"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.