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Simulation Based Structural Analysis of an Oil Tanker Bulkhead
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 194 (2017) 517 – 521
10th International Conference on Marine Technology, MARTEC 2016
Simulation Based Structural Analysis of an Oil Tanker Bulkhead Md. Shahidul Islam1 , Md.Humayun Kabir1 , Sharat Chandra Paul1, a Department
of Naval Architecture and Marine Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh b Offshore and Marine Engineering Ltd., Bangladesh
Abstract Traditional way of ship structural design is to follow the rule books of classification societies. These rule books certainly ensure enough structural integrity of ships. But in this age of scientific and engineering progress, reliable structural analysis programs and tools are available. This paper shows how these programs can be used to verify classification society rules and to accurately design in small details. A bulkhead of an oil tanker is chosen for this investigation. The analysis is done taking Von Misses stress and factor of safety as design criteria. The results of this investigation aim to help the traditional structural engineers to design ship structures in a more effective and optimized way. © by Elsevier Ltd. This is an openLtd. access article under the CC BY-NC-ND license c 2017 2017Published The Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology. Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology. Keywords: Structural Analysis; Classification Society; Finite Element Method; Bulkhead
1. Introduction Provision of ’adequate’ strength in a ship at a reasonable cost, has always been one of the most challenging task for the ship designers. Structural engineers design ship structures on the basis of their experience and basis ship during conceptual design. In real phase, they have to comply with the applicable rules as well as minimum requirements of the concerned classification society. Over the years the classification societies have been providing the necessary standards to ensure the adequacy of strength against all demands that can be envisaged during service life of the ship. The structural engineers also have to pay a good attention to the purpose of the ship and economic aspect. With the advancement of Numerical analysis methods and availability of computing facility at affordable costs, use of such methods provides more reliable and direct assessment. Finite Element Analysis (FEA) is a universally known structural analysis tool in use today to check such structural concerns [? ? ]. This paper presents a procedure of Finite Element Analysis of ship structure as a way to ensure its structural safety. This type of analysis will help the concerned people to improve the factor of safety of a structure as well as to eliminate the possible weak points from the structure in an effective way. 2. Problem description During the operation of a vessel, it faces different kinds of loading. Sometimes it will be fully loaded, sometimes will be empty and sometimes partially loaded. In this paper a worst or critical condition is considered where one side ∗
Corresponding author. Tel.:+01674145995 E-mail address: [email protected]
1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology.
doi:10.1016/j.proeng.2017.08.179
518
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
of a bulkhead of an oil tanker remains fully loaded and other side is empty. This type of loading condition is applicable for collision bulkhead and also for engine room bulkhead. The density of crude oil is 870 to 920 kg/m3 . But here water has been considered. Bulkhead position in the Oil Tanker and image of the bulkhead under construction are given below in Figure 1(a) and Figure 1(b) respectively.
Fig. 1: (a) Position of bulkhead in the oil tanker;(b) Figure of the bulkhead under construction
This Oil Tanker has just launched on 1st July, 2016. The principle particulars of the Oil Tanker are length over all 93.5 m, breadth 17m, draft 4.5 m and deadweight 4980 tonne. The thickness of plates at upper portion is 9 mm and at lower portion it is 12 mm. Vertical stiffeners are of 9 mm thickness. Brackets have size 400×400×12 FL- 90. There are different sized horizontal stiffeners at different water level and shown in Figure 3. All these scantlings are selected based on GL Rule book [? ]. 3. Designing the model A 3D model of the bulkhead has been created using modelling software named Rhino 3D. Some considerations have made in the model. As the bulkhead is symmetric with respect to the centre line of the vessel, half of its construction has been represented in the model. The 3D model view has shown below in Figure 2(a).Only the half of the bulkhead is taken for analysis due to symmetry. Upper side of the bulkhead will be welded with the deck and so will be the lower side with the double bottom. Again the side toward the ship centre line will be welded to centre longitudinal bulkhead and the other side will be welded to the inner shell plate. As there will be structures at all sides around the bulkhead, it is considered that all sides are fixed with those structures. Boundary conditions are shown in Figure 2(b).
Fig. 2: (a) 3D model of the bulkhead;(b) Applied boundary conditions
After the complete structure is modelled, it is needed to generate mesh which is a very important task. Meshing means discretization of geometry into small pieces called finite elements. The structure has many curved edges, sharp corners and meshing in such areas is very difficult because all the elements must satisfy the minimum criteria like aspect ratio, internal angle etc. The FE code used for the analysis is ABAQUS/Standard[? ]. This is a widely used
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
commercial finite element code for structural and other analyses. This code was preferred, because it is versatile, provides an extensive library of elements and several options for nonlinear solutions. The shell elements used for the modelling are four node quadrilaterals and three-node triangles. Four-node elements use a full integration scheme based on a variation of the Assumed Strain Method. They are intensively tested in the past for their accuracy and convergence, in both linear and nonlinear applications. Three-node elements use the reduced integration scheme since they predict thick shell behaviour more accurately than fully integrated triangles. Extensive shape tests were performed on all elements used in order to ensure their conformity with specific geometric constraints. These tests are essential for an accurate prediction of node displacements and stress/strain convergence [? ? ]. A meshed bulkhead is shown in Figure 3(a) & 3(b).
Fig. 3: (a) 3Generated mesh in the bulkhead;(b) zoomed view of mesh
As it is an oil tanker so the bulkhead is subjected to hydrostatic pressure. Hydrostatic pressure acts normal to the submerged surface of the structure, with its magnitude varying linearly with height. 4. Analysis and Discussion To investigate stress distribution over the bulkhead structure for finding the weak point, the model has been analysed using simulation software ABAQUS. Distributed stress over the structure has shown in Figure 4(a) from the analysis. Figure 4(b) shows maximum stressed region at the bracket corner with zoomed view.
Fig. 4: (a) Stress before modification; (b) Stress after modification
For convergence of results, this model has been analysed several times by increasing the elements numbers. The results achieved from these convergence tests are shown in Table-1. Here maximum stress has occurred in the joint of bracket and horizontal stiffener for all analysis and Figure 4 (b) shows the maximum stressed region marked with red circle. In all these cases the maximum stress remained almost same. The overall factor of safety is less than 2. It can be improved by reducing the stress on that region. As stress is inversely proportional to the area so it is possible to lessen the stress by modifying the bracket area. The bracket
519
520
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
Table 1: Results from different analysis of primary model Analysis No. 1 2 3
Elements No.
Max. stress (N/mm2 )
Min. stress (N/mm2 )
Factor of safety
9000 17000 46000
153.73 152.004 150.98
0.00019 0.00028 0.00365
1.53 1.55 1.57
design has been modified and the model again has been analysed with the new bracket. Initial and modified design of brackets is shown in Figure 5(a) and 5(b).Table-2 shows the results of convergence tests for the modified design and Figure 6(a) and 6(b) show the stress distribution. Table 2: Convergence test of the modified model
Analysis No. 1 2 3
Elements No.
Max. stress (N/mm2 )
Min. stress (N/mm2 )
Stress in the joining (N/mm2 )
Factor of safety
15000 24500 46000
108.00 137.80 138.90
0.00406 0.00490 0.00283
87.482 98.485 101.758
2.69 2.38 2.31
Fig. 5: (a) Initial bracket ; (b) Modified bracket
Fig. 6: (a) Stress before modification ; (b) Stress after modification
Stresses in the joining region of other brackets and stiffeners were also high in the initial model. But with the modified bracket stresses over these regions have also been reduced. A comparison of stresses over these regions between the two models has been shown in Figure 7.
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
Fig. 7: Comparison of stress in the joining of bracket
5. Conclusion This paper has described the method of using finite element analysis for the investigation of structural performance of an oil tanker bulkhead. There were two objectives of this study, evaluation of level of factor of safety that classification society provides and application of finite element analysis to modify structural design in small details. It is observed from this study that classification society rules usually provide structures with factor of safety more than 2.5. But due to design inadequacy, which class rule does not instruct, this target of factor of safety may not be achieved. Such as, for this example ship, only 1.5 factor of safety achieved near bracket. An overall 2.3 factor of safety is achieved from the example minor adjustment in bracket. Other option of bracket orientation is currently under investigation. Ongoing study includes effect of addition of vertical stiffener to resist bending tendency during movement and welding of the edges of the bulkhead, finding out the best possible orientation of stiffeners and comparison of bulkhead designs for oil tanker, general cargo vessel, bulk carrier etc. Acknowledgements The authors wish to thank Higher Education Quality Enhancement Project of University Grant Commission, Bangladesh for providing research facility under CP#3131. References [1] [2] [3] [4] [5]
Suman Kar, D.G. Sarangdhar, G.S. Chopra, Analysis of ship structures using ANSYS, ANSYS Conference, 2008. Joˇsko Parunov, Tomislav Uroda, Ivo Senjanovic, Structural Analysis of a General Cargo Ship, Brodogradnja, 61 1(2010) 28-33. Germanisher Lloyd, Rules for Classification and Construction, 2009. Simulia ABAQUS, Analysis User Guide, 2013. M. Mahendran, Applications of finite element analysis in structural engineering, Proceedings International Conference on Computer Aided Engineering, Chennai, India, (2007) 38-46. [6] Dimitris Servis, George Voudouris, Manolis Samuelides, Apostolos Papanikolaou, Finite element modeling and strength analysis of hold No. 1 of bulk carriers, Marine Structure, Elsevier, 16 8 (2003) 601-626.
521
ScienceDirect Procedia Engineering 194 (2017) 517 – 521
10th International Conference on Marine Technology, MARTEC 2016
Simulation Based Structural Analysis of an Oil Tanker Bulkhead Md. Shahidul Islam1 , Md.Humayun Kabir1 , Sharat Chandra Paul1, a Department
of Naval Architecture and Marine Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh b Offshore and Marine Engineering Ltd., Bangladesh
Abstract Traditional way of ship structural design is to follow the rule books of classification societies. These rule books certainly ensure enough structural integrity of ships. But in this age of scientific and engineering progress, reliable structural analysis programs and tools are available. This paper shows how these programs can be used to verify classification society rules and to accurately design in small details. A bulkhead of an oil tanker is chosen for this investigation. The analysis is done taking Von Misses stress and factor of safety as design criteria. The results of this investigation aim to help the traditional structural engineers to design ship structures in a more effective and optimized way. © by Elsevier Ltd. This is an openLtd. access article under the CC BY-NC-ND license c 2017 2017Published The Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology. Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology. Keywords: Structural Analysis; Classification Society; Finite Element Method; Bulkhead
1. Introduction Provision of ’adequate’ strength in a ship at a reasonable cost, has always been one of the most challenging task for the ship designers. Structural engineers design ship structures on the basis of their experience and basis ship during conceptual design. In real phase, they have to comply with the applicable rules as well as minimum requirements of the concerned classification society. Over the years the classification societies have been providing the necessary standards to ensure the adequacy of strength against all demands that can be envisaged during service life of the ship. The structural engineers also have to pay a good attention to the purpose of the ship and economic aspect. With the advancement of Numerical analysis methods and availability of computing facility at affordable costs, use of such methods provides more reliable and direct assessment. Finite Element Analysis (FEA) is a universally known structural analysis tool in use today to check such structural concerns [? ? ]. This paper presents a procedure of Finite Element Analysis of ship structure as a way to ensure its structural safety. This type of analysis will help the concerned people to improve the factor of safety of a structure as well as to eliminate the possible weak points from the structure in an effective way. 2. Problem description During the operation of a vessel, it faces different kinds of loading. Sometimes it will be fully loaded, sometimes will be empty and sometimes partially loaded. In this paper a worst or critical condition is considered where one side ∗
Corresponding author. Tel.:+01674145995 E-mail address: [email protected]
1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 10th International Conference on Marine Technology.
doi:10.1016/j.proeng.2017.08.179
518
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
of a bulkhead of an oil tanker remains fully loaded and other side is empty. This type of loading condition is applicable for collision bulkhead and also for engine room bulkhead. The density of crude oil is 870 to 920 kg/m3 . But here water has been considered. Bulkhead position in the Oil Tanker and image of the bulkhead under construction are given below in Figure 1(a) and Figure 1(b) respectively.
Fig. 1: (a) Position of bulkhead in the oil tanker;(b) Figure of the bulkhead under construction
This Oil Tanker has just launched on 1st July, 2016. The principle particulars of the Oil Tanker are length over all 93.5 m, breadth 17m, draft 4.5 m and deadweight 4980 tonne. The thickness of plates at upper portion is 9 mm and at lower portion it is 12 mm. Vertical stiffeners are of 9 mm thickness. Brackets have size 400×400×12 FL- 90. There are different sized horizontal stiffeners at different water level and shown in Figure 3. All these scantlings are selected based on GL Rule book [? ]. 3. Designing the model A 3D model of the bulkhead has been created using modelling software named Rhino 3D. Some considerations have made in the model. As the bulkhead is symmetric with respect to the centre line of the vessel, half of its construction has been represented in the model. The 3D model view has shown below in Figure 2(a).Only the half of the bulkhead is taken for analysis due to symmetry. Upper side of the bulkhead will be welded with the deck and so will be the lower side with the double bottom. Again the side toward the ship centre line will be welded to centre longitudinal bulkhead and the other side will be welded to the inner shell plate. As there will be structures at all sides around the bulkhead, it is considered that all sides are fixed with those structures. Boundary conditions are shown in Figure 2(b).
Fig. 2: (a) 3D model of the bulkhead;(b) Applied boundary conditions
After the complete structure is modelled, it is needed to generate mesh which is a very important task. Meshing means discretization of geometry into small pieces called finite elements. The structure has many curved edges, sharp corners and meshing in such areas is very difficult because all the elements must satisfy the minimum criteria like aspect ratio, internal angle etc. The FE code used for the analysis is ABAQUS/Standard[? ]. This is a widely used
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
commercial finite element code for structural and other analyses. This code was preferred, because it is versatile, provides an extensive library of elements and several options for nonlinear solutions. The shell elements used for the modelling are four node quadrilaterals and three-node triangles. Four-node elements use a full integration scheme based on a variation of the Assumed Strain Method. They are intensively tested in the past for their accuracy and convergence, in both linear and nonlinear applications. Three-node elements use the reduced integration scheme since they predict thick shell behaviour more accurately than fully integrated triangles. Extensive shape tests were performed on all elements used in order to ensure their conformity with specific geometric constraints. These tests are essential for an accurate prediction of node displacements and stress/strain convergence [? ? ]. A meshed bulkhead is shown in Figure 3(a) & 3(b).
Fig. 3: (a) 3Generated mesh in the bulkhead;(b) zoomed view of mesh
As it is an oil tanker so the bulkhead is subjected to hydrostatic pressure. Hydrostatic pressure acts normal to the submerged surface of the structure, with its magnitude varying linearly with height. 4. Analysis and Discussion To investigate stress distribution over the bulkhead structure for finding the weak point, the model has been analysed using simulation software ABAQUS. Distributed stress over the structure has shown in Figure 4(a) from the analysis. Figure 4(b) shows maximum stressed region at the bracket corner with zoomed view.
Fig. 4: (a) Stress before modification; (b) Stress after modification
For convergence of results, this model has been analysed several times by increasing the elements numbers. The results achieved from these convergence tests are shown in Table-1. Here maximum stress has occurred in the joint of bracket and horizontal stiffener for all analysis and Figure 4 (b) shows the maximum stressed region marked with red circle. In all these cases the maximum stress remained almost same. The overall factor of safety is less than 2. It can be improved by reducing the stress on that region. As stress is inversely proportional to the area so it is possible to lessen the stress by modifying the bracket area. The bracket
519
520
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
Table 1: Results from different analysis of primary model Analysis No. 1 2 3
Elements No.
Max. stress (N/mm2 )
Min. stress (N/mm2 )
Factor of safety
9000 17000 46000
153.73 152.004 150.98
0.00019 0.00028 0.00365
1.53 1.55 1.57
design has been modified and the model again has been analysed with the new bracket. Initial and modified design of brackets is shown in Figure 5(a) and 5(b).Table-2 shows the results of convergence tests for the modified design and Figure 6(a) and 6(b) show the stress distribution. Table 2: Convergence test of the modified model
Analysis No. 1 2 3
Elements No.
Max. stress (N/mm2 )
Min. stress (N/mm2 )
Stress in the joining (N/mm2 )
Factor of safety
15000 24500 46000
108.00 137.80 138.90
0.00406 0.00490 0.00283
87.482 98.485 101.758
2.69 2.38 2.31
Fig. 5: (a) Initial bracket ; (b) Modified bracket
Fig. 6: (a) Stress before modification ; (b) Stress after modification
Stresses in the joining region of other brackets and stiffeners were also high in the initial model. But with the modified bracket stresses over these regions have also been reduced. A comparison of stresses over these regions between the two models has been shown in Figure 7.
Shahidul Islam et al. / Procedia Engineering 194 (2017) 517 – 521
Fig. 7: Comparison of stress in the joining of bracket
5. Conclusion This paper has described the method of using finite element analysis for the investigation of structural performance of an oil tanker bulkhead. There were two objectives of this study, evaluation of level of factor of safety that classification society provides and application of finite element analysis to modify structural design in small details. It is observed from this study that classification society rules usually provide structures with factor of safety more than 2.5. But due to design inadequacy, which class rule does not instruct, this target of factor of safety may not be achieved. Such as, for this example ship, only 1.5 factor of safety achieved near bracket. An overall 2.3 factor of safety is achieved from the example minor adjustment in bracket. Other option of bracket orientation is currently under investigation. Ongoing study includes effect of addition of vertical stiffener to resist bending tendency during movement and welding of the edges of the bulkhead, finding out the best possible orientation of stiffeners and comparison of bulkhead designs for oil tanker, general cargo vessel, bulk carrier etc. Acknowledgements The authors wish to thank Higher Education Quality Enhancement Project of University Grant Commission, Bangladesh for providing research facility under CP#3131. References [1] [2] [3] [4] [5]
Suman Kar, D.G. Sarangdhar, G.S. Chopra, Analysis of ship structures using ANSYS, ANSYS Conference, 2008. Joˇsko Parunov, Tomislav Uroda, Ivo Senjanovic, Structural Analysis of a General Cargo Ship, Brodogradnja, 61 1(2010) 28-33. Germanisher Lloyd, Rules for Classification and Construction, 2009. Simulia ABAQUS, Analysis User Guide, 2013. M. Mahendran, Applications of finite element analysis in structural engineering, Proceedings International Conference on Computer Aided Engineering, Chennai, India, (2007) 38-46. [6] Dimitris Servis, George Voudouris, Manolis Samuelides, Apostolos Papanikolaou, Finite element modeling and strength analysis of hold No. 1 of bulk carriers, Marine Structure, Elsevier, 16 8 (2003) 601-626.
521