BQok Ref. LY95001
WEGEMT WORKSHOP
IDAM~AG •SIB]E
CIF SHOPS
Friday, 20 October, 1995 venue The Technical University of Denmark Lyngby, Denmark hosted by The Department of Ocean EngineeringWEGEMT Workshop on
Damage Stability of
Ships
Proceedings of a one-day Workshop held on Friday 20 October 1995
TU Denmark, Lyngby
Hosted by the Department Ocean Engineering,
TU Denmark
Published by WEGENIT
Publication reference number LY95001
C-ABOUT WEGEMT
WEGEMT is a European Association of 37 universities in 17 countries. It was formed in 1978 with the aim of increasing the knowledge base, and updating and extending the skills and competence of engineers and postgraduate students working at an advanced level in marine technology and related science.
WEGEMT achieves this aim by encouraging universities to be associated with the Foundation, to operate as a network and therefore actively collaborate in initiatives relevant to this aim
WEGEMT considers collaborative research, education and training activities at an advanced level and the exchange and dissemination of information, as activities which further the aim of the Association.
NB For "marine technology and related science", this includes all aspects of
offshore oil & gas exploration and production, shipping and shipbuilding. underwater technologies and other interdisciplinary areas concerned with the oceans and seas.
,ABOUT THE PUBLICATION
This publication represents a series of lecturers' notes which were presented at a one-day Workshop on Damage Stability of Ships first presented at TU Denmark, Lyngby on Friday 20 October 1995.
ISBN - 1 900453 00 2
Published by WEGEMT
Copyright © 1995 WEGEMT. All Rights Reserved. No part of this publication may
be reproduced, stored in a retrieval system or distributed in any form or by any means without the prior written consent of the publisher.
This volume has been made available so that it contains the original authors' typescripts. The method may from time to time display txpographical limitations. It is hoped however, that they do not distract the attentions of the reader. Please note that the views expressed are those of the individual authoi(s) and the publishers cannot
Programme for the Workshop
1. Collision damage statistics and Probabilistic damage stability calculations in preliminary ship design
Dr J Juncher-Jensen and P Andersen, DTH Lyngby
2. Overview of the Joint Nordic Project
Dr T Svensen, Project Manager DNV
3. Mathematical modelling of capsizal resistance of a damaged ship
Dr D Vassalos, Strathclyde University
4. Experimental studies on the capsize safety of passenger/ro-ro ships
S Velscho & M Schindler, Danish Maritime Institute
5. Recent developments, trends and proposals on damage stability criteria
Professor M Pawlowski, Dansk University & V Aanesland, .AR'RINTEK
6. Panel discussion and recommendations
COLLISION DAMAGE STATISTICS AND
PROBABILISTIC DAMAGE STABILITY CALCULATIONS IN PRELIMINARY SHIP DESIGN
J. JUNCHER JENSEN, J. BAATRUP & P. ANDERSEN
Department of Ocean Engineering Technical University of Denmark
Building 101 E, DK-2800 Lyngby, DENMARK
ABSTRACT
The recent MO Resolution MSC 19(58) points towards a more rational way of obtaining subdivisions in ships to ensure a sufficient stability in damaged conditions.
In the preliminary phase of ship design it is important to know how the attained subdivision index and the possible oil outflow in a collision are influenced by the actual positions of the watertight bulkheads. This information should be given in form of sensitivity factors yielding the change in attained index, subdivision and oil outflow for specified changes in the position of user-defined bulkheads.
In the paper such a procedure will be described. The formulation is quite general implying that future improvements in the damage stability regulations can be easily implemented. Furthermore, information of oil outflow from damaged cargo tanks is included. By that, both the probability of zero outflow, average hypothetical oil outflow as well as mean local outflow are presented. The procedure can thus be used to compare the environmental damage to be expected from different type of oil tankers in a given collision scenario.
INTRODUCTION
Recently the assessment of damage stability of ships has received a great deal of attention due to the tragic losses of several Ro-Ro ferries: the "European Gateway' (1982), the "Herald of
Free Enterprise" (1987) and the "Estonia" (1994). Of course the international maritime society is seriously concerned and puts substantial effort in both deriving rational procedures for assessing the residual stability of a damaged ship and setting up reasonable minimum requirements.
A significant development in the rules and regulations for damage stability assessment can be foreseen in the next few years, most certainly within the framework of a probabilistic
-description of the damages a ship may suffer in a grounding or a collision accident.
The probabilistic damage stability regulations for dry cargo ships of length greater than
100
mn issued by the International Maritime Organization (IMO) in 1990 [1] provide the most updated, generally accepted version of such a procedure. Whereas the probabilistic concept is simple and gives a single measure of the probability of surviving a collision accident its implementation in actual design poses a number of problems. The main problems are:(i) Establishment of realistic probability distributions for the location and extent of damages.
(ii) Extension to cover damage statistics for grounding.
(iii) Definition of the probability of damage of a given compartment or group of compartments.
(iv) Realistic criteria for the probability of surviving a given damage. (v) Definition of suitable loading conditions.
(vi) Proper account of openings, downflooding points and crossflooding pipes. (vii) Specification of realistic permeabilities of flooded compartments.
(viii) Definition of minimum requirements taking into account the number of people on board and the possible damage to the environment (oil outflow).
(ix) Proper account of the water ingress process.
Some of the problems are neglected in the present regulations [1] by simply demanding for example specific loading conditions and permneabilities. Other problems are covered by other regulations: grounding and oil outflow. The remaining items are treated in a more or less rigorous manner. allowing for some ambiguity in the application of the regulations. This is a very undesirable situation as the resulting safety measure, the attained subdivision index, should be the same for a given ship, irrespective of the design office or computer software
I I
Figure 1: General arrangement of a Ro-Ro ship (student's exercise).
The wireframe model of the outer hull is defined interactively in the programn package. It is possible to monitor important hydrostatic data such as displacement, center of buoyancy
and center of flotation during definition and modification of the model.
When the wireframe model is completed the topology of the hull can be established automatically and a surface model using N-sided patches is generated. Usually the surface model is only used for visual presentation of the hull while the hydrostatic properties are calculated using a longitudinal integration technique. However, a panel integration procedure is also available, [5]. The wireframe model can be transferred to any CAD/CAE package able to read points and B-splines from an IGES file. The drawing on Figure 1, showing the Ro-Ro ship used as an example in final section, is made by a transfer to AutoCAD.
The internal subdivision of the hull is described by compartments, modelling all closed volumes e.g. cargo holds, engine room, water ballast tanks and void spaces. A compartment is a collection of volumes with each volume defined by two transverse bulkheads, a number of longitudinal surfaces and of course the outer hull. The longitudinal surfaces are defined by a number of transverse polygons representing the shape of the surface at given longitudinal positions. These transverse polygon curves form a skeleton which is skinned to give the exact shape of the surface. By this approach rather complicated volumes and hence compartments can be defined. For each compartment an intact and a damage volume permeability are given.
doing the calculations. Therefore IM40 has released an explanatory note, lIMO Resolution A.684(17), [2] which eliminates some of the ambiguity, but still accepts different modelling of the damaged compartments.
In this paper an outline will be given of a computer program package for ship design including a probabilistic damage stability module in accordance with [I], but so versatile that improvements in the regulations can easily be accommodated. In addition the expected oil outflow in a collision is determined, using the procedure described in [3]. The computer software is especially intended for use in the preliminary design phase where the compartment layout can still be changed. Therefore, the procedure includes a sensitivity analysis for the attained subdivision index with respect to the vertical cefitre of gravity and on selected bulkhead and deck positions.
The designer may from these results relatively easily determine a compartment layout satisfying the requirement to the attained subdivision index.
In the next section a short description of the geometry definition of the hull and compartment layout in the present procedure is given. The following sections discuss some of the pertinent aspects in the probabilistic damage calculations and a possible extension to oil outflow estimation. The final section deals with an application of the procedure in preliminary ship design.
GEOMETRY DEFINITON
The first part in the description of the geometry is the definition of the outer hull. In the computer program package used in the present study the hull is defined by a number of two-and three dimensional curves. The available curves are station curves, contour curves, water lines, knuckle lines, buttocks and generic curves. A wireframe model of the outer hull surface can easily be defined using these curves. Both symmetrical and asymmetrical hulls can be represented. It is important to notice that the wireframe model is a connected wireframe, i.e. the curves are "glued " together at the intersection points. This forces e.,g. a water line
intersecting a station to change shape if the shape of the station curve is modified.
All the curves in the wireframe are represented by Ferguson splines [4] which is an interpolating spline containing the offset points.
xVJ.3 2 --Y12
p=
f
c(y)
f
a(x) dxdy(4)
0 x1 + y12
where J = x2 - xi. Performing the integrations the results quoted in [1] are exactly obtained
except for compartments bounded by either the forward or aft end of the ship, where p. is calculated slightly differently, [4]. For instance
J. ./2 fl384J4 j-)Y2
Po= f c~y) f
a(x)
dxdy + f
c(y)
f
a(x)
dxdy
(5)
a 0 0 4
for the aft compartment. The first integral is seen to include damages aft of the aft end which seems strange, but could be argued with the need to include some damages to the stem not included in the damage location parameter x. Integration of Eq. (5) yields exactly the result quoted in [1].
A more rational calculation of pc would be to apply Eq. (4) also for the fore and aft compartments. Thereby, c(y) should be normalized such that
J, 1 -y/2
f
c(y)
f a(x) dxdy = 1
(6)
0 y/2 yielding
c&Y)=C(1
-tY
(7)
where 2 1 = 08j 0O,12 (8)The fact that a more rational approach could be followed indicates that future changes in the damage statistics are very likely. Improvements in the damage statistics for the transverse
penetration and for the vertical extent of damage are even more urgently needed as the present formulas in [1] can result in negative probabilities or ship designs which may suffer
This allows the designer to use correct permeabilities whenever applicable. It is assumed that the fluid contents in a compartment can move freely between all volumes used to define the compartment. It should be mentioned that the entire internal subdivision is semi parametric allowing the designer to make major changes with limited effort.
The loading conditions are defined by a combination of fixed masses and liquid content in user-selected compartments. This ensures correct values for the center of gravity and free surface effects.
PROBABILISTIC DAMAGE STABILITY CALCULATIONS
The damage statistics applied in [I] is based on recorded data for 296 rammed ships. A quite
extensive discussion of this damage statistics is given in [2], but the actual derivation of it is omitted. However, it may easily be shown, [6] that it is derived using a joint probability density function f(x,y) in the form
(1) ftx,y) = a(x)c(y)
where x and y are the dimensionless longitudinal position and extent, respectively of the damage. The functions a(x), c(y) are
a(x)
={04
+ 1.6x; < x : •1 1/2 (2)and
c0,) = 1- ; J (3)
and it is readily verified that Eq. (2) and Eq. (3) fit reasonably well with the damage statistics shown in [2] using Jm = 0.24.
The probability p, that only the compartment bounded by x, and x, (x1 < x2) is damaged
a catastrophic accident if rammed by a ship with a bow height larger than the stipulated minimum bow height of the ramming ship.
Therefore, the current work taking place in a Nordic Project on Safety of Passenger/RoRo Vessels should be followed with great attention. Some preliminary results indicate that using the theories outlined in [9], reasonable collision damage statistics may be obtained for ships sailing in specific routes. This is a very important development as it points toward a rational procedure for estimating the collision damage statistics without releying solely on actual recorded collision events. The procedure takes into account the ship traffic in the area, navigational aids and the structural layout of the ships and thus gives the ship designer and the maritime administrations a tool able to estimate the benefit of various changes in the ship structures and in the operational profile.
For the complex compartment layout in actual ships, some simplifications must be done in order to determine the probability of damaging a single compartment or group of compart-ments. The explanatory notes [2] deal very extensively with this matter, but also demonstrate that various approaches, yielding different results, all are in accordance with the regulations
[U].
From a computational point of view the most convenient of the acceptable procedures is to divide each compartment into fictitious, rectangular boxes. Thereby, the designer does not need to specify the transverse depth of a compartment which may be difficult, as shown in [2].
Furthermore, the use of fictitious compartments usually leads to the largest value of the attained subdivision index as beneficial effects of all kinds of recesses are taken into account. The computer module for probabilistic damage stability implemented in the present program
therefore makes use of an automatic subdivision into fictitious rectangular boxes.
The most time consuming part of the probabilistic damage stability calculation is deter-mination of the GZ-curve for each damage configuration. As the same damage configuration can appear many times when fictitious compartments are used the GZ-cur-ve is of course stored when first calculated. In the preliminary design phase the vertical center of gravity is often not known precisely and therefore damage stability results for different values of the intact GM are useful. Such results are easily obtained without the need for further damage stability calculations as
GZ(8) = G'Z(O) + GG' sin 8 (9) The sensitivity of the attained subdivision index to user-selected re-positions of bulkheads is calculated automatically. Thereby, the designer is guided towards bulkhead locations satisfying the requirements in [1]. Also some additional local requirements as discussed in [7] may be considered.
Proper account of openings and pipe connections between compartments are extremely important and may be the most tedious point to deal with due to the complexity of the piping system in ships. As mentioned in [8], the bilge piping system effectively connects a large number of tanks in a damage condition, where a number of pipes are damaged. In the preliminary design phase the topology of the ship is seldom documented in such detail as to automatically extract information of these openings and the designer must therefore carefully either define such openings for a compartment or specify a probability of survival equal to zero for any damage conditions involving that compartment.
Nearly as important as the openings are the permeabilities assigned to the flooded compartments. These values are fixed in the regulations [1], but as discussed in e.g-. [6], [8] more accurate values should be aimed at including both volumetric and surface permeabilities. Finally, the number of loading conditions may need some extension. Today only full loading and partial loading must be analysed. This seems to be too few and may lead to designs which are overly safe in one condition and disproportionately below the average requirement in the other condition. An extension to three loading conditions may be better. In this context one may also look at the specification of these loading conditions. It seems much more relevant to use actual loading conditions including liquid cargo content in tanks, rather than fictitious loading conditions based solely on the hydrostatics of the ship.
PROBABILISTIC ASSESSMENT OF OIL OUTFLOW
The present regulations concerning prevention of oil outflow are given in Regulation I13F of Annex I of MARPOL73/78. For purposes of comparison between different tanker designs three outflow parameters are defined: Probability of zero outflow, mean outflow and extreme
In the master's programme students are generally taught Ship Design and Construction in their fourth and fifth semester. They have prior to that learned the basics which include intact stability and to a very limited extent damage stability. But they have no or very limited experience with actual ship constructions. Some experience of this they will gain from the literature, mainly journals and periodicals, where they study general arrangements and descriptions of ships already built. This enables them to make a conceptual design of the ship of the type and size which they are assigned.
In the conceptual phase of design the subdivision of the ship is carried out with consider-ation mainly for the use of various compartments, i.e. engine room, cargo space, tanks, etc.. The experienced naval architect will, however, already in this stage give some thoughts to the damage stability, keeping in mind that positions of bulkheads could be subject to minor modifications in the preliminary stage of design. When the designer has little experience, or is working with a novel and innovative design it may call for calculations for ]arge number of configurations to establish the optimum subdivision. It is therefore important that the procedure used will help the inexperienced as well as the innovative designer in obtaining an overview of the influence of the positions of watertight subdivisions on the attained sub-division index A thus giving guidelines towards the most favourable subsub-division.
As described previously the damage stability analysis is done for two loading cases (partial and full load) for a series of user-selected values of intact metacentric heights GM. For ships of the type where damage stability is crucial such calculations will put restrictions to the lowest acceptable value of GM in the intact condition and the designer should assess the most appropriate pair of restrictions on the basis of his knowledge regarding the operational profile of the ship. It is of course vital that in each of the two loading conditions a suitably safety level is chosen.
(local) outflow. A weighted single, so-called Pollution Index E, can be defined from these parameters and used for overall assessment of a given design relative to a reference double hull design.
The damage statistics applied is specified separately for collision and grounding. For collision the damage statistics used in the MARPOL regulations differs quite significantly from the damage statistics applied in the damage stability regulations [1]. From a rational point of view such differences must be expected to diminish in future modifications of the regulations, although some correlation of the damage statistics with ship type may be included.
The current damage statistics used in oil outflow calculations for collisions can yield a negative probability for damaging a group of compartments for certain compartment layouts. This can easily be demonstrated by examples. The same is true with the damnage statistics for damage stability calculations, but to a much lesser degree. In both cases this is due to approximations in the formulas for the damage statistics, and future modifications should remove this undesirable phenomena.
A similar development of a rational procedure as mentioned for collision damage statistics may also be expected for grounding accidents and this may be the way to remove the inconsistencies in the current regulations.
In the program package, the damage statistics specified in [1] for damage stability analysis is also applied for prediction of oil outflow. This procedure has previously been suggested, [3]. However, other damage statistics may quite easily be implemented as well.
APPLICATION OF THE PROCEDURE
Because the procedure has been developed by a University institute it has been implemented for two groups of users:
Wi Students doing their first ship design in the institute's course on Ship Design and Construction.
GMI (m] - original 1---: modified 21 I' ! A<R \ A, >R
itact
req
0
-[
0 2 GMf [m] 3Figure 3: Combinations of metacentric heights for full and partial loadings, GMf and GMP, satisfying A = R for both the original and modified design.
REFERENCES
[1] Resolution MSC 19(58), "On the Adoption of Amendments to SOLAS Convention, regarding Subdivision and Damage Stability of Cargo Ships", Report of the MSC on
its 58th Session, MSC 58/25, Annex 2, ]IMO, London, UK. 1990.
[2] Resolution A.684(17), "Explanatory Notes to the SOLAS Regulations on Subdivision and Damage Stability of Cargo Ships of 100 Metres in Length and Over", W[O,
1.0 A :original modified
1//
full 0 0 1 2 GM:m] 3Figure 2: Attained subdivision index A as function of intact metacentric height GM for the partial (AP) and full load (A,) conditions.
The output of such an analysis for the two loading conditions is shown in Figure 2. It is now up to the designer to decide for the design conditions fulfilling
A = 12 (Ap + A) > R (10)
where R is the required index, [1]. Such calculations must be done for many compartment layouts covering larger as well as smaller modifications in the internal subdivision of the ship. The computer program makes such modifications easily possible. If for instance the depth of a deck or the height of a double bottom is modified all compartments involved are automati-cally modified too. Such a situation is illustrated in the following, where a Ro-Ro ship design made by students as an exercise is modified by moving the main deck 0.5 m upwards. The general arrangement of the ship is shown in Figure 1.
The attained subdivision indices (Af, Ap) as function of intact GM are shown in Figure 2 for the full and the partial load case both before and after the modification of the depth to the main deck. From these results the designer must select a pair of conditions which fulfil Eq. (10). These combinations are shown in Figure 3.
[3] Pawlowski, M., "Oil Spill Prevention with New Ship Types in the Light of the Probabilistic Concept", Proc. WEMT'95, Copenhagen, Denmark 17-19 May, 1995, pp 181-209.
[4] Ferguson, J., "Multivariable Curve Interpolation", JA CM, 1112, pp. 221-228, 1964.
[5] Schalck, S. and Baatup, J. "Hydrostatic Stability Calculations by Pressure Inte-grations", Ocean Engineering, Vol. 17, No. 1-2, pp. 155-169, 1990.
[6] Jensen, J. Juncher, "Damage Stability Rules in Relation to Ship Design", Proc.
WEMT'95, Copenhagen, Denmark, 17-19 May, 1995, pp 71-96.
[7] Sen, P. and Gerigk, M.K., "Some Aspects of a Knowledge-Based Expert System for Preliminary Ship Subdivision Design for Safety", Proc. PRADS92, Vol. 2, pp. 1187-97. Eds. Caldwell, J.B. and Ward, G., Elsevier Publ. Ltd. UK., 1992.
[8] Koelman, H.J. "Freedom is just Another Word for Nothing Left to Loose", Proc.
WEMT'95, Copenhagen, Denmark 17-19 May, 1995, pp 45-56.
[9] Petersen, P. Temdrup. "Collision and Grounding Mechanics", Proc. WEMT'95, Copenhagen, Denmark 17-19 May, 1995, pp 125-157.
WEGEMT WORKSHOP: DAMAGE STABILITY OF SHIPS
DTU, Lyngby, 20th October 1995
"A NEW SAFETY STANDARD FOR PASSENGER/RORO VESSELS"
by Tor E. Svensen, Det Norske Veritas Classification AS
ABSTRACT
The paper presents the main objectives of the recently initiated joint North-West
European project "Safety of Passenger/RoRo Vessels". A critical examination of existing damage stability standards is made and 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. In particular the principle of creating a second barrier of defence against technical or human failure is discussed. Methods of performing a risk analysis on passenger/RoRo vessels are presented and the possible role of Formal Safety Assessment as part of vessel approval and certification procedures discussed.
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/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. However, 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 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/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.
Phase 2 of the project will examine how safety assessment procedures can be applied to the passenger/RoRo type of design. The safety assessment will be applied to any new ruleframework 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 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. IMO 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 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 of floodable length calculations. It 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. Present damaged stability requirements for RoRo 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 IMO, 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/RoRo vessels the requirements contained within this new probabilistic
framework A.265 are more stringent than the deterministic requirements in SOLAS-60 and therefore A.265 has generally not been much used on passenger/RoRo vessels. The next major step in the development of stability standards came in 1992 with the introduction of SOLAS part B-1 (in Chapter 11-1), containing a probabilistic standard for cargo vessels.
The IMO has worked on the harmonisation of stability standards for several years. Despite this we are 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 safety standard when compared with both A.265 and the new probabilistic standard for cargo vessels (SOLAS part B-1, 1992).
Following ajoint research project, the Nordic countries presented to the 38th session of SLF a draft 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 THE 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 be filled with water on the vehicle deck and capsize if the waveheight is above 0.5 - 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.
WHAT ARE THE M4hIV SHOR TCOMIVGS ?
The principal shortcomings of the present SOLAS standard for passenger/RoRo vessels in international unrestricted trade can be listed as follows:
The possibility that the vehicle deck may be flooded is not included in the calculations. The type B freeboard definition used on 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 Im. Even vessels built to SOLAS-90 standard are unlikely to survive in waves much above 1.5 -2m. Clearly this is inadequate for operation of large
passenger/RoRo vessels in unrestricted waters.
* Shift of cargo is not included as a risk. All RoRo cargo is assumed to remain
safely 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 for a vessel subject to unsymmetrical flooding after damage and rolling heavily in waves.
* The present SOLAS-90 standard for Passenger/RoRo vessels 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 internal waterline in flooded compartments. This is a very doubtful assumption.
WHY PROBABIL IS TIC STABILITY CRITERIA ?
They 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 passenger/RoRo vessels is that it should not destroy the basic principles behind the RoRo concept. It 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 for a new stability standard for passenger/RoRo vessels can be listed as follows:
1) Should be based upon the probabilistic method.
The basic requirements for a new stability standard for passcnger/RoRo vessels can be listed as
follows:
I) Should be based upon die probabilistic method.
2) Major risks such as flooding of the RoRo deck and cargo shift should be included. 3) A method for managing residual risk (i.e. preventing rapid capsize in those
damage cases where the vessel does not survive) should be included
The framework. The purpose of the framework is to control that the risk of sinking or capsizing
as a result of damage to the vessel or malfunction 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 a way that the number of fatalities are kept as
low as practically possible. The risk may be expressed as: RISK = Prohbability - Consequence
Consequence
Max. tolerable consequence
Uacpal riskiUnacceptable
Reducing probability
Acceptable risk Reducing consequence
If the risk is too large, it may be reduced by reducing the probability, reducing the 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
=
2(p
*s)
taken over all damage cases and combinations of damage cases where A = attained subdivision indexp = probability of damage
s = probability of survival with given damage A to be greater than a specified value
Calculation otfcapsize index (C):
C = * c) *•p taken over all damage cases and combinations of damage cases
where C = attained capsize index p = probability of damage
c = probability of capsizing with given damage (measure of residual risk) C to he less than a specified value
This latter probabilistic index relatingt 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 damagce in those cases where the vessel will not survive. The indices may be illustrated in the fiollowing diagram:
CONTROLLED SINKING
-
Attained capsize index "C"IControlled
sinking (not capsize)A
I
Attained subdivision index "A"MAIN ISSUES TO BE STUDIED:
Issues that will 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 be combined with the distribution of ship types to calculate the risk of impact with different ship types. Using first principles methods the probability of size of damage in terms of vertical extent, damage length and penetration will be
determined. The method will examine and, if possible, take into account the actual ship structural design of the RoRo, thus giving credit to a collision resistant
structure.
" Flooding and dynamic 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 vessel to survive. Model 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.
Damage stability calculation 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 stages of flooding, permeability. The key issue here will be to develop calculation methods that reflect the actual design solutions.
" Cargo shift: Development of a deterministic requirement for reserve stability as a
function of cargo type, number of lanes, deck layout etc.
5. SAFETY ASSESSMENT FOR PASSENGER/RoRo 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 offshore 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 for a passenger/RoRo vessel will involve the following elements:
* System definition * Hazard identification
* Frequency analysis and consequence modelling * Risk presentation
" Evaluation using risk criteria
* Selection of risk reduction measures
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 of 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 on a more limited scale and will concentrate on 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 Hazard
Identification, FMECA and SWIFT to an existing vessel. Identification of shortcomings in present design practices and applicable rules. This work will focus on prevention of single failures resulting in major accidents and smaller accidents escalating into major accidents.
" Quantitative risk analysis. Development of procedures for complete quantitative
risk analysys on passenger/roro vessels. Data collection and application to an existing vessel and a new design on one or more routes.
* Risk assessment 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 of procedures for safety assessment on individual vessels. The
purpose is to develop recommendations for 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.
It is generally believed that by following the above described procedure this will result in the most cost effective solutions.
A pre-study already carried out in the project has concluded that:
I) Techniques 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 identified.
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 vessel has concluded that the risk to the individual 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 significantly higher than for other means of transport.
6. CONCLUSIONS
The 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 be to 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:
1. SOLAS '29: International Conference on Safety of Life at Sea. 1929. 2. SOLAS '60: International Conference on Safety of Life at Sea. 1960. 3. SOLAS' 74: International Convention for the Safety of Life at Sea.
London, 1974.
4. IMO Resolution A.265 (VIII). Regulations on Subdivision and Stability of Passenger Ships as Equivalent to Part B of Chapter II of the International convention for the Safety of Life at Sea, 1969. IMO. London, 1974.
5. SOLAS 90: Ch. 1I-1, Part B-i: Subdivision and damage stability of cargo ships.
6. LLOYD, C.J.: "Research into Enhancing the Stability and Survivability of RoRo Passenger Ferries -Overview Study", Joint RTNA/DTp International Symposium on the Safety of RoRo Passenger Ships, London, April 1990
7. Vassalos, D. Dr.: "Capsizal Resistance Prediction of a Damaged Ship in a Random Sea", Joint RINA/DTp International Symposium on RoRo Ships' Survivability, London, November 1994.
8. Dand, I.W.: "Factors Affecting the Capsize of Damaged RoRo Vessels in Waves", Joint RINA/DTp International Symposium on RoRo Ships' Survivability., London, November 1994.
9. Velschou, S. and Schindler, M.: "RoRo Passenger Ferry Damage Stability Studies -a Continuation of Model Tests for a Typical Ferry", Joint RINA/DTp
International Symposium on RoRo Ships' Survivability, London, November
1994.
APPENDIX
Project Description for Joint Nordic Project
"Safety of Passenger/RoRo Vessels"
1. 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 damaged and flooded condition. The project will specifically address the issue of identify'ing and testing a second line of defence against technical or human failure. This will form the basis of proposals for new and extended Nordic and international rules.
2. SCOPE OF WORK
The scope of work is defined in principal tasks as follows:
Phase 1 : Stability:
Task 1:
Damage stability modelling 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 stages of flooding and permeability. Recommendations with proposals 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 collisions 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 will be utilised in two ways: 1) to enhance the existing method based upon purely historical data, allowing the actual traffic and 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. T ask 2. '
Large Scale Flooding: M,/odel test investigations to quantify' how water ingress on the
RoRo deck depends upon damage size, freeboard. GM, deck layout and seastate. Progressive tests in which the flooding is free to develop will be performed for GM values close to capsize for variations in freeboard, size and location of damage,
different seastates and different deck layouts. The primary purpose of the tests is to determine the amount and distribution of water trapped on the RoRo deck as a function of time and to determine the relative water level at the damage opening. The test results will also be used to validate the theoretical model developed under Task 5. Task 3 :
Dynamic Effects in Waves: Model 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 Task 5 and to develop a proposed simplified method for describing the amount of reserve stability required in order for the vessel to survive without capsizing in a given seastate.
Task 4 :
Cargo Securing and Cargo Shift: Existing rules and regulations for cargo stowage
and securing will be reviewed. Maximum heeling moments caused by cargo shift will be calculated in a deterministic way as a function of cargo type, width and number of lanes etc. A method for calculation of accelerations and forces acting on 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.
Task 5:
Development of M~athematical Model for Capsize Predictions: A mathematical model
for assessing the capsize safety of passenger/RoRo, vessels will be finalised. The model will be used for a systematic parametric investigation to identify and quantify the effect of key influencing 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. The model will further be used towards the development of relationships between ship design and environmental parameters and stability related parameters to be used as basis for deciding on appropriate levels regarding new probabilistic criteria.
Task 6 :
Framework for New Damage Stability Standard: Requirements to damage stability
assessment for RoRo ships will be developed and formulated, taking into account risk factors relevant for damage stability. The framework shall address all risks relevant to damage stability, like collisions. groundings, structural failures, etc. The prime goal is to provide a second line of defence against technical and human failures, such that adequate constructional features and technical/functional requirements may be provided. The framework shall describe procedures for formulation of requirements based on damaged GZ curve and other relevant damage stability parameters, together with assumed damage and damage statistics. Basis for the procedures shall be
statistics on weather conditions and traffic density for a certain service area. 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
minimising the risk of capsize shall be included. Example on formulation of such regulations shall be given.
Task 7:
Example Design : One or more example design will be developed in close
co-operation with a design consultant and/or shipyard in order to exemplify the proposed new rule framework and how this may be satisfied.
Phase 2 : Safety Assessment
Task 8:
Risk assessment (stability): A risk study will be made based upon the new set of rule
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. The
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 specific parts of IMO 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 TIMEPLAN, ORGANISATION AND BUDGET
Phase 1 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 with Phase I. A pre-study on the application of safety assessment procedures to passenger/RoRo ships will be completed by April 1995. However, the main effort in Phase 2 is from mid-1995 and the work will be finalised by February 1996.
The work on the project is split into tasks under the overall management of Det Norske Veritas Classification. The institutions and companies which are responsible for the tasks in Phase I are shown in the project organisation chart below
minimising the risk of capsize shall be included. Example on formulation of such regulations shall be given.
Task 7 :
Example Design : One or more example design will be developed in close
co-operation with a design consultant and/or shipyard in order to exemplify the proposed new rule framework and how this may be satisfied.
Phase 2 : Safety Assessment
Task 8:
Risk assessment (stability): A risk study will be made based upon the new set of rule
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 ofprocedures for safety assessment on individual vessels. The
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 specific parts of IMO 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 TIMEPLAN, ORGANISATION AND BUDGET
Phase 1 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 with Phase 1. A pre-study on the application of safety assessment procedures to passenger/RoRo ships will be completed by April
1995. However, the main effort in Phase 2 is from mid-1995 and the work will be finalised by February 1996.
The work on the project is split into tasks under the overall management of Det Norske Veritas Classification. The institutions and companies which are responsible for the tasks in Phase I are shown in the project organisation chart below:
Main cornerstones of new probabilistic stability framework:
SOLAS 1992, B-1)
TASK 1
Stability ______ __ _ ___ TASK 2.1
Modelling Collision
Methodsli -down flooding -damage size Damage
Methods light structures
-air pockets - damage water I
- intenmed. stages distribution -cross flooding (p-factor) -asymetric flooding
-time calculations Flooding
Prediction
NEW PROBABILISTIC
STABILITY FRAMEWORK
STASK 3 - residual Dynamic stability Effects TASK 4 capabilityTaSKe - heling mom. (s-factor) TASK 5
Cargo
-securing stdCriteriaShift
IDevelopm.
C
s
Attained capsize index "C"rn
Controlled sinking (not capsize) "S"A
Attained subdiv.F
-]index "A" 1995-11-06CAPSIZAL RESISTANCE OF DAMAGED RO-RO FERRIES:
Modelling and Application
DRACOS VASSALOS
Ship Stability Research Group
Department of Ship & Marine Technology University of Strathclyde, Glasgow
ABSTRACT
This paper presents a summary of the research work undertaken over the past few months in association with the Joint R&D Project between the Nordic countries, UK, France and Germany in so far as it outlines the fundamental thinking behind the approaches adopted and highlights some of the promising early findings. Representative results are also presented and discussed.
INTRODUCTION
Accident statistics clearly indicate that collision is the highest risk for passenger vessels, with
25% of accidents leading to water ingress of which more than 50% result in ship loss. Limiting
understanding of the ensuing complex dynamics related to the dynamic behaviour of the vessel and the progression of flood water through the damaged ship in a random sea state resulted in approaches for assessing the damage survivability of ships that rely mainly on hydrostatic properties. Furthermore. in case of serious flooding of ships with large undivided deck spaces, such as Ro-Ro vessels, the loss could be catastrophic as a result of rapid capsize, rendering evacuation of passengers and crew impractical, with disastrous (unacceptable) consequences. The need for a methodology to reduce the risk ensuing from collision damage to-a level As Low As Reasonably Practicable cannot be overemphasised. The tragic accidents of the Herald of Free
Enterprise and meire recently of Estonia were the strongest indicators yet of the existing gaps in
undivided deck spaces. They have also brought about a realisation that 'ship survival' might have to be addressed separately from 'passenger survival' in that the deterioration in the stability of such vessels when damaged could be 'catastrophic' rather than one of graceful degradation. It would appear, therefore, that the approach to assessing realistically the damage survivability of passenger ships and indeed any ships, must derive from a logical framework such as that offered by the probabilistic method and must, of necessity, offer the means of taking into consideration meaningfully both the operating environment and the hazards specific to the vessel in question.
This, in turn, necessitates the development of suitable "tools" and procedures for dealing in a systematic manner with the main problems and uncertainties pertaining to serious flooding of passenger ships tollowing collision damage.
Deriving from the above, one of the tasks should be to quantify the probability of damage with water ingress in a given service area and, the second, to quantify the consequences of damage by identifying and analysing all the important factors using probabilistic methods. However, even though it is self-evident that the risks involved can be reduced by reducing either the probability of damage or the consequences of damage or both, there is a level beyond which consequences cannot be tolerated. In this case, risk cannot be reduced by reducing the probability of damage alone. The need arises, therefore, for a methodology whereby key questions are addressed and answers sought concerning definition of acceptable risks, definition and nanagelment of maximum tolerable consequences and procedures for dealing with residual risks. The Joint R&D Project has been set to address this need urgently, by bringing together all the available expertise in the relevant areas. However, the wisdom of attempting to provide answers within what amounts to several months, forms in itself another interesting question. Details of the Project are provided by Dr Tor Svensen in paper 2 of these proceedings.
This paper deals with Task 5 of the Joint R&D Project, pertaining to the development and validation of numerical tools for assessing the damage survivability of passenger/ Ro-Ro vessels,
leading to the development of survival criteria.
BACKGROUND UK Ru-Ru Research Programme
In the wake of the Herald of Free Enterprise disaster, the need to evaluate the adequacy of the various standards in terms of providing sufficient residual stability to allow enough time for the orderly evacuation of passengers and crew in realistic sea states has prompted the Department of Transport to set up the Ro-Ro Research programme comprising txwo phases. Phase I addressed the residual stability of existing vessels and the key reasons behind capsizes. To this end theoretical studies were undertaken into the practical benefits and penalties of introducing a number of devices. [1], for improving the residual stability of existing Ro-Ro's. In addition, model experiments were carried out by the British Maritime Technology Ltd. [2] and the Danish Maritime Institute. [3] in order to gain an insight into the dvnamic behaviour of a damaged vessel in realistic environmental conditions and of the progression ot flood water through the ship.
