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A Study on Support Arrangement of a Cargo Tank for Tank Type A LPG Ships

The article contains info about practical support arrangement of a cargo tank for Tank Type-A Liquefied Petroleum Gas carriers.

Purpose of the Study

  • To provide information regarding optimised and practical design application to the support arrangement of a cargo tank.
  • Consequences of reduction in number of supports with respect to scantlings of the cargo tank and double bottom structures
Tank support
Works in the tank
Man under the tank
Tank support
Scheme of the Tank
Typical Prismatic Tank Type A LPG

Tank support arrangement & type of supports

Support Arrn’t at Mid C/T I/B

Tank support arrangement

Scheme of tank support arrangement
Example of a tank support arrangement
  • Vertical supports arranged at each web frames.
  • Transverse supports arranged at CL of each web frames.
  • Anti-pitching supports located at mid length of a cargo tank.

Vertical Support

Vertical supports - scheme
Example of vertical supports
Tank support construction
Support construction
  • To be designed to prevent hull structures from excessive stress concentration.
  • Vertical supports are subject to horizontal forces due to friction.
  • Strength of wood and resin to be checked in view of compressive strength.
Vertical Support of the Tank
Type of Vertical Support

Transverse Key & Support

To prevent possible damage due to thermal expansion and contraction:

Extruded part
Extruded part, key, to be fitted at cargo tank side
Gap in the support construction
At least 1~2 mm gap between wood and steel plate
Scheme of support
Scheme of support between upper deck and cargo tank

The collision forces acting on the cargo tank should correspond to acceleration:

  • 0,5 g in the forward direction,
  • 0,25 g in the aft direction.
Scheme: Anti-pitching Key & Support
Anti-pitching Key & Support

Anti-floating Key & Support

  • To be suitable to withstand an upward force caused by an empty cargo tank in a hold space due to flooding.
  • Normally, arranged at upper slope area of a cargo tank.
  • Some design is arranged at end bulkheads of a cargo tank.

Investigation of existing designs of Tank Type A LPG ships

Type of support

Photo of the tank construction
Design of tank support

Existing Tank Type A Designs

  • Principal particulars.
  • Design parameters.
ShipABCD
Cargo volume, m382 00060 00038 00023 000
LBP, m212195172155
Breadth, m36,632,229,226,2
Depth, m2220,818,215,3
Scantling draught, m12,5512,110,48,4
Block coeff., Cb0,780,7670,7790,734
Number of cargo tank4433
Tank length, m37,9232,9542,336,65
Design density, t/m30,610,690,700,68
Web frame spacing, m3,363,23,22,4
  • Number of keys/supports.
ShipABCD
Vertical supports (P & SB)Tank148144112154
Hull148144112154
Transverse keys & supports (CL)Tank82826482
Hull82826482
Anti-pitching keys & supportsTank16161212
Hull16161212
Anti-floating SupportTank828242
Hull4282
Total574566460578
Block-scheme: optimization of supports
Procedure for Optimization of Supports
  • Model extent is no. 3 plus half of tank no. 2 and 4 (1/2 + 1 + 1/2).
  • The supports are interconnected by beam element for material of wood between cargo tank and hull structure.
  • All structural elements are modelled based on the net scantlings.
Scheme and 3D model of LPG carrier
Global Cargo Hold FE Modeling

Design Load Cases

Support TypeLoad caseLoading ConditionTank loadSea pressure
staticSta. + dynstaticSta. + dyn
Vertical supportLC1Full load condition
Draught: Tsca.
X
(10-8 level)
XLC1
LC2Full load condition
Draught: Tsca.
XX
(10-8 level)
LC2
LC3Full load condition
Draught: Tact.
X
(10-8 level)
XLC3
Transverse supportLC4Heeled condition
of 30 deg.
X
(10-8 level)
XLC3
  • Design load cases based on actual loading manual.
  • The loads are calculated for a 20 years return period in the North Atlantic.
  • Double bottom structures must be considered for:
    • maximum net internal loads (downwards), LC1 & LC3;
    • maximum net external loads (upwards), LC2.

Verification of Applied Loads

Vertical support
Load caseLC1LC2LC3
Tank weight, ton1 1891 1891 189
Cargo weight, ton19 92013 33719 920
Total weight, ton21 10914 52621 109
Sum of reaction force, ton21 11014 53021 108
Model of the tank
Weight of tank
Transverse support
Load caseLC4
Volume of tank, m321 779
Max. transverse acceleration, ay0,6605*g
Applied transverse load, ton8 762
Sum of reaction force, ton8 761
3D model of transverse load
Transverse load

Ship A, 82 000 m3 LPG – Vertical Supports

Scheme: Vertical support arrangement
Vertical support arrangement of ship A
Calculated reaction forces in supports (ton)
Pos.123457891011Total
LC1PortGIR.B1 06854542437753750836441755293421 110
GIR.A855455383355446444357389461727
StbdGIR.A822426375350444442353379421708
GIR.B1 076551428380547518368420551948
LC2PortGIR.B27431227127545743126827032317614 530
GIR.A385389401417571565417402390305
StbdGIR.A386372388403557550403389365311
GIR.B283317272275466438269271323190
LC3PortGIR.B1 4415914523865044753644285761 30221 108
GIR.A1 060365247201220212193248367968
StbdGIR.A1 009337247201220221201248325950
GIR.B1 4445964563915114833694335741 306
MaxGIR.A1 060455401417571565417402461968
GIR.B1 4445964563915475183694335761 306
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Reaction force distribution in supports

Max. reaction force & Design loads:

  • Type VA: 596 ton => 650 ton.
  • Type VB: 1 060 ton => 1 100 ton.
  • Type VC: 1 444 ton => 1 500 ton.
Support type
Type of support
Reaction force
Reaction force
Girder B
Reaction force: Girder B
Girder A
Reaction force: Girder A

Effect of Hull Girder Bending – LC1 & LC3

Girder Bending – LC3
Girder A
Effect of Girder Bending – LC3
Girder B

Sagging moment will increase support forces close to TBHD.

Interaction of cargo tank and double bottom structure
Interaction of cargo tank and double bottom structure due to sagging moment
LC3123457891011
PortGIR.B107 %97 %93 %89 %87 %86 %89 %93 %96 %108 %
GIR.A109 %101 %100 %99 %95 %96 %98 %99 %101 %110 %
StbdGIR.A109 %100 %100 %99 %96 %97 %99 %100 %99 %112 %
GIR.B107 %97 %94 %90 %87 %87 %89 %93 %96 %108 %

Effect of Hull Girder Bending – LC2

Girder A LC2
Girder A LC2
Girder B LC2
Girder B LC2

Hogging moment will increase support forces in way of mid supports.

Scheme of hogging moment
Hogging moment
LC2123457891011
PortGIR.B71 %109 %121 %130 %133 %135 %132 %123 %109 %60 %
GIR.A74 %97 %98 %100 %102 %102 %100 %99 %97 %69 %
StbdGIR.A76 %97 %98 %99 %102 %102 %100 %99 %97 %71 %
GIR.B71 %109 %120 %130 %132 %134 %132 %122 %109 %62 %

Transverse Supports

Transverse support arrangement.

Support arrangement
Example of transverse support arrangement
Pos.1234567891011Sum
LC4Upp.4783602542242051982062232614142 823
Low.8396886215906065385044854885805 938
Low. adj.6345204694464574073803673694388 761

Max. reaction force & Design loads.

Upper transverse support:

  • Type UA: 261 ton => 300 ton.
  • Type UB: 360 ton => 400 ton.
  • Type UC: 478 ton => 500 ton.

Upper transverse support:

  • Type LA: 469 ton => 500 ton.
  • Type LB: 634 ton => 650 ton.

Type of support.

Scheme of transverse support arrangement of ship A
Transverse support arrangement of ship A

Reaction Force at Transverse Supports

Transverse support arrangement.

Reaction force
Reaction to transverse support arrangement

Distribution of reaction force at keys.

Reaction force distribution
Distribution of reaction force
  • For upper part, high force at keys close to end bulkheads and gas dome.
  • For lower part, high force at keys close to end bulkhead and sump well.
  • Reaction force at lower supports is higher than upper ones.
      Applied transverse load:
    • upper part 32 %;
    • lower part 68 %.

Case studies on support arrangements

Vertical Support Arrangement – Case Study

Case Study 1
Vertical support arrangement – case 1
Case Study 2
Vertical support arrangement – case 2
Case Study 3
Vertical support arrangement – case 3
Case Study 4
Vertical support arrangement – case 4

Support Arrangement – Case 2

Case 2
Support arrangement

Vertical supports is fitted at swash bulkhead location.

Some supports at GIR.A are removed at every second.

Anti-pitching support is relocated with combined type with vertical support.

Reaction Force Distribution – Case 2

Case 2
Reaction force distribution

Reaction force is mainly redistributed into mid supports, location 4, 6 and 8.

LC1 is critical to location 2 & 10 of GIR.A.

LC1 graph
LC1

LC2 is critical to location 4, 6 & 8 of GIR.A.

LC2 graph
LC2

LC3 has little variation.

Strength - Case 2
Strength of C/T and D/B – Case 2

Shear strength capacity due to access opening.

Consequences on a cargo tank and double bottom structures

Transverse Support Arr’n – Case 1

Support arrangement
Support arr’n – Case 1

Some of upper transverse keys are removed at every second.

Lower transverse keys are kept.

Reaction Force Distribution – Case 1

Case 1 - force distribution
Force distribution

Reaction force at upper keys shows high increase, max. 55 % at location 8.

Upper keys
Reaction Force Distribution – Upper keys

Type of support is changed.

Lower keys have little variation in reaction force.

Lower keys
Reaction Force Distribution – Lower keys

Conclusion

Effect of hull girder bending in view of reaction force.

Block-scheme - effect of hull girder
Effect of hull girder

When some of the supports with low utilization are removed.

Block scheme - low utilization
Low utilization removed
Footnotes
Sea-Man

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Ноябрь, 09, 2022 48 0
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