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

DNV’sHull Structure course for

World Wide Shipping

7 - Hull Structure – cargo areaBottom

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 13


Longitudinal stress

Hot spotBilge keel

16.03.2004Slide 14


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






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






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






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

16.03.2004Slide 30

2. Bottom

Questions ?

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

Documents

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