DNVC Comfort Class, A new concept ensuring acceptable noise and vibration levels on board High Speed Vessels

"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 C O N C E P T O F C O M F O R T

Comfort isjdefined as "A State o f 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, V I B R A T I O N , C L I M A T E 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 2 x 1 0 * 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

Locations

comfort rating number (crn)

Locations

Less than 50m LOA More than 50m LOA

Locations

1 2 3 1 2 3

Passenger localities 75 70 65 65 60 55

Navigation Bridge 65 60 55 60 55 50

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 o f 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.

Locations Private Accommodation comfort r a t i n g n u m b e r ( 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 o f 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 o f 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 o f firesh/outside air per person supplied to a space, expressed in 1/s.

Designated Type

Table 6 Onboard Climate Classification

Climate Parameters

(30 niirt inè^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

It is required individual room temperature control o f 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 E U 7 / F 8 5 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

Motion sickntM r«gion

-, A-ij •

/

/ /

/ /

,/

^//

[aulcy et. al.

1

0^1 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 c t 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 M a c h i n e r y - Bjdcaulles - V « n t U » t l o n faas - Eihaust system - vnc - S«« - Hind

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 I N CABIN B Y STRUCTUREBORNE SOLFND FROM

SOURCEI

TOTAL CONTRIBUTION FROM SOURCE I OCTAVEBAND SOUND PRESSURE LEVELS

RESULTING NOISE LE^/EL I N CABIN

1 1

NOISE LEVEL I N dB(A) I N CABIN TOTAL CONTRIBUTION FROM SOURCE I I TOTAL CONTRIBUTION FROM SOURCE I I I OTHER SOURCES

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 i f 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

ves-sels. 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).

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