1 Deterministic Fatigue Analysis [Compatibility Mode]

DETERMINISTIC FATIGUE ANALYSIS PROCEDURE

DETERMINISTIC FATIGUE ANALYSIS PROCEDURE

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Prof. S. Nallayarasu Department of Ocean Engineering Indian Institute of Technology Madras-36

DETERMINISTIC FATIGUE ANALYSIS PROCEDURE OBJECTIVE OF FATIGUE ANALYSIS The objective of fatigue analysis is to ensure that the jacket tubular connections are safe against fatigue damage and have adequate fatigue life of 25 years against fatigue loads. loads This analysis covers the Deterministic fatigue analysis of wellhead jacket structure including topside. The fatigue analysis includes the tubular joints as per API RP 2A F ll i Following F Fatigue ti lload d iis considered. id d 

Wave loads on tubular members

This deterministic fatigue analysis method is used for Jacket platforms is not sensitive to the dynamic effects.

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE MASS MODEL The mass model includes dead load, equipment, conductors, appurtenances, grout, marine growth, entrapped water, and added mass. A lumped mass model is sufficient to obtain global structure response. The equipment mass included in the model has been consider all equipment i t supported t d by b the th structure t t d i during any given i operation ti on the platform. If the equipment q p mass is p produced to vary y significantly g y for different operations during the platform life, it is appropriate to perform independent analyses and combine fatigue damage.

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE MASS MODEL – PLATFORM INFORMATION Water Depth Platform Orientation Leg Spacing (m) Number of Legs Dia. of Legs (mm) Leg Batter

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50m MSL North 10 x 10 4 1829 1:8

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE JACKET MODEL (Nodal Plots)

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE DESIGN CRITERIA Assumptions The various assumptions made in the computer modelling/structural analysis and design is listed as below. 



Members are assumed to be coincident at work points. Brace offsets are introduced into the computer model where actual eccentricities fall outside the API RP 2A allowable, i.e. 25% of the chord diameter or 305mm. Drag and inertia forces on individual members are calculated using Morison’s Equation.

Fatigue life of Tubular Joints The design fatigue life considered is 25 years. S N Curves S-N C The tubular joints are analysed for WJT curve as per API RP 2A. WJT curve corresponds to simple welded joints. 22-Jul-13

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Hydrodynamic Coefficients The drag (Cd ) and inertia (Cm) coefficients are tabulated below. Drag coefficient

Inertia coefficient

Cd

Cm

Clean Members

0.55

2.2

F l dM Fouled Members b

0 88 0.88

22 2.2

Type

Note : Includes 10% increase to account for wave loads on anodes. Wave Kinematics factor is taken as 1.00. 1 00

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE LOADING The applied cyclic loads are represented such that the effects of load distribution along the member are included in the member end stresses. Distributed loads on brace members need to be considered only between intersection points. Loads attributed to conductors and appurtenances such as launch cradles, mudmat framing, J-tubes, risers, skirt pile guides, anodes, etc., are considered. The choice of wave theory as well as drag and mass coefficients are determined as they may differ from those used in strength analyses for design wave loads. When the loading varies significantly for different operations during the platform life. it is appropriate to perform separate analyses and combine the fatigue damages from each joints. Marine growth may have a detrimental effect on fatigue life of members due to the increase in local and global wave loading. Marine growth affects platform added mass, member drag diameter, and drag coefficient. 22-Jul-13

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE DIRECTIONAL DISTRIBUTION OF FATIGUE WAVES

Pd = Directional Distribution 22-Jul-13

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE DIRECTIONAL WAVE SCATTER DATA DIRECTIONAL WAVE SCATTER DATA Direction Angle (deg) Direction Distribution Percentage (Pd)

N

NE

E

SE

S

SW

W

NW

315

0

45

90

135

180

225

270

5

2

20

15

25

15

10

8

100

%

%

%

%

%

%

%

%

%

Total

315360 126144 1261440 946080 1576800 946080 630720 504576 6307200

Number of Occurrences in 1 year

NH 

365 * 24 * 60 * 60  6307200 5

TZ - mean zero zero-crossing crossing period (5 sec)

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE JOINT OCCURRENCE DATA Max Wave Height, H (m) 0.0 to 2.0 2.0 to 4.0 4.0 to 6.0 6.0 to 8.0

Max Wave Height,

Joint Occurrence data (North) Max Wave period T(s) T(s) 0-2 2 to 4 4 to 6 6 to 8 Mean 1.0 3.0 5.0 7.0 1.0 0 31536 94608 47304 3.0 0 25229 63072 31536 5.0 0 0 15768 6307 7.0 0 0 0 0 Total 0 56765 173448 85147

173448 119837 22075 0 315360

Joint Occurrence data (North West) Max Wave period T(s) T(s)

0 2 0-2

2 to 4

4 to 6

6 to 8

H (m)

Mean

1.0

3.0

5.0

7.0

0.0 to 2.0

1.0

0

50458

151373

75686

277517

2.0 to 4.0

3.0

0

40366

100915

50458

191739

4 0 to 4.0 t 6.0 60

50 5.0

0

0

25229

10092

35320

6.0 to 8.0

7.0

0

0

0

0

0

0

90824

277517

136236

504576

Total

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Total

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Total


DETERMINISTIC FATIGUE ANALYSIS PROCEDURE JOINT OCCURRENCE DATA Max Wave Height, H 0.0 2.0 4.0 6.0

(m) to 2.0 to 4.0 to 6.0 to 8.0

Max Wave Height, H 0.0 2.0 4.0 6.0

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(m) to 2.0 to 4.0 to 6.0 to 8.0

Joint Occurrence data (West) Max Wave period T(s) T(s) 0-2 2 to 4 4 to 6 6 to 8 Mean 10 1.0 30 3.0 50 5.0 70 7.0 1.0 0 63072 189216 94608 3.0 0 50458 126144 63072 5.0 0 0 31536 12614 7.0 0 0 0 0 Total 0 113530 346896 170294 Joint Occurrence data (South West) Max Wave p period T(s) ( ) T(s) 0-2 2 to 4 4 to 6 6 to 8 Mean 1.0 3.0 5.0 7.0 1.0 0 94608 283824 141912 3.0 0 75686 189216 94608 5.0 0 0 47304 18922 7.0 0 0 0 0 Total 0 170294 520344 255442

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Total 346896 239674 44150 0 630720

Total 520344 359510 66226 0 946080


DETERMINISTIC FATIGUE ANALYSIS PROCEDURE JOINT OCCURRENCE DATA Max Wave Height,

T(s)

0-2

2 to 4

4 to 6

6 to 8

H( (m) )

Mean

1.0

3.0

5.0

7.0

0.0 to 2.0

1.0

0 157680 473040 236520

867240

2.0 to 4.0

3.0

0 126144 315360 157680

599184

4.0 to 6.0

5.0

0

0

78840

31536

110376

6.0 to 8.0

7.0

0

0

0

0

0

Total

0 283824 867240 425736 1576800

Max Wave Height, H 0.0 2.0 40 4.0 6.0

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Joint Occurrence data (South) Max Wave period T(s)

(m) to 2.0 to 4.0 to 6 6.0 0 to 8.0

Total

Joint Occurrence data (South East) Max Wave period T(s) T(s) 0-2 2 to 4 4 to 6 6 to 8 Mean 1.0 3.0 5.0 7.0 1.0 0 94608 283824 141912 3.0 0 75686 189216 94608 50 5.0 0 0 47304 18922 7.0 0 0 0 0 Total 0 170294 520344 255442

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Total 520344 359510 66226 0 946080


DETERMINISTIC FATIGUE ANALYSIS PROCEDURE JOINT OCCURRENCE DATA Max Wave Height, H 0.0 2.0 4.0 6.0

(m) to 2.0 to 4.0 to 6.0 to 8.0

Max Wave Height, H 0.0 2.0 40 4.0 6.0

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(m) to 2.0 to 4.0 to 6 6.0 0 to 8.0

Joint Occurrence data (East) Max Wave period T(s) T(s) 0-2 2 to 4 4 to 6 6 to 8 Total Mean 1.0 3.0 5.0 7.0 1.0 0 126144 378432 189216 693792 3.0 0 100915 252288 126144 479347 5.0 0 0 63072 25229 88301 7.0 0 0 0 0 0 Total 0 227059 693792 340589 1261440 Joint Occurrence data (North East) Max Wave period T(s) T(s) Mean 1.0 3.0 50 5.0 7.0 Total

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0-2 1.0 0 0 0 0 0

2 to 4 4 to 6 6 to 8 3.0 5.0 7.0 12614 37843 18922 10092 25229 12614 0 6307 2523 0 0 0 22706 69379 34059

Total 69379 47935 8830 0 126144


DETERMINISTIC FATIGUE ANALYSIS PROCEDURE ANALYSIS TECHNIQUES

 Fatigue loads based on the spectral scatter diagram were transformed into a single wave scatter diagram to define the wave exceedance diagram  The wave scatter data is used to generate directional wave scatter data by decomposing wave height, wave period combination for each direction.  Only y the tubular members of the jjacket structure were considered;; the fatigue g damage was calculated for 8 hotspots around the perimeter of the member  The basis for the S-N fatigue calculation is the stress range due to the cyclic loads, without considering the mean stress level, therefore only the wave loads were applied to the model.  Each E h combinations bi ti off wave height h i ht and d wave period i d has h unique i number b off occurrences (n) which is used for the calculation of fatigue damage.  Stress Concentration Factors (SCF) are then calculated for each joint at 8 points along the interface between chord and brace.  The accumulated fatigue damage was calculated based on the long term distribution of the hotspot stresses and S-N curve associated with the detail under consideration.  These SCF values are used to obtain the Hot Spot Stress range (HSSR). Efthymiou equations are used for tubular connections and DNV equations are used for inline thickness transitions.  Fatigue damage is then calculated using Palm-Miners Rule for the cumulative fatigue damage for all sea states.  The Airy’s wave theory has used to generate the member loads used in the analysis. analysis 22-Jul-13

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Stress range The stress ratio R is the relationship between the maximum stress and the minimum stress to which a structure is subjected. The stress range is the difference between these two extremes,

Maximum stress

  max

Minimum stress

  min

St Stress ratio ti

R

Cyclic Stress range

 min  max     max   min

Mean stress

  mean 

Cyclic Stress amplitude

  amp 

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 max   min 

2  max   min  2


DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Stress at a joint location Cyclic stress range

1   1  ( 2 )

 2   1  ( 3 )  3   1  ( 4 )  4   1  ( 5 ) 5   1  ( 6 )  6   1  ( 7 )  7   1  ( 8 ) Maximum stress range

 max   5

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE ANALYSIS FLOW CHART STATIC ANALYSIS Geometry + Gravity loads +Environmental loads

Pile Soil Interaction psiinp.J1

sacinp.J1_static

FATIGUE ANALYSIS Static Analysis

Fatigue input occurrence data

psicsf

ftginp.J1 (MMN)

Fatigue Analysis ftglst

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE DETERMINISTIC FATIGUE ANALYSIS USING SACS Deterministic fatigue analysis requires fatigue loads to be expressed as a finite number of discrete events. A discrete numbers of waves of varying i h height i ht and d period i d are provided id d over a 25 year period. i d The subsequent deterministic fatigue analysis consists of the following steps:  Calculate the directional distribution of each direction.  Carry out a static analysis with non linear soil file, file creating the PSICSF files.  Use the Fatigue module to determine the cumulative fatigue damage for each joint from the PSICSF file combined with fatigue input. 22-Jul-13

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Carry Out Static Analysis With Pile-soil Interaction The simple static analysis with non linearised foundation is carried out in order to generate the PSICSF file, which are required for the final fatigue analysis. Pile head has been considered as a boundary condition. The static analysis includes the following,   

Jacket Model including top sides. Hydrodynamic coefficients & Marine growth Fatigue wave loads ( for all 8 directions)

The Airy’s wave theory was used to generate the member loads used in the analysis. analysis 8 load cases (LOADCN) are generated for each wave. wave The wave forces were calculated based on 8 positions (steps) through each wave, which results in a total of 512 analysed load cases. ‘AL’ mentioned for Load case generate for each wave position. position wave spectrum data for that direction: spectrum type, individual wave height, dominant period, and spectrum parameters. 22-Jul-13

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Static Analysis Input file

Kinematic factor

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Wave Height

Wave period

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Wave Direction

Wave Step size


Static Steps & Critical Position

DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Carry out Fatigue Analysis Create Fatigue Run Input File The fatigue run input file defines the number of waves of each height impinging on the structure from each direction, the design life of the structure, S-N curve options, SCF options, member selections, joint overrides, spectrum options and so on. General Options The options line specifies a 25 design life for fatigue, with yearly wave f frequencies, i a default d f l S-N S N curve off API RP 2A, 2A no fatigue f i checks h k for f non-tubular members, Efthymiou SCF formulation with load path dependence, and so on. The JNTSEL and GRPSEL lines can be used to exclude tubular deck members. 22-Jul-13

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Carry out Fatigue Analysis Transfer function Options A long term stress distribution was derived based on the defined wave exceedance and the results of the wave/structural analysis, analysis for 8 wave headings with even probability for all directions and 8 waves per direction. The wave forces were calculated based on 8 positions (steps) through each wave, which results in a total of 512 analysed load cases.  Each fatigue case (FTCASE) corresponds to occurrence based on scatter wave height & wave period.  Each fatigue case (FTCASE) is defined by 2 TRFN lines that contain the 8 load case for one occurrence (simillary for 64 occurrences).  Dynamic Amplification is taken as 1.00  Selection of Stress Calculation type is Maximum and Minimum stress for analysis ‘MMN’ method.

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Fatigue case type : 'MMN‘ MMN Method  Deterministic analysis, the stresses are the difference between the maximum and minimum values at each joint due to several positions of a single wave. wave  To use this option, a SACS analysis is performed for several positions of the load.  Load case on the 'FTCOMB' lines should be for a different position of the same wave.  ‘MMN’ – For each wave, the stress range is to be taken as the difference of the maximum and the minimum HSS of the load cases specified p on the FTCOMB input p line.  For example, the user may run Seastate to produce load cases for 4 positions of the wave causing maximum shear and 4 positions causing minimum shear.. For example, the following input file may be used when 4 maximum and 4 minimum wave crest positions are saved.

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Fatigue Analysis Input file Design Life

Life safety factor S-N curve

No. of Occurrences Stress Calc. Type

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SCF option

DETERMINISTIC FATIGUE ANALYSIS PROCEDURE Combine Analysis for fatigue life determination This final step in carrying out the fatigue analysis requires the following files:  Fatigue run input file  PSICSF file In the Post Processing section (“Post”) of the Run file Wizard, select Fatigue. St t the Start th wizard i d and d choose h th fatigue the f ti run input i t file. fil Th wizard The i d will ill prompt you to select the PSICSF file. Run the analysis, and examine the FTGLST file for a listing of the joints in order of decreasing damage. damage A damage value greater than one indicates a failure to reach the desired design life.

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DETERMINISTIC FATIGUE ANALYSIS PROCEDURE DETERMINISTIC FATIGUE ANALYSIS RESULTS Joint Name

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Joint Type

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Fatigue Damage


Fatigue life

1 Deterministic Fatigue Analysis [Compatibility Mode]

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