07 - DNVs Hull Str for WW - Hull Str - Cargo Area - Bottom

DNV’s Hull Structure course for World Wide Shipping 7 - Hull Structure – cargo area Bottom

Hull Structural Breakdown Bottom 1. 2. 3. 4. 5. 6.

Slide 2

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

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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

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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

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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

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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

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Cracking in hopper knuckle

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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

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2.

Questions ?

Slide 30

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Bottom