Structural Design and Response in Collision and Grounding

damage results. Surprisingly, the tank spacing as well asthe characteristics of both the transverse bulkhead and thelongitudinal 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. Reducing the spacing and increasing thescantlings of longitudinal girders and transverse floorsalso affected damage results, but not as effectively. Thesame applied to longitudinal stiffeners.4.3.3 Structural ModificationsDAMAGE was found to provide a good tool forcomparative studies based on the validation cases and thesensitivity analysis. The next step in the study was toinvestigate the effect of structural modifications on thedamage extent in selected grounding scenarios. The studywas limited to eight grounding scenarios, and thestructural changes were limited to changes in scantlingsof a conventional double-bottom structure. The analysiswas intended to provide insight into the effects of themodifications and scenarios to help design a probabilisticanalysis, which will be the next step in the study.The base ship used in the analysis was a 150,000DWT double-hull tanker. The grounding scenarios wereselected to represent high, medium and low rockelevations relative to the ship’s baseline. One of thescenarios had a sharp rock tip (30 degree semi -apexangle), whereas the shape of the rock was kept constant inthe other scenarios (45 degree semi-apex angle). Twovelocities were analyzed: 7 knots to represent port speedand 14 knots to represent service speed of the tanker. Therock shapes and elevations are illustrated in Figure 11.More details on the base ship and the grounding scenarioscan be found in [17].Eight structural modifications, all designed accordingto the requirements of ABS SafeHull (97/98), whereinvestigated. The scantlings met the minimumrequirements except for the dimension underinvestigation. The modifications included:1. Increase in the outer bottom plate thickness.2. Increase in the inner bottom plate thickness.3. Increase in both outer bottom and inner bottomplate thickness.4. Additional longitudinal girder.5. Two additional longitudinal girders.6. Reduced spacing of transverse floors.7. Decrease in double bottom height.8. Increase in double bottom height.The steel weight (longitudinal structure only) wascalculated for each design, and the effectiveness of adesign modification was measured in terms of thereduction in inner bottom rupture as a function of the steelweight. Changes in the transverse structure caused by themodifications were not taken into account.Some conclusions based on the selected scenariosinclude:• In the low rock elevation case, which representsraking type grounding, none of the designs had innerbottom 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 and no effecton the oil outflow assuming that the vessel survivesthe raking damage.• In the sharp rock tip, high-rock elevation case therock penetrated through the entire cargo block in alldesigns at service speed. Even at port speed alldesigns had serious damage. However, increasedouter bottom or inner bottom thickness, twoadditional girders, or reduced spacing of transversefloors did reduce outer bottom and inner bottomrupture. This was the only grounding scenario whereincreasing double bottom height had very little effecton the inner bottom rupture.• In high and medium rock elevation cases most of thedesign modifications improved structuralperformance. Reducing the double bottom heightresulted in the worst performance. The design withtwo additional longitudinal girders performed best inreducing inner bottom rupture, but it also had theheaviest steel weight among the studied designs. Ifthe steel weight was taken into account, increasingthe thickness of the outer bottom was found to bemost effective in reducing the inner bottom rupture.Increasing both the inner bottom and outer bottomthickness reduced the inner bottom rupture more butat the cost of a higher steel weight.4.4 Ongoing and Future WorkThe work discussed above was carried out to provide abasis for a probabilistic analysis, which will take intoaccount a range of possible grounding scenarios.However, to add credibility to the results furthervalidation is needed. Data needs for validation work willbe discussed later in the paper. Further review of theresults from the presented validation cases is ongoing.The analysis on the effect of structural modificationsdiscussed above measured the performance in terms ofthe damage extent without taking into account thesubdivision of the vessel. Therefore the observed changesin the damage extent may have no effect on the resultingoil outflow. To determine the effect of the structural7