07 - dnvs hull str for ww - hull str - cargo area - bottom
TRANSCRIPT
16.03.2004Slide 2
2. BottomHull Structural Breakdown -Bottom
Bottom
Transverse bulkheadLongitudinal bulkheadWeb frames
1. Side2.
Deck3.
4.
5.
6.
16.03.2004Slide 3
2. BottomStructural functions of 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
16.03.2004Slide 4
2. BottomStructural build up of bottom –single skin tanker
Bottom platingw/longitudinals
Web frameCL girder
Bilge
Keel plate
16.03.2004Slide 5
2. BottomStructural build-up of a double bottom structure
Bottom plating withlongitudinals
Buttress
Inner bottom plating (tank top) with longitudinals
Transversegirder / floor
CL double bottom girder
Outboard girder(margin girder)
Hopper plating withlongitudinals
Hopper web plating
16.03.2004Slide 6
2. BottomFunction: 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
16.03.2004Slide 7
2. BottomFunction: Watertight integrity
Bottom plating with longitudinals are also acting as flange for the transverse web frame
Transverse bottom girder/web frame is supported at thelongitudinal bulkheads (max. shear force towards long. bhds.)
16.03.2004Slide 8
2. BottomBottom is supported by ship side and longitudinal bulkhead
Double span for double bottom without CL longitudinal
bulkhead
Shear stress in double bottom floordue to external seapressure
16.03.2004Slide 9
2. BottomFunction: Flange in hull girder
Global bending moment induces longitudinal stresses in the bottom plating and longitudinals
Longitudinal stresses (+/-) are acting in the bottom plating and longitudinals due to bending of hull girder
Section A-A
16.03.2004Slide 10
2. BottomDouble bottom structure
The double bottom is a grillage structure built up by transverse girders/floors and longitudinal girders
Double bottom transversegirder (web frame) as a single I-beam
Sea pressure
Shear force
For the transverse girder, high shear stresses are induced towards the ship side and longitudinal bulkhead
With few longitudinal girders, double bottom stresses resulting from external sea pressure are mainly transferred in the transverse direction
Shear force
16.03.2004Slide 11
2. BottomCharacteristic damages
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
16.03.2004Slide 12
2. BottomBilge keel cracking
Oil Tanker285,690 DWT built 1990
Crack in hull plating i.w.o. bilge keel terminations
Crack in hull plating in way of bilge keel toes
Bilge keel
16.03.2004Slide 14
2. BottomBilge keel cracking
10-15mm
Web frame/BilgeBracket
1600
Bilge Keel
Pad plate
200
Ship side
100
All measures in mm
125Edges to be grinded
smooth
25100
16.03.2004Slide 15
2. BottomCharacteristic damages
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
16.03.2004Slide 16
2. BottomCracking in bottom longitudinals
Bottom long. flat bar connection
Bottom long. tripping bracket
connection
Similar cracking in bottom longitudinals is alsovalid for double hull tankers
16.03.2004Slide 17
2. BottomCause for cracking in bottom longitudinals
1. Local stress from lateral dynamic sea loading
2. Longitudinal stresses from hull girder bending
Bottom longitudinals are subject to both:
MM
WebWeb/Trans bhd
p
16.03.2004Slide 18
2. BottomConsequences of cracks in bottom longitudinals:
-Leakage of oil
- Crack may propagate further into bottom plating and induce a larger transverse fracture
16.03.2004Slide 19
2. BottomExample: Cracks in inner bottom
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)
Oil Tanker95,371 DWT
Crack in tank top plating at toes of transverse bulkhead buttress P/S
Crack in bracket toe
16.03.2004Slide 20
2. BottomCracking in double bottom longitudinals
Cracks in flatbar connections for bottom and innerbottom longitudinals
16.03.2004Slide 21
2. BottomCause for cracking in double bottom longitudinals
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
16.03.2004Slide 22
2. BottomIllustration – double bottom flatbarconnections
Tensile stresses in critical structural details
The double bottom structure is exposed to large forces both in ballast and loaded condition
16.03.2004Slide 23
2. BottomCharacteristic damages
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
16.03.2004Slide 24
2. BottomCorrosion of bottom structures
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 top plating
16.03.2004Slide 25
2. BottomCorrosion of bottom structures
- 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
AF
L =σ
Increased risk for fatigue cracking and buckling ofbottom panels if general corrosion has developedover the cross section
Longitudinal stress
Area
Force
16.03.2004Slide 26
2. BottomCharacteristic damages
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
16.03.2004Slide 27
2. BottomCracking in hopper knuckle
Crack in hopper knuckle at web frame connections
16.03.2004Slide 28
2. BottomCause for cracking in hopper
knuckle
Bending moment
- 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)
σ L
σ L
Bending stress in double bottom girderBending stress in
inner bottom plating
16.03.2004Slide 29
2. BottomCause for cracking in hopper
knuckle
- 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
- This effect together with the knuckle being a geometric ‘hard point’ at web frame connections, induce very high stresses in the knuckle point
Un-balanced stress component
Membrane stress from bending of transverse girder
Resulting membrane stress in hopper plating