motion sickness is influenced by a number of factors such as translational and rotational oscillations, constant speed rotations about an off-vertical axis, Coriolis stimulation and movements of the visual scene. Whilst some types of motion can be reliably predicted to result in more nausea than others, motion sickness is neither explained nor predicted by dynamics. Biological factors such as age and sex also have an influence.
Acceleration, exposure time, and frequency of motion can be used to derive motion sickness indices, such as the Motion Sickness Dose Value (MSDV) described in ISO 2631. Motion sickness may be expected to occur in some unacclimatized people at accelerations greater than 2 to 3 m/s2 at roll periods of around 10 seconds for normal ship type hulls.
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SECTION 7 Ship Vibration and Noise Guidance Notes