Collision Analysis of Stiffened Plates

E-mails: amieralali@hotmail.com, nassersalam59@hotmail.com ABSTRACT Structural design of ships against collision requires prediction of the extent of damage to stiffened plates subjected

Collision Analysis of Stiffened Plates

Amier Alali, Yehia Abdel-Nasser, A Aliraqi, S A Swielm and Sulaiman Al-Shaye

Automotive & Marine Engineering Department, College of Technological Studies, Public

Authority for Applied Education &Training (PAAET ), Kuwait

E-mails: amieralali@hotmail.com, nassersalam59@hotmail.com

ABSTRACT

Structural design of ships against collision requires prediction of the extent of damage to stiffened plates subjected to impact In ship structures, stiffened plates are furnished with vertical

or horizontal stiffeners to sustain conventional loads such as shearing, bending and local buckling The consideration of collision in ship structural design is especially important for tankers where accidents may cause serious environmental pollution In predicting the extent of collision damage, FE modeling of stiffened plates using ABAQUS software is applied to demonstrate collision scenario Typical stiffened plates of tankers in service with different configurations of stiffeners are used to examine absorbed energy for each one The aim of this paper is to select the proper stiffener shape for absorbing more deformation energy These analyses of stiffened plates will guide ship designers to properly select effective stiffener absorbing higher deformed energy when simulate full scale ship against collision

Keywords: Stiffened panels, Tankers, Collision, FEM

1 Introduction

The serious consequences of ship grounding and collision necessitate the development of structural design and requirements for subdivision to reduce damage and environmental pollution and improve safety The consideration of the crashworthiness in design is necessary for tankers where accidents may cause serious environmental damage As Kuwait is a major exporting member of OPEC, and Kuwait’s economy depends on crude oil production and other oil products which provide well over 80% of Kuwaiti’s national revenues, the transportation of such products

is very important to the economy, and as KOTC is the main carrier for such shipments it is very important to keep the company’s vessels in good working condition and reduce probability of collision There are two ways in dealing with ship collisions The first one is to prevent the occurrence of extreme loads and accidents This can be achieved by using onboard monitoring equipment and well-trained crews In addition, the surveillance of sea routes, especially in high traffic areas near harbors, channels and offshore structures contributes a lot in minimizing

accident occurrence The second aspect is to increase the absorbed energy of structural

components This is done by developing stiffening systems that may bear damage within the limits of a required safety of the structure and environment

The International Maritime Organization (IMO) is responsible for regulating the design of oil tankers and other ships to provide for ship safety and environmental protection [1] Collision analysis models were first developed for analyzing the design of ships transporting nuclear materials The crashworthiness of these ships under worst case conditions was the primary

concern A totally inelastic, right angle collision with the struck ship at rest was considered the

“worst case” The most popular of these approaches is the one proposed by Minorsky [2]

Hutchison generalized the Minorsky method to include all horizontal degrees of freedom (surge, sway, yaw) and hull membrane resistance [3] The virtual masses of both struck and striking ships were developed in matrix form, including the added mass terms The computer program DAMAGE can be used to predict structural damage in the different accident scenarios [4]

Pedersen and Zhang derive expressions for absorbed energy uncoupled from internal mechanics [5] They apply three local coordinate systems to the striking ship, the struck ship and the impact point separately Since most deformation in collisions is local, instead of modeling the whole struck ship, they developed a collision model for analyzing minor collisions, which are defined as collisions without rupture of cargo boundaries A similar model is used by Crake and Brown [6]

A simplified collision model (SIMCOL) performs a collision scenario [7] There are three major ship-to-ship collision classifications: puncture, raking and penetrating Servis et al and Naar [8] also provide some excellent general guidance They used LSDYNA to model full-scale collision tests Their work identifies variable values that provide results consistent with their test results Paik provides many techniques for modeling crushing of ships experimentally and numerically [9] Kitamoura O et al [10], and Lee J.W et al [11] have developed very large FE element models and compared results with experimental results In the field of material failure, the

selection of material behavior until fracture is important, where this behavior influences the accuracy of non-linear finite element simulation Ehlers, S [12] has introduced material relation

to assess the crashworthiness of ship structures In the framework of selecting an efficient FE code for simulating ship collisions, several criteria should be met These criteria relate to the modeling capabilities for both the internal and the external collision mechanics In this aspect, the code must be capable of modeling ship motions during and after the collision (external

mechanics), as well as the deformation and collapse of the structures (internal mechanics)

2 Finite Element Analysis

The paper addresses the numerical simulation of impacts on ship structural components

by applying FEM This research work is concerned with the internal deformation of structural components It aims to know which stiffener arrangement absorbs higher internal energy, when large-scale impact is modeled The FE simulation of a collision encompasses a number of

individual problems, which should be given appropriate attention These problems are: The selection of a mesh and type of element, which should be fine enough, especially at the contact areas to acquire accurate results and to represent real failure modes during impact Coarse mesh may apply for areas located far from collision region to reduce CPU time Other problem such as modeling of the material damage criteria is explained in Refs [12 and13]

ABAQUS/Explicit Version 6.8-4 code is used to simulate the stiffened panel of a struck ship with different stiffening systems when subjected to accidental load The stiffened plates are

modeled using four nodes, thin shell double curved elements (S4R) The accidental load from a striking ship is simulated by assuming the ship' bow as a V-shape and modeled with rigid

elements This assumption is chosen because we are interested in comparing the effect of the collision on the different stiffened plates with no reference to the striking body Different

configurations of stiffening systems of struck ship are considered The absorbed energy and contact force for each stiffening system are calculated after damage

3 Simulation of Impact on Ship Structural Components

Structural components such as stiffened plates of ship side or bulkheads may be used to investigate the effect of impact on structural behavior In this work, stiffened plates with vertical stiffeners and horizontal stiffeners are examined Various FE models for such configurations have been developed The purpose of these models is to predict the extent of damage observed in real collision and to investigate the effect of the selection of stiffening system on the absorbed internal energy due to collision

The stiffened panel considered here consists of 3.4x4m plate, stiffened transversely or longitudinally with L-shaped stiffener having scantling of 150x100x9.5mm as shown in Fig.1 and the plate thickness is to be taken 15mm The material is assumed elastic-plastic of yield strength steel of 340 N/mm2 with isotropic hardening to a strength of 347 N/mm2 at plastic strain

of 0.025 In this aspect, the damaged response depends on element dimensions so fine mesh is

recommended at the region of contact

Different boundary condition may select around edges of stiffened panel and for each boundary condition, result of simulation will change In this work, the stiffened panel is constrained from all movements along its longitudinal edges (only one rotation in z direction is allowed) while it is free on its ends This assumption of boundary condition is similar to a real behavior of stiffened panels in ship sides during collision

Fig.1-a Longitudinally stiffened plate Fig.1-b Transversely stiffened plate

z x

y

The striking bow is rigid and free to move in right angle with a translational velocity along the -y-axis equal to 3.6 m/sec This velocity is applied at the reference point of the bow Point masses are assumed to model the mass of the ship and the added mass Virtual mass for the striking ship is acting at the reference point of the striking bow However, this mass is not

representing real mass of striking ship, real higher masses will be investigated The contact

surface during collision, elements based surface are applied to define contact region Middle region of both stiffened panel and rigid bow are considered as surfaces contact A dynamical explicit analysis at a time 0.25 sec is running

3.1 Collision Analyses of Transversely Stiffened Plates

In the first analysis three configurations of transversely stiffened plates are modeled with

L, flat and ,box stiffeners, in addition to modeling of the smeared plate (T=17.8mm) Scantlings

of stiffeners are determined on the basis of keeping cross sectional area and flexural rigidity for each stiffener are equal to those of L-stiffeners Four stiffeners with equal frame space are

furnished in the stiffened plate Fig 2 shows scantlings configuration for the stiffeners

Figure 3 shows deformed shape and internal energy for the analyses of transversely

stiffened plates for all configurations It is noticed that the three stiffened panels absorb small amount of the internal energy till their failure This because the stiffeners in the transverse

direction have negligible contribution to the impact load Consequently the panel absorbs small internal energy In contrast, the smeared plate absorbs higher internal energy Stiffened plates for three configurations are collapsed at a time about 0.15sec due to plastic deformation The failure is observed when the stiffness of element of the contact region has reached zero value The

displacement of point at the center of gravity of rigid bow is calculated for all stiffened panels as

shown in Fig.4 Almost same values of displacement are observed for the three stiffened plates Figure 5 shows contact force against time for all configurations The force-time relationship has

same behavior for all configurations However, the smeared plate shows higher contact force than other stiffened plates

Fig.2 Scantlings of stiffeners

B-160x50x6.4mm F-174x13.4mm

L- 150x100x9.5mm

Plate thickness, T=15mm

Figure 6 shows effect of mass of striking bow on the internal energy It is noticed when increasing mass of striking bow; deformed internal energy will slightly increase

3.2 Collision Analyses of Longitudinally Stiffened Plates

In the second analysis, the stiffened plates are longitudinally stiffened with L, flat and box stiffeners, in addition to the modeling of the smeared plate As mentioned before scantlings of

Fig.3 Internal energy –time relationship of a transversally stiffened panel

Fig 6 Effect of mass of striking bow

stiffeners are determined on the basis of keeping area and flexural rigidity are equal to those of L-stiffener Also, fine mesh for contact elements in the region of collision is applied Figure 7

shows absorbed internal energy for longitudinally stiffened plates Small difference is noticed in

the absorbed internal energy for all configurations as shown in Fig.7 However, Fig.8 shows higher contact force for smeared plate than other stiffened plates Stiffened plates furnished with Box or L stiffeners attained similar behavior of contact force

Figure 9 shows penetration displacement of the striking bow into the stiffened plates

through period of collision time Same penetration depth of striking bow is observed for both

stiffened plates and the smeared plate

Fig.7 Internal energy –time relationship of a longitudinally stiffened panel

Fig.8 Force-Time relationship Fig.9 Displacement-Time relationship

The paper refers to the simulation of stiffened plates collisions The following related aspects are considered to be essential for such a research work:

• The selection of boundary condition to simulate real collision scenario is necessary to investigate failure modes observed in real impact

• The striking mass is important to generate kinetic energy required to generate deformed energy on struck ship

• The longitudinally stiffened of the struck ship using different stiffeners configurations shows little difference in the absorbing energy However, the modeling with smeared plate shows higher contact force

• The validation of this study has to be executed onto real scale ship

Acknowledgments

This research is carried out for the part of (research No TS-08-05) of PAAET (Public Authority for Applied Education & Training, Kuwait) The authors are grateful for PAAET for their financial support

5 References

1) IMO, 1995, “Interim Guidelines for Approval of Alternative Methods of Design and

Construction of Oil Tankers under Regulation 13F(5) of Annex I of MARPOL 73/78”, Resolution MEPC.66 (37), September 14

2) Minorsky, V.V., 1959 “An Analysis of Ship Collisions with Reference to Protection of Nuclear Power Plants ”, Journal of Ship Research

3) Hutchison, B.L., 1986, “Barge Collisions, Rammings and Groundings - an Engineering

Assessment of the Potential for Damage to Radioactive Material Transport Casks”, Report No SAND85-7165 TTC-05212

4) Simonsen, B.C.,1999, “Theory and Validation for the DAMAGE Collision Module”, Joint MIT Industry Program on Tanker Safety, Report No 67, June

5) Pedersen, P.T and Zhang, S., 1998 “On Impact Mechanics in Ship Collisions”, Marine Structures, Vol 11, pp 429-449

6) Rawson, C., Crake, K and Brown, J., 1998, "Assessing the Environmental Performance of Tankers in Accidental Grounding and Collision", SNAME Transactions 106, 41-58

7) Chen, D., 2000, “Simplified Collision Model (SIMCOL)”, Dept of Ocean Engineering, Virginia Tech, Master of Science Thesis, May

8) Servis, D et al., 2001, “The Implementation of Finite Element Codes for the Simulation of Ship- Ship Collision”, 2nd International Conference on Collision and Grounding of Ships,

Copenhagen, Denmark, July

9) Paik, J.K., 2007, "Practical Techniques for FE Modeling to Simulate Structural

Crashworthiness in Ships", Ship and offshore structures, Vol 2 ,pp 69-80

10) Kitamura, O., July, 2001, “FEM Approach to The Simulation of Collision and Grounding Damage”, 2nd International Conference on Collision and Grounding of Ships, Copenhagen, Denmark

11) Lee J W., Petershagen, H., Rorup, J., Paik, H Y & Yoon, J H., "Collision Resistance and Fatigue Strength of New Oil Tanker With Advanced Double Hull Structure", Practical Design of Ships and Mobile Units, 1998, Elsevier Science B.V

12) Ehlers, S "Material Relation To Assess The Crashworthiness Of Ship Structures", Doctoral Dissertation, Helsinki University of Technology, Faculty of Engineering and Architecture, Department of Applied Mechanics, Nov.,2009

13) Lehmann, E & Yu, X., 1998," On Ductile Rupture Criteria for Structural Tear in The Case of Ship Collision and Grounding", Practical Design of Ships and Mobile Units, Elsevier Science B.V