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PRADS 2007

Dynamic Stability Criteria Based on the Simulation of Full Scale Ac-cidents

Author Name(s): Stefan Krueger, Florian Kluwe

Hamburg University of Technology (TUHH), Institute of Ship Design and Ship Safety

Hamburg, Germany

Abstract

It has become obvious that modern ships suffer fromproblems related to their seakeeping-behavior, which ismainly related to large amplitude roll motion in headand following seas.

As these effects are not covered by the existing intactstability criteria, an additional concept was developed.

This new concept allows us to quantify the risk of theoccurrence of large roll angles by calculating a capsiz-ing index.

The question left open so far was, how much stabilitywould be required for a certain ship to meet a distinct

safety level against capsizing.

This paper presents an approach addressing this ques-tion based on the analysis of some real capsizing acci-dents analyzed by applying various criteria proposed inthe past, in comparison to the new intact stability index.

For this purpose our simulation code, ROLLS, wasapplied, taking into account relevant effects such ascargo shift, additional heeling moments and water in-gress.

Keywords

Ship safety; parametric rolling; intact stability; capsiz-ing; probability; stability criteria; seakeeping perform-

ance

Introduction

In a concerted effort to address shipping accidents, a

number of capsizing criteria have been proposed in thelast decades, either based on model tests and simula-tions or on empirical observations. A brief introductionto a selection of these criteria introduced by German

research groups is given in the first section of this paper.All presented criteria are intended to reduce the capsize

risk of ships in heavy weather. Most of these criteria donot take into account dynamic effects of ships travelingin a rough seaway.

New techniques such as numerical motion simulationsin the time domain have improved our knowledge onthe phenomena and the situations in which ships areendangered with respect to large roll angles. Today this

increased knowledge allows us to address exactly thosedynamic aspects lacking in most of the older criteria.

This concern seems to be absolutely necessary, as mod-ern hull designs seem to be even more endangered byphenomena like parametric roll than traditional designs.Moreover the mean ship size and speed have increasedin the last decades, also contributing to the fact that the

current intact stability rules are not able to guarantee asufficient safety level for all ships.

Therefore, a new concept was developed called Insuffi-

cient Stability Event Index {ISEI} based on a largedatabase of ships simulated in various sea states. Anintroduction to this new approach is given in the secondpart of this document.

To get an idea of the capabilities the new index has withrespect to ship safety in heavy seas, a number of realcapsizing accidents were re-investigated. The loading

condition of the ship present at the time of the accident,

which can always be clearly identified as not safe,was analyzed with a set of intact stability criteria includ-ing the new index. Finally an attempt was made to iden-tify the stability increase necessary to omit the individ-

ual accidents. All three examples of accidents chosenand clearly related to insufficient intact stability arepresented in the third part of this document. More acci-dents are currently being investigated at TUHH, but nofinal results are available thus far.

Overview on Selected Capsizing Criteria

Empirical Criteria Related to Righting Levers

The aim of these criteria is to ensure sufficient safety of


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ships in heavy weather by identifying significant, stabil-ity related characteristics of the ships lever arm curves.

Wendelss concept of Balancing Righting and Heeling

Levers:

Wendel and his group developed a concept whereby the

stability of ships should be evaluated on the basis of anindividual balance of righting and heeling levers (Arndt

(1960) .The dynamic effects of capsizing as such aredisregarded in this concept, but the stability reduction istaken into account by using the mean value of the crestand trough condition lever arms instead of the stillwater

righting lever, which is questionable from todays pointof knowledge. The theoretical background of Wendel'sconcept is described in Wendel (1954) or Arndt (1960).The German Navys stability standard BV1033 is basedon this criterion.

The C-Factor Concept for Container Vessels Largerthan 100m in Length:

With the introduction of container vessels the averagebeam-to-depth-ratio of the world merchant fleet grewsignificantly from ca. 1.60 in 1960 to ca. 1.9 in 1980.An increased beam-to-depth ratio leads to larger initial

stability, whereas added form stability is significantlyreduced. Therefore, Blume and Wagner carried out anumber of model tests for container vessels. Based onthe results, Blume tried to establish a criterion for theminimum stability of vessels in rough weather, (Blumeand Hattendorf, 1987a). The findings lead to the devel-

opment of the C-factor concept, which enhances theoriginal Rahola-criteria.

For example, this task is done by replacing the static

requirement for the righting lever at 30 degrees beinglarger or equal 0.2 m according to Rahola by the con-stant value divided by C, where C is calculated as fol-

lows (Blume, 1987):

LC

C

KG

T

B

DTC

WP

B 1002

= [1]

Here, Tdenotes the draft,D a modified depth includinghatches,KGis the center of gravity above base line. CBand CWPdenote the block- and the waterline-coefficient,

respectively.

The C-factor today is part of the IMO Code on IntactStability for certain types of vessels above 100m in

length, but as the overall code, it is not mandatory. Fi-nally the problem still remains that the C-factor is re-lated to the still water righting lever curve, which is notsufficiently representative for seakeeping problems.

Capsizing Criteria Derived from Model Tests or Simu-

lations

The Kastner/Roden Criterion for a Minimum GM toPrevent Pure Loss Failures:

Based on model tests carried out on the inland lake

Ploen in Germany by Kastner (1962), a method wasdeveloped to determine a minimum GM required to

prevent the vessel from capsizing in rough weather. Theauthors observed the interesting phenomenon that a

clear limiting GM seemed to exist, distinguishing be-tween ships being safe or unsafe with respect to capsiz-ing. The criterion is based on the probability densityfunction for the time to capsize determined during themodel tests. The authors then asked for a cumulatedprobability of 95% for the complement event not cap-

sized. Then the time interval Tk is determined whichthe ship must survive to fulfill the requirement givenabove. Assuming that the ship always capsizes in thelargest wave akoccurring during Tkthe capsizing prob-ability is linked to the probability of occurrence of thatwave.

Now a maximum wave height ak can be determined

which has lead to the capsizing in a specific situation,e.g. during a model test. Now, assuming a probabilityfor a non-capsize, a related wave height ank the shipneeds to survive in order to be sufficiently safe can bedetermined in the same way. The authors then con-cludes that the GM- value of the vessel must be in-creased by the ratio which is defined by the these two

wave heights:

nk

k

nk

k

a

a

GM

GM= [2]

This ratio is somewhat doubtful from todays point ofknowledge as the assumptions made clearly fail in thecase where the GM gets close to zero.

Soedings Concept of Simulating Rare Events by Artifi-cially Amplified Wave Heights:

In principle event probabilities can be determined sim-

ply by counting them during model tests or numericalsimulations. But, as extreme events (e.g. capsizing) arerare, it is difficult to determine significant values for

capsizing probabilities during model tests and numericalsimulations due to the limited duration and the resultingsmall number of occurrences.

Therefore, Soeding and Tonguc (1986) suggest thesimulations be run in artificially high waves. By assum-ing Rayleigh-distributed amplitudes, the capsizing prob-ability can be extrapolated to the actual wave height ofinterest by the following relationship:

25.1)ln(

25.1)ln(2

2

+

+=

act

sim

act

sim

p

p

H

H [3]

Here H denotes the actual (act) or the simulated (sim)wave height, respectively.Pdenotes the capsizing prob-ability, using the same indices. However, the proposedcriterion does not provide a procedure to determine theenlargement factor for the wave height. Additionally theconcept does not include any threshold values for thecapsizing probability.

The Blume-Criterion

Blume and Hattendorff (1987b) developed this criterion

to evaluate the ship safety with respect to capsizing infollowing and stern quartering seas by model tests. For


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each run during the model test the maximum roll angleis registered. Then the residual area ER below the still

water lever arm is calculated, limited by the maximumroll angle and the point of vanishing stability (see Fig.1). If the ship capsizes during the run, ER is set to zero.Finally a ship is regarded as safe against capsizing if itfulfills the following requirement:

03>

sER [4]

Here RE denotes the residual area averaged by all runs,

srepresents the standard deviation ofER. By this a sta-bility limit, represented by either a minimum GM or bya limiting maximum wave height can be determined.

Fig. 1: Residual area below the righting leer curve

The Insufficient Stability Event Index (ISEI)

After some incidents related to parametric rolling withcontainer vessels become known at the end of the lastdecade, a German research group was established to

develop dynamic stability criteria, which should bebased on numerical simulations. The simulation code

ROLLS, originally developed by Kroeger (1987) andPetey (1988),was chosen to serve as the basis for theevaluation of seakeeping related problems,. The codewas validated and further enhanced by Cramer and

Krueger (2005).A research program was established inwhich a large number of model tests for different mod-

ern hull forms were carried out with tailored wave se-quences to validate the simulation code. It was con-cluded that the simulation code was able to predict allrelevant phenomena related to the problem of insuffi-cient stability in waves with sufficient accuracy. There-

fore, it was decided to develop a concept for minimumstability based exclusively on numerical motion simula-tions. Based on the numerical simulations, the followingmain findings were made or confirmed:

Both model tests and simulations confirmed

that critical situations endangering the shipwith respect to large roll amplitudes are ob-served in head as well as following seas.

No capsizing events were found in beam seasat zero speed.

The most dangerous scenarios appeared to bethose where the ship was traveling in followingand stern quartering seas.

In head and head-quartering seas, large rollingangles were observed, but capsizing usually didnot occur. This is due to the fact that criticalresonances are connected to relatively low val-

ues of GM in following seas, and to high GMvalues in head seas. The model tests were con-ducted close to potentially critical resonances.

Unlike the expectations by previous authors, wave-lengths significantly shorter than the ship length could

endanger the vessel, whereas wavelengths significantlylarger than ship length did not initiate large roll ampli-tudes.

In contradiction to previous criteria, it was decided todetermine all possible scenarios that may lead to a dan-gerous situation, but not to quantify just how dangerous

a specific situation actually is. When defining limitingstability values, it is of importance to assess the prob-ability of a specific loading condition being dangerousor not for the vessel. For this application it is not ofpractical interest to get the exact capsizing rate duringthe simulation, but it is singularly important to know ifthe ship did fail. Based on this, the concept is aimed

towards determining long-term probabilities rathershort-term probabilities. Thus, the concept requires amethodology to distinguish between being safe or un-safe for a ship in a specific situation without countingthe actual up-crossing rates.

Given that such a methodology is available, the totallong term probability for a dangerous situation happen-ing in a specific loading condition can be defined, then

by the insufficient stability event index (ISEI), which isdefined by the following equation (see also Krueger and

Kluwe (2006)):

13/113/1

0 0

2

0

13/1

),,,(

),(

1 3/1

max

min

dTdHddvvTHp

THpISEI

ssdang

T H

v

vv

sea

s

=

= = =

=[5]

Here psea denotes the probability of occurrence of a

specific sea state defined by the significant wave heightH1/3 and the characteristic (peak) period T1, whereaspdang represents the probability for the actual loadingcondition leading to a dangerous situation under thecondition of a specific sea state.

The two-dimensional probability density function iscalculated from a scatter table presented by Soeding

(2001).Taking the discrete values from the scatter tablefor each of the intervals for H1/3and T1, the integrationof equation [5] can easily be transformed into a summa-tion of the respective values.

The probability that the actual loading condition leads to

a dangerous situation in the seastate given by H1/3andT1then can be written as follows:

),,(

)(

),,,(),,,(

13/1

13/113/1

THvp

p

vTHpvTHp

sv

sfailsdang

=

[6]

In this equation,p()denotes the probability the ship istraveling at a course of -degrees relative to the domi-nating wave propagation. It is assumed that p() isindependent from the actual values of H1/3and T1.p()

can be taken from full-scale observations (see Krueger,Hinrichs, Kluwe and Billerbeck (2006)). Thenpv(H1/3,T1,,vs) denotes the probability that the ship is


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traveling at a speed of vs knots. As p() is selectedindependently from the seastate, pv(,vs|H1/3,T1,) is a

conditional probability depending on all four parame-ters, as not all speeds are physically possible in a spe-cific situation. Krueger, Hinrichs, Kluwe and Billerbeck(2006) determine the maximum possible ship speed inthe given environmental conditions at full engine outputand the minimum speed at engine idle speed from sys-

tematic propulsion calculations. Within the range ofpossible speeds [vmin,vmax] the probability of occurrenceis assumed equally distributed as more accurate data islacking.

The failure probability pfail(H1/3,T1,,vs) is determinedfrom the time series of the numerical simulation byapplying the Blume-criterion mentioned above. Given

the loading condition fulfills the Blume-Criterion in theactual situation, pfail(H1/3,T1,,vs) is set to 0, whichmeans that the loading condition is sufficiently safe forthe given conditions. In case the Blume-criterion failsfor the current situation,pfail(H1/3,T1,,vs)is set to 1. Thisequation means that decision is taken only betweensafe and unsafe by setting the failure probability to

0 or 1, respectively.

All situations in which the failure criterion is set to 1contribute to the overall long-term probability. Formally

this does not deliver a correct capsizing probability,which is the reason that the result is called capsizingindex. Yet taking into account the practical considera-tions, it seems to be more important for us to identify

dangerous situations than to determine the exact failurerate in a specific situation that is known to be danger-

ous.

Furthermore, it should be noted that our method explic-

itly treats head sea and following sea cases only. There-fore, we restrict the contributing courses to a 45-degreesector of encounter angles, port and starboard in headand following seas. Consequently, it is then useful to

split the ISEI in a head sea and a following sea index.The ISEI then can be written as follows:

))(,)(),()((

))((

))(),((

))(,)(),()((

))((

))(),((

13/1

1 1 1

13/1

13/1

1 1 1

13/1

1 3/1 , ,

1 3/1 , ,

kiTjHlvp

kp

iTjHp

kiTjHlvp

kp

iTjHp

ISEIISEIISEI

v

N

i

N

jj

N

k

N

l

sea

v

N

i

N

jj

N

k

N

l

sea

headfollowing

T H

Bl

h hv

T H

Bl

f fv

+

=

+=

= = = =

= = = =

[7]

In the formula, the summation on the limiting waveheights starts at jBl, which is the smallest significantwave height for the given significant period T1 wherepfailequals 1. The encounter angles run from =-/4to=+/4 for the following sea cases and from=3/4 to

=5/4 for head seas. The speed summation runs fromthe minimum speed possible in that condition to themaximum speed possible. The indices h and f indicate

head and following seas, respectively.

For practical applications, it is useful to find those com-binations of H1/3, T1, and vs, which represent the limit

between safe and unsafe. This solution can be mostefficiently achieved by finding the limiting significantwave height for a given combination of parameters T1, and vs according to the Blume-criterion. In cases wherethe Blume-criterion does not deliver suitable results

typically due to large angles of vanishing stability, theoccurrence of a certain maximum roll angle may besimultaneously taken into account.. The more conserva-tive value is taken for the decision between safe andunsafe. The results may be plotted in the form ofpolar diagrams as presented in Fig. 2. Each polar dia-gram presents the limiting wave heights for a specific

significant period (or the related significant deep waterwave length), giving an overview about critical situa-tions (see Cramer and Krueger (2005) and Krueger(2002)). Typically the simulations, with a duration of10000 seconds in real time, are repeated five times, eachwith different wave realizations.

Fig. 2: Graphical visualization of dangerous scenarios bythe limiting significant wave height according to the

Blume-criterion

The ISEI-concept allows the identification of ship de-signs and ship types, which are vulnerable for insuffi-cient stability events in following or head seas. At this,the ISEI-concept takes into account all relevant phe-

nomena occurring in head and following seas that mayendanger the vessel with respect to minimum stability.Unfortunately, there is no limiting value for the ISEIthus far making it difficult to actually apply the conceptwith respect to the determination of minimum stabilityrequirements. In order to define threshold values for the

ISEI-concept, Krueger and Kluwe (2006) suggestedanalyzing the safety levels for a large number of exist-ing ships by using the ISEI as the quantitative meas-urement.

The Simplified ISEI

To evaluate a ships safety by applying the ISEI conceptrequires expert knowledge with respect to numerical

seakeeping simulations while being relatively time con-suming. Therefore, it would be very useful to calculate

the capsizing index without the necessity of applyingtime domain simulations to obtain pfail. Currently, we

are working on a concept, which replaces the simula-tion-based calculation procedure forpfailby a simplified


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approximation method, mainly based on lever arm al-terations between wave crest and wave trough condi-

tions. A first, but in some aspects unsatisfying, approachis presented in Krueger and Kluwe (2006).

Investigation of Real Capsize Accidents

In order to test and calibrate the newly developed con-

cept, a number of real capsize accidents are currently re-investigated by TUHH. The criteria described in thefirst section of this paper are applied to the operatingcondition of the ship at the time of the accident, which

always clearly is an unsafe situation. Secondly thecriteria are used to estimate a loading condition wherethe respective vessel can be considered as safe. Forboth conditions the ISEI is calculated in order to dem-onstrate whether the new concept is able to distinguishbetween safe and unsafe loading conditions. Three ex-amples for this work are given below. More cases are

currently being investigated.

The Capsizing of SS Fidamus (1950)

Fig. 3: General Arrangement of SS Fidamus

On January 31, 1950, the 743 BRT vessel SS Fidamuscapsized in heavy weather bound from Wismar to Ant-

werp close to Langeoog, 54 N, 7 E. The vessel wasloaded with ca. 900t potash (angle of repose ca. 35

Deg.). The vessel was traveling in following seas of ca.40 m significant wave-length,H1/3was ca. 2.0 m and thevessels speed was ca. 9.5 knots. The vessel suddenlyheeled to more than 30 degrees and remained there with

a steady list of ca. 35 to 40 degrees. Water ingress thenlead to capsizing within 10 minutes. The information

given above was taken from the final report issued on1950-06-27 by the maritime casualty investigationboard of Bremerhaven (Seeamt Bremerhaven (1950)).

The floating condition prior to the accident could bereconstructed approximately as follows: The ship had atotal displacement of ca. 1541 tons, resulting in a draft

of 4.69m at the aft perpendicular (a.p.). The trim was1.12m by stern. Interestingly enough the ship did not

carry any ballast water, although this was strongly rec-ommended in the stability booklet. The resulting right-ing levers are shown in Fig. 4.The initial GM in stillwater conditions amounts ca. 0.30 m. Based on theselever arm curves we can conclude that without anyexternal heeling moment, the vessel would immediately

heel to about 30 degrees if it stays long enough on thewave crest.

Fig. 5: Simulated time series of SS Fidamus including

entrapped water (blue: starboard side, black: port side).

The red curve shows the roll angle (positive starboard)

During the original investigations it was supposed that

the vessel suffered from insufficient stability. It is offurther importance that in an expertise made on behalfof Seeamt Bremerhaven, Kempf (1950) concluded thatthe vessel was traveling in a 1:1 following sea reso-nance, where the rolling period of the vessel (for smallangles) was determined as 11.8 s by Kempf. The en-

counter period was determined to 11.1s. It was con-

cluded that the low stability of the vessel, further re-duced in the crest position, resulted in the large heelingangle. Another supposition was that entrapped waterbetween the hatch, forecastle and bulwark had lead to aheeling moment, which in turn, perhaps together with acargo shift resulted in an intermediate equilibrium float-

ing condition at about 35- 40 degrees coinciding with alocal minimum of the static still water righting levercurve. Water ingress in the forecastle was made respon-sible that the vessel finally capsized during the originalinvestigations.

Fig. 6: Polar diagram showing limiting wave heights with

respect to a maximum roll angle of 40 degrees. Significant

wave length is 40 m

Fig. 4: Lever arm curves for SS Fidamus

(cyan: still water, green: wave trough, red: wave crest)

The numerical simulations carried out with our time-domain code ROLLS (see Kroeger (1987) and Petey(1988)), show that the vessel is permanently rolling with


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a maximum of ca. 45 degrees. As the static angle ofvanishing stability in still-water conditions is beyond 90

degrees, it is theoretically not possible to capsize thevessel without any additional heeling moment. Ourinvestigations show that for the dynamically rollingvessel, additional water ingress is not necessary for thefinal capsizing, but that the entrapped water between thehatchway coaming and the bulwark produces a suffi-

cient heeling moment as the time series in Fig. 5 clearlydemonstrates.. The following conclusions can be drawnfrom the time domain simulations:

Fig. 7: General Arrangement of MV Lohengrin (taken

from Die deutsche Handelsflotte (1963))

For nearly all situations the ship is traveling in follow-ing seas large heeling angles beyond 40 can be ob-served, as shown in Fig. 6.It can be shown that a rela-tively small additional heeling moment due to water

between the hatchway coaming and bulwark is suffi-cient to cause the final capsizing of the vessel. Once alist of ca. 40 has been reached, the roll motion oscil-lates around this heeling angle while the amount ofwater trapped is reducing very slowly. The vessel cap-sizes as reported by the surviving crewmembers if thevessel stays a sufficiently long time in this situation. In

some of the simulated situations, the water-outflow wassufficiently fast enough to prevent the vessel from cap-sizing. The vessel then returns to the upright position.This exact behavior was reported from a voyage beforethe accident by surviving crewmembers.

By comparing the results for different intact stability

criteria as presented in Table 1, it can clearly be seen

that all criteria considered the case where the vessel didactually capsize as dangerous, whereas all criteria con-sider the 0.50m GM case as determined by the Kastner-Roden- criterion as clearly safe. Additionally, it can bestated that a direct ISEI of 0.2 represents a condition,clearly proven to be unsafe. On the other hand, an ISEI

of 0.0008 represents a condition considered to be safeby all other criteria.

The Capsizing of MV Lohengrin (1963)

On January 14th, 1950 the 955 BRT vessel MV Lohen-grin capsized in heavy weather bound from Igge-sund/Sweden to Kiel. The vessel was loaded with ca.

1195 t cellulose in bales. Half a year before the acci-dent, the vessel was converted resulting in higher hatch

coamings and an increased VCG of the cargo hold vol-ume. Based on the information obtained from the

Seeamt Flensburg (1964) the most probable floatingcondition at the time of the accident shows a draft of4.69m at a.p. with a total displacement of ca. 2000 tonsand an initial GM of 0.13 m. The resulting lever armcurves are shown in Fig. 8.On the day of the accidentthe vessel entered the Kiel Fjord at about 14.00 hrs. The

waves encountered the vessel from abaft. Significantwave period was ca. 6 seconds, the wave height was

reported to be about 2 meters. At about 14.15 hours thevessel heeled to 40-45 degree starboard side and re-mained there with a steady list of the same size. Theship finally capsized and sank ca. 1.5 hours later.

The dynamic analysis by means of numerical simula-

tions clearly shows that the reason for the loss of MVLohengrin can be consistently explained by insufficientstability and a pure loss on the crest situation. The ac-tual stability at the time of the accident was even belowthe recommended Rahola-criteria. As the deckhousewas weathertight, the vessel could find an intermediate

equilibrium while resting on the superstructures. Thanks

Table 1: Results for different criteria in safe and un-

safe loading condition for SS Fidamus

Criterion GM=0.30 m GM=0.50 m

Kastner/Roden

Capsizing time [s] 487 2386618

SoedingCapsize Probability 0.25/Roll Cycle 0.1 - 0.7E-6 /year

Blume (Modified)

E_R - 3 S E_R = S = 0 1.772 mmRad

ISEI (direct) 0.20078 0.0008

Empirical Criteria

Crest lever < 0.05 > 0.05

Crest range < 16 Deg > 16 Deg.

Blume c- factor none fulfilled all fulfilled

Fig. 8: Lever arm curves for MV Lohengrin

(cyan: Stillwater, green: wave trough, red: wave crest)

Fig. 9: Polar diagram showing limiting wave heights withrespect to the Blume-criterion. Significant wave length is

60 m


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to this fact, most of the crew could be saved. This in-termediate equilibrium is possible only if an additional

heeling moment acts on the ship, for example due tocargo shift. Our investigations show that a shift of TCGof the cargo by 4cm would be enough to keep the vesselin the listed position. In general, in the given loadingcondition the vessel was theoretically unsafe in all fol-lowing sea situations.

Again, comparing all criteria, the results are unique. Allcriteria consider the case where the vessel did actuallycapsize as dangerous. The GM value originally pre-dicted by the Kastner-Roden-criterion for this case

seems to be too low. For the GM-value of 0.305m,which was selected from the simulations in artificially

amplified waves, all criteria consider this case as safe.Additionally, it can be stated that a direct ISEI of 0.189

represents a condition, which has clearly proven to beunsafe, whereas an ISEI of 0.002 represents a condition,

which is considered to be safe by all criteria.

The Capsizing of SS Irene Oldendorff (1951)

Fig. 10: Side view of SS Irene Oldendorff

On December 31, 1951, the vessel SS Irene Oldendorffcapsized in heavy weather bound from Emden to Ystad

in the Hubert Gat. The vessel was carrying ca. 2750 tonsof coke, of which ca. 440 t has been carried on deck.

The wreck was found later at the position 53 38 27N and 6 17 10 E. The last known position of thevessel was close to buoy J/E 1 in the Hubert Gat. Ac-cording to our findings based on the data of the Seeamt

Bremerhaven (1952) the vessel had a draft of 5.4 m a.p.,trimmed 0.108 m by head at the beginning of the voy-

age (Emden Lock). This amount equals a displacementof 4575t.

The simulation-based analysis shows that the reason for

the loss of SS Irene Oldendorff can be consistentlyexplained as an intact stability accident due to the lossof stability in extreme weather conditions. The loss of

stability in this particular situation can be clearly relatedto water entrapped in the coke deck cargo, which could

not escape fast enough through the freeing ports. Thesimulations have shown that even if the superstructuresof the vessel are regarded as weather-tight, the stabilityvanishes completely on the crest for waves of 80m inlength and 5m in height. Dynamically, the vessel easilyreaches heeling angles beyond 40 degrees. These steep

waves, however, are quite extreme. Taking all of thisinto account the capsizing sequence very likely was asfollows: Due to the low stability in wave crest condi-tions a large heeling angle must have occurred, shiftingthe vessel to an intermediate equilibrium at ca. 45.Water ingress, cargo shift or both must then have lead to

the final capsize.

The simulations have shown that capsizing is hardlypossible for the stability the ship had when she left Em-den lock. This fact is clearly stated in the polar-diagram(Fig. 11,left side) for the Emden Lock situation wheresignificant wave heights of ca. 7-8m are required toendanger the vessel. These extreme (significant) waveheights are hardly possible in the Hubert Gat. Assuming

an amount of 90 tons of water was entrapped in thecoke, the polar-diagram changes significantly (Fig. 11,right side). Now, the limiting wave height is shifted to a

much lower value of about 4-5 meters for the operatingcondition of the vessel. All calculations are based on theassumption that the deckhouse can be considered as atleast temporarily weather-tight. Otherwise, it seemsimpossible that the crew could have manned the rescueboats.


safe loading condition for MV Lohengrin


Kastner/Roden

Capsizing time [s] 6024 -

Soeding

Capsize Probability - 0.4E-7/year

Blume (Modified)

E_R - 3 S -3.509 mmRad 51.9 mmRad

ISEI (direct) 0.18911 0.002

Empirical Criteria

Crest lever < 0.05 > 0.05

Crest range < 16 Deg > 16 Deg.


Fig. 11: Polar Diagram showing limiting wave heights

according to

Table 3 shows that all investigated criteria consider the

case as dangerous, where the vessel did actually capsize.On the other hand all criteria consider the 0.39m-GMcase as clearly safe. Therefore, the direct ISEI of 0.158represents a condition, which has clearly proven to beunsafe. An ISEI of 0.0011 represents a condition, whichis considered to be safe by all criteria. All criteria exceptthe C-factor and the Blume-criterion do not consider the

situation at Emden lock as sufficiently safe where GMwas ca. 0.20 m. This condition is related to an ISEI of0.0139. The C-factor concept may suffer from the factthat the vessel is actually a Shelter Decker, which haslarge freeboard and may not be covered by the concept.

All these considerations for the Emden lock conditionmust lead to the conclusion that the vessel did not haveenough maximum righting lever. Without the deckhouse


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being weather-tight, the maximum righting lever occursat 30 degree and amounts ca. 0.2 m.

Although the large freeboard results in a significant

range of positive righting levers, this does not lead tosufficient stability as in the given situation large heelingmoments acted upon the ship. They can only be coveredby a sufficiently large maximum righting lever and notby range alone. Taking all findings into account, the SSIrene Oldendorff -accident can be regarded as a case,

which did occur close to the limit that distinguishes aship from being safe or unsafe.

Conclusions

In the recent years a large number of ships was investi-gated with respect to their dynamic behavior in waves

by means of numerical simulations in the time domain.Based on this database, a new intact stability conceptwas developed called Insufficient Stability Event Index(ISEI).

The new concept is based on long-term probabilitiestaking into account the probability of occurrence for

sea-state, course and ships speed. The actual failurecriterion for the ship in a specific operating condition isimplemented via a safe/unsafe-decision based onthe Blume-criterion and the maximum roll angle ob-

served during the simulation.

Additionally, a number of intact stability accidents, allleading to the total loss of the vessel, were investigated

to get a more precise picture of the complex failuremodes leading to the capsizing of a ship.

A set of intact stability criteria, presented at the begin-ning of this paper, was then applied to the respectiveloading condition of the ship. Based on these criteria asafe loading condition was estimated and subse-

quently the newly developed ISEI was calculated forboth conditions. The results show that the values ob-

tained for the ISEI differ significantly between the twocalculated loading conditions. This observation indi-cates that the new concept is able to give clear advice asto whether the ship is safe or unsafe in a specific condi-

tion. Based on this work it will be possible in the nearfuture to define threshold values for the ISEI.

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safe loading condition for SS Irene Oldendorff


Kastner/Roden

Capsizing time [s] 13800 No capsize found

Soeding

Capsize Probability -

0.3E-6 for h>9m

none for h 0.05

Crest range 10 < 16 Deg > 16 Deg.


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adopt pub prads2007 capsizing final tuhh sdit

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