Resistance to Accidental
Ship Collisions
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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…..
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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.”
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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”
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
5
SHIP COLLISION
How much energy has
been dissipated?
What is the extent of
damage to the platform?
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
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6
Grane- impact events to be
simulated on Row 2
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Analysis and Design for Robustness of Offshore Structures
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Grane - potential impact locations Row A
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Analysis and Design for Robustness of Offshore Structures
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8
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
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
9
SHIP COLLISION
Design principles
 Strength design
Installation resists collision without deformation- ship
deforms and dissipates major part of energy
 Ductility design
Installation deforms and dissipates major part of
energy- ship remains virtually undamaged
 Shared energy design
Both ship and installation deform and contribute
substantially to energy dissipation
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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.
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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.
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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.
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
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13
Stern corner -column collision
Distribution of energy dissipation- ductile vs. strength design
Weak column left (Alt. 1)
Strong column right (Alt.2)
Column
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Stern corner
Analysis and Design for Robustness of Offshore Structures
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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
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
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 vs + 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
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
16
External collision mechanics
Collision energy to be dissipated as strain energy
Compliant installations
(semi-subs, TLPs, FPSOs,
Jackups)
2

v 
1  i 
vs 
1
2
E s  (m s  a s )v s 
m  as
2
1 s
mi  a i
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
=
=
=
=
=
=
=
NUS July 12-14,
z 2005
=
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
Analysis and Design for Robustness of Offshore Structures
distance
from pivot
point to point of contact
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17
Ship collision- dissipation of strain energy
Rs
Es,i
Es,s
dws
Ship
E s  E s,s  E s,i  
Ri
Installation
w s,max
0
R s dw s  
dwi
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
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
18
SHIP COLLISION
Force-deformation curves for supply vessel
(TNA 202, DnV 1981)
50
Broad side
D = 10 m
= 1.5 m
Impact force (MN)
40
D
30
D
20
Stern corner
Stern end
D = 10 m
= 1.5 m
10
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
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
NUS – Keppel Short Course
19
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
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
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20
SHIP COLLISION
Supply vessel - stern corner force/distribution
Force [MN]
16
• Total force
12
8
4
0
0
0,25
0,5
0,75
1
Deformation [m]
Contact area
a(m)
b (m)
0.35
0.65
3.0
0.35
1.65
6.4
0.20
1.15
5.4
NUS July 12-14, 2005
• Local force
Force (MN)
b
a
subset of total force
distributed over
smaller area
Analysis and Design for Robustness of Offshore Structures
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SHIP COLLISION
Strength design of large diameter columnssupply vessel stern impact
Stern end impact
Contact area
Force
(MN)
a (m)
b(m)
0.6
0.3
5.6
0.9
0.5
7.5
2.0
1.1
10
b
a
Stern corner impact
Contact area
Force
(MN)
a(m)
b (m)
0.35
0.65
3.0
0.35
1.65
6.4
0.20
1.15
5.4
NUS July 12-14, 2005
For strength design the
column shall resist
maximum local
concentrations of the
collision force imposed
by the deforming
supply vessel. The
forces are assumed
uniformly distributed
over a rectangular area
b
a
Analysis and Design for Robustness of Offshore Structures
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22
Energy dissipation modes
in jackets
Plastic
Elastic
Plastic
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Analysis and Design for Robustness of Offshore Structures
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23
Local denting tests with tubes
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Analysis and Design for Robustness of Offshore Structures
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24
Yield line model for local denting
Measured
deformation
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Analysis and Design for Robustness of Offshore Structures
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25
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
NUS July 12-14, 2005
D
b w
) = (22 + 1.2 ) ( d )
t
D D
1.925
b
3.5+
D
Approximate
4
1
N 3

(1 - [1 ] ) expression including
3
4
Np
effect of axial force
Analysis and Design for Robustness of Offshore Structures
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26
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
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Analysis and Design for Robustness of Offshore Structures
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27
Relative bending moment capacity of
tubular beam with local dent
(contribution from flat region is conservatively neglected)
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
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SHIP COLLISION
Plastic resistance curve for bracings
collision at midspan
P
w

Collapse model for beam with fixed ends
NUS July 12-14, 2005
Ru = 1 - ( w 2 + w arcsin w
)
D
D
D
Ro
w
<1
D
Ru =  w
Ro 2 D
w
>1
D
Analysis and Design for Robustness of Offshore Structures
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29
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
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1
1,5
2
2,5
3
3,5
w
4
Deformation
Analysis
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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
Analysis and Design for Robustness of Offshore Structures
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information is available.
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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.
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Analysis and Design for Robustness of Offshore Structures
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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
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Analysis and Design for Robustness of Offshore Structures
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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)
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Analysis and Design for Robustness of Offshore Structures
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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
t
 0.02  0.65 ,

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  5t : length of plastic zone
Analysis and Design for Robustness of Offshore Structures
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Critical deformation for tensile fracture in yield hinges
c
w
 1
d c 2c f
displacement factor
c1
c
l
W
WP
cr
 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 

c f  

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

=
fy
2
2
1  f cr  f y 
E  ε cr  ε y 
critical strain for rupture
yield strain
E

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
=
elastic section modulus
=
2hw twice the web height for stiffened plates
Analysis and Design for Robustness of Offshore
Structures
36
NUS
12-14,
2005
=
h
height of cross-section for symmetric I-profiles
= July
plastic
section
modulus
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=
critical strain for rupture
Tensile fracture in yield hinges
• Proposed values for ecr and H for
different steel grades
Steel grade
S 235
S 355
S 460
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cr
20 %
15 %
10 %
H
0.0022
0.0034
0.0034
Analysis and Design for Robustness of Offshore Structures
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37
Global integrity during impact
 Normally, it is unlikely that the installation will turn into a global
collapse mechanism under direct collision load, because the collision
load is typically an order of magnitude smaller than the resultant design
wave force.
 Linear analysis often suffices to check that global integrity is maintained.
 The installation should be checked for the maximum collision force.
 For installations responding predominantly statically the maximum
collision force occurs at maximum deformation.
 For structures responding predominantly impulsively the maximum
collision force occurs at small global deformation of the platform. An
upper bound to the collision force is to assume that the installation is
fixed with respect to global displacement. (e.g. jack-up fixed with respect
to deck displacement)
NUS July 12-14, 2005
Analysis and Design for Robustness of Offshore Structures
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38