Structural Design and Response in Collision and Grounding

More documents

Recommendations

Info

force and extent using closed-form solutions, but theydiffer in the detail in which the structure is defined and itsbehavior is analyzed. The methods also differ in theirtreatment of ship motions. The program DAMAGE wasselected for further testing and analysis, because it isapplicable for a wider range of grounding scenarios.4.2.1 DAMAGE [10]The computer program DAMAGE was developed at MITunder the Joint MIT-Industry Program on Tanker Safety.This project, lead by Professor Tomasz Wierzbicki, wasinitiated in 1991, and in addition to the programDAMAGE the project has produced more than 70technical reports about prediction of grounding andcollision damage. The program DAMAGE Version 5.0can be used to predict structural damage in the followingaccident scenarios:• Ship grounding on a conical rock with arounded tip (rigid rock, deformable bottom)• Ship-ship collision (deformable side,deformable bow)Compared to previous models for prediction of groundingand collision damage, a major advantage of DAMAGE isthat the theoretical models are hidden behind a moderngraphical user interface (GUI) [10]. The program hasbeen developed with the objective of making crashanalysis of ship structures feasible for engineers that donot have any particular experience in the field ofcrashworthiness. Figure 4 shows the input screen fordefining the “ship-ground interaction”, the relativeposition and velocity of ship and ground, coefficient offriction, etc.event, the contact force between ship and ground inducesheave, roll and pitch motion on the ship, and eventuallycauses the ship to stop.The primary mechanisms of energy dissipation are:1. A change in potential energy of the ship and thesurrounding water as the ship is lifted by theground reaction.2. Friction between the ground and hull.3. Deformation and fracture of the hull.Since the lifting of the ship off the rock generally causes areduction in crushing and tearing forces it is important toinclude the coupling between the global ship motions(external dynamics) and the local damage process(internal mechanics). Detailed descriptions of the modelfor the external dynamics can be found in References [11]and [12].For the model to be applicable to optimization forstructural crashworthiness it is important that it capturesthe effect of :• Material strength and ductility• Dimensions and arrangement of major structuralcomponents such as bulkheads, decks, girdersand large stiffeners.The theoretical model is based on a set of super-elementsolutions, i.e. closed form, analytical solutions for theenergy absorption of rather large structural componentsundergoing large deformations. All structural componentsare assumed to follow the same overall mode ofdeformation around the rock. This way it is possible totake into account the structural resistance of all typicalmembers in a ship bottom. The fracture criterion is basedon analytical solutions for plastic response of amembrane. Figure 5 shows a window from an animationof the ship motion over the rock.A description of the procedure including detailedderivations of the super-element solutions can be found inReferences [13] and [14].Figure 4 - Input screen for defining the “Ship-groundInteraction” in the program DAMAGE [10]Initially, the ship is assumed to be on a straight-linecourse with a known velocity and trim. The rock is atsome distance to port or starboard of the ship’s centerline,and at some height above the ship’s baseline. During theFigure 5. DAMAGE simulation of ship motion over arock (only selected structural elements are shown).4
4.2.2 Method by Wang [15]Dr. Wang developed a method to predict structuralresistance in raking type grounding in his doctoral workunder Professor Hideomi Ohtsubo’s supervision. Themethod was further developed and published in 1997[15].Figure 6 - Four Failure Modes in the Method by WangThe method includes closed-form solutions for fourfailure modes: stretching (beam mode), denting, tearingand concertina tearing illustrated in Figure 6.The rock is modeled as a wedge, and the ship’sbottom structure is modeled with periodic structuralmembers where the period corresponds to transverseframe spacing. Only horizontal ship motions areconsidered in the calculation.The grounding damage is calculated by combiningthe failure modes to model periodic resistance of thestructure. The resistance of the transverse structure ispredicted with a beam model. The denting failure mode isassumed for the plate immediately behind the transversestructure. As the wedge advances in the plating and acrack develops at the tip of the wedge, the rupture ismodeled by tearing failure mode. Concertina tearing isused to model the accordion like behavior of the plating,which can lead to cracks at other locations than the tip ofthe wedge. The entire calculation can be carried out byhand or with a simple spreadsheet program, but theanalyst must decide how the failure modes are combinedto achieve the final damage.4.3 ApplicationDAMAGE was selected for further testing and analysis,because of its applicability to a wider range of groundingscenarios. Although the method by Wang is elegant in itssimplicity, its application in the current formulation islimited to raking type damage only. The majorlimitations of the current version of DAMAGE are thetype of obstruction (pinnacle only), and the structuralmodel (the structure is modeled for the cargo block only).4.3.1 Validation of DAMAGESimonsen presented verification of the theory behindDAMAGE models by comparing calculated results withUS NAVY 1/5-scale grounding experiments, with largescalegrounding experiments carried out in theNetherlands, and with an actual grounding of a VLCC[14,16].Based on the limited number of validation casesDAMAGE is found to predict the damage extent well. Inthe case of a VLCC grounding, the difference between thecalculated damage length of 177 meters and the observeddamage length of approximately 180 meters is only 1.7%.The result is sensitive to the transverse location of therock relative to the ship’s centerline as is illustrated inFigure 7. As the rock moves away from the centerline theeffect of ship motions increase and the predicted damagelength increases.DAMAGE predictions for average rock penetrationand grounding force are also excellent. Predictions forminimum and maximum values are not as good. Figures8-10 illustrate a comparison of DAMAGE results with alarge-scale grounding test carried out in the Netherlandsby ASIS. Large differences in the penetration at theinitial stages of grounding are probably due tosimplifications in the global motion calculations.Rock Penetration (m)5432-50 0 50 100 150Longitudinal Position (m)e=0e=5e=10Figure 7 - Rock Penetration vs. Rock EccentricityVertical Penetration (m)1.41.210.80.60.40.20DAMAGE0 0.5 1 1.5 2 2.5 3Time (sec)MeasuredFigure 8 - Vertical Penetration (ASIS Test 2)5
Page 1 and 2: Brown, A.J., Tikka, K., Daidola, J.
Page 3: 3 PROBABILISTIC FRAMEWORKFigure 2 i
Page 7 and 8: damage results. Surprisingly, the t
Page 9 and 10: ybeginning of time step nStriking S
Page 11 and 12: location, COG, and detailed structu
Page 13: Ship Type150,000 dwtbulk carrierLen
Page 16 and 17: 6.2 DATA FOR MODEL VALIDATIONThe se
Page 18 and 19: Force (N)8.00E+087.00E+086.00E+085.
Page 20 and 21: Figure 37 - Macro flow diagram for
Page 22 and 23: enhance resistance to collision. Th
Page 24 and 25: Figure 42 - Load versus lateral ind
Page 26 and 27: Accidental Grounding and Collision

Structural Design and Response in Collision and Grounding

Create successful ePaper yourself

Delete template?

Save as template?