Fatigue Assessment Analysis of a Jack-up Platform Pile Leg Structure

7/25/2019 Fatigue Assessment Analysis of a Jack-up Platform Pile Leg Structure

1/9

Fatigue Assessment Analysis of a

Jack-up Platform Pile Leg Structure

Zuo Xin

College of Shipbuilding Engineering, Harbin Engineering University, Harbin

150001, China

ABSTRACT

Fatigue has long been recognized as an important consideration for designing offshore

structures.. In this paper, fatigue life assessment for welded tubular joints of a jack-up

platforms is numerically assessed as part of mitigation for platform life time. The analysis

procedures are presented for numerical fatigue assessment methods based on S-N curve

approach for ABS standard utilizing the spectral (stochastic) method. The objective of this

analysis is to verify that the pile leg of a jack-up platform has the appropriate fatigue life

referring to the expected design life of twenty (20) years. The results are discussed and

summarized through tables.

KEYWORDS: Fatigue Assessment, Jack-up Platform, Pile Leg, Tubular Joints

INTRODUCTION

Generally, jack-up platform structures are used for production drilling and exploration of

ocean oil which is an important structure style in the offshore industry over the last 40 years.

Fatigue is inevitable when it operates for a long time. Fatigue is the process of damage

accumulation in material due to stress fluctuation caused by variation of loads in service time.

The fatigue failure occurs when accumulated damage has exceeded a critical level [1].

Research on the fatigue of offshore structures has attracted much attention during recent

decades [2-5]. Nolte and Hansford [2] developed closed-form mathematical expressions for

determining the fatigue damage of structures due to ocean waves. Almar-Naess [3] discussed

the most important subjects related to fatigue of the offshore steel structures, such as

calculation of fatigue stresses and fatigue lives. Dover and Madhava Rao [4] reviewed and

summarized the knowledge in the area of SCF (stress concentration factor), fatigue and

fracture mechanics of the tubular joints, damage assessment and reliability analysis of joints.

Etube et al. [5] presented a modeling of jack-up response for fatigue calculation. By analyzing

a mathematical model to obtain the transfer function of the dynamic response for a typical

- 1025 -
http://www.ejge.com/Index.htm


2/9

Vol. 20 [2015], Bund. 3 1026

jack-up platform, they found out that the complex leg-soil interaction can be adequately

modeled using springs and assuming a rigid foundation.

In this study, the fatigue analysis was performed using SESAM software, in which,

stochatic method was employed, and the water depth was taken as 100m. In the calculation ofthe cumulative fatigue damage of the leg of the platform, the Middle East sea states were used.

The critical joints of the chords and the braces were selected and reviewed for the fatigue life

calculation.

APPROACH

This fatigue analysis by using spectral analysis method was based on a dynamic analysis

in Sestra of SESAM software, and the methodology and process in this fatigue analysis are

described as follows:

In the dynamic analysis, effect of equipment is taken into account with loads, and other

weights are reflected by mass elements. RAOs of legs are calculated through frequency

domain spectral analysis method by using Wajac of SESAM. In these calculations, range of

wave periods is from 4.40s to 30.12s, and sixteen wave approaching directions(see table 1)

are considered. Wave periods are select based on:

Eigenfrequencies (eigenperiods) of the platform;

Cancellation and attenuation effects;

With the high wave energy;

Including a reasonable range of wave periods with high wave energy.

Table 1:Wave periods of loads calculation

T(s)4.40 4.61 4.97 5.20 5.54 5.97 6.35 6.80 7.53

8.34 9.12 10.91 12.21 15.00 17.82 24.37 30.12

The stress range transfer functions of the leg's structure are generated from a dynamic

structural analysis in each wave heading by using Sestra of SESAM.

Fatigue analysis through stochastic method for the selected joints are carried out by using

Framework of SESAM.

FINITE ELEMENT MODEL

The idealized model of the jack-up platform is shown in Figure 1 and Figure 2. Air gap

used in the analysis is 10m for the water depth of 100m, and the length of the leg penetrated

into seabad is taken as 3.40m. Pinned type supports at the spud cans are used and the leg pin

point is conservatively assumed at 3.40m below the mudline. Members and locations of the

selected chords and braces are shown in Figure 3.

Fatigue critical locations identified by reviewing the stress responses under the design

operating conditions are found near the lower guides, spud cans and waterline,
http://www.ejge.com/Index_ejge.htm


3/9

Vol. 20 [2015], Bund. 3 1027

Figure 1:The whole model of jack-up Figure 2:Leg model

Figure 3:Fatigue critical locations


4/9

Vol. 20 [2015], Bund. 3 1028

FATIGUE LOADING

Dead loads

Maximum elevated weight is 9800t, which includes 5000t of ship hull and 3000t of VDL,

and also included in the analysis are the leg weight of 1500t and spud can weight of 300t.

Drag coefficient

The contribution of hydrodynamic loads due to individual k-braces and interior braces is

lumped to the three leg chords.

Wave loads were applied to the Finite Element model as a series of distributed loadsproducing the correct force and overturning moment. Loads are computed using Morisons

equation. Wave particle kinematics are computed based on Airy wave theory.

The equivalent chord drag and inertia coefficients are found to be: CD1= 1.2, CM1 = 2.

The brace drag and inertia coefficients are found to be: CD2= 0.65, CM2 = 2. The jet line

drag and inertia coefficients are found to be: CD3= 1.95, CM3 = 6.

STRESS TRANSFER FUNCTIONS

Stress concentration factors

Stress concentration factors (SCFs) are calculated according to DnV Rules for all joints,

and the calculated results are included in table 1.

Stress transfer functions

Since the fatigue damage is primarily due to cyclic load action, the non-cyclic loads

including buoyancy and gravity are not considered in this fatigue analysis. The frequency of

current and wind loads are largely lower than that of wave loads, so the effect of them can be

ignored, and only the fatigue damage resulted form wave loads are calculated.

The relationship between stress range and wave height was assumed to be linear.

Although this relationship is more close to a quadratic polynomial, it is sure that the linear

approximation is accurate enough for the fatigue assessment. RAOs of legs of the jack-up

platform are calculated by using Wajac of SESAM, and the stress transfer functions are

calculated by using Sestra of SESAM.

Rayleigh damping may be given for all forced response methods:

1 2C M K = + ,


5/9

Vol. 20 [2015], Bund. 3 1029

where:

C is the damping matrix;

M is the mass matrix;

K is the stiffness matrix;

1is the Coefficient of proportionality related to the system mass matrix;

2is Coefficient of proportionality related to the system stiffness matrix.

According to the natural period of the Jack-up platform, the values of 1 and 2 are

chosen as 1 = 0.3 and 2 = 0.

STOCHASTIC FATIGUE ASSESSMENT

Long term distribution of sea states

An annual wave scatter diagram provided for the Middle East sea state. Based on the

diagram, a distribution of the sea states defined by the mid-range significant wave height,

averaged zero crossing period (weighed by their occurrences) and the corresponding percent

occurrences are derived. All these data are imported into Framework of SESAM for fatigue

analysis.

Wave energy spectral density function

The Pierson-Moskowitz spectrum for fully developed seas is selected for the current

analysis. A convenient form of the spectrum is expressed as follows:

( ) ( ) ( )2

5 41exp

4

s z z z

H TS f T f T f

=

,

where Hs is the significant wave height, Tz is the average (mean zero-crossing) waves period,

and ( )

S f is the P-M power density function for wave surface elevation (unit:m2/Hz).

Stress range response spectra

The stress range response spectra are obtained by:

( ) ( ) ( )2

S f T f S f =

,

where T(f) is the stress range transfer function.

The statistical properties of the response were then evaluated as follows:

The nth order spectral moment:


6/9

Vol. 20 [2015], Bund. 3 1030

( )0

n

nm f S f df

= ,

The standard deviation of the stress range:

0rms m=

,

The effective frequency of the stress response:

2

0

e

mf

m=

.

Distribution of stresses

The stress range history at a joint can be assumed to be a narrow-banded Gaussian

Process whose peaks are Rayleigh distributed. Knowing the standard deviation of the stress

range for a given sea state, a Rayleigh probability density function ( )rp against stress

range can be obtained:

( )2

0 0

exp2

r rr

p

m m

=

.

The total area under the function is unity. It is shown that the probability of the

occurrence at the stress range greater than 5 standard deviations becomes negligible small.

Thus, the stress range from 0 to 5 0m

can be divided into small stress range blocks. The

probability of the occurrence of the stress block i is the corresponding strip area under the

function and can be found by:

( ) ( ) ( )1

1

2 2

1 11 1

0 0

exp exp2 2

ri

ri

ri ri

r ri ri r ri r r

p p p dm m

+

+ +

= = < < = =

.

Miners cumulative fatigue damage

The number of cycles applied in a year from sea state j and at stress range block i is calculated by:

( )331536 10 %ji ri en P P f= ,

where P% is the total fraction occurrences of the wave in a year for j sea state and ( )riP is the

probability of the occurrence of the stress range block i and fe is the effective stress response frequency

for j sea state.


7/9

Vol. 20 [2015], Bund. 3 1031

ABS fatigue criteria

The fatigue stress under constant amplitude loading is expressed through a S-N curve.

The S-N curves given in ABS Guide for The Fatigue Assessment of Offshore Structures[6]

are be used here. The effect of thickness and the relative corrosiveness of the environment inwhich the structural detail is being subjected to variable stress are considered.

The fatigue performance of a structural detail depends on member thickness. For the same

stress range the details fatigue strength may decrease, as the member thickness increases.

This effect (also called the scale effect) is caused by the local geometry of the weld toe in

relation to the thickness of the adjoining plates and the stress gradient over the thickness. The

basic design S-N curves are applicable to thicknesses that do not exceed the reference

thickness tR = 22 mm (7/8 in). For members of greater thickness, the following thickness

adjustment to the S-N curves applies:

R

q

f

tS S

t

=

,

where Sis unmodified stress range in the S-N curve;

t is plate thickness of the member under assessment;

q is thickness exponent factor (= 0.25).

Damage accumulations

The Middle East sea state has been used in this analysis. The cumulative fatigue damage

in one year is then calculated by summing each stress range block i, each sea state j and each

wave direction.

ji

j

j i ji

nD P

N= ,

wherePj is the appearance probability of the sea state j.

Then the fatigue life is calculated by 1/Y D= .

RESULTS AND CONCLUSIONS

Fatigue lives for the operating water depth of 100m are summarized as follows:

Table 2:The results and conclusions of fatigue assessment

Element

ID

Joint

IDWeld Side

SCF

S-N Curve DamageLife

(years)SCFax SCFipb SCFopb

BM2017 1277 BRACE-SID 17.131 3.802 5.078 ABS-D-CP 0.188 107

BM2227 1580 BRACE-SID 17.131 3.802 5.078 ABS-D-A 0.163 123


8/9

Vol. 20 [2015], Bund. 3 1032


















BM1800

1553

BRACE-SID

17.131

3.802

5.078

ABS-D-CP

0.043

469



Using the fatigue analysis procedures discussed in this report, the shortest fatigue lives of

a jack-up platform have been conservatively assessed to be 107 years for 100m water depth

based on ABS fatigue criteria.

REFERENCES

[1] Naser Shabakhty, Pieter van Gelder. Reliability analysis of jack-up platforms based on fatigue

degradation. Proceedings of OMAE 02 21st International Co

and Artic Engineering June 23-28, 2002,Oslo, Norway

[2] Nolte, K.G. and J.E. Hansford, 1976. Closed-form expressions for determining the fatigue

damage of structures due to ocean waves. In: Proceedings of the Eighth Annual Offshore

Technology Conference, Offshore Technology Conference Dallas, TX, 2: 861-872.

[3] Almar-Naess, A., 1985. Fatigue Handbook: Offshore Steel Structures. Trondheim.

[4] Dover, W.D. and A.G. Madhava Rao, 1996. Fatigue in Offshore Structures vol. 1. A.A.

Balkema, Rotterdam/Brookfield.


9/9

Vol. 20 [2015], Bund. 3 1033

[5] Etube, L.S., F.P. Brennan and W.D. Dover, 1999. Modeling of jack-up response for fatigue

under simulated service conditions. Marine Structures, 12: 327-348.

[6] ABS Rules for Building and Classing Mobile Offshore Drilling Units 2008 Edition.

[7] Chinese Classification Society Rules for the Classification and Construction of Mobile Offshore

Drilling Units, 2005.

2015 ejge
http://www.ejge.com/copynote.htmhttp://www.ejge.com/Index.htmhttp://www.ejge.com/copynote.htmhttp://www.ejge.com/Index_ejge.htm

Fatigue Assessment Analysis of a Jack-up Platform Pile Leg Structure

Documents

Transcript of Fatigue Assessment Analysis of a Jack-up Platform Pile Leg Structure