Structural Design and Response in Collision and Grounding

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• Benchmarking and development of collisionmodels• Definition of grounding and collision scenariosincluding striking bow geometry and bowstiffness.• Innovative design concepts2 MOTIVATIONThe serious consequences of ship grounding and collisionnecessitate the development of regulations andrequirements for the subdivision and structural design ofships to reduce damage and environmental pollution, andimprove safety.The International Maritime Organization (IMO) isresponsible for regulating the design of oil tankers andother ships to provide for ship safety and environmentalprotection. Their ongoing transition to probabilisticperformance-based standards requires the ability topredict the environmental performance and safety ofspecific ship designs. This is a difficult problemrequiring the application of fundamental engineeringprinciples and risk analysis.IMO first introduced probabilistic standards indamage stability regulations for passenger ships [1] andlater for cargo ships [2]. IMO’s first attempt to apply aprobabilistic methodology to tankers was in response tothe US Oil Pollution Act of 1990 (OPA 90). In OPA 90the US required that all oil tankers entering US watersmust have double hulls. IMO responded to this unilateralaction by requiring double hulls or their equivalent.Equivalency is determined based on probabilistic oiloutflow calculations specified in the "Interim Guidelinesfor the Approval of Alternative Methods of Design andConstruction of Oil Tankers Under Regulation 13F(5) ofAnnex I of MARPOL 73/78” [3], hereunder referred to asthe Interim Guidelines.All of these regulations use probability densityfunctions (pdfs) to des cribe the location, extent andpenetration of side and bottom damage. These pdfs arederived from limited historical damage statistics [4], andapplied identically to all ships without consideration oftheir structural design. Figure 1 is the IMO probabilitydensity function used to define the longitudinal extent ofbottom damage from grounding in oil outflowcalculations. The histograms represent the statistical datacollected by the classification societies and the linear plotrepresents IMO's piece-wis e linear fit of the data. OtherIMO pdfs are constructed in a similar manner.Probability Densityfb25.04.03.02.01.00.04.5LONGITUDINAL EXTENT FOR BOTTOM DAMAGE0.50.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Damage Length/Ship LengthTanker Data - 63 CasesPiecewise Linear FitFigure 1 - Damage Probability Density Function [4]A major shortcoming in IMO’s current oiloutflow and damage stability calculationmethodologies is that they do not consider the effect ofstructural design or crash-worthiness on damageextent [5,6]. The primary reason for this exclusion isthat no definitive theory or data exists to define thisrelationship.Other deficiencies include:• Damage pdfs consider only damage significantenough to breach the outer hull. This penalizesstructures able to resist rupture.• Damage extents are treated as independent randomvariables when they are actually dependent variables,and ideally should be described using joint pdfs.• Damage pdfs are normalized with respect to shiplength, breadth and depth when damage may dependto a large extent on local structural features andscantlings vice global ship dimensions.It is logical and essential that crashworthiness isconsidered in oil outflow and damage stabilitycalculations. An analytical method is required todefine the relationship between structural design anddamage extent in collision and grounding. This is aprimary objective of the work of this panel.A method that considers crashworthiness must besensitive to at least the basic parameters defining uniquestructural designs while maintaining sufficient generalityand simplicity to be applied by working engineers in aregulatory context for a ship in worldwide operation. Itshould not require detailed finite element analysis or belimited to a single accident scenario.An additional advantage of performance-basedmodels is the potential for their application todifferent and innovative designs. This is a secondprimary objective of this panel.0.52
3 PROBABILISTIC FRAMEWORKFigure 2 illustrates the process proposed to predict theprobabilistic extent of damage in collision as a function ofship structural design [7]. A similar process is proposedfor grounding.Probability given collisionPoint puncture, raking puncture,penetrating collisionPdf's:Striking ship speedStriking ship displacementStriking ship draft & bow heightStriking ship bow shape and stiffnessCollision striking location & angleStruck ship design variables:Type (SH,DH,IOTD,DS,DB,DS)LBP, B, DSpeed & displacementSubdivisionStructural designMonte CarloSimulationSpecificcollisionscenario'sPdf parametrics for extentof damage as a functionof struck ship designExtent ofDamageCalculationRegressionanalysisjoint pdf forlongitudinal, vertical andtransverse extent ofdamage:Figure 2 - Process to Predict Probabilistic Damage [7]The process begins with a set of probability densityfunctions (pdfs) defining possible grounding or collisionscenarios. Using these pdfs, a specific scenario isselected in a Monte Carlo simulation, and combined witha specific ship structural design to predict damage. Thisprocess is repeated for thousands of scenarios and a rangeof structural designs until sufficient data is generated tobuild a set of parametric equations relating probabilisticdamage extent to structural design. These parametricequations can then be used in oil outflow or damagestability calculations.1210864204.5Bottom Damage Longitudinal Extent PDF0.5DAMAGE Generated PointsMARPOL Standard-20 0.2 0.4 0.6 0.8 1Length of Damage / Ship LengthFigure 3 – Bottom Damage pdf [6]0.5A damage pdf generated in an early application ofthis approach is shown in Figure 3 [6]. This pdf wasgenerated for a MARPOL single hull tanker typical of thestruck ships represented in the data used to develop thecurrent MARPOL damage pdfs.4 GROUNDING MODELS4.1 Background and PlanThe Exxon Valdez accident in 1989 and the regulatoryevents following it lead to active research on structuralbehavior in grounding. The earlier studies had beenmostly empirical, the best-known being a study by Card[8] who surveyed 30 grounding incidents to determine theeffectiveness of a double bottom in reducing pollution.More recent studies have included large numericalsimulations, small and large scale experiments, and thedevelopment of simplified methods. Many of thesestudies were supported by the project “Protection of OilSpills from Crude Oil Tankers” carried out by theJapanese Association for the Structural Improvement ofShipbuilding Industry (ASIS). In the United States, theCarderock Division of the Naval Surface Warfare Center(NSWCCD) conducted grounding experiments, and theMIT-Joint Industry Project on Tanker Safety developedsoftware to analyze structural damage in grounding.Development of analytical models for the software wassupported by experimental studies. Active research efforton grounding analysis has been carried out also inEurope, mainly in Denmark and the Netherlands.The Specialist Panel V.4 of the 1997 InternationalShip and Offshore Structures Congress (ISSC 1997) [9]reviewed the state-of-the-art of the research andconcluded that nonlinear finite element analysis coupledwith calculation of ship motions had reached a level atwhich a fairly accurate prediction of the structuralresponse was possible. However, the report noted thatsince the method is time consuming and requires a highlevelof expertise, it is not suitable for the design orregulatory environment. The report also concluded thatthe simplified methods that existed at the time requiredfurther validation.This study concentrates on evaluation of the existingsimplified methods, which have been further developedsince the ISSC 1997 report. The objective is to assesstheir suitability for design and regulatory work.4.2 Models Included in the Current StudyThe methods evaluated in this study include DAMAGE, asoftware tool developed by the MIT-Joint Industry Projecton Tanker Safety, and a simple analytical methoddeveloped by Dr. Wang at the University of Tokyo(method by Wang). Both methods calculate grounding3
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Structural Design and Response in Collision and Grounding

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