STRUCTURAL CONSEQUENCE ANALYSIS OF AGED DOUBLE HULL OIL TANKERS INVOLVED IN SHIP SHIP COLLISION

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1 Proceedings of the ICTWS th International Conference on Thin-Walled Structures ICTWS September 2 October 2014, Busan, Korea ICTWS STRUCTURAL CONSEQUENCE ANALYSIS OF AGED DOUBLE HULL OIL TANKERS INVOLVED IN SHIP SHIP COLLISION Samy Youssef, Serdar Turgut Ince, Yang Seop Kim, Muhammad Faisal, Jeom Kee Paik The Korea Ship and Offshore Research Institute (The Lloyd s Register Foundation Research Center of Excellence), Pusan National University Busan, Republic of Korea Samy Youssef Marine Engineering Technology Department, Arab Academy for Science, Technology and Maritime Transport, Alexandria, Egypt Serdar Turgut Ince Naval Architecture and Marine Engineering Department, Yildiz Technical University, Istanbul, Turkey ABSTRACT Ships undergo a reduction in structural safety in later life, although that safety is quite adequate at the design stage. Aged tankers might be subjected to a very serious structural damage if involved in collision accidents. Therefore, this study aims to investigate the structural response characteristics of tankers subject to two types of damage; age-related damage in terms of corrosion and accidentrelated damage in terms of collision. A time dependent corrosion wastage modelling technique for various ship ages is applied to evaluate the amount of corrosion damage of the target structure in terms of annual corrosion rate for each structural item. The LS- DYNA nonlinear finite element method (NLFEM) is used to simulate the suggested collision scenario. Thus, the structural respo nse characteristics in terms of two parameters; energy absorption capability of the struck structure and the penetration of the striking bow for various collision limit criteria, are computed. As an illustrative example, a Suezmax class double hull oil tanker is considered for the purpose of the present study. 1. INTRODUCTION Aging of ship structures may be defined as the progressive deterioration of structures as a result of the normal operational use and environmental influences (Wang et al. 2009). The structural degradation mechanisms that may pose threats to the structural integrity of aging steel structures can be in forms of corrosion, fatigue cracks, local dents and changes in material properties. Although the ship and offshore structures have well designed inspection and maintenance systems, there is obvious evidence of corrosion, particularly for aged structures for which maintenance practices have been less optimal (Melchers 2008). As ships get older, they may suffer serious structural damages when involve in grounding or collision accidents. This is because of the reduction in their structural crashworthiness due to progressive structural degradation in terms of corrosion. Therefore, this study aims to investigate the structural response characteristics of ships subject to two types of damage; agerelated damage (i.e., corrosion) and accident-related damage (i.e., collision). A Suezmax class double hull oil tanker with various ages involved in a collision with another ship is employed for the purpose of this study. Fig. 1 shows the general procedure of the present study. The target structure is defined in terms of its main particulars as well as the structural design characteristics. Then, Finite element (FE) models are established for both colliding ship structures; by identifying the structural geometry, material, element s type, mesh size, failure mode and surrounding water effect etc. In order to predict the corrosion damage (i.e., agerelated damage) in terms of the thickness reduction for the primary structural supporting member of the objective structure, a time-dependent corrosion wastage model is applied to predict the corrosion damage. On the other hand, the collision scenario in which the target ships are involved, is defined in terms of various collision parameters such as colliding ship's speed, displacement, loading conditions, collision angle and impact location along the struck ship length. To predict the accident-related damage to the virtual corroded 1

2 structure, the proposed scenario is simulated using explicit nonlinear software LS-DYNA (Hallquist 2010). Based on the simulation results, the structural response characteristics in terms of collision energy-absorption by the struck ship and striking bow penetration are investigated. Figure 1. Procedure of the present study. 2. TARGET STRUCTURE A Suezmax-class double hull oil tanker (157,500 DWT) is selected as the struck ship. Her main dimensions are presented in Table 1. A bow portion of an Aframax-class double hull oil tanker (95,000 DWT) is employed to play the role of the striking ship. Table 1. Main dimensions of the struck ship (in m) Item Dimension Length overall Length between perpendiculars Moulded breadth 48.0 Moulded depth 23.7 Design draft (moulded) 16.0 Double side width 2.64 Double bottom height 2.64 Transverse frame spacing CORROSION DAMAGE Various types of corrosion models used for marine structures are found in the literature. Time-dependent corrosion wastage model (TDCWM) is one of them, which can provide a more accurate figure for annual corrosion wastage (Paik and Thayamballi 2002, Mansour and Ertekin 2003, Paik et al. 2003, Paik 2004, Paik et al. 2004, Paik and Kim 2012). Paik et al. (2003) performed an extensive statistical analysis of a large number of corrosion measurements for the structures of singleand double-hull tankers; floating, storage, and off-loading units (FSOs); and floating, production, storage, and off-loading units (FPSOs), to develop a set of time-dependent corrosion wastage models for 34 different structural member groups by type and location, considering plating, and webs and flanges of stiffening. These models are established according to a coating life of 5 years, 7.5 years and 10 years, with two types of corrosion rate; average and severe level. The design life of double hull oil tankers is assumed to be 25 years specified by IACS (2006). In corrosion loss measurement data, information on the coating life is normally unclear. Paik et al. (2004) considered that the 5-year coating life may represent an undesirable situation, whereas 10 years or longer is a relatively more desirable state of affairs. In this regards, a time-dependent corrosion wastage model (Paik et al. 2003) with coating life of 10 years in severe corrosion rate level (see Fig. A1), is applied to predict the thickness reduction for the target ship s structural members of different ages; 15 years, 20 years and 25 years. 4. FINITE ELEMENT MODELS A full size of the Suezmax-class double hull oil tanker (i.e., objective ship) is modelled. All structural members are modelled by Type 24-Elastic/Plastic Isotropic with piecewise linear plasticity that was found in LS-DYNA material library (Hallquist 2010). The strain rate effects and complete material fracture are considered by adopting the Cowper-Symonds model (1957). Belytschko-Tsay shell elements (Belytschko et al. 2006) with four-noded plate-shell elements and five section points through the thickness are used for plating and supporting members. A fine mesh with a size of 50 mm is applied around the mid-cargo tank (i.e., contact region) and medium mesh (200 mm) is applied in other areas around the fined mesh ones to achieve acceptable computational time. The fracture strain value ε f = 0.1 is defined which is commonly used before in the maritime industry (Paik et al. 1999, Servis et al. 2002, Sajdak and Brown 2005, Paik et al. 2009). Fig. 2 shows the FE model of the Suezmax-class double hull oil tanker (i.e., the objective ship). The striking bow is comparatively more rigid than ship side. Thus, at the collision events, absorbing the energy of the striking ship bow may be negligible. In this study, the striking ship bow is treated and modelled as a fully rigid body (Törnqvist 2003). However, the situation in which the striking will not have any plastic deformation might lead to a worst case scenario, and thus a conservative result with respect to large damage caused to the struck vessel, could be expected. Fig. 3 shows the finite element model of the Aframax-class double hull oil tanker s bow portion. On the basis of the employed (TDCWM) model (see section 3), the corrosion damage is predicted by applying the element thickness reduction method for all structural members of the target ship s FE-model. In this study, four models of the struck ship are established with different scantlings; with gross (as-built) scantlings to 10 years (coating life), 1 5 years, 20 years and 25 years. 2

3 Figure 2. FE-model of the struck ship. Figure 4. Proposed collision scenario. The effect of the surrounding seawater is taken into consideration as a virtual added mass of the struck ship in surge, sway and yaw (Pedersen and Zhang 1998, Törnqvist 2003, Sajdak and Brown 2005). In addition, MCOL (Ferry et al. 2002) is employed in this study, which is a subroutine under an LS-DYNA program that can calculate the rigid body motion for external dynamics of ships. Each time step MCOL uses resultant forces and moments of rigid bodies and updates the ship positions and returns the new positions to LS-DYNA. The program can calculate added mass effects and buoyancy forces from a linear restoring force approximation. Figure 3. FE-model of the striking ship s bow portion. 5. COLLISION DAMAGE In this study, the ship ship collision scenario is considered in which the collision angle between the side structure and the bow is taken to be 90 degrees. The impact location is at midlength of the struck ship. The loading condition of the striking and struck ship is assumed to be normal ballast and fully laden, respectively. The speed of the striking ship at the initiation of the collision is considered to be 4 knots, while the struck ship is considered to be at standstill (berthing in harbor). Concerning the worst collision scenario (i.e., more severe for the struck ship), the first contact point of the striking ship bulbous bow is assumed to be in between two stringers and two transverse web frames in the middle cargo tank of the struck ship (Paik et al. 2009) as shown in Fig. A2. The proposed scenario is applied for the four corroded models of the struck ship (see section 3). Using the nonlinear finite element software LS-DYNA (Hallquist 2010), the suggested scenario is simulated and analysed where the calculations are carried out in the time domain using the explicit analysis method. Fig. 4 shows a setup for the proposed collision scenario. 6. STRUCTURAL ANALYSIS To investigate the structural response characteristics of the target ship that subjected to two types of damage; age-related damage (i.e., corrosion) and accident -related damage (i.e, collision), the numerical results for the studied four models of the struck ship to be involved in collisions, are used. In this section, collision performance of the target ship is evaluated based on three types of design criteria and compared between the suggested scantlings (i.e., ship ages). Design Criteria Against Collision In terms of accidental limit states associated with ship collisions, Paik et al. (2009) compared the collision strength performance for a double-hull oil tanker structures designed by pre-csr and CSR methods based on three types of design criteria are as follows: rupture of outer side shell plate, penetration of the striking bow until the location of inner side shell plate equivalent to double-side breadth, and rupture of inner side shell plate. In this study, structural response characteristics based on the aforementioned three criteria is evaluated and compared 3

4 between each scantling. In addition, the total energy absorbed by the struck structure and the maximum penetration of the striking ship bow portion into the struck structure, are also investigated. Structural Response Characteristics Based on the collision simulation results generated by LS- DYNA, the transverse extent of the damage (i.e., penetration) to the struck ship structure and the energy absorbed by the struck ship structure is calculated for each collision scenario taking into consideration the aforementioned three criteria. Figs. 5 and 6 compares the collision resultant force and absorbed energy versus penetration curves of the struck structure for each scantling, respectively. The suggested design criteria against collision may be given by two parameters, namely the energy-absorption capacity of the structure and the penetration of the striking bow until the corresponding criteria are satisfied (Paik et al. 2009). Therefore, the critical values of the absorbed energy and penetration are computed depending on the aforementioned limit criteria for four different corrosion scantlings as indicated in Table A1 where E cr = energy-absorption capability, d cr = penetration until the event of the corresponding criterion. According to the computational results, the energyabsorption capability (i.e., critical energy) of the struck ship structure and the critical penetration of 15 years, 20 years and 25 years scantlings are compared to gross (new -built) scantlings. Based on rupture of outer side shell criterion, it is observed that the energy-absorption capability of 15 years, 20 years and 25 years scantlings is decreased by 12%, 22% and 31%, respectively. On the other hand, the penetration in all cases has a slight difference (a few millimetres) that can be ignored comparing to the size of colliding vessels. According to the second criterion (i.e., penetration of the striking bow until the location of inner side shell plate equivalent to double-side breadth), the energy-absorption capability of 15 years, 20 years and 25 years scantlings is reduced by 10%, 20% and 33%, respectively. For the rupture of inner side shell criterion, a similar trend is found in the energy-absorption capability of 15 years, 20 years and 25 years scantlings, which is decreased by 7%, 8% and 16%, respectively. In contrast, the penetration in cases of 15 years, 20 years and 25 years scantlings is increased by 2%, 10% and 15%, respectively. In the case of the end of kinetic energy releasing, similar trends are found in the total energy absorbed by the struck ship structure as well as the maximum penetration. In general, the results are expected due to the reduction in the global structural crashworthiness of the struck ship along its life span. As expected, the energy-absorption capability decreases as the ship going older. That is because of the strength reduction caused by the corrosion damage. On the other hand, the striking bow penetration into the struck ship structure becomes larger as the ship going older. This is also expected due to the stiffness reduction of the structural members caused by the corrosion damage. Resultant force(mn) Figure 5. Collision resultant force versus penetration. Absorbed energy(mj) New built 15 years 20 years 25 years Figure 6. Collision energy absorption versus penetration. 7. CONCLUDING REMARKS Penetration(m) New built 15 years 20 years 25 years Penetration(m) This study aims to investigate the structural response characteristics of ageing tanker subjected to two types of damage; age-related damage in terms of corrosion and accident-related damage in terms of collision. For the corrosion damage, a time-dependent corrosion wastage model was applied to predict the scantlings for the 4

5 target ship s structural members of different ages; 15 years, 20 years and 25 years. For the collision damage, the non-linear explicit finite element software; LS-DYNA was employed to simulate the four studied cases. The collision resistance capabilities of the target ship structure were examined based on three design criteria against collision for various scantlings (i.e., ship ages). Furthermore, the reduction in collision performance (i.e., energy-absorption capacity) of the aged structures is presented as a percentage of the performance with gross (new -built) scantlings. Based on the findings of this study, the following conclusions can be drawn; when the ship being older, the structural strength will be partially lost due to the corrosion damage exposed in form of thickness reduction, the energy-absorption capability of ship structure involved in collision decreases as a result of ship aging, the corrosion damages for the aged structure leads to stiffness reduction of the structural members, the struck ship structure subjected to more penetration by the striking bow as the ship being older, and Both of reduction in the energy-absorption capability and stiffness of the struck tanker structure, may lead to serious impacts such as, loss of watertight integrity, increasing into the cargo outflow probability and damage to the environment due to the oil pollution. Therefore, periodic surveys are important to detect corrosion problems and help estimate the remaining thickness of the structure (i.e., strength or integrity) prior to their needing replacement even in later life. The present study considered one specific collision scenario. If the scenario is changed, the details of structural response may of course become different. In this regard, consequence analysis of randomly selected collision scenarios including different striking ship types, is being carried out for various scantlings. In future work, the risk will then be calculated and assessed. ACKNOWLEDGMENTS This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, Ict & future Planning (MSIP) (Grant no.: ). REFERENCE Belytschko, T., Liu, W.K. and Moran, B., 2006, Nonlinear Finite Elements for Continua and Structures, Chichester (England), John Wiley & Sons, UK. Cowper, G.R. and Symonds, P.S., 1957, Strain Hardening and Strain-rate Effects in the Impact Loading of Cantilever Beams, Brown University Division of Applied Mathematics, Report No. 28. Ferry, M., Le Sourne, H. and Besnier, F., 2002, MCOL- Theoretical Manual, Technical Report 01-52, French Shipbuilding Research Institute, France. Hallquist, J.O., 2010, LS-DYNA3D Theory Manual, Livermore Software Technology Corporation, California, USA. IACS, 2006, Common Structural Rules for Double Hull Oil Tankers, International Association of Classification Societies, London, UK. Mansour, A.E. and Ertekin, R.C., 2003, Report on the Committee V.3: Collision and grounding, Proceeding of the 15th International Ship and Offshore Structures Congress, San Diego, USA. Melchers, R.E., 2008, Chapter 4: Corrosion Wastage in Aged Structures, In: Condition Assessment of Aged Strucures, edited by Paik J.K. and Melchers R.E., CRC Press, NewYork, USA. Paik, J.K., 2004, Corrosion Analysis of Seawater Ballast Tank Structures, International Journal of Maritime Engineering, Vol. 146, PartA1, pp Paik, J.K., Chung, J.Y., Choe, I.H., Thayamballi, A.K., Pedersen, P.T. and Wang, G., 1999, On the Rational Design of Double Hull Tanker Structures Against Collision, SNAME Transactions, Vol. 107(1), pp Paik, J.K. and Kim, D.K., 2012, Advanced Method for the Development of an Empirical Model to Predict Timedependent Corrosion Wastage, Corrosion Science, Vol. 63(1), pp Paik, J.K., Lee, J.M., Hwang, J.S. and Park, Y.I., 2003, A Timedependent Corrosion Wastage Model for the Structures of Single/Double Hull Tankers and FSOs/FPSOs, Marine Technology, Vol. 40(3), pp Paik, J.K., Park, H.J. and Samuelides, E., 2009, Collision Accidental Limit States Performance of Double-Hull Oil tanker Structures: Pre-CSR Versus CSR Designs, Marine technology, Vol. 46(4), pp Paik, J.K. and Thayamballi, A.K., 2002, Ultimate Strength of Ageing Ships, Journal of Engineering for the Maritime Environment, Vol. 216(1), pp Paik, J.K., Thayamballi, A.K., Park, Y.I. and Hwang, J.S., 2004, A Time-variant Corrosion Wastage Model for Seawater Ballast Tank Structures of Ships, Corrosion Science, Vol. 46(2), pp Pedersen, P.T. and Zhang, S., 1998, On Impact Mechanics in Ship Collisions, Marine Structures, Vol. 11(10), pp Sajdak, J.A.W. and Brown, A.J., 2005, Modelling Longitudinal Damage in Ship Collisions, SSC-437, Ship Structural Committee, Washington DC., USA. Servis, D.P., Samuelides, M., Louka, T. and Voudours, G., 2002, The Implementation of Finite Element Codes for the Simulation of Ship ship Collisions, Journal of Ship Research, Vol. 46(1), pp

6 Törnqvist, R., 2003, Design of Crashworthy Ship Structures, PhD thesis, Technical University of Denmark, Lyngby, Denmark Wang, G., Boon, B., Brennan, F.P., Garbatov, Y., Ji, C., Parunov, J., Rahman, T.A., Rizzo, C., Rouhan, A., Shin, C.H. and Yamamoto, N., 2009, Report on the Committee V.6: Condition Assessment of Aged SHips and Offshore Structures, Proceeding of the 17th International Ship and Offshore Structures Congress, Seoul, Korea. APPENDIX Table A1: Energy-absorption capacities of the struck structure for various design criteria against collision Criterion Rupture of outer shell plate Scantling (Ship age) 0 10 years 15 years 20 years 25 years E cr (MJ) d cr (m) E cr-i years / E cr-gross d cr-i years / d cr-gross Penetration until doubleside breadth E cr (MJ) d cr (m) E cr-i years / E cr-gross d cr-i years / d cr-gross Rupture of inner shell plate E cr (MJ) d cr (m) E cr-i years / E cr-gross d cr-i years / d cr-gross Total absorbed energy E (MJ) d (m) E i years / E gross d i years / d gross

7 Figure A1. Annual corrosion rate value (mm/year) with 10 years coating life (A = air, B = ballast, O = oil, S = seawater, H=horizontal member, V = vertical member, W = web, F = flange) (Paik et al. 2003). Figure A2. Contact point illustration. 7