WEGEMT
20th October 1995"A NEW SAFETY STANOARD FOR PASSENGER/RORO VESSELS" by Tor E. S'en.sen, Det iVorsk4 Ventas Class Ulcation AS
ABSTRACT
ihe paper presents the main objectives of the recently initiated joint North-West
luropeaa project 'Safcty of Passenger/RoRo Vessels". Acritical examination of existing damage stabiuty standards is made arid the principal risks not covered are discussed. Important aspects such as damage stability modelling methods watertight integrity, intermediate stages of flooding and dynamic effects are discussed and some possible solutions outlined. Recent events have shown that passenger/RoRo vessels are vulnerable when subject to large scale flooding and that stability and survivability requirements must be improved. [n particular the principle of creating a second barrier of defence against technical or human failure is discussed, Methods of performing a risk analysis on passeriger/RoRo vessels are presented and the possible role of Formal Safety Assessment as part of vessel approval and certification procedures discussed.
I. INTRODUCTtON
Immediately following the "Estonia disaster, the Nordic countries together with the
tJnitcd Kingdom and some major Classification Societies established a project to take a fundamental new look at the stability and survivability requirements for
Passenger/RoRo vessels. The aim of the project is 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 RoRo vessels with particular reference to the damaged and flooded condition. 1-lowever,it has been recognised by the project group that other risks should be considered in an overall assessment. In particular, it is considered important to ensure that otherrisks
are not increased as a consequence of different design solutions that are intrinsically safer from a stability consideration.
'Ihe project has been split into two phases with Phase i addressing the most urgent issue of improving the stability of Passenger/RoRo vessels when subject to large scale flooding. The project will specifically address the issue of identifying and testing a second line of defence against technical or human failure. In practice this means that a single failure or incident should not lead to catastrophic consequences. The results of the project will form the basis of proposals for new and extended Nordic and International rules. 1995-W-16 TECHNISCHE UN1VERSITEJT Laboratorium spoor Scheepshydromechanlca &rchiof Mekelweg 2, 2628 CD Delft 1eI: 015 78687 Fax: 015 78133S
Phase 2 ot' the project will examine how safety assessment procedures can be applied to the passeriger/RoRo type of design. The safety assessment will be applied to any
new ruteframework 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 minar accident should not be allowed to escalate into a major accident, The safety
assessment study to be carried out in Phase 2 of the project will also describe a
framework for how a safety assessment procedure should be carried out and documented on a new design.
The project objectives and organisation is briefly described in the enclosed Appendix.
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 of 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 afler the Tl'l'ANIC (lisaster was the new requirements for life saving appliances. Anew
conference held in 1929 resulted in requirements for subdivision in terms of floodable length calculations. lt is important to note that the principal focus at the 1929
convention was on intact stability and floodable length requirements.
The first damage stability requirements were introduced following the 1948
convention. Prescrit damaged stabilIty requirements for RaRo vessels arc 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 OZ curve) alter damage. These requirements were made effective from 1990, and for the first time in the history of the 1MO, they were made retroactive to existing ships.
The first probabiListic damage stability rules for passenger vessels were introduced in 1967 as an alternative ta the deterministic requirements in SOLAS-60. For most of the passenger/RoRo vessels the requirements contained within this new probabilistic framework A.265 are more stringent than the deterministic requirements in SO LAS-60 and therefore A265 has generally not been much used on passenger/RoRo vessels. The next major stcp in the development of stability standards came in 1992 with the introduction of SO1,AS part B-1 (in Chapter 11-1), containing a probabilistic standard for cargo vessels.
The 1MO has worked on the harmonisation of stability standards for several years. Despite this we arc still faced with substantially different requirements for different ship types. When comparing the different standards for different ship types, the fact is that the present deterministic stability standard for passenger/RoRo vessels probably
represents the lowest satèty standard when compared with both A.265 and the new probabilistic standard for cargo vessels (SOLAS part B-1, 1992).
Following a joint research project, the Nordic countries presented to the 38th session of SLF a drafl probabilistic damage stability regulation for passenger vessels. In this
proposal the survival capability is solely based upon the GZ curve characteristics and the margin line and bulkhead deck are not considered. This proposal may represent the first step towards a new probabilistic standard for passenger/RoRo vessels. However, before this can become an acceptable standard, there are some important
further problems that need to be addressed as outlined below.
3. PRESENT STABILITY REQUIREMENTS FOR RORO VESSELS
* WHAT ARE TElE PROBLEMS?
In principle all existing RoRo 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/RoRo vessel with a standard SOLAS side damage will rapidly he filled with water on the vehicle deck and capsize if the waveheight is above O5- 1 0m. This
has been somewhat improved with SOLAS-90, but is still considered inadequate.
The number of recent major disasters with passenger RoRo 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.
WHA 7 A R E TI/E MA IN SHORTCOMINGS?
The principal shortcomings of the present SOLAS standard for passenger/RoRo vessels in international unrestricted trade can be listed as follows:
l'ue possibility that the vehicle deck nay be flooded is not includeti in the calculations, The type B freeboard definition used ori RoRo vessels means that only compartments below the freeboard deck are considered in the damage stability calculations. Although recent tragedies have involved flooding through
the bow doors it is well recognised that side collision with damage to
compartments below the freeboard deck as well as opening to the freeboard deck itself represents one of the most likely accident scenarios. The extensive work carried out after the "Herald of Free Enterprise" accident clearly demonstrated that most existing designs will not survive a standard SOLAS side damage in waves above 1m. liven vessels built to SOLAS-90 standard are unlikely to survive in waves much above l. - 2m. Clearly this is inadequate for operation of large passenger/RoRo vessels in unrestricted waters.
L9O5.LO-L
Shill of cargo is' not includgd as a risk. All RaRo cargo is assumed to remain
satèly in the original position and there is no additional heeling moment applied to the calculations due to shift of cargo. This is considered to be an unrealistic
assumption or a vessel subject to unsymmetrical flooding after damage and rolling heavily in waves.
The present SOLAS-90 standard for Passenger/RoRo 'essels is entirely deterministic, A new probabilistic standard should be developed. This will be more logical and will provide a more objective measure of the survival capability. Such a risk based method is consistent with current thinking on safety analysis and risk management.
In addition, the fact is that most vessels operate with watertight doors open during the voyage and this is contrary to the assumptions made when damage stability
calculations are approved. Some of the damage stability modelling methods in use make only static assumptions with respect to the internat waterline in flooded compartments, This is a very doubtful assumption.
WHY PROBABILISTIC STABILiTY CRITERIA?
1'hcy allow 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 combining the results of damage scenarios to one compartment or a group of compartments with a probability of the vessel surviving the damage, it is possible to calculate the attained subdivision index A In practice this 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.
4. REQUIREMENTS FOR A NEW STABILITY STANDARD.
An important requirement for a new stability standard for passeriger/RoRo vessels is that it should not destroy the basic principles behind the RoRo concept. lt should be recognised that the RoRo design is part of a highway system. Any new regulations must recognise this basic principle and not result in rigid deterministic requirements making the RoRo concept totally unworkable.
The basic requirements tòr a new stability standard for passenger/RaRe vessels can be listed as follows:
1995W. iO. 16
Majar risks such as flooding of the RoRo deck and cargo shiji should
be included.
4 method förmanaging residual risk (I.e. pre venting rapid capsize in those damage cases where the vessel does not survive,) should be included
The fn*mework The purpose of the framework is to control that the risk of sinking or capsizing as a result of damage to the vessel or malfùnction of vessel's system or system components, whether due to technical or human failures, is brought down to an acceptable level. At the same time the residual risk should be managed in such u way that the number of tìtalites are kept as low as practically possible.
The risk may be expressed as:
RISK Probability * Consequence
Consequence
Probability
Max. tolerable consequence
Acceptable risk
Unacceptable risk
Reducing probability
If the risk is too large, it may be reduced by reducing the probability, reducingthe consequence, or a combination of these, There will always be a limit of max. tolerable consequence, for example rapid capsize with loss of many lives. Above this limit, the risk can not be reduced by reducing the probability alone,
In practice the proposed new stability framework for passenger/RoRo vessels will tentatively be based upon the following probabilistic calculation procedures:
Calculation of attained subdivision Index (A):
A =
* s)
taken over all damage cases and combinations of damagecaseswhere A = attained subdivision index p = probability of damage
s probability of survival with given damage
A to be greater than a specified value Calculation of capsiie index (C):
C = E(p *c)
taken over all damage cases and combinations of damage cases where C = attained capsize indexp = probability of damage
c = probability of capsizing with given damage (measure of residual risk)
C to be less than a specified value
'I'his latter probabilistic index relating to a given capsize probability is introduced in order to ensure that future designs are constructed in such a way that they will sink in a controlled manner without capsizing after major damage in those cases where the
vc55c1 will not survive, The indices may be illustrated in the following diagram:
CONTROLLED 8INKNG I995-1O-6 Attained capeize Index "C" Controlled sinking (not capsize) Attained gubdivIIon Index "A"
¡vIA ¡W JSSUIS Y'Q BE STUDIED.
Issues that wiil be specifically studied in the project in order to address present shortcomings in the probabilistic methods as already Introduced for other types of vessels are listed below. It is intended that the results will provide a reliable and well documented basis for a new ruleframework.
Damage extent: Instead of using old damage statistics a new method will be developed based upon calculating the risk of collision on a given route. This will he combined with the distribution of ship types to calculate the risk of impact with different ship types. lJsing first principles methods the probability of size cl'
damage in terms of vertical extent, damage length and penetration will be
determined. The method wilt examine and, it' possible, take into account the actual ship structural design of the RaRo, thus giving credit to a collision resistant
structure.
Flooding and jymimic effects in waves A critical parameter in a new probabilistic framework is how much water will enter the vehicle deck after a given collision damage and how large reserve is required on the GZ curve in order for the vesse! to survive, Mode! tests are carried out to determine the time function of water entry as a function of waveheigth, GM, freeboard, damage size and other relevant
parameters. Combined with the development of a theoretical model prediction of vessel motions and capsize it is expected that the project will arrive at clear criteria for survival to be used in a proposed new framework. Implicitly by introducing waveheigth as a parameter for survival will be the opportunity to introduce service restrictions operating in more protected waters.
Damc stability calculation methods:. Critica! examination of the priricip les of
damage stability modelling with particular reference to the basic assumptions and calculation methods employed, such as symmetrical flooding, intermediate stages of flooding, permeability. The key issue here will be to develop calculation methods that reflect the actual design solutions.
Cargq.hiÍ: Development of a deterministic requirement for reserve stability as a
function of cargo type, number01'lanes, deck layout etc.
5. SAFETY ASSESSMENT FOR
PASSENGERJRoRo VESSELS Phase 2 of the project is devoted to the development of the safety assessment methodology to passenger/RoRo vessels. These methods and procedures have been used in the oftshorc industry, the nuclear power industry and the chemical process industries for many years. The number of applications in the shipping industry
are to
date very limited.
Briefly the safety assessment procedure fora passenger/RoRo vessel will involve
the
tòllowing elements:
u System definition
Ha"trd identification
Frequency analysis and consequence modelling Risk presentation
Evaluation using risk criteria
Selection of risk reductionmeasures
A major difference between the offshore industry and the shipping industry is that individual ships within a class of ships are generically very similar. This is likely to result in procedures which are simpler to implement than what we have seen to date in the offshore industry. We are therefore likely to end up with a type of safety
assessment where most ot'the requirements for redundancy, prevention of escalation etc. are covered by rules for design and constructions. Individual safety assessments
for new designs will only be carried ori a more limited scale and will concentrateon
those items that are specific for the particular vessel design and operation. Phase 2 of the project will focus on the following items:
Qualitative risk analysis. Application of existing techniques such
as Eic'rd
Identification, FMECA and SWIFI' to an existing vessel. Identification of shortcomings in present design practices and applicable rules. This work wilt focus on prevention of single failures resulting in major accidents and smaller accidents escalating into majoraccidents.Quantitath'c' risk analysis. Development of procedures for complete quantitative risk analysys on passenger/rota vessels. Data collection and application to ari existing vessel and a new design on one ormore routes.
Riskas.s essmtn for new stability framework The purpose will be to document that the new framework has resulted in a significant reduction in the probability'
of
capsize and sinking compared with the existing stability rules.Development ofprocedures for safety assessment on individual vessels. The purpose is to develop recommendations for a rational procedurerecogriising that most safety aspects will be covered by prescriptive rules and concentratingon design aspects that are deviating from the rule basis.
ft is generally' believed that by following the above described procedure this will result in the most cost effective solutioris.
A pre-study already carried out in the project hasconcluded that;
1) 'I'echniques for qualitative risk analysis
on passenger/RoRo vessels are already available, have already been used and can be implemented immediately. Quantitative techniques require some further development and data sources need to be identíted.
However, it should be possible to have both techniques implemented within a timescale of a few months,
2) Future uses of risk assessment for passenger/RoRo vessels will be at two
levels:
Assessment of the risks of passenger/RoRo vessels in general, to indicate the need for new initiatives in safety regulation, to target them cost effectively, or to
estimate their benefit if they were adopted.
Assessment of individual vessels to indicate the need for safety measures in their design and operation, and to provide a basis for Safety Cases for them.
3) An initial risk analysis of a passenger/RoRo vesse! has concluded that the risk to the indivìdual passenger is no higher than for other means of public transport. However, the risk that many lives will be lost in a single accident is signitica.ntly higher than for other means oftransports
6. CONCLUSIONS
1'he development of a new safety standard for new passenger/RoRo designs with particular focus on stability and survivability in the damaged and flooded condition is considered essential in the light of recent tragic accidents. The main aim will beto
develop a second barrier of defence against technical or human failure. The main features of this new standard will, be:
new stability framework based upon probabilistic methods, allowing a more
objective assessment of survivability
criteria for managing residual risk to ensure that the probability of rapid capsize is as low as practically possible
new rule developments based upon safety assessment
REFERENCES:
L SOLAS '29: f nternational Conference
on Safety of Life at Sea. 1929.
SOLAS '60: International Confeeiice on Safety of Life at Sea. 1960. SOLAS' 74: International Convention fbr the Safety of Life at Sea. London, 1974.
[MO Resolution A.265 (VIII). Regulations on Subdivision and Stabilityof Pussenger Ships as Equivalent to Part B of Chapter II of the tnternational convention for the Safety of Life at Sea, 1969. [MO. London, 1974.
SOLAS 90: Ch. LI-1, Part B-1: Subdivision and damage stability of cargo
ships.
LLOYD, C,J,: "Research into Enhancing the Stability and SurvivabUity of RoRo Passenger Ferries - Overview Study", Joint RINAIDTp International Symposium on the Safety of RaRo Passenger Ships, London, April 1990 Vassalos, D. Dr.: "Capsizal Resistance Prediction of a Damaged Ship in a
Random Sea", Joint RINA/DTpEntemnational Symposium on RoRo Ships' Survivability, London, November 1994.
Dand, [W.: "f'actors Affecting the Capsize of Damaged RoRo Vessels in Waves", Joint RINA/DTp International Symposium on RoRo Ships' Survivability, London, November 1994.
Velsehou, S. and Schindler, M.: "RaRo Passenger Ferry Damage Stability Studies - a Continuation of Model Tests for a Typical Ferry", Joint R1NA)DTp
Enternational Symposium on RaRo Ships' Survivability, London, November
1994.
APPENDIX
Project Description for Joint Nordic Project
"Safety of Passenger/RoRo
Vessels"
f. OBJECTIVES
The main objective of the project is to investigate technical aspects relating to safety
and survivability of RoRo vessels with particular reference to the damagedand flooded condition. '[he project will specifically address the issue of identifying and testing a second line of defence against technical or human failure. '[his will form the basis of proposals for new and extended ordic and international rules.
2.
SCOPEOF WORK
'['he scope ot' work is defined in principal tasks as follows:
Phase 1: Stability:
'Task I:
Damage stability #wdeiling methods: Critical examination of the principles of damage stability modelling with particular reference to the basic assumptions and calculation methods employed, such as symmetrical flooding, intermediate suiges of flooding and permeability. Recommendations with propos.als for improvements in modelling methods including realistic conditions for treatment of compartment boundaries and penetrations.
Task 2.1:
Damage Extent.' Development of method to predict the size and extent of damage on passenger/RoRo vessels as a result of coUisions with other vessels. The method will be 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 wilt be utilised in two ways: I) to enhance the existing method based upon purely historical data, allowing the actual traffic arid normal RoRo ship structures to be taken into account and 2) to determine estimates for the statistical distribution of ship damages to be used in proposed new rules, Task 2.2:
Large Scale Flooding: Model test investigations to quantify how water ingress
ori the
RoRo deck depends upon damage size, freeboard, 0M, deck layout arid seastate. Progressive tests in which the flooding is free to develop will be performed for(1M values close to capsize for variations in freeboard, size and location ofdamage, 1995-IO-[6
different seastates and different deck layouts. The primary purpose of the tests is to determine the amount and distributiono'water trapped on the RoRo deckas a
function of time und to determine the relative water level at the damage opening. '['he test results will also be used to validate the theoretical model developed under Task 5.
Tusk 3
Dynamic Effects'in Waves,' Mode! tests will be performed in order to develop
a
method for calculating the dynamic effects acting upon a vessel in a seaway when subjected to flooding after damage. Alternative amounts of water on deck, different GM values, different seastates, vessel headings and speeds will be investigated. The results of the investigations will be used to verify the theoretical model developed under '['ask 5 and to develop a proposed simplified method for describing the amount of reserve stability required in order for the vesse! to survive without capsizing in a given seastate.
'['ask 4:
cargo Securing andcargo Shift.' Existing rules and regulations for cargo stowage
and securing will be reviewed. Maximum heeling moments caused by cargo shift will he calculated in a deterministic way as a function of cargo types width and number of lanes etc. A method fur calculationofccelcratìons and forces actingon the cargo onboard a damaged vessel moving in an open sea will be developed as a function of the relative heeling angle. Finally, a deterministic calculation method will be
developed for predicting the heeling moment caused by cargo shift as a function of the relative heeling angle.
'lask 5:
Development of Mathematical Mode/for Capsize Predictions: A mathematical model for assessing the capsize safety of passeriger/RoRo vessels will be finalised. '['he
model will be used for a systematic parametric investigation to identify and quantify the effect of key intluericing factors on vessel survivability. Calibration of the model will be undertaken using the results of the model tests performed in Tasks 2.2 and 3. '['he model will further be used towards the development of relationships between ship design arid environmental parameters and stability related parameters to be used as basis for deciding on appropriate levels regarding new probabilistic criteria.
Task 6:
/'rameworkJr iVew Damage Stability Standard: Requirements to damage stability assessment for RoRo ships will be developed and formulated, taking into account risk factors relevant tòr damage stability. The framework shall address all risks relevant to
damage stability, like collisions, groundings, structural failures, etc. 'l'he prime goal is to provide u second line of defence against technical and human failures, such that adequate constructional features and technical/functional requirements may he
provided. '['he t'ramework shall describe procedures for formulation of requirements
based on damaged OZ curve and other relevant damage stabilityparameters, together with assumed damage and damage statistics. Basis for the procedures shall be
statistics on weather conditions and traffic density for a certain service arca, All relevant effects, including large scale flooding, cargo shift, dynamic behaviour in waves, etc., shall be taken into account. Procedures for managing residual risks by
minimisirig the risk of capsize shall be included. Example on formulation of such regulations shall be given.
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Example Design : One or more example design will be developed in close
co-operation with a design consultant andlor shipyard in order to exemplify the proposed new rule framework and how this may be satisfied.
Phase 2 : Safety Assessment
Task 8:
Risk assessment 'stahillty,: A risk study will be made based upon the new set of rute framework for existing designs, proposed new designs and other designs (dry cargo, passenger). Risk reduction factors for designs developed within the new framework will be documented.
Task 9 & 'task 10: Safety Assessment:
Development of procedures for safety assessment on individual vessels. 1'he purpose is to develop a rational procedure recognising that most safety aspects will be covered by prescriptive rules and concentrating on design aspects that are
deviating from the rule basis,
Safety assessment on speciJìc parts of 1MO rules for design ofpassenger/RoRo vessels. The purpose will be to identify shortcomings and propose new ruLes This work will focus on prevention of single failures resulting in major accidents and smaller accidents escalating into major accidents.
3. PROJECT TEMEPLAN, ORGANISATION AND BUDGET
Phase i of the project will be carried out over a period of approximately one year with the main task of developing a new rule framework completed by early 1996.
Phase 2 is being carried out in parallel wìth Phase 1. A pre-study on the application of safety assessment procedures to passenger/RoRo ships will he completed by April
1995. However, the main effort in Phase 2 is from mid-1995 and the work will be uinalised by February 1996.
The work on the project is split into tasks under the overall management of Det Norskc Ventas Classification, The institutions and companies which are responsible For the tasks in Phase i arc shown in the project organisation chartbelow:
Project Organisation T 2 TatT'J Tc22 P1 TiciL.Lc D.4 c ,
The sponsors of the project are the following organisatiors::
The Danish Maritime Authority The Finnish Maritime Authority The Norwegian Maritime Directorate 'I'hc Swedish Maritime Authority The Danish Shipowners Association
The Finnish & A land Shipowners Association The Norwegian Shipowners Association The Swedish Shipowners Association
Marine Safety Agency (UK) Bureau Ventas
Germanischer LLoyd
Det Norskc Ventas (Project Manager) 1MO (Observers)
The budget for the project is NOK 11 million (US $ 1.5 million), with tentatively 80 % allocated to Phase I and 20% allocated to phase 2.
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