Analysis of Offshore Structure Having Different Bracing Pattern Using Sap2000
Neeta Chavhan1*, Bhalge PS2 and Vaidkar NS3
1Professor and PG Student, India
2Hod, Department of Civil Engineering, India
3Prof., Department of Civil Engineering, India
Submission: July 12, 2019; Published: August 12, 2019
*Corresponding Author: Neeta Chavhan, Professor and PG student, SYCET, Aurangabad, India
How to cite this article: Neeta Chavhan, Bhalge PS, Vaidkar NS. Analysis of Offshore Structure Having Different Bracing Pattern Using Sap2000. Civil Eng Res J. 2019; 9(1): 555751. DOI: 10.19080/CERJ.2019.09.555751
In this paper time history analysis is performed for offshore steel structures for El-Centro data for 31sec. The effect of slope (different angle 0degree, 20degree and 30degree) is studied for various loading condition and the effect bracings (single bracings, knee bracings, cross bracings) for different loading are also studied. For FEA analysis SAP 2000 is used which observed very effective for analysis.
Keywords: Offshore structure; SAP 2000; El-Centro; Bracings
The total number of offshore platform in various bays, gulf and oceans of the world is increasing year by year, most of which are of fixed jacket-type platforms located in 30m to 200m depth for oil and gas exploration purposes . Fixed offshore platforms are subjected to different environmental loads during their lifetime. These loads are imposed on platforms through natural phenomena such as wind, current, wave, earthquake, snow and earth movement. Among various types of environmental loading, wave forces loading is dominated loads . The results of these investigations highlight the importance of accurately simulating nonlinear effects in fixed offshore structures from the point of view of safe design and operation of such systems. It is necessary to design an offshore structure such that it can respond to moderate environmental loads without damage and is capable of resisting severe environmental loads without seriously endangering the occupants [3-5]. The standard design of the structure is carried out using the allowable stress method. However, it is important to clarify the effects on nonlinear responses for an offshore structure under the severe wave conditions. Offshore structures may be analyzed using static or dynamic analysis methods. Static analysis methods are sufficient for structures, which are rigid enough to neglect the dynamic forces associated with the motion under the time-dependent environmental loadings [6-9]. On the other hand, structures which are flexible due to their particular form and which are to be used in deep sea must be checked for dynamic loads.
The main objective of this study is to examine the behavior of Off-shore structure with different bracing system with different angles provided to structure.
Single bracing for 0° structure
a) Double bracing for 0° structure
b) Knee bracing for 0° structure
c) Single bracing for 20° structure
d) Double bracing for 20° structure
e) Knee bracing for 20° structure
f) Single bracing for 30° structure
g) Double bracing for 30° structure
h) Knee bracing for 20° structure
All above cases are analyzed for time history analysis
Figure 1. The analysis is carried out using SAP 2000 software.
The comparison is made between different bracing systems.
They study of platform is a fixed Jacket-Type platform currently
installed in the Suez gulf .
Response spectrum analysis is a procedure for computing the
statistical maximum response of a structure to a base excitation
Figure 2. Each of the shaking modes that are considered may
be assumed to respond independently as a single-degreeof-
freedom system . Spectra which determine the base
acceleration applied to each mode according to its period (the
number of seconds required for a cycle of vibration).
The time history analysis calculates the building responses
at discrete time steps using discretized record of synthetic
time history as base motion . If three or more-time history
analyses are performed, only the maximum responses of the
parameter of interest are selected.
The studied platform is a fixed Jacket-Type platform currently
installed in the Suez gulf, Red sea, 1988 shown in Figure 3, The
offshore structure is a four legs jacket platform, consists of a
steel tubular-space frame [12-14]. There are diagonal brace
members in both vertical and horizontal planes in the units to
enhance the structural stiffness [15-17].
The Platform was originally designed as a 4-pile platform
installed in 110 feet (110’ =33.5m) water depth.
a. Outer diameter 0.762m
b. Inner diameter 0.664m
c. Wall thickness for platform 0.05m
d. Spacing of horizontal member 5.6m
e. X-braced with tubular member diameter 0.32m
f. Density of various materials considered for design,
in this we compared the single system bracing system in
different degree that is (0degree, 20degree, 30 degree) and we
come to know that 30degree model is preferable to the forces
applied in time history Table 1.
i. In this Graph 1 Mode Shear vs Time Period highest time
period is observed (6.1sec.) for knee bracing.
ii. In this Graph 2 Mode Shear vs Natural Frequency
highest natural frequency (14Hz) is observed for double
iii. In this Graph 3 Deformation-x20 Degree Highest
deformation is observed 0.24mm for single bracing.
Therefore, double bracing is most effective as compared to
single and knee bracing
iv. In this Graph 4 0.6mm highest deformation is observed
for single bracing Therefore double bracing is most effective
as compared to single and knee bracing
v. In this Graph 5 Base shear 295kN for o degree highest
base shear is observed for single bracing Therefore double
bracing is most effective as compared to single and knee
bracing due to less deformation.
In this paper, various types of bracings are studied subjected
to dynamic load and it is observed that deformation in y direction
is 25% less in double bracing and 15% less in knee bracing. But
deformation in X direction is observed more in knee bracings.
In addition to this, the natural frequency is observed more in
knee bracings. For base shear, it is observed that base shear is
15% more in single bracings than the cross bracings and knee