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"DNVC dOMFORT CLASS", A New Concept Ensuring Acceptable
Noise and Vibration Levels on Board High Speed Vessels
By Kai A. Abrahamsen
DNVC A/S,
1322 Havik,
Norway
Abstract:
Noise, vibration and sea induced motion are probably the most important parameters
determining the comfort on board high speed ferries. Other parameters are climate, air
pollution, lighting, seating comfort, space, availability of safety measures, crew and interior
appearance.
High speed passenger vessels are inherently more noisy than conventional passenger
vessels, due to high power to weight ratios, short transmission paths and restricted space
and weight allowance for noise reducing measures. The high speed and moderate size of
such vessels make them potentially vulnerable to strong sea induced motions.
In order to assist owners and yards to improve the comfort on board high speed passenger
vessels, the DNVC "Comfort Evaluation" may be of valuable assistance. The "Comfort
Evaluation comprises the DNV "Comfort Class" and a service for calculation of the
"Sea Comfort Index".
"Comfort Class" is a voluntary class notation specifying comfort criteria for noise,
vibration and indoor climate. The "Sea Comfort Index" is a systematic approach to
evaluation of the probability of motion sickness among passengers and crew.
1. INTRODUCTION
The future success of the liigh speed vessel industry depends on the ability of liigh
speed vessels to cany passengers safely and comfortably at high speed over exposed i
waters. jHè safety aspect is being taken care of by ordinary classification rules as well
as regulations specified by IMO and national authorities.
Comfort has traditionally been regarded as an important property of a design, but has
fi^equently been dealt with in a rather random way. Owners, shipyards and designers
have had difficulties in communicating due to lack of accepted criteria and inadequate
knowledge in this field. Some projects have been able to specify certain criteria based
on previous experience, others have had to rely on the ability of all parties involved to
deal with a rather rough and often imprecise specification. This unsatisfactory situation
is the reason for the development of the DNV Comfort Evaluation which comprises
the DNV "Comfort Class" and a service for calculation of the "Sea Comfort Index".
"Comfort Class" is a voluntary class notation specifying comfort criteria for noise,
vibration and indoor climate. The class is issued when the fulfilment of the criteria has
been verified by measurements. Noise , vibration and climate are classified according
to a comfort rating firom 1 to 3, which reflects "acceptable" to "high" levels of
comfort. In addition to state requirements to defined comfort related standards, the
rule text describes measurement procedures, international standards to be followed and
the instrumentation to be used for measurements. This information is important for the
tnie assessment of vessel comfort, but is often missed in the building specification.
"Sea Comfort Index" is a measure of the probability of motion sickness among
passengers and crew. For two different vessels operating in the same area, the "Sea
Comfort Index" will give an objective evaluation of the seakeeping performance of
vessels in relation to human comfort. Sea comfort rating of this kind is particularly well
suited for relative comparison between different vessel designs and routes. The "Sea
Comfort Index" is initially offered as an advisory service, but will at a later stage be
incorporated in the "Comfort Class".
THE CONCEPT OF COMFORT
Comfort isjdefined as "A State of Physical Well Being". The overall perception of
comfort DA board a passenger ship depends on a number of different factors associated
with the on board environment, safety, facilities, design and space, see table 1.
Livestigations on the relative importance of environmental factors among seafarers, ref /
GOETHE et. al. (1978) & SAN (1978)/, have shown that noise, vibration and sea bduced
motions are rated as the clearly most troublesome factors. Other factors such as climate,
air pollution, lighting, etc., were rated as troublesome by substantially fewer of the
subjects.
Although the referenced investigations dealt with able seamen on board large merchant
ships, one may assume that the same situation rouglily will apply to passengers on board
fast ferries. High speed, compact design, light weight structures and Wgh power
requirements are factors that make high speed ferries vubierable to unfavourable motions
as well as high noise and vibration levels. Hence, it is hnportant to take care of these
properties during the design of a vessel.
The ability of a vessel to operate comfortably under varying climatic conditions is
important. Many high speed vessels operate in countries with highly variable clhnate or
they are transferred between different parts of the worid depending on season. For such
vessels it is important to have a documented ability to maintam a satisfactory interior
climate under varying environmental conditions.
Environment
;>
* Sea Induced Motion
* Noise
* Vibration
* Indoor Climate
* Illumination
* Odour
Safety Design
*
*
Emergency equipment
Alarms
Escape routes
Crew appearance
Information
Architectural design
Window vision
Outward appearance
Cabin layout
Furniture
Facilities Space
Restaurants/Cafeterias
Shops
Sanitary facilities
Information/Assistance Service
Entertainment
Spaciousness of interior
Passenger density
Loftiness
Cabin size
Table 1, Aspects innuencing the overall perception of comfort on board a
ship.
3. CRITERIA FOR NOISE, VIBRATION, CLIMATE AND SEA
INDUCED MOTIONS
3.1 General t
Intemaiional standards have been used as foundation for the "Comfort Class" rules,
but have not necessarily been adhered to.. When determining criteria to comfort on board
high speed vessels, due consideration has to be given to technical and practical
limitations inlierent in the design and construction of the vessels. Otherwise rather
unrealistic criteria would be derived. It is therefore important to see the criteria in
relation to the situation on board a liigh speed vessel and not be confused by what one
could require in a different situation. The concept of comfort will be relative to what it
is practical to achieve for a particular application. Hence, the comfort criteria for high
speed vessels may have to be adjusted i f fiiture design developments improves the
attainable comfort levels significantly.
The criteria for noise and vibration discussed below apply to steady state normal transit
conditions. It is self evident that short and infrequent exposures should be considered
separately. The criteria for sea mduced motions apply to time averaged exposures.
The noise , vibration and climate criteria are divided into three groups depending on
the level of comfort achieved, i.e. comfort rating number (cm) 1, 2 and 3,
where crn (3) represents the hignest comfort level and crn (1) represents an
acceptable level of comfort.
The lowest crn-number achieved for noise or vibration will determine the overall
rating for noise and vibration. This means that a vessel meeting crn (2) for vibration
and crn (3) for noise will be denoted cni (2). A separate crn-number Avill be given for
the indoor climate when relevant.
Noise Criteria
Airborne Noise is defined as pressure fluctuations detectable by the ear in the firequency
range 20 H^'to 20 000 Hz. It is measured in decibels, dB, and defined as: 1
I . /
Sound Pressure Level = 20 log (P / Pref), dB
where: P - Sound pressure in Pa
Pref - Reference pressure 2x10 * Pa
A reduction in noise level of 3 dB is just detectable by the human ear, although this actually
represents one halve of the initial noise energy. A drop in the noise level of 10 dB is
perceived subjectively as a halving of the loudness.
The measured sound pressure level is usually subject to a fi^equency weighting called
A-weighting and denoted as dB(A). The A-weighting is a fi-equency response curve
approximating the ear sensitivity to various fi-equencies. Hence, the A-weighted noise level
is a measure of the noise as it is perceived by the human ear.
The following noise criteria have been derived for high speed passenger vessels:
Table 2 . High Speed, Light Craft
Maximum Noise Levels in dB(A)
Locations
comfort rating number
(crn) Locations
Less than 50m L O A More than 50m L O A
Locations
1 2 3 1 2 3
Passenger localities 75 70 65 65 60 55
Navigation Bridge 65 60 55 60 55 50
Working places / shops / kiosk 80 75 70 70 65 60
The choice of comfort rating nmnber will necessarily be a compromise between the desire
for a low noise level, technical feasibility and cost considerations.
In additioi^\to the requhements to high speed vessels for passenger transportation
given in tafcle 2 above, separate criteria have been derived for yachts. Yachts are used
for recreation purposes and the passengers are usually on board for a relatively long
time period. Also, the weight and space allowance for noise reducing measures are
usually more generous than for the commercial type of high speed passenger craft.
Hence, the criteria are significantly stricter for yachts than for other high speed light
craft.
Koisc levels in dB(A) |
Locations
comfort rating number
(crn) Locations
1 2 3
Sleeping rooms 45 40 35
Lounges / Saloons 50 45 40
Outdoor Recreation Areas 65 60 55
Navigation Bridge 60 55 50
It is also important to realise that the IMO resolution A.468(XII) 1981,"'Code on Noise
Levels on Board Ships", or national authorities may apply in the crew areas. These criteria
have been set to protect the crew fi"om hearing damage and to avoid disturbance of
communication, work performance and rest.
In addition to the noise criteria shown above, the "Comfort Class" also contains
requirements to sound insulation and impact sound (stepping noise). These will, however,
seldom be relevant for high speed vessels, but may have significance for yachts.
The requirements are stated as tlie sum of the relevant noise criterion hp and the
weighted apparent sound insulation index, ref ISO 717. This has been done because a
low background noise level will require a stricter requirement to sound insulation m
order to achieve a satisfactory level of comfort.
Cabin to cabia (crew) 88
Cabin to cabin (passenger) 90
Cabin to corridor 87
Cabin to stairways 100
Cabia to engine rooms 100
Cabin to public spaces 100
Mach./ techn spaces to passenger corridor 100
For the cabins in general, the normalised impact sound pressure level is not to exceed
50 dB. For areas with wooden or marble deck covering, the above requirement may be
relaxed to 55 dB due to constructional limitation. Such covering materials should
preferably not be used above passenger cabins.
For cabins located below dance floors, show rooms and gymnasium, a normalised
impact sound pressure level is not to exceed 45 dB.
3.3 Vibration Criteria :
Vibration on board ships may have three types of detrimental effects :
Fatigue damage to the structure
Cause damage to or impair proper fijnctioning of machinery and equipment
Annoyance and discomfort to crew and passengers
Only the comfort aspect of vibration will be treated in this paper.
Vibration is defined as mechanical motion in the fi-equenqr range 1 Hz to 100 Hz. The
vibration lijjiits are given in vibration velocity, peak amplitude. I f RMS (Root Mean
Square) yjÜues are measured, each fi^equency component may be converted to peak
amplitude by multiplication o{^/2 . .
It should be noted that ISO 6954 defines a conversion factor to be multiplied with the
time averaged peak values. The obtained "max. repetitive value" should be compared
to the guideline. In the "Comfort Class" rules, the time averaged peak values are to be
directly compared to the given limits, since a conversion factor is already incorporated
in the limits.
The ranges outlined apply to each single firequency component of vertical, fore and aft
and arthwartship vibration which is to be assessed separately.
1 / V:,:Table;5 ;̂ffi
Vibration level in mm/s peak for single frequency components above 5 Hz
Locations
comfort rating number
(crn)
• 1 2 3
Passenger localities 5.0 4.0 2.0
Navigation Bridge 5.0 4.0 - 2.0
Offices 5.0 4.0 2.0
Control Rooms 6.0 5.0 3.0
For fi-equencies below 5 Hz the requirements follow constant acceleration curves
corresponding to the acceleration at 5 Hz.
Again somewhat stricter criteria have been derived for yachts.
Locations
Private Accommodation
comfort rat ing number
( c m )
3.0 2.0 LO
Navigation Bridge 4.0 2.5 L5
For frequencies below 5 Hz the requirements follow constant acceleration
corresponding to the acceleration at 5 Hz.
curves
3.4 Climate
"On board Climate" is defined as a general name for the physical factors that influence
human beings inside a vessel or installation at sea.
Ambient temperature, temperature gradient, air velocity, humidity and carbon dioxide
concentration are used as descriptors for indoor climate. The "Comfort Class" rules
outline standards, conventions, guidélines and specifications for the purpose of
categorisation of a vessel's interior climate in relation to the performance of the on
board Heat, Ventilation, and Air Conditioning ( HVAC ) plant.
The rules apply to passenger vessels with a dead-weight exceeding 100 tons or 50 m
and to cargo vessels exceeding 300 tons in dead-weight. Hence, only larger high speed
vessels will have to comply.
The requirements to interior climate are related to the main class issued for the ship.
The requirements are divided in groups for specified locations. All the locations
specified in the tables below are to comply with the criteria in order to be assigned a
Comfort Class notation.
1 i - Si: :: :::; : .•.•:-:x-M-::-x-\-::-:->:-.-:-:-:-:-:-:!:y^
Type A
l i i i l f i i i ;
Cabin accommodation spaces for crew and passengers
~ j
Type B Public spaces excluding toilettes and spaces intended for passage only
TypeC Hospital Areas
TypeD Navigation BridgeAVheel house. Engine Control room, Office Areas,
Crew Messes/Recreation rooms
The requirements to air quality at different localhies and comfort ratings are shown in
table 8. The following definitions apply for table 8:
Temperalure: The average temperature of a specific number of temperature
measurements in a particular space, recorded during 30 minutes, expressed in degree
Celsius.
Relative humidity: The quotient of the vapour content in the air and the saturation
vapour content of that air expressed m percent.
Air velocity: The measured mean absolute velocity of a mass of air in motion.
Ambient outside air temperature: The actual air temperature measured out of direct
sun exposure outside of the vessel, expressed in degree Celsius
Draught: The unwanted local cooling of the body caused by air movement.
Vertical gradient: Vertical air temperature difference.
Air operative temperature: A measure of the equivalent heat loss fi-om a human body
caused by convection and radiation that the actual temperature causes, expressed in °C.
(It can be approximated by the globe temperature).
Air supply quantity: The nominal quantity of firesh/outside air per person supplied to a
space, expressed in 1/s.
Concentration of CO-,: The volume quotient of CO^ to air expressed in ppm.
Designated Type Table 6 Onboard Climate Classification
Climate Parameters (30 niirt inè̂ Mt viaiüés)
Air Opernlive Vertical • Max Air Relative Air supply Conc. 0 UaOC Tein|>eralure Air air Humidity quanta Max.
c m Operat. velocity Rl) Min Outside*' ̂ €02 comfort Summer Winter Temp. Summer Winter Tresh air rating Gradient per person
number rci [°C/ml [m/sj [%] llit./sl [ppmvl A Cabin 1 26(+/-2.0| 221+2.5/-1.01 4.0 0.40 <60 - 7 1200
Accommodaiion 2 24[+/-1.51 22t + 2.0/-1.0) 3.5 0.35 <55 >20 10 1000 Spaces 3 22(+/-1.0| 23(+/-l.51 2.5 0.25 <50 >30 10 1000 Public spaces intended Tor liigii piiysical 1 26[+/-2.5] 22[+3.0/-2.01 4.0 0.40 <65 - 7 1200
B i activity and or spaces 2 24I+/-2.01 22I+2.5/-2.01 3.5 0.30 <65 >20 10 1000 " 1 such as: 3 22(+/-l.51 23[ + 2.0/-1.5] 2.5 0.25 <60 >30 12 800
Dance Lounge, Disco Gymnasium •
Public spaces intended for medium physical
B o activity and or spaces 1 26(+/.2.01 22(+2.5/-2.01 4.0 0.40 <65 - 7 1200 " L such aa: 2 24(+/-1.5I 22(+/-2.0I 3.5 0.30 <60 >20 10 1000
Show Lounge, Dining 3 22(+/-1.0) 23(+/-1.5J 2.5 0.25 <60 >30 12 800 Room, Atrium, Casino shopping area, Bars
Public spaces intended for low physical 1 261+/-1.5! 22( + /-2.0) 3.5 0.35 <65 - 7 1200
B 3 activity and or spaces 2 241+/-1.01 22(+/-1.51 3.0 0.25 <60 >20 10 1000
B 3 (uch aa: 3 22I+/-0.5) 23t+/-1.01 2.5 0.20 <55 >30 12 800 Conference Room Library, Carè rooms Sealing area
1 25I+/-2.0) 22[+2.5/-1.0] 3.5 0.35 <60 >20 7 1000 Hospital 2 24(+/-l.5] 221+ 2.0/-1.0] 3.0 0.25 <55 >30 10 1000
c Ward Rooms 3 22(+/-I.01 23{+/-1.0) 2.5 0.15 <50 >30 12 800
1 251+/-2.5] 22(+/-3.01 3.0 0.25 <65 - 7 1200 Office 2 241+/-2.0I 22(+/-2.51 3.0 0.25 <60 >20 10 1000
D Wheelhoitse 3 221+/-I.5] 231+/-2.0J 2.5 0.20 <55 >30 12 800 Eng. eontr. room
I t is required individual room temperature control of spaces designated type A, B,
and D.
In order |b achieve the designated comfort rating, the maintainability and the
redundapcy of the system is to fulfil certam minimum requirements. The requirements
are stated in the fiill rule text.
Air filters in air handling units or fan-coil units supplying air to designated spaces shall
have a minimum filtration efficiency* according to the following European or US
standards:
Spjcc crn Filter Performance - new filter
1
2
3
E U 3 / G 8 0
E U 5 / F 4 5
E U 6 / F 6 0
9 0 % of PM > 7-9 micron
9 0 % of PM > 3-4 micron
1 E U 3 / G 8 0
B 2 E U 5 / F 4 5
3 E U 5 / F 4 5
1 EU7 /F85
2 E:U7/F85
3 E U 7 / F 8 5 9 0 % of PM > 1 micron
1 E U 5 / F 4 5
D 2 E U 5 / F 4 5
3 E U 6 / F 6 0
Table 9, Filter requirements.
* Airborne particles are inlierently difficult to measure accurately and it is difficult to isolate
the source of the particles. The particles in the supply air which often dominate on board
vessels can be reasonable checked by surveying the supply air tllters instead of measuring the
particulate concentration in the air.
3.5 Criteria for Sea Induced Motions
Low frequent (below 1 Hz) naotions on board a vessel may cause motion sickness. This
undesirable effect may range from slight discomfort tlirough dizziness and nausea to
vomiting and complete disability. These symptoms vary from subject to subject in severity
and duration and may change for the same subject depending on circumstance and
habitation. Tolerance varies considerably with age, sex, vision, fear, head movement,
odours, activity and the ingestion of certain foods and drinks. Tliere is also a tendency to
adapt with frequent exposure.
The calculation of the "Sea Comfort hidex" is based on the boundaries stated in ISO 2631.
This standard defines severe discomfort boundaries related to vertical accelerations and
time of exposure. The ISO boundaries and results from two other investigations are
plotted in figure 1. It is evident that designers should try to limit motions in the 0.1 Hz to
0.315 Hz region in particular.
3.15
2.5
^ 2.0
1 1.6 V)
E 1.25
.S 1.0 ra b 0.B
^ 0.63
0.5
0.'.
0.315
0,25
0.2
0.16
0.125
0.1
Motion sickntM r«gion -, A-
ij
•
/ / / / /
,/ m
mln ^// ' / •
V
7 /
f
\ - \
mln_ V' / Z
f f f
— -
\ Ih 0
1 i — 2Sh
/ 1—
ISO 263 1 / 3
— Goto (19831
[aulcy et. al.
1 0̂ 1 0.12S 0.16 0.2 0.25 0.315 0.4 0.5 0.63 0,8 1.0
Frequency IHiI
Figure 1 Severe discomfort boundaries from ISO 2631/3-1985. Data from /GOTO
(1983) and McCauley ct al. / are included for comparison.
4. NOISE CONTROL
4.1 Geneml
The "ComfiaVt Class" requirements have to be verified througli measurements. It may,
however, be advantageous to cany out calculations at an early project stage in order to
ensure that necessary noise and vibration control measures are included. Different aspects
of noise control and noise calculations are.outlined below.
For vessels without any noise control consideration noise levels in the range fi-om 80
dB(A) to above 90 dB(A) may occur. With noise control measures included in the design,
noise levels in the range fi-om about 65 dB(A) to 75 dB(A) are possible . The above applies
to the noisiest position on board, which usually will be in the aft ship and directly above or
next door to the engine rooms. Larger vessels will usually be considerably less noisy, due
to longer transmission paths for the noise and because of more favorable arrangement of
the passenger localities.
4.2 Noise Mechanisms
On board a high speed vessel there are numerous noise and vibration sources. The most
significant sources are the wateijets/propellers, the main engines, gears, shafting systems
and auxiliary machinery including lifting fans for SES vessels, see figure 2. Further, there
are various secondary sources like hydraulics, ventilation fans, exhaust system, HVAC, sea
and wind. The relative importance of the different sources depends on the actual design of
the vessel, type of equipment and installation.
The noise originates at the source and is transmitted tlirough the structure or through an
air or a fluid path (e.g. hydraulics). When arriving at the receiving position the noise is
influenced by the radiation and absorption properties of the materials used in that position,
as well as the size and the shape of the room.
Noise control on board high speed vessels is a complex task. Light weight structures, high
power requirements and short transmission paths fi-om machinery to passenger locations
make Iiigli speed vessels inherently more noisy than other passenger vessels. Strict weight
and space restrictions limit the use of conventional noise and vibration reducing measures.
On board a high speed vessel a multitude of sourceSj transmission patlis and radiating
surfaces may" have importance for the resulting noise. Efficient noise control depends on
detail knowledge about source strengtli, layout, structural design and interior materials.
Otherwise a noise control eflFort may be unnecessary expensive, weight intensive or even
wasted. The necessary knowledge can be obtamed from measurements, provided a
prototype or sister vessel is available, or by using an analytical approach at the design
stage.
H a t e r j e t
P r o p e l l e r
S h a f t s
A u x i l i a r y Machinery
- Bjdcaulles
- V«ntU»t lon faas
- Eihaust system
- vnc
- S«« - Hind
Figure 2, Noise and vibration sources
4.3 Noise Calculations
Noise may be estimated in several different ways, at different levels of complexity and accuracy. .
Early in a design process it may be worthwhile to carry out a review of the design in
order to highlight possible problem areas based on previous experience from similar
vessels. The findings may be used to adjyst the arrangement of the vessel, to highlight
areas of fiirther attention and to make a preliminary assessment of necessary noise
reducing measures. This type analysis is rather rough and only meant as an early
project guidance.
Further analysis may either be based on statistics from similar vessels or on direct
calculations on the design in question.
A statistical analysis is carried out using statistics from similar vessels in combination
with calculations of the effect of significant differences between the proposed design
and vessels in our data base. This involves calculation of insertion losses for the
machinery isolation systems, evaluation of source strengths and calculation of
differences in structural damping due to different transmission distances between the
various sources. The analysis will also comprise calculation of airborne sound
transmission from machinery compartments to passenger locations. In positions where
the noise criteria are expected to be exceeded, noise reducing measures will be
proposed.
Alternatively the noise levels may be calculated by direct calculations oii one particular
design. DNV has developed a method based on waveguide theory specifically for this
purpose, the Noise Prediction Program NV590, ref /NILSON (1984) and ANDRESEN et
al. (1986)/. By modeling the ship as a cross sectional element model the transmission losses
can be calculated. Further the program calculates resulting noise levels in specified
positions on board a vessel. In addition infomiation is provided about dominating noise
sources transmission paths and radiating surfaces. Figure 3 presents a flowchart for the
program In positions where the noise criterion will be exceeded noise reducing
measures will be proposed.
Lp
SOURCE I
Lv
SOURCE I
AIRBORNE SOUND TRANSMISSION THROUGH STRUCTURES
Lp INDUCED
NOISE LEVEL CABIN BY
AIRBORNE SOUND
FROM SOURCE
ATTENUATION I N
STRUCTURE
Lv BULKHEAD
Lv
DECK
Lv CEILING
Lv FLOOR
Lp INDUCED NOISE LEVEL IN CABIN BY STRUCTUREBORNE SOLFND FROM
SOURCEI
TOTAL CONTRIBUTION FROM SOURCE I OCTAVEBAND SOUND PRESSURE LEVELS
RESULTING NOISE LE^/EL I N CABIN
1 1
NOISE LEVEL IN dB(A) IN CABIN
TOTAL CONTRIBUTION FROM SOURCE I I
TOTAL CONTRIBUTION FROM SOURCE I I I
OTHER SOURCES
re 3, Flow chart for the NV590 noise prediction program
5.1
VIBRATION CONTROL
General
Vibration is due to a source acting with a dynamic force on the structure. The complete
global structure may tlien vibrate (hull vibration) or the vibration may have local character
(local vibration) or act on the source itself (source vibration). High levels of vibration may
be due to a strong source and/or a weak structure (forced vibration). Alternatively
structural properties may cause a natural frequency to couicide with a source forcing
frequency and thus lead to strongly amplified vibration (resonant vibration).
Vibration depends very much on structural design, position and source installation.
Vibration levels will vary a lot from vessel to vessel and from position to position. High
speed vessels with resiliently mounted machmery and waterjets or high speed propellers are
not particularly prone to vibration problems. Most high speed ferries will have vibration
levels below some 4 mm/s to 5 mm/s on passenger decks, unless resonances occur or
particularly strong sources are present.
Certain types of vibration may be experienced on board particular vessel designs only. E.g.
vibration on board SES or ACVs which is due to the flexibility of the air cushions.
However, such phenomena can usually be taken care of by suitable control systems.
5.2 Vibration Mechanisms
Fault free sources mounted correctly on a sound foundation will normally not cause
excessive forced vibration. Factors leading to high dynamic forces from a source may be
unbalance, shaft misalignment, shaft resonance, propeUer pressure impulses, some sort of
shaft resonance or a mechanical fault.
On board high speed vessels the shaft systems are usually rotating with relatively high
speed and cardan shafts and/or flexible couplings are often used to take up the motion of
resiliently mounted machinery. Therefore the shaft systems become critical in respect of
vibration. It is important that shaft resonances are avoided, that cardan shafts are correctly
installed and that misalignment is avoided. When aligning shafts, the relative deflection of
resiliently mounted maciiinery imder load (e.g. main engine/gear) and long term set of
resilient elements should be taken into account.
In order to .ensure low vibration levels all sources should be mounted to the structure at
points of siifiBcient stiflBiess. Tliis applies to resilient as well as rigidly mounted sources.
Again the shaft systems may be critical and it is important that the structural stifftiess at the
bearings is satisfactory.
For resiliently mounted machinery due care should be taken in choosing mounts having a
resonance frequency below the first forcing frequency of the mounted equipment.
Additionally the stability of the equipment may be increased by lowering the centre of
gravity of the equipment and mcreasing the distance between the outer mounts.
Structural resonances, local as well as global, should be avoided in order to acliieve vessels
with moderate vibration levels. The only way to assure that resonances are avoided for a
new design, is to perform calculations. Detailed finite element calculations of a complete
hull are expensive. It is usually feasible to make a rough assessment of the possibilities of
resonances from empirical data and/or by using simple analytical methods. Detailed
calculations vM then only have to be performed if the probability for resonances are
indicated by the simpler approach. Also, it will usually be necessary to compute a detailed
analysis of a section of a vessel only.
5.3 Vibration Calculations
In order to assure a vessel with moderate vibraüon the practical factors mentioned above
should be observed. Further it is recommended that the following approach is adhered to :
- Natural frequencies of the propulsion shaft and other major shafts ought to be calculated
and the probability of resonances assessed. Such calculations will usually be offered by the
machinery manufacturers or classification societies.
- The structure should be subjected to a simple analytical assessment to determine the
probability of structural resonances.
- I f tliere exists a probability for a structural resonance a detailed calculation using finite
elements (e.g. SESAM) should be carried out or the forcing fi-equency of the source in
question altered.
Vibration control is described in greater detail in /DNVC (1985)/. I f a vibration problem
occurs on board a new built or existing vessel, the best way to solve tlie problem will be to
have a trouble shooting measurement survey carried out by an experienced consultant.
6. WAVE^BWUCED RESPONSES
6.1 General
The wave induced motions of a vessel depend on the seakeeping performance as well as
the actual sea-state experienced by the vessel. Hence when selecting a vessel for a
particular route, it is important to evaluate these factors.
The lliree most important variables contributmg to seasickness are:
- Vertical acceleration
- Exposure thne
- Encounter frequency
The relation between seasickness, vertical acceleration level and encounter frequency is
based on the ISO standard (ISO 2631). The calculation of the vertical acceleration as a
flinction of the encounter frequency is carried out by state-of-the-art ship motion
programs. These calculations will produce hydrodynamic transfer fiinctions for the
vessels. Alternatively, these transfer functions can be obtained from measurements.
The long term climate in the area (or route) where the ship operates is described in
terms of combinations of characteristic wave heights and periods with associated
probabilities of occurrence.
When this environmental information is combined with the transfer fiinctions, the
probability distribution of different combinations of vertical accelerations and encounter
frequencies are obtained.
The last step is to specify the estimated exposure time for the vessel and calculate the
Sea Comfort Index by combining the physical information about the seasickness
probabilities for given combinations of vertical accelerations levels and encounter
frequencies with the long tenn probability distribution for these combinations.
6.2 Calculation of seakeeping performance for high speed vessels
DNV has in cooperation with Marintek developed a computer program for numerical
prediction of wave-induced motions and sectional forces for vessels with high speed
(Froude Number > 0.4). The computer program is called FASTSEA and the main feahires
of the program are summarized in figure 4
FASTSEA A new computer program for high speed vessels. Developed In a joint research.project by Det norske Veritas (DnV) and MARINTEK with funding from DnV and NTNF.
Features: Monohull and catamaran vessels Air cushions (SES) Foil systems
Figure 4, Description of the FASTSEA program
The vessel is assumed to have a high forward speed, and the incident waves may have an
arbitrary propagation direction relative to the vessel. Both monohulled and multihulled
vessel types, with or without foils, may be analysed, and the area between the hulls may be
replaced by an air cushion in order to simulate a SES. An unsteady lifting-line theory is
employed to predict the lift forces and torsional moments acting on the foil system. Since
linear theory is used, the program is best suited to describe the behaviour of vessels in
moderate sea states, with wave amplitudes and wave-induced motion responses small
relative to the wavelength and cross sectional dimensions of the vessel. The main theory
and assumptions will not be outlined, but details, may be found in ref / Z H A O and
F A L T I N S E N (1990), F A L C H (1991), and N E S T E G A R D (1990)/. The main output
features of the program are displayed in figure 5. /SVENSEN (1994)/ presents examples
on the use of the program.
OUTPUT FEATURES
Seastate
All response modes Acceleration In all three directions at any point in the vessel Sea-sickness probability Shear forces and bending moments on the hull Computation of mean sinkage and trim Resistance
Figure 5, Output features of the FASTSEA program
Requirements related to comfort may be difficult to fijlfill for fast ferries for long exposure
periods. But since the main advantage, of a high speed vessel is the reduced transit period,
the increased response levels due to the high speed may be compensated for by means of
the reduced exposure period.
The example in figure 5 is made to demonstrate the importance of choosing the most
suited design for the actual route. Alternative 1 has good performance at moderate sea-
states, but if the probability of encountering rough seas is high m the actual area, the right
choice could have been alternative 2 despite the fact that tliis vessel had poorer
performance for some sea states. Most important is that altemative 2 is below the sea
sickness liniit.
7. REFERENCES
ANDRESEN K . A.C. NILSSON and E. BRUBAKK (1986) "Noise prediction and
prevention." 2nd International symposium on sliipboard acoustics ISSA'86, pages 433-459.
DNVC A/S,(1985) "Vibration control in ships" Handbook, Section for noise and vibration.
FALCH S.,(1991) "Seakeeping Characteristics of Foil Catamarans". Marintek,
MT60-91-0005, (In Norwegian).
GOETHE H., ZOM E., HERRMANN R and SCHEPERS B.F., (1978) "Die
psycho-physische Belastnung des Personals modemer SeeschiflFe als aktuelles Problem der
Scliiffahrtsmedizin." Zbl. Bakt. Hyg., lAbt. OrigB , 166 pages 1-36.
GOTO D . (1983) "Characteristics and evaluation of motion sickness incidence on-board
ships." PRADS83, 2nd Int. Symp., Tokyo & Seoul.
McCAULEY M.E., ROYAL I.W., WYLIE CD., OHANLON J.F. and MACKIE R R
1976) "Motion sickness incidence: Exploratory studies of habituation, pitch and roll".
Technical Report 1733-2, Human Factors Research Inc.
N E S T E G A R D A,(1990) "Motions of Surface Effect Ships". A.S Veritas Research Report
NILSSON A-C. (1984) "A method for the prediction of noise and velocity levels in ship
constructions." Journal of Sound and Vibration 94(4).
SAN.Sjöfartens Arbetarskyddsnamd,(1978), "Kartlagging av arbetsmiljön innom sjöfarten"
SVENSEN T.(1994),"Hydrodynamic Loads and Structural Analysis of Large High Speed
Craft", D N V C Paper Series 1994.
ZHAO R and FALTINSEN O.M., (1990) "Seakeeping of High-Speed
Catamarans". Marintek, MT24-90-0074.