Structural Design and Response in Collision and Grounding

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enhance resistance to collision. These fall into twocategories:Figure 39 - Energy Absorption of Structural Components(Standard VLCC) [40]Figure 40 - Energy Absorption Capacity [40]The contribution of various structural components fora ULCC with a more conventional double side structure isshown in Figure 39 [40].8.1.2 ApplicationWith the knowledge acquired from understanding theprocess identified by the analytical evaluations, it ispossible to postulate hull structural features that may• Conventional hull structural arrangements that aremodified to enhance energy absorption.• Unconventional hull structural arrangements thatinclude structurally actuated mechanisms for energyabsorption.As part of the research project for improved tankersafety against collision and grounding, the Association forStructural Improvement of the Shipbuilding Industry(ASIS) of Japan conducted a study of varying structuralcharacteristics of a conventional VLCC design andapplying a more unconventional structural feature as well[40]. The more conventional alterations included use ofhigh strength steel, additional shell stringers, alongitudinal strut in the outboard cargo tank supportingthe double side, and a double side unidirectionalstiffening system. As the side-shell has shown to be asignificant contributor to energy absorption, anunconventional new skin composed of double plate panelsinternally stiffened by web frames, essentially a coredpanel skin providing additional net shell thickness, wassubstituted for the conventional stiffened plate. Theresults are shown in Figure 40. Other ideas of the moreconventional nature included:• Bulkhead stools on the wing tank side to reduce hardspots and increase damage length.• Reduced stiffness of web frames to increase damagelength.• Increased side-shell plating thickness, especiallyfrom the ballast waterline up, to increase membranetension in the side-shell.• Decreased spacing of side-shell longitudinals andadded intermediate vertical web frames on the sideshell to increase the integrity of the side-shell.• Increased number of horizontal stringers in the wingtank to increase side skin integrity and provideadditional material for membrane tension.Although the practicability and acceptance from aregulatory point of view of very unconventionalapproaches may be seriously in doubt, it is neverthelesspossible to postulate alternatives, and in fact the greateststep in the level of possible energy absorption is likelywith an unconventional approaches:• Controlled pressure fluid chamber in the wing tankswherein the venting of fluid under pressure to otherspaces when compressed during collision results inenergy dissipation.• Introduction of material in wing ballast-tanks thatdoes not reduce ballast capacity significantly, butincreases energy absorption, like ultra-large open-cell
foam. Permeability and inspection are possibleproblems with this approach.• Truss structure separating wing tank skins. Such anarrangement can enhance the integrity of the innerhull longer as the truss deforms while supporting theouter skin and more slowly loading the inner skin.• Pre-planned failure or “crumple zones” as have beendeveloped by the automobile industry for the morecritical and predictable areas of damage.• A foam release system actuated in the event of animpending collision within the expected area ofcontact to increase stiffness and energy absorption.8.2 Bow Impact8.2.1 Analytical IndicationsThe significance of the energy absorption in bow damageand the effects of the bow interacting with the struckship’s side structure make it a critical element inconsidering collision results. Calculations assuming rigidbows with vertical stems and no bulbs have simplifiedanalyses and fostered understanding, but do not depict thecurrent and likely future nature of bows. Ship resistance,powering and seakeeping requirements are likely tocontinue to foster the existence of bows with raked stemsand bulbs into the future.Analytical evaluations do point a way to consideringbows. If they are deformable, they will account for asizeable portion of the total energy absorption in acollision. If they can be made to deform in a manner thatdoes not cause premature failure of important struck shipenergy absorbing structure, such as the side shell, then thesame attributes of a rigid vertical stem bow will beachieved at the struck vessel.8.3 ApplicationThe bows of ships incorporate a significant amount ofstiff horizontal fore and aft reinforcing structural elementsthat enhance the rigidity of the structure. Alternatively,arrangements with transverse stiffening coupled withappropriate spacing can lead to significant foldingdeformation, which can effectively reduce or eliminatethe protrusions of the upper bow and bulb as well as theirstructural hard-spots. Recent studies [44] have confirmedthat lower crushing pressure of a bulbous bow ispreferable to avoid the early rupture of the strucksideshell and the crushing strength should not exceed thatof the side structure.8.4 Grounding8.4.1 Analytical Indications – GroundingThe potential phases of grounding are depicted in Figure41. In grounding and raking, the vessel has forwardspeed. In stranding it does not.Model testing and computer finite element simulationof a stranding for a 280000 dwt tanker double bottomresulted in the lateral load versus indentation of up to 2mshown in Figure 42. The peak point, point 1, denotes thebuckling of the center longitudinal girder, and point 2, thefracture of the outer bottom plating. Point 3 indicatesfracture of the center girder-floor connection. It isinteresting to note that the response shown in Figure 42 issimilar to that predicted for collision, with the principaldifference being the nature of the obtrusive loading,which may be a line load for a collision and a point loadfor a grounding.Raking bottom damage is of significantly differentcharacter than the grounding damage heretoforediscussed. In raking damage, the obstruction ruptures thebottom structure and then continues destructive shearing,tearing and buckling as the vessel continues to move in itsdirection of motion, Figure 43. The length of bottomraking is based on an energy balance concept where thekinetic energy of the vessel is entirely absorbed by thedestruction of bottom structure.Figure 41 - Model of the grounding process [41,42]An energy analysis for grounding which takes intoaccount energy due to the tearing that occurs duringtracking-type collisions presumes that in such cases thedamage will be long and shallow, involving plate tearing,but relatively little volume distortion of the innerstructure. However, Figure 42 indicates that if there is avertical component to the striking force, after fracture ofthe outer plating at point 2, there is an increase in stiffnessleading up to point 3 and beyond as more girders andfloors come into contact with the obstruction. Figure 44also indicates a retention of stiffness.23
Page 1 and 2: Brown, A.J., Tikka, K., Daidola, J.
Page 3 and 4: 3 PROBABILISTIC FRAMEWORKFigure 2 i
Page 5 and 6: 4.2.2 Method by Wang [15]Dr. Wang d
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 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

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