Resistance to Accidental
Ship Collisions
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
1
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
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
2
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…..
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
3
NORSOK STANDARD
DESIGN AGAINST ACCIDENTAL LOADS
• 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.”
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
4
NORSOK STANDARD
DESIGN 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”
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
5
Recent trends:
Location sometimes close to heavy traffic lanes
AtoN North
12 nm radius
Gjøa SEMI
AtoN South
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
6
Present trend for supply vessels:
bulbous bows & increased size
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
7
The outcome of a collision may be this….
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
8
..or this….
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
9
Principles for ALS structural design
Energy dissipation
illustrated for FPSO/ship collision
Ductil e
design
Shared-energy
design
Strength
design
ship
installation
Relative strength - installation/ship
Strength design
- FPSO crushes bow of vessel
(ref. ULS design)
Ductility design
- Bow of vessel penetrates
FPSO side/stern
Shared energy design - Both vessels deform
Fairly moderate modification of relative strength may change the
design from ductile to strength or vice verse
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
10
SHIP COLLISION
Design 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.
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
11
SHIP COLLISION
Design 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.
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
12
SHIP COLLISION
Design 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.
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
13
Grane - potential impact locations -
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
14
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
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
15
External collision mechanics
Central collision (force vector through centre of gravity of platform and ship)
Conservation of momentum
vc =
ms v s + m p v p
ms + m p
Common velocity end of impact ms v s + m p v p = ( ms + m p ) vc
Conservation of energy
1/2 m s v 2s + 1/2 m p v 2p = 1/2 ( m s + m c ) v c2 + E s + E p
Energy to be dissipated by ship and the platform
2
v
(1 - p )
vs
2
E s + E p = 1/2 ms v s
m
1+ s
mp
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
16
External collision mechanics
Collision energy to be dissipated as strain energy
2

v 
1  i 
vs 
1
2
E s  (ms  a s )vs 
m  as
2
1 s
mi  a i
Compliant installations
(semi-subs, TLPs, FPSOs,
Jackups)
1
2
(m s  a s )v s
2
Fixed installations (jackets)
Es 
Articulated columns

v 
1  i 
vs 
1
E s  (m s  a s ) 
m z2
2
1 s
J
2
ms
as
vs
mi
ai
vi
J
=
=
=
=
=
=
=
ship mass
ship added mass
impact speed
mass of installation
added mass of installation
velocity of installation
mass moment of inertia of installation (including added mass)
with respect to effective pivot point
standard for offshore structures
Lysaker November
z 22-23,
= 2006
distance fromNORSOK
pivot point
to point of contact
Norwegian Structural Steel Association
17
Ship collision- dissipation of strain energy
Rs
Es,i
Es,s
dws
Ri
Ship
Installation
E s  E s, s  E s,i  
w s,max
0
dwi
R s dw s  
w i, max
0
R i dw i
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
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
18
SHIP COLLISION - according to NORSOK
Force-deformation curves for supply vessel
(TNA 202, DnV 1981)
50
Broad side
D = 10 m
= 1.5 m
Impact force (MN)
40
Force – deformation
curves from 1981 –
derived by simplified
methods
D
30
Now: NLFEA is available!
D
20
Stern corner
Stern end
D = 10 m
= 1.5 m
10
Analysis of bulbous bow
required
D
Bow
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Indentation (m)
Note: Bow impact against large diameter columns only
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
19
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
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
20
Finite element models
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
21
Material modeling
900
800
Effective stress [MPa]
700
600
500
400
300
Mild steel curve fit
200
High strength steel curve fit
High strength steel data points
100
Mild steel data points
0
0
0,05
0,1
0,15
0,2
0,25
0,3
Plastic strain [-]
 Bow: Mild steel – nominal fy = 235 MPa, apply fy = 275 MPa
 Column: Design strength fy = 420 MPa
 Strain hardening included – relatively more for bow
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
22
Impact location 1
Max strain 12%
Bow is crushed – relatively small deformations in column
Max. column strain – 12% - at bulb location
Strain level close to rupture
Lysaker November 22-23,Column
2006
NORSOK
standard for offshore location
structures is 7%
strain
at
superstructure
Norwegian Structural Steel Association
23
Force deformation curve for bow
Bulb
Bow superstructure
 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
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
24
Pressure-area relation for design
 Pressure-area relation analogy with ice design is found from
collision analysis
 Provide recommendation for design against impact
pressure-area curve
40
35
30
Pressure (MPa)
Total
collision
force
distributed
over this
area
P=7.06A-0.7
25
20
15
10
5
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Area (m^2)
Plots of collision force
intensity
Lysaker November 22-23, 2006
Pressure-area relation for design
NORSOK standard for offshore structures
Norwegian Structural Steel Association
25
Ship collision with FPSO
• Only the side of one tank is modeled
• Three scenarios established w.r.t.
draughts
Scenario 1
Lysaker November 22-23, 2006
Scenario 2
Scenario 3
NORSOK standard for offshore structures
Norwegian Structural Steel Association
26
SHIP COLLISION
Contact force distribution for strength design of large
diameter columns
Total collision force
distributed over this
area
Area with high force
intensity
Deformed stern corner
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
27
Bow collision with braces
Can the brace be designed to crush the bow?
Medium strength bow - tube
undamaged
Lysaker November 22-23, 2006
Strong bow- tube and bow deforms
NORSOK standard for offshore structures
Norwegian Structural Steel Association
28
Ship
collision
with
oblique
brace
30
14000
25
12000
10000
20
8000
15
6000
10
Force [KN]
Energy [MJ]
Deformation energy & Collision force
4000
Total Energy
Total Contact force
5
0
0
500
1000
1500
2000
2500
2000
0
3000
Deformation [mm]
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
29
Ship
collision with
brace
20
18
16
14
12
10
8
6
4
2
0
12000
10000
8000
6000
4000
Total Energy
Total Contact force
0
500
1000
1500
2000
2500
3000
Force [KN]
Energy [MJ]
Deformation energy & Collision force
2000
0
3500
Deformation [mm]
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
30
Ship collision with brace
Energy dissipation in bow versus brace resistance
Fcstl. deck
Energy dissipation in bow if brace resistance R0
Contact location
1st deck
10 m
> 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
Brace must satisfy the
following requirement
1.5
0.5
fyt D
2
  factor
3
Joints and adjacent structure must be strong enough to support the
reactions from the brace.
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
31
Energy dissipation modes
in jackets
Plastic
Elastic
Plastic
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
32
Local denting tests with tubes
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
33
Yield line model for local denting
Measured
deformation
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
34
Resistance curves for tubes subjected to denting
20
18
16
b/D =
R/(kRc)
14
12
10
8
2
1
0.5
0
6
4
2
0
0
0.1
0.2
0.3
0.4
0.5
wd/D
2
t
R/( fy
4
D
b w
) = (22 + 1.2 ) ( d )
t
D D
Lysaker November 22-23, 2006
1.925
b
3.5+
D
Approximate
4
1
N 3

(1 - [1 ] ) expression including
3
4
Np
effect of axial force
NORSOK standard for offshore structures
Norwegian Structural Steel Association
35
Resistance curves for tubes subjected to denting
20
18
16
b/D =
R/(kRc)
14
12
10
8
2
1
0.5
0
Include local denting
6
4
If collapse load in bending, R0/Rc < 6
neglect local denting
2
0
0
0.1
0.2
0.3
0.4
0.5
wd/D
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
36
Relative bending moment capacity of
tubular beam with local dent
(contribution from flat region is conservatively neglected)
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
37
SHIP COLLISION
Plastic resistance curve for bracings
collision at midspan
P
w

Collapse model for beam with fixed ends
Ru = 1 - ( w 2 + w arcsin w
)
D
D
D
Ro
w
<1
D
Ru =  w
Ro 2 D
w
>1
D
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
38
SHIP COLLISION
Elastic-plastic resistance curve for bracings
collision at midspan
Factor c includes the effect of elastic flexibility at ends
6,5
6
5,5
5
4,5
0.2
4
0
3,5
R/
R
3
Bending & membrane
Membrane only
F-R
k
k
w
Rigid-plastic
0,3
0.1
0.5
1
2,5
c 
2
0.05
1,5
1
0,5
0
0
0,5
1
Lysaker November 22-23, 2006
1,5
2
2,5
3
3,5
4
w
Deformation
NORSOK standard for offshore structures
Norwegian Structural Steel Association
39
Example: supply vessel impact on brace
628
762 x 28.6 mm
= 23.3 m
508
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
40
Example: supply vessel impact on brace
10
8
8
6
6
USFOS
4
2
4
2
Simple model
1.0
0.8
Normalised force N/NP
Energy dissipation
Energy dissipation [MJ]
Impact force [MN]
10
0.6
0.4
0.2
0.0
0
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0
0.2
0.4
0.6
0.8
1.0
-0.2
Normalised moment M/MP
Displacement [m]
Kinetic energy absorbed by brace prior to rupture: 6 ~ 7 MJ
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
41
Strength of connections
(NORSOK N-004 A.3.8)
 Provided that large plastic strains can develop in the impacted member, the
strength of the connections that the member frames into has to be checked.
 The resistance of connections should be taken from ULS requirements in
NORSOK standard for tubular joints and Eurocode 3 or NS3472 for other
joints.
 For braces reaching the fully plastic tension state, the connection shall be
checked 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 of
the 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.
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
42
Strength of adjacent structure
 The strength of structural members adjacent to the impacted
member/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 be
modified accordingly.
 Since, the physical behaviour becomes more complex with
mechanisms 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.
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
43
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
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
44
Local buckling of tubes undergoing
large rotations
M/Mps
ov
1.0
A2 (24)
ov
D/t -ratio
0.5
A5 (48)
A8 (96)
10
20
30 y
Bending moment versus rotation of beam (reproduced form Sherman 1986).
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
NORSOK standard for offshore structures
Lysaker
November
22-23,
2006
local
buckling
occurs.
Norwegian Structural Steel Association
45
Ductility limits
Ref: 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.
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
46
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
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
47
Y
max
Y h
Y h
M
M
Stress
distribution
Strain
Approximate stress
distribution
Stress-strain distribution - bilinear material
50

k
45
40
Hardening parameter H = 0.005
Strain 
35
Maximum strain
cr/Y
= 50
= 40
= 20
30
25
20
P
x

15
No hardening
10
5
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
x/
Axial variation of maximum strain for a cantilever beam
with circular cross-section
Lysaker November 22-23, 2006
Assumption: Bilinear
NORSOK standard for offshore structures
Norwegian Structural
Steel Association
stress-strain
relationship
48
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)
 14cf f y  κ  2 
  
β
 c1  d c  


1
3
where
β
Dt
235 f y
2
 c 
 axial flexibility factor
c f  

1 c 
dc =
=
c1 =
=
c =
k
characteristic dimension
D for circular cross-sections
2 for clamped ends
1 for pinned ends
non-dimensional
spring stiffness as

 0.5 =the smaller distance from location of collision load to adjacent joint
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
49
Critical deformation for local buckling
(NORSOK N-004)
Local buckling may be assumed to occur when

w
1 

1
dc
2c f 


2
14c f f y  κ  



1
3
d  
c1β
 c  

For small axial restraint (c < 0.05)
3.5f y
w

dc
c1β 3
 κ 

d 

 c 
2
Note: Local buckling does not necessarily imply that energy dissipation ceases complet
cf

c 


1 c 


dc =
=
c1 =
=
c =

k 0.5
2
axial flexibility factor
characteristic dimension
D for circular cross-sections
2
for clamped ends
1
for pinned ends
non-dimensional spring stiffness as
=the smaller distance from location of collision
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
50
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)
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
51
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)
 cr  0.02  0.65
Lysaker November 22-23, 2006
t
,

  5t : length of plastic zone
NORSOK standard for offshore structures
Norwegian Structural Steel Association
52
Critical deformation for tensile fracture in yield hinges
c
w
 1
d c 2c f
displacement factor
c1
c
 1  4c
w c f ε cr
1  2  
W  εY   κ 



c w   c lp 1  c lp   41 
c1   3   WP  ε cr   d cr 
plastic zone length factor
 ε cr  W
  1
H
ε
W
y
P


c lp 
 ε cr  W
  1
H 1
ε
W
 y
 P
axial flexibility factor
 c 

cf  

1

c


non-dim. plastic stiffness
H
Ep
E
=
2
for clamped ends
=
1
for pinned ends
cr
non-dimensional spring stiffness
εy 
=

/ c1  1

2
1  f cr  f y 
E  ε cr  ε y 
=
fy
2
critical strain for rupture
yield strain
E
kl

0.5l the smaller distance from location of collision load fy
=
yield strength
fcr
=
strength corresponding to cr
to adjacent joint
dc
=
D
diameter of tubular beams
W
=
elastic section modulus
=
2hw twice the web height for stiffened plates
NORSOK standard for offshore structures
53
November
22-23,
2006
WLysaker
=
plastic
section
modulus
=
h
height of cross-section for symmetric I-profiles
P
Norwegian Structural Steel Association
cr
=
critical strain for rupture
Tensile fracture in yield hinges
Determination of H
fcr
fcr
HE
E
HE
E
cr
cr
Determination of plastic stiffness
f
HE

Erroneous determination of plastic stiffness
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
54
Tensile fracture in yield hinges
• Proposed values for ecr and H for
different steel grades
Steel grade
S 235
S 355
S 460
Lysaker November 22-23, 2006
cr
20 %
15 %
10 %
H
0.0022
0.0034
0.0034
NORSOK standard for offshore structures
Norwegian Structural Steel Association
55
Tensile fracture in yield hinges
comparison with NLFEM
20%
NORSOK
15%
Strain
ABAQUS fine
USFOS beam
10%
ABAQUS
5%
USFOS shell
0%
0.0
0.5
1.0
1.5
2.0
Displacement [m]
Lysaker November 22-23, 2006
NORSOK standard for offshore structures
Norwegian Structural Steel Association
56