Lysaker November 22-23, 2006 NORSOK standard for offshore structures Norwegian Structural Steel Association 1 Resistance to Accidental Ship Collisions

Lysaker November 22-23, 2006 NORSOK standard for offshore structuresNorwegian Structural Steel Association

1

Resistance to Accidental Ship Collisions


2

OutlineGeneral principlesImpact scenariosImpact energy distributionExternal impact mechanicsCollision forcesEnergy dissipation in local dentingEnergy dissipation in tubular membersStrength of connectionsGlobal 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.”

• SS

NORSOK STANDARDDESIGN AGAINST ACCIDENTAL LOADS


5

NORSOK STANDARDDESIGN AGAINST ACCIDENTAL LOADS

• “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

Strengthdesign

Shared-energydesign

Ductiledesign

Relative strength - installation/ship

ship

installation

En

erg

y d

issi

pat

ion

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

C entral co llision (fo rce vecto r th rough cen tre o f g ravity of p latfo rm and ship )

C onserva tion o f m om entumm+m

vm+vm=vps

ppss

c

C om m on velocity end o f im pact v)m+m( = vm + vm cpsppss

C onserva tion o f energy E + E + v )m+m( 1/2=vm 1/2 + vm 1/2 ps2ccs

2pp

2ss

E nergy to be d issipated by sh ip and the p latfo rm

m

m+1

)v

v-(1

vm1/2=E+E

p

s

s

p

2

2ssps


17

External collision mechanicsC o l l i s i o n e n e r g y t o b e d i s s i p a t e d a s s t r a i n e n e r g y

C o m p l i a n t i n s t a l l a t i o n s

( s e m i - s u b s , T L P s , F P S O s ,J a c k u p s )

ii

ss

2

s

i

2ssss

am

am1

vv

1

)va(m2

1E

F i x e d i n s t a l l a t i o n s ( j a c k e t s ) 2ssss )va(m

2

1E

A r t i c u l a t e d c o l u m n s

J

zm1

v

v1

)a(m2

1E

2s

2

s

i

sss

m s = s h i p m a s sa s = s h i p a d d e d m a s sv s = i m p a c t s p e e dm i = m a s s o f i n s t a l l a t i o na i = a d d e d m a s s o f i n s t a l l a t i o nv i = v e l o c i t y o f i n s t a l l a t i o nJ = m a s s m o m e n t o f i n e r t i a o f i n s t a l l a t i o n ( i n c l u d i n g a d d e d m a s s )

w i t h r e s p e c t t o e f f e c t i v e p i v o t p o i n tz = d i s t a n c e f r o m p i v o t p o i n t t o p o i n t o f c o n t a c t


18

Ship collision- dissipation of strain energy

dws dwi

RiRs

Ship Installation

Es,sEs,i

maxi,maxs,

is,

w

0 ii

w

0 ssss,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)

Imp

act

forc

e (M

N)

Broad sideD = 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 [-]

Effe

ctiv

e st

ress

[MP

a]

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 superstructureBulb


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)

Pre

ss

ure

(M

Pa

) 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

areaArea with high forceintensity

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]

En

erg

y [M

J]

0

2000

4000

6000

8000

10000

12000

14000

Fo

rce

[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]

En

erg

y [

MJ

]

0

2000

4000

6000

8000

10000

12000

Fo

rce

[K

N]

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 0.5y

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

)]N

N-[1

4

1 -(1

3

4 )

D

w( )

D

b1.2+(22 = )

t

D

4tf( / R 3

p

D

b+3.5

1.925

d2

y Approximate expression including effect of axial force


36

Resistance curves for tubes subjected to denting

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

Mre

d/M

P

D

wd


38

SHIP COLLISIONPlastic resistance curve for bracings

collision at midspan


39

SHIP COLLISIONElastic-plastic resistance curve for bracings

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

Bending & membraneMembrane 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/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= 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

N o r m a l i s e d m o m e n t M /M P

Norm

alise

d fo

rce N

/NP

0

2

4

6

8

1 0

0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0

D is p la c e m e n t [m ]

Impa

ct fo

rce [

MN

]

0

2

4

6

8

1 0

Ener

gy d

issip

ation

[MJ]

U S F O S

S im p le m o d e l

E n e rg y d is s ip a tio n

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 otherjoints.

For braces reaching the fully plastic tension state, the connection shall bechecked for a load equal to the axial resistance of the member. The designaxial stress shall be assumed equal to the ultimate tensile strength of thematerial.

If the axial force in a tension member becomes equal to the axial capacity ofthe connection, the connection has to undergo gross deformations. Theenergy dissipation will be limited and rupture has to be considered at a givendeformation. A safe approach is to assume disconnection of the memberonce the axial force in the member reaches the axial capacity of theconnection.

If the capacity of the connection is exceeded in compression and bending,this does not necessarily mean failure of the member. The post-collapsestrength of the connection may be taken into account provided that suchinformation 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 canprovide 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 isrecommended to consider a design which involves as few membersas possible for each collision scenario.


44

Ductility limitsRef: 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

ov

A2 (24)

10 20 30 y

M/Mps

1.0

0.5

A8 (96)

A5 (48)

ov

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 largerthan the plastic bending moment and maintain this for a significant rotation. Forintermediate slenderness (D/t ~40 –60) the plastic bending moment can be achieved, butlocal buckling takes place after some rotation. Tubes with high slenderness can not evenreach the plastic bending moment, but experiences a dramatic drop in the capacity oncelocal 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 toughnesspresence of defectsstrain ratepresence of strain concentrations

Critical strain of section with defects - assessment by fracture mechanics methods.

Plastic straining preferably outside the weld - overmatching weld material


48

StrainStressdistribution

Approximate stressdistribution

M

Y max hY hY

0

5

10

15

20

25

30

35

40

45

50

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35x/

Str

ain

Hardening parameter H = 0.005

Maximum strain

cr/Y

= 50 = 40 = 20

No hardening

P

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


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

where

yf235

tDβ

2

fc1

cc

axial flexibility factor

dc =characteristic dimension =Dfor circular cross-sectionsc1 =2for clamped ends =1for pinned endsc =non-dimensional spring stiffness as 0.5=the smaller distance from location of collision load to adjacent joint


50

Critical deformation for local buckling (NORSOK N-004)


51

Tensile FractureThe degree of plastic deformation at fracture exhibits a significant scatter and depend upon the following factors:material toughnesspresence of defectsstrain ratepresence 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


53

Critical deformation for tensile fracture in yield hinges

1/εc4c12c

c

d

w1crfw

f

1

c

c

d i s p l a c e m e n t f a c t o r2

crcrPlplp

1w d

κ

ε

ε

W

W14c

3

21c

c

1c

Y

p l a s t i c z o n e l e n g t h f a c t o r

1HW

W1

ε

ε

HW

W1

ε

ε

c

Py

cr

Py

cr

lp

a x i a l f l e x i b i l i t y f a c t o r2

fc1

cc

n o n - d i m . p l a s t i c s t i f f n e s s

ycr

ycrp

εε

ff

E

1

E

EH

c1 = 2 for clamped ends

= 1 for pinned ends

c = non-dimensional spring stiffnessl 0.5l the smaller distance from location of collision load

to adjacent joint

W = elastic section modulus

WP = plastic section modulus

cr = critical strain for rupture


54

Tensile fracture in yield hingesDetermination of H


55

Tensile fracture in yield hinges

• Proposed values for ecr and H for different steel grades

Steel grade cr HS 235 20 % 0.0022S 355 15 % 0.0034S 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]

Str

ain

NORSOK

ABAQUS fine

USFOS beam

ABAQUS

USFOS shell

Documents

Lysaker November 22-23, 2006 NORSOK standard for offshore structures Norwegian Structural Steel Association 1 Resistance to Accidental Ship Collisions