Structural Design and Response in Collision and Grounding

modifications on oil outflow the DAMAGE calculationsshould be coupled with outflow analysis.The next step in the study is to carry out aprobabilistic analysis of damage in grounding scenariosdefined by probability density functions of the inputparameters. This analysis will be coupled with outflowcalculations. The objective of the study will be to providea tool for comparative studies between vessel designs.5 COLLISION MODELS5.1 Background and PlanIn 1979, the Ship Structure Committee (SSC) conducted areview of collision research and design methodologies[18]. They concluded that the most promising simplifiedcollision analysis alternative was to extend Minorsky’soriginal analysis of high-energy collisions by includingconsideration of shell membrane energy absorption. Asimple and fast model is important in probabilisticanalysis because thousands of different scenarios must berun to develop statistically significant results.A more recent review of the literature and of theapplicability of available methods for predicting structuralperformance in collision and grounding was made in the1997 International Ship and Offshore Structures Congress(ISSC 97) by Specialist Panel V.4 [9]. Their report states:“Knowledge of behavior on a global level only (i.e., totalenergy characteristics like the pioneering Minorskyformula) is not sufficient. The designer needs detailedknowledge on the component behavior (bulkheads,girders, plating, etc.) in order to optimize the design foraccident loads.”The approach taken in this study is to progressivelyincrease the complexity of SIMCOL (Simplified CollisionModel), the Ad Hoc Panel’s baseline collision model,until results with sufficient accuracy and sensitivity todesign characteristics are obtained. In order to assess themodel’s consistency and sensitivity, the model is appliedin a series of collision scenarios with a range of tankersizes and designs. SIMCOL results are compared toresults obtained using three other collision models:DAMAGE [10,19], ALPS/SCOL [20,21], and a TechnicalUniversity of Denmark (DTU) model. The DTU modeland ALPS/SCOL were also further developed during thisstudy. Determining the ideal trade-off between simplicityand sufficiency is a primary concern of this process.5.2 Models Included in the Current StudyThree types of collision were identified for investigationby the panel:• Low energy puncture at a point• Low energy raking puncture• Penetrating Collision – right angle and obliquesufficient to penetrate outer hull with significantdamage extending in at least 2 directions(penetration, horizontal, vertical)Work thus far is focused on models for penetratingcollision.Each collision model must consider two verydifferent aspects of the collision event:• External problem – includes the globalcharacteristics of the striking and struck ships andtheir motion before, during and after collision;• Internal problem - includes the internal deformationmechanics and structural response of the struck shipand the striking ship bow during collision.The four models studied by the panel take differentapproaches to solving and coupling these two problems.Although not a formal validation, comparison andagreement between these models provides useful insightinto their performance and increases confidence in thevalidity of their results.5.2.1 Simplified Collision Model (SIMCOL) [22]SIMCOL uses a time -domain simultaneous solutionof external dynamics and internal deformation mechanicssimilar to that originally proposed by Hutchison [23].Figure 12 shows the SIMCOL simulation process. Theexternal sub-model uses a three degree-of-freedomsystem for ship dynamics shown in Figure 13. Theinternal sub-model determines reacting forces from sideand bulkhead structures using detailed mechanismsadapted from a 1981 Rosenblatt study [24,25]. Itdetermines the energy absorbed by the crushing andtearing of decks, bottoms and stringers using theMinorsky correlation [26] as modified by Reardon andSprung [27].Go to the next time step:i = i+1At time step iBased on current velocities, calculate thenext positions and orientation angles of theships, and the relative motion at impactCalculate the change of impact locationalong the struck ship and the increment ofpenetration during the time stepCalculate the average reaction forces duringthe time step by internal mechanismsCalculate the average accelerations of bothships, the velocities for the next time step,and the lost kinetic energy based on externalship dynamicsNoMeet stoppingcriteria ?YesCalculate maximumpenetration and damagelengthFigure 12 - SIMCOL Simulation Process8