Design Development of Corrugated Bulkheads TSCF 2010 Shipbuilders Meeting 27 October 2010 Nippon Kaiji Kyokai 1 Topics Purpose of corrugated bulkheads Structural types of corrugated bulkheads Types of damages Design development Latest rule requirements Conclusion 2 1
Typical Hull Structure of Tankers Small Large 10,000 t 20,000 t 30,000 t 50,000 t 100,000 t DWT 3 Purpose of corrugated bulkheads Plane surface as cargo tank boundary - Complete cargo tank washing - Minimize the cargo residue - Maximum cargo tank capacity 4 2
Next Topic Purpose of corrugated bulkheads Structural types of corrugated bulkheads Types of damages Design development Latest rule requirements Conclusion 6 Structural types of corrugated bulkheads Types of cargo tank bulkhead Vertically Corrugated BHD Vertically Corrugated BHD Horizontally Corrugated BHD Horizontally Corrugated BHD Small Large Small Large Tank cleaning Tank capacity : Very good : Good 7 3
Structural types of corrugated bulkheads ships 60 Types of Longitudinal Corrugated Bulkheads ships 60 Types of Transverse Corrugated Bulkheads 50 40 Horizontal Vertical 50 40 Horizontal Vertical 30 30 20 20 10 10 0 0 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Horizontally Corr. Longitudinal BHD Transverse BHD Horizontally Corr. Vertically Corr. Number of ships (ClassNK) delivered in 1990-2008 Vertically Corr. 8 Next Topic Purpose of corrugated bulkheads Structural types of corrugated bulkheads Types of damages Design development Latest rule requirements Conclusion 9 4
Typical damages of corrugated bulkheads Corru. BHD Cargo Tank Inner Bottom Stress concentration at toes of fillet welding Corru. BHD Inner Bottom 10 Typical damages of corrugated bulkheads Cause of damages Defects of welding (Overlapping, Undercutting) Lack of supporting structures Lack of continuity (misalignment) 11 5
Next Topic Purpose of corrugated bulkheads Structural types of corrugated bulkheads Types of damages Design development Latest rule requirements Conclusion 12 Design development 1. Design of supporting structures Stiffeners under corrugation flange Floors and girders under corrugation flange 13 6
Design of supporting structures Effectiveness of supporting structure Case 1 2 3 4 5 d0 Depth of stiffener / d0 0.31 0.46 0.62 0.92 2.06 1.4 1.2 Stiffener Floor d Stress (mises) 1.0 0.8 0.6 0.4 0.2 Floor d 0 =d(double btm depth) 0.0 0.0 0.5 1.0 1.5 2.0 2.5 Stiffener depth/d 0 Relative stress at the corner of corrugation 14 Design of supporting structures Outcome; Stress of end connection decreases by deeper supporting structure Half of corrugation depth is considered as effective support depth Floors/girders are suitable as supporting structure Suitable supporting structure under flange plate of corrugated bulkhead gives higher reliability 15 7
Design development 2. Application of full penetration welding Corrugated BHD Fillet welding Full penetration welding Detail stress distribution of corrugation corner Full penetration welding has been recently applied at the corner of corrugation of lower end connection with weld toe grinder 16 Design development Effectiveness of full penetration welding Fixed Tensile load Gap Fixed Fixed Comparative stress evaluation z - by bending load and tensile load - with various x gaps 17 8
Von-Mises stress caused by bending / tensile load Bending load Full penetration Gap 0mm (Fillet) Gap 2mm Tensile load 18 Von-Mises stress by tensile load 3.5 3.0 2.5 Tensile load Bending load A Gap 2.0 1.5 1.0 0.5 0.0 Full Pene. Gap_0mm Gap_2mm Gap_4mm Gap_6mm Gap_8mm Relative stress at Position A (Stress of Full Penetration = 1.0) 19 9
Experimental test of stainless steel (SUS316L) Tensile test (as weld) Tensile test (as weld) Higher tensile load bending test (as weld) Weld toe grinding Tensile test (Grinder) Tensile test (Grinder) (ClassNK Research Institute) Higher tensile load 20 Application of full penetration welding Outcome; Cracks by bending load initiate from weld toe regardless of root gap Cracks by tensile load initiate from weld toe or root Root gaps lead to higher stress at weld toe and root by tensile load Gap control and satisfactory throat thickness are important Full penetration welding contributes ; - to mitigate the risk of gap control - to give satisfactory throat thickness Grinder or TIG welding which gives more smooth surface further contributes to lower the stress concentration at weld toe Full penetration welding and smoothed weld toe at corrugation corner gives higher reliability 21 10
Design development 3. Verification of designs with gussets and shedder plates Shedder plate Shedder plate 1200 mm 1500 mm Gusset plate 300 mm 1500 mm Gusset plate Lower stool Lower stool 300 mm Lower stool Impact by gusset and shedder plates were investigated 22 Impact to the design by gusset and shedder plates 1. Reduced stress at critical point Lower stool 2. Stress concentration at the crossing of shedder plates Lower stool Lower stool Bracketed stiffener 3. Extended enclosed space leads to a loss of cargo capacity 23 11
Reinforcement by gusset and shedder plate Outcome; Gusset and shedder plates are often used in the design of corrugated bulkheads in bulk carriers and recommended in CSR for tankers. Gusset and shedder plates definitely contribute to lower the stress However, following deep considerations are necessary ; - Manholes and air holes to avoid formation of any gas pocket - Bracketed Stiffener where adjacent shedder plate cross - Careful workmanship of welding in narrow space Extended gusset plate leads to some loss of cargo capacity Better to keep just as recommendation instead of mandatory requirement in tanker designs 24 Next Topic Purpose of corrugated bulkheads Structural types of corrugated bulkheads Type of damages Design development Latest rule requirements Conclusion 25 12
CSR Requirements Prescriptive requirements : Local strength (Plate thickness, Section Modulus) Arrangement of supporting structures Full penetration welding to lower end of vertical corrugated bulkhead connections Corner Part Full penetration welding + Grinder Recent practice All Part Full penetration welding CSR 26 CSR Requirements Requirements of Finite Element Analysis (FEA): Overall stress and buckling analysis by Coarse Mesh Detail stress analysis of lower end connection of vertically corrugated bulkhead by Fine Mesh Target Tank 27 13
Sensitivity of mesh size at corrugation corner Parametric analysis to know the sensitivity of mesh size Coarse Mesh Fine Mesh (50mmx50mm) Very Fine Mesh (17mm x 17mm) Web Flange Coarse mesh 50mm Fine mesh Very fine mesh X mm 28 Sensitivity of mesh size at corrugation corner P S MR Tanker Load Case : B4-1 (Zig-Zag) Thickness : 22mm(net) S.G.(t/m 3 )=0.8, 1.025, 1.3, 1.5, 1.85 γ=1.85 γ=1.50 γ=1.30 γ=1.025 Von-Mises stress depending on mesh size N/mm2 Coarse Mesh Very Fine Mesh Fine Mesh γ=0.80 350 300 250 200 150 100 50 Distance from corrugation corner (mm) 900 800 700 600 500 400 300 200 100 0 0 Element stress strongly depends on the mesh size near the corner Definition of mesh size is important when detail analysis is required 29 14
Impact on scantlings by CSR Fine Mesh Case studies to investigate the impact of CSR criteria - MR Tanker - Transverse Corrugate Bulkhead (Vertical) - Critical Load Case : B4-5a (Zig-Zag) - S.G. = 1.025 P S Ships for study Ship A : New design vessel which fully complies with CSR Ship B : Non-CSR ship with successful service experiences without any damage records Ship C : Non-CSR ships with damage records Ship C_with bracket : After reinforcement of ShipC without any further damage records 30 Impact on scantlings by CSR Fine Mesh N/mm2 800 700 600 500 400 300 200 100 Ship A CSR-T requirement (Coarse mesh, HT32) Ship C CSR-T requirement (Fine mesh, HT32) Ship C_with bracket Ship B ship A (fine) ship B (fine) ship C (fine) ship C_withbracket ship A (coarse) ship B (coarse) ship C (coarse) 0 (mm) from the corner -600-400 -200 0 200 400 600 31 15
Impact on scantlings by CSR Fine Mesh CSR contribute to more robust corrugated bulkhead designs Feedback from Japanese shipyards Considerable scantling increase even from designs having demonstrated history of successful service experiences Required thickness tends to reach to the maximum allowable thickness for the fabrication of cold-bending Modification of corrugation span by fitting upper/lower stools or reduction of design cargo density has been required, which inevitably leads to a loss of cargo capacity and deadweight Further rule development considering such as edge treatment and anti-fatigue steel is anticipated 32 Conclusion Corrugated bulkheads are essential structure for product / chemical tankers Sufficient service records have proved the advantage Rules and designs have been improved to cover complexities of fabrications and operations However it has been also reported that designers need to modify the initial design to save unacceptable scantling increase due to latest CSR requirements Continuous update of rules allowing establishment of new technologies should also be performed, while watching valuable service experiences of existing designs 33 16
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