Open Journal of Marine Science, 2011, 1, 36-42
doi:10.4236/ojms.2011.12004 Published Online July 2011 (http://www.SciRP.org/journal/ojms)
Seismic Response of a Typical Fixed Jacket-Type Offshore
Platform (SPD1) under Sea Waves
Khosro Bargi, S. Reza Hosseini, Mohammad H. Tadayon, Hesam Sharifian
School of Civil Engineering, College of Engineeri n g, University of Tehran, Tehran, Iran
Received May 6, 2011; revised May 20, 2011; accepted June 5, 2011
Abstract
Offshore platforms in seismically active areas should be designed to service severe earthquake excitations
with no global structural failure. In seismic design of offshore platforms, it is often necessary to perform a
dynamic analysis that accounts for nonlinear pile soil structures interaction effects. This paper summarizes
the nonlinear dynamic analysis of a 3-D model of a typical Jacket-Type platform which is installed in Persian
Gulf (SPD1), under simultaneously wave and earthquake loading has been conducted. It is assumed that they
act in the same and different directions. The interaction between soil and piles is modeled by equivalent pile
length theory. The structure is modeled by finite element method (Ansys Inc.). It be concluded that when the
longitudinal components of the earthquake and wave are in different directions, an increase on the response
of platform can be seen.
Keywords: Fixed Offshore Platform, Nonlinear Dynamic Analysis, Earthquake, Wave, Seismic Response
1. Introduction
Fixed offshore platforms are subjected to different envi-
ronmental loads during their life time. These loads are
imposed on platforms through natural phenomena such
as wind, current, wave, earthquake, snow, ice and earth
movement. Among various types of environmental load-
ing, wave forces and earthquake loading are two domi-
nated loads in seismically active regions.
According to API-RP2A 1997 (2.2) [1], environmental
loads, with the exception of earthquake, should be com-
bined in a manner consistent with the probability of their
simultaneous occurrence during the loading condition
being considered. Earthquake load, where applicable,
should be imposed on the platform as a separate envi-
ronmental loading condition. In addition DNV 1980
(5.2.4) [2] suggests that loads due to earthquake nor-
mally need not be considered to act simultaneously with
other environmental loads.
Yamada et al. (1989) [3] studied the seismic response
of offshore structures in random seas and concluded the
hydrodynamic damping effect of random seas. Jain
(1996) [4] considered a simple cantilever beam model
for a bottom fixed offshore steel tower under sea waves
and earthquake forces and concluded that hydrodynamic
damping forces are proportional to the square of the rela-
tive velocities between the waves and the structure. Jin et
al. (1997) studied the stochastic response of a two di-
mensional Jacket-Type platforms under simultaneously
acting waves and earthquakes, acting in the same direc-
tion and get similar conclusions. Etemad et al. (2004) [5]
studied time history analysis of a Jacket platform under
waves and earthquake loads assuming to act in different
directions. Their results show that waves and earthquake
applied in different directions, can introduce sever situa-
tions.
In this study a jacket-type offshore platform which is
installed in Persian Gulf has been modeled for the as-
sessment of seismic response of structure under simulta-
neously acting wave and earthquake loads. The soil
which surrounds piles is modeled by equivalent pile
length theory. Fifth order stokes wave theory was used
and the recorded earthquake time history-displacement
involves 3 records with different energy levels (El Centro,
Kobe, Tabas) which are provided from
http://PEER.Berkeley.edu.html, were used.
2. Case Study
In this study, the studied platform is a fixed Jacket-Type
platform which is located in Persian Gulf. It has 6 main
legs with the height of 78.1 m. The total mass of the

K. BARGI ET AL.
37
platform is 4334 tons and the water depth in the location
of installed platform is 70.2 m
Regarding to the information of waves height with the
returning period of one year for studied zone, a fifth or-
der stokes wave theory with the height of 6.7 m and the
period of 8.6 s used in this study. The mentioned wave is
the most critical wave with respect to platform particu-
lars (geometry and spacing).The applied earthquake re-
cords which are used in this study involve Tabas, El
Centro and Kobe with peak ground acceleration 0.836 g,
0.313 g and 509 g respectively and provided from PEER.
Respectively, Figure 1 show longitudinal and hori-
zontal components of time-history of earthquake dis-
placement which are used in this study.
3. Modelling and Analysis Procedure
Nonlinear seismic response analysis of the pile supported
Jacket-Type offshore platforms developed in this study
was performed using the general purpose finite element
analysis software ANSYS 9.
In order to model jacket, tubular elements and act of
the wave load on studied model, “PIPE59” from ele-
ments library of software has been used, which it has the
ability to model wave load with the use of some wave
theories such as stokes fifth order theory, buoyancy, hy-
drostatic, added mass and etc. Drag and inertia coeffi-
cients which are used in this study are 0.7 and 2 respec-
tively.
In the present finit element model, the pile members
are subdivided into elastic-plastic pipe elements of
“PIPE20” from elements library of software. “PIPE20” is
a two-nodded uniaxial plastic straight pipe element with
six degrees of freedom at each node. Only large deflec-
tion capabilities of this element are used in this model.
Neither Shear deflections nor other plastic features of the
-1
-0.5
0
0.5
1
1.5
0 10203040
Time (sec)
Displacement
ER Y
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0 10203040
Time (sec )
Displacement
ER X
-0.1
-0.05
0
0.05
0.1
0.15
0 10203040
Time (sec)
Displacement
ER Y
-0.15
-0.1
-0.05
0
0.05
0.1
010203040
Time (sec )
Displacement
ER X
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0 10203040
Time (sec)
Displa ce men t
ER Y
-0.1
-0.05
0
0.05
0.1
0.15
010203040
Time (sec )
Displacement
ER X
(a) (b)
(c) (d)
(e) (f)
Figure 1. Time history Longitudinal Component of (a) El Centro; (c) Kobe; (e) Tabas and Time history Horizontal Compo-
nent of (b) El Centro; (d) Kobe; (f) Tabas.

K. BARGI ET AL.
38
element are used in this model.
For calculating the length of equivalent pile, jacket
modeled with total length of the pile and influenced soil
and pile interaction and design wave on model. After-
wards the base shear and displacement on mud line level
was calculated. With comparing results, this length was
calculated 16.5 m that approximately is 12D (12* pile
diameter).
In order to corresponding operation of pile and jacket
elements, their degrees of freedom coupled at each node
in horizontal plane.
Figure 2 shows a schematic model of studied platform
in this study.
The mass of deck and some other masses such as
Grout, Boat landing, anodes, mud mat, bumpers and etc
were applied on studied model as concentrated masses
on related nodes with the use of “MASS21” from soft-
ware element library. “MASS21” is a point element has
up to six degrees of freedom on which only translational
degrees of freedom are used in this model. In the present
study, in order to model the geometric non-linearity at
the structural level and inelastic buckling of the struc-
tural members, large displacement formulation in AN-
SYS with bi-linear kinematic hardening material model
are used.
Node 589
Figure 2. Schematic model of Jacket-Type platform.
In this finite element model, interaction between soil-
pile has been modeled by equivalent pile length theory.
A dynamic analysis is normally mandatory for every
offshore structure, but can be restricted to the main
modes in the case of stiff structures. The first step in a
dynamic analysis consists of determining the principal
natural vibration mode shapes and frequencies of the
undamped, multi-degree-of-freedom structure up to
given order (30th to 50th). First rigid structures have a
fundamental vibration period well below the range of
wave periods (typically less than 3sec.) [8], first and
second modes are effective on structure behavior and
higher order mode shapes having less effects on structure
behavior.
Then transient dynamic analysis was done with im-
posing wave and current design loads. Results show that
model horizontal displacement has less than 5% differ-
ence with fact.
Analysis shows that model behavior has good corre-
sponding with fact. Afterwards, in order to reduce the
calculation time, model was used with equivalent pile
length.
In this study, exceed than 90 difference analyses in
order to investigate results and model verification were
performed.
In order to model verification, 20 first modes of struc-
ture compared with initial design modes. It shows that
first and second modes correspond with fact and high
order modes have approximately 35% - 50% difference,
because jacket modeled with equivalent pile length. This
comparison is show in Table 1.
Table 1. Comparison of periods.
Mode Model Periods (sec)SPD1 Periods (sec)% Error
1 2.02 2.05 −1.4
2 1.89 1.9 −0.5
3 0.77 1.52 −49.0
4 0.47 0.82 −42.1
5 0.44 0.75 −41.1
6 0.32 0.53 −39.1
7 0.32 0.5 −36.4
8 0.30 0.49 −38.3
9 0.30 0.47 −36.0
10 0.29 0.46 −36.5

K. BARGI ET AL.
39
4. Seismic Response Analysis of Studied
Platform Subjected to Earthquake
and Wave Loads
Wave and earthquake phenomena occur at stochastic
direction. Studied structure is symmetric around Y direc-
tion, therefore according to Figure 3, for analysis, four
directions for earthquake and wave loads imposed on
structure are selected.
In order to evaluate the response of studied platform
under earthquake and wave loads, simultaneously four
studies regarding to wave and earthquake directions have
been performed.
In the first study, only earthquake load analysis was
performed at four directions (Figure 3). Regarding to the
random features of sea waves, since it is possible that the
direction of wave and earthquake loads to be different, in
the second analysis, earthquake longitudinal component
fixed at zero direction simultaneously acts with wave
load at four directions. For tertiary analysis, wave load
component fixed at zero direction simultaneously acts
with earthquake longitudinal component at four direc-
tions. For all conditions, both earthquake longitudinal
and horizontal components were used. Finally, analysis
for 100 years wave was performed at four directions.
The results are shown as displacement-time history at
node 589 (one of the deck’s nodes) as shown in Figure 2.
Figure 4 show the comparison between displacements
under earthquake load (blue line) and the combination of
earthquake and wave loads (pink line) and 100 years
wave (yellow line) in X and Y directions respectively for
each analyses critical condition.
90
º
X
Y
Earthquake
X Direction
45
º
−45
º
0
º
X
Y
Wave
Direction
90
º
45
º
−45
º
0
º
Figure 3. Wave and earthquake imposed directions.
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1020304050
Ti m e (s ec)
Displacement (m )
ER-45
WA0 ER-45
WA90
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1020304050
Ti m e (s ec)
Displacement (m )
ER45
WA0 ER45
WA0
El Centro-X El Centro-Y
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0 10203040
Tim e ( s ec)
Displacement ( m )
ER45
W A0 E R45
WA90
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0 10203040
Ti m e (s ec)
Displacement (m )
ER0
WA0 ER0
WA0
Tabas-X Tabas-Y
(a) (b)
(c) (d)

40 K. BARGI ET AL.
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1020304050
Tim e (sec)
Displacement (m)
ER0
WA 90 ER0
WA90
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 1020304050
Tim e ( sec)
Displacement (m)
ER45
WA0 ER-45
WA0
Kobe-X Kobe-Y
(e) (d)
Figure 4. Panels (a), (c), (e) Response of platform in X direction at node 589, Panels (b), (d), (f) Response of platform in Y
direction at node 589. (Loads: pink line: earthquake; blue line: earthquake and wave; yellow line: 100 years wave).
API-RP2A and DnV has also suggested evaluating the
behavior of fixed offshore platforms under earthquake
and wave loads separately. According to the results, it
can be concluded that the displacement for earthquake
load alone is less than the displacement for the combina-
tion of earthquake and wave loads.
This study shows significant difference between drift
under simultaneously wave and earthquake loads com-
pared with regulations criteria (for earthquake load). This
difference is shown in Table 2.
For example, the response of jacket under wave load-
ing and El-Centro seismic loading was studied on severe
direction separately. This comparison was done for other
seismic records and other conditions. Those results are
shown on result section.
This method of analysis considers the wave in one di-
rection is constant and earthquake lateral component
direction is applied 45 degrees by 45 degrees. The drift
of node 589 is evaluated in X direction. Figure 5 shows
the result of effect of non-directional act of wave and
earthquake components on jacket.
According to the Figure 6 the result is that the maxi-
mum drift on X direction occurs when the seismic lateral
component is applied on 45 degrees and water wave is
applied on zero degree.
Table 2. Difference betw een drifts.
TabasKobe Elcentro Earthquake
YXY X Y X Direction
1.751.711.942.65 1.94 1.97
Difference ratio between wave
and earthquake compared with
earthquake alone
5.746.812.132.15 2.41 2.74
Difference percent ratio between
wave and earthquake compared
with 100 years wa v e
Figure 7 shows the comparison effect of non-direc-
tional of water wave and seismic lateral component such
as conditions discussed above, but Y direction drift is
studied.
-0.2
-0.1 5
-0.1
-0.0 5
0
0.05
0.1
0.15
0 10203040
Tim e (sec)
Disp lacemen t (m )
50
ER0
WA 0 E R45
WA0 ER-45
WA0 ER0
WA 0 E R90
Figure 5. The comparison of X direction Drift when water
wave lateral component is applied on zero degree and seis-
mic lateral comp onent is applied 45 degrees by 45 degrees.
0
20
40
60
80
100
120
P ercentage of
Differences
0101.5 14.271.948.4
ER0 WA0
ER45WA0 ER-45 WA0 ER0 WA0 ER90
Figure 6. Comparison of Percentage changes in relative
displacement in X direction on node 589 for Figure 5.

K. BARGI ET AL.
41
According to Figure 8 the result is that the maximum
drift on Y direction occurs when the seismic lateral
component is applied on –45 degrees and water wave is
applied on zero degree.
Figure 9 shows the comparison of non-directional ef-
fect of water wave and seismic lateral component applied
-0. 1
-0. 05
0
0.05
0.1
0.15
0.2
0.25
0.3
0 10203040
Time (sec)
Displace ment (m)
50
ER0
W A 0 ER45
WA0 ER-45
WA0 ER0
W A 0 ER90
Figure 7. The comparison of Y direction Drift when the
water wave lateral component is applied on zero degree and
seismic lateral component is applied 45 degrees by 45 de-
grees.
0
20
40
60
80
100
120
P ercentage of
Differenc es
15.80107.4 44.563.3
ER0 WA0
ER45 WA0 ER-45 WA0 ER0 WA0 ER90
Figure 8. Comparison of Percentage changes in relative
displacement in Y direction on node 589 for Figure 7.
-0.1 5
-0. 1
-0.0 5
0
0.05
0.1
0.15
0 102030405
Time (sec)
Displacement (m)
0
W A45 ER0
WA0 ER0
W A-45 E R0
W A90 ER0
ER0
Figure 9. Comparison of drifts on X direction when seismic
lateral is applied on zero degree and water wave direction
changes 45 degrees by 45 degrees.
simultaneously on jacket. In this case the direction of
seismic lateral component is constant on zero degree and
water wave direction changes 45 degrees by 45 degrees.
Drifts are compared in X direction for node 589.
Figure 10 shows that maximum drift on X direction
when the seismic lateral component is applied in zero
degree and water wave is applied in 45 degrees. The
same comparison for drift in Y direction for node 589 is
given below.
Figure 12 shows that maximum drift on Y direction
when the seismic lateral component is applied in zero
degree and water wave is applied in 45 degrees.
5. Conclusions
The nonlinear dynamic behavior of Jacket-type platform
under simultaneously acting of wave and earthquake
loads was studied in this paper. The following results are
obtained.
At first, the earthquake loads were applied alone at
four different directions (Figure 3). Then wave and lon-
gitudinal component of earthquake were applied simul
taneously in the same and different four directions (Fig-
ure 3).
0
10
20
30
40
50
60
70
80
P ercen t age of
Differences
064.2 74.271.9 73.5
ER0 WA90
ER0 WA-45 ER0 WA0 ER0 WA45 ER0
Figure 10. Comparison of Percentage changes in relative
displacement in X direction on node 589 for Figure 9.
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0 102030405
Time (sec)
Displacement (m)
0
WA45 ER0
WA0 ER0
WA-45 ER0
WA90 ER0
ER0
Figure 11. Comparison of drifts on Y direction when seis-
mic lateral is applied on zero degree and water wave direc-
tion changes 45 degrees by 45 degrees.

K. BARGI ET AL.
42
0
5
10
15
20
25
30
35
40
P erc entage of
Differenc es
035.9 23.824.8 38.6
ER0WA90
ER0 WA-45 ER0 WA0 ER0 WA45 ER0
Figure 12. Comparison of Percentage changes in relative
displacement in Y direction on node 589 for Figure 11.
The results comparison shows that the maximum dis-
placement response of platform under combination of
two loads (earthquake and wave loads) are more than
maximum displacement response of earthquake load
alone.
6. References
[1] API Recommended Practice 2A-WSD, “Recommended
Practice for Planning, Designing and Constructing Fixed
Offshore Platform-Working Stress Design,” 20th Edition,
Official Publication, US, 1996.
[2] Det Norske Veritas (DNV) “Result for the Design, Con-
struction and Inspection of Offshore Structures,” Oslo,
1977. (Reprint with correction 1981).
http://www.dnv.com
[3] A. K. Jain, “Dynamics of Offshore Structures under Sea
Waves and Earthquake Forces,” American Society of
Mechanical Engineers, Offshore Technology, Vol. 1,
1996, pp. 191-198.
[4] D. Y. Jin and T. Matsui, “Stichastic Response Analysis of
Jacket-Type Ocean Platforms under Simultaneously Act-
ing Waves and Earthquakes,” American Society of Me-
chanical Engineers, Safety and Reliability, Vol. 2, 1997,
pp. 297-302.
[5] “P0144: Earthquake and Station Details,” 1978.
http://peer.berkeley.edu/svbin/Detail?id=P014
[6] “P0006: Earthquake and Station Details,” 1940.
http://peer.berkeley.edu/svbin/Detail?id=P0006
[7] “P1046: Earthquake and Station Details,” 1995.
http://peer.berkeley.edu/svbin/Detail?id=P1046
[8] ESDEP WG 15A, “Lecture 15A.1: Offshore Structure:
General Introduction,” University of Ljubljana, Ljubljana,
2003.