microsoft powerpoint - ship collision

Lysaker November 22-23, 2006 NORSOK standard for offshore structures

Norwegian Structural Steel Association

1

Resistance to Accidental

Ship Collisions



2

Outline�General principles

�Impact scenarios

�Impact energy distribution

�External impact mechanics

�Collision forces

�Energy dissipation in local denting

�Energy dissipation in tubular members

�Strength of connections

�Global integrity



3

DESIGN AGAINST ACCIDENTAL LOADS

• Verification methods

– Simplified (“back of the envelope methods)• Elastic-plastic/rigid plastic methods (collision/explosion/dropped

objects)

• Component analysis (Fire)

– General calculation/Nonlinear FE methods• USFOS, ABAQUS, DYNA3D…..



4

• General

– “The inherent uncertainty of the frequency and magnitude of the

accidental loads as well as the approximate nature of the methods for

their determination as well as the analysis of accidental load effects shall

be recognised. It is therefore essential to apply sound engineering

judgement and pragmatic evaluations in the design.”

NORSOK STANDARD




5

NORSOK STANDARD


• “If non-linear, dynamic finite element analysis is applied

all effects described in the following shall either be

implicitly covered by the modelling adopted or subjected to

special considerations, whenever relevant”



6

Recent trends:Location sometimes close to heavy traffic lanes

Gjøa SEMI

12 nm radius

AtoN North

AtoN South

Gjøa SEMI

12 nm radius

AtoN North

AtoN South



7

Present trend for supply vessels:

bulbous bows & increased size



8

The outcome of a collision may be this….



9

..or this….



10

Principles for ALS structural designillustrated for FPSO/ship collision

S treng th

d esign

S ha red-energ y

d esign

Ductil e

d es ign

Relativ e strength - installat io n/ship

ship

in stallation

Energy dissipation

�Strength design - FPSO crushes bow of vessel

(ref. ULS design)

�Ductility design - Bow of vessel penetrates

FPSO side/stern

�Shared energy design - Both vessels deformFairly moderate modification of relative strength may change the

design from ductile to strength or vice verse



11

SHIP COLLISIONDesign principles- analysis approach

�Strength design:

The installation shape governs the deformation field of the

ship. This deformation field is used to calculate total and

local concentrations of contact force due to crushing of

ship.The installation is then designed to resist total and

local forces.

Note analogy with ULS design.



12

SHIP COLLISIONDesign principles - analysis approach

�Ductility design:

The vessel shape governs the deformation field of the

installation. This deformation field is used to calculate

force evolution and energy dissipation of the deforming

installation.

The installation is not designed to resist forces, but is

designed to dissipate the required energy without collapse

and to comply with residual strength criteria.



13

SHIP COLLISIONDesign principles - analysis approach

�Shared energy design:

– The contact area the contact force are mutually dependent

on the deformations of the installation and the ship.

– An integrated, incremental approach is required where the

the relative strength of ship and installation has to be checked

at each step as a basis for determination of incremental

deformations.

– The analysis is complex compared to strength or ductility

design and calls for integrated, nonlinear FE analysis.

– Use of contact forces obtained form a strength/ductility

design approach may be very erroneous.



14

Grane - potential impact locations -



15

Collision Mechanics

• Convenient to separate into

� External collision mechanics– Conservation of momentum

– Conservation of energy

� Kinetic energy to be dissipated as strain energy

� Internal collision mechanics– Distribution of strain energy in installation and

ship

� Damage to installation



16

External collision mechanics

Central collision (force vector through centre of gravity of platform and ship)

Conservation of momentumm+m

vm+vm=v

ps

ppss

c

Common velocity end of impact v)m+m( = vm + vm cpsppss

Conservation of energy E + E + v )m+m( 1/2=vm 1/2 + vm 1/2 ps

2

ccs

2

pp

2

ss

Energy to be dissipated by ship and the platform

m

m+1

)v

v-(1

vm1/2=E+E

p

s

s

p

2

2

ssps



17

External collision mechanicsCollision energy to be dissipated as strain energy

Compliant installations

(semi-subs, TLPs, FPSOs,

Jackups)ii

ss

2

s

i

2

ssss

am

am1

v

v1

)va(m2

1E

++

+

−

+=

Fixed installations (jackets) 2

ssss )va(m2

1E +=

Articulated columns

J

zm1

v

v1

)a(m2

1E

2

s

2

s

i

sss

+

−

+=

ms = ship mass

as = ship added mass

vs = impact speed

mi = mass of installation

ai = added mass of installation

vi = velocity of installation

J = mass moment of inertia of installation (including added mass)

with respect to effective pivot point

z = distance from pivot point to point of contact



18

Ship collision- dissipation of strain energy

dws dwi

RiRs

Ship Installation

Es,sEs,i

∫∫ +=+=maxi,maxs,

is,

w

0ii

w

0ssss,s dwRdwREEE

The strain energy dissipated by the ship and installation equals the total

area under the load-deformation curves, under condition of equal load.

An iterative procedure is generally required



19

SHIP COLLISION - according to NORSOKForce-deformation curves for supply vessel

(TNA 202, DnV 1981)

Note: Bow impact against large diameter columns only

0

10

20

30

40

50

0 0.5 1 1.5 2 2.5 3 3.5 4

Indentation (m)

Impact force (MN)

Broad side

D = 10 m

= 1.5 m

Stern end

D = 10 m

= 1.5 m

Bow

Stern corner

D

D

D

�Force – deformation

curves from 1981 –

derived by simplified

methods

�Now: NLFEA is available!

�Analysis of bulbous bow

required



20

Supply vessel bow ~ 7500 tons

displacement Dimension: Length:

L.O.A. 90.70m

Lrule 85.44m

Breadth mld 18.80m

Depth mld 7.60m

Draught scantling 6.20m



21

Finite element models



22

Material modeling

� Bow: Mild steel – nominal fy = 235 MPa, apply fy = 275 MPa

� Column: Design strength fy = 420 MPa

� Strain hardening included – relatively more for bow

0

100

200

300

400

500

600

700

800

900

0 0,05 0,1 0,15 0,2 0,25 0,3

Plastic strain [-]

Effective stress [MPa]

Mild steel curve fit

High strength steel curve fit

High strength steel data points

Mild steel data points



23

Impact location 1

�Bow is crushed – relatively small deformations in column

�Max. column strain – 12% - at bulb location

�Strain level close to rupture

�Column strain at superstructure location is 7%

Max strain 12%



24

Force deformation curve for bow

�The crushing force in the bulb is larger than the superstructure for the

crushing range analyzed

�The crushing force increases steadily for the superstructure

�The bulb attains fast a maximum force followed by a slight reduction

Bow superstructure

Bulb



25

Pressure-area relation for design

� Pressure-area relation analogy with ice design is found from

collision analysis

� Provide recommendation for design against impact

Plots of collision force

intensity

Pressure-area relation for design

pressure-area curve

0

5

10

15

20

25

30

35

40

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Area (m^2)

Pressure (MPa) P=7.06A-0.7Total

collision

force

distributed

over this

area



26

Ship collision with FPSO

• Only the side of one tank is modeled

• Three scenarios established w.r.t.

draughts

Scenario 1 Scenario 2 Scenario 3



27

SHIP COLLISIONContact force distribution for strength design of large

diameter columns

Total collision force

distributed over this

area

Area with high force

intensity

Deformed stern corner



28

Bow collision with braces

Can the brace be designed to crush the bow?

Strong bow- tube and bow deformsMedium strength bow - tube

undamaged



29

Ship

collision

with

oblique

brace

Deformation energy & Collision force

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500 3000

Deformation [mm]

Energy [MJ]

0

2000

4000

6000

8000

10000

12000

14000

Force [KN]

Total Energy

Total Contact force



30

Ship

collision with

braceDeformation energy & Collision force

0

2

4

6

8

10

12

14

16

18

20

0 500 1000 1500 2000 2500 3000 3500

Deformation [mm]

Energy [MJ]

0

2000

4000

6000

8000

10000

12000

Force [KN]

Total Energy

Total Contact force



31

Ship collision with braceEnergy dissipation in bow versus brace resistance

Energy dissipation in bow if brace resistance R0

Contact location > 3 MN > 6 MN > 8 MN > 10 MN

Above bulb 1 MJ 4 MJ 7 MJ 11 MJ

First deck 0 MJ 2 MJ 4 MJ 17 MJ

First deck - oblique brace 0 MJ 2 MJ 4 MJ 17 MJ

Between f'cstle/first deck 1 MJ 5 MJ 10 MJ 15 MJ

Arbitrary loaction 0 MJ 2 MJ 4 MJ 11 MJ

1.5 0.5

y

2f t D factor

3≥ ⋅Brace must satisfy the

following requirement

Joints and adjacent structure must be strong enough to support the

reactions from the brace.

10 m

1st deck

Fcstl. deck



32

Energy dissipation modes

in jackets

Elastic

Plastic

Plastic



33

Local denting tests with tubes



34

Yield line model for local denting

Measured

deformation



35

Resistance curves for tubes subjected to denting

0

2

4

6

8

10

12

14

16

18

20

0 0.1 0.2 0.3 0.4 0.5

wd/D

R/(kRc)

2

1

0.5

0

b/D =

)]N

N-[1

4

1 -(1

3

4 )

D

w( )

D

b1.2+(22 = )

t

D

4

tf( / R

3

p

D

b+3.5

1.925

d

2

y⋅

Approximate

expression including

effect of axial force



36

Resistance curves for tubes subjected to denting

0

2

4

6

8

10

12

14

16

18

20

0 0.1 0.2 0.3 0.4 0.5

wd/D

R/(kRc)

2

1

0.5

0

b/D =

If collapse load in bending, R0/Rc < 6

neglect local denting

Include local denting



37

Relative bending moment capacity of

tubular beam with local dent(contribution from flat region is conservatively neglected)

0

0,2

0,4

0,6

0,8

1

0 0,2 0,4 0,6 0,8 1

wd/D

Mred/M

P

D

wd



38

SHIP COLLISION

Plastic resistance curve for bracings

collision at midspan

l

w

P

Collapse model for beam with fixed ends

1 < D

w

D

w

D

w+)

D

w(-1 =

R

R 2

o

uarcsin

1 > D

w

D

w 2 =

R

R

o

u π



39

SHIP COLLISION

Elastic-plastic resistance curve for bracings

collision at midspanFactor c includes the effect of elastic flexibility at ends

Bending & membrane

Membrane only

k kw

F - R

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

5,5

6

6,5

0 0,5 1 1,5 2 2,5 3 3,5 4

Deformation

R/

0

1

0.1

0.2

0,3

0.5

0.05c=∞

w

Rigid-plastic



40

Example: supply vessel impact on brace

628

508

762 x 28.6 mm

l= 23.3 m



41

Example: supply vessel impact on brace

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Normalised moment M/MP

Norm

alised force N/N

P

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Displacement [m]

Impact force [MN]

0

2

4

6

8

10

Energy dissipation [MJ]

USFOS

Simple model

Energy dissipation

Kinetic energy absorbed by brace prior to rupture: 6 ~ 7 MJ



42

Strength of connections (NORSOK N-004 A.3.8)

• Provided that large plastic strains can develop in the impacted member, thestrength of the connections that the member frames into has to be checked.

• The resistance of connections should be taken from ULS requirements inNORSOK standard for tubular joints and Eurocode 3 or NS3472 for other

joints.

• For braces reaching the fully plastic tension state, the connection shall bechecked for a load equal to the axial resistance of the member. The design

axial stress shall be assumed equal to the ultimate tensile strength of the

material.

• If the axial force in a tension member becomes equal to the axial capacity ofthe connection, the connection has to undergo gross deformations. The

energy dissipation will be limited and rupture has to be considered at a given

deformation. A safe approach is to assume disconnection of the member

once the axial force in the member reaches the axial capacity of the

connection.

• If the capacity of the connection is exceeded in compression and bending,this does not necessarily mean failure of the member. The post-collapse

strength of the connection may be taken into account provided that such

information is available.



43

Strength of adjacent structure

• The strength of structural members adjacent to the impactedmember/sub-structure must be checked to see whether they can

provide the support required by the assumed collapse mechanism.

• If the adjacent structure fails, the collapse mechanism must bemodified accordingly.

• Since, the physical behaviour becomes more complex withmechanisms consisting of an increasing number of members it is

recommended to consider a design which involves as few members

as possible for each collision scenario.



44

Ductility limits

Ref: NORSOK A.3.10.1

� The maximum energy that the impacted member can dissipate will –

ultimately - be limited by local buckling on the compressive side or

fracture on the tensile side of cross-sections undergoing finite rotation.

� If the member is restrained against inward axial displacement, any local

buckling must take place before the tensile strain due to membrane

elongation overrides the effect of rotation induced compressive strain.

� If local buckling does not take place, fracture is assumed to occur when

the tensile strain due to the combined effect of rotation and membrane

elongation exceeds a critical value



45

Local buckling of tubes undergoing

large rotations

θov

A2 (24)

10 20 30 θ/θy

M/Mps

1.0

0.5

A8 (96)

A5 (48)

θov

Bending moment versus rotation of beam (reproduced form Sherman 1986).

D/t -ratio

Tubes with low slenderness (~20-30) can achieve a bending moment equal to or larger

than the plastic bending moment and maintain this for a significant rotation. For

intermediate slenderness (D/t ~40 –60) the plastic bending moment can be achieved, but

local buckling takes place after some rotation. Tubes with high slenderness can not even

reach the plastic bending moment, but experiences a dramatic drop in the capacity once

local buckling occurs.



46

Ductility limitsRef: NORSOK A.3.10.1

� To ensure that members with small axial restraint maintain moment

capacity during significant plastic rotation it is recommended that

cross-sections be proportioned to Class 1 requirements, defined in

Eurocode 3 or NS3472.

� Initiation of local buckling does, however, not necessarily imply that

the capacity with respect to energy dissipation is exhausted,

particularly for Class 1 and Class 2 cross-sections. The degradation of

the cross-sectional resistance in the post-buckling range may be taken

into account provided that such information is available

� For members undergoing membrane stretching a lower bound to the

post-buckling load-carrying capacity may be obtained by using the

load-deformation curve for pure membrane action.



47

Tensile Fracture

Plastic deformation or critical strain at fracture

depends upon•material toughness•presence of defects•strain rate•presence of strain concentrations

Critical strain of section with defects

- assessment by fracture mechanics methods.

Plastic straining preferably outside the weld

- overmatching weld material



48

StrainStress

distribution

Approximate stress

distribution

M

εY εmax σhσY σhσY

0

5

10

15

20

25

30

35

40

45

50

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

x/l l l l

Strain εε εε

Hardening parameter H = 0.005

Maximum strain

εcr/εY = 50

= 40

= 20

No hardening

P

l

x

Axial variation of maximum strain for a cantilever beam

with circular cross-section

Assumption: Bilinear stress-strain relationship

Stress-strain distribution - bilinear material

M

ε κ



49

Local buckling does not need to be considered

if the follwowing conditions is met

Assumption: Membrane tension larger than compression in rotation

(NORSOK N-004)

3

12

c1

yf

d

κ

c

f14cβ

≤

l

whereyf235

tDβ =

2

fc1

cc

+= axial flexibility factor

dc = characteristic dimension

= D for circular cross-sections

c1 = 2 for clamped ends

= 1 for pinned ends

c = non-dimensional spring stiffness as

κ l ≤ 0.5 l =the smaller distance from location of collision load to adjacent joint



50

Critical deformation for local buckling

(NORSOK N-004)L o c a l b u c k l i n g m a y b e a s s u m e d t o o c c u r w h e n

−−=

2

c

3

1

yf

fc d

κ

βc

f14c11

2c

1

d

w l

F o r s m a l l a x ia l r e s t r a i n t ( c < 0 .0 5 )

2

c

3

1

y

c d

κ

βc

3.5f

d

w

=

l

N o te : L o c a l b u c k l i n g d o e s n o t n e c e s s a r i l y im p l y t h a t e n e r g y d i s s i p a t i o n c e a s e s c o m p le t e l y

2

fc1

cc

+= a x i a l f l e x i b i l i t y f a c t o r

d c = c h a r a c t e r i s t i c d im e n s i o n

= D f o r c i r c u l a r c r o s s - s e c t i o n s

c 1 = 2 f o r c l a m p e d e n d s

= 1 f o r p i n n e d e n d s

c = n o n - d im e n s i o n a l s p r i n g s t i f f n e s s a s

κ l ≤ 0 .5 l = th e s m a l l e r d i s t a n c e f r o m l o c a t i o n o f c o l l i s i o n



51

Tensile Fracture

The degree of plastic deformation at fracture exhibits a

significant scatter and depend upon the following factors:

•material toughness

•presence of defects

•strain rate

•presence of strain concentrations

Welds normally contain defects. The design should hence ensure that

plastic straining takes place outside welds (overmatching weld material)



52

Tensile Fracture

• The critical strain in parent material depends

upon:

• stress gradients

• dimensions of the cross section

• presence of strain concentrations

• material yield to tensile strength ratio

• material ductility

• Critical strain (NLFEM or plastic analysis)

zoneplasticoflength:5,t

65.00.02 tcr ≥+= ll

ε



53

Critical deformation for tensile fracture in yield hinges

( )1/εc4c12c

c

d

w1crfw

f

1

c

−+= c

displacement factor2

crcrP

lplp

1

wd

κ

ε

ε

W

W14c

3

21c

c

1c

−+

−=lY

plastic zone length factor

1HW

W1

ε

ε

HW

W1

ε

ε

c

Py

cr

Py

cr

lp

+

−

−

=

axial flexibility factor2

fc1

cc

+=

non-dim. plastic stiffness

−

−==

ycr

ycrp

εε

ff

E

1

E

EH

c1 = 2 for clamped ends

= 1 for pinned ends

c = non-dimensional spring stiffness

κl ≤ 0.5l the smaller distance from location of collision load

to adjacent joint

W = elastic section modulus

WP = plastic section modulus

εcr = critical strain for rupture

E

fε

y

y = yield strain

fy = yield strength

fcr = strength corresponding to εcrdc = D diameter of tubular beams

= 2hw twice the web height for stiffened plates

= h height of cross-section for symmetric I-profiles



54

Tensile fracture in yield hingesDetermination of H

E

H E

εcr

fcr

E

H E

εcr

fcr

Determination of plastic stiffness

H E

f

ε

Erroneous determination of plastic stiffness



55

Tensile fracture in yield hinges

• Proposed values for ecr and H for

different steel grades

Steel grade εεεεcr H

S 235 20 % 0.0022

S 355 15 % 0.0034

S 460 10 % 0.0034



56

Tensile fracture in yield hingescomparison with NLFEM

0%

5%

10%

15%

20%

0.0 0.5 1.0 1.5 2.0

Displacement [m]

Strain

NORSOK

ABAQUS fine

USFOS beam

ABAQUS

USFOS shell

microsoft powerpoint - ship collision

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