Structural Design and Response in Collision and Grounding

Figure 42 - Load versus lateral indentation for 280000dwt tanker double bottom [42]Figure 43 - Bottom raking due to grounding8.4.2 ApplicationIn resisting both grounding and raking damage, it is clearthat the bottom shell and the internal grid of thetransverse floors, longitudinal girders and bulkheads iscritical.In the case of raking damage, the longitudinalstiffness of floors and their ability to absorb energy indistortion are more critical than their support of the shell.As in this case the distorting force is applied in theweakest direction of the floor, the situation is furthercomplicated. Clearly, the more floors and the smaller thespans between their longitudinally resistive structure, thebetter. On the other hand, this arrangement will serve tolimit the lateral deflection and therefore, the energyabsorption in a stranding. Other ideas have included:• Increase the scantlings of the lower one-third part ofthe hull.• Slope the inner bottom, higher at the collisionbulkhead and lowering to the regulatory requirementat the end of the first cargo tank.• Preclude web frame and bulkhead damagepropagation through the inner bottom by introducinga discontinuity in the rigidity between the two. Abulkhead stool would be an example.9 CONCLUSIONSFigure 44 - Raking force versus penetration length byexperiments [41]The almost two-year effort of the panel has achieved orpartially achieved many of the original objectives statedin the introduction to this paper:• Simplified grounding models have been investigated,compared and assessed. DAMAGE 4.0 is shown tobe an excellent tool for comparative analysis ofgrounding damage, and based on limited validation,provides good results for cases with similar idealizedrock geometry. More validation is required.Limitations include the ability to consider onlyconventional ship geometries and an idealizedpinnacle geometry. Grounding on other rockgeometries, reefs, etc., cannot be analyzed usingDAMAGE, but should be considered in the future asan important part of a complete probabilistic analysis.Only a preliminary attempt at probabilistic analysisusing DAMAGE was completed.• Four simplified collision models have beeninvestigated, compared and assessed. The modelsprovide similar penetration results for the casesconsidered. Although some limited validation hasbeen accomplished for specific mechanisms used inthe models, more validation is required. Theprediction of longitudinal extent of damage,particularly at transverse bulkheads, remains largelyunexplored. This is very important for oil outflowand damage stability calculations. This prediction24