Examination of the intact stability and the seakeeping behaviour

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5 Evaluation Transverse acceleration on the bridge in ballast arrival loading condition [m/s 2 ] 16 14 12 10 8 6 4 2 0 9 2468 TEU in accident 10 8 4 13 12 5 7 6 CE in accident 2 3 1 15 14 = CE 4 6 8 10 12 14 GM solid [m] 11 Accident situation 1 Accident situation 2 Accident situation 3 Figure 5.1: Transversal accelerations on the bridge against GM solid 5.2 Consequences Normally it is expected that large vessels with a length of over 200 m or even 300 m are relatively safe in heavy sea and do not experience an exceptional seakeeping behavior. But the analysis reveals, that most of the considered container vessels apparently have signicant problems with their seakeeping behavior in combinations of certain loading and environmental conditions. In exception to the above stated the three largest vessels, namely Vessel No. 11, Vessel No. 14 and Vessel No. 15, experience smaller rolling angles and accelerations during the examination (see gure 5.1). Simply looking at this result, it could be concluded, that these large vessels generally do not have problems in the considered seaways. But with Vessel No. 14, the Chicago Express takes part of the examination, being the vessel in accident, which is described with accident situation 1 (refer to table 3.1). During this accident very high rolling angles and transversal accelerations on the bridge occurred in a dierent loading condition (compare BSU report [1]). The examined ballast arrival condition diers from the accident loading condition. The Chicago Express now has a smaller displacement and GM as well as a dierent trim. This implies that the other large vessels, Vessel No. 11 and Vessel No. 15 may also be endangered to experience equally high high transversal accelerations in a dierent loading condition. This assumption is analysed further in chapter 6.2. The problems in the seakeeping behavior occur due to the following reasons: The shaped hull form and the signicant bow are of the analysed vessels favours the impact of direct, exciting heeling moments through the heavy sea. On the other hand, the excessive stability causes high restoring moments of the heeled vessel. This results in signicant transversal accelerations (see [8] for more details). Furthermore the examination shows that the problem of excessive rolling angles and transversal 44
5.3 Recommendations accelerations seems not to be a pure stability problem. Rather high accelerations occur for a wide scope of examined GM solid values (see gure 5.1). Regarding the occurring maximum transversal accelerations, a comparison with an usual value for the dimensioning of the container lashing equipment is interesting, since a reference value for the maximum transversal accelerations acting on humans on the bridge does not exist. For example according to the DNV rules [12], the transversal dynamic acceleration taking eect on container lashings on deck, shall be taken not smaller than ∼ 0, 5 g. The calculated accelerations on the bridge partly exceed the triple of that value. In the same time the normalized mean value of the occurring acceleration amplitudes mostly exceeds a value of ∼ 0, 5 g (refer to the respective histograms in chapter 4). Such high transversal accelerations are considered being denitely not acceptable. 5.3 Recommendations The simulation results show, that the ballast arrival loading condition of container vessels is not a safe seagoing condition. The risk of encountering excessive rolling angles and very high transversal accelerations on the bridge in heavy sea is increased signicantly. Based on the simulation results, the following approaches to reduce the risk of accidents can be exemplied. 5.3.1 Stability Concerning the stability of the vessels, no universally valid GM value, which reduces the risk of accidents, can be derived from the analysis. The seakeeping behavior of a ship apparently has a lot of important additional inuence factors. For instance two of the factors which have to be considered, are the ship's trim and the hull form. Altogether the factors can form a critical ship situation consisting of the ship's stability, the ship's trim, the ship's hull form and so on. To identify such critical situations for each loading condition, seakeeping calculations have to be done with adequate methods for each vessel individually. A general prediction of a critical situations is not possible until now. For three of the vessels in ballast arrival loading condition, the most critical situation is determined in chapter 6.1. 5.3.2 Roll damping The enlarging of the roll damping, no matter how the damping is done, reduces the roll motions and transversal accelerations on the bridge. There are dierent ways to increase the roll damping of a ship. According to Abdel-Maksoud [11] the following possibilities t for this purpose: ˆ Enlarge the bilge keel area → roll damping by ow separation at the bilge keels ˆ Increase the ship's speed at low speeds → roll damping by shear stress on the hull, angular incoming ow on the rudder and immersed transom ˆ Integrate a roll damping tank → roll damping by e.g. a sloshing uid For a vessel already in service, the bilge keels could be modied easily. Though such a modication would not have a deciding inuence on the transversal accelerations. In appendix B.1 a graph can be found, where the bilge keel area is changed for Vessel No. 13, being the vessel with the highest occurring transversal acceleration value during the examination. In the graph the vessel in ballast arrival condition encounters the seaway of accident situation 2. it follows, that the enlarging of the bilge keel area by 50 %, just provides a reduction of the transversal accelerations of about 10 %. A general advantage of bilge keel is, that they also function with zero speed. 45
Page 1:
Dezember 2010 SCHRIFTENREIHE SCHIFF
Page 5:
1st Examiner: Prof. Dr.-Ing. Stefan
Page 9 and 10:
Contents List of Figures List of Ta
Page 11 and 12: Contents A.14 Vessel No. 14 . . . .
Page 13 and 14: List of Figures 2.1 Calculation mod
Page 15 and 16: List of Tables 3.1 Accident situati
Page 17 and 18: Nomenclature Symbol Unit Meaning A
Page 19 and 20: 1 Introduction In the years 2008/20
Page 21 and 22: 1.2 Following objectives for the di
Page 23 and 24: 2 Theory 2.1 Description of the uti
Page 25 and 26: 2.1 Description of the utilised sea
Page 27 and 28: 2.1 Description of the utilised sea
Page 29 and 30: 2.2 Environmental conditions period
Page 31 and 32: 3 Data input The thesis is written
Page 33 and 34: 3.6 Free surface correction 3.6 Fre
Page 35 and 36: 3.10 Bridge height 3.10 Bridge heig
Page 37 and 38: 4 Examination For each vessel the s
Page 39 and 40: 4.2 Vessel No. 02 4.2 Vessel No. 02
Page 41 and 42: 4.3 Vessel No. 03 ˆ The documented
Page 43 and 44: 4.5 Vessel No. 05 accident situatio
Page 45 and 46: 4.6 Vessel No. 06 4.6 Vessel No. 06
Page 47 and 48: 4.7 Vessel No. 07 ˆ The stability
Page 49 and 50: 4.9 Vessel No. 09 Figure 4.16: Tran
Page 51 and 52: 4.10 Vessel No. 10 4.10 Vessel No.
Page 53 and 54: 4.11 Vessel No. 11 ˆ The limiting
Page 55 and 56: 4.13 Vessel No. 13 Figure 4.24: Tra
Page 61: 5 Evaluation Based on the determina
Page 65 and 66: 6 Detailed examination The evaluati
Page 67 and 68: 6.1 Variation of the GM values acce
Page 69 and 70: 6.2 Rolling behavior of the large v
Page 75 and 76: 7 Conclusions Summing up the result
Page 77 and 78: A Vessel data A.1 Vessel No. 01 Tab
Page 79 and 80: A.3 Vessel No. 03 A.3 Vessel No. 03
Page 87 and 88: A.11 Vessel No. 11 A.11 Vessel No.
Page 93 and 94: B Variation of the bilge keel area
Page 95 and 96: C Variation of the ship's speed 16
Page 97: Bibliography [1] BSU; Federal Burea
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