Side Bottom Deck Transverse bulkhead Longitudinal bulkhead Web frames
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2.
Bottom
Structural functions of bottom
2.
Bottom
Watertight integrity - Resist external sea pressure - Resist internal pressure from cargo and ballast Flange in hull girder - Bottom plating and longitudinals act together as the lower flange in the hull girder beam
Slide 3
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Structural build up of bottom – single skin tanker
Bilge Bottom plating w/longitudinals Keel plate Slide 4
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CL girder
Web frame
2.
Bottom
Structural build-up of a double bottom structure
Inner bottom plating (tank top) with longitudinals
2.
Bottom
Buttress
Hopper plating with longitudinals
Hopper web plating
Outboard girder (margin girder) Slide 5
CL double bottom girder
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Bottom plating with longitudinals
Transverse girder / floor
Function: Watertight integrity External loads induce shear forces and bending moments in the bottom longitudinals, acting as single beams (between each web frame)
Bottom longitudinal as a single beam between two web frames Slide 6
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2.
Bottom
Function: Watertight integrity Bottom plating with longitudinals are also acting as flange for the transverse web frame
Transverse bottom girder/web frame is supported at the longitudinal bulkheads (max. shear force towards long. bhds.) Slide 7
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2.
Bottom
Bottom is supported by ship side and longitudinal bulkhead
2.
Bottom
Double span for double bottom without CL longitudinal bulkhead
Shear stress in double bottom floor due to external sea pressure Slide 8
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Function: Flange in hull girder
2.
Bottom
Global bending moment induces longitudinal stresses in the bottom plating and longitudinals
Section A-A Slide 9
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Longitudinal stresses (+/-) are acting in the bottom plating and longitudinals due to bending of hull girder
Double bottom structure
2.
Bottom
The double bottom is a grillage structure built up by transverse girders/floors and longitudinal girders With few longitudinal girders, double bottom stresses resulting from external sea pressure are mainly transferred in the transverse direction Sea pressure
Shear force
Shear fo
Double bottom transverse girder (web frame) as a16.03.2004 Slide 10 single I-beam
rce
For the transverse girder, high shear stresses are induced towards the ship side and longitudinal bulkhead
Characteristic damages
2.
Bottom
1. Bilge keel terminations – crack in hull plating 2. Fatigue cracking in bottom longitudinal connections to web frame and transverse bulkhead 3. Corrosion of bottom structures 4. Hopper knuckle – cracks
Slide 11
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Bilge keel cracking
2.
Bottom
Oil Tanker 285,690 DWT built 1990 Crack in hull plating i.w.o. bilge keel terminations
Bilge keel
Crack in hull plating in way of bilge keel toes
Slide 12
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Bilge keel cracking
Hot spot
Bilge keel
Longi
Slide 13
2.
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tudina l stres
s
Bottom
Bilge keel cracking
2.
Web frame/Bilge Bracket All measures in mm
125 Edges to be grinded smooth Ship side
Pad plate
10-15mm Bilge Keel 200 1600 100 25 100
Slide 14
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Bottom
Characteristic damages
2.
Bottom
1. Bilge keel terminations – crack in hull plating 2. Fatigue cracking in bottom longitudinal connections to web frame and transverse bulkhead 3. Corrosion of bottom structures 4. Hopper knuckle – cracks
Slide 15
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Cracking in bottom longitudinals
Bottom long. flat bar connection
Similar cracking in bottom longitudinals is also Slide 16 16.03.2004 valid for double hull tankers
2.
Bottom
Bottom long. tripping bracket connection
Cause for cracking in bottom longitudinals
Bottom
2.
Bottom longitudinals are subject to both: Web/ Trans bhd
Web
M
M p
1. Local stress from lateral dynamic sea loading
2. Longitudinal stresses from hull girder bending Slide 17 16.03.2004
Consequences of cracks in bottom longitudinals:
2.
Bottom
-Leakage of oil - Crack may propagate further into bottom plating and induce a larger transverse fracture
Slide 18
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Example: Cracks in inner bottom
2.
Bottom
Oil Tanker 95,371 DWT Crack in tank top plating at toes of transverse bulkhead buttress P/S
Crack in toe of big brackets connecting transverse bulkhead and tank top plating (in various cargo tanks along ships length)
Crack propagating through tank top plating (a few cases)
Slide 19
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Crack in bracket toe
Cracking in double bottom longitudinals
2.
Bottom
Cracks in flatbar connections for bottom and inner bottom longitudinals
Slide 20
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Cause for cracking in double bottom longitudinals
2.
Bottom
In a ballast condition there is a net overpressure in the double bottom ballast tank (full ballast tank and empty cargo tank) In a loaded condition there will be a negative net pressure on the double bottom (empty ballast tank, full draft and full cargo tank)
This effect may cause yield stress in hot spots at flat bar connections Due to the dynamic +/- variation of stresses, low cycle fatigue may occur Slide 21
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Illustration – double bottom flatbar connections
2.
Bottom
Tensile stresses in critical structural details
The double bottom structure is exposed to large forces both in ballast and loaded condition Slide 22
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Characteristic damages
2.
Bottom
1. Bilge keel terminations – crack in hull plating 2. Fatigue cracking in bottom longitudinal connections to web frame and transverse bulkhead 3. Corrosion of bottom structures 4. Hopper knuckle – cracks
Slide 23
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Corrosion of bottom structures
2.
Bottom
Local corrosion (pitting): may occur all over the bottom plating, but area below and around bell-mouth is particularly exposed
Pitting is also applicable for double hull tankers i.w.o. tank top16.03.2004 plating Slide 24
Corrosion of bottom structures
2.
Bottom
- Pittings and local corrosion may cause leakage, in general not any structural problem - General corrosion will reduce the bottom sectional area, which can lead to an increased stress level: 1. Higher risk for fatigue cracks in bottom longitudinals 2. Higher risk for buckling of plate fields in the bottom Longitudinal stress
Force
F σL = A Area
Increased risk for fatigue cracking and buckling of bottom panels if general corrosion has developed over the cross section Slide 25
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Characteristic damages
2.
Bottom
1. Bilge keel terminations – crack in hull plating 2. Fatigue cracking in bottom longitudinal connections to web frame and transverse bulkhead 3. Corrosion of bottom structures 4. Hopper knuckle – cracks
Slide 26
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Cracking in hopper knuckle
Slide 27
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Crack in hopper knuckle at web frame connections
2.
Bottom
Cause for cracking in hopper knuckle
2.
Bottom
- Bending of double bottom due to external and internal dynamic loads induces membrane stresses in the inner bottom (flange in the double bottom transverse girder)
σL
Bending moment
Bending stress in inner bottom plating Slide 28
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Bending stress in double bottom girder
σL
Cause for cracking in hopper knuckle
2.
Bottom
- Inner bottom membrane stresses are transferred into the hopper plating - The turn of the stress direction (inner bottom to hopper plating) results in an unbalanced stress component Resulting membrane stress in hopper plating
Membrane stress from bending of transverse girder Un-balanced stress component
- This effect together with the knuckle being a geometric ‘hard point’ at web frame connections, induce very high stresses in the knuckle point Slide 29
