API RP 2A-WSD-2007 Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms-Working Stress Design
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RECOMMENDED PRACTICE FOR PLANNING, DESIGNING AND CONSTRUCTING FIXED OFFSHORE PLATFORMS—WORKING STRESS DESIGN 269
Vessel supports should remain integral at least until process
blowdown is complete.
Stringers to which equipment is attached may have signifi-
cantly different natural periods than the surrounding struc-
ture. Their dynamic response may therefore need to be
assessed separately.
C18.9 ACCIDENTAL LOADING
C18.9.1 General
The following commentary presents general guidance and
information for consideration of vessel collision.
C18.9.2 Vessel Collision
All exposed elements at risk in the collision zone of an
installation should be assessed for accidental vessel impact
during normal operations.
The collision zone is the area on any side of the platform
that a vessel could impact in an accidental situation during
normal operations. The vertical height of the collision zone
should be determined from considerations of vessel draft,
operational wave height and tidal elevation.
Elements carrying substantial dead load (that is, knee
braces), except for platform legs and piles, should not be
located in the collision zone. If such elements are located in
the collision zone they should be assessed for vessel impact.
C18.9.2a Impact Energy
The kinetic energy of a vessel can be calculated using
Equation C18.9.2-1.
E = 0.5 a m v
2
(C18.9.2-1)
Where:
E = the kinetic energy of the vessel,
a = added mass factor,
= 1.4 for broadside collision,
= 1.1 for bow/stern collision,
m = vessel mass,
v = velocity of vessel at impact.
The added mass coefficients shown are based on a ship-
shaped or boat-shaped hull.
For platforms in mild environments and reasonably close
to their base of supply, the following minimum requirements
should be used, unless other criteria can be demonstrated:
Vessel Mass = 1,100 short tons (1,000 metric tons)
Impact Velocity = 1.64 feet/second (0.5 meters/second)
The 1100-short-ton vessel is chosen to represent a typical
180-200-foot-long supply vessel in the U.S. Gulf of Mexico.
For deeper and more remote locations, the vessel mass and
impact velocity should be reviewed and increased where nec-
essary. In shallow areas, it may be possible to reduce this cri-
teria where access to the platform is limited to small
workboats.
18.9.2b Energy Absorption
An offshore structure will absorb energy primarily from
the following:
a. Localized plastic deformation (that is, denting) of the
tubular wall.
b. Elastic/plastic bending of the member.
c. Elastic/plastic elongation of the member.
d. Fendering device, if fitted.
e. Global platform deformation (that is, sway).
f. Ship deformation and/or rotation.
In general, resistance to vessel impact is dependent upon
the interaction of member denting and member bending. Plat-
form global deformation may be conservatively ignored. For
platforms of a compliant nature, it may be advantageous to
include the effects of global deformation.
C18.9.2c Damage Assessment
Two cases should be considered:
1. Impact (energy absorption and survival of platform).
2. Post-impact (platform to meet post-impact criteria).
Primary framework should be designed and configured to
absorb energy during impact, and to control the consequences
of damage after impact. Some permanent deformation of
members may be allowable in this energy absorption.
The platform should retain sufficient residual strength after
impact to withstand the one-year environmental storm loads
in addition to normal operating loads. Special attention
should be given to defensible representation of actual stiff-
ness of damaged members or joints in the post-impact assess-
ment. Damaged members may be considered totally
ineffective providing their wave areas are modeled in the
analyses.
Where adequate energy absorption can be calculated for
individual members, further checking is not required. In cases
where very stiff members (grouted legs or members) cause
the main energy absorption to be in the vessel, the supporting
braces for the member, the joints at each end of the member,
and the adjacent framing members should be checked for
structural integrity resulting from the impact loads.
Bracing members: A number of research studies have
been performed to evaluate the force required to locally dam-
age tubular members [2, 3]. O. Furnes [3], reported on these
experimental test results and found the relationship between
force and dent depth to be:
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270 API RECOMMENDED PRACTICE 2A-WSD
P
d
= 15 M
p
(D/t)
1/2
(X/R)
1/2
(C18.9.2-2)
Where:
P
d
= the denting force,
M
p
= the plastic moment capacity of the tube,
= F
y
t
2
/4 with F
y
being the yield strength,
D,R = the diameter and radius of the tube, respectively,
t = the wall thickness,
X = the dent depth.
Alternatively, C. P. Ellinas [4], reported the relationship
to be:
P
d
= 40 F
y
t
2
(X/D)
1/2
(C18.9.2-3)
The energy used in creating the dent is the integral of the
force applied over the distance or:
(C18.9.2-4)
Combining Equation C18.9.2-2 and C18.9.2-4 yields:
E
d
= 14.14 M
p
X
3/2
/ t
1/2
(C18.9.2-5)
Substitution of M
p
yields:
E
d
= 3.54 F
y
(tX)
3/2
(C18.9.2-6)
and introducing the relationship X = D/B to solve for various
D/t ratios yield:
E
d
= 3.54 F
y
(tD/B)
3/2
(C18.9.2-7)
Where:
B = brace diameter/dent depth
The energy required to cause a dent of limited depth may
be equated with the kinetic energy from the vessel impact.
Table C18.9.2-1 lists required tubular thickness of various
diameters for B = 8, 6, and 4 (corresponding to dents 12.5,
16.7, and 25 percent of the member diameter). Values have
been tabulated for F
y
= 35 and 50 ksi. If the dent should be
limited to D/8 (B = 8), then, from Table C18.9.2-1 the
required wall thickness for a 36-inch diameter 50 ksi tubular
is 0.94 inches.
Note that for small diameters, the required thicknesses get
quite large resulting in low D/t ratios. Much of the test data
falls in the D/t region of 30 to 60; projection of the results
outside of these ranges should be considered with caution.
Forces developed from Equation C18.9.2-2 applied to hori-
zontal and vertical diagonal members commonly found in
offshore jackets indicates that, in most situations, these mem-
bers would experience plastic deformation at the member
ends before the full denting force could be reached. Because
of this, the designer should consider the relative trade-offs
between increasing the wall thickness and diameter so that
the brace will be locally damaged rather than entirely
destroyed. In most normal operating conditions, the loss of a
brace in a redundant structure at the waterline is not cata-
strophic provided the leg to which the brace was attached
remains relatively undamaged. Other members connecting to
the same joint need to withstand forces resulting from the
impact. Where other brace members significantly overlap the
impacted member at the joint, the integrity of the connection
should be evaluated.
For structures with limited redundancy, such as minimal
structures, the loss of a waterline brace may be catastrophic.
Also, some decks have critical knee braces in the vessel
impact zone. These braces should be designed to withstand
vessel impact if the loss of the structure is unacceptable.
Jacket leg members: Energy absorption in jacket leg
members occurs mainly through localized denting of the
tubular shell and elastic/plastic bending of the member.
Denting should be minimized to ensure sufficient member
capacity for the platform post impact considerations. This is
accomplished through the selection of appropriate D/t ratios
for jacket legs. Using the U.S. Gulf of Mexico energy level
for broadside vessel impacts, dent depths for various D/t
ratios may be computed and the axial capacity of the dam-
aged member may then be compared to the undamaged case.
Figures C18.9.2-1 through C18.9.2-4 present the percentage
reduction in axial capacity of dented legs for both straight and
bent (L/360) conditions for 35 and 50 ksi yield strengths.
C18.9.2d Fendering
Fendering devices may be used to protect platform appur-
tenances (for example, risers, external conductors, etc.) or
parts of the structure. Fendering should be designed to with-
stand vessel impact without becoming detached from the
structure.
Clearances between fendering and protected elements of
the installation should be adequate to ensure integrity of pro-
tection throughout the energy absorption process of vessel
impact.
Supports for fendering systems should be designed to
avoid concentrating loads on primary structural members (for
example, legs).
C18.9.2e Risers and Conductors
Evaluation of risers and conductors is essential when
such elements are external to the structure. Clear warnings
are suggested for those sides of the platform where such
elements are located and not protected by some form of
fendering.
E
d
P
d
xd
0
x
∫
=
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RECOMMENDED PRACTICE FOR PLANNING, DESIGNING AND CONSTRUCTING FIXED OFFSHORE PLATFORMS—WORKING STRESS DESIGN 271
Table C18.9.2-1—Required Tubular Thickness to Locally Absorb Vessel Impact
Broadside Vessel Impact Condition
F
y
= 345 MPa (50 ksi) F
y
= 240 MPa (35 ksi)
B*= 8.0 6.0 4.0 8.0 6.0 4.0
Diameter (inch) Wall Thickness, t (inch) Wall Thickness, t (inch)
12.0 2.834 2.125 1.417 3.595 2.696 1.797
14.0 2.429 1.822 1.215 3.081 2.311 1.541
16.0 2.125 1.594 1.063 2.696 2.022 1.348
18.0 1.889 1.417 0.945 2.396 1.797 1.198
20.0 1.700 1.275 0.850 2.157 1.618 1.078
22.0 1.546 1.159 0.773 1.961 1.471 0.980
24.0 1.417 1.063 0.708 1.797 1.348 0.899
26.0 1.308 0.981 0.654 1.659 1.244 0.830
28.0 1.215 0.911 0.607 1.541 1.155 0.770
30.0 1.134 0.850 0.567 1.438 1.078 0.719
32.0 1.063 0.797 0.531 1.348 1.011 0.674
34.0 1.000 0.750 0.500 1.269 0.952 0.634
36.0 0.945 0.708 0.472 1.198 0.899 0.599
38.0 0.895 0.671 0.447 1.135 0.851 0.568
40.0 0.850 0.638 0.425 1.078 0.809 0.539
42.0 0.810 0.607 0.405 1.027 0.770 0.514
44.0 0.773 0.580 0.386 0.980 0.735 0.490
46.0 0.739 0.554 0.370 0.938 0.703 0.469
48.0 0.708 0.531 0.354 0.899 0.674 0.449
50.0 0.680 0.510 0.340 0.863 0.647 0.431
52.0 0.654 0.490 0.327 0.830 0.622 0.415
54.0 0.630 0.472 0.315 0.799 0.599 0.399
56.0 0.607 0.455 0.304 0.770 0.578 0.385
58.0 0.586 0.440 0.293 0.744 0.558 0.372
60.0 0.567 0.425 0.283 0.719 0.539 0.359
62.0 0.548 0.411 0.274 0.696 0.522 0.348
64.0 0.531 0.399 0.266 0.674 0.505 0.337
66.0 0.515 0.386 0.258 0.654 0.490 0.327
68.0 0.500 0.375 0.250 0.634 0.476 0.317
70.0 0.486 0.364 0.243 0.616 0.462 0.308
72.0 0.472 0.354 0.236 0.599 0.449 0.300
Note: the table lists the required wall thickness for selected values of D, B and F
y
based on Equation C18.9.2-7. Values are derived assuming a broadside
impact of a 1000-metric-ton vessel moving at 0.50 meters/sec. All energy is assumed to be absorbed by the member.
*Where B = Diameter/X (dent depth).
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--`,,```,,,`,,,,,,,,,,,,,,`,``,`-`-`,,`,,`,`,,`---
272 API RECOMMENDED PRACTICE 2A-WSD
Figure C18.9.2-1—D/T Ratio versus Reduction in Ultimate Capacity,
48, 54, and 60 Inch Legs—Straight with L = 60 Feet, K = 1.0, and F
y
= 35 ksi
Figure C18.9.2-2—D/T Ratio versus Reduction in Ultimate Capacity,
48, 54, and 60 Inch Legs—Straight with L = 60 Feet, K = 1.0, and F
y
= 50 ksi
60 Inch leg
54 Inch leg
48 Inch leg
Legend
D/T
0.0
50.0
100.0
150.0
200.0
20.0 30.0 40.0 50.0 60.0 70.0
Percent Reduction in Ultimate Capacity
60 Inch leg
54 Inch leg
48 Inch leg
Legend
D/T
0.0
50.0
100.0
150.0
200.0
20.0 30.0 40.0 50.0 60.0 70.0
Percent Reduction in Ultimate Capacity
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RECOMMENDED PRACTICE FOR PLANNING, DESIGNING AND CONSTRUCTING FIXED OFFSHORE PLATFORMS—WORKING STRESS DESIGN 273
Figure C18.9.2-3—D/T Ratio versus Reduction in Ultimate Capacity,
48, 54, and 60 Inch Legs—Bent with L = 60 Feet, K = 1.0, and F
y
= 35 ksi
Figure C18.9.2-4—D/T Ratio versus Reduction in Ultimate Capacity,
48, 54, and 60 Inch Legs—Bent with L = 60 Feet, K = 1.0, and F
y
= 50 ksi
60 Inch leg
54 Inch leg
48 Inch leg
Legend
D/T
0.0
50.0
100.0
150.0
200.0
20.0 30.0 40.0 50.0 60.0 70.0
Percent Reduction in Ultimate Capacity
60 Inch leg
54 Inch leg
48 Inch leg
Legend
D/T
0.0
50.0
100.0
150.0
200.0
20.0 30.0 40.0 50.0 60.0 70.0
Percent Reduction in Ultimate Capacity
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274 API RECOMMENDED PRACTICE 2A-WSD
C18.10 REFERENCES
1. Interim Guidance Notes for the Design and Protection of
Topsides Structures Against Explosion and Fire, The Steel
Construction Institute, April 1993.
2. G. Foss and G. Edvardsen, “Energy Absorption During
Ship on Offshore Steel Structures,” OTC 4217, 1982.
3. O. Furnes and J. Amdahl, “Ship Collisions with Offshore
Platforms,” Intermaric ‘80, September 1980.
4. C.P. Ellinas and A.C. Walker, “Effects of Damage on Off-
shore Tubular Bracing Members,” IABSE, May 1983.
5. NFPA 68.
6. C.P. Ellinas, W.J. Supple, and A.C. Walker, Buckling of Off-
shore Structures, Gulf Publishing Company.
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08/07
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