Horizontal Force (MN)32.521.510.50DAMAGEMeasured0 0.5 1 1.5 2 2.5 3Time (sec)Vertical Force (MN)21.510.50DAMAGEMeasured0 0.5 1 1.5 2 2.5 3Time (sec)Figure 9 - Comparison of Horizontal Forces (ASIS Test2)Figure 10 - Comparison of Vertical Forces (ASIS Test 2)Figure 11 - Sketch of Four Ground<strong>in</strong>g Scenarios4.3.2 Sensitivity AnalysisThe sensitivity of DAMAGE results to changes <strong>in</strong>ground<strong>in</strong>g parameters was studied <strong>and</strong> presented <strong>in</strong> [17].The base ship used <strong>in</strong> the sensitivity analysis was a237,000 DWT s<strong>in</strong>gle-hull VLCC <strong>and</strong> the ground<strong>in</strong>g casestudied corresponded to the VLCC validation case.In the sensitivity analysis only one parameter waschanged at the time <strong>and</strong> the other parameters were kept attheir <strong>in</strong>itial values. The objective of this analysis was notto study the effect of the structural design on the damageextent, rather the objective was to <strong>in</strong>vestigate thesensitivity of the DAMAGE model to the parametersdef<strong>in</strong><strong>in</strong>g the ground<strong>in</strong>g event.As can be expected the results are very sensitive tothe ground characteristics <strong>and</strong> to the parameters def<strong>in</strong><strong>in</strong>gthe ship-ground <strong>in</strong>teraction (rock elevation, rock shape<strong>and</strong> transverse location, ship velocity <strong>and</strong> displacement,friction coefficient <strong>and</strong> trim angle).The parameters affect<strong>in</strong>g the global ship motions(longitud<strong>in</strong>al center of floatation <strong>and</strong> the longitud<strong>in</strong>al <strong>and</strong>the transverse metacentric height) had little effect on6
damage results. Surpris<strong>in</strong>gly, the tank spac<strong>in</strong>g as well asthe characteristics of both the transverse bulkhead <strong>and</strong> thelongitud<strong>in</strong>al bulkhead had very little effect on damageresults. The results were also not very sensitive tomaterial characteristics.Damage results were sensitive to the thickness of theouter bottom. Reduc<strong>in</strong>g the spac<strong>in</strong>g <strong>and</strong> <strong>in</strong>creas<strong>in</strong>g thescantl<strong>in</strong>gs of longitud<strong>in</strong>al girders <strong>and</strong> transverse floorsalso affected damage results, but not as effectively. Thesame applied to longitud<strong>in</strong>al stiffeners.4.3.3 <strong>Structural</strong> ModificationsDAMAGE was found to provide a good tool forcomparative studies based on the validation cases <strong>and</strong> thesensitivity analysis. The next step <strong>in</strong> the study was to<strong>in</strong>vestigate the effect of structural modifications on thedamage extent <strong>in</strong> selected ground<strong>in</strong>g scenarios. The studywas limited to eight ground<strong>in</strong>g scenarios, <strong>and</strong> thestructural changes were limited to changes <strong>in</strong> scantl<strong>in</strong>gsof a conventional double-bottom structure. The analysiswas <strong>in</strong>tended to provide <strong>in</strong>sight <strong>in</strong>to the effects of themodifications <strong>and</strong> scenarios to help design a probabilisticanalysis, which will be the next step <strong>in</strong> the study.The base ship used <strong>in</strong> the analysis was a 150,000DWT double-hull tanker. The ground<strong>in</strong>g scenarios wereselected to represent high, medium <strong>and</strong> low rockelevations relative to the ship’s basel<strong>in</strong>e. One of thescenarios had a sharp rock tip (30 degree semi -apexangle), whereas the shape of the rock was kept constant <strong>in</strong>the other scenarios (45 degree semi-apex angle). Twovelocities were analyzed: 7 knots to represent port speed<strong>and</strong> 14 knots to represent service speed of the tanker. Therock shapes <strong>and</strong> elevations are illustrated <strong>in</strong> Figure 11.More details on the base ship <strong>and</strong> the ground<strong>in</strong>g scenarioscan be found <strong>in</strong> [17].Eight structural modifications, all designed accord<strong>in</strong>gto the requirements of ABS SafeHull (97/98), where<strong>in</strong>vestigated. The scantl<strong>in</strong>gs met the m<strong>in</strong>imumrequirements except for the dimension under<strong>in</strong>vestigation. The modifications <strong>in</strong>cluded:1. Increase <strong>in</strong> the outer bottom plate thickness.2. Increase <strong>in</strong> the <strong>in</strong>ner bottom plate thickness.3. Increase <strong>in</strong> both outer bottom <strong>and</strong> <strong>in</strong>ner bottomplate thickness.4. Additional longitud<strong>in</strong>al girder.5. Two additional longitud<strong>in</strong>al girders.6. Reduced spac<strong>in</strong>g of transverse floors.7. Decrease <strong>in</strong> double bottom height.8. Increase <strong>in</strong> double bottom height.The steel weight (longitud<strong>in</strong>al structure only) wascalculated for each design, <strong>and</strong> the effectiveness of adesign modification was measured <strong>in</strong> terms of thereduction <strong>in</strong> <strong>in</strong>ner bottom rupture as a function of the steelweight. Changes <strong>in</strong> the transverse structure caused by themodifications were not taken <strong>in</strong>to account.Some conclusions based on the selected scenarios<strong>in</strong>clude:• In the low rock elevation case, which representsrak<strong>in</strong>g type ground<strong>in</strong>g, none of the designs had <strong>in</strong>nerbottom rupture. At service speed the outer bottomruptured throughout the entire length of the ship. Inthis type of scenario, the structural modificationshave little impact on the damage extent <strong>and</strong> no effecton the oil outflow assum<strong>in</strong>g that the vessel survivesthe rak<strong>in</strong>g damage.• In the sharp rock tip, high-rock elevation case therock penetrated through the entire cargo block <strong>in</strong> alldesigns at service speed. Even at port speed alldesigns had serious damage. However, <strong>in</strong>creasedouter bottom or <strong>in</strong>ner bottom thickness, twoadditional girders, or reduced spac<strong>in</strong>g of transversefloors did reduce outer bottom <strong>and</strong> <strong>in</strong>ner bottomrupture. This was the only ground<strong>in</strong>g scenario where<strong>in</strong>creas<strong>in</strong>g double bottom height had very little effecton the <strong>in</strong>ner bottom rupture.• In high <strong>and</strong> medium rock elevation cases most of thedesign modifications improved structuralperformance. Reduc<strong>in</strong>g the double bottom heightresulted <strong>in</strong> the worst performance. The design withtwo additional longitud<strong>in</strong>al girders performed best <strong>in</strong>reduc<strong>in</strong>g <strong>in</strong>ner bottom rupture, but it also had theheaviest steel weight among the studied designs. Ifthe steel weight was taken <strong>in</strong>to account, <strong>in</strong>creas<strong>in</strong>gthe thickness of the outer bottom was found to bemost effective <strong>in</strong> reduc<strong>in</strong>g the <strong>in</strong>ner bottom rupture.Increas<strong>in</strong>g both the <strong>in</strong>ner bottom <strong>and</strong> outer bottomthickness reduced the <strong>in</strong>ner bottom rupture more butat the cost of a higher steel weight.4.4 Ongo<strong>in</strong>g <strong>and</strong> Future WorkThe work discussed above was carried out to provide abasis for a probabilistic analysis, which will take <strong>in</strong>toaccount a range of possible ground<strong>in</strong>g scenarios.However, to add credibility to the results furthervalidation is needed. Data needs for validation work willbe discussed later <strong>in</strong> the paper. Further review of theresults from the presented validation cases is ongo<strong>in</strong>g.The analysis on the effect of structural modificationsdiscussed above measured the performance <strong>in</strong> terms ofthe damage extent without tak<strong>in</strong>g <strong>in</strong>to account thesubdivision of the vessel. Therefore the observed changes<strong>in</strong> the damage extent may have no effect on the result<strong>in</strong>goil outflow. To determ<strong>in</strong>e the effect of the structural7