Examination of the intact stability and the seakeeping behaviour

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5 Evaluation Furthermore the increase of the ship's speed always reduces the roll motion. But at rst the vessel has to be able to signicantly increase the speed, which is not self-evident due to the high wave forces slowing the ship with each wave. On the other hand a higher ship's speed in heavy seaway causes high slamming loads on the ship's bow structure, when heading into the waves. This eect may cause severe damages on the structure, especially for vessels with a large bow are. At last with a higher speed and heading into the waves, the vessel may be endangered of encountering a critical 2:1 resonance. Therefore the augmentation of the ship's speed to improve the roll damping is suitable to only a limited number of cases. The inuence of ship's speed is also determined for Vessel No. 13. The associated graph in appendix C.1 shows that the ship's speed has not a signicant inuence, too. For this analysis, the vessel in ballast arrival condition encounters again the seaway of accident situation 2. At last the integration of a roll damping tank has the advantage, that it also functions with zero ship's speed. But it reduces the ship's payload due to the used space for the tank and reduces the intact stability due to the free surface of the tank. 5.3.3 Lines of the ship The design of the lines of the ship has a strong inuence on the seakeeping behavior. As mentioned before, the shape of the frames or rather the shape of the bow are in combination with the wide transom, governs the induced rolling moments. Already during the very early design phase, the following compromise has to be determined. On one hand the vessel is intended to carry as much cargo as possible which results in aring hull forms. On the other hand this often contradicts with the demand of a good seakeeping behavior. 5.3.4 Other The operating behavior of the crew has also an inuence. Besides the increase of the ship's speed, the risk of accident can be reduced by placing the vessel parallel to the main wave direction at zero speed. In beam sea a lot of the wave energy induced into the ship is transformed into a drift motion in beam direction. Therefore the roll motions of the vessel are reduced. A problem in this position can be, that the ship's stern turns into the waves due to the poor course keeping ability at zero speed. When this happens to container vessels with their wide and at transom, the stern experiences often high slamming loads. Furthermore drifting in beam sea is obviously not advisable inshore. 46
6 Detailed examination The evaluation discussed in chapter 5 discloses, that further investigations are necessary for the better understanding of the high transversal acceleration problem. For this reason the following detailed examinations are carried out. At rst, the inuence of the ship's stability on the transversal accelerations is analysed. Furthermore it has to be examine, if the large vessels, Vessel No. 11 and Vessel No. 15, are also endangered to encounter high acceleration and rolling angle values in a loading condition similar to the one of Vessel No. 14 (Chicago Express) when it experienced severe ship motions in accident. 6.1 Variation of the GM values In this chapter the inuence of the initial stability (namely GM) on the occurring transversal acceleration for three out of the 15 examined vessels is determined. In order to consider the probable inuence of the ship's size, a small, a mid-sized and a large vessel are chosen. Based on the ballast arrival loading condition, the value of GM solid is varied by changing the vertical center of gravity KG of the vessel. The calculations are performed without considering the inuence of free surfaces, for the reasons named in chapter 3.6. The displacement as well as the ship's trim of the analysed ballast arrival condition are kept constant. For each GM value all three accident situations according to chapter 3.12 are simulated. In addition it is estimated, if such a GM solid or respectively KG value is achievable from a technical point of view. For instance a total ship's center of gravity KG at height of the inner bottom is not realistic. The feasible minimum KG depends on the light ship's vertical center of gravity, the position and lling of the ballast water and fuel oil tanks in the lower part of the ship as well as cargo in the holds. A shifted GM directly aects the seakeeping behavior. Hence to represent well the dierences, a new set of transfer functions (RAOs) according to chapter 2.1.1 has to be calculated for each GM value. 6.1.1 Small vessel Vessel No. 13 was chosen as smallest vessel for the detailed stability analysis. Its relevant data are listed in table 6.1. More main dimension data of this vessel can be found in the appendix A.13 on page 71. The GM is varied around the ballast arrival GM solid = 6.98 m within a range of about GM ∼ = 3 ... 12 m. In gure 6.1 the resulting transversal accelerations on the bridge are shown related to the dierent GM solid values. All three accident situations result in a comparable curve slope. In accident situation 1 and 2 the highest accelerations occur for the GM value of the ballast arrival loading condition. In accident situation 3 the highest acceleration occurs for a higher GM of about 8 m. With a higher or a lower stability the vessel would experience signicantly smaller accelerations. Though just a GM solid value of 8 ... 9 m is realistically reachable for this vessel by e.g. lling ballast water in tanks in low positions. Higher values in this simulation can only be reached by decreasing the vertical center of gravity of the lightship weight, which is considered being impossible for an existing vessel. 47
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 and 62: 5 Evaluation Based on the determina
Page 63: 5.3 Recommendations accelerations s
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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