A new stability standard for passenger/Ro-Ro vessels

Paper presented at the

International Conference

WATERTIGHT INTEGRITY & SHIP SURVIVABILITY

21

& 22 NOVEMBER 1996 LONDON

ThCSCIE UiVERSITEFT

..aboratorium voor

Scheephydromochanica

Archof

Mekeiweg 2. 2628 CD DeIft

iL Th- 187j

- Fai

015 - 18183e PAPER NO.3.

A NEW STABILITY STANDARD FOR PASSENGER/RO-RO VESSELS

by T E Svensen

Head of Section tor Environmental Loads,

Marine Technology Department, Det Norske Ventas, and

S Rusaas, Principal Surveyor, Det Norske Ventas

t

i

A NEW STABILITY STANDARD FOR P.ASSENGERJRO-RO VESSELS Tor E Svensen

Head of Section for Environmental Loads, Marine Technology Department, Det Norske Ventas

and

Sigmund Rusaas

Principal Surveyor, Det Norske Ventas

SUMMARY

The paper presents the main objectives of the joint North-West European project "Safety of Passenger/Ro-Ro Vessels. A critical examination of existing damage stability standards s presented and the principal risks not covered are discussed. Important aspects such as damage stability modelling methods, watertight integrity, intermediate stages of flooding, dynamic effects and survival criteria for damaged vessels are discussed and some possible solutions outlined. Recent events have shown that passenger/Ro-Ro vessels are vulnerable when subject to large scale flooding and that stability and survivability requirements must be improved. In particular the principle of creating a second barrier of defence against technical or human failure s discussed. Results and conclusions from three example designs are presented and discussed.

AUTHORS' BIOGRAPHIES

Dr Tor Svensen obtained a degree in Naval Architecture and Shipbuilding from the University of Newcastle upon Tyne in 1978 and subsequently gained a PhD in 1983. He has held various positions in research and

development and engineering consultancy. Since 1993 Dr Svensen has been Head of Section for Environmental

Loads at DNV n Oslo, responsible for internal and

external work in the fields of hydrodynamics, stability and probabilistic methods and, for the past 18 months, has also been the Project Manager of the joint North-West European research project into Ro-Ro ferry safety entitled "Safety cf Passenger/Ro-Ro Vessels"

Mr Sigmund Rusaas is a Principal Surveyor at Det Norske Ventas. He has been employed with the Society since 1970, and has been engaged in the stability of ships and ocean vehicles since 1977. His main areas of respon-sibility

has been stability

calculations and approval, development of regulations and related research. For the past 18 months Sigmund Rusaas has also been involved as Task Manager for the development of Framework for new damage stability standard in the joint North-West European research project into Ro-Ro ferry safety entitled

Safety of Passenger/Ac-Re Vessels".

1. INTRODUCTION

Immediately following the 'Estonia" disaster, the Nordic countries together with the United Kingdom and some major Classification Societies established a project to take a fundamental new look at the stability and survivability requirements for Passenger/Re-Re vessels. The aim of the project has been to come up with proposals for new design requirements leading to improved safety for new

vessels. The project was set up to primarily address

technical aspects relating to safety and survivability of Ro-Ro vessels with particular reference to the damaged and

flooded condition. However, it was early recognised by the project group that other risks should be considered in an overall assessment. In particular, it s considered important to ensure that other risks are not increased as

a consequence of different design solutions that are

intrinsically safer from a stability consideration.

The project has been split into two phases with Phase i addressing the most urgent issue of improving the stability of Passenger/Ro-Ro vessels when subject to large scale flooding. The work has focused on establishing an entirely

new risk based stability standard. This includes the

development of survivability criteria to ensure a near zero probability that future designs will capsize, even after large scale damage. Important aspects such as damage stability modelling methods, watertight integrity, collision damage extent and dynamic effects in waves have been studied in the project.

The results of the project will form the basis of proposals for new improved international rules.

Phase 2 of the project has examined how safety assess-ment procedures can be applied to the Passenger/Re-P.o type of design. The safety assessment has included a qualitative analysis focusing on traditional hazard assess-ment techniques to ensure that no future designs will be constructed and operated in such a way that a single failure may result in a major accident. Similarly, a minor accident should not be allowed to escalate into a major accident. The safety assessment study has also included a quantitative risk analysis providing documentation of major risk factors and cost effective means of reducing these risks.

The project has been organised as a jointly funded

research project with the following sponsoring

Lhv. f-sir*ä

G&m Ud

note that the principal focus at the 1 929 convention was on intact stability and floodable length requirements. The first damage stability requirements were introduced following the 1948 convention. Present damaged stability requirements for Ro-Ro vessels are generally based upon the same deterministic principles, although some

important improvements have been made. Most notably these improvements involve requirements to residual

stability (range, height and area of GZ curve)

after

damage. These requirements were made effective from 1990, and for the first time in the history of the MO, they were made retroactive to existing ships.

The first probabilistic damage stability rules for passenger vessels were introduced in 1967 as an alternative to the deterministic requirements in SOLAS-60. For most of the passenger/Re-Re vessels the requirements contained within this new probabilistic framework A.265 are more stringent than the deterministic requirements in SCLAS-60 and therefore A.265 has generally not been much used on passenger/Re-Re vessels.

The next major step in the development of stability

standards came in 1992 with the introduction of SOLAS

part 8-1 (in Chapter lI-1), containing a probabilistic

standard for cargo vessels.

The 1MO has worked on the harmonisation of stability standards for several years.

Despite this we are still

faced with substantially different requirements for cifferent ship types. When comparing the different standards for different ship types, the fact is that the present deterministic stability standard for passenger/Ro-Ro vessels probably represents the lowest safety standard when compared with both A.265 and the new probabilistic standard for cargo vessels (SOLAS part B-i, 1992).

Fig. i Project Organisation

STASI UTY

Rasel

RISK Rase2 Tasx i Task2 T3B adL3

SeFirq

in 3 Task 4 rç TaskS i1as:r n asi Cniia Tas6 Fcre.axf

i:

Task 7 Task S

I9

Fassiiity Sey Aasre1

Sh siIIty ies Earc4e csie E Task 2.1: onj Task 22 1 OPN

d Sr'

eu-Vaitas OtN T-r Gern uo Data in I SSPA Usw. L. Uiv

',

SSPA I Task -L Bu Vait Task L

Sqe

T S

The Danish Maritime Authori. i ne Finnish Maritime Authority. The Norwegian Maritime Directorate. The Swedish Maritime Authority. The Danish Shipowners Association.

I ne Finnish & Aland Shipowners Association. The Norwegian Shipowners Association. The Swedish Shipowners Association.

Marine Safety Agency (UK). Bureau Ventas.

Germanischer LLoyd.

Det Ncrske Ventas (Project Manager).

The timescales for the project has been a total of 18 months and the technical work tasks have been carried out with the assistance of a selecticn of the best available

experts within Europe.

2. 1MO STABILITY REQUIREMENTS - A BRIEF HISTORICAL REVIEW

Historically, most changes in international regulations for ship design and operation have been introduced as a result of major disasters with a large loss cf life. The first notable of such disasters was the well known sinking of the TITANIC.

Probably the most important outcome of the international conference held after the TITANIC disaster was the new requirements for life saving appliances. A new conference held in 1929 resulted in requirements for subdivision in terms cf floodable length calculations. t is important to

Following a joint research project, the Nordic countries presented to the 38th session of SLF a draft probabilistic damage stabiUty regulation for passenger vessels. In this proposal the survival capability is solely basad upon the GZ curve characteristics and the margin line and bulkhead deck are not considered. This prcposal may represent the first step towards a new probabiiistic standard for passenger/Ro-Ro vessels. However, before this can become an acceptable standard, there

are some

important further problems that need to be addressed as outlined below.

3. WEAKNESSES IN PRESENT DETERMINISTIC

STABILITY RULES

In principle all existing Ro-Ro vessels satisfying the SOLAS 2-compartment standard has an adequate stability margin for surviving a damage provided the weather is calm and there is no cargo shift. In practice it has been clearly demonstrated in the work carried out after the accident of the HERALD OF FREE ENTERPRISE that a modern passenger/Ro-Ro vessel with a standard SOLAS side damage will rapidly be filled with water on the vehicle deck and capsize if the waveheight is above 0.5 - 10m. This has been somewhat improved with SOLAS-go, but is still considered inadequate.

The existing deterministic regulations imply several weaknesses with several major risk factors uncovered. The number of recent major disasters with passenger Ro-Ro vessels have clearly confirmed their extreme vulnerability when water is allowed to enter the vehicle deck. Combined with cargo shift the outcome can be rapid capsize without much time for passengers to evacuate the

vessel. The most prominent risks nct covered in the

present deterministic regulations are:

Damage beyond the deterministic damage assumptions:

The one-compartment standard represents probably the greatest single potential danger to ro/ro

passenger ships, or to any other passenger ship. Calculations using probabilistic methods indicates that if a vessel collides by another vessel, it is more than 70% probability that the damage will include at least one watertight bulkhead.

The B/5-line penetration have for decades been

regarded as the standard to be used for damage stability, and is used in most damage stability regulations, both for passenger- and cargo ships. The

damage statistics behind 1MO Res. A.265 have

shown, however, that abt. 50% of the damages are penetrating beyond the 8/5-line.

Bottom damage penetrating the double bottom:

The bottom damages are only

implicitly covered in

SOLAS, through a requirement that there should be a double bottom, the height of which is left up to the admini-stration. Damages penetrating through the double bottom are in principle potential dangerous, even if the

require-ment to floodable length ensures some degree of float-ability (but not necessarily stfloat-ability) in flooded condition.

Flooding through open or damaged access doors or

other openings:

Such flooding may cause large amounts of water to enter

the Ro-Ro deck with

catastrophical consequences. Flooding through other openings may have the similar effect, but such scenarios will most probably develop slower, and the flooding thereby easier to contain.

Watertight doors left open:

The risk implied by watertight doors left open need special consideration. It ¡s common procedure in many Ro-Ro/ passenger ships to leave these doors open, and the cnly precaution to be taken is that they are to be closed from

the bridge following a damage. These dcors have

considerable dimensions, and it may be expected that large amounts of water may pass through these doors

before they can be closed. Deformations due to the

damage, and possible loss of power may make the dcors impossible to close in an emergency situation.

Cargo Shift:

Cargo shift represents a major risk factor for Ro-Ro

vessels, in particular ¡f there is no longitudinal subdivision.

Internal flooding:

Internal flooding may have several causes, but are seldom the sole cause far serious accidents. The risk should be addressed, however, especially potential flooding to the Ro-Ro deck.

Dynamic effects due to wave action:

The survival criteria are based on calculation of GZ curve in still water only. Flooding of the Ro-Ro deck is complete-ly neglected, and wave action is oncomplete-ly implicitcomplete-ly covered

by height, area and range of GZ curve. There are

indications that the residual stability criteria are sufficient for a wave height of 1-2 meters. even after the SOLAS 90 standard.

In addition it should be pointed out that the reliability of the damage stability calculaticns are greatly influenced of the methods and assumptions employed. The wncie

damage stability modelling has in many ways been

ideaiized, with the most common assumption being that n all stages of flooding the internal waterline is common to all compartments included in the damage.

Other idealizations usually made is that light structures

are generally neglected, (large) cross flooding ducts

usually regarded as giving instantaneous flooding, air pockets in cross flooded tanks neglected, etc. The permeabilities applied in the different rooms also have great influence on the results. The SOLAS Convention stipulates standard permeabilities to be used for different categories of rooms. The validity of these permeabiities, in particular for ro/ro spaces, may be highly questioned.

TASK 1

Stability

Modelling

Methods

-- light structuresdown flooding - air pockets - intermed. stages - cross flooding - asymetric flooding - time calculations

NE\I PROBABILISTIC

STAB 1LITY FRAMEVVORK

A.265

SOUkS 1992, B-1

Nordic Proposa!

Common to all the weaknesses listed above is the lack of a second barrier of defence. In particularthe

combina-tion of collision with

rapid water ingress and open

watertight doors may be dangerous. Another weak point is flooding of the Ro-Ro deck, li such flooding reaches a critical level, rapid capsize without warning may be the resuit. The regulations do not stipulate any requirements

to the behaviour of the vessel if the damage is only

slightly larger than the described extent.

-damage size - damage water distribution

(p-factor)

S

A

Fig.2 Main Cornerstones of New Stability Framework

TASK 2.1

Collision

Damage

TASK 2.2

Flooding

Prediction

Attained capsize index "C" Controlled sinking (not capsize) "S' Attained subdiv. index "A"

4. MAIN ELEMENTS IN A NEW PROBABILISTIC

STABILITY STANDARD

The proposed new damage stability framework is based on the probabilistic concept ofsurvival. This means that the standard of survivability is expressed in terms of the probability that the vessel will survive, given a damage

with water ingress has taken piace. A damage is a

random event, and it is not possible to state which parts

L

TASK 3

-residual stability

Dynamic

Effects

- survival capability

(s-factor)

TASK 4

Cargo

Shift

- heeling mom. - securing std.

TASK 5

Criteria

D ev e 10pm.

of the vesse! is actually being flooded. The probability of

flooding of a certain compartment or combination of

compartments may however be determined based on probability distribution on damage extent and position along the vesse!.

The probabilistic method allows the risk of a particular event, such as collision and flooding, to be combined with the probability of

survival to give a resulting

index

describing a weighted survival capability. By summing the results of damage scenarios to one compartment or a group of compartments with a probability of the vessel surviving the damage for all possible damage cases, it is possible to calculate the attained subdivision index A for the complete vesse!.

This may be illustrated as follows:

Calculation of attained subdivision index (A):

A =

* s)

taken over

all

damage cases and

combinations of damage cases; where A = attained subdivision index;

pl = probability that a damage has caused

flooding of compartment or group of

compartments no. i, assuming collision has taken place;

s = probability of survival, assuming a

certain damage has taken place. The attained index, A, to be greater than a specified value The probability of collision causing flooding, p1, calculated on the basis of the following distributions:

* longitudinal position (a) damage length (p) damage penetration (r) damage vertical position (y)

n practice this attained index A is a survival index for the

complete design. The most important point about a

probabilistic method is that it is less arbitrary and provides a more objective measure of the survival capability of the vessel in the case of damage compared with a standard deterministic method. By using a risk-based method those events that have a likelihood of occurring carry a heavier

weight and conversely those events with a very low

probability of occurring have a small influence upon the final result.

The Probability of survival, s1 is in the present SOLAS regulations based upon characteristic of the CZ curve and expressed as a function of GZ curve height, range and area, In order to cater for the special situation on Ro-Ro vessels where there is a risk of flooding the large open vehicle spaces, it

will be proposed within the new

framework that this risk is covered in a modified formulation of the s-factor:

The modified s-factor Is expressed as:

s, = sa

* SW

sa = s1 - factor based on static GZ-curve (SOLAS'90) function of (heel, GZmax, range, area, prob. of progressive flooding)

SW = s - factor based on water on deck. The proposed new factor sw is a function of:

Signiíicant waveheight. Damage extent. Damage location. Freeboard. CZ curve characteristics. Loading condition. Permeability.

lt should be noted that the calculation of sw is only

applicable for large open spaces such as Ro-Ro decks. In order to determine an appropriate calculation method for Sw, the following developments have been undertaken within the project:

Development of numerical model for time simulation of water ingress and vessel motions in waves.

Model experiments to determine water inflow

coefficients and capsize boundaries.

Verification of numerical model against model

experiments.

Parameter study using numerical model.

Identification of critical parameters for vessel survival in waves.

Development of equivalent 'static" calculation method for determining vessel survival limits after damage. Parameter study to determine uniqueness of criticai boundary curves for survival.

5. TECHNICAL CORNERSTONES OF THE NEW

STABILITY FRAMEWORK

In order to facilitate the development of a new framework

for

stability assessment based upon a

probabilistic method, a series of work tasks were defined within the project. The main purpose of performing these work tasks has been to address the shortcomings as already describ-ed above and to provide proposals for improvements and new methods to be employed. The principal objectives

and main results of the technical work tasks can be

summarised as follows:

Damage

stability

modelling methods: Critical examination of the principles of damage stability modelling with particular reference tothe basic assumptions and calculation methods employed, such as symmetrical flooding, intermediate stages of flooding and permeability. Recommendations with proposals for improvements in modelling methods including realistic conditions for treatment of compartment boundaries and penetrations. Recommendations on dimensioning of cross flooding ducts, air pipes in order to achieve satisfactory cross flooding.

Damage Extent: Development of method to predict the size and extent of damage on passenger/Re-Ac vessels as a result of collisions with other vessels. The method is based upon analytical techniques taking into account frequency estimates of collisions due to ship traffic in the area and rational models for consequences of given ship

collisions. The method has been utilised to

validate existing statistical distributions for damage length and

penetration used within the MO probabilistic stability

framework, and to propose necessary modifications. In addition, new distributions for vertical extent of damage have been proposed.

Large Scale Flooding:

Model test investigations to quantify how water ingress on the Ro-Ro deck depends upon damage size,

freeboard, GM, deck layout and

seastate. The primary purpose of the tests has been to determine the amount and distribution of water trapped on the Ro-Ro deck as a function of time and to determine critical water levels before capsize. The test results has

also

been used to

validate the theoretical model

developed under Task 5. The model tests have been performed for two different vessel designs and for a range

of internal ship configurations including centrecasing, sidecasing and open deck layout.

Dynamic Effects ¡n Waves:

Model tests have been performed in order to develop capsize boundary curves for a vessel in a seaway when subjected to flooding after damage. Alternative vessel configurations, freeboards, GM values, seastates, vessel headings and speeds have been investigated. The resuits have been used to verify the theoretical model developed under Task 5. and to develop a proposed simplified method for describing therequired

stability characteristics in order for the vessel to survive without capsizing in a given seastate.

Cargo Securing and Cargo Shift:

Development of recommended standards for cargo stowage and securing. Development ci a method for calculation of accelerations and forces acting on the cargo onboard a damaged vessel moving in an open sea

as a function of the relative

heeling angle and incorporating a deterministic calculation method for predicting the heeling moment caused by cargo shift as a function of the relative heeling angle.

Development of Survival Criteria for Ro-Ro Vessels ¡n

Damaged Condition:

Development of mathematical model for assessing the capsize safety of passenger/Ac-Ro vessels. Development of relationships between ship design and environmental parameters and stability related parameters to be used as basis for deciding on

general-B

ised survival criteria for use

n new probabilistic

framework.

Framework for New Damage Stability Standard:

Development of a new framework for assessing the

stability using probabilistic methods and utilising the main results and recommendations from the other tasks. The role of the individual task results as cornerstones in the proposed new stabiiity framework is illustrated in figure 2.

6. EXAMPLE DESIGNS

In order to test and demonstrate the consequences and feasibility of the new framework tnree example designs were made:

2500 Passenger Cruise Ferry. loco Passenger Cargo Liner. BOO Passenger Handy Size Ferry.

An important part of the example designs were also to provide a basis for setting a requirement to the

Subdivision Index and Capsize Index.

The selection of vessels

were made 10

reflect a

representative spread in size and ship types, each of which is discussed in the following.

6.1 SHIP A: 2500 PASSENGER CRUISE FERRY

This type was selected to represent the class of ships with extreme number of persons at risk. A layout with 12main

compartments, a small lower hold, wide side casings, fully drive-through trailer deck was adopted. Freeboard and GM were substantially increased. Ref. Fig. 6

The calculations were made for the following variations: "Basic case", i.e. an idealized case where effect of progressive flooding is neglected. Furthermore, it was assumed that equalization would occur within 10 seconds, and no intermediate conditions need to be studied.

Progressive flooding included as a limiting faccr. Asymmetrical flooding during the process of inflow. Combined progressive flooding and unsymmetrical flooding considered.

Closed side-lanes, alternative lay-out (base case only).

The results show an attained index of 0.916 for the base case, but reduced to 0.794 when combined progressive flooding and unsymmetrical flooding is considered. In

practical terms it is anticipated that an attained index of approximately 0.60 is the maximum achievable following the new framework.

The full results for the variations are shown in the

following table:

V

/

/

2 TABLE i

N

6

Fig. 6a Example Vesse! A

2

Basic case Progressive

flooding included Asymmetric flooding considered Combined asymmetric and progressive flood-ing considered

Closed side lanes considered

Capsize index C 0.047 0.157 0.047 0.176 0.047

Attained index A 0.916 0.837 0.877 0.794 0.923

SHIP DATA SUBDIVISiON DATA

Length between perp.: Breadth: Max. draught: 169m 31m 66m Ls: Design GM:

Significant wave height:

185m 3.Om 4.Om

Height to main Ro-Ro deck: 100m Required index acc. to A.265 0.80

Number of passengers: 2500

Number of crew: 230 Attained index:

Trailer lanes: 1300m

Deadweight: 4000t - openings disregarded: 0.92

Gross tonnage: 520001 - openings included: 0.84

Machinery: Diesel-electric

Service: Short international Capsize index: 0.05

MAIN CHARACTERISTICS Side casings

High freeboard to trailer deck High GM

o 500 I

CROSS SECTION

PAZ TRAILER 4310 8 CZEV

Fig. 6b Example Vessel A

PAZ XC 14 T S PA5 SA 5E S ICARO) CREA CEA OWL

6,6 M

E PAZ 10 27800 PIJ8L IC 9 24400 pUELIC 8 21000 PAZ 7 8300 PAZ 6 l500 S TRA I LER 14110 14 30000 1 3 s000 PAX/PUOL IC 12 33200

FAX PAZ PAZ

11 :osoo 4 10000 3 7200 2 4800 PAZ VAX PAX PAZ

6.2 SHIP B: loco PASSENGER CARGO CARRIER This type was selected to represent the class of ships with three trailer decks and moderate numbers of persons at risk. A layout with 16 main compartments, a small lower hold, side or 'B/4" casings, fully drive-through train/trailer deck was adopted. Freeboard and GM were substantially increased. Ret. Fig. 7

The calculations are carried out including the effect

of progressive flooding through necessary openings for

escape and ventilation. Furthermore, the cross flooding

ducts are made sufficiently wide to not penaiize the

s-factor.

The results for this vessel show:

Attained subdivision index: 0.737

Capsize index: 0.110

Fig. 7a Example Vessel B

SHIP DATA SUBDIVISION DATA

Length between perp.: Breadth: Max. draught: 183m 29m 6.Orn Ls: Design GM:

Significant wave height:

i 88m

30m 40m

Height to main Ro-Ro deck: 9.30 Required index acc. to A.265 0.70

Number of passengers: 1000

Number of crew: 60 Attained index: 0.73

Trailer lanes: 2000m

Deadweight: 7900t

Gross tonnage: 37000t

Machinery: Diesei-electric

Service: Short international Capsize index: 0.11

MAIN CHARACTERISTICS

Longitudinal bulkhead from baseline up to level 153m Trains on main deck

Lower hold for 150 private cars Dangerous cargo in open air

\/

/

6.3 SHIP C: 800 PASSENGER HANDY SIZE

FERRY

This type was selected to represent the smaller type of ships, with one trader deck anc one deck for carriage of

cars.

A layout with 14 main compartments, a small lower hcld for cars and fully drive-through trailer deck with

small side casinas was adopted. Freeboard and GM were substantially increased. Rei. Fig. 9

''1

Fig. 7b Example Vessel B

10

I

The calculations are carried out including the effect of

progressive flooding through necessary openngs for

escape and ventilation. Furthermore, the cross flooding effect is taken into account.

The results are:

Attained subdivision index: O .846

ni

r

lii

H

D

Fig. Sa Example Vessel C

Dt

SHIP DATA SUBD1 VISiON DATA

Length between perp.: 102m Ls: 11094m

Breadth: 21m Design GM: 2.75m

Max. draught: 4.6m Significant wave height: 40m

Height to main Ro-Ro deck: 700m Required index acc. to A.265 0.64

N umber of passengers: 800

Number of crew: 50 Attained index: 0.85

Trailer lanes: 500m

Deadweight: 1300t

Gross tonnage: 5000t

Machinery: Diesel-electric

Service: Sheltered (20 NM) Capsize index: 0.15

MAIN CHARACTERISTICS Small Side casings

No subdivision on trailer deck High GM

I _-..J

R1

i i

b'i

AR CONOITION[NG

SR1DCE

CREW CABiJS

PASSENGERI PUBLIC SPACES

Fig. 8b Example Vessel C 12 740Cl (C LO T LO Cui Cuy Cuy.

C

CU t PASSENGER CABINS

PASSENGER PUBLIC SPACES

PASSENGER CABINS

PASSENGER PUBLIC SPACES

CREW CABtS

CREW FUELtC SPACES

I\ I

'k t k £ k I \

45m

6.4 MAIN FINDINGS FROM THE EXAMPLE DES GNS

Common to all three example designs were that they are designed with moderate sized ro-ro spaces in the lower holds. In order to increase survivability they have been designed with reserve buoyancy above the trailer deck,

either in form of side casings or, as is the case for

example A, utilization

of space high up as reserve

buoyancy. Furthermore, in order to cope with the more

stringent requirements to cross flooding, the double

bottom height is increased, allowing for ample cross flooding ducts.

The metacentric height and freeboard

is increased compared with to-days designs.

The main findings from the calculations of the example designs may be summarized as follows:

The rule framework provides a logical way of evaluat-Ing survivability, giving the designer, in principle, free hands to make his design. The philoscphy follows the cargo ship convention, but the amount of analysis work has increased substantially.

A high level of survivability is achievable as a technical exercise. Practical considerations, e.g. to emergency escape, ventilation, piping systems etc. will limit the achievable level.

The example designs show a

practical attainable index ranging from 0.73 to 0,85. lt may be noted that it does not seem to be the usual correlation between ship size and attainable index, in fact the smallest ship obtained the highest index. (but also the highest capsize index). The sample is of

course too small to draw any firm conclusion on where the level should be, but there are indications that an attained index of abt. 0.80 and capsize index of abt. 0.10 is achievable using the proposed framework. Sensitivity with respect to wave height was less than anticipated. This may be explained by the fact that the survivability factor sa' is not a function of the wave height. Another factor contribution to this was that most of the index was achieved through one and two-compartment damages. which were all showing very

high critical wave height. The variation with wave

height was then only visible for the three-compartment damages, which accounted for only a small portion of the total index.

Sensitivity with respect to GM is high. This is expected because GM is an important parameter both to the GZ curve and critical amount of water on deck. The GM's used n the example designs are close to 3.0 m, a figure which is expected to

be up against the

maximum tolerable from a comfort and sea-keeping point of view.

The capsize index approach appears effective against rapid capsize due to loss of stability and/or excessive asymmetry. lt seems difficult, however, to bring the capsize index lower than approx. 0.10.

Handling of openings in the new framework becomes very complicated. This may be explained with lack of adeQuate computational tools for the moment. The approach

itself should not be regarded as more

complicated than the traditional method.

There may be a conflict of interest between evacuation and watertight integrity. A high level of survivability may be restricted by a need for evacuation routes. This is one of the main factors limiting the achievable subdivision index, and has to be born in mind when setting the required level.

SOLAS escape routes are mainly concerned with fire scenarios. lt

may be necessary

to differentiate between fire escape and flooding escape. This is

however a general problem connected to

safety regulations in general, and is not a particular problem

for the new framework. The problem has come

particularly in focus when a high level of survivability is aimed at.

Openings present very different risks, from spilling over half-height

car deck gates,

to

grey water

scuppers. They should be treated differently with

regard to seriousness, e.g. size.

Guidelines for treating

ship internal systems are

needed. In a probabilistic framework endless numbers of critical penetration depths may be generated, one

for each position of pipe connecting two tanks or

rooms.

The proposed level of requirement for instantaneous flooding may prove to be impractical lt is correct and necessary to address transient flooding, but the 10 secs interval should be discussed.

Cost effects are dependent on how it is possible to utiiize volumes and deck areas. Increased beam may be utilized for cargo or accommodation, or it may be a waste of space. In any case, volumes below the main deck will increase substantially, and it will be difficult to utilize economically. A cost increase in the range of 5% is found ¡n the examples. The total cost increase from the rule proposal can be determined only after the required level has been set.

7. CONCLUSiONS

The project "Safety of Passenger/Ro-Ro Vessels has

focused on the development of a new safety standard for new passengeriRo-Ro designs with particular focus on stability and survivability in the damaged and flooded condition. The project has demonstrated a unique ability and willingness of maritime administrations, shipowners associations and classification societies from several countries to work together in order to achieve the main objectives of the project.

The main conclusions from the project

may be

summarised as follows:

A probabilistic stability standard for Ro-Ro vessels is possible and a draft framework has been prepared. New probabilistic stability standard for Ro-Ro vessels will be a special case cf the harmonized standard for all ship types.

Risk analysis

has concluded that

collision with

subsequent flooding and capsize is the dominant risk contributor.

The example designs have demonstrated that it is

passible to design ships with substantial higher

degree of survivability that existing ships, taking

account of large scale flooding, water on deck, cargo shift, improved damage stability modelling and other

features of the new framework.

-REFERENCES

SOLAS '29: International Conference on Safety of Life at Sea. 1929.

SOLAS '60: International Conference on Safety of Life at Sea. 1960.

SOLAS '74: International Convention for the Safety of Life at Sea. London, 1974.

14

1MO Resolution A.265 (Vili). Regulations on Subdivi-sion and Stability of Passenger Ships as Equivalent to Part B of Chapter II o the International convention for the Safety of Life at Sea, 1969. 1MO. London,'74. SOLAS 90: Ch. II-1, Part B-1: Subdivision and damage stability of cargo ships.

LLOYD, CJ: Research into Enhancing the Stability and Survivability

of Ro-Ro Passenger Ferries

-Overview Study', Joint RINA'DTp International Symposium on the Safety of Ro-Ro Passenger Ships, London, A6ril 1990

VASSALOS, D Dr: Capsizal Resistance Prediction

of

a Damaged Ship

in

a Random Sea', Joint

RINNDTp International Symposium on Ro-Ro Ships Survivability, London, November 1994.

DAND, I W:

Factors Affecting the Capsize of

Damaged Ro-Ro Vessels in Waves', Joint RINAJDTp International Symposium on Ro-Ro Ships' Survivability, London, November 1994.

VELSCHOU, S and SCHINDLER, . M: 'Ro-Ro Passenger Ferry Damage Stability Studies - a Continuation of Model Tests for a Typical Ferry', Joint RINAIDTp lnternational Symposium on Rc-Rc Ships' Survivability, London, November 1994.

i

TABLE 2

Task

Document

Number

Document Name

Odginator

Latest Issue

Rep-Taski-COl Damage Stability Modelling

Vol 1: Main Report Knud E. Hansen 1995.11.01

Vol 2: Result from Time Domain Simulations SSPA 1995.10.95

Vol 3:Damage Stability Calculations Knud E. Hansen 1995.11.01

2.1 pac-COl Probabilistic Analysis of Collision Damages

with Alicatiof1 to Ro-Ro Passenger Vessels

Technical University of

Denmark

1995.12.06

2.1 95-0419 Damage and Pcnctration Analysts Det orske Venus 1995.07.11

2.2 DVfJ 95,016 Testing of Large Scale Flooding Danish Maritime 1995.11.13

Main report Institute

Appendix 1 Vol 1+2

3 MT6O F96-CO92 Model Tests, Part I Marirtiek 1996.05.13

Model tests, Part II 1996.07.16

Main report Appendices

4 REP-T04-0O1 Cargo Securing and Cargo Shift Mariterm AB 1995.07.07

5 Review of Recent R&D Work on the Damage University of Strsthciyde May 1995 Stability and Survivability of Passenger/Ro-Ro

Vessels

5 A Theoretical Investigation of the Capsizal University of Strsthclyde March 1996 Resistance of Passenger/Ro-Ro Vessels and

Proposal for Survival Criteria

6 REP-T06-0O1 Framework for new Damage Stability Standard Germanisher Lloyd 19%-0821

Bureau Venias Det Norske Venias

7 REP-T07.001 Example Designs Kvmer Masa Yards 1996.08.14

7 P-3823 Example Design Delt&Marin 1996.09.13

Rev. A

8 _{Feasibility Study for Safety Asscssntent of DNV Technica 1995.03.16}

Ro-Ro Passenger Vessels

Safety Assessment of Passenger/Ro-Ro Vessels 1996.01.05

Methodology Report

QRA of M/S Prinsesse Ragnhild 1996.07.04

Hazard Assessment of M/S Prinsesse Ragnhild 1996.02.28

8 95-3462 Collision and Grounding risk for Prinscssc DNV Industry 1995.08.11 Ragrihild

95-3463 Route Data for Ro-Ro Passenger Ferries operating to/from Scandinavia