Final - Dissertation - Kok - Hoy - Seng - 16006 - LJF in SACS

Enhancement of Structural Reliability of Jacket Platforms in Malaysian Waters
for Life Extension
By
Kok Hoy Seng
16006
Dissertation submitted in partial fulfillment of
The requirements of the
Bachelor of Engineering (Hons)
(Civil)
January 2015
Universiti Teknologi Petronas

Bandar Seri Iskandar
31750 Tronoh
Perak Darul Ridzuan
CERTIFICATION OF APPROVAL
Enhancement of Structural Reliability of Jacket Platforms in Malaysian Waters

for Life Extension
By
Kok Hoy Seng
16006
A project dissertation submitted to the
Civil Engineering Programme
Universiti Teknologi PETRONAS
In partial fulfillment of the requirement for the
BACHELOR OF ENGINEERING (Hons)
(CIVIL)
Approved by,
_________________________
(Dr Montasir Osman Ahmed Ali)
UNIVERSITI TEKNOLOGI PETRONAS

TRONOH, PERAK
January 2015
CERTIFICATION OF ORIGINALITY
This is to certify that I am responsible for the work submitted in this project, that the
original work is my own except as specified in the references and acknowledgements,
and that the original work contained herein have not been undertaken or done by
unspecified sources or persons.
___________________________
(KOK HOY SENG)

ABSTRACT
Most of the jacket platforms that belongs to PETRONAS Carigali Sdn Bhd have been
operated more than their service life for Enhancement of Oil Recovery (EOR).
However, it is uncertain to claim the platforms are safe for life extension. Hence,
enhancement of structural reliability becomes a necessity to justify the platform is safe
throughout the EOR period. In this paper, the effect of local joint flexibility on
enhancement of structural reliability of offshore jacket structure in Malaysia waters
will be studied. Rigid joint assumptions made during software modelling of jacket
structures, had been practiced for past decades. While, standards such as API RP2A-
WSD only applies local joint flexibility to the fatigue life analysis. However, past
researches show that tubular joints of offshore structure in reality are not fully rigid
but possesses flexibilities. In this project, pushover analysis will be perform on the
F9JT-A platform model using SACS 5.3 software .100 years return periods of storm
were considered as environmental loading and pile soil interaction were included in
the pushover analysis for intact and structure with local joint flexibilities(LJF). LJF
(Fessler and Buitrago LJF methods) were introduced to all joints of the jacket structure,
to determine the effects of LJF to the reserve strength ratio (RSR). The Buitrago
method shows better results compare to Fessler method in improvement of RSR.
Buitrago method shows a maximum of 21.2% improvement in RSR on 90o loading
direction, while Fessler show a maximum improvement of 6.43% in RSR on 270bo
loading direction when compared with intact RSR. While for structural reliability
analysis, reliability index and probability of failure were obtained through FORM and
MCS method, results from Buitrago method shows better result compare to Fessler
method with a maximum improvement of 8.817% for reliability index and a maximum
reduction of 98.98% for probability of failure when compared with intact structure.
While the Fessler method shows a maximum improvement of 3.86% in reliability
index and a maximum reduction of 79.67% in probability of failure when compared
with the intact structure.
i
ACKNOWLEDGEMENT
First and foremost, I would like to express my appreciation and thanks to Dr Montasir
Osman Ahmed Ali, my FYP supervisor, for his full support, guidance and availability
in advising me during my Final Year Project.
Special thanks my co-supervisor, Ir. Mohamed Mubarak bin Abdul Wahab for his
support, technical knowledge and time in the contribution of this final year project.
I would also like to take this opportunity to thank Mr Jerin Mathew George, Master
student under Prof Kurian which assisted and mentored me on the application of SACS
software.
Also, I would like to thank Ir. Andrew Chin Kim Ping, my industrial training
supervisor, for his opinion and industrial technical knowledge on this project.
Finally, I would like to thank my family, for their sacrifices made on my behalf and
unconditional support.
Here, I might not be able to mention everyone who helped me in the completion of
this project, but I will always keep your kindness in my memory. Thank you.
ii
TABLE OF CONTENTS
ABSTRACT i
ACKNOWLEDGEMENT ii
TABLE OF CONTENTS iii
LIST OF FIGURES v
LIST OF TABLES vi
ABBREVIATIONS AND NOMENCLATURES vii
CHAPTER 1: INTRODUCTION 1
1.1 Background of study 1
1.2 Problem statement 2
1.3 Objectives 2
1.4 Scope of study 3
CHAPTER 2: LITERATURE REVIEW 4
2.1 Structural Reliability Assessment 4
2.2 Fixed Jacket Structure 5
2.3 Pushover Analysis 5
2.4 The Ultimate Strength Assessment of Offshore Structures 8
2.5 Reserve Strength Ratio (RSR) 8
2.6 Enhancement of Structural Reliability 9
2.7 First-Order Reliability Method (FORM) 10
2.8 Monte Carlo Stimulation (MSC) Method 10
2.9 Critical Analysis 11
CHAPTER 3: METHODOLOGY AND PROGRESS 14
F9JT-A Jacket Structure description 14
3.1 Methodology 15
3.2 Project Activities 24
3.3 Key Project Milestone 25
3.4 Gantt Chart 26
CHAPTER 4: RESULTS AND DISCUSSION 28
4.1 Results 28
4.1.1 RSR of intact structure, JF joint flexibility and BF local joint flexibility
introduced structure. 28
4.1.2 The results of displacement in X, Y and Z axis for 0o, 45 o and 90 o
environmental loading direction. 33
4.1.3 The results on effect of joint flexibility on structural reliability of F9JT-A
jacket platform. 43
4.2 Discussion 45
iii
4.2.1 To determine the Reserve Strength Ratio (RSR) of the intact F9JT-A
platform model using SACS 5.3 software. 45
4.2.2 To determine the effect of local joint flexibilities to the RSR improvement on
the F9JT-A jacket structure using SACS 5.3. 45
4.2.3 To determine the effect of joint flexibility on structural reliability of F9JT-A
platform through First-Order Reliability Method (FORM) and Monte-Carlo
Stimulation (MCS) methods using MATLAB. 47
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS 49
5.1 Conclusion 49
5.2 Recommendations 50
References a
APPENDIX A d
APPENDIX B m
iv
LIST OF FIGURES
FIGURE 2-1 Non-linear curve component description 7
FIGURE 3-1 F9JT-A in SACS view 14
FIGURE 3-2 The Omni-directional lateral loads that are applied to the structure during
Pushover analysis. 15
FIGURE 3-3 General joint geometry. 18
FIGURE 3-4 Flow chart of Methodology 23
FIGURE 3-5 Project activities of FYP 24
FIGURE 4-1 The plot between base shear, base shear at collapse and RSR of intact
structure. 29
FIGURE 4-2 The plot between base shear, base shear at collapse and RSR of intact structure
with JF flexibility 30
FIGURE 4-3 The plot between base shear, base shear at collapse and RSR of intact structure
with BF flexibility. 31
FIGURE 4-4 The comparison of designed base shear between intact structure and structure
with JF and BF joint flexibility implemented. 31
FIGURE 4-5 The comparison of base shear at collapse between intact structure and
structure with JF and BF joint flexibility implemented. 32
FIGURE 4-6 The comparison of RSR between intact structure and structure with JF and BF
joint flexibility implemented. 32
FIGURE 4-7 Location of joint 7436 33
o
FIGURE 4-8 Load factor vs displacement curve in X- axis for 0 direction. 34
o
FIGURE 4-9 Load factor vs displacement curve in X- axis for 45 direction 35
FIGURE 4-10 Load factor vs displacement curve in X- axis for 90o direction 36
o
FIGURE 4-11 Load factor vs displacement curve in Y- axis for 0 direction 37
o
FIGURE 4-12 Load factor vs displacement curve in Y- axis for 45 direction 38
FIGURE 4-13 Load factor vs displacement curve in Y-axis for 90o direction 39
o
FIGURE 4-14 Load factor vs displacement curve in Z- axis for 0 direction 40
o
FIGURE 4-15 Load factor vs displacement curve in Z- axis for 45 direction 41
FIGURE 4-16 Load factor vs displacement curve in Z- axis for 90o direction 42
FIGURE 4-17 Reliability Index of Intact structure, JF and BF joint flexibility introduced
structure 43
FIGURE 4-18 Probability of Failure Pf 44
v
LIST OF TABLES
TABLE 2-1 Critical analysis on past research papers 11
TABLE 2-2 Critical analysis on past researches and gap between this project. 12
TABLE 3-1 Project key milestone for FYP 1 25
TABLE 3-2 Project key milestone for FYP 2 25
TABLE 3-3 Grantt Chart for FYP 1 26
TABLE 3-4 Grantt Chart for FYP 2 27
TABLE 4-1 The results of pushover analysis for intact structure of F9JT-A 28
TABLE 4-2 The results of pushover analysis for intact structure of F9JT-A with JF method.
29
TABLE 4-3 The results of pushover analysis for intact structure of F9JT-A with BF method.
30
TABLE 4-4 Reliability index of Intact structure, and both JF and BF joint flexibility
introduced structure. 43
TABLE 4-5 Probability of Failure of Intact, and both JF and BF joint flexibility introduced
structure 44
vi
ABBREVIATIONS AND NOMENCLATURES
BF Buitrago local joint flexibility
D Outer Chord Diameter
E Young’s modulus of elasticity
FORM First-Order Reliability Method
I The moment of inertia, A is the Area of short-flex element
JF Fessler local joint flexibility
L Flex-element length
LJFAX Local joint flexibility of axial
LJFOPB Local joint flexibility of Out-of plane bending moment equation
LJFIPB Local joint of In-plane bending moment equation
(LJFm) In-plane or Out of plane bending local flexibility
(LJFp) Axial loading local joint flexibility
MCS Monte-Carlo Stimulation
RSR Reserve Strength Ratio
SACS Structural Analysis Computer System
vii
Chapter 1: INTRODUCTION
1.1 Background of study
More than 60% of the platforms from PETRONAS Carigali Sdn Bhd (PCSB) had been
operating for more than 20 years while some have exceeded 30 years compare to their
initial designed service life of 20 to 25 years. The extension of service life demand for
these operating platforms however had been increasing due to enhanced of oil recovery.
Because of this, upgrading, modification and work-over demand will certainly
increase the loads subjected to the jacket structure where the platform was not initially
designed for. Moreover, the increase in environmental met-ocean loading, seismic
loading, shallow gas and other challenges will also significantly affect the jacket
structure throughout the years of its service life (Nichols, 2006).
Structural reliability analysis becomes necessary to ensure the structure is safe. The
probability of a system perform its purpose in a specified period without failing is
known as reliability. Choi et al. (2007) stated the structural reliability study; focus on
the calculation and prediction of the probability of limit state violations at any stage of
the structure service life. The probabilistic approach is based on the theoretical
foundation of Probability Distribution Factor (PDF) information. In addition, the uses
of random variables, process and field are introduced to represent uncertainty. To
further enhance the structural reliability, local joint flexibilities is one of the approach.
The reliability of the platform for service life extension can be determined by assessing
the reliability index and probability of failure which is through Reserve Strength Ratio
(RSR). RSR is determined by dividing the ultimate strength to the design strength. The
ultimate strength can be determined through pushover analysis. The pushover analysis
is used as the capacity of the jacket structure depends significantly in non-linear range
of deformation of the behaviour of structure members and the foundation interaction
with soil.
As significant number of platforms in Malaysia are extending their service life for
continuous production, this research would be a valuable for PETRONAS, our national
oil company.
1
1.2 Problem statement
The existing jacket structure are safe regard to overloading due to wind, current and
wave loading, provided that the load is not significantly different from the load the
structure was initially designed for ( Ersdal, 2005). However when the structure is used
beyond its service life, it would be uncertain to claim the structure is still safe for the
coming years of operations.
Offshore jacket platforms possess some flexibilities in the joints in reality and there
were a lot researches have been conducted on the effect of local joint flexibilities on
the overall structure behaviour, and it was included recently in fatigue analysis.
However, this local joint flexibilities effect was not included in most the offshore
structures’ finite element analysis such as in-place analysis and nonlinear pushover
analysis.
Hence, enhancement of structural reliability of the jacket platform by considering the

effect of local joint flexibilities to determine the actual strength of the jacket platform
in Malaysian waters is necessary to requalify the strength of jacket platform.
1.3 Objectives
The main objective of this project is to analyse the factors that contribute to the
enhancement of the structural reliability of jacket platforms in Malaysian waters for
life extension.
This main objective can be subdivided into few sub-objectives as follows:
i) To determine the Reserve Strength Ratio (RSR) of the intact F9JT-A

platform model using SACS 5.3 software.
ii) To determine the effect of the local joint flexibilities to the RSR
improvement on the F9JT-A jacket structure using SACS 5.3.
iii) To determine the effect of joint flexibility on structural reliability of F9JT-
A platform through First-Order Reliability Method (FORM) and Monte-
Carlo Stimulation (MCS) methods using MATLAB.
2
1.4 Scope of study
The scope of this project is limited to the following constrains:
i) F9JT-A jacket platform provided by PETRONAS Carigali will be used in

this project.
ii) The static non-linear pushover analysis using SACS 5.3 is used in this
project.
iii) Deterministic value of wind, wave and current loading as environmental
loading will be consider in this project to evaluate response of the jacket
platform.
iv) Only both Fessler and Buitrago local joint flexibility methods will be
include during pushover analysis for structure with local joint flexibilities.
v) Structural Reliability Analysis will be performed using FORM and Monte
Carlo Stimulation (MCS) methods through MATLAB.
3
Chapter 2: LITERATURE REVIEW
This chapter will introduce the concept of structural reliability assessment, pushover
analysis, ultimate strength assessment, reserve strength ratio (RSR), enhancement of
structural reliability, First Order Reliability Method (FORM) and Monte Carlo
stimulation (MCS).
2.1 Structural Reliability Assessment
The capability of the offshore structure to meet its purpose under any condition is
known as structural reliability. Determination of whether the limit-state of structure is
exceeded is how the reliability analysis evaluates the probability of failure of a
structure. The confidence interval of structural response and probability distribution
function are very important for reliability analysis as mention by Choi S.K et al. (2007)
in their book.
The limit state, whereby the structure exceeded its specific limit and is unable to carry
the load which is initially design for is considered as unreliable. There are two types
of limits state, ultimate-limit states and serviceability limit-state. The ultimate limit-
state is very unlikely to occur is caused by progressive collapse, plastic mechanism,
fire, fracture, fatigue, deterioration and corrosion. As for the serviceability limit-state,
is caused by leakage, local damage, excessive vibration and deflection which are not
as critical as ultimate limit-state.
In term of structural system reliability assessment, pushover analysis has a huge role
to assess the resistance capacity of the offshore structure. Onoufriou, and Forbes (2001)
in their research focus on three main part of the jacket platform for pushover analysis
besides the failure mechanism and application of loads, which include the
superstructure, substructure and the foundation.
The reliability of the platform for extended usage can be determined by the reliability
index and probability of failure of the platform structure, and this will determine
whether the platform structure is suitable and reliable for extended usage.
4
2.2 Fixed Jacket Structure
Fixed jacket structure or offshore jacket platform is a structure that is totally made up
of tubular steel frame with pile foundation that is located on shallow sea. While the
structure members of the jacket consist of X, Y and K joints and members. Production
facilities, living quarter and helideck are all included on the superstructure of the jacket
platform. The purposes of jacket structure are to process the crude oil/gas from
reservoir and pumped to shore through pipelines after process. The design of jacket
structure depends on various requirements, which include fatigue and strength and it
is design to have a typical service life of 10 to 25 years (Randall, 2010).
2.3 Pushover Analysis
Pushover analysis which has the same function as the non-linear analysis which is used
to determine the Ultimate Strength of the jacket platform is carried out to determine
whether it is safe for continuous usage of the existing jacket structure by determine the
RSR of the structure.
Asgarian and Lesani (2007) mention that to determine the ultimate strength of the
jacket structure, pushover analysis is the most general method. Buckling, member
failure due to yielding, joint failure and pile soil failure, are the important assessment
for offshore platforms are all included in the pushover analysis. During the pushover
analysis, loads are pushed to the jacket structure until the jacket collapsed or targeted
displacement is achieved.
Onoufriou and Forbes (2001) found the capacity and response of the whole structure
of jacket offshore platforms significantly rely on the deformation of structure member
in the non-linear range in their research. Most critical member will be determined by
pushover analysis. However, the effects of possible component strength variation
result in different combination of elements and failure sequence will not be considered.
Non-linear pushover analysis assesses the non-elastic range of the structure in order to
determine the weakest joint/point of the structure together with the failure mechanism
of the structure. In the research of Krawinkler and Seneviratna (1998), the weakest
5
point, which is hidden from the elastic analysis is shown by non-linear pushover
analysis and at same time provides a more reliable result for the assessment of jacket
structure.
According to krawinkler (1994), pushover analysis is used for evaluating the design’s
solution as this analysis does will not provide a good solution. However, pushover
analysis will analyse and forecast the load and failure mechanism happens to the
elements of the structure. Two and three dimensional model is analysed in the analysis,
which will account for both linear and nonlinear response of the structure. The
structure was pushed to a targeted displacement by applying lateral loads which
represent the relative inertia forces that developed at location of substantial masses.
While the deformation and internal strength calculated through the pushover analysis
are then comparing to the available capacities.
Onoufriou and Forbes (2001) performed the pushover analysis by applying gravity
load to the structure, followed by lateral loads which are applied incrementally to the
structure until the structure eventually collapses. Beside material properties, joint
failure is the main focuses in their analysis although there are many cases where failure
of member will occur first before joint failure because the joint failure will affect the
estimation of ultimate load and failure mechanism of the jacket structure.
During the pushover analysis, points were considered as hinges when they reached the
bending strength in the application of lateral loads. The analysis will continue even it
exceeded the targeted displacement, which will result in a base shear vs. displacement
response curve. Non-linear curve component description is shown is Figure 2-1.
6
FIGURE 2-1 Non-linear curve component description (V.J. Kurian at el, 2013)
2.3.1 Advantages of pushover analysis:
Pushover analysis is a method that considered the redistribution of internal

forces, this is vital when the structure unable to resist the internal forces in the
elastic range. Pushover Analysis also evaluates more comprehensive and
realistic compared to linear elastic analysis. Hence, when dynamic analysis or
static linear elastic analysis unable to obtain targeted information on response
characteristic from a structure, pushover analysis is used.
The response characteristic provided by pushover analysis according to

krawinkler (1994) includes:
i) Deformation demand estimation of element that will deform

inelastically to release the ground motion energy transmitted to the
structure.
ii) The behaviour of structural system affected by the individual
member strength deterioration.
iii) Strength discontinuities identification that will cause the dynamic
characteristic to change in inelastic zone.
iv) Identification and focus on critical regions which have high
deformation demand through detailing.
7
Pushover analysis will also check the load transfer across connection between
ductile materials with realistic forces. Most importantly, it will cover all the
elements from the structure, which include structural or non-structural
elements which will cause distribution of significant loads.
2.4 The Ultimate Strength Assessment of Offshore Structures
WestLake et al. (2006) in their paper, state that non-linear finite element analysis, also
known as collapse analysis or pushover analysis will be used for the ultimate strength
assessment of offshore structures. This analysis will assess the capacity of the entire
system of the structure.
WestLake et al. (2006) in their research, found that each directional environmental
loading, will cause the structure to have different RSR. While the environmental
loading direction which causes the lowest RSR to the structure will be the main focus.
The plastic deformation of piles, members and joints are allow in this ultimate strength
assessment, and the components of the structure are allow to undertake load above the
yield strength. In addition, the loads applied to the structure are all redistributed to all
the structure members until the structure eventually collapses.
2.5 Reserve Strength Ratio (RSR)
According to Titus and Banon (1988) (Bolt et al, 1996) RSR is the term used for
offshore platform ultimate strength measurement. It is the measurement of the ability
of the structure to withstand overloading as compared to the initial designed load of
the offshore platform. RSR value obtained through the pushover analysis will
determine whether the jacket platform is reliable for the continuous usage for the
industry. However, the RSR is greatly affected by the load combination and the
environmental loading direction subjected to the jacket structure.
8
2.6 Enhancement of Structural Reliability
In current practice, the tubular joints/connections are all assumed to be fully rigid
during analysis for offshore jacket platforms. However, their true behaviour is
essentially flexible. (Nichol et al. 2006)(Masoud et al, 2009). This is due to lack of
knowledge on how the actual behaviour of tubular joints can be represented in frame
test and large scale component.
Present-day practice with no flexibility on tubular joint, will give inaccurate joint
response of the structure in the analysis result. Hence, joints should be represented
with finite linear elastic flexibility where it represent the accurate way of joint behave
in practice, which is suggested by Structural engineering mechanics.
There are extensive data showing that all the tubular joints are flexible, that differ
depends on geometry load case and joint types. Masoud et al (2009) state the
flexibilities of the connection should be considered to obtain accurate stiffness and
strength of the platform as connections are not perfectly rigid. Masoud et al (2009)
also obtained a significant result in the research by comparing a fully rigid structure
and a structure which includes flexibilities on connection. Besides, Masoud et al (2009)
also found that effect of flexibilities of joint become apparent in non-linear analysis
where the structure undergo plastic region.
Local joint flexibility (LJF) is now introduced to the fatigue analysis in a very reliable
and cost-effective manner (MSL, 2002) (Nichol et al (2006). Local joint flexibilities
had been implemented by introducing short “flex-element” at the end of the brace
which connect to the surface of the chord. To verify this method, a T-joint was created
using SACS software which has the same geometry as the test specimen was selected
from a database that contain data of full-scale failure test on tubular joints. Analysis
was carried out for both with and without flex-element T-joint. The result from the test
shows the predicted of T-joint with flex-element’s deformation is close to the test
result from database, while the rigid joint model’s result is not matching at all.
Research had been done on a platform by MSL (2002), where a more accurate fatigue
life prediction was obtained that had a similar result with the result obtained from
under water inspection when the flex-element was introduced to the jacket structure
finite element model.
9
2.7 First-Order Reliability Method (FORM)
FORM is probabilistic method that is used to evaluate the reliability of a system.

Sokheang (2014) in his research found that, the component probability of failure can
be determined by FORM method. The determination of whether the limit state function
is an uncorrelated normal variables, linear function or linear first order approximation
with equivalent normal variables represent the non-linear limit state function are
evaluated using FORM method.
2.8 Monte Carlo Stimulation (MSC) Method
The Monte Carlo Stimulation method is a simple random sampling method that is use
to determine uncertainty. The approximate probability of an event can be determine
using this method as MSC contain statistical analysis of trial output, variable reduction
techniques and digital generation of random variables and function (Choi et al ,2007).
The structure probabilistic characteristic response can be determined by stimulation
through generated sampling set for the analysis of structural reliability according to
probability density function.
10
2.9 Critical Analysis
This section discuss the analysed critical analysis based on past research papers on
implementing joint flexibilities to the offshore jacket platforms and the gaps between
this project and past researches, are shown in table 2-1 and table 2-2.
TABLE 2-1 Critical analysis on past research papers
Authors
Onoufriou.T and Forbes. V.J MSL Engineering Ltd (2002)
(2001)
What System Reliability Assessment of The effects of local joint
they Fixed Jacket Platforms. They also flexibility on the reliability of
Studied study the various system effects fatigue life estimation by
( deterministic and probabilistic comparing fatigue life predicted
effects) and their relative for rigid joint structure and
contributions to the overall flexible joint structure in the
system reliability North Sea
Methods Pushover Analysis under extreme Pushover Analysis using SACS,
Environmental Loading include Local joint flexibility
(LJF), hydrodynamic loads
Remarks Uncertainties on pushover A more accurate fatigue life
prediction based on assumption prediction with a closer
made ( foundation effects, joint agreement results from
failure, extreme loading and underwater inspections
fatigue conditions)
11
TABLE 2-2 Critical analysis on past researches and gap between this project.
Authors Gap
Nichols et al (2006) Masoud et al (2009)
What Study the fatigue The effect of joint flexibility Enhancement of
they analysis on the on overall behaviour of two structural
Studied structure based on jacket platforms, effect of reliability of
rigid and flexible joint flexibility on natural jacket platforms
joints frequency of vibration of the in Malaysia
structure and the process of Waters
plastic hinge formations
Methods Pushover Analysis Nonlinear static and Using SACS, to
using SACS, Dynamic analysis implement joint
Implement Local flexibility (JF and
Joint Flexibility BF) to specific
(Buitrago Method) joints of the
on various joints and structure to
tested on the tubular determine the
joint prototype. Reserve Strength
of the jacket
structure and on
the same time
determine the
effects of joint
flexibility to the
RSR.
Remarks The predicted Joints are not perfectly rigid,
fatigue life and the flexibility should be
increased, and the implemented to obtain
result from the accurate strength and
analysis is closer to stiffness of the platform.
the result of full Recommend to take joint
scale test on the flexibility into account in
tubular joint. design and analysis of
12
offshore structure. Flexible
connections shows higher
displacements and inter-
storey drifts, lower base
shear cause by low stiffness
and strength of jacket
structure. Overestimation of
lateral capacity of structure if
joint flexibility is not
included.
13
Chapter 3: METHODOLOGY AND PROGRESS
This chapter will describe method use in this project, from how information was
sourced, the project carried out and plan.
F9JT-A Jacket Structure description
Kumang Cluster F9JT-A platform is a jacket platform, located in Sarawak in the South
China sea, 200m away from the MLNG plant offshore Bintulu Sarawak. F9JT-A is
typical unmanned four legged fixed jacket structure which operates in shallow water
with water depth of 94.8m. There are total six decks at the topside of the structure,
which consist of helideck, main deck, mezzanine deck, cellar deck, sub cellar deck and
SNV access deck. (MMC Oil & Gas Engineering)
FIGURE 3-1 F9JT-A in SACS view
14
3.1 Methodology
Non-linear analysis can be used to determine the ultimate strength of the jacket
structure, as it will consider the large deflection and plasticity of material in the
analysis.
In this research, pushover analysis was chosen as the suitable analysis to determine the
ultimate resistance capacity of the jacket structure. Non-linear Pushover analysis has
been used for many years which include both onshore and offshore for researches to
determine the structural behaviour especially in the failure mechanism and identify the
weakest point of the structure in the inelastic range.
The pushover analysis was performed by subjecting the structure to lateral loads.
These lateral loads were the environmental loads that include the wind, wave and
current that will be applied to the jacket structure, as referring to Omni-directional
loading from API-RP2A-WSD. For the structure used in this project, load will be
applied from 8 directions which are the 0 degree, 45 degree, 90 degree, 135 degree,
180 degree, 225 degree, 270 degree, and 315 degree as shown in figure 3-2.
FIGURE 3-2 The Omni-directional lateral loads that are applied to the structure during Pushover
analysis.
15
During the analysis, the chosen direction designed storm loads were applied to the
structure and the lateral loads were factored incrementally until the structure collapse
where the ultimate strength of the structure reached.
The reliability of the jacket structure can be represented by the Reserve Strength Ratio
of the jacket structure via the pushover analysis by converting the jacket platform
resistance capacity into the Reserve Strength Ratio.
The RSR defined by Titus and Banon(1988)(Bolt et al,1996) as:
Ultimate Platform Resistance

RSR = (3.1)
Design Load
Detailed steps are shown in the following to determine the Reserve Strength Ratio,
implementing joint flexibilities to the jacket model using SACS 5.3 software.
i) F9JT-A platform model was obtained. Further modifying of the structure

model was done when there is a necessity using the SACS 5.3 software.
ii) Load subjected to the jacket model were as according to the initial designed
data. During load application process, the combination of live loads, dead
loads and environmental loads were determined so as to find the
combination of loads which give the most significant effect to the structure.
iii) The non-linear pushover analysis was done by applying the lateral loads
from all the 8 directions using SACS 5.3.
iv) The RSR was determined using pushover analysis for the intact structure.
v) Implement local joint flexibility to the jacket platform model using JF and
BF local joint flexibility options to the specific joints of the jacket model.
vi) Determine the RSR via pushover analysis for the structure with the
flexibility introduced to the specific joint of the jacket structure.
vii) The results obtained will be compared
viii) Interpretation of results.
16
Implementation of local joint flexibility
Buitrago equations and Fessler equation method were used in this project. Both Fessler
and Buitrago equation are the option provided by SACS software to implement the
joint flexibility to the structure.
JF option that use Fessler equation is the equation originally used by SACS software
on tubular joints to implement joint flexibility. The Fessler local joint flexibility
equations provided by SACS Collapse Manual are as follow:
1.95γ2.15 (1−β)1.3 sin2.19 ∅

LJFAX = (3.2)
ED
85.5γ2.2 exp(−3.85β)sin2.16 ∅
LJFOPB = (3.3)
ED3
134γ1.73 exp(−4.52β)sin1.22 ∅
LJFIPB = (3.4)
ED3
Brace diameter(d)
β = Chord diameter(D) (3.5)
Chord diameter(D)
γ = 2∗Chord thickness(T) (3.6)
∅ = Chord − brace interection angle
Where:
LJFAX is the local joint flexibility of axial
LJFOPB is the local joint flexibility of Out-of plane bending moment equation
LJFIPB is the local joint of In-plane bending moment equation.
While BF option (Buitrago Joint flexibility method), involved inserting a short flex-
element at the end of the selected brace, the flex-element is connected with both brace
and surface of the chord. In this project, SACS 5.3 software will automatically
implement the method when the BF option is selected.
17
Buitrago local joint flexibility Equations (DNV-OS-J101 (2004)) are as follow:
L
I = E(LJF (3.7)
m)
L
A = E(LJF (3.8)
p)
f
LJFp = axial
ED
(3.9)
f
LJFm (LJFIPB )=ED
IPB
3 (3.10)
f
LJFm = ED
OPB
3 (3.11)
Where:
I is the moment of inertia, A is the Area of short-flex element
L= flex-element length
(LJFm)= In-plane or Out of plane bending local flexibility
(LJFp)= Axial loading local joint flexibility
E=Young’s modulus of elasticity
D= Outer Chord Diameter
FIGURE 3-3 General joint geometry. (DNV-OS-J101 (2004))
18
Tubular joints’ parametric expression for calculation are shown in following.
According to DNV-OS-J101 (2004), for single-brace joint (Y), the non-dimensional influence
factor expression for local joint flexibility are:
faxl = 5.69τ−0.111 exp(−2.251β)γ1.791 sin1.700 Ѳ
fipb = 1.39τ−0.238 β−2.245 γ1.898 sin1.240 Ѳ (3.12)
fopb = 55τ−0.220 exp(−4.076β)γ2.417 sin1.883 Ѳ
For X joint, the non-dimensional influence factor expression for local joint flexibility are:
δ
faxl1 1 = 8.94τ−0.198 exp(−2.759β)γ1.791 sin1.700 Ѳ
Ѳy
fipb1 = 67.60τ−0.063 exp(−4.056β)γ1.892 sin1.255 Ѳ
1
Ѳ
x1
fopb = 73.95τ−0.300 exp(−4.478β)γ2.367 sin1.926 Ѳ (3.13)
1
δ
fopb
1
1
= τ−0.1 (−353 + 1197β − 1108βsinѲ − 40βγ + 50γsinѲ)
Ѳy
fopb2 = τ−0.1 (26 + 75β2 − 8.5β2 sinѲ + 85β2 γ − 7.4γsinѲ)
1
Ѳx2
fopb = τ−0.1 (2249 − 5879β + 5515βsinѲ + 221βγ − 358γsinѲ)
1
For K joint, the non-dimensional influence factor expression for local joint flexibility are:
i) Gapped Joints
δ
faxl1 1 = 5.90τ−0.114 exp(−2.163β)γ1.869 ϛ0.009 sin1.869 Ѳ1 sin−0.089 Ѳ2
Ѳ y1
fipb = 52.2τ−0.119 exp(−3.835β)γ1.934 ϛ0.011 sin1.417 Ѳ1 sin−0.108 Ѳ2
1
Ѳ x1
fipb = 49.7τ−0.251 exp(−4.165β)γ2.449 ϛ0.004 sin1.865 Ѳ1 sin0.054 Ѳ2
1
δ
faxl2 1 = 3.93τ−0.113 exp(−2.198β)γ1.847 ϛ−0.056 sin0.837 Ѳ1 sin0.784 Ѳ2 (3.14)
Ѳ Ѳ
y2
fipb y1
= fipb − 1.83τ−0.212 β−2.102 γ1.872 ϛ0.020 sin1.249 Ѳ1 sin0.060 Ѳ2
1 1
Ѳ x2
fipb = 4.37τ−0.295 exp(−3.814β)γ2.875 ϛ−0.149 sin0.885 Ѳ1 sin1.109 Ѳ2
1
19
Where δ and Ѳy and Ѳx = Axial Deflection and IPB and OPB Rotations
Subscripts 1 and 2 =Brace 1 and Brace 2
ii) Overlapped Joints
δ
faxl1 1 = 3.91exp(−2.265β)γ2.010 ϛ−0.009 sin1.811 Ѳ1 sin−0.029 Ѳ2
Ѳy
fipb1 = 1.86β−2.093 γ1.766 ϛ−0.029 sin0.711 Ѳ1 sin0.036 Ѳ2
1
Ѳ x1
fipb = 54.2exp(−3.959β)γ2.403 ϛ0.001 sin1.865 Ѳ1 sin−0.009 Ѳ2
1
δ
faxl2 1 = 0.48β−1.269 γ2.032 ϛ0.072 sin0.949 Ѳ1 sin0.954 Ѳ2 (3.15)
Ѳ y2
fipb = 0.75β−3.000 γ2.063 ϛ1.079 sin0.533 Ѳ1 sin0.586 Ѳ2
1
Ѳx
fipb2 = 1.16β−2.068 γ2.550 ϛ0.117 sin1.090 Ѳ1 sin1.089 Ѳ2
1
Ϛ= Absolute value of g/D
faxl = LJFaxl*ED ; fipb =LJFipb*ED3 ; fopb =LJFopb*ED3
20
Structural Reliability Analysis
For the structural reliability analysis, it will be computed by using MATLAB, the
FORM and Monte Carlo Stimulation (MCS) methods will be used to determine the
reliability of the results. FORM, which is also referring as the First-Order Reliability
Method, is a further development of First-Order Secondary Moment method (FOSM).
The FORM will be used to determine the reliability index of the jacket structure.
According to Choi at al. (2007), the approximate limit-state function at the mean is
written as:
ḡ(x)=g(µx)+ ∇g(µx)T(Xi- µxi ) (3.16)
Where µx = { µx1 , µx 2 , µx 3 , µx 4 .. µx n }T and ∇g (µx) is the gradient of g evaluated at

µx. The mean value of approximate limit-state function is:
µḡ= g(µx) (3.17)
The limit-state approximation function standard deviation is:
Ϭḡ =√(Var[ḡ(x)] (3.18)
n 1/2
∂g(µx ) 2
Ϭḡ =[∑ ( ) Ϭ2xi ]
i=1 ∂x1
While the reliability index β is defined as:
µḡ
β=Ϭ (3.19)
ḡ
However, if the limit-state function is nonlinear, mean value method will be used to
linearize the original limit-state function to obtain the approximate limit-state surface
at the mean value point. While the β in equation 3.23, will be known as Mean Value
First-Order Secondary Moment method (MVFOSM). The complex probability
problem will be change by MVFOSM to a simpler problem that forms relationship
between mean, standard deviation and the reliability index.
If the failure surface is a hyper plane, which can be defined as a linear-failure function
for independent variables of n-dimensional space:
21
ḡ(x)=c0+∑ni=1 ci xi (3.20)
µḡ=c0+c1µx1 + c2µx2 +…. cnµxn (3.21)
Ϭḡ =√∑ni=1 c12 Ϭ2xi (3.22)
MVFOSM reliability index β is defined as:
µḡ
β=Ϭ (3.23)
ḡ
Whereas the Monte Carlo Method, will be used to generate random variables through
predetermined probability distribution function.
The probability of failure provided by Monte Carlo Stimulation is as follows:
Nf
pf = (3.24)
N
Where Nf represent the trials number when g (‘) is violated out of N experiment
conducted while g (‘) represent failure for the samples of random variables.
As for the enhancement of the structural reliability, will be focusing on the joints of
the jacket structure. In this project a MATLAB code known as Finite Element
Reliability Using Matlab (FERUM) will be used to perform the reliability analysis.
22
The simplified methodology of the project is shown in the following flow chart (Figure
3-4)
Start
Using SACS 5.3 to do pushover analysis for intact F9JT-A

jacket platform for 00, 450 ,900 ,1350 ,1800 ,2250 ,270a 0, 270b 0,
3150 environmental storm loadings (Wind, Wave and Current)
Determine the Reserve Strength Ratio (RSR) of the jacket platform for
all the 9 environmental conditions
Introduce joint Flexibility factor to the F9JT-A jacket model

to specific joints (All joints, Primary Joints, Secondary Joints)
Run pushover analysis with the joint

flexibility together with the platform and
determine the new RSR
NO The joints selected

contributes to the
enhancement of
structural reliability
YES
Reliability Analysis using MATLAB
Use FORM and Monte Carlo Stimulation method to determine the
Structural Reliability of the Jacket structure for both intact and joint flexibility
introduced structure.
FIGURE 3-4 Flow chart of Methodology
23
3.2 Project Activities
Activities slated throughout this project are illustrated in the chart below:
Preparation
•Research studies and Literature review
stage
•Obtain the F9JT-A platform SACS model

Stage 1 •SACS software learning and familiaization
•Practice Pushover analysis using SACS software for F9JT-A platform model
Stage 2 • Complete the Extended Proposal
•Perform Pushover analysis on F9JT-A platform model

•Determine the RSR value of the platform
Stage 3
•Determine factors that can improve the RSR value of the platform
•Introduce the joint flexibility to the jacket structure

•Determine the RSR of each condition
Stage 4
•Compare the RSR of the intact structure and the RSR obtained with joint
Stage 5 flexibility
•Introduce JF and BF joint flexibility methods to one joint at a time for all the
8 loading direction
Stage 6
•Determine the differences of results between BF and JF methods
•Start Mat-Lab for structural reliability analysis using FORM and MCS
Stage 7
methods
Stage 8 •Data processing and dicussion
•Finalized final report

Stage 9
FIGURE 3-5 Project activities of FYP
24
3.3 Key Project Milestone
FYP 1
TABLE 3-1 Project key milestone for FYP 1
Key Activities Week

Choose FYP tittle 1
Practice SACS software 3
Submission of Extended Proposal 6
Determine the RSR of Intact Structure 8
Proposal Defence 9
Introduce Joint Flexibility to the F9JT-A structure 10
Submission of Interim report 13
FYP 2
TABLE 3-2 Project key milestone for FYP 2
Key Activities Week

Introduce Joint flexibility to one joint at a time to the F9JT-A Structure 1
Submission of Progress Report 7
Evaluate Results 8
Structural Reliability Analysis 9
Pre-Sedex 12
Submission of Dissertation (Soft Bound) 12
Submission of Technical Paper 13
Viva 14
25
3.4 Gantt Chart
TABLE 3-3 Grantt Chart for FYP 1
26
TABLE 3-4 Grantt Chart for FYP 2
FYP2
Week
Main Task 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Continue Non-linear pushover analysis that include JF and BF joint
flexibilities for various joints
Refine Literature review and Methodologies
Tabulation of Results for the effect of joint flexibility to the F9JT-A
Platform
Submission of Progress Report
Structural Reliability Analysis of F9JT-A Platform
Detail Analysis of results obtained
Prepare Draft Report
Pre-Sedex
Submission of Dissertation
Submission of Technical Paper
Viva
27
Chapter 4: RESULTS AND DISCUSSION
4.1 Results
4.1.1 RSR of intact structure, JF joint flexibility and BF local joint flexibility
Results of intact structure
Results obtained from pushover analysis for intact structure are tabulated in table 4-1.
TABLE 4-1 The results of pushover analysis for intact structure of F9JT-A
Intact Base Intact Base Shear Intact

o
Direction( ) shear(kN) at Collapse (kN) RSR
0 8842 38415.24 4.344632
45 19909.13 35834.59 1.799907
90 10020.02 35769.84 3.569837
135 10189.84 33969.4 3.333654
180 10018.5 38037.34 3.79671
225 10013.65 27543.84 2.750629
270a 10319.15 30484.16 2.954135
270b 10320.49 30485.84 2.953914
315 10012.52 29398.06 2.93613
Referring to table 4-1, base shear, base shear at collapse and RSR are tabulated with
respect to the direction of environmental loading. It can be seen that the highest RSR
for the intact structure at 0o direction with RSR value of 4.344632, while the lowest
RSR obtained is at 45o with RSR value of 1.799907. Graph illustration are shown is
figure 4-1
28
Intact
90
40000
135 30000 45
Intact Base shear(kN)
20000
180 10000 0 Intact Base Shear at

Collapse (kN)
0
Intact RSR
225 315
270b 270a
FIGURE 4-1 The plot between base shear, base shear at collapse and RSR of intact structure.
Results with local Joint flexibility introduced
Structure with local joint flexibility introduced in each and every joints are referred in
this report for local joint flexibility introduced structure, while structure with local
joint flexibility introduced to one joint at a time can be referred in APPENDIX A
Results of intact structure with Fessler local joint flexibility introduced.
Results from pushover analysis for intact structure with JF (Fessler) local joint
flexibility introduced to all the joints, are tabulated in table 4-2.
TABLE 4-2 The results of pushover analysis for intact structure of F9JT-A with JF method.
JF base JF base shear at

o
Direction( ) shear(kN) collapse (kN) JF RSR
0 8841.45 36346.1 4.110875
45 19912.35 35770.48 1.796397
90 10015.46 31822.33 3.177321
135 10189.1 33961.37 3.333108
180 10018.15 38015.33 3.794646
225 10014.29 27546.73 2.750742
270a 10323.29 30499.67 2.954453
270b 10325.16 32460 3.143777
315 10019.11 29326.94 2.9271
As shown in table 4-2, base shear, base shear at collapse and RSR are tabulated with respect
to the direction of environmental loading. It can be seen that the highest RSR for the structure
that apply JF local joint flexibility to all the joints of the structure is at 0o direction with RSR
value of 4.110875, while the lowest RSR obtained is at 45o with RSR value of 1.796397. Graph
illustration are shown is figure 4-2.
29
.
JF
90
40000
135 30000 45
20000 JF base shear(kN)

10000
180 0
JF base shear at collapse
0
(kN)
JF RSR
225 315
270b 270a
FIGURE 4-2 The plot between base shear, base shear at collapse and RSR of intact structure with JF
local joint flexibility
Results of intact structure with Buitrago local joint flexibility introduced.
Results from pushover analysis for intact structure with BF (Buitrago) local flexibility
introduced to all the joints, are tabulated in table 4-3.
TABLE 4-3 The results of pushover analysis for intact structure of F9JT-A with BF method.
BF base BF base shear at

o
Direction( ) shear(kN) collapse (kN) BF RSR
0 8876.44 39838.36 4.488101
45 19913.51 35764.16 1.795975
90 10022.3 43369.63 4.327313
135 10190.54 34006.52 3.337068
180 10019.87 38034.8 3.795937
225 10016.07 27549.04 2.750484
270a 10323.45 32439.93 3.142354
270b 10327.04 32456.9 3.142904
315 10020.17 29310.02 2.925102
Based on table 4-3, base shear, base shear at collapse and RSR are tabulated with
respect to the direction of environmental loading. It can be seen that the highest RSR
for the structure that apply BF local joint flexibility to all the joints of the structure is
30
at 0o direction with RSR value of 4.488101, while the lowest RSR obtained is at 45o
with RSR value of 1.795975. Graph illustration are shown is figure 4-3.
BF
90
50000
135 40000 45
30000
20000 BF base shear(kN)
180 10000 0
BF base shear at collapse
0
(kN)
BF RSR
225 315
270b 270a
FIGURE 4-3 The plot between base shear, base shear at collapse and RSR of intact structure with BF
local joint flexibility.
The comparison of base shear, base shear at collapse and RSR between the intact
structure, JF and BF introduced to the structure are illustrated in figure 4-4, 4-5 and 4-
6.
Comparison of Base Shear

90
20000
135 15000 45
10000
5000 Intact Base shear(kN)

180 0
0 BF base shear(kN)
JF base shear(kN)
225 315
270b 270a
FIGURE 4-4 The comparison of designed base shear between intact structure and structure with JF and
BF local joint flexibility implemented.
31
Comparison of Base Shear at Collapse
90
50000
135 40000 45
30000
20000 Intact Base Shear at
Collapse (kN)
180 10000 0
BF base shear at collapse
0
(kN)
JF base shear at collapse
(kN)
225 315
270b 270a
FIGURE 4-5 The comparison of base shear at collapse between intact structure and structure with JF
and BF local joint flexibility implemented.
Comparison of RSR
90
5
135 4 45
3
2
180 1 0 INTACT RSR

0 BF RSR
JF RSR
225 315
270b 270a
FIGURE 4-6 The comparison of RSR between intact structure and structure with JF and BF local joint
flexibility implemented.
32
4.1.2 The results of displacement in X, Y and Z axis for 0o, 45 o and 90 o
environmental loading direction.
Joint 7436, has been taken as the joint for result tabulation in this report to represent
the displacement of the structure. Joint 7436 is a joint located in the centre point of the
top most layer of the jacket part of the F9JT-A platform.
FIGURE 4-7 Location of joint 7436
33
i) The following figure 4-8, 4-9, 4-10 are the load factor vs displacement
curve for 0 o, 45 o and 90 o in the X- axis.
The load factor vs displacement curve in X- axis for 0o direction is shown in figure 4-
8 below:
Load Factor vs Displacement (cm) in X-axis

6
4
Load Factor
3 Intact
JF
BF
0
-100 0 100 200 300 400 500 600
Displacement (cm)
FIGURE 4-8 Load factor vs displacement curve in X- axis for 0o direction.
Based on figure 4-8, it can be seen that the BF local joint flexibility introduced
structure have the highest load factor with lower displacement, while the JF local joint
flexibility introduce structure has a larger displacement with respect to load factor, as
comparing both with the intact structure for displacement in the X- axis for 0o direction.
34
The load factor vs displacement curve in X- axis for 45o direction is shown in figure
4-9 below:

3
2.5
2
Load Factor
1.5 Intact
JF
BF
0.5
0
-50 0 50 100 150 200 250 300 350
Displacement (cm)
FIGURE 4-9 Load factor vs displacement curve in X- axis for 45o direction
Referring to figure 4-9, it can be seen that the JF local joint flexibility introduced
structure have the highest load factor with lower displacement, while the BF local joint
comparing both with the intact structure for displacement in the X- axis for 45o
direction.
35
The load factor vs displacement curve in X- axis for 90o direction is shown in figure
4-10 below:

4.5
3.5
3
Load Factor
2.5
Intact
2 JF
BF
1.5
0.5
0
-50 0 50 100 150 200 250 300
Displacement (cm)
FIGURE 4-10 Load factor vs displacement curve in X- axis for 90o direction
As shown in the figure 4-10 above, it can be seen that the BF local joint flexibility
introduced structure has the highest load factor with highest displacement, while the
intact structure has a larger displacement with respect to load factor, as comparing with
the JF local joint flexibility introduce structure for displacement in the X- axis for 90o
direction.
36
ii) The following figure 4-11, 4-12, and 4-13 are the load factor vs
displacement for 0 o, 45 o and 90 o in the Y- axis.
The load factor vs displacement curve in Y-direction for 0o direction is shown in figure
4-11 below:
Load Factor vs Displacement(cm) in Y-axis

6
4
Load Factor
3 Intact
JF
BF
2
0
-60 -50 -40 -30 -20 -10 0 10
Displacement (cm)
FIGURE 4-11 Load factor vs displacement curve in Y- axis for 0o direction
Based on figure 4-11 , it can be seen that the BF local joint flexibility introduced
structure have the highest load factor with lower displacement, while the JF local joint
flexibility introduce structure has a lower displacement with respect to load factor, as
comparing with the intact structure for displacement in the Y- axis for 0o direction.
37
The load factor vs displacement curve in Y- axis for 45o direction is shown in figure
4-12 below:
Load Factor vs Displacement (cm) in Y-axis

3
2.5
2
Load Factor
1.5 Intact
JF
BF
0.5
0
-50 0 50 100 150 200 250 300 350 400
Displacement (cm)
FIGURE 4-12 Load factor vs displacement curve in Y- axis for 45o direction
As referring to the figure 4-12, it can be seen that the JF local joint flexibility
introduced structure has the highest load factor with lower displacement, while the BF
local joint flexibility introduce structure has a larger displacement with respect to load
factor, as comparing with the intact structure for displacement in the Y- axis for 45o
direction.
38
The load factor vs displacement curve in Y- axis for 90o direction is shown in figure
4-13 below:
Load Factor vs Displacement (cm) in Y- axis

4.5
3.5
3
Load Factor
2.5
Intact
2 JF
BF
1.5
0.5
0
-50 0 50 100 150 200 250 300 350 400
Displacement (cm)
FIGURE 4-13 Load factor vs displacement curve in Y-axis for 90o direction
Based on figure 4-13 above, it can be seen that the BF local joint flexibility introduced
structure has the highest load factor with lower displacement, while the JF local joint
flexibility introduce structure has a lower capacity of displacement with respect to load
factor, as comparing with the intact structure for displacement in the Y-axis for 90o
direction.
39
iii) The following figure 4-14, 4-15 and 4-16 are the load factor vs
displacement for 0 o, 45 o and 90 o in the Z- axis.
The load factor vs displacement curve in Z- axis for 0o direction is shown in figure 4-
14 below:
Load Factor vs Displacement (cm) in Z-axis

6
4
Load Factor
3 Intact
JF
BF
0
-40 -30 -20 -10 0 10
Displacement (cm)
FIGURE 4-14 Load factor vs displacement curve in Z- axis for 0o direction
Based on figure 4-14, it can be seen that the BF local joint flexibility introduced
comparing with the intact structure for displacement in the Z- axis for 0o direction.
40
The load factor vs displacement curve in Z- axis for 45o direction is shown in figure
4-15 below:

3
2.5
2
Load Factor
1.5 Intact
JF
BF
1
0.5
0
-6 -5 -4 -3 -2 -1 0
Displacement (cm)
Based on figure 4-15, it can be seen that the JF local joint flexibility introduced
structure has the highest load factor with lower displacement, while the BF local joint
flexibility introduced structure has a lower displacement with respect to load factor, as
41
The load factor vs displacement curve in Z-axis for 90o direction is shown in figure 4-
16 below:

4.5
3.5
3
Load Factor
2.5
Intact
2 JF
BF
1.5
0.5
0
-80 -60 -40 -20 0
Displacement (cm)
Referring to figure 4-16, it can be seen that the BF local joint flexibility introduced
flexibility introduced structure has a lower displacement with respect to load factor, as
42
4.1.3 The results on effect of joint flexibility on structural reliability of F9JT-A
jacket platform.
TABLE 4 Reliability index of Intact structure, and both JF and BF local joint flexibility
Reliability Index, β
o
Direction( ) Intact JF BF
0 7.625077 7.45978 7.717915
45 4.127333 4.118397 4.117322
90 6.996537 6.572673 7.613445
135 6.752041 6.752041 6.755798
180 7.206213 7.206213 7.20554
225 6.000648 6.000648 6.00043
270a 6.289692 6.290119 6.530508
270b 6.289395 6.53224 6.531178
315 6.26537 6.253084 6.250357
Comparison of Reliability Index, β

90
8
135 6 45
2 Intact
180 0
0 JF
BF
225 315
270b 270a
FIGURE 4-17 Reliability Index of Intact structure, JF and BF local joint flexibility introduced
structure
Referring to table 4-4 and Figure 4-17, it can be seen that, the highest and lowest
reliability index value for all Intact, and both JF and BF local joint flexibility
introduced structure are on the same loading direction, which is 0o loading direction
for the highest reliability index value while lowest is on 45o loading direction. The
highest reliability index value for Intact, JF and BF are 7.625077, 7.45978 and
7.717915 respectively. While the lowest reliability index value for intact, JF and BF
are 4.127333, 4.118397 and 4.117322 respectively.
43
TABLE 4-5 Probability of Failure of Intact, and both JF and BF joint flexibility introduced structure
Probability of Failure
o
Direction( ) Intact JF BF
0 1.22E-14 4.34E-14 5.94E-15
45 1.83E-05 1.91E-05 1.92E-05
90 1.31E-12 2.47E-11 1.33E-14
135 7.29E-12 7.29E-12 7.1E-12
180 2.88E-13 2.88E-13 2.89E-13
225 9.83E-10 9.83E-10 9.84E-10
270a 1.59E-10 1.59E-10 3.28E-11
270b 1.59E-10 3.24E-11 3.26E-11
315 1.86E-10 2.01E-10 2.05E-10
Comparison of Probability of Failure , Pf

0.000025
0.00002
0.000015
0.00001
0.000005
0
0 45 90 135 180 225 270 315 360
Intact JF BF
FIGURE 4-18 Probability of Failure Pf
Based on table 4-5 and Figure 4-18, it can be seen that, the highest probability of failure
for all Intact, JF and BF are on 45o loading direction with probability of failure value
of 1.83E-05, 1.91E-05 and 1.92E-05 respectively. As for the lowest probability of
failure for all Intact, JF and BF structure are on 0o loading direction with probability
of failure value of 1.22E-14, 4.34E-14 and 5.94E-15 respectively.
44
4.2 Discussion
4.2.1 To determine the Reserve Strength Ratio (RSR) of the intact F9JT-A
platform model using SACS 5.3 software.
The RSR values of the intact structure were obtained through Pushover Analysis using
SACS 5.3 software and the results had been shown in table 4-1. In this project, 100
years return of storm condition which include wind, wave and current have been
applied to the intact structure incrementally until the structure collapsed. It can be seen
that, the highest RSR obtained for the F9JT-A intact structure was 4.344 for 0o
environmental loading direction while the lowest RSR value obtained was 1.799 on
45o environmental loading direction. While for 90o, 135o, 180o, 225o, 270ao, 270bo and
315o with RSR value of 3.569, 3.333, 3.796, 2.750, 2.954, 2954 and 2.936 respectively.
Different loading directions cause different RSR of the jacket structure due to different
failure mechanisms in different directions, which may cause by topside failure, jacket
members or joint failure, foundation failure or whole system failure.
4.2.2 To determine the effect of local joint flexibilities to the RSR improvement
on the F9JT-A jacket structure using SACS 5.3.
Based on the results obtained by implementing all the joints to be flexible through
methods suggested by Fessler and Buitrago, the Fessler method gives a less significant
results on improvement of RSR value as compare with the results obtained through
Buitrago method. There are total of 3 out of 9 environmental loading from pushover
analysis gives positive percentage in the differences between the intact structure and
the structure that implement the Fessler equation for local joint flexibility (JF method).
The highest positive difference between the JF method and the intact structure in term
of RSR is 6.427% for 270bo, while the other 2 environmental direction were the 225o
and 270bo with RSR improvement of 0.0041% and 0.01075% when compare with the
RSR of the intact structure.
While for the RSR obtained using the Buitrago local joint flexibility equations method
(BF method) gives more significant results on the improvement of the RSR when
compare with the intact structure and the JF method. There are total of 5 out of 9
environment loading directions where the RSR were improved, with the most
significant of 21.22% of improvement of RSR for the 90o environmental loading
direction, while the other environmental loadings that have RSR improvement
includes 0o, 135o, 270ao and 270bo with RSR improvement of 3.3%, 0.1023%, 6.3714%
and 6.3979% respectively when it is compare with the intact structure.
45
Even though there were improvement in term of RSR when the local joint flexibility
(JF and BF) were included to the structure, there were RSR values that had a slight
decrease in certain environmental loading directions with the maximum decrease in
percentage of RSR for JF and BF methods were 10.99% and 0.375% respectively. It
can be seen that although there were some decrease RSR values for BF method, the
results of BF were far more better as compare with the JF method.
As mentioned by Onoufriou and Forbes (2001), the deformation of the structure will
have significant impact on the strength of the jacket platform. In reality, structure’s
joints are not absolutely rigid when the structure is subjected to lateral loading, it will
clearly shows flexibility at the joints. During Pushover analysis the loads were pushed
incrementally to the structure in the lateral direction, thus by allowing the joints to
have flexibilities will allow the loads to be redistributed to the whole jacket more
effective than considering the joints to be rigid, which will in certain direction of
loading cause increment and decrement in RSR values when it is compared with intact
structure.
The effect of local joint flexibilities are significant when the structure is in the
nonlinear behavior which is in the plastic region. It can be seen from the results, that
the structure stiffness decrease when joint flexibilities were introduced and the
structure was allowed to have a higher displacement compare with the intact structure
when subjected to lateral loading. This is because the structure with local joint
flexibilities were allowed to deflect more than the intact structure. Based on the results
obtained, it can also be seen that the intact structure with rigid joints, have
underestimated and overestimated the strength of the structure on certain loading
directions, knowing the fact that the joints of the structure are flexible in reality. Thus,
to determine a more accurate results of RSR and structure deformation, local joint
flexibility should be introduced.
Moreover, when the structure with flexibilities is more ductile as it is allowed to deflect
more than the intact structure. The load factor that were applied to the structure with
respect to displacement will also increase and on the same time increase the
displacement for certain loading directions, this means the structure with joint
flexibilities can sustain higher loading compare to the intact structure. This means the
structure with local joint flexibility relatively stronger than the intact structure,
especially the Buitrago local joint flexibility introduced structure.
46
4.2.3 To determine the effect of joint flexibility on structural reliability of F9JT-
A platform through First-Order Reliability Method (FORM) and Monte-Carlo
Stimulation (MCS) methods using MATLAB.
Based on reliability index results obtained as shown in table 4-4, the highest
occurrence of percentage in reliability enhancement was from BF local joint flexibility
introduced structure when compare with the intact structure with total of 5 out of 9
occurrences of enhancement on loading directions of 0o,90o,125o,270ao and 270bo. The
highest percentage of enhancement for Buitrago local joint flexibility introduced
structure is on 90o loading direction with 8.817% of enhancement. While for 0o, 135o,
270ao and 270bo have the reliability enhancement of 1.217%, 0.0556%, 3.82875%,
and 3.8443% respectively. There are also 4 out of 9 occurrence of decrease in
percentage for the value of reliability index. However, the decreased reliability index
was not significant when the BF local joint flexibility introduced structure was
compared with the intact structure. This can be seen when the maximum percentage
of decrement was 0.24256% on 45o loading direction. While for 180o, 225o and 315o
the decrement in the reliability index are 0.0093%, 0.0036% and 0.2396% respectively.
Meanwhile, for the Fessler local joint flexibility introduced structure has no significant
results when the reliability index was compared with the intact structure. There were
total of 2 out of 9 occurrence on the enhancement of reliability index, with highest
percentage of enhancement of 3.8612% on 270bo loading direction and 0.00679% for
315o loading direction. While the highest decrement in percentage of reliability index
was on 0o loading direction with the 2.1678 %.
By referring to table 4-5, Buitrago local joint flexibility introduced structure gives the
lowest probability of failure when it is compared with the Fessler and intact structure.
There were total of 5 occurrence for lowest probability of failure for BF joint flexibility
introduced structure. They were the 90o, 0o, 270bo, 270ao and 135o environmental
loading directions with their percentage differences of -98.984%, -51.364%, -79.525%,
-79.394% and -2.5566% respectively when compared with the intact structure.
While the highest probability failure when compare with the intact structure were
observed from Fessler local joint flexibility introduced structure. There were total of
4 occurrence with high probability of failure. They were 90o, 0o, 315o and 45o loading
directions with percentage difference of 1783.62%, 255%, 8.196% and 3.957% when
compared with the intact structure.
Reliability of a structure is the ability of the structure to perform its purpose without
failing. In this project the reliability is a function of RSR. Thus the higher the RSR,
the lower the probability of failure, higher reliability as a result. Hence, both results of
reliability index and probability of failure were control by the RSR value obtained
from objective 2. The higher the RSR value of specific structure on specific loading
direction, the higher the reliability index and the lower the probability of failure, vice
versa.
47
Since Buitrago local joint flexibility introduced structure shows the higher RSR values
occurrence when it was compare with both intact and Fessler local joint flexibility
introduced structure, hence, making it the structure which has the highest reliability
index and lowest probability of failure. On the same time, it also means the structure
is more reliable and has a lower tendency to fail when Buitrago local joint flexibility
is introduce to the structure compare to Fessler local joint flexibility and the rigid intact
structure.
48
Chapter 5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
To conclude this project, all three objectives have been achieved. The RSR of intact
structure, the effect of local joint flexibility using Fessler equations and Buitrago et al
equations to the RSR, and the effect of local joint flexibility on the structural reliability
of F9JT-A jacket platform have been studied in this project.
Although assumption of rigid joints for jacket structure has been a practice in the
offshore industry, however neglecting joint flexibility of the structure will lead to
underestimate and overestimating the strength of the structure in the real situation. This
is because the joints of jacket platforms are normally welded, may possess some
flexibilities cause by the welding methods on connection during fabrication and
modifications.
Based on the RSR results obtained from this project, the local joint flexibility method
suggested to be apply for the enhancement of structural reliability during pushover
analysis is the Buitrago local joint flexibility method. This is because it provide a
significant results with the maximum of 21.22% of improvement in terms of RSR
value and this method was also suggested by various researchers for fatigue analysis.
In addition to that, based on the results obtained from reliability index and probability
of failure, it can be concluded that reliability of the structure increased significantly
with maximum of 8.817% for reliability index when Buitrago local joint flexibility
was introduced. While the probability of failure of the structure reduced by maximum
of 98.98% when the Buitrago local joint flexibility was introduced to the platform
when it was compared with the intact F9JT-A jacket platform.
From all the results in this project, the load distributions and deformation effects due
to local joint flexibility have a significant impact to the result from analysis of the
F9JT-A platform. The effect of joint flexibility is recommended to be taken into
various analysis for offshore jacket platforms and Buitrago local joint flexibility
method is suggested as it enhanced the structure’s reliability.
49
5.2 Recommendations
As for recommended future work of this project / similar project are as follow:
.
i) It is suggested to researchers that carry out similar researches that,
experimental full scale test on the tubular joints should be carry out to
determine the exact capacity and deflection and to validate the results from
results obtained from finite element software that estimate the ultimate
strength of the tubular joints.
ii) Lower scale factor can be used depend on the limitation of research
facilities if full scale test is not achievable.
iii) Different types of joints including X, Y and K-joints with high sensitive
sensor attached should be used for the full scale test. Finite element
software should be used to model the exact material properties and
dimension of the joint that gone through the full scale test to be analyse and
determine the accuracy of the finite element analysis.
iv) Latest finite element software such as SACS 5.7, USFOS 8.7, SESAM,
ANSYS or ABAQUS should be used for the non-linear pushover analysis
that equipped with latest refined theories to generate a more accurate results
in future researches.
50
References
1) API RP2A-WSD ‘Recommended Practice for Planning, Design and
Constructing Fixed Offshore Platforms’, 21st Edition, Dec 2000.
2) B. Asgarian, M. Lesani (2007). Pile-soil interaction in pushover analysis of

jacket offshore platforms using fiber elements. Journal of constructional steel
research 65 (2009) 209-218.
3) Bolt, H. M., Billington, C. J., & Ward, J. K. (1996). A Review of The Ultimate
Strength of Tubular Framed Structures. Health and Safety Executives.
Retrieved from www.hse.gov.uk/research/othpdf/200-399/oth365.pdf
4) Choi.S.K, V. Ramana. Grandhi, and Robert A. Canfield. (2007) ‘Reliability

Based Structural Design’, Springer-Verlag London Limited.
5) DNV OS-J101 “OFFSHORE STANDARD, DESIGN OF OFFSHORE WIND

TURBINE STRUCTURES” JUNE 2004. Retrieved from
http://homes.civil.aau.dk/rrp/BM/BM8/m.pdf
6) Ersdal, G. (2005).Assessment of existing offshore structures for life extension.

University of Stavanger, Norway. Retrieved from
www.icrard.org/upload/gerhard-Avhandling_kompl.pdf
7) FERUM; Finite Element Reliability Using Matlab

http://www.ce.berkeley.edu/projects/ferum/
8) Krawinkler, H., & Seneviratna, G. D. P. K. (1998). Pros and cons of a pushover

analysis of seismic performance evaluation. Engineering Structures, 20(4–6),
452-464. doi: http://dx.doi.org/10.1016/S0141-0296(97)00092-8
9) Krawinkler H. (1994) Pushover analysis: Why, how, when and when not to use
it. Structural Engineers Associatopn of California;.Retrieved from
nisee.berkeley.edu/elibrary/Text/LIB050223
10) Kristina.W, Louis.Q, Jan.W, Holger.H, Hans-Gerd.B (2009). Stimulation

Method for Joint Flexibility in Modern Offshore Support structures.
Fraunhofer Institute for Wind Energy and Energy System Technology (IWES).
Retrieved from
http://proceedings.ewea.org/offshore2009/allfiles2/221_EOW2009presentatio
n.pdf
11) Kurian .V.J, M.M.A. Wahab, M. C. Voon, S. S. Goh, and M. S. Liew .(2013).
Reliability of Jacket Platforms in Malaysian Waters. Sensitivity Study using
a
Pushover Analysis. Civil Engineering Department. Universiti Teknologi
PETRONAS. Retrieved from eprints.utp.edu.my/10127/1/1569763331.pdf
12) M.Masoud, A.Z.Hossein, A.Pejman and M.A.Assareh (2009) Effect of Joint

Flexibility on Overall Behavior of Jacket Type Offshore Platforms. Khajeh
Nassir Toosi University of Technology. Tehran, Iran. Retrieved from
thescipub.com/PDF/ajeassp.2009.25.30.pdf
13) MMC Oil & Gas Engineering. Conceptual Design & Engineering Services for
Kumang Cluster Development Project. Retrieved 10 December 2014, from
http://www.mmcog.com/projects/conceptual-design-engineering-services-for-
kumang-cluster-development-project
14) MSL Engineering Ltd (2002). The effects of local joint flexibility on the
reliability of fatigue life estimate and inspection planning. Offshore
Technology Report 2001/056. Retrieved from
www.hse.gov.uk/research/otopdf/2001/oto01056.pdf
15) Nichols.N.W, Petronas Carigali Sdn Bhd; t.k.goh, Petronas Research &
Scientific Services Sdn Bhd; h. Bahar, Petronas Carigali Sdn Bhd.(2006)
‘Managing Structural Integrity for Aging Platform’, Proceedings, SPE Asia
Pacific Oil and Gas Conference and Exhibition, Adelaide, Australia, No
SPE101000
16) Onoufriou, T., & Forbes, V. J. (2001). Developments in structural system

reliability assessments of fixed steel offshore platforms. Reliability
Engineering & System Safety, 71(2), 189-199. doi:
http://dx.doi.org/10.1016/S0951-8320(00)00095-8
17) Randall, Robert E. (2010). Elements of Ocean Engineering. Society of Naval

Architects and Marine Engineers (SNAME). Online version available
at:http://app.knovel.com/hotlink/toc/id:kpEOE0000A/elements-ocean-
engineering/elements-ocean-engineering
18) SACS 5.3 Collapse Manual
19) T. Sokheang.(2014). System Reliability of an Existing Jacket Platform (Failure

Paths and System Reliability Index). University Teknologi PETRONAS.
20) Thoft-Christensen.P. (1978). Introduction to Reliability of Offshore Structures.

Aalborg University, Denmark Retrieved from
b
http://vbn.aau.dk/files/17204971/chapter_8_introduction_to_reliability_of_of
fshore_structures
21) WestLake. H.S, Puskar.F.J, O’Connor. P.E, and Bucknell.J.R. (2006). The
Role of Ultimate Strength Assessments in the Structural Integrity Management
(SIM) of Offshore Structures. Offshore Technology Conference, Houston,TX,
U.S.A., 1-4 May 2006.Retrieved from https://www.onepetro.org/conference-
paper/OTC-18331-MS
c
APPENDIX A
d
Joint flexibility introduced to single chosen joint at a time for JF and BF methods.
(JF) Fessler equation joint flexibility method:
201
JF base JF base shear at 201 JF %

Direction shear(kN) collapse (kN) RSR differences
90 10019.96 35777.95 3.570668 0.023272 Positive
45 19909.12 35928.98 1.804649 0.263455 Positive
0 8841.93 37831.91 4.278694 -1.51771 Negative
315 10012.53 29354.55 2.931781 -0.1481 Negative
270a 10319.19 30455.04 2.951301 -0.09591 Negative
270b 10320.47 30485.82 2.953918 0.000128 Positive
225 10013.64 27543.84 2.750632 9.99E-05 Positive
180 10018.49 38037.34 3.796714 9.98E-05 Positive
135 10189.84 33876.07 3.324495 -0.27475 Negative
202

90 10019.82 35765.38 3.569463 -0.01047 Negative
45 19909.11 35573.16 1.786778 -0.72945 Negative
0 8841.87 37832.77 4.27882 -1.5148 Negative
315 10012.53 34539.44 3.449622 17.48872 Positive
270a 10319.12 30484.04 2.954132 -0.0001 Negative
270b 10320.49 30491.25 2.954438 0.017746 Positive
225 10013.64 27543.84 2.750632 9.99E-05 Positive
180 10018.49 38037.11 3.796691 -0.0005 Negative
135 10189.8 33852.51 3.322196 -0.34371 Negative
203

90 10019.91 35773.33 3.570225 0.010855 Positive
45 19909.11 35852.17 1.800792 0.049159 Positive
0 8841.91 37770.12 4.271715 -1.67833 Negative
315 10012.53 29354.57 2.931783 -0.14803 Negative
270a 10319.14 30484.06 2.954128 -0.00023 Negative
270b 10320.43 30491.25 2.954455 0.018327 Positive
225 10013.65 27543.63 2.750608 -0.00076 Negative
180 10018.49 38037.39 3.796719 0.000231 Positive
135 10189.84 33880.57 3.324936 -0.2615 Negative
e
204

90 10019.98 35769.66 3.569833 -0.0001 Negative
45 19909.11 35900.36 1.803213 0.183638 Positive
0 8841.67 34792.87 3.935102 -9.42613 Negative
315 10012.52 29345.53 2.930884 -0.17869 Negative
270a 10319.15 30484.06 2.954125 -0.00033 Negative
270b 10320.52 30490.96 2.954402 0.016504 Positive
225 10013.64 27543.84 2.750632 9.99E-05 Positive
180 10018.49 38037.33 3.796713 7.35E-05 Positive
135 10189.82 33886.81 3.325555 -0.24293 Negative
206

90 10018.93 35739.92 3.567239 -0.07278 Negative
45 19908.72 33804.72 1.697986 -5.66261 Negative
0 8842.2 39794.31 4.500499 3.58756 Positive
315 10013.41 29353.73 2.931442 -0.15967 Negative
270a 10319.7 32475.31 3.146924 6.526075 Positive
270b 10321.65 32391.12 3.138173 6.23778 Positive
225 10013.65 27544.43 2.750688 0.002142 Positive
180 10018.4 38039.51 3.796965 0.006703 Positive
135 10190.05 33845.37 3.321414 -0.36718 Negative
207

90 10019.87 35768.81 3.569788 -0.00138 Negative
45 19909.13 35932.39 1.80482 0.272921 Positive
0 8841.84 37900.47 4.286491 -1.33823 Negative
315 10012.66 29354.53 2.931741 -0.14947 Negative
270a 10319.16 30484.1 2.954126 -0.00029 Negative
270b 10320.5 32403.69 3.13974 6.29085 Positive
225 10013.69 27543.83 2.750617 -0.00044 Negative
180 10018.5 38037.34 3.79671 0 Negative
135 10189.88 33848.39 3.321765 -0.35662 Negative
f
208

90 10018.59 35645.42 3.557928 -0.33361 Negative
45 19909.8 44819.41 2.251123 25.06883 Positive
0 8841.68 39780.86 4.499242 3.558639 Positive
315 10014.29 29355.2 2.931331 -0.16344 Negative
270a 10320.4 32451.85 3.144437 6.441901 Positive
270b 10321.42 30494.66 2.954502 0.019918 Positive
225 10014.41 36014.25 3.596243 30.74254 Positive
180 10018.44 38036.03 3.796602 -0.00285 Negative
135 10189.67 33968.95 3.333665 0.000344 Positive
210

90 10020.02 35770.75 3.569928 0.002544 Positive
45 19909.12 35894.37 1.802911 0.166872 Positive
0 8841.81 39774.82 4.498493 3.541393 Positive
315 10012.43 29354.66 2.931822 -0.14673 Negative
270a 10319.08 30484.01 2.95414 0.000186 Positive
270b 10320.57 32279.87 3.127722 5.883977 Positive
225 10013.64 27544 2.750648 0.000681 Positive
180 10018.58 38036.71 3.796617 -0.00245 Negative
135 10189.84 33879.12 3.324794 -0.26577 Negative
301

90 10019.85 35685.62 3.561492 -0.23376 Negative
45 19908.86 35932.58 1.804854 0.274811 Positive
0 8842.31 54820.28 6.199769 42.69951 Positive
315 10012.86 37893.5 3.784483 28.89358 Positive
270a 10319.1 30483.17 2.954053 -0.00276 Negative
270b 10320.82 30491.97 2.954414 0.01691 Positive
225 10013.57 27544.15 2.750682 0.001924 Positive
180 10018.48 38037.4 3.796724 0.000357 Positive
135 10189.73 33837.79 3.320774 -0.38636 Negative
g
302

90 10019.92 35736.01 3.566497 -0.09358 Negative
45 19908.86 35932.58 1.804854 0.274811 Positive
0 8843.37 39495.45 4.466109 2.796003 Positive
315 10012.86 29354.66 2.931696 -0.15102 Negative
270a 10319.76 30491.61 2.954682 0.018526 Positive
270b 10320.62 32622.23 3.160879 7.006463 Positive
225 10013.88 27543.87 2.750569 -0.00219 Negative
180 10018.51 38040.91 3.797063 0.009286 Positive
135 10189.79 33969.52 3.333682 0.000844 Positive
303

90 10012.48 35777.62 3.573303 0.097073 Positive
45 19909.26 35894.83 1.802921 0.167452 Positive
0 8842 38047 4.302986 -0.95858 Negative
315 10014.68 29354.69 2.931166 -0.16906 Negative
270a 10318.87 30483.18 2.95412 -0.0005 Negative
270b 10321.64 30490.96 2.954081 0.005651 Positive
225 10014.18 28253.8 2.821379 2.572135 Positive
180 10018.5 38041.82 3.797157 0.011778 Positive
135 10189.82 33812.97 3.318309 -0.46031 Negative
304

90 10020.05 35776.65 3.570506 0.018739 Positive
45 19909.25 33773.91 1.696393 -5.7511 Negative
0 8841.86 49682.27 5.618984 29.33163 Positive
315 10014.64 29354.84 2.931193 -0.16815 Negative
270a 10319.15 30488.9 2.954594 0.015549 Positive
270b 10321.55 30486.04 2.95363 -0.00961 Negative
225 10013.91 27544.05 2.750579 -0.00183 Negative
180 10018.61 38935.66 3.886334 2.360555 Positive
135 10189.91 33843.04 3.321231 -0.37267 Negative
h
(BF)Buitrago et al joint flexibility equations method:
Joint 201
BF base BF base shear at 201 BF %

90 10019.94 33608.46 3.354158 -6.04171 Negative
45 19909.12 35903.62 1.803376 0.192685 Positive
0 8841.9 37864.24 4.282365 -1.43321 Negative
315 10012.53 29354.57 2.931783 -0.14803 Negative
270a 10319.2 30485.23 2.954224 0.003025 Positive
270b 10320.45 30491 2.954425 0.017314 Positive
225 10013.64 27543.64 2.750612 -0.00063 Negative
180 10018.49 38037.71 3.796751 0.001073 Positive
135 10189.84 33968.85 3.3336 -0.00162 Negative
Joint 202

90 10019.71 35714.29 3.564404 -0.15221 Negative
45 19909.1 35897.41 1.803065 0.175456 Positive
0 8841.82 37962.35 4.2935 -1.17692 Negative
315 10012.54 29354.62 2.931786 -0.14796 Negative
270a 10319.1 30484.94 2.954225 0.003043 Positive
270b 10320.48 30491.27 2.954443 0.017908 Positive
225 10013.64 27543.85 2.750633 0.000136 Positive
180 10018.49 38037.07 3.796687 -0.00061 Negative
135 10189.77 33846.99 3.321664 -0.35967 Negative
203

90 10019.86 35773.66 3.570275 0.012276 Positive
45 19909.1 35895.81 1.802985 0.170991 Positive
0 8841.88 39797.48 4.50102 3.599561 Positive
315 10012.52 29354.61 2.93179 -0.1478 Negative
270a 10319.14 32460.06 3.145617 6.48183 Positive
270b 10320.39 30491.27 2.954469 0.018781 Positive
225 10013.65 27543.83 2.750628 -3.6E-05 Negative
180 10018.49 38037.41 3.796721 0.000284 Positive
135 10189.84 33880.07 3.324887 -0.26297 Negative
i
204

Direction
shear(kN) collapse (kN) RSR differences
90 10019.96 35769.7 3.569845 0.000207 Positive
45 19909.11 35893.88 1.802887 0.165555 Positive
0 8841.82 37858.22 4.281723 -1.44799 Negative
315 10012.48 29354.56 2.931797 -0.14757 Negative
270a 10319.15 30484.97 2.954213 0.002657 Positive
270b 10320.52 30490.95 2.954401 0.016471 Positive
225 10013.65 27543.84 2.750629 0 Negative
180 10018.49 38037.32 3.796712 4.72E-05 Positive
135 10189.82 33887.53 3.325626 -0.24082 Negative
206

90 10018.38 35729.98 3.566443 -0.09508 Negative
45 19908.43 35904.28 1.803471 0.198 Positive
0 8842.29 39830.58 4.504555 3.680919 Positive
315 10013.77 34764.5 3.47167 18.23964 Positive
270a 10318.71 32494.17 3.149054 6.598166 Positive
270b 10320.8 32572.07 3.155964 6.840066 Positive
225 10013.39 27544.56 2.750773 0.005211 Positive
180 10018.34 38037.88 3.796825 0.003017 Positive
135 10190.1 33819.85 3.318893 -0.44279 Negative
208

90 10018.59 35760.34 3.569398 -0.01229 Negative
45 19909.79 35938.71 1.805077 0.287233 Positive
0 8841.67 39791.16 4.500412 3.58557 Positive
315 10014.29 29302.88 2.926107 -0.34138 Negative
270a 10320.4 32493.84 3.148506 6.579628 Positive
270b 10321.42 30496.11 2.954643 0.024674 Positive
225 10014.41 27544.12 2.750449 -0.00657 Negative
180 10018.44 38036.01 3.7966 -0.0029 Negative
135 10189.67 33968.95 3.333665 0.000344 Positive
j
301

90 10019.8 35709.84 3.563927 -0.16555 Negative
45 19908.89 33946.4 1.705088 -5.26804 Negative
0 8842.43 36896.88 4.172708 -3.95716 Negative
315 10013.02 29353.99 2.931582 -0.15489 Negative
270a 10319.06 30479.07 2.953667 -0.01583 Negative
270b 10320.91 32422.96 3.141483 6.349835 Positive
225 10013.52 27544.54 2.750735 0.00384 Positive
180 10018.51 38037.57 3.796729 0.000505 Positive
135 10190.03 33839.02 3.320797 -0.38567 Negative
302

90 10019.94 35740.79 3.566966 -0.08042 Negative
45 19908.89 33946.4 1.705088 -5.26804 Negative
0 8843.95 39822.55 4.502801 3.640559 Positive
315 10013.13 29279.85 2.924146 -0.40817 Negative
270a 10319.9 32398.59 3.139429 6.272357 Positive
270b 10320.65 32626.79 3.161312 7.021109 Positive
225 10013.84 27539.8 2.750174 -0.01656 Negative
180 10018.53 38039.65 3.796929 0.005774 Positive
135 10189.76 33969.78 3.333717 0.001904 Positive
303

90 10019.91 35778.77 3.570768 0.026063 Positive
45 19909.68 35889.98 1.80264 0.151805 Positive
0 8841.97 38134.62 4.31291 -0.73015 Negative
315 10014.62 29317.65 2.927485 -0.29443 Negative
270a 10315.96 39485.08 3.827572 29.5666 Positive
270b 10321.7 30490.51 2.95402 0.003594 Positive
225 10014.33 27543.64 2.750423 -0.00752 Negative
180 10018.51 38041.88 3.797159 0.011836 Positive
135 10189.81 33297.15 3.267691 -1.9787 Negative
k
304

90 10020.07 35775.72 3.570406 0.015939 Positive
45 19909.32 35898.48 1.803099 0.177335 Positive
0 8841.78 37666.09 4.260012 -1.9477 Negative
315 10014.56 29354.98 2.93123 -0.16688 Negative
270a 10318.55 30487.2 2.954601 0.015788 Positive
270b 10321.55 30486.17 2.953643 -0.00919 Negative
225 10014.01 27544.16 2.750562 -0.00243 Negative
180 10018.66 38034.61 3.796377 -0.00877 Negative
135 10189.94 33968.8 3.333562 -0.00275 Negative
l
APPENDIX B
m
Guide to include local joint flexibility in SACS model and collapse input.
100 years environmental loads for 45o loading direction in SACS’ Datagen:
Introducing local joint flexibility:
Edit in option from SACS’ model file (SACINP.F9JT-A)
Edit in collapse input: to include BF local joint flexibility
n
To include local joint flexibility for joints 201,202,203, 204

Final - Dissertation - Kok - Hoy - Seng - 16006 - LJF in SACS

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Enhancement of Structural Reliability of Jacket Platforms in Malaysian Waters

for Life Extension

Kok Hoy Seng

Dissertation submitted in partial fulfillment of

The requirements of the

Bachelor of Engineering (Hons)

Universiti Teknologi Petronas

Enhancement of Structural Reliability of Jacket Platforms in Malaysian Waters

Kok Hoy Seng

A project dissertation submitted to the

Civil Engineering Programme

Universiti Teknologi PETRONAS

In partial fulfillment of the requirement for the

BACHELOR OF ENGINEERING (Hons)

(Dr Montasir Osman Ahmed Ali)

UNIVERSITI TEKNOLOGI PETRONAS

(KOK HOY SENG)

BF Buitrago local joint flexibility

D Outer Chord Diameter

E Young’s modulus of elasticity

FORM First-Order Reliability Method

I The moment of inertia, A is the Area of short-flex element

JF Fessler local joint flexibility

LJFAX Local joint flexibility of axial

LJFOPB Local joint flexibility of Out-of plane bending moment equation

LJFIPB Local joint of In-plane bending moment equation

(LJFm) In-plane or Out of plane bending local flexibility

(LJFp) Axial loading local joint flexibility

MCS Monte-Carlo Stimulation

RSR Reserve Strength Ratio

SACS Structural Analysis Computer System

1.1 Background of study

Hence, enhancement of structural reliability of the jacket platform by considering the

This main objective can be subdivided into few sub-objectives as follows:

i) To determine the Reserve Strength Ratio (RSR) of the intact F9JT-A

The scope of this project is limited to the following constrains:

i) F9JT-A jacket platform provided by PETRONAS Carigali will be used in

2.1 Structural Reliability Assessment

2.3 Pushover Analysis

2.3.1 Advantages of pushover analysis:

Pushover analysis is a method that considered the redistribution of internal

The response characteristic provided by pushover analysis according to

i) Deformation demand estimation of element that will deform

2.4 The Ultimate Strength Assessment of Offshore Structures

2.5 Reserve Strength Ratio (RSR)

FORM is probabilistic method that is used to evaluate the reliability of a system.

2.8 Monte Carlo Stimulation (MSC) Method

TABLE 2-1 Critical analysis on past research papers

F9JT-A Jacket Structure description

FIGURE 3-1 F9JT-A in SACS view

The RSR defined by Titus and Banon(1988)(Bolt et al,1996) as:

Ultimate Platform Resistance

i) F9JT-A platform model was obtained. Further modifying of the structure

1.95γ2.15 (1−β)1.3 sin2.19 ∅

∅ = Chord − brace interection angle

LJFAX is the local joint flexibility of axial

LJFIPB is the local joint of In-plane bending moment equation.

I is the moment of inertia, A is the Area of short-flex element

faxl = LJFaxlED ; fipb =LJFipbED3 ; fopb =LJFopb*ED3