American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com

| Mahar, Jasem Alhasan

| and | Adnan, Mohamed Ibrahim

Tishreen University | Department of water engineering and irrigation | Lattakia | Syria |

ABSTRACT

Background The efficiency and safe design of marine platforms depend on a complex structure of mutual influence between these

facilities and the surrounding environment and the degree of resistance to different loading patterns during the entire investment

period. Fixed jacket platforms are huge steel framed structures used for the exploration and extraction of oil and gas from the earth’s

crust. Jacket type structures are appropriate for relatively shallow water depth. Objectives: the objective of this study is to select

specific parameters for the elements of a fixed platform (jacket) and conduct an static analysis study to resist these elements for the

influence of the basic loads that are subject to it within the environment of the Syrian coast , using the software SACS. Methods: the

jacket is modeled in SACS finite element program and static analysis is conducted with fixed boundary conditions. Critical conditions

are taken into account, which include structure and equipment weight, wind load, hydrodynamic load using Morison equation.

Results: Linear static analysis is performed for the four legged jacket considering (3) loading directions. The maximum base shear

and overturning moment are calculated in normal environmental conditions and stormy. as well as the unit check values, for all

member are obtained it. Conclusions: the environmental conditions of the Syrian coast are suitable for the construction of jacket

offshore, therefore a great importance to support the Syrian national economy.

Keywords

Unity Check, Jacket,

Linear Static Analysis

SACS software

1. INTRODUCTION

Offshore platforms are huge steel or concrete structures used for the exploration and extraction of oil and gas from the

earth’s crust. Offshore structures are designed for installation in the open sea, lakes, gulfs, etc., many kilometers from

shorelines. these structures may be made of steel, reinforced concrete or a combination of both [7].

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 100 ft (32 m) to 650 ft (200 m) depth for oil and gas exploration

purposes [8]. The analysis, design and construction of offshore structures compatible with the extreme offshore

environmental conditions is a most challenging and creative task. Over the usual conditions and situations met by land-

based structures, offshore structures have the added complication of being placed in an ocean environment where

hydrodynamic interaction effects and dynamic response become major considerations in their design [6].

The jacket structure typically consists of tubular members of various diameters and wall thicknesses. the air gap between

the sea surface and the bottom of the topside structure is made high enough to prevent waves from hitting the topside

structure. At the bottom, the jacket is normally outfitted with a temporary foundation which supports the jacket until the

permanent foundation is installed. Bracing configurations consist of the vertical, horizontal and diagonal members, who

connect jacket legs forming a stiff truss system, transfer the loads acting on the platform to its foundation [4]. The

primary function of a jacket structure is to support the weight of the topside structure by transferring the weight to the

foundation. The jacket structure is 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 [9]. 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. On

the other hand, structures which are flexible due to their particular form are to be used in deep water must be checked

for dynamic loads [2]. The calculation of the wave loads on vertical tubular members is always of major concern to

engineers. The analysis of wave effects on offshore structures, such as wave loads and corresponding responses, are of

ORIGINAL ARTICLE

ANALYSIS OF OFFSHORE JACKET STRUCTURE IN THE SYRIAN

COAST BY USING COMPUTER MODELING

and Applied Sciences are the property of Atlantic Center Research Sciences, and is protected by copyright laws CC-BY. See: http://creativecommons.org/licenses/by-nc/4.0/.

American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com

great importance to ocean engineers in the design, and for the operational safety of offshore structures, especially

recently when such studies are motivated by the need to build solid marine structures in connection with oil and natural

gas productions [3]. The effects of various wave patterns on offshore structure have been investigated by numerous

researchers in the past [10, 11].

From the simulations of wave loading and structural analysis on few model tests, it can be concluded that the developed

programs are able to reproduce results from the model tests with satisfactory accuracy [5].

The present paper deals the static responses of an four legged jacket platform under the environmental conditions of the

Syrian coast, in order to determine the design parameters necessary for the stability of a typical platform at a specific

depth by using the computer program SACS. SACS (structural analysis computer systems), a design and analysis software

for offshore structures and vessels, is used for the modeling and analysis of the jacket.

2. MATERIALS AND METHODS

2.1 Study site and data availability:

Environmental data of the platform model for static analysis are based on study conducted by the Institute Kaspmornii

proekt, to implement the technical project to expand the port of Lattakia, which is located in the northwestern part of

Syrian territory. The wave system was determined in the study area based on the processing of wind data taken from the

overall weather maps of the Eastern Mediterranean between 1975 and 1951 (25 years).

Wind speeds at 10m above mean sea level from three main geographical directions with 1 and 100- years return period

are shown in table (1).

Table 1: The table presents the wind speeds with 1 and 100- years return period.

Geographical

Direction(from)

Wind Velocity (m/s)

1-year

100-years

south

11,0

South-west

14,0

North-west

10,0

2.2 Data processing:

The values of the significant wave heights and their periods were calculated in the study area Depending on the CEM

relationship for wave prediction. The wave height and period of wave with 1 and 100- year return period shown in table

(2).

Table 2: The table presents the wave height and period of wave with 1and 100- year

return period.

Geographical

Direction(from)

wave

1-year 100-year





(m) 



(s)





(m) 



(s)

south

1.98 6.1

4.4 8.1

South-west

2.4 6.4

6.7 9.94

North-west

1.5 5.3

4.4 8.2

2.3 Environmental loads:

Water force can be classified as forces due to waves and forces due to current. Wind blowing over the ocean’s surface

drags water along with it, thus forming current and generating waves. The forces induced by ocean waves on platform

are dynamic in nature. However, it is the accepted practice to design shallow water platforms by static approach. As a

water depth increases and platforms become flexible, dynamic effect becomes significant [9].

2.4 Waves:

Regular wave theories used for calculation of wave forces on fixed offshore structures are based on the three

parameters water depth (d), wave height (h) and wave period (T) as obtained from wave measurements adapted to

different statistical models, as The figure1 presents. Wave plus current kinematics (velocity and acceleration fields) are

American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com

generated using 5th order Stokes wave theory, the forces on individual structural elements are calculated using Morison

equation, based on hydrodynamic drag and mass coefficients (Cd , Cm) and particle velocity and acceleration obtained by

the 5th order Stokes wave theory. Stokes 5th order wave is defined by providing wave height and period in the input

data with the wave type specified as Stokes in the SACS options [7].

2.5 Current loads:

The wave induce an orbital motion in the water in which they travel, and these orbits are closed but experience a slight

drift forward to wind surface effects. The current is actually induced by wave. A current in the wave direction tends to

stretch the wavelength, typical wind and current profile, as The figure1 presents, is consider in this study [1,6].

Figure 1: The figure presents the wave coordinate system and typical wind and

tidal current profile.

2.6 Wind loads:

When a structure is placed in the path of the moving air so that wind is stopped or is deflected from its path, then all or

part of the kinetic energy is transformed into the potential energy pressure. Wind forces on any structure therefore result

from the differential pressure caused by the obstruction to the free flow of the wind. These forces are functions of the

wind velocity, orientation, area, and shape of the structural elements. Wind forces on a structure are a dynamic problem,

but for design purposes, it is sufficient to consider these forces as an equivalent static pressure [1].

2.7 Analysis Software

SACS (structural analysis computer systems), a design and analysis software for offshore structures and vessels, is used

for the modeling and analysis of the jacket. SACS is an integrated suite of finite element based software that supports the

analysis, design and fabrication of offshore structures, including oil, gas, and wind farm platforms and topsides [7]. Its

ability to dynamically iterate designs allows users to perform advanced analysis, comply with offshore design criteria, and

visualize complex results. SACS provides reliable beam member code checking and tubular joint code checking capacity；

therefore it is very suitable for topsides structures consisting of plate girders and tubular columns/ braces [4].

2.8 Modeling Data

The jacket platform is four-legged jacket for the purpose of supporting 6960 KN maximum operation weight located in

The Syrian Coast, at a water depth of 79.5 m. The total height of the sea platform (102.5m) intended to support the

loads of the upper section (main, secondary). The jacket footprint at sea floor is (22.112m × 25.46m) and topside footing

dimension is (13.96m x 9.16m) at level (+2). The dimensions of the main section (20mx18m) at level (23) and the

secondary section (15 mx18m) at the level (+15.3). The bracing pattern used is (X- pattern). All the members are tubular

with outside diameter varying from 320mm to 1230 mm and wall thickness from 12.5mm to 44.5 mm. Computer model

of the four legged platform is shown in figure 2.

The steel s420 is used for legs, piles and primary members and steel grade s355 is used for secondary members. The

properties of are taken from NS-EN10025-3.

American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com

The design loading models depend on permanent loads, variable loads, current and wind loads and wind loads.

Figure 2: The figure presents the 3D Model in SACS and test.

3. RESULTS AND DISCUSSION

Linear static analysis is performed for the four legged jacket considering 3 loading directions (135, 180 & 45) deg,

depending on the dominant trends of wind waves affecting the Syrian coast (southwest, west, northwest). Post, a sub

program of SACS vi, is used to calculate element stresses and compare them to allowable stresses. The (APIRP2A-WSD)

code is selected to check stresses in the elements [1].

Six load combinations will be added into the model. Three of them are corresponding to operating storms and the other

three are corresponding to extreme storms. Load factor 1.1 will be used for environmental loads. Live loads will be

included with a 0.75 factor in maximum storm load combinations, in order to check the efficiency of the structure. The

maximum base shear and overturning moment for ULS_A (for operational condition) and ULS_B (for extreme condition)

analysis are shown in table (2) and table (3).

The stresses and forces (shear forces, axial and bending stresses) to which each element of the platform is subjected, as

well as the unit check values, are calculated after the analysis. The result of unit check values for all members are

shown in figures (3,4).

Table 2: The table presents the maximum base shear and overturning moment for

ULS_A Analysis.

Load

Direction(Deg)

Base Share

Overturning Moment

187.973KN

13154.158 KN.M

135

357.994 KN

24991.627KN.M

180

263.994KN

18265.729 KN.M

Table 3: The table presents the maximum base shear and overturning moment for

ULS_B analysis.

Load

Direction(Deg)

Base Share

Overturning Moment

912.209 KN

63894.941 KN.M

135

1706.071KN

120899.492 KN.M

180

837.998 KN

59707.098 KN.M

American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com

Figure 3: The figure presents the four-legged jacket’s member check results, Row (A,B).

Figure 4: The figure presents the four-legged jacket’s member check results, Row (1,2).

American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com

The comparison of base shears shows that the maximum base shear occurs in (135°) and the maximum overturning

moment in (135°) in both operating and extreme conditions.

Unity check has been performed and found that the ratio of actual stress to allowable stress is less than unity for all

members; thus the structure is safe.

5. CONCLUSION

Typical jacket in Syrian coast is modeled in SACS. It is analyzed for environmental and operating conditions for the all

load combinations given in APIRP2A-WSD code and the base shear and overturning moments are found. The worst-case

occurs while the environmental loads act from the south-west of the structure.

The environmental conditions of the Syrian coast are suitable for the construction of jacket offshore, therefore a great

importance to support the Syrian national economy.

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Cite this article: Adnan, Mohamed Ibrahim, and Mahar, jasem alhasan.

ANALYSIS OF OFFSHORE JACKET

STRUCTURE IN THE SYRIAN COAST BY USING COMPUTER MODELING. Am. J. innov. res. appl. sci. 2018; 7(2): 91-96.

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