LIU Kun,WANG Zi-li,ZHANG Yan-chang,TANG Wen-yong

(1 State Key Lab.of Ocean Engineering,Shanghai Jiao Tong University,Shanghai 200240,China;

2 School of Naval Architecture and Ocean Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,China)

1 Introduction

The main purpose of ship collision research is to improve the crashworthiness of the hull structure.It has important significance for the environmental protection,economy and the life safety.In recent years,with the improvement of ship collision research methods,the research on the ship crashworthiness has developed a lot.Kitamura[1]designed two new crashworthiness structures,they are frame panel and corrugated panel.Lee[2]designed the new double hull structures in VLCC which improved the side structure crashworthiness,they are NOAHS and NOAHS II.Based on the thin circular tube characteristics of dynamic progressive buckling under axial load,Wang[3]proposed a VLCC side structure crashworthiness structure CCT.Wang and Jiang[4-5]designed two kinds of single hull side crashworthiness structure,they are FCT and IFP.Some researchers have proposed different kinds of crashworthiness structures,such as Y-shaped,tube-shaped,hat-shaped,diamond-shaped longitudinal,honeycomb sandwich panels,and so on,and these structures improved the ship’s crashworthiness[6-10].

The crashworthiness structure design is to use reasonable structure design method or introduce special energy absorbing unit,so as to improve the collision performance of the struc-ture.Overall,there are two kinds of methods to improve the collision performance,one is to use the new crashworthiness structures,and the other is to optimize the traditional design of the hull structure that must ensure regular strength and not increase the structural weight.This paper uses the first method,which uses the new crashworthiness structure to improve the crashworthiness.

Sandwich Plate System(SPS)is a three layers composite sandwich panel which is composed of two layers of metal panels and elastic body core[11].It has the advantages of high specific strength,high stiffness,light,damping,low maintenance cost,convenient repairing and excellent impact resistance,etc.This paper applied the SPS to the hull side structure,analyzes the collision performance of SPS side structure by the finite element software ABAQUS,and then compared it with the traditional side structure.

2 Description of the collision scenario

2.1 Collision scenario

The collision scenario chooses a single hull ship impacted by a 10 000 DWT ship with a bulbous bow.The impact velocity is 6 m/s and impact angle is 90°.

Fig.1 Collision scheme

2.2 Collision scheme

In order to analyze the collision performance of SPS side structure,which is equivalent by the conventional side structure,both of them are impacted by a striking ship which is simplified as a rigid ball.The collision scheme is shown in Fig.1.

3 The collision performance of conventional side structure

3.1 Finite element models

The area of struck side is one cargo tank in the length and from the bilge to the upper deck in the depth,the size of which is 13.5 m×8 m,and both ends of the area are fixed.In order to analyze the collision performance of struck structures,we select four collision positions shown in Fig.1 and Fig.2.Position a is located at the center of two frames on the 2 deck,position b is located at the center of two frames on the longitudinal,position c is located at the intersection of frame and 2 deck,position d is located at the intersection of frame and longitudinal.

Structural collision simulations were performed using the commercial code ABAQUS.The structural material is marine low carbon steel,and the mechanical properties of the material(Tab.1)used in the finite element models are obtained from the Ref.[12],which were ob-tained from in-house quasi-static tensile tests carried out on material of marine low carbon steel.The Cowper-Symonds model is selected as material constitutive model,and the material strain hardening effect is considered.

Fig.2 Finite element models and collision positions

Tab.1 The main factors of materials in simulation

3.2 Calculation results

The position b is selected as the typical impact position,and the simulation model is calculated by the dynamic nonlinear finite element software ABAQUS,the results are as follows.

3.2.1 Collision force

Fig.3 shows the relationship between the penetration and the collision force.It can be seen from the figure:1)The curve has the obvious characters of volatility and nonlinearity.2)It can reflect the damage deformation process of the struck structures.Before the Point A(t=0.26 s),it is the load region where the collision force increased with the penetration and the structure deformation is mainly in membrane tension of shell plate and bending deformation of frame and longitudinal.At the Point A,the collision force reached the maximum which is called peak load,when the shell plate began to burst.After Point A is the unloading region where the collision force decreased with the penetration and the structure deformation is mainly in tearing of shell plate and longitudinal.

Fig.3 Collision force-penetration curve

Fig.4 shows the damage deformation of the side structure at the limited penetration(t=0.26 s).We can see that the structure deformation has the significant localized characters under impact loading,which mainly occurs in the collision area.The deformation of shell plate is membrane tensile,the stringer is bending,and the 2 deck and deck beams are buckling.Because of the collision position,the 1 deck is not deformed obviously,and the frame mainly takes bending deformation.

Fig.4 Damage deformation of the struck side structure(t=0.26 s)

3.2.2 Energy absorption

Fig.5 shows the relationship between energy absorption of struck structural components and collision penetration.We can see from the figure that it has the same tendency of all the curves of structural components,under the impact load.The curves firstly increase with the penetration,and gradually stabilize after reaching the maximum value.In addition,the shell plate absorbs the most energy,so it is the main energy-absorption component.Frames,2 deck and longitudinal also absorb much energy,while other components absorb relatively less energy.That gives us guidance that if we want to improve the energy absorption,we can do something about the shell plate.The energy absorption of structural components at the limited penetration which refers to the penetration when the shell plate of the struck structure began to burst is shown in Tab.2.

Fig.5 Energy absorption-penetration curves

Tab.2 Energy absorption of structural components

4 The collision performance of SPS side structure

4.1 The concept design of SPS side structure

In this paper,the crashworthiness design object is side structure and based on the SPS,so we called it SPS side structure.The design idea of SPS side structure is that the shell plate is replaced by the upper and lower panels of SPS,and the longitudinal is replaced by the core of SPS.The thickness of the upper and lower panels is the same,while the height of core is less than or equal to the original height of web of longitudinal.The value of the dimension is determined in accordance with the principle of equivalent quality.In this case,the shell plate between the side stringers and the 1 deck is replaced by the SPS,the thickness of the upper and lower panels is 5 mm,and the height of core is 30 mm,other dimensions is the same as the conventional side structure.

Fig.6 Plated grillage and SPS

The finite element models are shown in Fig.7,all the models are using shell element except the core of SPS which are using solid element.The core uses Polyurethane,whose density is 1 200 kg/m3,elastic modulus is 820 MPa,Poisson’s ratio is 0.44,yield stress is 26 MPa,and critical stress is 235 MPa.The materials of other side structure components are the same as Tab.1.

4.2 Calculation results

Dynamic nonlinear finite element software ABAQUS is used to calculate the collision models and the results are shown as follows.

Fig.7 The finite element models

Fig.8 Damage deformation of the struck side structure(t=0.283 s)

Fig.8 shows the damage deformation of the SPS side structure at the limited penetration(t=0.283 s).Compared with the Fig.4,we can see that the deformation modes of the SPS struck side structures are basically the same with conventional structures,but the deformation area and degree at the limited penetration of SPS struck side structure are more serious than those of the conventional structures.It is because the SPS side structures.

Fig.9 is the collision force-penetration curve.We can see that it has the same characters of volatility and nonlinearity with the conventional side structure(Fig.3)and the peak value is 14.51 MN,16.3%more than that of the conventional side structure(12.51 MN).Meanwhile,it can reflect the damage deformation process of the struck structures.When reaching the point A,the lower panel of SPS first began to burst,and reaching the point B,the upper panel burst.The core of SPS burst at point C,when the SPS shell plate began to burst,and structure is in the unloading region.

Fig.10 shows the relationship between energy absorption of SPS side structure components and collision penetration.We can see that it has the same tendency with the conventional side structure(Fig.5&Tab.2)that all the curves of structural components,under the impact load.The curves firstly increase with the penetration,and gradually stabilize after reaching the maximum value.Because the limited penetration of the SPS side structure(1.675 m)is deeper than the conventional structure(1.543 m),the deformation area and degree are more serious than the conventional,it absorbs more energy.The shell plate absorbs the most energy,so it is the main energy-absorption component.Frames,2 deck and longitudinal also absorb much energy,while other components absorb relatively less energy.That is also the same with before.

Fig.9 Collision force-penetration curve

Fig.10 Energy absorption-penetration curves

The energy absorption of structural components at the limited penetration is listed in Tab.3.

Tab.3 Energy absorption of structural components at the limited penetration

4.3 Comparison of collision performance between SPS and conventional side structure

The collision performance of the conventional side structure is compared with the SPS side structure in Tab.4.The energy absorption of two side structures on four different positions at the limited penetration is listed.It shows that with the same weight,on the four collision positions,the limited penetration of SPS side structure is increased by 8.2%,8.6%,64.8%and 5.8%to the conventional side structure,the energy absorption is improved by 20.2%,34.1%,115.9%and 25.5%,respectively.So the collision performance of SPS side structure is better than the conventional side structure.The shell plate(when compared,the stiffeners is included in the shell plate of conventional side structure)of the two side structures is the main energy-absorption component,so it is the effective method to increase the energy absorption of shell plate to improve the crashworthiness.It absorbs less energy when collision occurs at the position A and position C,because the stiffness of those positions is higher,where it transfers the loading to other components and makes them involved in the deformation earlier,and leads the structure burst earlier,so the structure absorbs less energy.

Tab.4 Comparison of two side structures

5 Conclusions

This paper applied the SPS unit in the struck side structure,analyzed the collision force,energy absorption and limited penetration,and then compared with the conventional side structure.The main conclusions are as follows.

(1)Based on the simulation results,the SPS side structure has better characteristics of resistance to deformation and energy absorption,so it can improve the collision performance of struck structures significantly.

(2)Shell plate is the main energy-absorbing structure both in conventional side structure and SPS side structure,so the most effective method to improve the structure energy absorption is to increase the energy absorption of shell plate.So the SPS is introduced to replace the conventional side structure to improve the structure energy absorption.

(3)SPS has great potential in ship structure protection.Through proper design,it can enhance the ship’s defensive capabilities and vitality;providing protection for the personnel and equipment inside the ship.

This work was financially supported by the National Natural Science Foundation of China(Grant No.51379093),the National Natural Science Foundation of China(Grant No.BK2011507)and the Open Research Fund Program of Jiangsu Key Lab.of Advanced Design and Manufacturing Technology(Grant No.CJ1305).

[1]Kitamura O.Comparative study on collision resistance of side structure[J].Marine Technology,1998,34(2):293-308.

[2]Lee J W,Petershagen H,Rorup J,et al.Collision resistance and fatigue strength of new tanker with advance double hull structure[C].Practical Design of Ships and Mobile Units,Oosterveld M W C and Tan S G,Editor,1998:133-139.

[3]Wang Zili,Gu Yongning.A crashworthy type of double hull structure of VLCC[J].Journal of Ship Mechanics,2002,6(1):27-36.

[4]Wang Zili,Jiang Jinhui.A crashworthy type of single hull structure based on foam cubic tube(FCT)[J].Shipbuilding of China,2004,8(2):51-56.

[5]Jiang Jinhui,Wang Zili.A crashworthy type of single hull structure based on IFP[J].Journal of Ship Mechanics,2004(5):80-85.

[6]Wang Zili,Zhang Yanchang.Single hull ship structure crashworthy design based on sandwich panel[J].Shipbuilding of China,2008,49(1):60-65.

[7]Zhang Yanchang,Wang Zili.Study on crashworthiness of honeycomb sandwich panel under lateral dynamic load[J].Journal of Jiangsu University of Science and Technology,Natural Science Edition,2007,21(3):1-5.

[8]Wang Zili,Gu Yongning.A crashworthy side structure for single-hull LPG carrier[J].Ship Engineering,2001(2):12-14.

[9]Jung Yeob kim,Jae Wook Lee.On the structure energy absorbing system for the double hull tanker[C].Proc.of the 7th International Marine Design Conference,2000:305-312.

[10]Jones N.Structural Impact[M].Cambridge:Cambridge University Press,1989.

[11]Zhou Hong,Zhang Yanchang,Yue Yalin,Liu kun.Numerical simulation technology for sandwich plate system crashworthiness[J].Computer Aided Engineering,2013,22(2):66-71.

[12]Wang Zili.Study on damage mechanism in ship collisions and structural crashworthiness[D].Shanghai.Shanghai Jiao Tong University,2000.

[13]Kitamura O.FEM approach to the simulation of collision and grounding damage[J].Marine Structures,2002,15:403-428.

[14]Kristjan Tabri,Joep Broekhuijsen,Jerzy Matusiak,Petri Varsta.Analytical modelling of ship collision based on full-scale experiments[J].Marine Structures,2008,22:42-61.

[15]Lorenzo Peroni,Massimiliano Avalle,Giovanni Belingardi.Comparison of the energy absorption capability of crash boxes assembled by spot-weld and continuous joining techniques[J].International Journal of Impact Engineering,2009,36:498-511.

[16]Andrew Negri,Paul Marshall.TBT contamination of remote marine environments:Ship groundings and ice-breakers as sources of organotins in the Great Barrier Reef and Antarctica[J].Journal of Environmental Management,2009,90:31-40.