Dnv hull structure course

Hull Structure Course DNV

2005

Consequenceof a crack in this detail?

Where is it likelyto find cracks?

How are theloads takenup by thestructure?

Hull Structure Course

Objective:

After completion of the course, the participantsshould have gained knowledge of basic hull strength and understanding of how to performbetter hull inspections.

Hull Structure Course

Purpose:

To train technical personnel about the basics of hull structure.

Target group is technical personnel within ship owner / manager organization in need of improved competence in structural matters, with special focus on Bulk Carriers and Oil Tankers.

Course breakdown:

Day 1 • Introduction• Single beams & loads• Structural connections• Hull structure failure typesDay 2• Fore & aft ship• Hull structural breakdown Oil TankerDay 3• Hull structural breakdown Bulk CarrierDay 4• Fore & aft ship• Hull structural breakdown Container Carrier

Agenda day 1

09.00-09.15 Welcome & Introduction09.15-09.45 Expectation & presentation of participants10.00-11.30 Beams + Buzz group11.30-12.30 Loads

12.30-13.15 Lunch

13.15-14.15 Structural connections14.15-15.45 Failure mode fatigue15.45-16.45 Buckling & Indent16.45-17.45 Corrosion17.45-18.00 Review questions

Agenda day 209.00 – 09.15 Answers to review questions09.15 – 10.30 Structural breakdown fore and aft ship10.30 – 10.45 Introduction to tank10.45 – 11.00 Coffee break11.00 – 11.45 Ship side & longitudinal bulkhead11.45 – 12.15 Webframes

12.15 – 13.00 Lunch

13.00 – 13.30 Case: Oil Tanker Part A13.45 – 14.30 Deck14.30 – 15.00 Bottom 15.00 – 15.15 Coffee break15.15 – 16.15 Case: Oil Tanker Part B16.15 – 16.45 Transverse Bulkhead16.45 – 17.00 Review quiz

Agenda day 309.00 - 09.30 Answers to review questions09.30 - 10.00 Introduction to Bulk 10.00 - 10.45 Side10.45 – 11.00 Coffee break 11.00 - 11.45 Bottom11.45 - 12.15 Deck

12.15 - 13.00 Lunch

13.00 - 13.45 Case: Side hold no 113.45 - 14.30 Transverse Bulkhead14.30 - 15.00 Hopper tank & topside tank15.00 – 15.15 Coffee break15.15 - 15.45 Hatch coaming & covers15.45 – 16.30 Case: Ore Carrier16.30 - 17.00 Review Quiz and closing

Agenda day 409.00 - 09.30 Answers to review questions from day 109.30 - 10.30 Structural breakdown fore and aft ship 10.30 - 11.00 Introduction – Container Carriers11.00 – 11.15 Coffee break 11.15 – 12.15 Bottom and Ship Sides

12.15 - 13.00 Lunch

13.00 – 14.00 Hatch Covers, Deck & Hatch Coamings14.00 – 15.00 Case: Container Carriers15.00 - 15.15 Coffee Break15.15 – 15.45 Bulkheads15.45 – 16.00 Closing16.00 – 16.30 Review Quiz

Basic Hull StrengthModule 2: Basic Hull Strength

Basic Hull StrengthObjectives

After completion of this module the participants should have gained:

1. Understanding of:The behaviour of simple beams with loads and corresponding

shear forces and moments.The applicable local and global loads on the hull girder and the

corresponding shear forces and bending moments.

Basic Hull Strength

Load

Simple beam properties

Tension

Compression

Shearforce

Shear area: The beam has to have a sufficient cross sectional area to take up the external load and transfer this towards the end supports.

Bending: When a beam is loaded it will bend dependent on its stiffnessand its end connections. A single load from above causes compressionstress on the upper side and tension stress on the lower side of the beam.

A

A

Section A-A

Bending moment

Basic Hull StrengthSimply supported beam - concentrated load

F

Single beam withconcentrated load,

simply supported ends

ShearForce

BendingMoment

F/2 F/2

M=Q x ℓ

Q=F/2

Q=F/2

ℓ

F

L/2

L/2

Basic Hull StrengthSimply supported beam – distributed load

p

L

Single beam withdistributed load,

simply supported ends

BendingMoment

ShearForce

Q=pL/2

pL/2 pL/2

Q=pL2

M=pL2/8

Basic Hull StrengthBeam with fixed ends - distributed load

L

ShearForce

BendingMoment

Single beamwith distributedload, fixed ends

p

M=pL2/24

M=pL2/12

pL/2pL/2

Q=pL/2

Q=pL/2

No rotation!

Basic Hull StrengthBeam with spring supported ends

p

Shear force and bending moment distribution varies with degree of end fixation (spring stiffness)

Degree of end fixation = 0

Spring Springkk

Degree of end fixation = 1

Simply supported

Fixed ends

Basic Hull Strength

Symmetrical load – full fixation

End fixation

Structural clamping – spring support

Basic Hull Strength

• Load on structure is important with regard to fixation bottom longs connection to transverse bulkhead

Beam – fixation at ends

Non symmetry in loads gives less fixation or even forced rotation

Symmetric load gives full fixation

LoadedEmptyEmptyEmpty Empty

Basic Hull StrengthAxial stress

Area

Force

Stress = ForceArea

σ = ε x E (Hook’s Law)

ε : Relative elongation

E : Youngs modulus (2,06E5 N/mm² - steel)

Basic Hull StrengthStress levels – elastic & inelastic region

Elastic region: σ < σyield

- A beam exposed to a stress level below the yield stress, will return to its original shape after the load is removed, Simple beam theory valid

σ

ε (elongation)

Yieldfracture

Inelastic regionIn-elastic region: σ = > σyield- A beam exposed to stresses above the yield stress will have a permanent deformation after removing the load (yielding, buckling, fractures)

Elastic region

σ = ε * E

Basic Hull StrengthHigh Tensile Steel (HTS)

Material grades NVA - NVE• Measure for ductility of material (prevent brittle fracture)• Material grade dependent on location of structure and

thickness of plate.

NVA

NVB

NVD

NVE

MS

HT28

HT32

HT36

HT40

Basic Hull StrengthBending stress - Simple beam with load

R1 R2

A

A

A

A

Section A-A

Area effective intransferring the bending of the beam

Distribution of stresscaused by bending

Max stress at flanges.Zero stress at neutral axis:

F

n.a

Basic Hull StrengthShear stress - Simple beam with load

R1 R2

A

A

A

A

Area effective intransferring load to the supports

Distribution of thestress

Max shear stress atneutral axisis of profile:

Section A-A

F

Basic Hull StrengthBending and shear stress flow

R1 R2

A

A

A

A

F

Section A-A

Shear stress is transferred in the web, τ

Compression

Bending stress is transferred in the flanges, σ

Tension

Basic Hull StrengthBeam stiffness and section modulus

As the axial stresses are transferred in the flange of a beam, it is the flange area that is governing a beam’s ‘bending stiffness’

n.a x

y

1yIZ x

x =Section modulus:

The ‘Section Modulus’ is expressing the beam’s ability to withstand bending

y1

Aflange

21

3 2121 yAblI flangex +=Moment of Inertia:

lb

XZM

=σBending Stress:

Basic Hull StrengthShear stress & shear area

The load is carried in shear towards the supports by the web

n.a x

y

thAs ⋅=Shear area :

sAQ

=τShear stress:

t h

QShear force :

Basic Hull Strength

Flatbar (slabs)Easy with regard to production, flatbar stiffeners have poor buckling strength properties, low section modulus mostly applied in deck and upper part of side - long. bhd.

Conventional profiles in ship structures

Angle bar (rolled and welded)Angle bar will twist when exposed to lateral load due to non-symmetric profile. This effect gives additional stress at supports due to skew bending. Angle bars are more prone to fatigue cracking than symmetrical profiles (Ref. sketch next page)

Due to the skew bending, which gives a moment in the web-plate at welded connection to the plate, angle bars are also more critical with regard to grooving (necking) corrosion.

Basic Hull Strength

An angle bar profile will twist when exposed to lateral loads due to asymmetric profile which gives additional stress at supports due to skew bending

Additional bending stress in web

POSTFEM 5.6-02 5 SEP 2SESAM

XY

Z

MODEL: T1-1 DEF = 2034: LINEAR ANALYSISNODAL DISPLACE ALLMAX = 1.46 MIN = 0

.696E-1

.139

.209

.278

.348

.418

.487

.557

.626

.696

.766

.835

.905

.9741.041.111.181.251.321.39

Side longs

internal pressure

Angle bar (rolled / built up)

Basic Hull Strength

Bulb profile (single / double bulb) Bulb profiles are favourable with regard to coating application.Single bulb which is most common will (as for the L-profile) have some skew bending when exposed to lateral load.

T- ProfileThe T-profile is symmetrical and will not be prone to skew bending. Favourable with regard to fatigue strength. The profilemay have large section modulus. Some T-profiles on single skin VLCC’s have been found critical with regard to buckling due to a high and thin web-plate with a small flange on top.

Conventional ship structure profiles

Basic Hull StrengthHierarchy of hull structures

Plate – Stiffener – Stringer / girder – Panel – Hull

Stresses in a hull plate due to external sea pressure, are transferred further into the hull structure through the hierarchy of structures.

Basic Hull StrengthLevel 1: Plate - simple beam

Water pressure

StiffenerPlating

A strip of platingconsidered as a beamwith fixed ends and evenly distributed load

PLATE AS A BEAM

NO ROTATION

Basic Hull StrengthLevel 2 Longitudinal - simple beam

Longitudinal between two web frames

Symmetric load fwd and aft of web frames gives no rotation -fixed ends

Max shear and bending moment at supports (web frames)

Basic Hull StrengthLevel 3 : Transverse web - simple beam

Beam with fixed ends and concentrated loads from the bottom longitudinals

BM

SFMax shear and bending moment towards ends (side & long bhd.)

Basic Hull StrengthLevel 3 Longitudinal girder withtransverse webframes

Longitudinal girder between two transverse bulkheads

Max shear and bending moment towards transverse bulkheads

Single beam with fixed ends and concentrated loads from the transverse web frames

Max Shear and bending moment towards ends

Basic Hull StrengthBeams, load transfer

Double bottom structure

Centre girder

Floor / transversebottom girderSide girder

Stiffeners supportedby floors

Loads taken up by the bottom plating are transferred through the hierarchy of structures into the hull

Basic Hull Strength

Single skin structure

CL girder

Transverse bottomgirder /web frame

Longitudinal bulkhead

Bottom longitudinals with plating

Loads taken up by the bottom platingare transferred through the hierarcyof structures into the hull

Beams, load transfer

Basic Hull StrengthDamage experience

• Level 1 Plate supported at stiffeners

• Level 2 Stiffener supported at webframe

• Level 3 Webframe supported at panel

• Level 4 Panel – hull girder

Consequences of damages level 1-4 above!

Basic Hull StrengthSingle beam VS Hull girder

A vessel’s hull has many of the same properties as a single beam.

Hence simple beam theory may be applied when describing the nature of a vessels hull

The term ‘Hull girder’ is used when thinking of the hull as a single beam

Single beam

Hull

Basic Hull StrengthHull girder bending

When a vessel’s hull is exposed to loading, it will bend similarly as a single beam

Basic Hull StrengthSingle beam VS Hull girder

Section A-A

Hull GirderShear stress, τ

Bending stress, σ

Compression

Tension

A

A

A

A

F

Deck and bottom acts as flanges in the ‘hull girder’, while ship sides and longitudinal bulkheads, act as the web

Basic Hull StrengthStress hierarchy in ship structure

Local stress : Plate / stiffenerGirder stresses: Webframes / Girders /FloorsHull girder stresses; Deck & bottom / Side /

long. Bhd.

Basic Hull StrengthCase Module 2: Loads Buzz Groups

• For a beam with fixed ends and evenly distributed load, i.e. from sea pressure, is it true that:– Bending stresses are zero at one location– Reaction forces are equal at both ends– No rotation at ends– Bending stresses are positive (tension) in one flange

and negative (compression) in the other in the middle of the span

– Shear stresses are highest in the middle of the span– Shear forces are carried by the web

Basic Hull StrengthCase Module 2: Beams Buzz Groups

• Is it correct that the transverse girders are supported by the longitudinal stiffeners?

• Are the longitudinals inside a tank structure for example bottom longitudinals between webframes normally fixed or simply supported?

Basic Hull StrengthSummary: Beams

• BM and Shear force• Stress axial / bending / shear• Section modulus / Moment of inertia / Shear area• Stress distribution Bending and shear• BM and SF distribution depending on load and

end fixation• Profile types and properties• Structural hierarchy plates-stiffeners-girder-panel

Basic Hull StrengthLoads acting on a ship structure

Basic Hull StrengthLoads acting on a ship structure

1. Internal loads: - Cargo- Ballast- Fuel- Flooding- Loading/unloading

2. External loads: - Sea- Ice- Wind

Anchor

Basic Hull StrengthStatic and Dynamic loads

Static local load: The local load, internal and externaldue to cargo / ballast pressure

Dynamic local load: External - dynamic wave loads, Internal - due to acceleration

Static global loads: Global Bending Moment and Shear Force

Wave loads: Dynamic Bending Moment and Shear Force

Basic Hull StrengthStatic and Dynamic loads

Total external local load acting on a vessel:

Max at the bottom

Note the relative size of static / dynamic pressure is not to scale!

Static Dynamic

Max around the waterline

Basic Hull Strength

Plotted sea pressure curve is a sum of the static and dynamic contribution

Constant in the midshiparea, increasing towards ends

Sea Pressure – static and dynamic contribution

Local sea pressure (example for a bottom longitudinal)

p (kN/m2)

aft fwd

Basic Hull Strength

• Global dynamic vertical and horizontal wave bending moments give longitudinal dynamic stresses in deck, bottom and side

Highest global dynamic loads for all longitudinal members in the midship area

Static and Dynamic loads

Basic Hull StrengthLoads on foreship

Bottom Slamming Pressure•Induced by waves in shallow draft condition (ballast condition)•Dominant for flat bottom structure forward

Bow Impact Pressure•Induced by waves, vessel speed, flare and waterline angle important factors•Dominant for ship sides in the bow at full draught

Basic Hull Strength

Green Seas Loading:• Dominant for hatch covers and fwd deck structure

(incl. deck equipment, doors, openings etc)

Loads on deck

Basic Hull StrengthWeights and buoyancy

Steel weight, equipment and machinery

Buoyancy

Weight distribution of cargo and fuel

Static Dynamic

Basic Hull Strength

Static internal load from cargo

Static external sea pressure

Dynamic internal load from cargo

Bulk Carrier typical load

Dynamic external sea pressure

Basic Hull Strength

Internal load

- External load

= Net load on double bottom

Static and dynamic sea pressure

Static and dynamic internal load from cargo

Net load on structure – ‘Ore hold’

Basic Hull Strength

Static and dynamic sea pressure

Net load on structure - empty hold

Net load from sea pressure

Basic Hull StrengthAlternate loading condition

Basic Hull StrengthWeights and buoyancy

Buoyancy and weights are not evenly distributed along a ships length…

…hence, a global shear force and bending moment distribution is set up on the hull girder

Basic Hull StrengthHull girder still water bending moment and shear force

Example: SF and BM distribution for a double hull tanker in a fully loaded condition

Basic Hull Strength

Total hull girder bending moment MTotal = Mstill water + M wave

Total BM acting on a vessel

Mtotal

Mstill water

Hog

ging

Sagg

ingBM

lim

its

Mwave

Basic Hull StrengthCase 2 Module 2 – Loads/Materials

• Where in the hull girder cross section of a hull girder are the local dynamic loads due to sea pressure highest?

• Where along the hull girder are the dynamic sea pressure loads highest?

• Where in the hull girder is the global dynamic bending moment highest?

• Does a vessel in sagging condition experience compression or tension in deck?

• A vessel in sagging condition experience flooding of aempty tank in midship. Will the hull girder bending moment increase or decrease?

Basic Hull StrengthSummary: Loads

• Static & dynamic• Internal & external• Load distribution• Net load• Longitudinal strength SF & BM

Basic Hull Strength

End of Module 2: Basic Hull Strength

Module 3:

Structural ConnectionsModule 3: Structural Connections

• Objectives of this Module:


• Knowledge about connections between structural elements• Understanding of the transfer of forces between structural elements

and the relevant stress distributions• Knowledge about how to improve the design of structural

connections

Module 3:

Structural ConnectionsContents

• Types of welds• Connections of stiffeners• Connections of girders/web frames• Connections between panels• Design details

Module 3:

Structural ConnectionsWeld Types

We will briefly touch upon the following types:

• Fillet welds• Full penetration welds (Full pen)

(Ref. Rules Pt.3 Ch.1 Sec.11)

Module 3:

Structural Connections

Fillet welds:

• The most common type

Transferring shear forces (between profile and plate)• Building welded sections• Connections to other members• NDT by magnetic particle or

dye penetrant

Leg length

Throat thickness

Throat thickness-measure 3.5 mm = leg length 5.0 mm

Weld Types – Fillet welds

Module 3:

Structural ConnectionsWeld Types – Full penetration

t

Throat thickness

Root Face 2-4 mm for full penetration welds

σ

Full penetration welds:

• To be used where stress level normal to the weld is high

Transferring shear forces and forces normal to the weld• Connections to other members in highly stressed

locations• NDT by ultrasonic, dye penetrant or magnetic particle

Gap <3 mm

Module 3:

Structural ConnectionsConnections of stiffeners

• What forces are to be transferred?

ShearForce

L

BendingMoment

Module 3:

Structural ConnectionsLoad from stiffener to webframe

How is theforces

transferredfrom the

stiffener to webframe

How are theforces

transferredfrom the

stiffener to webframe

Module 3:


a) c)

+ +

d)

+

Connections of stiffeners

Web

fr.

Web

fr.

Web

fr.

Stiffener

b)

+

Module 3:


= =

Effect of brackets on the max bending stress

No or negativeeffect

Module 3:


Common crack locations in longitudinal

= =

Longitudinal

StiffenerWeb-plating

Module 3:


σx

Stress distribution for different details

Static stress in stiffener on top

σx

ballast loaded

Module 3:


Common crack locations

= =

Longitudinal

StiffenerWeb-plating

Design improvement

Module 3:

Structural ConnectionsEnd-brackets on girders - forces

Full Centre Tank

EmptyWingTank

Net loadNet load

Module 3:


iiii

a

ii

End-brackets on girders

i)

ii)

iii)

iv)

iiib)

Transverse welding of flange outside curved area

High Stress Areas

High Stress Areas

Flange attachedand supported

Improved design

High Stress AreasSoft bkts. recommended

Increased stress at support bkts.

Module 3:


Crack

Original Design

Bracket with thickness 20 to 25 mm

Original thickness 16mm Insert 20 to 25 mm

Stringer connection to inner side

Repair

Inner side

Ship side

Trv. Bhd.

Stringer

Module 3:


Girder bracket

End-brackets on girders

Typical crack location

Ref. iii b) previous fig.

Module 3:

Structural ConnectionsCross-Ties

Full Centre Tank

Full Centre/Empty Wing at full draught = Max. Compression in Cross Tie

Empty Centre/Full Wing at ballast draught = Max. Tension in Cross Tie

EmptyWingTank

Empty Centre TankFullWingTank

Module 3:


Out of plane forces

Knuckles

Deformation/low stiffness

helikopter

Module 3:


Support as close to the knuckle as possible

Knuckles

Module 3:


Vertical Brackets

Knuckles

Module 3:

Structural ConnectionsKnuckles

Crack in shell plate at knuckle:

New Brackets

Module 3:


Crack Locations

Stress ConcentrationsIn way of Webs

Knuckles

Module 3:

Structural ConnectionsKnuckles

Preferred design:

• No misalignment in the connection.

• No lugs or scallops

Module 3:

Structural ConnectionsIntersecting Hull Elements

Connecting area ~ t · tConnecting area ~ (a+b) · t

tt

b

a

Panel 1

Panel 2

Crossing Panel - No bracket Crossing Panel - With bracket

Module 3:


WINGTANK

DIESELSUPPLYTANK

TOP SIDETANK NO. 7

CRACKS

EN

GIN

E R

OO

MB

ULK

HE

AD

CRACKS

ENGINE ROOM BULKHEAD

A

A

EXISTING BRACKETTO BE REMOVED

NEW BRACKETS INLINE WITH BOTTOMPLATE IN TOP SIDETANK

Section A-AENGINE ROOM BULKHEAD

iii

ADDITIONALBRACKET

SLANTING TANK TOPPLATING

TO BE IN LINE


LONGITUDINAL BULKHEAD


TANK TOP

STR LON

GIT

UD

INA

L

BU

LKH

EA

D

BKT.

iv

Intersecting Hull Elements

Cracks

Reinforcements

A - A

Module 3:

Structural ConnectionsNotches, Drain/Lightening Holes

i)

Common notch in way of weld

Crack iii)

Notch away from weld

Reduced risk of cracking

Module 3:

Structural ConnectionsSummary module 3

• Welding• Connection stiffener – girder• Girder – panel• Cross tie• Knuckles• Intersection of plates / panels• Cut-outs and notches

Module 5

Hull Structural Breakdown

Oil Tanker Bulk Carrier

Container Ship

Hull Structural BreakdownOil Tanker – Bulk Carrier – Container Ship

Objective of Module 5:


• Understanding of hull structural design for Oil Tankers, Bulk Carriers and Container Ships through application of basic hull

strength theory

• Knowledge of typical structural damages and their consequences

Contents of Module 5

1. Fwd and aft structural parts

2. Oil Tankers – structures in cargo area

3. Bulk Carriers – structures in cargo area

4. Container Ship – structures in cargo area

Fore ship Contents – Fwd and aft structural parts

1. Hull structure breakdown – fwd part of ship

2. Hull structure breakdown – aft part of ship

3. Case

Structural functions of fore ship

1. Watertight integrity (local strength)

- Resist external sea pressure / Bow impact / bottom slamming

- Resist internal pressure from ballast

2. Web in hull girder (global strength)

- Side plating act as the web in the hull girder beam

Fore ship

Stringer decks

Breast hook

Chain lockerCollision bhd.

Bulbous bow

Side webframes

Fore ship Structural build up fore ship

Vertical side frames Horizontal side longs

Fore ship Structural build up fore ship

Structural functions of fore ship

• Shell side must withstand static and dynamicloads from external sea pressure.

• Bow impact and bottom slamming introduce additional loads

• Internal pressure from ballast

Fore ship

Plate supported by side longs

Side longs supported at webframes

Webframes supported at stringer flats

BM and SF distribution for a single beam with distributed load and fixed ends

Fore ship

Structural build up fore peak Horizontal stiffening

Structural build up fore peak Horizontal stiffening

Reduced efficiency

due to flare angle

Fore ship

Plate supported by side frames

Side frames supported by stringer flats

BmSF

Fore ship

Structural build up fore peak Vertical stiffening

Functions of fore peak global strength

Side plating is acting as web in the hull girder beam


Cont.

• Ship side / longitudinal swash bulkhead carry global shear forces from net load in fore peak to the collision bhd.

Full draught with empty fore peak

most critical

Fore ship

Functions of fore peak Global strength

2. Deck and Bottom in hull girder (global strength)- The global bending moments are always zero at fwd / aft end.

- The longitudinal stresses in deck and bottom are moderate in fore structure

- If large flare – wave induced compression stresses in deck may critical

Fore ship

Hull damages in fore ship

Characteristic damages fore ship1. Corrosion – lost ship side fore peak

2. Buckling of stringers

3. Bow impact

4. Damages to the wave breaker

5. Bottom slamming

Fore ship specially prone to hull

damages.

Of top 10 damages on tankers are 6 of

them in the fore ship!

Fore ship

Lost shipside

Heavy local

corrosion

• Local heavy corrosion – increase stress level - reduced buckling strength

• local buckling stiffener collapse – web frame buckling/collapse

• Side longs double span – overload and collapse

Experience feedback

Oil Tanker 357 000 DWT built 1973

20 years

Fore ship

• Shell side lost its watertight integrity

• Lost buoyancy – increased fwd. draught –impact on longitudinal strength

• Reduced shear carrying capacity for hull girder

• Collision bhd. Exposed to dynamic sea loads

Lost shipside - Impact of functionOil Tanker 357 000

DWT built 1973

20 years

Fore ship

Buckling of stringer in fore peak tank

Oil Tanker302,419 DWT built 1992

Buckling of stringers in fore peak tank(after 1 year)

Buckling in stringer no 1, 2 & 3 in fore peak tank. Stringer no 1 shown, other stringers similar buckling pattern

Fore ship

Stringer as beam

Local web buckling due to lateral load axial stress in web

Buckling of stringer due to high shear / compression stresses

Experience feedback

Buckling of stringer in fore peak tank



Fore ship

Buckling of stringerImpact of function

• Buckled / deformed stringers may develop cracks penetrating the shell – cause leak – impact on trim – draught

• If stringers are significantly reduced in strength the webframes loose their support.

• Side longitudinals loose their support at webframes.

• Side longitudinals with excessive loads may collapse and ship side collapse –flooding of fore structure.



Fore ship

Bow Impact DamageContainer ship

1 yearFore ship

A recent damage in 2001…..Occurred during the first year of operation


1 yearFore ship


1 yearFore ship

Sea Pressure: ”Evenly” distributed

Bow impact: Peak pressure

β

α

h0

Important factors:

Flare angle, αWaterline angle, βHeight above waterlineVessel speedRoll and pitch


1 yearFore ship

Local plate buckling

Container ship1 yearFore

ship

• Buckled plating may lead to leakage

• Damages to longitudinals may reduce their load carrying capacity

• Damages to stringers and webs could lead to reduced support of longitudinals which again may lead to ship side collapse and flooding.

Bow Impact DamageImpact of function

Bottom slamming fore shipBulk Carrier 220 000DwtBuilt 1997

• Bottom plate set in

• Bottom longs tripped ( L-profiles)

• Webframes buckled between longs and access holes

Fore ship

Bottom slamming fore shipFeeder

L = 100 m Fore ship

Plates set in and puncturedFloors twisted and damagedMostly for small ships in ballast condition

Slamming Pressure

Slamming Pressure

Bottom slamming fore shipFeeder

L = 100 m Fore ship

Parametres:

= Ballast draught forward. Increasing ballast draught decreases slamming load.

= Breadth of flat bottom. “V” shape forward reduces slamming load.

= Distance from FP. Pitch component of relative velocity, and therefore slamming load, decreases with distance from FP

XBT

B

BF

Bottom slammingImpact of Function

• Bottom longs tripped will not efficiently support plate– Bottom plate + longs will be set in– In plane buckling capacity significantly reduced

• not critical in this area due to low vertical bending moment

• Bottom floors buckled, webframes reduced their load carrying capacity

• Loss of watertight integrity – flooding possible scenario – impact on trim - draught

Fore ship

Contents – Fwd and aft structural parts

1. Hull structure breakdown – fwd part of ship

2. Hull structure breakdown – aft part of ship

3. Case

Aft ship

Structural build up aft ship

Transom stern plate

Engine room bulkhead

Floors

Webframes

Aft ship

Structural build up aft ship

Engine room platform

Side plate & longitudinals

Webframe side

Webframe deck

Aft ship

Structural build up aft peak tank

Vertical side frames Horizontal side longs

Aft ship

Structural functions of aft ship

Loads are taken up by the hull plating, stresses are transferred from plate to stiffener

• Shell must withstand static and dynamic sea pressure, bottom slamming may introduce additional loads

• Internal pressure from ballast • Dynamic impulses from the propeller

Aft ship

Functions of aft ship

Side plating is actingas web in the hull

girder beam

Global loads are acting on the hull

girder beam

Web in hull girder (global strength)

Cont.

Ship side together with the longitudinal swash bulkheads are taking up global shear forces from net load on the hull girder in the aft end

High shear forces fwd. of engine room

full load conditions

Aft ship

Functions of Aft ship

2. Deck and Bottom in hull girder (global strength)- The global bending moments are always zero at fwd / aft end

- The longitudinal stresses in deck and bottom are moderate in fore peak

Aft ship

• Ensure adequate stiffness for:– Main engine support (double bottom engine room)– Steering gear support (steering gear flat / aft peak)– Rudder horn (aft peak structure)

Aft ship Functions of Aft ship

Hull damages in aft ship

Characteristic damages for the aft ship:1. Buckling of engine room stringers

2. Stern Slamming

3. Cracks due to vibration

4. Cavitation damages to the rudder

Aft ship

BucklingOil TankerBuilt 1992

Buckling of stringers in engine room(after 1 year)

Buckling of side stringer 7700 mm above baseline in engine room (P/S)

Buckling of stringers aft in engine room 7100 / 11150 mm above baseline

Aft ship

External sea pressure

Buckling

Bending + shear exceed the buckling capacity of the plate

Bending moment

Aft ship

Buckling Impact of function

Aft ship

• Stiffeners may loose their support and areas may be overloaded

• Collapse of panels and leakage may be a possible scenario

• Flat stern structure is prone to be high stern slamming impact load - the wider beam, the higher impact pressure and total load on the stern

Stern Slamming Container ShipAft ship

Scallop and stiffener connection to outer shell longitudinals in ballast tanks in after body area were found fractured in several locations.

Repaired connection area/ scallop


F

F


• Side longitudinals may loose their support at web frames

• Crack may penetrate the shell plating - loss of watertight integrity - flooding possible scenario

Stern SlammingImpact of function

Container ShipAft ship

Cracks in aft peak tank due to vibrationsAft ship

Vibrational cracks

Cracks in Trans. at Steering Gear Flat

Supporting structure below oscillating machinery

Passage doors in engine room area

Cracks in aft peak tank due to vibrations

Crack in weld between web frame and shell side

Aft ship

Crack

Crack caused by vibration of the web frame due to

impulses from the propeller

Crack start in scallop

Repair;

Additional intercostals to change natural frequency for side webs

• The supporting structure may get less effective• If the cracks are in the side shell frames or

webs, this may lead to crack in the shell plate and thereby leakage.

Vibration damagesImpact of function

Aft ship

Rudder CavitationAft ship Typical on Container Ship

Typical repair;• Grind the affected area

• Pre-heat

• Re-weld

Pressure distribution(suction)

Positive pressure

U = speed ofambient water

Pressure distribution due to shape of profile

Pressure distribution due to thickness of profile

Pressure distribution aroundtypical rudder profile

Cavitation of rudder blade depend on:

• Shape of profile• Thickness of profile• Rudder angle • Speed of water over profile

Aft ship Rudder Cavitation

• Stainless steel shielding– Preferred solution welded

with continuous weld in small pieces – not slot welds

Aft ship Rudder Cavitation

Aft ship

This is how it may end if the shielding is not

welded properly

Rudder Cavitation

• Cracks may occur which could lead to reduced rudder support and maneuverability

Rudder CavitationImpact on function

Aft ship

End of Module 5 Fore & aft ship

18.02.2005Slide 1

Oil Tankers

Oil Tankers - Hull Structure

18.02.2005Slide 2

Oil Tankers Contents – Oil tankers

1. Introduction

2. Hull structural breakdown – function of hull elements:• Side, bottom, deck, transverse bulkhead, longitudinal bulkhead,

web frames including relevant hull damages for all structural elements

3. Case

18.02.2005Slide 3

Oil Tankers Characteristics for Oil tankers

- High number of tanks – good capability of survival - Low freeboard, green seas on deck- Pollution / public attention / fire explosion hazards - Fatigue - Liquid cargo – sloshing in wide tanks and stability aspect -Hull inspection environment- Fully utilizes BM limits hogging/sagging (double hull tankers)

Any proposals?

18.02.2005Slide 4

Oil Tankers Size categories of tankers

Oil TankersType DWTULCC 320,000+ VLCC 200 - 320,000Suezmax 120 - 200,000Aframax 75 - 120,000Panamax 55 - 70,000Products 10 - 50,000

Source: INTERTANKO

18.02.2005Slide 5

Oil Tankers Size categories of tankers

Panamax (55 - 75,000 dwt):• Max size tanker able to transit the Panama Canal• L(max): 274.3 m• B(max): 32.3 m• Typical vessel: 60,000 dwt, L=228,6m, B=32,2m, T=12,6m

Aframax (75 – 120,000 dwt):• AFRA= Average Freight Rate Assessment• Traditionally employed on a wide variety of short and medium-haul crude oil trades• Biggest tanker in US ports is 100,000 dwt• Typical vessel: 100,000 dwt, L=253,0m, B=44,2m, T=11,6m

Source: INTERTANKO Age distribution

Age distribution

18.02.2005Slide 6

Oil Tankers

Suezmax (120 – 200,000 dwt):• Notation is soon to become redundant as the project of deepening the Suez Canal to 18,9m is completed• Typical vessel: 150,000 dwt, L=274,0m, B=50,0m, T=14,5m

VLCC (200 – 320,000 dwt):• Were prompted by the rapid growth in global oil consumption during the 60’s and the 1967 closing of the Suez canal

• Today the most effective way of transporting large volumes of oil over relatively long distances

• Typical vessel: 280,000 dwt, L=335,0m, B=57,0m, T=21,0m

Size categories of tankers

Source: INTERTANKO

Age distribution

Age distribution

18.02.2005Slide 7

Oil Tankers

ULCC (320,000+ dwt):• Most ships of this type built in the mid to late 70’s• Ordered to take advantage of the economies of scale in a buoyant market• Less than 40 of these ships remaining• Rather inflexible, may enter very few ports• Typical vessel: 410,000 dwt, L=377,0m, B=68,0m, T=23,0m

Size categories of tankers

Source: INTERTANKO

18.02.2005Slide 8

Oil Tankers

- Old design, build up to 1993

Single Skin Oil Tanker

Ship data:L = 310mB = 56m

D = 31,4m284,497 DWT

18.02.2005Slide 9

Oil Tankers Single bottom with side ballast tanks


D = 19,2m88,950 DWT

- Built in the 80’s, considered as ‘single skin’

18.02.2005Slide 10

Oil Tankers

- Common VLCC design of today

Double Hull – Two Longitudinal Bulkheads


D = 26,8m298,731 DWT

18.02.2005Slide 11

Oil Tankers Double Hull – CL Longitudinal Bulkhead


D = 23,2m159,681 DWT - Common Aframax and

Suezmax design of today

18.02.2005Slide 12

Oil Tankers Double Hull – no CL bulkhead

Ship data:L = 218mB = 32,2mD = 19,7m

63,765 DWT- Older design

18.02.2005Slide 13

Oil Tankers

Nomenclature for a typical double hull oil tanker

18.02.2005Slide 14

Oil Tankers

-A vessel’s hull can be divided into different hull structuralelements

- Each element has its own function contributing to the integrityof the hull

- In order to assess the structure of an oil tanker, one needs to understand the function of each structural element

Structural breakdown of hull

18.02.2005Slide 15

Oil Tankers Damages and repairs

WWW.witherbys.com

18.02.2005Slide 16

Oil Tankers Function of hull elements

Bottom:

Deck:

Transverse bulkhead:

Longitudinal bulkhead:Webframes:

Ship side:

18.02.2005Slide 17

Oil Tankers Hull Structural Breakdown

1.

2.

3.

4.

5.

6.

SideBottomDeck

Transverse bulkheadLongitudinal bulkheadWeb frames

18.02.2005Slide 18

Oil Tankers

End of Oil Tanker session

18.02.2005Slide 1

Oil Tankers 1. Side

Hull Structural Breakdown -Ship side

1.

2.

3.

4.

5.

6.

SideBottomDeck


18.02.2005Slide 2

Oil Tankers 1. Side

Structural build up of ship side – single skin tanker

Cross ties

Transversebulkhead

Side plating withlongitudinals

Web frameStringers

18.02.2005Slide 3

Oil Tankers 1. Side

Structural build-up of a double hull ship side

Side plating withlongitudinals

Stringers

Web frame

Inner side platingwith longitudinals

18.02.2005Slide 4

Oil Tankers 1. SideStructural functions of ship side

Watertight integrity

- Take up external sea loads and transfer these into the hull girder

- Resist internal pressure from cargo and ballast

Web in hull girder


18.02.2005Slide 5

Oil Tankers 1. Side

Full centre tank

Loads on the ship side - example

Ballast condition

Full wing tank

Net force

Water Line

Fully loaded condition

Net force

Water Line

18.02.2005Slide 6

Oil Tankers 1. SideLocal function: Watertight integrity

External loads induces shear forces and bending moments in the side longitudinals as single beams (between each web frame)

Side long.as a single beam between two web frames BM and SF distribtion for a single beam

with evenly distributed load and fixed ends

18.02.2005Slide 7

Oil Tankers 1. SideLocal function: Watertight integrity

-Side longs are supported at the web frames

- Web frames are supported at the cross ties and at the deck and bottom

Part of web frame supported at two cross ties, shear max towards supports

Shear force

Bending moment

18.02.2005Slide 8

Oil Tankers 1. SideDouble hull ship side

• The structural functions of a double hull ship side is the same as for a single hull:

As there are no cross ties, side web frame is supportedat the deck and bottom

High shear stress

18.02.2005Slide 9

Oil Tankers 1. SideGlobal function: Web in hull girder

Global shear forces resulting from uneven distribution of cargo and buoyancy are taken up in the ship side plating

Shear stress distribution resulting from global loads for midship section

Area effective intransferring shear force

18.02.2005Slide 10

Oil Tankers 1. SideStringers in a double side

• Stringers contribute to the stiffness of the double hull ship side, which means:

High shear stress in stringer towards thetransverse bulkhead

15mm

20mm

25mm

20mm

15mm

18.02.2005Slide 11

Oil Tankers 1. Side

Characteristic damages for ship side:

1. Cracks in side longitudinals at web frames

2. Cracks in cut-outs for longitudinals

3. Cracks in side longitudinals at transverse bulkheads

4. Indents of side shell and stiffeners

18.02.2005Slide 12

Oil Tankers 1. SideCrack in side longitudinals


Cracking in side longitudinal web frame connection

(after 3 years)

Crack in side longitudinal tripping bracket connection to web frame (various wing tanks)

Side longitudinal flatbar connection to web frame

18.02.2005Slide 13

Oil Tankers 1. SideCause for cracking in side longitudinals

Dynamic loads (seaand cargo) are forcingthe side longitudinal to flex in and out

•High alternating bending stresses towards the end supports (web frames)

•Highly stressed areas created around geometric’hard points’ (bracket toes, scallops, flat bars)

18.02.2005Slide 14

Oil Tankers 1. Side

More Stress concentration factors ;

• Kg : Gross Geometry (from FEM analysis)

• Kw : Weld Geometry (typical 1,5)

• Kn : Unsymmetrical Stiffeners (L& bulb-profiles)

Stress concentration factors

18.02.2005Slide 15

Oil Tankers 1. SideStandard repair proposal longs / webframes

18.02.2005Slide 16

Oil Tankers 1. SideCracks in web frame cut outs

Cracks around openings for side longitudinals in web

framesCracks

18.02.2005Slide 17

Oil Tankers 1. Side

Cause for cracking in cut outs for longitudinals

Sea loads induce shear stresses in the web frame

Shear stress

Shear stress

High shear stresses around openings etc, where shear area is

reduced

18.02.2005Slide 18

Oil Tankers 1. SideConsequence of crack in web frame

Re-distribution of shearstresses in web frame

Side longitudinals loose their support

May lead to overloadingof adacent structure

How does this damage impact on the function of the web frame?

18.02.2005Slide 19

Oil Tankers 1. Side

Crack in side longitudinal at transverse bulkhead

Side longitudinal connectionsto transverse bulkheads

Cracks in side longitudinal connection to stringers at transverse bulkhead

18.02.2005Slide 20

Oil Tankers 1. Side

Seapressure

Relative deflections occur betweenthe ’rigid’ transverse bulkhead and the flexible web frame construction

Why cracking at transverse bhd.?

Ship side

The relative deflection induces additionalbending stresses at the end connection of side longitudinals to the transverse bulkhead. Alsoimportant at wash bulkheads.

18.02.2005Slide 21

Oil Tankers 1. SideFEM plot of double hull oil tanker

Loading condition:External dynamic sea pressure at full draught

Relative deflection

18.02.2005Slide 22

Oil Tankers 1. SideConsequence of damage

Cracks in side longitudinals:- oil leakage and pollution- longitudinal may break off- in worst case (a series of cracks in

same area) could induce a larger fracture (loss of ship side)

Suggestions?

leakage

18.02.2005Slide 23

Oil Tankers 1. SideIndents of side shell with stiffeners

The terms ’indents’ and ’buckling’ should not be mixed up with eachother, as the cause for these damages are different:

-Indents: Mainly due to contact damages

-Buckling: Due to excessive in-plane stresses

Mainly from contact damages:

18.02.2005Slide 24

Oil Tankers 1. SideConsequense of indents

18.02.2005Slide 25

Oil Tankers 1. SideConsequense of indents

Sharp indents may lead to cracks and possible leakage

Large area set in (plating and stiffeners) gives reduced buckling capacityAdjacent areas may then be overloaded

18.02.2005Slide 1

Oil Tankers 2. Bottom

Hull Structural Breakdown -Bottom

1.

2.

3.

4.

5.

6.

SideBottomDeck


18.02.2005Slide 2



• 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

Structural functions of bottom

18.02.2005Slide 3


Structural build up of bottom –single skin tanker

Bottom platingw/longitudinals

Web frameCL girder

Bilge

Keel plate

18.02.2005Slide 4


Structural 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

18.02.2005Slide 5


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

Function: Watertight integrity

Cont.

BM and SF distribtion for a single beam with distributedload and fixed ends

Fixation?

18.02.2005Slide 6


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.)

Function: Watertight integrity

BM

SF

p L

18.02.2005Slide 7


Bottom is supported by ship side and longitudinal bulkhead

Shear stress in double bottom floordue to external seapressure

Double span for double bottom without CL longitudinal

bulkhead

18.02.2005Slide 8

Oil Tankers 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

σ L

σ L

18.02.2005Slide 9

Oil Tankers 2. BottomDouble bottom structure

Load distribution in double bottom

girder system

18.02.2005Slide 10

Oil Tankers 2. BottomLoad response double bottom

Cont.

Stresss flow shortest way to

support


support

18.02.2005Slide 11


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

Double bottom structure

Net load

Shear force

High shear stresses in floors & girders in way oftransv. Bhd. And hopper tank

With few longitudinal girders, double bottom stresses resulting from the net load on the girder system are mainly transferred in the transverse direction

Shear force

18.02.2005Slide 12

Oil Tankers 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

18.02.2005Slide 13

Oil Tankers 2. BottomBilge keel cracking


Crack in hull plating i.w.o. bilge keel terminations

Crack in hull plating in way of bilge keel toes

Bilge keel

18.02.2005Slide 14


Longitudinal stress

Hot spotBilge keel

18.02.2005Slide 15


10-15mm

Web frame/BilgeBracket

1600

Bilge Keel

Pad plate

200

Ship side

100

All measures in mm

125Edges to be grinded

smooth

25100

Full pen. weld

18.02.2005Slide 16

Oil Tankers 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

18.02.2005Slide 17


Cause 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

18.02.2005Slide 18


Consequences of cracks in bottom longitudinals:

-Leakage of oil

- Crack may propagate further into bottom plating and induce a larger transverse fracture

18.02.2005Slide 19

Oil Tankers 2. BottomExample: Cracks in inner bottom

Oil Tanker95,371 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 in bracket toe

Crack propagating through tank top plating (a few cases)

18.02.2005Slide 20


Cracking in double bottom longitudinals

Cracks in flatbar connections for bottom and innerbottom longitudinals

18.02.2005Slide 21


Cause 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

18.02.2005Slide 22


Illustration – double bottom flatbarconnections

Tensile stresses in critical structural details

The double bottom structure is exposed to large forces both in ballast and loaded condition

18.02.2005Slide 23

Oil Tankers 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

18.02.2005Slide 24

Oil Tankers 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

18.02.2005Slide 25

Oil Tankers 2. BottomCracking in hopper knuckle

Crack in hopper knuckle at web frame connections

18.02.2005Slide 26


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)

Cause for cracking in hopper knuckle

σ L

σ L

Bending stress in double bottom girderBending stress in

inner bottom plating

18.02.2005Slide 27


- 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

Cause for cracking in hopper knuckle

- 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

18.02.2005Slide 1

Oil Tankers 3. Deck

Hull Structural Breakdown -Deck

1.

2.

3.

4.

5.

6.

SideBottomDeck


18.02.2005Slide 2

Oil Tankers 3. DeckStructural functions of deck

Flange in hull girder

- Deck plating and longitudinals act as the upper flange in the hull girder beam

18.02.2005Slide 3

Oil Tankers 3. Deck

Structural build up of deck –single skin tanker

Deck CL girderDeck platingw/longitudinals

Transverse deckgirder / Web frame

18.02.2005Slide 4

Oil Tankers 3. Deck

Longitudinal stresses (+/-) are set up in the deck plating and longitudinals due to bending of hull girder

Function: Flange in hull girder

Hull girder bending moment induces longitudinal stresses in the deck plating and longitudinals

σL

σ L

18.02.2005Slide 5

Oil Tankers 3. DeckLongitudinal stresses in deck

Longitudinal stresses from bending of hull girder is maximum at midship

Bending moment

Max

Midship area most susceptible to fatigue cracking and buckling

18.02.2005Slide 6

Oil Tankers 3. DeckCharacteristic damages

1. Cracks in deck longitudinals

2. Crack in deck plating

3. Corrosion of deckhead

4. Buckling of deck

18.02.2005Slide 7

Oil Tankers 3. Deck

Deck longitudinal connection to web frames

Cracking in deck longitudinals

Deck longitudinal connection to

transverse bulkhead

18.02.2005Slide 8

Oil Tankers 3. DeckCracking in deck longitudinals

Oil Tanker135,000 DWT built 1991Crack main deck plating

Crack in underdeck support for hose handling crane (P/S, midship area)

18.02.2005Slide 9

Oil Tankers 3. Deck

The wave induced excitation of the hull girder leads to dynamic axial stress in the deck longitudinals

Cause for cracking in deck longitudinals

The cyclic variation of axial stress may lead to fatigue cracksinitiating at hot spots

+_+

_

A loaded condition will normally induce compression stress in the deck (sagging)

A ballast condition will normally induce tension stress in the deck (hogging)

18.02.2005Slide 10

Oil Tankers 3. DeckCracks in deck longitudinals

- May result in oil spill on deck- Corrosion is highly influencing the fatigue life of a detail- A crack could develop further in the deck plating (brittle fracture)

18.02.2005Slide 11

Oil Tankers 3. DeckOpenings in deck

Longitudinal stress-flow around manhole in deck

Increased stress level around openings in deck!

σ

σ

Kg.Kw. σ

18.02.2005Slide 12

Oil Tankers 3. Deck

Example: crack in scallop in deck longitudinal

Scallop in deck longitudinal is close to access opening in deck. This will give an additional accumulated stress in the longitudinal, which is believed to be the cause for the damage.


Crack main deck plating (after 3 years)

18.02.2005Slide 13

Oil Tankers 3. DeckCrack in deck plating

Tanker for Oil99328 DWTbuilt 1996

Crack in deck plating

Crack in deck plating at hose saddle support (midship area)

18.02.2005Slide 14

Oil Tankers 3. DeckCorrosion of deckhead

The ullage space (deckhead) is an area susceptible to general corrosion

18.02.2005Slide 15


Reduced sectional area in deck may lead to plate buckling

A reduction of the deck transverse sectional area due to general corrosion will lead to an increased stress level in deck

AF

L =σ

Longitudinal stress

Area

Force

Longitudinal stress distribution

σ L

σ L

Higher stress level in deck

Long. stress distribution(with reduced decksectional area)

n.a.

18.02.2005Slide 16


Higher stress level in deckdue to general corrosion

σ L

σ L

AF

L =σ

Longitudinal stress

Area

Force

A reduction of the deck transverse sectional area due to general corrosion will lead to an increased stress level in deck may lead to buckling problems

18.02.2005Slide 17


Flatbars have poor buckling capacity

L-profiles have good buckling capacity

18.02.2005Slide 18

Oil Tankers 3. DeckBuckling in deck

Buckling of a plate field (plating with stiffeners)

Buckling in deck is most likely to occur in the midshipregion where the hull girder bending moment is at its maximum

18.02.2005Slide 19

Oil Tankers 3. DeckCause for buckling in deck

Buckling in deck is a result of in plane compression forces in excess of the buckling capacity of the deck plate field

Such a situation may occur if the transverse section of the deck is reduced due to general corrosion and the vessel is in a fully loaded (sagging) condition

The deck buckling may take the form of one plate between two deck longitudinals or in worst case a complete plate field (both deck plating with stiffeners)

Buckling of complete plate field

18.02.2005Slide 20

Oil Tankers 3. DeckCorrosion of deckhead / buckling:

- heavy corrosion of deck may lead to buckling

- small buckles (plate between stiffeners) is a strong warning sign that longitudinal stresses are high

- large buckles (plate field) may lead to loss of global strength and in worst case a total collapse of the hull girder

Remember max 10% diminution of deck transverse sectional area!

18.02.2005Slide 1

Oil Tankers 4. Transverse

bulkhead

Hull Structural Breakdown -Transverse bulkhead

1.

2.

3.

4.

5.

6.

SideBottomDeck

Transverse bulkheadLongitudinal bulkheadWebframes

18.02.2005Slide 2


bulkheadStructural build up of transverse bulkhead

Stringers

Transverse bulkheadplating w/stiffeners

Buttress

18.02.2005Slide 3


bulkhead


- Resist internal pressure from cargo and ballast (cargo boundary)

- Safety against collapse if water ingress (boundary for flooding)

Hull girder stiffness

- Transverse bulkhead is an important contributor to the hull girder transverse stiffness

Structural functions

18.02.2005Slide 4


bulkhead

The transverse bulkhead must withstand internal pressure loads from cargo and ballast

The distribution of cargo and ballast introduces alternate loading on sections of the transverse bulkheads (single skin tanker)

Functions of transverse bulkhead

Typical fully loaded condition (single skin)

Typical ballast condition (single skin)

18.02.2005Slide 5


bulkheadFunction: tank boundary

Stringer

Stiffener

Shear force

Bending moment

18.02.2005Slide 6


bulkheadFunction: tank boundary

One sided loading on the transverse bulkhead introduces stresses in the transverse bulkhead as a panel

Bulkhead will flex out and high stresses occur at end connections towards deck and bottom

18.02.2005Slide 7


bulkhead

Transverse bulkheads are an important contributor to the hull girder strength

Function: transverse stiffness

Transversestiffness

Seapressure

Seapressure

18.02.2005Slide 8


bulkheadCharacteristic damages

1. Stringer toes – cracking

2. Bottom longitudinal bracket connection to transverse bulkhead - cracks

3. Cracking of transverse bulkhead stiffeners connection to stringers

18.02.2005Slide 9


bulkheadCracking in stringer toe

Cracks in stringer toes and heel

18.02.2005Slide 10


bulkheadCracking in stringer toe

18.02.2005Slide 11


bulkhead

Full cargo tank

Cause for cracking in stringer toe

Full cargo tankSeapressure

Compression/tension stresses from one sided loading

Very high alternating bending stresses in stringer toe

18.02.2005Slide 12


bulkheadCracks in stringer

May cause contamination of ballast water and small oil spills

Stringer flange

Stringer webLongitudinal bulkhead

Crack

18.02.2005Slide 13


bulkhead

17.

Cracks in toe of transverse bulkhead bracket ending at bottom longitudinals(wing tanks, midship area)

Cracks in bottom longitudinals

18.02.2005Slide 14


bulkheadCause - cracks in bottom brackets

One sided loading at the transverse bulkheadinduce high local alternating bending stresses at the bracket toe

Crack in brackettoe (hot spot)

18.02.2005Slide 15


bulkheadDouble btm at transverse bulkhead

Similarily, one sided alternate loading at the transverse bulkhead alsoinduces high stresses for a double bottom structure

Critical areas

Modern designs have nolongitudinal girders in double bottom giving largerelative deflection

18.02.2005Slide 16


bulkhead

Crack in transverse bulkhead stiffeners connection to stringers

Connection of stringer to transversebulkhead with associated brackets

18.02.2005Slide 17


bulkheadCause for cracking in transverse bulkhead stiffeners

One sided internal loading from cargo and ballast sets up a shear stress distribution in the bulkhead stiffener

Highly stressed areas arecreated around geometric’hard points’ at stiffenerend connections to thestringer

-may cause ballast water contamination and possible oil spills

18.02.2005Slide 1

Oil Tankers 5. Longitudinal

BulkheadHull Structural Breakdown -Longitudinal bulkhead

1.

2.

3.

4.

5.

6.

SideBottomDeck

Transverse bulkhead

Web framesLongitudinal bulkhead

18.02.2005Slide 2


BulkheadStructural build up of longitudinal bulkhead

Cross ties

Longitudinal bulkhead platingwith stiffeners

Web frame

18.02.2005Slide 3


BulkheadStructural functions of long.bhd


- Resist internal pressure from cargo and ballast (cargo boundary)

- Safety against collapse if water ingress (boundary for flooding)

Web in hull girder

- Contributes to hull girder longitudinal stiffness

18.02.2005Slide 4


BulkheadFunction : Cargo boundary

Internal loads induce shear forces and bending moments in the longitudinal bulkhead longitudinal (between each web frame)

Stresses are loaded onto the web frames and further into the hull girder structure

18.02.2005Slide 5


BulkheadFunction: Web in hull girder

Longitudinal bulkhead together with ship side is taking up global shear forces from wave induced loads and weight/buoyancy distribution along the vessel length

R1 R2

A

A

A

A

F

Section A-A SF

Shear force distributionresulting from global loads for midship section

18.02.2005Slide 6


BulkheadCharacteristic damages

1. Cracks in bulkhead longitudinals connection to stringers at transverse bulkhead

2. Shear buckling of longitudinal bulkhead

18.02.2005Slide 7


BulkheadCrack in long.bhd longitudinalsconnection to stringers

Connection of longitudinal bulkhead longitudinals to stringerswith associated brackets

18.02.2005Slide 8


Bulkhead

High bending stresses towards the supports (transverse bulkheads)

Cause for cracking in long.bhdat stringer connections

Fully loaded condition Ballast condition

Longitudinal bulkhead is flexing depending on theloading condition (filling of tanks)

18.02.2005Slide 9


Bulkhead

Cause for cracking in long.bhdstringer connections

Hot spot

May cause contamination of ballast water and small oil spills

Full ballast tank

18.02.2005Slide 10


BulkheadShear buckling of longitudinal bulkhead

Shear buckling is most likely to occur in areas towards the transverse bulkheads, butmay also occur in other areas depending onthe thickness of the bulkhead plating

18.02.2005Slide 11


BulkheadShear buckling of longitudinal bulkhead

Longitudinal shear forcedistribution – an example

SF maximum at transverse bulkheads

18.02.2005Slide 12


BulkheadCause for shear buckling

Result of excessive shear stress in the bulkhead plating

Corrosion increases possibility for shear buckling

Shear buckled panels will have a reduced shear strength, which may lead to an overload of adjacent areas

SFSF

Shear buckling (middle and upper area ofbulkhead most exposed due to corrosionrisk and reduced original scantlings)

18.02.2005Slide 1

Oil Tankers 6. Web frames

Hull Structural Breakdown -Web frames

1.

2.

3.

4.

5.

6.

SideBottomDeck

Transverse bulkhead

Web framesLongitudinal bulkhead

18.02.2005Slide 2


Structural build up of web frame

Cross tie

Web frame flange

Web frames

18.02.2005Slide 3

Oil Tankers 6. Web framesFunction of web frames

- Web frames are supports for the longitudinal stiffeners

- Web frames contributes to the hull girder transverse strength

18.02.2005Slide 4

Oil Tankers 6. Web framesFunction of web frame

• Web frames are supports for the longitudinals

• Web frames take up local loads from the longitudinal stiffeners and transfer them further into the hull girder

• Web frames keep the cross sections together and contribute to the transverse stiffness

Internalpressure

Seapressure

18.02.2005Slide 5

Oil Tankers 6. Web framesCharacteristic damages

1. Corrosion / buckling of web frame

2. Corrosion / cracking of cross tie connection

3. Cracking of tripping bracket connection to web frame flange

18.02.2005Slide 6

Oil Tankers 6. Web framesShear buckling of web frame

High shear stress

SF

SF

18.02.2005Slide 7

Oil Tankers 6. Web framesTYP. WEB SEC. (SHEAR STRESS)

LC 2Shear buckling may occur in areas where shear stress is high

18.02.2005Slide 8

Oil Tankers 6. Web framesShear buckling of web frame:

Corrosion of web frame increases the risk for shear buckling

Corroded cut outs and openings in web frame are exposed to buckling, because of the reduced shear area (high τshear)

18.02.2005Slide 9

Oil Tankers 6. Web framesCorrosion of cross tie

Weld connection of straight part of cross tie to the side and

longitudinal bulkhead

18.02.2005Slide 10

Oil Tankers 6. Web framesCorrosion of cross tie

Cross ties are subject to bothcompression and tension stress depending on loading condition

Corrosion

Reduced Buckling capacity

Increased stress level

Cross tie collapse?

+/- Axial stress

18.02.2005Slide 11


Crack in tripping bracket connection to web frame flange

Weld connection of large curved flanges and tripping brackets on webframes

18.02.2005Slide 12


Cause for cracking in web frame flange

- If exposed to compression, the flange will bend inwards

Deflection patternof free flange

Cracks occur due to additionalbending stresses from the presenceof a tripping bracket in the curved

part of the flange

- If flange is exposed to tension, the flange will bend outwards

18.02.2005Slide 13

Oil Tankers 6. Web framesFEM plot of cross tie with deflections

18.02.2005Slide 14

Oil Tankers 6. Web framesCracks in web frame

• Webframe support for longidudinals – reduced support – excessive load on longitudinals

• Increased loads on adjacent webframes

• May lead to loss of stiffened panel

18.02.2005Slide 1

Bulk

Carriers Bulk Carriers - Hull Structure

18.02.2005Slide 2

Bulk

Carriers Contents – Bulk Carriers

1. Introduction to Bulk carrier hull structure

2. Hull structural breakdown – function of hull elements:• Side, bottom, deck, transverse bulkhead, longitudinal bulkhead,

web frames including relevant hull damages for all structural elements

3. Case

18.02.2005Slide 3

Bulk

Carriers Characteristics for Bulk Carriers

• Single skin / hopper & top-wing tanks• Heavy cargoes• Large net load on double bottom • High shear stresses shell side• Sensitive to leakage - total structural loss• High loading rate• Transverse strength • Green seas• Not much public attention (no vetting)• Low survival capability when flooded• High number of vessels lost

18.02.2005Slide 4

Bulk

Carriers Bulk Carrier loading flexibility

• Bulk Carrier HC/EA– Any hold empty at full draught

• Bulk Carrier HC/E – hold 2,4,6 …. Empty– Given combination of holds empty at full draught

• Bulk Carrier HC– Any hold empty at 80% of full draught

• Bulk Carrier– Any hold empty at 60% of full draught

Red

uced

flex

ibili

ty

18.02.2005Slide 5

Bulk

Carriers History

• Built in 1954 - Cassiopeia

• First bulk carrier with hopper tank – topside tank cross section

18.02.2005Slide 6

Bulk

Carriers Bulk Carrier particulars

5 cargo holds

7 cargo holds

9 cargo holds

18.02.2005Slide 7

Bulk

Carriers Nomenclature

18.02.2005Slide 8

Bulk

Carriers Nomenclature

18.02.2005Slide 9

Bulk

Carriers

- A vessel’s hull can be divided into different hull structural elements

- Each element has its function in the structure

- In order to assess the structure of a Bulk Carrier youneed to understand the function of the structural element you are looking at


18.02.2005Slide 10

Bulk

Carriers Typical damages and repairs

WWW.witherbys.com

18.02.2005Slide 11

Bulk

Carriers

5. Topside tank1. Side

2. Bottom

3. Deck

4.

Transverse bulkhead

Structural breakdown of Bulk Carrier

7. Hatch coaming & cover

6. Hopper tank

18.02.2005Slide 12

Bulk

Carriers Hull Structural Breakdown

1.

2.

3.

4.

5.

6.

SideBottomDeckTransverse bulkheadHopper tankTopside tank

7. Hatch cover & coaming

Bulk Carrier

Hull Structural Breakdown -Ship side

1.

2.

3.

4.

5.

SideBottom Deck

Transverse bulkheadHopper tankTopside-tank

1. Side

6.

Bulk Carrier Structural functions of ship side


- Resist external sea pressure

- Resist internal pressure from cargo and ballast



1. Side

Bulk Carrier Structural build up of ship side 1. Side

Side frames

Lowerbracket

Side plating

Upperbracket

Bulk Carrier Structural functions of ship side

Watertight integrity (local strength)

1. Side

Loads are taken up by the hull plating, stresses are transferred into the vertical side frames – further into the upper and lower bkt’s further into the topwing tank and hopper tank structure

Ship side must withstand static and dynamic loads from external sea pressure as well internal pressure from cargo and ballast

Bulk Carrier Functions of ship side 1. Side

Watertight integrity (local strength)

Lateral loads induces shear forces and bending moments in the vertical side frames. The side frame is a single beam supported at hopper / twt bkt’s

BmSF

Bulk Carrier

Net load down cause rotation of hopper tank structure. additional moment in the mid-field and upper end

Functions of ship side 1. Side

Ore hold load response;

SF Bm Bm

Bulk Carrier

Net load up cause rotation of hopper tank structure. additional moment in the mid-field and lower end

Functions of ship side 1. Side

Empty hold load response;

SF Bm Bm


Side plating is actingas web in hull girder

beam

Global loads areacting on the hull

girder beam


Cont.

Ship side is taking up global shear forces resulting from the hull girder bending moment and weight/buoyancy distribution along the vessel length

Bulk Carrier

Bend

ing

mom

ent

Hog

ging

Sagg

ing

0

Shea

r for

ce

0

Function of ship side (longitudinal shear strength)Sh

ear f

orce

(t-m

)

Shear Distribution at a cross section Cont.


Shear force distributionresulting from global

loads for midshipsection

Web in hull girder (global strength)- Global shear forces are distributed in the ship side plating Cont.

Bulk Carrier Hull damages in ship side 1. Side

Two characteristic damages for ship side:1. Cracks in side frames at lower / upper bracket connection

2. Corrosion of side frames and lower bkt. – detached bkt’s

Bulk Carrier 1. Side

Vertical side frame lowerbkt. commection

Crack in side longitudinal web frame connection

Cracking in vertical side frame:

Bulk Carrier

The dynamic loads from the sea are taken up by the side plates supported by the vertical side frames and load is transferred to the upper and lower bkt’s. This gives peak of bending moment and shear in way of lower bkt. connection.

Cause for cracking in vertical side frames lower bkt. connections 1. Side

1a. The sniped termination of the bracket flange creates a local stress concentration, which may develop cracks from the toe of the bracket

1a.

1b.

In this point a high bending stress in flange and a stress concentration due to weld (overlap) increase the risk for fatigue cracks.

1b.

Bulk Carrier

Crack in side longitudinal web frame connection Possible consequence

• As these cracks develop, the lower end fixation of the side frame is reduced:– higher bending moment in the middle of the frame– some of the load will be carried by adjacent frames

• Crack through stiffener:– beam simply supported lower end, profile may buckle at mid-

field

• Side shell may crack.

• Adjacent frames crack – panel collapse, possible water flooding.

1. Side

Bulk Carrier

Side frames and bkt’s are prone to corrosion, both general corrosion as well as grooving corrosion which may result in :

• Local corrosion and grooving

• General wastage.

• Fractured/detached frames• Fracture in plating/bracket toes

Corrosion of side frames and lower bkt. connection 1. Side

Bulk Carrier

Torig T-min T-subst T-CoatHold 1:Aft end of Hold 1:Upper bracket web 13,0 9,8 10,6 11,2Frame web, middle and upper part 13,0 9,8 10,6 11,2Frame web, Lower part 13,0 11,2 11,6 11,2Lower bracket web 15,0 11,3 12,2 12,7Frame flange thickness, middle and upper part 20,0 15,0 16,3 N/AFrame flange thickness, lower part 20,0 15,0 16,3 N/ALower bracket flange thickness 20,0 15,0 16,3 N/AMiddle part of Hold 1:Upper bracket web 13,0 9,8 10,6 11,2Frame web, middle and upper part 13,0 9,8 10,6 11,2Frame web, Lower part 13,0 9,9 10,7 11,2Lower bracket web 15,0 11,3 12,2 12,7Frame flange thickness, middle and upper part 20,0 15,0 16,3 N/AFrame flange thickness, lower part 20,0 15,0 16,3 N/ALower bracket flange thickness 20,0 15,0 16,3 N/AForward end of Hold 1:Upper bracket web 13,0 9,8 10,6 11,2Frame web, middle and upper part 13,0 9,8 10,6 11,2Frame web, Lower part 13,0 13,9 NB! N/ALower bracket web 15,0 16,9 NB! N/AFrame flange thickness, middle and upper part 20,0 15,0 16,3 N/AFrame flange thickness, lower part 20,0 15,0 16,3 N/ALower bracket flange thickness 12,5 9,4 10,2 N/A

Upper Bracket

Low er Bracket

Middle and upper part of Frame

Low er part of Frame

Revised Minimum Thickness List

Bulk Carrier Corrosion of side frames and lower

bkt. Connection – Consequences 1. Side

• Local grooving of side frame support bkt’s – Shear area of profile web reduced– If angle bar specially critical

• Detached lower side frames– Frames simply supported, increase BM –

buckling– Side plate rupture top of hopper tank - flooding

• General corrosion of side frames reduce the shear area and section modulus. – Bending moment stress level increases– Stiffeners may collapse in buckling

Bulk Carrier Damage impact on function 1. Side

1. Cracks in vertical side frame- may increase moment in field for frame- may increase loads on adjacent frames- may cause water ingress leakage- may develop to panel collapse - flooding – stability - strength (loss of ship)

2. Corrosion of side frames- As above

Bulk Carrier 2. Bottom

Slide1

Hull Structural Breakdown -Bottom

1.

2.

3.

4.

5.

SideBottom Deck

Transverse bulkheadHopper tankTopside-tank6.


Slide2

1. Watertight integrity (local strength bottom / inner bottom)

- Resist external sea pressure (bottom)

- Resist internal pressure from cargo/ballast & fuel oil

2. Carry net load on double bottom girder structure

- Inner bottom / bottom plate & stiffn. are girder flanges

- double bottom floors / girders are webs in double bottom girders

2. Bottom flange in hull girder (global strength)

- Bottom and inner bottom structure is the bottom flange in the hull girder



Slide3

Structural build up of bottom

Longitudinal girders

Floor

Pipe tunnel


Slide4



Cont.

Bottom plate must withstand static and dynamic loads from external sea pressure as well internal pressure from ballast or fuel oil

Inner bottom plate must withstand static and dynamic loads from cargo hold as well as static and dynamic pressure from ballast or fuel oil


Slide5


• Stress distribution in a double bottom structure

• Forces are taken up by the stiffest structure

• Middle of hold more stresses in transversedirection

• Towards bhd. – more stresse in longitudinal direction


Slide6

Functions of inner bottom (local stiffener level)

Cargo hold boundary (local strength)

External loads induce shear forces and bending moments in the inner bottom longitudinals as single beams (between floors)

BM and SF distribtion for a single beam with distributed load and fixed ends

Cont.


Slide7

Load response double bottom

Cont.


support


support


Slide8

• girders & floors carry the net load to hopper tank and transverse bulkhead

• floors carry most of the loads in middle of hold

• longitudinal girders carry most of the load towards transverse bulkhead

• length / width ratio is important for the distribution of loadsbetween girders & floors

• The stiffest elements are taking most of the load / stresses seek the shortest way to supports

Double bottom girders load response


Slide9

Functions of double bottom girder

Net Load on double bottom

Longitudinal girders represented by springs

Simple beam model


Slide10

Floors / girders- design

High Shear force – No cut-outs / increased

thickness

Long. Db. girder

Floor


Slide11

Functions of bottom

Bottom structure is acting as web in hull

girder beam


girder beam


Cont.

The bottom and inner bottom longs and longitudinal girders are carrying the vertical bending moments from still water and wave induced bending moments along the vessel length


Slide12

Still water bending moment [intact]Max allowable bending moment [intact]

Moment diagramB

endi

ng m

omen

t

TM

Reduced global bending but high double bottom

stresses


Slide13

Highly stressed areas

Bottom plate/longs middle of empty holds (compression )

Bottom plate in loaded holds (tension)

Inner bottom plate middle of loaded holds (compression )

+

Double bottom bending

Tanktop

Bottom

NA

Inner bottom level

Global bendingBottom

Deck


Slide14

Hull damages bottom / inner bottom

Three characteristic damages for bottom are:1. Cracks in inner bottom plate in way of knuckle to hopper tank

2. Crack / Corrosion of floors – girders in ballast tanks

3. Indents of inner bottom plate due to cargo handling


Slide15

Fractures

Cracks in way of hopper knuckle

• Heavy ballast condition – tension in inner bottom plate


Slide16

Cracks in way of hopper knuckle

Hopper plate

Inner bottom plating


Slide17

• Loss of watertight integrity – leak ballast –cargo

• Cracks extending from one webframe to another severe impact on double bottom strength

Cracks in way of hopper knuckleImpact on function


Slide18

A

A

Inner bottomFractures

Doublebottomfloor

Hoppertransverseweb

Side girder

View A-ATransverse fractures inhopper web platingpossibly extending intothe hopper sloping plate

Innerbottom

Floor ortransversewebplating

Fracture in thefloor/web of thehopper transverse

DamageFull penetration weldconnection to the innerbottom and hopperplating

Collar plate

Edge chamfered forfull penetration weld

Reinforcement A Intermediatebrackets (i.e.between floors)Alternatively, may

stop at longitudinalswhere fitted

Reinforcement BFace plate oftransverseweb

Scarfing brackets

Inner bottom

View B-B

Repair

Fractures in connection offloors i.w.o. hopper


Slide19

Crack in floor

Damage

Floor or transverse web frame

Longitudinal

Bottom shell plating, inner bottom plating, side shell plating or hopper sloping plate

Buckling and/or fracturing

Fractures

Fractures

Repair ALug

New plating ofenhanced thickness

Repair B

Full collar plate

• Floor in way of high shear stress

• Connection at bottom longitudinals


Slide20

Crack in floor impact on function

• Loss of support of longs – increased stresses at adjacent floors – longs

• Large crack in floor – increased stresses in adjacent floors - girders


Slide21

Indents of inner bottom plate


Slide22

Indents of inner bottom plateImpact on function

• Difficult with discharge of cargo – cleaning• Severe indents – cracks – leak• Impact on buckling capacity of panel


Slide23

Fracture in longitudinals at stool connection

Damage Cause

Damage due to stress concentrations and large relative deflections (bulkhead stool - first floor) leading to accelerated fatigue in this region.

Stool

Inner bottomlongitudinal

Fractures

Bottom shell longitudinal


Slide24

Repair

Too large brackets may cause further problems.

Stool

Additionalbrackets withsoft toes

Where required the longitudinal to becropped and part renewed



Slide25

Damage

Stool

Inner bottom

Bilge well

Fracture

Fracture

Repair

Modified bracketswith soft toes

Additional bracketwith soft toes

Where required the longitudinals to becropped and part renewed


Bulk Carrier 3. DeckHull Structural Breakdown - Deck

1.

2.

3.

4.

5.

SideBottom Deck



Bulk Carrier 3. DeckStructural functions of deck



2. Transverse strength of the hull girder

3. Upper flange in hull girder (global strength)

Bulk Carrier 3. DeckStructural build up of deck

• Main deck outside line of hatches• Deck between hatches

• Longitudinal hatch coaming

• Transverse hatch coaming

• Deck webframe



Deck plate must withstand static and dynamic loads from green sea pressure as well as internal pressure from ballast tank


• Stress distribution in deck

Bulk Carrier 3. Deck

Flexing in transverse direcction

Structural functions of deck

• Deck between hatches


• The element contributing to transverse strength:– Deck plate and transverse stiffener between hatches– Hatch end girder– Upper stool tank

Bulk Carrier 3. DeckFunctions of deck

Deck structure is acting as web in hull

girder beam


girder beam

2. Upper flange in hull girder (global strength)

Cont.

The deck plating and longs outside line of hatches are carrying the vertical bending moments from still water and wave induced bending moments along the vessel length

Bulk Carrier 3. DeckHull damages deck

Characteristic damages for deck are:1. Cracks in deck plate at end of longitudinal hatch coaming

2. Buckling of deck between hatches

3. Crack in deck plate in way of hatch corner

Bulk Carrier 3. DeckCrack in deck plate at

hatch coaming end

• Longitudinal stresses are going into the side hatch coamings

• At the toe of the bkt. There is a local stress concentration

Possible consequences:- Water leak to cargo

- Long crack – longitudinal strength problem

Bulk Carrier 3. DeckBuckling of deck between hatches

• Ore carrier (250 000 DWT) Local buckling of deck plates and transverse stiffeners.

• Deck plates and transv. Stiffn. buckled•


2 adjacent holds filled

• Buckling caused by excessive stresses in transverse direction deck between hatches

Bulk Carrier 3. Deck

• Possible consequences of buckling of deck between hatches:- Ships transverse strength severely affected- Ships sides comes in- Hatch coamings deformed - Loss of weather tight integrity

Buckling of deck between hatches

Bulk Carrier 4. Bhd.Hull Structural Breakdown -

Bulkhead

1.

2.

3.

4.

5.

6.

SideBottomDeckTransverse bulkheadHopper tankTopside tank


Bulk Carrier 4. Bhd.Structural functions of bhd.

1. Cargo hold boundary (local strength)

- Resist internal pressure from cargo / ballast

- Resist water flooding

2. Transverse strength of the hull girder

Bulk Carrier 4. Bhd.Structural build up of deck

Corrugated bhd.

Lower stool

Upper stool

Bulk Carrier 4. Bhd.Structural build up of deck

Lower stool diaphragm

Upper stool diaphragm

Hatch coaming bkt

Shedder plate



Transverse bhd. plate must withstand static and dynamic loads from bulk cargo and ballast

The bulkhead must also withstand the water pressure from flooding of cargo hold without collapse

Bulk Carrier 4. Bhd.


Design load conditions

• Water flooding

• ” Light cargo ” full hold

SF Bm

High stress lower / upper end & midfield


flange

Web

Structural functions of bhd.


One sided load on bhd. Introduce a moment in lower stool.

Size of moment incrase by narrow lowerstool ( s – on sketch)

High stress at intersection lower stooldiaphrame and longitudinal girders

Narrow stool – high shear stress in diaphrames

s

Moment


Loaded holdEmpty hold

Moment onlower stool

Structural functions of bhd.

• Transverse bhd. Supports the double bottom long. girders


Net load from cargo

• Transverse bhd. Carryglobal shear from double bottom to shipside


• Upper and lower stool transverse strenght of hull

Flexible part

Bulk Carrier 4. Bhd.Hull damages transverse

bulkhead

Two characteristic damages for transverse bulkheads:1. Collapse of bulkhead due to corrosion in lower stool diaphrames.

2. Shear buckling of corrugated bulkhead due to excessive corrosion

Bulk Carrier 4. Bhd.Collapse of transverse bulkhead

Capesize Bulk Carrier 9 holds – 20 years • Loaded with pellets alternate holds

• Bhd. Hold 8/9 collapsed at bottom

• Hatch coamings / covers pulled down

• Inspection revealed heavy corrosion in lower stool

• Void space – humidity – heating in double bottom below.

s

Moment

Heavy corrosion


Casualty information

Collapse of transverse bulkhead

Bulk Carrier loaded with pellets1. Transverse bulkhead collapsed at

connection between lower stool and tank-top

SF Bm

LOWER STOOL DIAPHRAME

3. Bulkhead collapsed due to insufficient shear area at connection to tank-top

2. Inspection revealed excessive corrosion at the lower end of the diaphrames in excess of 50%.


Collapse of transverse bulkheadImpact on function

• No boundary between cargo holds

• Transverse strength of hull girder lost

• Watertight integrity lost upper deck

• To be repaired before leaving port

Bulk Carrier 4. Bhd.Shear buckling transverse

corrugated bulkhead

Capesize bulkcarrier 7,5 years found with shear buckling on transverse corrugated bulkehad observed during routine inspection.

Experience feedback

Buckling cause

2 adjacent holds filled


• Hatch end coaming will be deformed – impact on weather-tightness - flooding

• Longitudinal girders in double bottom is getting less support at transverse bulkhead – more stresses in the floors.

• Hopper tanks will rotate more – loads on side frames will increase

• Vessels transverse strength will be severely affected.

• Vessel may capsize!

Shear buckling transverse corrugated bulkhead impact on function

Bulk Carrier 5. Hopper tankHull Structural Breakdown –

Hopper tank

1.

2.

3.

4.

5.

SideBottom Deck


Bulk Carrier 5. Hopper tankStructural functions hopper tank



- Resist sea pressure on ship side

2. Give support for side structure and double bottom

3. Web in hull girder (global strength)- Side plating / hopper tank sloping plate are part of the web in the hull girder beam - Hopper tank bottom plate and lower part of side plate are part of the bottom flange in the hull girder

Bulk Carrier 5. Hopper tankStructural build up hopper tank

Hopper tank sloping plate

Bottom side girderoutboard

Hopper tank side plate

Bilge plate

Bulk Carrier 5. Hopper tankStructural build up hopper tank

Vertical side framesupporting bkt.

Hopper transverse web frame

Bulk Carrier 5. Hopper tankStructural functions of hopper

tank


Cont.

Hopper tank sloping plate must withstandstatic and dynamic loads from bulk cargo and ballast

Plate – Stiffener – Web frame – Panel – Hull girder

1. Watertight integrity (local strength)Bottom and side plate must withstand static and dynamic loads from external sea pressureand from internal ballast

Bulk Carrier 5. Hopper tankStructural function

Local loadsDesign load conditions

• Ballast pressure• Ore load

Pressure due to ballast

Pressure due to cargo

Bulk Carrier 5. Hopper tank

Structural function Hopper tank Local loads


Similar for side longs and bottom longs

High stress at webframeconnection & midfield



Sea pressure

Sea pressure

Full load condition empty hold

Combined effect of pressure on ship side and on double bottom gives compression stresses in hopper plate

Bulk Carrier 5. Hopper tankStructural function of webframe

Local loads

SF BMHopper

tank webframe

Concentrated loads from

hopper longs

Areas with high shear stress

Back

Bulk Carrier 5. Hopper tankFunctions of hopper tank

global loads

Global loads are acting onthe hull girder beamWeb in hull girder (global strength)

Ship side, hopper tank and top-wing tanks is taking up global shear forces from wave induced loads and weight/buoyancy distribution along the vessel length

Bulk Carrier 5. Hopper tankGlobal function of hopper tank

Glo

bal s

hear

forc

e

Shear flow distribution in hopper tank

Note the shear force is distributed between hopper tank sloping plate and ship side


High shear stress in hopper tank plate and outboard double bottom girder towards bulkheads

Global response of hopper tank

NET LOAD ON DOUBLE

BOTTOM GIRDER

Bulk Carrier 5. Hopper tankGlobal response of hopper tank

Effect of side pressure and net load on double bottom gives torsion of hopper tank, specially in loaded ore hold

Net load on double

bottomSea

pressureShear

stress

Bulk Carrier 5. Hopper tankHull damages Hopper tank

Characteristic damages for hopper tanks:1. Cracks in way of knuckle line between hopper tank sloping plate

and inner bottom plate

2. Crack in webframe in way of sloping plate lower long. Connection to webframe

Bulk Carrier 5. Hopper tankCrack in webframe at hopper

tank / inner bottom knuckle

Stress concentration in way of scallop

Heavy ballast condition

Net load down and out on shell side

Bulk Carrier 5. Hopper tankRepair method

• Close scallop by doublerplate, (reduce local stress concentration)

• Fit bracket in line with inner bottom (reduce effect of hard spot where inner bottom welded to webframe)

Or:• Vertical brackets fwd. / aft

of webframe (distribute the stresses in way of the webframe)

Bulk Carrier 5. Hopper tankCrack in webframe at lower end sloping plate

Webframe cracked at scallop for longitudinal

High Shear stress


• Crack will reduce webframe strength • Hopper tank longitudinals will transfer more

load to the adjacent webframes• Hopper tank longitudinal may loose its support

– double span of stiffener• May develop cracks in adjacent webframes• May develop cracks in hopper tank plate –

water flooding of cargo hold

Crack in webframe impact on function

Bulk Carrier 6. Topside tankHull Structural Breakdown –

topside tank

1.

2.

3.

4.

5.

SideBottom Deck



Bulk Carrier 6. Topside tankStructural functions topside

tank



- Resist sea pressure on ship side

2. Give support for side structure and hatch coaming

3. Web in hull girder (global strength)- Side plating / top-wing tank sloping plat are part of the

web in the hull girder beam - topside tanks upper part is part of the upper flange in

the hull girder beam

Bulk Carrier 6. Topside tankStructural build up topside tank

Deck plating & longs

Topside tank, vertical strake

Topside tank, sloping plate & longs

Topside tank, side plate& longs

Bulk Carrier 6. Topside tankStructural build up topside tank

Topside tank transveres webframe, deck

Topside tank transveres webframe, side

Topside tank transveres webframe, sloping plate

Vertical side framesupporting bkt’s, upper

Bulk Carrier 6. Topside tankStructural functions of topside

tank tank


Cont.

topside tank sloping plate must withstandstatic and dynamic loads from bulk cargo and ballast


1. Watertight integrity (local strength)Deck and side plate must withstand static and dynamic loads from external sea pressureand from internal ballast

Bulk Carrier 6. Topside tankStructural function

Local loadsDesign load conditions

• Ballast pressure• Light bulk cargo / ballast

Pressure due to ballast ( cargo)

• Sea pressure

Bulk Carrier 6. Topside tank


BM and SF distribtion for a single beam with distributed load from external and sea-presure and fixed ends

High stress at webframeconnection & midfield

Topside tank lower side long.

Cont.



Sea pressure on long

+Load from vert.

stiffener

Resulting BM and SF

External sea pressure Distributed load on side frame

BM & SF upper end vertical side frame

BM

SF

Back

Bulk Carrier 6. Topside tankGlobal strength function of

topside tank, bending moment

Upper part of ship side and sloping plate areimportant contributors to the top flange in the hull girder beam

Global loads are acting on thehull girder beam from cargo distribution and wave loads

Flange in hull girder (global strength)

Cont.

Bulk Carrier 6. Topside tankGlobal strength function of

topside tank, shear

Global loads are acting onthe hull girder beamWeb in hull girder (global strength)

Cont.

Ship side, hopper tank and top-wing tanks is taking up global shear forces from wave induced loads and weight/buoyancy distribution along the vessel length

Bulk Carrier 6. Topside tankGlobal Strength topside tank

Glo

bal s

hear

forc

e

Shear flow distribution topside tank

Note the shear force is distributed between hopper tank sloping plate and ship side

Bulk Carrier 6. Topside tankStrength topside tank

Full load condition

Ore hold

Sea pressure

Topside tank rotate up and out

Net load on double bottom and side pressure rotate hopper tank as shown

Cont.

POSTFEM 5.6-02 18 MAR 3SESAM

X YZ

MODEL: T1-1 DEF = 1002: LINEAR ANALYSISGAUSS D-STRESS SIGMXSURFACE: 1MAX = 85.8 MIN = -168

-156-144-131-119-107-95.2-83.2-71.1-59-47-34.9-22.8-10.71.3313.425.537.549.661.773.8

FEM - PLOT


High shear stress in topside tank sloping plate and ship side towards transverse bulkheads due to global shear stresses and torsion of topside tank

Global response of topside tank

Bulk Carrier 6. Topside tankHull damages topside tank

Characteristic damages for topside tanks:1. Buckling deformation due to overpressure of ballast tank

2. Crack in lower side long in topside tank

3. Heavy corrosion topside tank webframe

Bulk Carrier 6. Topside tank1. Overpressure of topside tank

• Vessels with high ballast pump capacity, filled to overflow through air pipes, with possible excessive pressure in topside tank

Typical location for overpressure buckling


• Deformed webframe has lost its strength and may not be able to support the side and sloping plate longs.– If longs are not efficiently supported at webframes

they may be excessively loaded in the mid-field, and may buckle, however normally a local strength problem

1. Overpressure of topside tank impact on function

Bulk Carrier 6. Topside tank2. Crack lower side long

Experience feedback

Fatigue crack through side long. Flange in

way of weld to flatbarstiffener on top


2. Crack lower side long. impact on function

• Crack impact on function– Crack through side long. may lead to

penetration of shell side, and cause leak of water.

• If side longs are cracked, the upper support for the vertical side frame is weakened

• Less fixation at upper end of vertical side frame will give higher stresses in the field and in way of lower end.

• The stresses in the vertical side frames may become excessive –could lead to collapse of side frame and water flooding.

Bm

SF

Bulk Carrier 6. Topside tank3. Heavy corrosion in topside tank

Vessel with vertical stiffener on ship side and sloping plate

Poor buckling strength exposed to longitudinal compression stresses

Calculation of allowable t-min values for side & sloping plate revealed marginal allowable reduction

Bulk Carrier 6. Topside tank4. Corrosion of webframes in topside

tank

Heavy local wastage of webframe in way of deck &

side longs


• Local corrosion of webframemay lead to deck longs lose their attachment to webframe– Span for deck longs two times

design value, Local strength requirement increase by 4-times (square of the stiffener span)

– Buckling capacity significantly reduced

4. Corrosion of webframes in topside tank consequence

May lead to global structural collapse !

Bulk Carrier 7. Hatch cover & coamingHatch cover & coaming

1.

2.

3.

4.

5.

SideBottom Deck



Bulk Carrier 7. Hatch cover & coaming

Structural functions of Hatch cover & coaming


- Resist dynamic loads from green seas, horizontal & vertical pressure

2. Hatch coaming supports the hatch covers

3. Hatch end coaming contributes to transverse strength

Bulk Carrier 7. Hatch cover & coamingStructural build up of deck

• Longitudinal hatch coaming, web & flange

• Hatch end coaming, web & flange

• Hatch end bracket

• Hatch side bracket


Structural functions Hatch cover & coamings


Cont.

Hatch cover & coaming plate must withstand dynamic loads from green sea pressure as well internal pressure from ballast in combined cargo / ballast hold.


2. Load on hatch covers (local strength)Hatch cover & coaming plate must withstand static and dynamic loads from deck cargo if this is allowed (containers / timber ).


• Longitudinal global stresses


• The longitudinal stresses in deck due to cargo distribution and wave loads will ”flow” into the longitudinal hatch coamings. The hatches in the midship region with full longitudinal stresses most exposed

High stress areas


• Transverse stresses



Q = q x l /2

M = q x l2 / 8

SF.

BM.

q

l

q x l /2

Transv. girder

Hatch cover with green seas load

Transverse girder single beam with distributed load

Structural function local load hatch cover

Bulk Carrier 7. Hatch cover & coamingHull damages hatch

cover/coaming

Characteristic damages for hatch cover & coaming are:1. Crack in hatch coaming flange

2. Shedder plate

3. Corrosion on hatch covers

Bulk Carrier 7. Hatch cover & coamingCrack in deck plate at

hatch coaming end

Crack in hatch coaming flange amidships

Note cut-outs for hatch cover hydraulic lifting jacks

Local high stress concentration due to square cut-outs and reduced cross section area


Crack in deck plate athatch coaming, consequence

- Crack in coaming may cause water leakage – damage to cargo

- Crack may propagate to main deck

- Impact on longitudinal strength

Bulk Carrier 7. Hatch cover & coamingCorrosion of hatch covers


• Moisture in cargo – some dry bulk cargoes may become liquified (Ref. IMO code for safe practice for solid bulk

cargoes BC code sec. 7 App. A)

• Reduced thickness of stiffeners and girders may cause collapse of stiffener / girder

• Possible flooding of cargo holds – impact on longitudinal strength and stability / trim

Corrosion of hatch coversImpact on function

18.02.2005Slide 1

Container

Ships Container Ships - Hull Structure

18.02.2005Slide 2

Container

Ships Contents – Container Ships

1. Introduction to Container Ship hull structure

2. Hull structural breakdown – function of hull elements:• Bottom, side, hatch, deck and hatch coaming and transverse

bulkhead including relevant hull damages for all structural elements

3. Case

18.02.2005Slide 3

Container

Ships Ship related characteristics

Feeder

Panamax

Open Top

Post Panamax

• Double Hull• Flexible hull girder – torsion• Critical hull girder strength – high tensile steel• High freeboard• Worlds largest engines (100 000 BHP)• High Speed• Light loads• Value of cargo up to 5 times value of ship• Liner Trade

Any proposals?

18.02.2005Slide 4

Container

Ships

Container Carriers,Bulk Carriers and Oil Tankers

Historical Fleet Development

1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 20020

1

2

3

4

5

6

Mill. TEU

0

100

200

300

400

500Mill. Dwt.

Container Carriers (TEU)Bulk Carriers (Dwt.)Oil Tankers (Dwt.)

Average growth 1997 - 2002:Container Carriers: Bulk Carriers: Oil Tankers:

+9.3 % +3.1 % + 3.2 %

2002 - Year-end figuresSource: Fairplay/Clarkson

18.02.2005Slide 5

Container

Ships It started in the late 50’s

18.02.2005Slide 6

Container

Ships The Container Ship Development

• Container ship era started late 60’s• 70 ship below 2000 TEU delivered before 1970• In the 70’s ships up to 3000 TEU• Big Panmax built in the 80’s, exceeding 4000 TEU• Post Panmax ships today designed with capacity

exceeding 8000 TEU• 10000 TEU now contracted at HHI

18.02.2005Slide 7

Container

Ships

Feeder

Panamax

Open Top

Post Panamax

• Loa 100 - 200 m long

• Service speed range is 18 to 22 Knots in general

• Cranes are often arranged to achieve flexible operating ability

• Damage stability criteria influence on hatch cover tightness and subdivision of hold area

• Fully aft located deckhouse can be seen often

• Mixed stowage (Russian stowage) in hold is common

Types of Container Ships

18.02.2005Slide 8

Container

Ships

Feeder

Panamax

Open Top

Post Panamax

• 3800 – 4800 TEU• Max Loa = 294 m• Service speed 24 knots• 11 rows in hold in general, but 12 rows is possible • 8 tiers in hold, 5 tiers on deck


18.02.2005Slide 9

Container

Ships

Feeder

Panamax

Open Top

Post Panamax

• Loa 270 m (5,500 TEU) to 340 m (9000 TEU) • 5,500 TEU has been popular size, but it’s a trend that the

ships become bigger and bigger• Service speed 25-26 knots• HT40 steel is often used to upper deck and hatch coaming


18.02.2005Slide 10

Container

Ships The Cargo

Total value = Ship + Cargo = 100 + 500 = 600 000 000 USD

18.02.2005Slide 11

Container

Ships The Cargo

Post Panamax Container Ship in Typhoon Babs - Pacific, October 98

• 300 containers lost • ab. 100 more damaged• Cargo claim ~ 50mUSD (or even higher)• New ship price ~ 92mUSD

18.02.2005Slide 12

Container

Ships

- A vessel’s hull can be divided into different hull structural elements

- Each element has its own function in the total hull integrity

- In order to assess the structure of a Container Ship you need to understand the function of the structural element you are looking at


18.02.2005Slide 13

Container

Ships Hull Structural Breakdown

1.

2.

3.

4.

5.

BottomSide

Deck & hatch coamingTransverse Bulkhead

Hatch

18.02.2005Slide 1

Container

Ships1. BottomHull Structural Breakdown

1.

2.

3.

4.

5.

BottomSide

Deck & hatch coamingHatch

Transverse Bulkhead

18.02.2005Slide 2

Container

Ships1. Bottom

1. Watertight integrity (local strength bottom / inner bottom)

- Resist external sea pressure (bottom)

- Resist internal pressure from ballast & fuel oil

2. Carry net load on double bottom girder structure

- Inner bottom / bottom plate & stiffn. are girder flanges

- Double bottom floors / girders are webs in double bottom girders


- Bottom and inner bottom structure is the bottom flange in the hull girder


18.02.2005Slide 3

Container

Ships1. BottomStructural build up of bottom

Longitudinal girders

Floor Hopper Tank

Bottom platingw/ longitudinals

18.02.2005Slide 4

Container

Ships1. BottomStructural functions of bottom

Stress distribution in a double bottom structure follows the hierarchy:

→ Plating

→ Longitudinals

→ Floors / girders

→ Bulkheads /side

18.02.2005Slide 5

Container

Ships1. BottomFunctions of inner bottom

Cargo hold boundary (local strength)

The internal loads from tanks induce shear forces and bending moments in the inner bottom longitudinals as single beams (between floors)


18.02.2005Slide 6

Container

Ships1. Bottom

External loads from container sockets induce shear forces and bending moments in the floors and girders

Functions of inner bottom

18.02.2005Slide 7

Container

Ships1. Bottom

Load response double bottom


support

18.02.2005Slide 8

Container

Ships1. Bottom

• girders & floors carry the net load to hopper tank and support- and water tight bulkhead

•longitudinal girders carry most of the load towards transverse bulkhead

• length / width ratio is important for the distribution of loadsbetween girders & floors

• the stiffest elements are taking most of the load / stresses seek the shortest way to supports

Double bottom girders load response

18.02.2005Slide 9

Container

Ships1. BottomFunctions of double bottom girder


Longitudinal girders represented by springs

18.02.2005Slide 10

Container

Ships1. BottomFunctions of double bottom girder


Bending Moment

Shear Force

18.02.2005Slide 11

Container

Ships1. Bottom


Bending Moment

ShearForce

Functions of double bottom girder

18.02.2005Slide 12

Container

Ships1. BottomFunctions of bottom

Bottom structure is acting as flange in hull girder beam

Global loads are acting on the hull

girder beam

Bottom flange in hull girder (global strength)

The bottom and inner bottom longs and longitudinal girders are carrying the vertical bending moments from still water and wave induced bending moments along the vessel length

18.02.2005Slide 13

Container

Ships1. Bottom

ENGINE ROOM

Post-Panamax Container ShipMoment & Shear Force Diagram

Ben

ding

Mom

ent [

tm]

Shea

r For

ce [

t]Functions of bottom

18.02.2005Slide 14

Container

Ships1. Bottom

Total hull girder bending moment = Mstill water + M wave

Total BM acting on a vessel

Mwave

Mstill water

Hog

ging

Sagg

ingBM

lim

its

18.02.2005Slide 15

Container

Ships1. BottomHighly stressed areas

•Bottom plate/longs middle of empty holds (compression )

•Bottom plate in loaded holds (tension)

•Inner bottom plate middle of loaded holds (compression )

Double bottom bendingGlobal bending

Deck

Inner Bottom

Bottom

NA

18.02.2005Slide 16

Container

Ships1. BottomHull damages bottom / inner

bottom

Characteristic damages for bottom are:

1. Crack at connection of longitudinals to floors

2. Indents of inner bottom plate

18.02.2005Slide 17

Container

Ships1. BottomCrack at connection of longitudinal to floor

• Floor in way of high shear stress

• Connection at bottom longitudinals

• Areas exposed to high fatigue loading

18.02.2005Slide 18

Container

Ships1. Bottom

• Loss of support of longitudinals – increased stresses in adjacent structure

• Large crack in floor – increased stresses in adjacent floors and girders

Crack of floorImpact on function

18.02.2005Slide 19

Container

Ships1. BottomIndents of inner bottom plate

18.02.2005Slide 20

Container

Ships1. BottomIndents of inner bottom plate

• Severe indents – cracks – leakage• Impact on buckling capacity of panel

18.02.2005Slide 21

Container

Ships1. BottomContact damages in bottom plate

18.02.2005Slide 22

Container

Ships1. BottomContact damages of bottom plate

Impact on function

• Severe indents – cracks – leakage• Impact on buckling capacity of panel

2. SideContainer


1.

2.

3.

4.

5.

BottomSide


Transverse Bulkhead

2. SideContainer

Ships Structural functions of ship side

1. Watertight integrity (local strength)- Resist external sea pressure- Resist internal pressure from ballast / fuel oil tanks

2. Carry net load on double side structure- Inner side / side plate are girder flanges- The webs act as web in double side girder

3. Web in hull girder (global strength)- Side plating and inner side act as the web inthe hull girder beam

2. SideContainer

Ships

Strength deck

Side shell

Side longitudinal

Side stringer

Longitudinal bulkhead

Side frame

Hopper structureFlat, recess or step

Structural build up of ship side

2. SideContainer

Ships Local function: Watertight integrity

External static and dynamic loads induces shear forces and bending moments in the side and inner side longitudinals as single beams (between each web frame)

Side long.as a single beam between twoweb frames


2. SideContainer

Ships

-Side longs are supported at the web frames

- Web frames are supported at the stringers and at the deck and bottom

Shear force

Bending moment

High Shear

Local function: Webs in a double side

L

2. SideContainer

Ships Local function: Stringers in a double side

Stringers contribute to the stiffness of the double hull ship side, which means:

High shear stress in stringers towards the transverse bulkhead

2. SideContainer

Ships Loads on the ship side

Min cargo / max draught

Net force from containers

Max cargo / min draught

Net force

2. SideContainer

Ships

Side plating is acting as web in hull girder beam


Ship side is taking up global shear forces resulting from the hull girder bending moment and weight/buoyancy distribution along the vessel length

Global function: Web in hull girder

Global loads are acting on the

hull girder beam

2. SideContainer

Ships Function of ship sideSh

ear f

orce

Shear Force Distribution

Bending moment

2. SideContainer

Ships Global function: Web in hull girder

Global shear forces resulting from the distribution of cargo and buoyancy are taken up in the ship side plating

Shear stress distribution resulting from global loads for midship section

Area effective intransferring shear force

2. SideContainer

Ships Hull damages in ship side

Characteristic damages for ship side:1. Indents in ship side

2. Fatigue Cracks in side longitudinals

3. Fatigue Cracks in web frame cut out

2. SideContainer

Ships Indents of side shell with stiffeners

The terms indents and buckling should not be mixed up with each other, as the cause for these damages are different:

Indents: Caused by lateral forces.

Buckling: Due to excessive in-plane stresses

Mainly from contact damages:

2. SideContainer

ShipsAcceptance CriteriaDeformations

Local Plate Indents (contact / slamming deformations);Maximum Depth S/12 provided;

smooth indent

no cracks

Small deformation (less than 15 deg) out of plane for stiffeners and girders

Less than

15deg

New IS 5.1 Technical survey Guide

2. SideContainer

Ships Consequense of indents

Sharp indents may lead to cracks and possible leakage

Large area set in (plating and stiffeners) gives reduced buckling capacityAdjacent areas may then be overloaded

2. SideContainer

Ships Fatigue cracks in longitudinals

• Cracks have been detected due to FO leakage to the sea

• 270 cracked longitudinals • Ship was 7 years of age

This could be the future problem in

many container ships!

2. SideContainer

Ships Fatigue cracks in longitudinals

Side longs connection to web frame & transverse bhd.

2. SideContainer

Ships Cause for cracking in side longitudinals

Fatigue Damages are caused by Dynamic Loading

2. SideContainer

Ships Cause for cracking in side longitudinals

•High alternating bending stresses towards the end supports (web frames)

•Highly stressed areas created around geometric’hard points’ (bracket toes, scallops, flat bars)

Ex. Panamax

Potential problem area

2. SideContainer

Ships

• Kg : Gross Geometry (from FEM analysis or standard values)

• Kw : Weld Geometry (typical 1,5)

• Kte : Eccentricity tolerance (production tolerances)

• Ktα : Αngular mismatch (production tolerances)

• Kn : Unsymmetrical Stiffeners (L & bulb-profiles)

Stress concentration factors

2. SideContainer

Ships

31⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛≈

KCN

σWhere:

N = Fatigue life (normally 20 years)

σ = Nominal Stress (dynamic stress amplitude)

K = Stress Concentration Factor

C = Constant (including the environment and mean stress level i.e. compression / tension)

Fatigue Life

2. SideContainer

Ships Standard repair proposal longs / web frames

2. SideContainer

Ships Consequence of damage

Cracks in side longitudinals:- oil leakage and pollution- longitudinal may break off- in worst case (a series of cracks in same area) could induce a

larger fracture (loss of ship side)

2. SideContainer

Ships

Cracks around openings for side longitudinals in web

framesCracks

Fatigue cracks in web frames

2. SideContainer

ShipsCause for cracking in cut outs for longitudinals

Sea loads induce shear stresses in the web frame

Shear stress

Shear stress

High shear stresses around openings etc, where shear area is

reduced

2. SideContainer

Ships Consequence of fatigue crack in webs

Re-distribution of shear stresses in web frame

Side longitudinals loose their support

May lead to overloadingof adjacent structure

How does the damage impact on the function?

2. SideContainer

Ships

• Fatigue is not an exact science– ±10% stress → ±30% fatigue life

• High tensile steel ≈ Mild steel • Corrosive environment → (Fatigue life / 2)• North Atlantic/Pacific → (Fatigue life / 2)• Symmetric profiles have longer fatigue life

“Rules of Thumb” Regarding Fatigue

2. SideContainer

Ships“Rule of thumb” regarding fatigue crack repairs

* Note! cracks in main deck / hatch opening corners to be specially considered

Years

• Workmanship has a significant impact on fatigue life• Repair as function of time for crack to develop:

0-5 Design improvement recommended, check misalignment, possible vibration related

5-10 Design improvement recommended

10-15 Repair to original standard normally acceptable, grinding out and re-welding may also be considered towards 15 years *

< 15 Repair by re-welding normally acceptable *

3. HatchContainer


1.

2.

3.

4.

5.

BottomSide


Transverse Bulkhead

3. HatchContainer

Ships Structural functions

1. Load on hatch covers (local strength)

• must withstand static and dynamic loads from containers

2. Allow for hull deformations

3. Weather tightness

• Resist water pressure

3. HatchContainer

Ships Structural build up

Pin stopper (Rolling / pitching)

Longitudinal stopper (Pitching) Hold down device

(Vertical support)

Support Pads (Vertical support)

3. HatchContainer

Ships

Hatch cover with container load

A-A

A

A

Bending Moment

Shear Force

Structural functions: Container load (local strength)

3. HatchContainer

Ships

Wind Transverse Acceleration

Structural functions: Container load (local strength)

Ph

3. HatchContainer

ShipsStructural functions: Allow for Hull Deformations

15010070Diagonal deflection

(mm)

+ 9000 TEU+ 7000 TEUPanamax.Ship Size

Hull deformation looking down at deck

3. HatchContainer

ShipsStructural functions: Allow for Hull Deformation

3. HatchContainer

ShipsStructural functions: Weather tightness

• Weather tight hatches are to have packing

• Some hatches are not weather tight, i.e. no packing. In case of non weather tight hatches, this is written in the Load Line report.

3. HatchContainer

Ships Hull damages

Characteristic damages related to the hatch cover are damages to the:

• Hatch Cover Support

3. HatchContainer

Ships Hull damages - hatch cover support

Damaged low friction pad

Heavily worn steel to steel

Damage due to corrosion and high forces

3. HatchContainer

Ships Hull damages - hatch cover support

Low friction bearing pad Lubripads for big ships

3. HatchContainer

Ships

• Damages to friction pad may cause an undesired stiff connection

• Introduction of new forces• Potential cracks in the coaming

Consequence of damage

4. Deck and coaming

Container


1.

2.

3.

4.

5.

BottomSide


Transverse Bulkhead

4. Deck and coaming

Container

Ships Structural build up

Hatch side coaming

Coaming stay

Hatch end coaming Hatch coaming top Hatch side coaming

4. Deck and coaming

Container




2. Carry and transfer loads from hatch (local strength)

- Coaming stays are main load carrying element

3. Global strength

-Bending and torsion

4. Deck and coaming

Container

Ships


Deck plate and hatch coaming must be watertight


4. Deck and coaming

Container

Ships

Hatch cover with container load

2. Carry and transfer loads from hatch (local strength)


4. Deck and coaming

Container

ShipsStructural functions: Container load (local strength)

Stays Support

4. Deck and coaming

Container

Ships

Vertical Bending Moment

Structural functions: Global Strength

What kind of global loads are we talking

about and which effects do they have?

4. Deck and coaming

Container

Ships

Horizontal Bending Moment


4. Deck and coaming

Container

Ships

Torsion


4. Deck and coaming

Container

ShipsStructural functions: Incorporate hull deformation

Deck plate and coaming must be strongenough to withstand the combination of all theloadcases!

A typical combination of stresses could be:• Max Still water bending moment (vertical + horizontal + torsion)• 45% vertical wave bending moment• 100% horizontal wave bending moment• 100% wave torsion

4. Deck and coaming

Container

Ships Hull damages

Characteristic damages related to deck & hatch coaming are:1. Hatch Coaming Stays

2. Hatch and Deck Corners

3. Knuckle at Side Hatch Coaming

4. Coaming Termination

4. Deck and coaming

Container

Ships

Hatch Coaming Stays

Cracks in hatch coaming stays

Upper deck

Coaming stay

Upper deck

High Dynamic stress due to friction between hatch

and bearing pad

4. Deck and coaming

Container

Ships Consequence of damage

• Hatch coaming may loose its transverse strength• The cracks may propagate into the deck

4. Deck and coaming

Container

Ships Cracks in Hatch corners

High global stress (vertical and horizontal bending) in addition to torsion may result in fatigue damages in the hatch corners

4. Deck and coaming

Container

Ships

High global stress (vertical and horizontal bending) in addition to torsion may result in fatigue damages in the hatch corners

Cracks in Hatch corners

4. Deck and coaming

Container

Ships

Hatch Corner

Insert Plate

Insert plate IWO hatch corners is to be 25 % thicker than adjacent deck plate

Forward Cargo Hold

Cracks in Hatch corners

4. Deck and coaming

Container

ShipsCracks in hatch corners Consequence

• Cracks in hatch and deck corners should be taken serious! (Contact MTPNO864 if in doubt)

• Crack in hatch corners could indicate a design problem. It is therefore most likely to find similar damages other places too.

• The cracks may develop rapidly in a highly utilized structure

Repair• Thickness increase• Edge grinding• Improved shape

4. Deck and coaming

Container

Ships

Upper Deck

Additional force due to knuckle brings stress concentration at upper deck connection

Cause of Damages:• The transverse member was arranged 100 mm away from the knuckle line• Fine mesh F.E. analysis results show high stress concentration factor of K

= 3.5 (75 mm offset distance and 20° of knuckle angle) at the knuckle point

Cracks in Hatch Coaming Knuckle

4. Deck and coaming

Container

Ships

Upper Deck

Hatch CoamingKnuckle in

Coaming

Upper Deck

Cracks


4. Deck and coaming

Container

Ships

The knuckle has to be supported. A possible

repair is insert of a support bracket


Consequence of crack

• May influence the load carrying characteristics of the hatch coamingwith regard to support of hatch

• Reduced longitudinal strength

4. Deck and coaming

Container

Ships Hatch Girder / Coaming Termination

4. Deck and coaming

Container


Crack

4. Deck and coaming

Container


Repair proposal– Meeting angle of bracket to be less than 15 degrees– Bracket toe and flange end to be grinded after

welding– Full penetration welding to be carried out for min.

500 mm IWO flange and 1000-1500 mm for bracket toe

ConsequenceCrack may develop and penetrate the deck

4. Deck and coaming

Container

Ships Damages to the wave breaker

4. Deck and coaming

Container

Ships

Possible buckling problems

Sea pressure

Damages to the wave breaker

4. Deck and coaming

Container

Ships

• Collapse of wave breaker could lead to damages to the containers or leakage into cargo hold

Damages to the wave breakerImpact of function

5. TransverseBulkhead

Container


1.

2.

3.

4.

5.

BottomSide


Transverse Bulkhead


Container

Ships

Transverse watertight bulkhead

Pillar or support bulkhead

Bulkhead stringer

Vertical girder

Box beam web (diaphragm)

Box beam

Structural build up


Container



- Watertight integrity

- Support of container stacks

- Support the bottom

- Support the stringers in ship side

2. Stiffness to the hull girder (global strength)


Container

Ships

Damaged condition

ShearForce

Bending Moment

Structural functions: Watertight integrity (local strength)

Watertight bulkhead


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Ships

High stress lower / upperend & midfield

Structural functions: Support of container stacks (local strength)

ShearForce

Bending Moment

Pillar bulkhead


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ShipsStructural functions: Support of container stacks (local strength)

TippingRackingStringer


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ShipsStructural functions: Support the bottom (local strength)

High compression


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ShipsStructural functions: Transverse strength of hull girder

Torsion

Deformation


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Ships Hull damages

Characteristic damages for transverse bulkheads:

1. Damages to cell guide

2. Damages to webs and stringers

3. Overstressed / buckled support bulkhead


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Ships Damages to cell guide


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Ships Damages to cell guide

Consequences of damages?• Difficulties in loading / unloading the cargo holds

• Loss of support of containers


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Ships Damages to webs

Damages to webs due to wrong loading of containers


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Ships

From Specification:“7th tier in cargo holds shall be suitable for 40ft long 9 feet 6 inches high container loading.”

Typical design


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Ships Typical design

8’6’’Bulkhead


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Ships Damages to webs


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Ships

Consequences of damages• Difficulties in loading / unloading the cargo holds

• Damages to webs and stringers could reduce thecontainer support

• Reduced vertical support of bottom

Damages to webs


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Ships Overstressed / buckled support bulkhead

Crack repaired by welding and additional stiffener

Critical area of support bulkhead

• The support bulkheads are highly stressed in shear and equivalent stress in the outer part

• Areas with lightening holes are to be specially checked


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ShipsOverstressed / buckled support bulkhead Impact of function

• Damages may lead to cracks and hence leakage from bottom / wing tank

• Containers may shift due to reduced support

• Reduced support of bottom and consequently other overloaded areas


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Ships

Dnv hull structure course

Health & Medicine

Transcript of Dnv hull structure course