Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011

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1 Proceedings of the ASME th International Conference on Ocean, Offshore and Arctic Engineering OMAE2011 Proceedings of ASME th InternationalJune Conference 19-24, 2011, on Ocean, Rotterdam, Offshore The and Netherlands Arctic Engineering OMAE2011 OMAE 2011 June 19-24, 2011, Rotterdam, The Netherlands OMAE OMAE ACCIDENTS IN SITUATIONS WITH LARGE STABILITY VALUES Prof. Stefan Krüger Christoph Michael Steinbach Nicolas Rox Institute for Ship Design and Ship Safety Hamburg University of Technology Hamburg, Germany Jörg Kaufmann Frenec John German Federal Bureau of Maritime Casualty Investigation Hamburg Germany ABSTRACT In the last two years a number of accidents with several dead or heavily injured crew members have been reported to the German Federal Bureau of Maritime Casualty Investigation caused by excessive roll motions with large transversal accelerations. All accidents happened on board of container vessels in situations of large stability and partial draft like in ballast or in laidup situations. Also the accident situations were similar in all cases. During harsh weather conditions the crew tried to keep the ship against the incoming sea at constant slow speed. It was possible to reconstruct the accident events and the occurrence of high transversal acceleration values by numerical seakeeping simulations. To figure out if container vessels have an inherent danger of such problems a numerical study on a group of fifteen container vessels of different sizes was carried out in the mentioned accident situations. In this paper the reported accidents and the numerical investigation on the accidents are described, furthermore the results of the extended numerical study are presented and a phenomenological conclusion is drawn from the results. NOMENCLATURE AP Aft Perpendicular a.bl above Baseline Bft. Wind Force according to Beaufort scale BL Baseline BSU Bundesstelle für Seeunfalluntersuchungen - German Federal Bureau of Maritime Casualty Investigation CL Center line DWD German national meteorological service FP Forward Perpendicular FS Full scantling MF Main frame RAO Response amplitude operator TEU Twenty-foot equivalent unit Address all correspondence to this author. 1 Copyright c 2011 by ASME

2 LIST OF SYMBOLS Symbol Unit Description A Enc [Deg] Angle of Encounter B [m] Ships breadth D [m] Ships depth H s [m] Significant wave height GM [m] Metacentric height GM c [m] Free surface corrected metacentric height L OA [m] Length over all L PP [m] Length between the perpendiculars T [m] Draft T D [m] Design draft T FS [m] Full scantling draft T s [s] Significant wave period T s,enc [s] Significant wave period of encounter a y [ m s 2 ] Transversal acceleration g [ m s 2 ] Gravitational constant v service [ m s ] Speed service [mt] Displacement λ [m] Wavelength ω [s 1 ] Circular frequency INTRODUCTION In 2002 the national maritime safety investigation law came into effect in Germany with the aim to improve the maritime safety on a national level. By order of this law every incident regarding vessels with a german port of call, a german citizen or incidents which have happened in german territorial waters must be reported to the German Federal Bureau of Maritime Casualty Investigation if a severe damage to a vessel, the environment or to the health of a person has occurred. In 2008 and 2009 three accidents were reported to BSU (see [1], [2]& [3]) with several dead and heavily inured crew members caused by large transversal accelerations on board of container vessels. During the progress of investigations the BSU requested reports on the seakeeping behaviour of the vessels at the Institute for Ship Design and Ship Safety at Hamburg University of Technology, to figure out how and why these high acceleration values have occurred. Noticeable about these three accident was the fact that they happened under similar circumstances. The vessels were in ballast or near ballast condition with high GM values in harsh weather, while the crew kept them at slow speed against the sea to avoid slamming and 2:1 resonance. The question which comes up is, why have such problems not been reported earlier? The ballast condition is a quite uncommon loading condition for container vessels but due to the economic situation in the last two years numerous container vessels have been laid-up or operated with small amount of cargo. The Accident Events Accident Situation No. 1 The first reported accident [1, BSU Report 510/08] happened on board of a 336m long Container Vessel (Vessel 1) of the 8600TEU Class built in 2005 at Hyundai Heavy Industries Ulsan Shipyard. At the afternoon of the 23rd September 2008 the vessel was forced to leave the harbour of Hong Kong. The Harbour was closed for safety reason, because of the approaching typhoon Hapugit. The vessel left the harbour at partial draft and a stability value of 7.72m GM (corrected). During its sea passage the vessel was hit by the rim of the typhoon. It was facing harsh weather conditions with wind speeds of 10Bft. in gusts 12Bft. and wave heights around 7.5m - 8.0m H s with a period of 10s. Around 20:00 the vessel started to roll about +/- 20 as the crew reported. In the following hours the crew tried to keep the vessel against the sea at slow speed (3-5kn) to control the roll motion and to keep the vessel free of the close coast line. At 02:45 the vessel was hit by a group of three waves and rolled violently to starboard. The master and one seaman on the bridge lost hold which caused the death of the seaman and the master heavy injuries. When the vessel was examinated one day later in the port of Hong Kong only little damage on some empty on-deck containers were detected. Accident Situation No. 2 The second accident happened [3, BSU Report] at the 15th September 2009 in the south china sea offshore of Hong Kong. The vessel is, compared to the first vessel, relatively small with 208m Length over all and a capacity of 2500 TEU, built in 1997 at Kvaerna Shipyard, Rostock. The vessel was in laid-up condition on the outside roads of Hong Kong when it had to leave its position seawards at the 14th around 14:00 due to typhoon alarm. At around 20:00 the weather conditions became very rough with wind around 10Bft. and waves of 7-8m. The crew struggled at this point to keep the course and had differences between heading 2 Copyright c 2011 by ASME

3 and course over ground up to 100. At a quarter past midnight the master and the 3rd officer were on the bridge. Due to the violent roll motion the 3rd officer lost hold and sustained heavy injuries from his fall, which he died of short time later. The examination of the vessel later on showed damage to the vessels superstructure. Accident Situation No. 3 The last reported accident happened at the 16th of October 2009 in the german bright close to the island of Borkum on board of a 2500 TEU container vessel very similar to Vessel 2. The vessel was on its way from Emden to Rotterdam only with light cargo after being laid up for two month. The weather conditions were worse than in the other two cases. The wind was blowing with 8 to 9Bft. causing a wind sea of about 6m. With a swell of 6.7m this lead to about 9m significant wave height, which was certified by a near by wave buoy. The vessel was rolling so extensivly that the pilot who was on board was thrown out of his seat and heavily injured. Summary of the reported Accidents To compare the affected vessels and to summarize the facts of the accidents and their consequences, the main data of the accidents are displayed in this section. In table 1 and table 2 the main dimensions and the loading condition in the particular accident situations of the three vessels can be compared. The values of speed and heading for vessels No.1 and No.2 were found by evaluating the voyage data recorders of the vessels. In case of vessel No.3 this was not possible. The values were taken from the witness reports of the crew. The values are given in table 3. The environmental conditions shown in table 4 were taken from reports of the german national meteorological service, the data based on weather reports, forecast and numerical models. From speed, heading and enviromental conditions a range for the angle of encounter and the belonging period of encounter can be evaluated for the accident situations. This data is given in table 5. TABLE 1. MAIN DIMENSIONS LOA[m] LPP[m] B[m] D[m] T FS [m] T D [mt] TEU[-] V service [kn] TABLE 2. LOADING CONDITION [mt] T FP [m] T MF [m] T AP [m] GM C [m] TABLE 3. HEADING AND SPEED OVER GROUND Heading[DEG] Speed[kn] Numerical Method The seakeeping analyses are carried out by using the program E4ROLLS originally developed by Kröger [4, Kröger 1987] and Petey [5, Petey 1988] at the Institute of Naval Architecture, University of Hamburg. The code has been improved and extensivly validated within the framework of two research projects [6, Billerbeck et al.] [7, Krüger et al.] [8, Clauss et al.] [9, Henning et al.], funded by the German Ministry of Education and Research (BMBF), which lead to the currently used version of E4ROLLS. E4ROLLS simulates the motion of intact and damaged ships in time domain in all six degrees of freedom in irregular long or short crested seaways, represented by a random superposition of an finite number of regular wave components. Four motions, namely heave, pitch, sway and yaw, RAO are used, calculated linearly by means of strip theory. The surge motion is simulated assuming a hydrostatic pressure distribution under the water surface for the determination of the surge-inducing wave forces. While the roll motion is simulated nonlinearly using equation 1. ΣM represents the sum over M wind, M sy, M wave and M tank, which are the roll moments due to wind, sway and yaw, waves and fluid in tanks and flooded compartments, respectively. d L and d Q are the linear and nonlinear damping coefficients following Blume [10, Blume,1979], leading to the nonlinear damp- 3 Copyright c 2011 by ASME

4 TABLE 4. ENVIRONMENTAL CONDITION WIND[Bft.] From[Deg] H s [m] From[Deg] T s [s] The acceleration becomes maximal if the sin-function is one. The amplitude of the roll motion can be estimated from the reports of the crew (at least 30 ), the center of motion is assumed in the waterplane and ω is assumed to be two times π times the frequency of encounter as the roll motion is a forced motion. a y,max = ω 2 h B Â + g sin(â) (4) This leads to the worst case maximum values given in table 6 for each of the vessels. TABLE 5. RANGE OF ENCOUNTER ANGLES & PERIODS TABLE 6. MAXIMUM ACCELERATIONS IN Y-DIRECTION BY ANALYTICAL ESTIMATION A Enc [Deg] T s,enc [s] a y [ m s 2 ] ing moment M d as given in equation 2. Angles ϕ,ϑ and ψ are the roll, pitch and yaw angles, respectively, while m is the mass of the ship and g the gravitational acceleration. The righting lever in the seaway H s is determined for every time step using effective wave [11, Grim,1961] as modified by [12, Söding,1987]. Θ xx and Θ xz are the moment of inertia about the longitudinal axis and the product of inertia, respectively, calculated for the actual mass distribution (light ship and cargo). A more detailed description of the code can be found in [13, Kluwe, 2010] To make sure that the numerical hull form is the same as the built form. The cross curves from the stability booklet are compared to the computed ones as well as the floating condition from the board computer is compared to the computed one. Table 7 shows the comparison between the floating conditions from the board computers and our calculations. The table shows little difference between computed and reference values. Therefore an acceptable accuracy of the numerical hull form description can be assumed. ϕ(t) = ΣM M d m h s (g ξ ) Θ xx Θ xz {ψsin(ϕ) ϑcos(ϕ)} Θ xz {( ϑ + ϑ ϕ)sin(ϕ) ( ψ + ψ ϕ)cos(ϕ)} Θ xx Θ xz {ψsin(ϕ) ϑcos(ϕ)} (1) M d = d L ϕ + d Q ϕ ϕ (2) Seakeeping analysis Basic Considerations A first guess of the possible occurred accelerations can be made by a rough analytical estimation if a sinusoidal roll motion is assumed. a y (t) = ω 2 h B Â sin(ωt + ε) + g sin(âsin(ωt + ε)) (3) FIGURE 1. LEVERARM CURVE OF VESSEL NO.1 IN STILL WATER SITUATION. 4 Copyright c 2011 by ASME

5 FIGURE 3. POLAR PLOT OF LIMITING WAVE HEIGHTS TO REACH 30 DEGREE ROLL ANGLE FOR VESSEL NO.1. FIGURE 2. LEVERARM CURVE OF VESSEL NO.1 IN STILL WATER, WAVE CREST AND TROUGH SITUATION WAVE- LENGTH=156m. TABLE 7. FLOATING CONDITION checked with long term statistic data from measurements. The limiting maximum wave height for the second accident in the specific sea area is H s,max = 7m. For the third accident the most probable wave period for the measured wave height of 9m is 9.5s. For the first accident multiple calculations with different wave lengths and wave heights were carried out. The waves for the simulations are generated by a JONSWAP-Spectrum based on H s and T s in frequency domain and a cos 2 spreading function in angular domain. Vessel T FP [m] T MF [m] T AP [m] GM[m] Vessel 1 Board Computer Calculations Vessel 2 Board Computer Calculations Vessel 3 Board Computer Calculations TABLE 8. MOST PROBABAL ENVIRONMENTAL CONDITION H s [m] T s [s] The calculated lever arm curves of all three vessels show no special characteristic, they follow GM 0 ϕ up to 20 in all three cases. Furthemore the lever arm curves were almost identical in still water, wave crest and wave trough conditions in the accident situations with the correspondent wave length, so that parametric excitation could be excluded. Both facts are exemplary displayed for Vessel No.1 in figures 1 & 2 Environmental Conditions The input data for wave height and direction for the seakeeping analysis were derived from the data given in table 4. The original data showed a range of possible wave heights and lengths which makes it necessary to identify the most probable situation. The wave data from the original reports were cross Results of the numerical Accident Investigation The evaluation of the vessels seakeeping behaviour leads to similar results for all three. The polarplots 3,4 and 5 show the results of the calculations for the most probable wave length. All three vessels tend to roll permanently around +/-20 in head seas in their accident stability situation. As soon as the waves start to come a little more from the side the rolling angle will increase, respectively the necessary wave height to reach a certain roll angle decreases. This is in good agreement with the statements of facts given by the crews. The plots indicate that the roll angles in the reported situations amounted to in all three cases. Furthermore they indicate that no narrow band effects like resonance lead to the accidents. The area where large roll angles can occur reaches over a wide area of headings and speeds. This indicates that in situations with headings and speeds neighbouring the most probable accident situations the accidents would most likely have happened as well. The evaluation of the 5 Copyright c 2011 by ASME

6 transversal accelerations at a point one meter above bridge deck at centerline in the accident situation lead to the histograms of acceleration distribution given in figure 6-8. They show that TABLE 9. MAXIMUM ACCELERATIONS IN Y-DIRECTION FROM NUMERICAL ANALYSIS a y [ m s 2 ] transversal accelerations between 1-1.2g occurred, which were responsible for the heavy injuries of the affected crew members. Causal for the violent roll angles and accelerations were supposedly the combination of poor roll damping due to the slow speeds and large direct wave moments caused by the excessive stability. The extreme values of roll angles and acceleration were summarised in table 9. FIGURE 5. POLAR PLOT OF LIMITING WAVE HEIGHTS TO REACH 35 DEGREE ROLLANGLE FOR VESSEL NO.3. Extended numerical Investigation In [14, Rox 2010] an extended numerical study has been carried out on a group of 15 different container ship designs to figure out how these vessels behave in the described accident situations. For each vessel the ballast arrival condition as given in the stability booklet is chosen as loading condition. Figure 9 gives an overview of the main dimensions of the vessels. Vessel No.14 in the extended study is indentical with vessels No.1 from the accidents mentioned above. Figure 10 shows the distribution of the bridge heights from waterline in ballast arrival condition over FIGURE 6. HISTOGRAM OF Y-ACCELERATION AT 150 DE- GREE ANGLE OF ENCOUNTER FOR VESSEL NO.1. FIGURE 4. POLAR PLOT OF LIMITING WAVE HEIGHTS TO REACH 35 DEGREE ROLLANGLE FOR VESSEL NO.2. L pp within the group. Speed and course as well as the environmental conditions used for the simulations are given in table 10. They are derived from the original accident situations given in table 3 and 4. The results of the simulations are condensed in the Figures 11 & 12. These figures indicate that nearly all the vessels in this study suffer from the same problem. While it is not possible to identify one of the situations as worst case scenario, the diagrams show that, except for vessels No.1, No.13 and No.14, all vessels experienced accelerations of 0.8g up to 1.4g and corresponding large roll angles. Although the three large container vessels No.11, No.14 and No.15 don t show critical seakeeping 6 Copyright c 2011 by ASME

7 FIGURE 9. DISTRIBUTION OF LPP OVER DISPLACMENT. FIGURE 7. HISTOGRAM OF Y-ACCELERATION AT 150DE- GREE ANGLE OF ENCOUNTER FOR VESSEL NO.2. FIGURE 10. DISTRIBUTION OF BRIDGE HEIGHTS OVER LPP. TABLE 10. INPUT DATA FOR SEAKEEPING ANALYSIS Situation Speed A Enc [Deg] H s [m] T s [s] FIGURE 8. HISTOGRAM OF Y-ACCELERATION AT 120 DE- GREE ANGLE OF ENCOUNTER FOR VESSEL NO.3. behaviour in this situations it is clear from the reviewed accidents that they could suffer from the same kind of the problem. Therefore the results for vessel No.14 in its accident floating condition is displayer in diagram 11 as well. This points out that for this typ of vessel smaller stability values are dangerous Detailed Investigation To figure out the influence of stability on the seakeeping behaviour a detailed investigation has been carried out on a subset of three of the fifteen vessels. Vessel No. 13 is a small vessel with a size similar to both small vessels from the original accident reports, Vessel No.1 is a midsize vessel of 274m and Vessel No.14. The GM value is varied while the floating condition remains the same as in ballast arrival condition. Figures 13 & 14 7 Copyright c 2011 by ASME

8 FIGURE 11. OVER GM. MAXIMUM TRANSVERSAL ACCELERATIONS FIGURE 13. INFLUENCE OF VARIATION OF GM ON ACCEL- ERATIONS FOR VESSEL NO.13. FIGURE 12. MAXIMUM ROLL ANGLE OVER GM. show that the stability value in ballast arrival situation is actually the worst case stability value for both vessel in this floating condition. Figure 15 shows that in case of vessel No.14 the partial loading stability value as in the original accident situation is dangerous for the ballast arrival floating condition as well. A change of stability and floating condition of vessel No.11 to a similar situation as the original accident situation of vessel No.14 also leads to transversal accelerations about 1g for this vessel. Conclusions It was possible to identify the reasons for the described accident events by numerical simulations. Furthermore an extended numerical study has been carried out which indicates that other container vessel designs would suffer the same problems in similar situations. The reason for the described phenomenon is a combination of poor roll damping and large direct wave moments FIGURE 14. INFLUENCE OF VARIATION OF GM ON ACCEL- ERATIONS FOR VESSEL NO.1. caused by excessive stability. The specific design considerations of container vessels are leading to high stability values. The minimum required amount of ballast water is in general given by the rule for minimum Draft at F.P. and the necessity of submergence of rudder and propeller. The demand of a maximum number of 14t homogeneous container intake makes it necessary to have a maximum number of bottom water ballast tanks. This combination leads to large stability values in ballast. To fulfill the IMO visibility rule at nominal container capacity, container vessels are usually equipped with high deck houses which makes the problem even worse. The insufficent roll damping in the reviewed accidents was triggered by the slow speeds, but to keep the vessel against the sea with slow speed is the standard conduct under these circumstances. As long as the crews have no information about the seakeeping behaviour of a vessel they do not 8 Copyright c 2011 by ASME

9 FIGURE 15. INFLUENCE OF VARIATION OF GM ON ACCEL- ERATIONS FOR VESSEL NO.14. have a chance to know possible dangerous situations in advance. Warnings of high accelarations at large stability as they are usually given in the cargo securing manual, should be given in the stability booklet as well to warn the crew of possible dangerous situations. These accidents show the necessity to develop stability rules which take dynamic effects into account for small and for large stability values. ACKNOWLEDGMENT We wish to thank the German Ministries BMBF and BMWI for their continuous funding of numerical and experimental treatment of stability problems in heavy weather. Without this funding, our work would not have been possible. REFERENCES [1] Kaufmann, J., Untersuchungsbericht 510/08 Toedlicher Personenunfall an Bord des CMS CHICAGO EXPRESS. Accident Investigatio report 510/08, German Federal Bureau of Maritime Casualty Investigation, Hamburg, Germany, November. [2] Krueger, S., Steinbach, C., Kaufmann, J., and John, F., Stability accidents in ballast/ laid- up conditions a new phenomenon?. In Proceedings 2010 PRADS, Vol. 1. Paper number [3] Kaufmann, J., Accident Report. Accident investigatio report, German Federal Bureau of Maritime Casualty Investigation, Hamburg, Germany, to be published. [4] Kroeger, P., Simulation der Rollbewegung von Schiffen im Seegang. Schriftenreihen Schiffbau(473). [5] Petey, F., Abschlussbericht zur Erweiterung des Vorhabens Leckstabilitaet im Seegang. Schriftenreihen Schiffbau. [6] Billerbeck, H., SinSee - Beurteilung der Schissicherheit in schwerem Seegang. Tech. rep., BMBF Statustagung, Rostock. Funded by the German Ministry for Research and Development. [7] Kruger, S., and Kluwe, F., Development of dynamic stability criteria from seakeeping simulations.. In Proceedings of the Ninth International Marine Design Conference (IMDC). [8] Clauss, G., Hennig, J., Cramer, H., and Brink, K.-E., Validation of numerical motion simulations by direct comparison with time series from ship model tests in deterministic wave sequences.. In Proceedings of OMAE 2005, ASME. Paper number [9] Hennig, J., Qualitative and quantitative validation of a numerical code for the realistic simulation of various ship motion scenarios.. In Proceedings of OMAE 2006, ASME. Paper number [10] Blume, P., Experimentelle Bestimmung von Koefzienten der wirksamen Rolldaempfung und ihre Anwendung zur Abschaetzung extremer Rollwinkel. Schiffstechnik, 26. [11] Grim, O., Beitrag zum Problem der Sicherheit des Schiffes im Seegang. Schiff & Hafen. [12] Soeding, H., Ermittlung der Kentergefahr aus Bewegungssimulationen. Schiffstechnik, 34. [13] Kluwe, F., Development of a Minimum Stability Criterion to Prevent Large Amplitude Roll Motions in Following Seas. PhD. Thesis, Schriftenreihe Schiffbau Rep. No. 648, Hamburg University of Technology, Hamburg, Germany. [14] Rox, N., Examination of the intact stability and the seakeeping behaviour of container vessels within the ballast condition. MS Thesis, Hamburg University of Technology, Hamburg, Germany. 9 Copyright c 2011 by ASME