Đánh giá khả năng chịu tải vượt mức thiết kế của kết cấu công trình biển cố định bằng thép khi gia hạn khai thác – áp dụng vào điều kiện việt nam tt tiếng anh

MINISTRY OF EDUCATION AND TRAINING
NATIONAL UNIVERSITY OF CIVIL ENGINEERING

VU DAN CHINH

ASSESSMENT OF OVER-DESIGNED LOAD-BEARING CAPACITY OF
FIXED STEEL OFFSHORE STRUCTURES IN EXTENDED DURATION
– APPLICATION FOR VIETNAMESE CONDITIONS

Major: Offshore Enginerring
Code number: 9580203

SUMMARY OF DOCTORAL DISSERTATION

Ha Noi - 2019


The Dissertation has completed at: National University of Civil Engineering

Academic Advisor: Prof.Dr. Pham Khac Hung

1st Reviewer: Assoc.Prof.Dr. Pham Van Thu

2nd Reviewer: Assoc.Prof.Dr. Bui Duc Chinh

3rd Reviewer: Assoc.Prof.Dr. Nguyen Van Vi

The Dissertation will be defensed in University Committee meeting
at……………………………………………………………………………………..
At ………. hh ……….. dd …………. Mm ………… yy ……….
The Dissertation can be found in National Library and NUCE Library

………………………………………………………………………………………….


1

INTRODUCTION
Idea of the dissertation topic
There are two reasons for the dissertation: (1) The demand of life extension for
existing fixed platforms in Viet Nam which are out of design life now and in the future
is necessary; (2) Life extension assessment methods for the structures haven’t fully
mentioned in current standards and these are the lastest trends worldwide for real
project applications.
Research Objectives and Contents
Objectives of the dissertation is to study to develope a method to assess the
fixed steel platforms suffering over-designed environmental loadings to predict
allowable life extension duration for the expired platforms in Vienamese sea
conditions.
Contents of the dissertation includes 4 chapters to provide overview of the
problems, to summarize theorical basis for structural analyses, to develope a structural
assessment method and to apply it to a typical structure under Vietnamese sea
conditions.
Subjects and Scope of the dissertation
The subjects are jacket platforms which are out of date in terms of design. The
scope are to develope an assessment method for the structures suffering over-designed
environmental loadings, according to full plastics and fatigue crack propagation at
cross sections of the structural elements, take account of random properties as wave
loads, cross-section dimension, Young’s modulus and yield strength of steel materials.
Scientific Basis
The dissertation builds an assessment method for the structures based on
scientific basis as below: Non-linear analysis of frame structures in terms of geometric

and physical aspects; Fatigue crack propagation analysis on hot spots of the structures;
Random Processes Theory; Probabilistic and Structural Realiability Methods.
Research Method
+ Theorical research: to study and assess elasto-plastic structures and analyse
fatigue under crack extension in random simulations.


2

+ Applicable research: to study and build up procedures and typical calculation
sheets, asscociated with commercial sofwares to assess fixed steel platform structures
suffering over-designed environmental loads according to probabilistic simulations.
+ Application for an actual structure to estimate and validate new approach of
the dissertation.
New Contributions
Scientific Aspect: The dissertation suggests a new approach to assess the safety of
expired over-designed load-bearing jackets in Vietnamese seas in the extended
duration. The method is built based on realiability model of full plastic conditions and
fatigue-caused crack limitation according to slow-speed propagation of cracks of the
structural main members.
Practical Aspect: In practical meanings, it serves directly to the life extension
and upgrading of the platforms, and is an important problem in Vietnam recently.
CHAPTER 1: OVERVIEW OF SAFETY ASSESSMENTS OF FIXED STEEL
PLATFORMS AND IDEAS OF THE DISSERTATION

1.1. Overview of the development of fixed steel platforms worldwide and in
Vietnam
1.1.1. Introduction about fixed steel platform structures

Fixed steel offshore structures (or jackets) are framed structures supported by

piles or gravity based foundations. The platforms are built in offshore zones to serve
oil and gas exploitation, other economical services and national security…In this
dissertation the main subjects are the piled structures.
1.1.2. The development and application range of fixed steel offshore structures

The fixed steel offshore structures are commonly used over the world with about
6800 jacket platforms distributed over 53 countries. Recently, in Viet Nam, there are
more than 70 fixed steel platforms, concentrated in 50m water depth areas. The
deepest one is Lan Tay Platform with water depth of 125 m, the smallest one is Thai
Binh Platform with water depth of 29m. In current situation, to safe cost, investors of
oil and gas fields in Viet Nam are trying to reduce new constructions and focus on two
main solutions:
+ To upgrade and connect fields to increase output;
+ To extend platform duration of exploitation compared with design life.
1.2. Current Standards applied in design and safety assessments of fixed steel
platform structures


3

There are 5 limit states applied in current standard systems in terms of design
such as API, DnV, ISO… as Ultimate Limit State (ULS), Fatigue Limit State (FLS),
Service Limit State (SLS), Progressive Collapsed Limit State (PLS) and Accidental
Limit State (ALS). These limit states shall be used to assess existing platforms. API
RP 2A re-checks the assessment procedure into 3 levels: Design Level, Global
Ultimate Strength and Risk Assessment.
1.3. Related Studies
1.3.1. Some Research over the world
a) Non-linear analysises for fixed steel platform structures using probabilistic
theory.

b) Random Fatigue Crack Propagation according to Fracture Mechanical Theory.
c) Research on safety assessments of the structures based on strength – fatigue
interactions using probabilistic methods by authors, such as Moan, T., (2002);
Gerhard Ersdal, (2005) or Surrey University, (2000).
1.3.2. Safety Assessments of offshore structures based on probabilistic methods
taking account of strength – fatigue interactions in Viet Nam
a) Assessment methods according to Prof. PHAM Khac Hung opinion;
b) Doctor Dissertation of Dr. PHAM Hien Hau (2010);
c) Doctor Dissertation of Dr. MAI Hong Quan (2014);
1.4. Research Ideas
According to the analysises of standards and related research, the dissertation
aims to study in order to develope a method to assess over-designed loading-bearing
capacity of expired fixed steel platform structures in terms of full plastic and fatigue
crack propagation conditions of main member cross sections. Based on the evaluation
results, extended durations of the structures will be decided with an acceptable risk
probability.
1.5. Main contents of the dissertation
- 1st Content: Research on prediction of appearance probability of an over-designed
environmental condition in Vietnamese seas in the extended duration.
- 2nd Content: Research on developing an assessment method for the structures in
the extended duration based on strength reliability, according to full plastic and
fatigue reliability conditions in terms of crack propagations on member cross
sections.
- 3rd Content: Research on Risk Assessment and Determination of Allowable
extended duration of the platform structures.
1.6. Dissertation Scope


4

-

-

-

-

-

To reduce complexity of the research but ensure practicality, the dissertation
suggests some limits as below:
Random characteristics: Young’s Modulus, Yield Limit of steel materials and
cross-section properties of main members are considered as random variables with
standard distributions; Waves are considered as Stationary, Ergodic Random
Processes.
Non-linear structural analysis method: The structures are analysed with large strain
theory; the structures area analyzed with quasi-static method while wave loads are
considering as quasi-static loading.
Global Strength Analysis: Using elasto-plastic model for structural materials; the
structures are collapsed if one or more main members (Legs, Piles) are fully plastic;
Not consider to plastic condition of joints.
Fatigue Analysis: Analyze the fatigue-caused crack propagation according to
Paris’s Law; with the assumption that crack propagation is slow, in a sea state the
propagation velocity of a crack is constant; shape of crack widening is semiellipses;
Other factors: Corrosions, Marine Growth, collosion-caused deformation and
geological conditions will consider with detailed survey data.

CHAPTER 2. THEORICAL BASIS OF STRUCTURAL ANALYSIS OF FIXED STEEL
PLATFORMS SUFFERING OVER-DESIGNED LOADS

2.1. Introduction
Chapter 2 will summarize analysis methods of fixed steel offshore structures
suffering over-designed loads and apply in order to find collapse mechanisms of these
typical platforms in Viet Nam seas. This content is to develope the assessment method
in chapter 3.
2.2. Analysis on Over-designed Parameters in the extended lifetime of Fixed Steel
Offshore Structural Platforms
The dissertation only considers two main over-designed factors in the extended
lifetime as below:
- Fatigue Cracks continuously develop, leading to narrow member cross sections;
- The structures meet over-designed storms;
Over-designed sea states are rised from smallest in the extended lifetime.
Corresponding to each case, load-bearing capacity will be analysed to assess risk and
estimate the allowable extended lifetime.
2.3. Theorical Basis for analysis of fixed steel offshore structural suffering over
design loading
2.3.1. Non-linear static analysis of fixed steel platform structures


5

Figure 2.1. Regular of Coordinates and Displacements of frame members
                 

                      


u
(2.1)

1 v  2 w2 
 
xx

x





2 






 



x 



x  



 To establish shape functions u(x), v(x), w(x) at tips point of the element based

on the elastic defelection equations depended on load bearing conditions. Nonlinear Elastic Stiffness Matrix of the element can be expressed as below:

k

uu

k uv

k uw 

K  k vu


k vv

kvw 




e



k

k

wu

wv



k

ww

(2.2)



Whereas, matrix blocks on main diagonal are:
    T

l

k




l



k

vv


0
l

k

ww







  EI y 
0



2

2

uu

  EA 



u



u



(2.3)

dx

 x  x 
T
l
T
2   


 dx
N v  v  
 v  v   x 



 



x
0

 x x  2
EI z   x  
T

T

0

v



EI

 v 
z
2 
x
x 2 



v

2

2

 w  w


x

2





x 2



dx 

T

T

N w  w  


x  x  dx 
 EI y 

 


(2.4)

EA

l

EA
0

 w dx
x

 w  w 

x
 x 

(2.5)

These two are coupling matrices between axial and lateral deformation and are
linear in rotation.
k

T

l

uw

k
uv

 k vu

  EA 
0

 v 
u





 x x 


T

k

w

l

u

  v 


  EA 

0

u





w

T


dx

(2.6)

 x  x  x 

(2.7)


6

The last integral give coupling matrices between the two directions of deflection:
l
 w v   T
(2.8)
T

k wv  k

vw

0



w

  EA 



v

 dx

 x  x x  x 

 Based on the material model, plastic stiffness matrix of the structures can be
expressed as below:

(2.9)
K p  K e  K e G (G T K e G ) 1G T K e
Whereas:
giT 

1

Plastic Surface:










0
g
G 
0 g 2





S i

 f ( N Qy Q

N Q
P

,

,

 N Q y

,

,

 

,



Q

yP

z

M
,

x

,

M

zP

xP

My

M





,

,

Qz M x

M
,

M y

,

z
)1

M

yP



0

M





z

i

(2.10)
(2.11)

zP

Herein N, Qy, Qz, Mx, My, Mz, NP, QyP, QzP, MxP, MyP, MzP related to member
force components and plastic forces of the cross sections.
For tubular elements:



3

Where:

M

P



Y

M y2  M z2

M

3

D d 




P

 D 
2

 ; NP

6

 N
 cos 

Y 




 1 


4 

2 N



2



(2.12)


P

0



d 
 




 D  

 , D and d are outer and iner

diameter of the element;
 Non-linear analysis of the structures according to Finite Element Method Loading
are increase step by step. For a step, static equations are solved to decided the
new states of structures for the analysis in the next step.
K p  V F

(2.13)

Herein, F is loading vector, V is joint displacement vector.
2.3.2. Fatigue Crack Propagation analysis at hot spots of fixed steek offshore
structures
The relationship between ratio of crack range and number of cycle range and
stress intensity range can be written according to Paris’s law as below:
da  C ( K )m

(2.14)

dNw

Deduced:

a

Nw 

c

da

(2.15)

 C ( K ( a ))m

a

th

Herein, ath is the crack depth related to K = K th, ac is the crack depth related to
collapsed time (m); Nw is the cycle number of loading; C is da/dN at K = 1 MPa.√m
(m/cycle), m is slope (from 2 to 10) depended on materials and types of


7

welds; K ( a )  K max  Kmin is stress intensity range, depended on a. Generally, stress
intensity factor can be expressed by the formula:
K  Y ( a , t d ). .  a



(2.16)

              







a

Y ( a , t d )   1, 08  0, 7







22,1 a

t

.  1, 0  1, 24.e



357 a

d

 3,17.e

t

d

td 


(2.17)





For complex joints with no for available formula, the stress instensity factor can
be determined by numerical methods.
At the ith year, related to jth wave group with number of cycle N wji, the crack
depth is extended:
a ji  C .  ji .Y ( j 1i ).  .a m N wji
(2.18)


a

j 1i



k

Crack depth at ith year is determined as: a  ai 1  aji

(2.19)

j1

ao is the initial crack depth, related to first stage fatigue limit. It is normally
determined through surveys. According to tests, ao of metals can be assumed from 0.25
to 1mm.
2.3.3. Fatigue cracks effects modeling
As NORSOK N-004, an ellipse crack with short axis of 2a, long axis of 2c can be
equivalent by a dent depth Dd:
D
1
A  1
ac 
(2.20)
d

D



1  cos 

2


c

A

   1  cos
2



2

D d

2




Where D is member out diameter, Ac is crack area, A is cross area.

Figure 2.2. Illustration of plastic stress distribution on dented member
Assuming that at a crack location, the equivalent section is modeling by a dent
depth Dd (Figure 2.2). Fully plastic function equation of the section presented by the formula:


 My
 (




M yP

M
)2(

z

M zP

1/ 2

1/ 2

)2




  f12

 f2 2 

0

(2.21)


8

Formulas of f1, f2 are detailed in dissertation report.
2.4. Analysis of Collapsed Mechanisms of Fixed Steel Offshore Structures when

suffering over design loading
The author collected data and analysed the collapsed mechanisms of some
typical existing platforms in Viet Nam seas with different structural types including
Thai Binh Platform at 29,2m water depth, Topaz-A Platform at 41,1m water depth,
Ruby B Platform at 50m water depth, JVPC-C1 at 52,5m water depth, Thăng Long at
68,1m water depth, Đại Hùng Platform at 111,42m water depth. Based on the analysis
results, there are some comments as below:
- Structures are only collapsed when one or more of main members are fully
plastic.
- There is no collapse due to lack of soil bearing capacity.
- The collapse wave heigh are usually at least of 1.3 time of design wave heigh.
However, the structures were no longer capable to operate or to be strengthening
to operate with a lower environmental condition.
For the life extension purpose, the loading bearing capacity of structures are
limited by one of the following conditions: (1) A first main member section is fully
plastic; (2) Fatigue crack depth is equal the pipe thickness. Actually, when one of these
conditions occurring, the structures have not collapsed yet and still have working
capacity.
2.5. Conclusion of Chapter 2
Chapter 2 is summarized theorical basis to analyse structures suffering over
design loading serving to build a method of structural assessment. Based on the
analysis results find out the collapse mechanisms of typical fixed steel platforms using
in Vietnamese sea areas. According to the collapse mechanisms analyses, the
limitations of over design loading bearing capacity are suggested to build the new
assessment method which will be presented in Chapter 3.
CHAPTER 3. METHODOLOGY FOR ASSESSMENT OF OVER LOADING
BEARING CAPACITY OF FIXED STEEL OFFSHORE STRUCTURES
EXTENDED DURATION

IN

3.1. Introduction
According to theorical basis presented in chapter 2, content of chapter 3 is to
build a new method to assess the over loading bearing capacity of the fixed steel


9

offshore structures in extended duration. Chapter 3 is also the main content which
presents research results of author of the dissertation.
3.2. Regulations for assessment of fixed steel offshore structures when suffering
over design loading
3.2.1. Analysis of random variables considered in the dissertation
- Wave loads on the structures are determined by Morision equation, whereas
wave profiles are considered as stationary, ergodic random processes.
- Young modulus, yield limits of materials are considered as random variables
with Gaussian distribution according to catalogue of producers.
- Diameters and thicknesses of tubular members are considered as random
variables with Gaussian distribution. In case of enough of survey data, the effects of
corrosion can be considered as random variables when assessing the structures.
To reduce the complexity, the dissertation just assigns random varibles for main
members and piles of the structures.
3.2.2. Regulations for assessment
The dissertation suggests 7 regulations for assessment of over loading bearing
capacity of the fixed steel offshore structures to make basis for life extension, whereas
extended duration is chosen from two cases:
+ The structures haven’t collapsed due to fatigue but only bear a T returned
period wave heigh with allowable risk probability, corresponding to extended duration
tb.
+ A main member section is occurred a fatigue crack with the depth is equal the

pipe thickness, corresponding to extended duration tm.
The allowable extended duration is chosen by min(tb, tm).
3.3. Probability of occurency of over design sea-states in Vienamese sea
conditions
3.3.1. Relationship between over design wave heights and the probability of
occurences in Vietnameses conditions
According to survey data, maximum wave height in Vietnamese sea conditions mainly comply with IFisher-Tippett distribution presented in the formula as below:
T

F ( H max )  P ( H max  Hmax

T




)  exp   exp  





 HT

max





 




1

2





(3.1)

Whereas probabilistic characters 1, 2 are determined by statistic analysis of
survey data. So the maximum wave height with T returned period can be epxressed:


10
HmaxT  1  2 ln(  ln(1  1 ))

(3.2)

T

3.3.2. Chart of relationship between maximum wave heights and the probability of
occurences in typical Vienamese sea areas

a) Bach Ho Field

b) Su Tu Trang Field

c) Thang Long – Dong Do Field
d) Thien Ung Field
Figure 3.1. Relationship between maximum wave heights and the
returned periods – Vienamese sea conditions
3.4. Methodology for assessment of over loading bearing capacity of the fixed
steel offshore structures
3.4.1. Fatigue Reliability based on crack propagation conditions
- For each sea-state in ith year occurs in duration of t, normally is 3h or 6h,
determining hot spots stresses;
- Due to the hot spots stresses are random processes, so stress ranges and related
number of cylces are determined by rain flow counting method (Figure 3.2):

Figure 3.2. Illustration of Rain flow counting method
- To determine equivalent stress ranges eqji :


11

K



aj 1i

,

td

1

th




 1 k'

N
 N wji 
j'1


eqj ' i

m

m

. j ' i 

wj ' i

(3.3)





Herein, m is slope depended on materials (seeing Chapter 2),  j ' i is stress ranges
corresponding to j’th effective wave group with number of cycle Nwj’i (j’=1k’)
into jth sea-state induces stress intensity range K large than threshold
(Figure 3.3) at the time of crack depth aj-1i, it means:
K
 j ' i  [ ji ] 
th

Y(a

j 1i

, t ).  a
d

(3.4)

j 1i

Figure 3.3. Illustration of Effective Stress Ranges
- For a sea-state number j with significant wave height Hsj and number of cylce Nwji
in ith year, probabilistic characters of a ji can be determined based on Paris’s law:
+ Expected value of aji at tj:
ji

a a

j 1 i

 C . Nwji

. m

eqji

. m

(3.5)

G ( a j 1i ,t d )

+ According to Taylor expansion, variance of aji can be calculated as:
Var ( a ji )  Var ( a j 1i

2

2

)m C N

2

w ji

  2( m 1) . 2 m





eqji

G(a

j 1 i

.Var (  

eqji

)2m


,t )
d

eqji

Var ( G ( a j 1i , t d 
))
G ( a j 1i , t d )


. 2( m1)

Whereas expected value and variance of G(ai-1,td) is determined:

22,1 





357
a
j 1i

 G(a

j 1 i

 1, 08  0, 7
,td )






td

Var (G(a j 1i , t d ))

Details of



0





2
a

a j 1i

a j 1 i

 
.  1,

t
 1,
3,17.e
24.e
.Var ( a j 1i )   2 .Var (t d )
d



td

j 1i

are presented in dissertation.

(3.6)

td

- Probabilistic character of the crack depth at the end of ith year:











a

(3.7)

j 1i

(3.8)


12

 

+ Expected value:

ai

(3.9)

aki

(3.10)

Var ( ai )  Var ( aki )

+ Variance:

variables with Gausian

- Assuming crack depths at a time point are random
distribution. Fatigue Reliability at the end of the ith year:
P  P ( a  t )  0,5 (  )
i

mi

d

(3.11)

i

 
a

Reliability Index:

a

t a
d

(3.12)

i

Var ( ai ) Var (t d )

i

3.4.2. Strength Reliability of main member sections based on fully plastic conditions
3.4.2.1. The relationship between fully plastic surface  of a main member sectioncủa
and the random variables
The relationship between approximate fully plastic surface  of member cross
sections and random variables according to polynomial surface type:
n

T

n

o  1 H max   2 F y   3 E    i1  3 D i  
i1 1
2n

i1 1

n



2 n
i1  3

i1  n 3 i1

T 2

2

 1 H max

 2 F y

Di







2
i1  n3

ti

1

T

T

  1 H max F y   2 H max E   3 F y E

(3.13)

i1 1
T

n

   i1  3 H max D i1  

T



i1  n  3

n

H max t



i1

i1 1







n
i1  4 n  3



E Di 
1

i1 1







n

n 1

Et 

i1  5 n  3

i1

i1 1

i1  2 n  3





1

t t

i1 1

 

1



2

i  7 n  2

i1 1





i1  3n3

Fyt

i1

D1 D i

n2
1

1







i1  7 n2

D 2 Di

1

2

i1 1
n

n ( n 1)
2

i1 1
n2

n( n 1)  n 2 1 i 1

1

1

i1 1

2

i  6 n 3 



i1 1

i1  6 n  3

n

 

F y Di

i1 1

 ...   6 n  3  n ( n 1) D n 1 D n    i  6 n  3 
n 1

t

i1 1

3E 
i1 1
n



D 1 t i ...i6n3
1

i1 1

n ( n1) n2

1

n ( n1)  n ( n1)

1

t 2 t i1  2  ...   2 n

D n ti1

2

2

 5 n3 t n 1tn

2

Whereas:
T

H max 

HT T
H

max

Var ( H

max

T

)

F

E  E

; Fy 

;E

; Di1 

yF y

Var ( Fy )

Var ( E)

D

max

1

t

D

i1

Var ( Di )

1

; ti1 

1

Var (ti )

t

(3.14)

i1

1

Herein i1 = 1÷n, n number of main members.
Factors in equation (3.13) are determined by regression method with minimum
mean square error condition. Total error of the regression model is estimated by
S
S
determination factor R2  R  1 e . For regression model with R2 near by 1, the
S
S
T

model is suitable with true model.

T


13

For each H value, to perform Monte Carlo simulation for random variables
and determine eq values according to (3.13) with the large enough number of trials nt:
2


nt 





100 z e / 2 Var( )
e 



(3.15)





Herein, e is acceptable error of the trials and ze is value corresponding to P ( z 
e

ze/2 )  , z is random variable with Gaussian distribution.
2

3.4.2.2. Strength Reliability of main member cross sections based on fully plastic
conditions
Strength Reliability of a main member section is evaluated by the Monte Carlo
simulation results with number of trials nti:
p
(3.16)
n
1

Pi1i  P(  i1i  0) 

ti

nt
i

where nt

p
i

th

1

is the total trials in i year with i1i ≤ 0

3.4.3. Assessment of over loading bearing capacity of structures at ith extended year
a)

Assessment based on fatigue condition

-

In case of fatigue reliability satisfying requirements, the crack depth is
determined by the following formula. The crack will be updated to the structural
model at the same location prior to analyse strength of the structure.
ai  ai  3 Var ( ai )

-

(3.17)

In case of fatigue reliability not satisfying requirements:
+ If the crack is on the brace member, it is assumed that the brace member is
released to chord member. The structural model will be updated to analyse
strength of structure.
+ If the crack is on the chord member, it is assumed that the chord is collapsed, it
means the structure haven’t enough load bearing capacity.

b)

Assessment based on strength condition

Reliability of structures based on strength conditions when suffering T year
returned period at ith year Pi1 estimated by the minimum value of reliability of n1 main
member cross sections.

14

Pi1  min Pi1i  với (i1=1÷n1)

(3.18)

Whereas, Pi1i are determined by the formula in section 3.4.2.2. If Pi1 are satisfied
the requirements, the structures will be considered as enough bearing capacity.
3.4.4. Risk Assessment for structures suffering over design wave loading
Risk probability when structures meet a wave height HmaxT :
Pr  (1  Pi H maxT ).P ( H  HmaxT )
Herein, Pi HmaxT are determined by formula (3.18) when

(3.19)
wave heighs are

increased in step by step.
According to DnV “Risk Acceptance Criteria and Risk Based Damage StabilityPart 1”, risk criteria divided into three zones. 1st zone: Acceptale Risk, where Pr ≤ 106
/1 year; Zone 2: As low as reasonably practicable, where 10 -6 ≤ Pr ≤ 10-3 /1 year;
Zone 3: Not Acceptable Risk, where Pr ≥ 10-3 /1 year.
3.4.5. Assessment of reducing of over loading bearing capacity of the structures and
prediction of maximum extended duration
The maximum extended duration are limited by minimum value of tb and tm,
where:
- tm are determined based on fatigue reliability, tm = i with:
P ( ai  t d )  0,5  ( a )  [P ]

(3.20)

i

- tb are determined based on acceptable risk probability Pr related to T year returned
period (T ≥ 100), tb = i’:
(3.21)
P H maxT  1  T ( Pr 1)
i'

3.4.6. Procedure of assessment of over loading bering capacity and extended duration
of fixed steel offshore structures


15


16


17

3.5.

Softwares using in dissertation
The dissertation uses commercial softwares: SACS, USFOS to analyse and uses

Jrain to determine the number of stresses cycles using rain flow counting method.
3.6.

Conclusion of Chapter 3

In chapter 3, the dissertation has developed a new method to assess over loading
bearing capacity of fixed steel offshore structures and to predict the extended durations
of the structures in Vietnamese sea conditions.
The method is fitted with calculation methods and softwares in the world. The
method can be used to assess offshore platforms for life extension in Vietnamese sea
areas when cooporating with survey data.
The method has some limitations, such as errors control, reduction of calculating
amount, calculating time... These things are needed to improve in the next researchs.
The method is performed through the combination of the softwares listed above
and calculation sheets of the author. The method will be applied in chapter 4 to assess
an actual fixed steel platform in Vietnamese sea conditions.
CHAPTER 4. A VIENAMESE CASE STUDY
4.1. Introduction
The method was developed in chapter 3 will be applied in chapter 4 to analyse
and assess over loading bearing capacity of a fixed steel offshore structure in Su Tu
Den field, Viet Nam offshore. Input data and analysis results are expressed in sections
as below.
4.2. Input data summaries
4.2.1. Structural Data
Platform Substructure is a jacket with four legs, piles in legs. The main
parameters are summarized in table 4.1.
Table 4.1. Structural data summaries
Items

Parameters

Topside

Living Quarter

Platform
40x20x9 (m)

Topside Weight

800 (T)

Legs

810x20,6 (mm)

Piles

720x20 (mm)

Braces

609x12,7(mm)

Function


18

4.2.2. Environmental data
Mean Still Water Level: 45,6m; heighest: +2,0m; lowest: -2,5m
Table 4.2. Maximum Wave Height Data
Parameters

Wave Direction

NE
E
SE
S SW
12,27 14,78 8,23 7,88 8,91
10,88 12,79 8,00 7,76 8,47

N
12,90
11,35

Max Wave Height (m)
Period (s)

W
11,61
10,40

NW
12,58
11,11

Table 4.3. Fatigue WaveData
Hs(m)
0,0
0,5
1,0
1,5
2,0
2,5

3,0
3,5
4,0
4,5

o

45
0,0131
0,0723
0,0785
0,0516
0,0388
0,0192
0,0050
0,0009
0,0000
0,0000

o

o

90
0,0041
0,0137
0,0209
0,0199
0,0144
0,0054

0,0003
0,0000
0,0000
0,0000

135
0,0007
0,0016
0,0018
0,0010
0,0003
0,0000
0,0000
0,0000
0,0000
0,0000

o

o

180
0,0008
0,0022
0,0019
0,0020
0,0014
0,0007
0,0003
0,0000

0,0000
0,0000

225
0,0056
0,0322
0,0614
0,0856
0,0803
0,0661
0,0462
0,0265
0,0145
0,0069

o

270
0,0080
0,0205
0,0006
0,0001
0,0000
0,0000
0,0000
0,0000
0,0000
0,0000

o

315
0,0130
0,0347
0,0066
0,0008
0,0001
0,0000
0,0000
0,0000
0,0000
0,0000

4.3. Structural Assessment according to current standards
4.3.1. 1st stage fatigue analysis results
Using SACS software to analysis fatigue in 1st stage according to spectral
method, the minimum fatigue life results are expressed in table 4.4.
Table 4.4. Minimum Fatigue Life Results
Joint
202L
101L

Member

Crack Location

101L-202L

Crown point, on

101L-201L

chord
Crown point, on

Life
(year)

Impacted Wave
Direction

27,01

225

o

53,75

225

o

chord

4.3.2. Strength checking based on WSD
The results of the most dangerous members for ULS (100 returned period) are
listed in table 4.5.

19

Table 4.5. Strength Checking Results for ULS
Member

Location

UC

Direction

0,912

Hmax (m)
14,78

102L-202L

Last bay of Row
A

004P-104L

Pile element of
Row B

0,973

14,78

East

East

4.3.3. Ultimate Global Strength Assessment Results
Table 4.6. Ultimate Global Strength Assessment Results
Design Wave
Direction

Collapsed
Wave Height
(m)

RSR
Height (m)

East

14,78

2,0

20,9

Collapsed Modal
[RSR]
The chord member
of last bay Row B is
unstable and fully
plastic.

1,6

4.4. Structural Assessment based on the method of dissertation
4.4.1. Prediction of crack propagation
According to results in table 4.4, joint number 202L has minimum fatigue life of
27 years, so it will be selected to analyse crack propagation in second stage in each
year to make basis for prediction of structural life extension.
4.4.2. 2nd stage fatigue analysis results
Crack propagation of member 102L-202L at the crown point of joint 202L is
analyzed corresponding to each short term sea-state in each year. Probabilistic
characters results of the stress range in every year are summarized in table 4.7. The
analysis results of fatigue crack sizes in every year are summarized in table 4.8.
Table 4.7. Probabilistic characters results of the stress range in every year

Year

µ

ai

Hs

1

2

3

4

(MPa)

(MPa)

(MPa)

(MPa)

(MPa2)

95,88

90,89

3

0,00165

4

0,00218

5

0,00268

89,88
79,24
89,88

79,24
84,73
75,39
84,73
75,39
84,73
69,5

90,89

0,0013

93,06
83,54
93,06
83,54
87,9
83,54
87,9
78,04
85,62
75,22

95,88

2

(m)
4,5
3,5

4,5
3,5
4,5
3,5
4,5
3,5
4,5
3,5

(MPa)

1

(m)
0,001

92,12
75,16
92,12
75,16
92,12
69,23

90,89
79,24
90,89
79,24
84,06
70,88

92,428
81,39
92,428
81,39
88,91
78,333
88,91
76,958
86,633
71,208

5,293
4,623
5,293
4,623
8,179
11,666
8,179
3,017
10,343
5,758



Var()


20
Year

ai

Hs

6

(m)
0,00362

7

0,00520

8

0,00837

(m)
4,5
3,5
4,5
3,5
2,5
4,5
3,5
2,5

Var()

1

2

3

4

µ

(MPa)

(MPa)

(MPa)

(MPa)

(MPa)

(MPa2)

84,9
68,56
80,27
64,78
54,3
78,42
61,96
54,3

78,31
68,54
76,3
66,77
51,97
70,16
61,65
51,97

86,94
66,06
82,41
63,31

80,76
68,9
78,56
64,61

75,62
60,98

74,31
60,44

82,728
68,015
79,385
64,868
53,135

74,628
61,258
53,135

11,463
1,294
5,033
1,530
1,357
8,857
0,348
1,357



Table 4.8. Fatigue crack analysis
Year
1
2

a

i

(m)
0,001
0,0013

3

0,00165

4

0,00218

5

0,00268

6

0,00362

7

0,00520

8

0,00837

Hs
(m)
4,5
3,5
4,5
3,5
4,5
3,5

4,5
3,5
4,5
3,5
4,5
3,5
4,5
3,5
2,5
4,5
3,5
2,5

µ



Var()
2

(MPa)

(MPa )

92,428
81,39
92,428
81,39
88,91
78,333

88,91
76,958
86,633
71,208
82,728
68,015
79,385
64,868
53,135
74,628
61,258
53,135

5,293
4,623
5,293
4,623
8,179
11,666
8,179
3,017
10,343
5,758
11,463
1,294
5,033
1,530
1,357
8,857
0,348

1,357

Average of
N in 1 year
49,35
15,52
49,35
15,52
67,86
38,79
67,86
46,55
81,43
131,90
113,51
217,24
146,82
341,38
39,60
222,09
535,35
39,60

µ

a

(m)

0,00121

Var(a)
(m)

2,2e-05

0,00155 2,61e-05
0,00198 4,96e-05
0,00247 5,34e-05
0,00328 9,89e-05
0,00468 0,000146
0,00759 0,000223

0,01371 0,000467

The reliability index in 8th year are calculated:
a

 
td

a

 3,53

Var ( a ) Var (t d )

So in the 8th year of extension, the fatigue reliability is approximate 0,9993.
4.4.3. Assessment Results of Over loading bearing capacity of the structure in
extended duration

According to the analysis results as above, the reliability of structural strength
and risk probability are assessed at the most dangerous member section (joint 202L of
member 102L-202L) in 7th and 8th years. The results are presented as below:
1st case: At the 7th year, the fatigue crack is equivalent with a dent depth Dd =
0,94cm. eq at the considered section is written as: