Export SCR Riser Analysis Report_Kim Young Tae

7/29/2019 Export SCR Riser Analysis Report_Kim Young Tae

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INDEPENDENCE HUB-MC920

EXPORT GAS SCR DESIGN

ANLALYSIS REPORT

GEM

20120941

KIM, YOUNG TAE


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Contents1. INTRODUCTION .......................................................................................................................... 4

1.1 General ..................................................................................................................................... 4

1.2 Executive Summary.................................................................................................................. 4

2. PROJECT DOCUMENT AND DESIGN CODES ......................................................................... 4

2.1 Project Documents ................................................................................................................... 4

2.2 Design Codes and Standards .................................................................................................... 5

3. DESIGN DATA (RISER DESIGN BASIS & METHODOLOGY) ............................................... 5

3.1 Steel Riser data ......................................................................................................................... 5

3.2 SCR Porch Location & Hang-Off Angles ................................................................................ 6

3.3 Flex-Joint .................................................................................................................................. 6

3.4 Strake Properties ....................................................................................................................... 7

3.5 Hydrodynamic Coefficient for Strength and Interference Analysis ......................................... 8

3.6 Hydrodynamic Coefficients for Fatigue Analysis .................................................................... 8

3.7 Internal Fluid Data, Export SCR .............................................................................................. 8

3.8 Environmental Data .................................................................................................................. 9

3.8.1 Sea Water Properties ............................................................................................................. 9

3.8.2 Soil Data ............................................................................................................................... 9

3.8.3 Current Data .......................................................................................................................... 9

4. DESIGN METHOD ...................................................................................................................... 11

4.1 General ................................................................................................................................... 11

4.2 Static Analysis ........................................................................................................................ 12

4.3 Dynamic Analysis .................................................................................................................. 12

4.4 Input Data for Shear7 Software .............................................................................................. 13

5. ANALYSIS ................................................................................................................................... 14

5.1 Free Strake (Bare pipe) Riser System [Current Case I] .......................................................... 14

5.2 Riser System with Strake [Current Case I] & [Nonlinear Stiffness] ...................................... 16

5.3 Riser System with Strake [Current Case II] & [Nonlinear Stiffness] ..................................... 19

5.4 Riser System with Strake [Current Case III] & [Nonlinear Stiffness] ................................... 20

5.5 Riser System with Strake [Current Case I] & [Single Value Stiffness] ................................. 24

5.5.1 Top vs. Bottom Strakes ....................................................................................................... 24

5.5.2 Top vs. Separated Strakes ................................................................................................... 25

5.5.3 Eddy & Hurricane Current (100-Year) Condition .............................................................. 27

5.5.4 DNV E & DNV C Curve based on Eddy Current ....................................................... 28

6. Summary ....................................................................................................................................... 29

7. References ..................................................................................................................................... 30


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Table 1 Strake Design ......................................................................................................... 4

Table 2 Fatigue Design Life with Strake (years)................................................................. 4Table 3 Current Speed for SEHAR 7 Beam 3 Example Cases ...................................... 10

Table 4 100 Year Loop Current Eddy Profile (DEEPSTAR IIA Project: "Steel Catenary

Riser Performance On A Floating Production System, 1996) ................................... 11

Table 5 100 Year Hurricane Current Profile (DEEPSTAR IIA Project: "Steel Catenary Riser

Performance On A Floating Production System, 1996) ............................................ 11

Table 6 Design Basis Recommended Value ...................................................................... 13

Table 7 SHEAR7 Recommended Value ............................................................................ 14

Table 8 OMFD & Fatigue Life for Free Straked Riser ..................................................... 16

Table 9 OMFD & Fatigue Life for Straked Riser [Current Case I & Non-Linear F.J.

Stiffness] ................................................................................................................... 18

Table 10 OMFD & Fatigue Life for Straked Riser [Current Case II & Non-Linear F.J.

Stiffness] ................................................................................................................... 19

Table 11 OMFD & Fatigue Life for Straked Riser [Current Case III & Non-Linear F.J.

Stiffness] ................................................................................................................... 22

Table 12 Fatigue Life for Current Case I~III .................................................................... 22

Table 13 OMFD & Fatigue Life Near TDP for Straked Riser [Current Case III & Non-

Linear F.J. Stiffness] ................................................................................................. 23

Table 14 Fatigue Life Near TDP for Current Case I~III ................................................... 23

Table 15 OMFD & Fatigue life with Different Allocation of Strakes ............................... 26

Table 16 OMFD & Fatigue Life Extreme GOM Condition [DNV C Curve] ................ 27

Table 17 Comparative Table DNV "E" vs. DNV "C" ....................................................... 28

Table 18 S-N curves in seawater with cathodic protection (DNV-RP-C203, 2012) ......... 28

Figure 1 Current Profile for 5 cases .................................................................................. 10

Figure 2 Fatigue Life of Free Strkaked Riser .................................................................... 15

Figure 3 Comparative Table for Current Case I&II .......................................................... 20


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1.INTRODUCTION1.1 General

This document presents the VIV fatigue analyses required to achieve the fatigue design life for the

export riser. Steel Catenary Risers (SCRs) is employed for the export lines. The detailed project

description is provided in the Riser Design Basis & Methodology.

1.2 Executive Summary SCR with Strakes Strakes type: 16D x 0.25D Strake Coverage: 80 %

Table 1 Strake Design

Type Start Stop

Strake 0ft 2000 ft.

Riser 2000 ft. 3000 ft.

Strake 3000 ft. 9000 ft.

Riser (Flow line) 9000 ft. ~

Table 2 Fatigue Design Life with Strake (years)

Current Type 80 % coverage

Beam 3 example 425.24

Eddy Current 177.6

Hurricane Currrent 585.86

2.PROJECT DOCUMENT AND DESIGN CODES2.1 Project Documents

The following design document shall govern the design of the export riser for the initial design.

Riser Design Basis & Method


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2.2 Design Codes and StandardsDet Norske Veritas (DNV )

DNV-RP-C203 Fatigue Design of Offshore Steel Structures

3.DESIGN DATA (RISER DESIGN BASIS & METHODOLOGY)3.1 Steel Riser data (p.13)

Steel Riser Data

Riser pipe outer diameter (in) 20.000

Wall Thickness (in) 1.210

Corrosion Allowance:

Internal (in) 0.05

External(in) none

Wall Thickness Tolerances:

Wall thickness tolerance range +20~-8%

Average dry weight (% of Nominal) 108%

Ovality +0.75%/-0.25%

Material Properties

Material Properties API X-65

Density(lb/ft3) 490

Minimum yield strength (ksi) 65

Young's Modulus (ksi) 29700

Shear Modulus (ksi) 11423

Tangent Modulus (ksi) 66.3

Anti-corrosion coating

Strake / Faring region

FBE Thickness (in) 0.016

Density(lbs/ft3) 87

Touchdown Zone region

TLPE Thickness (in) 0.1


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Density(lbs/ft3) 87

Bare Pipe Region

FBE Thickness (in) 0.016

Density(lbs/ft3) 87

S-N Curve (p.40) DNV E curve (single slope)

SCF (p.40) 1.2

3.2 SCR Porch Location & Hang-Off Angles (p.14)

Identification

Porch Co-ordinates Azimuth

Angle

Hang-Off

Angles

(deg)X(ft) Y(ft) Z(c)

Gas Export

SCR17.50 120.67 15.00 325 12

3.3 Flex-JointSingle value stiffness for flex-joint will be adjusted for this analysis according to the Riser Design

Basis & Methodology, but Flex-joint stiffness curve data will be considered for the analysis too.

The details of data is as following:

Table-Flex-Joint Stiffness Single Value data for SCR analysis (p.18)

Riser Type 20-inch SCR

Fatigue Analysis (small angle) 25 kips-ft.

Table-Flex-Joint Stiffness Curve data for 20-inch Export SCR Analysis (p.16)

Alternating F.J. Angle

(deg)

Max Design Rotation Stiffness

(kips-ft./deg)

Unit

(kips-ft.)

0.01 436.411 4.36411

0.02 358.097 7.16194

0.03 318.974 9.56922

0.04 293.836 11.75344

0.05 275.711 13.78555

0.06 261.734 15.70404


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0.07 250.471 17.53297

0.08 241.108 19.28864

0.09 233.139 20.98251

0.1 226.234 22.6234

0.2 185.637 37.1274

0.3 165.355 49.6065

0.4 152.324 60.9296

0.5 142.928 71.464

0.6 135.682 81.4092

0.7 129.844 90.8908

0.8 124.990 99.992

0.9 120.859 108.7731

1 117.280 117.28

1.5 104.466 156.699

2 96.234 192.468

3 85.720 257.16

4 78.965 315.86

5 74.094 370.47

6 70.338 422.0288 64.794 518.352

10 60.797 607.97

15 54.155 812.325

20 49.887 997.74

25 46.810 1170.25

3.4 Strake Properties (p.19)The strake type for achieving the VIV suppression is adopted for export riser. The data is presented

as following:

Strake Properties

Section weight in air (lbs. /ft.) 48.4

Section weight in water (lbs. /ft.) 5.3

Barrel Outside diameter (in) 22.362

Barrel thickness 0.098


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Strake Height (0.25D) (in) 5.591

Strake Pitch (16D) (in) 357.8

3.5 Hydrodynamic Coefficient for Strength and Interference Analysis (p.20)

parameter

strength and interference analysis

Bare pipe Straked section

Normal drag 1.2 2.6

Tangential drag 0.0 0.05

Normal inertia 2.0 2.5

Normal added mass 1.0 1.5

Tangential added

mass0.0 0.05

3.6 Hydrodynamic Coefficients for Fatigue Analysis (p.20)

parameter

Fatigue load cases

Bare pipe Straked section

Normal drag 0.7 2.6

Tangential drag 0.0 0.05

Normal inertia 2.0 2.5

Normal added mass 1.0 1.5

Tangential added

mass0.0 0.05

3.7 Internal Fluid Data, Export SCR (p.21)Load case & parameter

shut-in condition

pressure (psig) 3250

density (lb./ft3) 12.5

normal operating conditions for fatigue assessment



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density (lb./ft3) 12.5

hydrotest conditions


density (lb./ft3) 64

installation conditions

pressure (psig) ambient

density (lb./ft3) void

3.8 Environmental Data3.8.1 Sea Water Properties (p.23)

Water depth 8000 ft.

Sea water density 64 lbs./ft3

3.8.2 Soil Data (p.26)undrained shear strength 50 lbs./ft2

submerged unit weight 20 lbs./ft3

Friction coefficients

Longitudinal 0.5

Transverse 1

Soil Stiffness (lbs./ft./ft.)

Vertical 23500 lbs./ft./ft.

Lateral (VIV purposes) 16215 lbs./ft./ft.

3.8.3 Current DataCurrent data was assumed based on the data of SHEAR 7 Beam 3 example. Five current cases

were prepared to perform SCR analysis because current profile in Beam 3 example cant be

convinced to represent GOM current.

Case I: Uniform current below half of the water depth with 1.0 ft. /s

Case II: Uniform current below half of the water depth with 0.8 ft. /s

Case III: Sheared current below from the top of the sea level


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Table 3 Current Speed for SEHAR 7 Beam 3 Example Cases

Case I Case II Case III

depth(ft.) current speed (ft./s)

-160 4.3 4.3 4.3

-532 4.29 4.29 3.892

-1068 2.42 2.42 2.42

-2000 1.49 1.49 1.39

-3892 1.01 1.01 1.49

-4000 1 1 1.49

-4800 1 0.8 1.35

-8000 1 0.8 1

Figure 1 Current Profile for 5 cases

Including the example current study, the additional case study with 100-year loop current eddy

profile and Hurricane current profile current data were performed to check for practical purpose.

These data shown below table have been taken from the Deepstar JIP. (INTEC, 2006)

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

00 2 4 6 8

depth

current speed (ft./s)

Current Profile

Case I

Case II

Case III

100-Year Loop Current

EDDY Profile (ft./s)

100-Year Hurricane Current

Profile (ft./s)


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Table 4 100 Year Loop Current Eddy Profile (DEEPSTAR IIA Project: "Steel Catenary Riser

Performance On A Floating Production System, 1996)

Depth (ft.)Case IV. 100-Year Loop Current EDDY

Profile (ft./s)

0 6.76

300 6.25

500 2.54

1000 2.37

1500 0.85

2000 0.34

3000 0.34

6000 0

Table 5 100 Year Hurricane Current Profile (DEEPSTAR IIA Project: "Steel Catenary Riser

Performance On A Floating Production System, 1996)

Depth (ft.)CASE V. 100-Year Hurricane Current

Profile (ft./s)

0 4.2

190 4.2

272 0

6000 0

4.DESIGN METHOD4.1 General

The VIV analysis of the Independence Hub SCRs shall be performed to assess the fatigue

performance of the SCRs to different five types of current events.

Fatigue analysis are performed considering the purpose of the report. Shear7 is applicable program

to analysis fatigue design. Shear7 however needs a third party software because it does not an

internal routine for computing the natural frequencies and mode shaped of a general SCR, Flexible

riser and Umbilical with significant bending stiffness or complex configuration.


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The solution for that is to import the natural frequencies and mode shapes through a FEA software

such like an OrcaFlex, Flexcom. Natural frequencies and mode shapes should be written with file

*.mds file name for input Shear7. OrcaFlex has the function to generate the *.dat file and *.mds

file from the result of the static analysis, then which will be used for run Shear7 software.

(Introduction to VIV and SHEAR7, 2013)

Analysis FEM Software

Static Analysis OrcaFlex 9.6c

Modal Analysis OrcaFlex 9.6c

Dynamic Analysis Shear7

4.2 Static AnalysisThe main purpose of the static analysis is to generate the equilibrium profile of riser under the

combined effects of self-weight, buoyancy, inner fluid, VIV suppression devices weight and

current. The result is presented by calculating modal analysis.

The undamped natural modes of the SCR line is generated from the modal analysis using the

OrcaFlex. From the modal analysis generate the modes table with mode types, periods and mode

shape with respect to each mode. For convenience 200 number of modes are considered and

transverse types among these are used to the input for dynamic analysis

OrcaFlex calculates the natural modes of the discretized model, not those of the real continuous

system. However the discretized modes are close to the continuous ones and for a mode number

the accuracy improves with increasing elements.

4.3 Dynamic AnalysisShear7 performs dynamic analysis with the result of the modal analysis that mean to calculate

fatigue damage using VIV analysis. The procedure of dynamic analysis is as following.


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4.4 Input Data for Shear7 Software

Table 6 Design Basis Recommended Value

No. of elements 2000

Mode cut-off value 0.7

Structural Damping 0.003

Strouhal No.1 0.18

BandwidthSingle 0.4

Multi 0.2

1 Design basis recommend the value of Strouhal code with 200 curve has Strouhal numbers of

0.24 for Reynolds numbers above 90,000, 0.17 for Reynolds numbers below 20,000, and

intermediate numbers in between. Strouhal code 200 however has been disabled in the Shear7 v4.7

and then the recommend value from Shear7 v4.7 user guide is adopted. (VandiverKim, 2012)

Shear7 User Guide Recommended Value

Strake and bare pipe riser have different value of St, Cl table, Band width, etc. The values table 7

is recommended from Shear7 User Guide.

natural frequencyand mode shape

Findingpotentially

excited modes

Input power foreach mode

Modes abovecutoff

Excitation lengthcalculation

initial lift anddrag coefficients

Calculatingmodal input

power & outputpower

Modal power

balance: A/D

Adjust CL if notconverging

RMS displacement

and acceleration

RMS stress andfatigue life

Program output


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Table 7 SHEAR7 Recommended Value

Type Strake Bare

Ca 2 1

St 0.1 0.18

Cl table 5 1

Damp Coefficient 0.4, 0.5, 0.2 0.2, 0.18, 0.2

5.ANALYSIS5.1 Free Strake (Bare pipe) Riser System [Current Case I]

An initial analysis for riser pipe with no strakes is performed to study the fatigue life and stress at

the location of flexible joint and touchdown point. The seabed properties are presented in the

design basis. Linear model with stiffness is sued, so stiffness of the flexible joint will effects the

fatigue life of the system.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0.00 2.00 4.00 6.00

DEPTH

RMS Stress

Free stiffness Infinity stiffness

Nonlinear

Stiffness

Simple Value

0

50

100

150

200

250

300

350

400

450

500

0.00 2.00 4.00 6.00

DEPTH

RMS Stress near Top


Nonlinear

Stiffness

Simple Value


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Figure 2 Fatigue Life of Free Strkaked Riser

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0.00 0.50 1.00 1.50 2.00

DEPTH

Damage Rate (1/yr)


Nonlinear

Stiffness

Simple Value

0

50

100

150

200

250

300

350

400

450

500

0.00 1.00 2.00

DEPTH

Damage Rate Near Top

(1/yr)


Nonlinear

Stiffness

Simple Value

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fatigue Life (yr)

Free stiffness Infinity stiffness Nonlinear

Stiffness

Simple Value


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Table 8 OMFD & Fatigue Life for Free Straked Riser

F.J Type OFMD x/L occurred OMFD Fatigue Life

Free stiffness 0.960 0.992 1.042

Infinity

stiffness29.400 0.000 0.034

Nonlinear

Stiffness2.888 0.000 0.346

Simple Value 0.961 0.992 1.041

The result shows the effect of the stiffness of the flexible joint on the hang-off region. Free stiffness

condition give the largest fatigue life with both of top and touch down point. Infinity stiffness has

the worst result regarding the fatigue life, furthermore the damage rate of the top point is larger

than the touch down point. Accordingly flexible joint to reduce bend stiffness should be designed.

The non-linear stiffness case with having various value responding the angle has also the negative

effect to the fatigue life.

5.2 Riser System with Strake [Current Case I] & [Nonlinear Stiffness]The analysis shows the effect of the VIV suppression device according to the length of the coverage

with strake. The analysis is performed with current case I and nonlinear stiffness.

0.00

0.20

0.40

0.60

0.80

1.00

0 2000 4000 6000 8000 10000

RMS Stress

Strake10 Strake30 Strake50 Strake70 Strake90


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0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 100 200 300 400 500 600 700 800 900 1000

RMS Stress Near Top


0.00

0.50

1.00

1.50

2.00

2.50

8000 8500 9000 9500 10000

RMS Stress Near TDP



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Table 9 OMFD & Fatigue Life for Straked Riser [Current Case I & Non-Linear F.J. Stiffness]

Strake

CoverageStrake10 Strake30 Strake50 Strake70 Strake90

OMFD 0.38835 0.32208 0.25515 0.11839 0.0008025

x/L

occurred

OMFD

9925.3 9955 9974.8 9984.7 0

Fatigue

Life2.57 3.1 3.92 8.45 1246.18

This result shows that more coverage with strake enhance the fatigue life. The fatigue life however

is not satisfy the design life considering safety factor (400 yr.) except the case of 90 percent

coverage. It can be explained that constant current speed of 1ft/sec below the half of the water

0

2000

4000

6000

8000

10000

0.00E+00 5.00E-03 1.00E-02 1.50E-02 2.00E-02

Damage Rate (1/yr)

Strake10

Strake20

Strake50

Strake85

Strake90


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depth gives constant shedding frequency. Single shedding frequency would increase RMS stress

in the focused power-in region.

5.3

Riser System with Strake [Current Case II] & [Nonlinear Stiffness]

Table 10 OMFD & Fatigue Life for Straked Riser [Current Case II & Non-Linear F.J. Stiffness]

Strake

CoverageStrake30 Strake50 Strake70 Strake90

OMFD 0.084774 0.063352 0.012645 0.000806

x/L

occurred

OMFD

9955 9974.8 9984.7 0

Fatigue

Life 11.79607 15.78482 79.08264 1241.465

0

2000

4000

6000

8000

10000

0.00E+00 2.00E-03

Damage Rate

(1/yr)

Strake30 Strake50

Strake70 Strake90

0

40

80

120

160

200

0.00E+00 2.00E-03

Damage Rate

Near Top (1/yr)

Strake30 Strake50

Strake70 Strake90

8000

8500

9000

9500

10000

10500

0.00E+00 2.00E-03

Damage Rate

Near Bottom

(1/yr)

Strake30 Strake50

Strake70 Strake90


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Figure 3 Comparative Table for Current Case I&II

Figure 3 present that less uniform current speed would increase the fatigue life of the riser system,

but still coverage strake less than 70 % is not enough large to satisfy the requirement of the design

life. But installation of the 90 % coverage of the strake could be solution to prevent the fatigue

damage from VIV.

5.4 Riser System with Strake [Current Case III] & [Nonlinear Stiffness]

Case I Current

Case II Current

-100

400

Strake30 Strake50 Strake70 Strake90Fatigue

Life

(yr)

Fatigue Life Current Case I & II

Case I Current Case II Current

0

2000

4000

6000

8000

10000

-1.00E-03 1.00E-03 3.00E-03 5.00E-03 7.00E-03 9.00E-03 1.10E-02 1.30E-02 1.50E-02

Damage Rate (1/yr)

Strake30

Strake50

Strake70

Strake90


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0

50

100

150

200

250

300

350

400

450

500

-1.00E-03 1.00E-03 3.00E-03 5.00E-03 7.00E-03 9.00E-03 1.10E-02 1.30E-02 1.50E-02

Damage Rate Near Top (1/yr)

Strake30

Strake50

Strake70

Strake90

8000

8500

9000

9500

10000

10500

-1.00E-03 1.00E-03 3.00E-03 5.00E-03 7.00E-03 9.00E-03 1.10E-02 1.30E-02 1.50E-02

Damage Rate Near Bottom (1/yr)

Strake30

Strake50

Strake70

Strake90


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Table 11 OMFD & Fatigue Life for Straked Riser [Current Case III & Non-Linear F.J. Stiffness]

Strake


OMFD 0.017647 0.0055057 0.0057614 0.0059242

x/L

occurred

OMFD

0 0 0 0

Fatigue

Life

56.66 181.63 173.57 168.80

Table 12 Fatigue Life for Current Case I~III

Strake


Current I 3.10 3.92 8.45 1246.18

Current II 11.80 15.78 79.08 1241.46

Current III 56.67 181.63 173.57 168.80

In case of sheared current the fatigue life is noticeably increased compared to the case I and II. It

is should be noted that the OMFD of current case III occurred at the location of flexible joint, not

touch down point. The vulnerable point of the fatigue is changed into the flexible joint location.

0.00 200.00 400.00 600.00 800.00 1000.00 1200.00 1400.00

Strake30

Strake50

Strake70

Strake90

Fatigue Life Current Case I ~III

Case III Current Case II Current Case I Current


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Although the sheared current reduced the fatigue damage rate of the touch down point but did not

so significantly on the flexible joint location.

Table 13 OMFD & Fatigue Life Near TDP for Straked Riser [Current Case III & Non-Linear F.J.

Stiffness]

Strake


OMFD 0.014274 0.00047415 0.000146850.00002712

4

x/L

occurred

OMFD

9955.00 9974.80 9984.70 9960.50

Fatigue Life 70.06 2109.04 6809.67 36867.72

Table 14 Fatigue Life Near TDP for Current Case I~III

Strake

Coverage Strake30 Strake50 Strake70 Strake90

Case I

Current3.10 3.92 8.45 135233.00

Case II

Current11.80 15.78 79.08 56293.00

Case III

Current70.06 2109.04 6809.67 36867.72


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As a result of this parametric study it is severely conservative to use the non-linear data of flexible

joint for fatigue analysis. According to the design basis single value stiffness shall be used for VIV

analysis. The analysis with simple value of flexible joint is performed and present the result as

follows.

5.5 Riser System with Strake [Current Case I] & [Single Value Stiffness]Top Strakes vs. Bottom Strakes with 70% coverage The Single value stiffness of the flexible joint

will reduce the fatigue damage rate. From this result of study most harsh current profile (Case I)is adopted for VIV design and this chapter will present several parametric study.

For designing the VIV suppression device it is most important to decide the length and location of

strakes considering both of safety and economic sense.

5.5.1 Top vs. Bottom StrakesStrakes installed from top give the considerably different result with bottom strakes. In this current

case bottom strakes could reduce the fatigue damage at the touch down zone, but the fatigue

damage rate at near flexible joint still remain large compared to the top strakes.

0.00 500.00 1000.00 1500.00 2000.00 2500.00 3000.00 3500.00 4000.00

Strake30

Strake50

Strake70

Fatigue Life Near TDP

Case III Current Case II Current Case I Current


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5.5.2 Top vs. Separated StrakesTop section covered 40% Strakes and bottom section with 30% strakes design is compared to the

continuous top section strakes with 70%. Top strakes design does not cover the uniform currentprofile area fully, it thus is not good for the fatigue life in touch down area. To suppress VIV

effectively the installation of strakes in the constant current area would be considered.

Adjustment of the strakes allocation along with riser system

As appears by below parametric study the design allocated more strakes in the bottom area enhance

the fatigue life than vice versa. From the result it is reasonable to design of strakes in the constant

current area including top strakes.

0

2000

4000

6000

8000

10000

0.00E+005.00E-021.00E-011.50E-012.00E-012.50E-01

FATIGUE DAMAGE RATE

Top vs. Bottom Strakes

Strake70Top Strake70Bottom

0

2000

4000

6000

8000

10000

0.00E+00 1.00E-03 2.00E-03 3.00E-03

Top vs. Seperated

Strakes

Stake40Strake30 Strake70Top


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Table 15 OMFD & Fatigue life with Different Allocation of Strakes

Strake

Coverage

Stake40

Strake30

Strake70

Top

Stake30

Strake40

Strake70

Bottom

Stake30

Strake50

Strake20

Strake60

OMFD 0.01739 0.11841 0.01333 0.20152 0.00321 0.00235

x/L 9939.60 9984.70 9935.20 514.80 9945.10 9945.10

Fatigue

Life57.49 8.45 75.00 4.96 311.12 425.24

As the result of the study the design with 20% and 60% each top and bottom separately of strakes

is the best solution. To confirm the design and compare as-built value of strakes for Independence

Hub additional check is performed based on the practical data with GOM current profile in harsh

environmental study. Examples of current

8000

8500

9000

9500

10000

10500

0.00E+00 1.00E-03 2.00E-03 3.00E-03

Seperated Strakes

with 80% Coverage

Stake30Strake50

Strake20Strake60

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0.00 0.20 0.40

RMS Stress

Stake30Strake50

Strake20Strake60


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5.5.3 Eddy & Hurricane Current (100-Year) ConditionTable 16 OMFD & Fatigue Life Extreme GOM Condition [DNV C Curve]

Strake Coverage Stake30Strake50Eddy Current

Stake20Strake60

Eddy Current

Stake30Strake50

Hurricane

OMFD 0.0060283 0.0056307 0.0017069

x/L 9945.10 9945.10 9945.10

Fatigue Life 165.88 177.60 585.86

As the result of the study with extreme condition of eddy and hurricane current, fatigue life is not

satisfied to the case of 100-year eddy current. To satisfy the eddy current condition the design of

strake would become very conservative.

0

2000

4000

6000

8000

10000

0.000 0.002 0.004 0.006

Fatigue Damage in Eddy

& Hurricane

Stake30Strake50Eddy

Stake30Strake50Hurr

Stake20Strake60EddyDNVE

0

2000

4000

6000

8000

10000

0.00 0.10 0.20 0.30 0.40

RMS Stress

in Eddy & Hurricane

Stake30Strake50Eddy

Stake30Strake50Hurr



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5.5.4 DNV E & DNV C Curve based on Eddy CurrentTable 17 Comparative Table DNV "E" vs. DNV "C"

Strake Coverage Stake20Strake60EddyDNV E Stake20Strake60EddyDNV C

OMFD 0.0056307 0.0015106

x/L occurred

OMFD9945.1 9945.1

Fatigue Life 177.59 661.98

RMS stress is calculated from the VIV analysis with Shear7, fatigue damage is then calculated based

on the specified S-N curve. When calculate the damage using the S-N curve, the stress range shouldbe defined. Stress Concentration Factor (SCF) scales the stress ranges, so SCF and S-N curve type

give significant effects the fatigue life. In this parametric study fatigue life calculated by S-N Curve

C are increased more three times than E curve. The DNV S-N curve is as follow figure.

Table 18 S-N curves in seawater with cathodic protection (DNV-RP-C203, 2012)


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6.SummaryAnalyses were performed for different cases of bared and Straked riser on several profile of current.

Hang-off region and touch down zone are vulnerable the fatigue damage from the analyses. Besides

fatigue life is very sensitive to the current profile. More accurate design data would be needed for

VIV suppression design from the result of this study. According to the as-built design the strake

coverage on the SCR of 8300 ft., which is about 80% of suspended catenary length, 10340 ft. (Conor

Galvin, 2007) As noted that current data give the effect to the fatigue life considerably, which current

data set is used for design very important.

30 current profiles were used to assess SCR VIV performance in FEED for the project. The result

of the FEED study is that the dataset was not sufficiently refined, yielding spurious damage

prediction. Therefor the dataset was refined further using filtered current data from a sample of

monitored data, recorded hourly for two years. And then large current profile dataset was used for

detailed design of the gas export SCR. The detail design gave the result that about 9,100 ft. of strakes

0

2000

4000

6000

8000

10000

-1.00E-031.00E-03 3.00E-03 5.00E-03 7.00E-03

Fatigue Damage with

DNV "E"& DNV "C"


Stake20Strake60EddyDNVC

0

2000

4000

6000

8000

10000

0.00E+001.00E-012.00E-013.00E-014.00E-01

RMS Stress

DNV "E"& DNV "C"


Stake20Strake60EddyDNVC


30/30

to achieve a satisfactory VIV fatigue life in the critical touchdown region. But during the project,

the strake coverage on the SCR was reduced slightly to a final as built design of 8,300 ft. by using

the welding procedure and S-N curve approaching a C curve with an SCF 1.1.

As a result applying the same DNV S-N curve the strake design also could satisfy all extreme cases

such as eddy current with high speed. For practical application real current profile which

investigated for I-Hub project and soil data would be required, then sensitivity study with soil could

be performed.

7.ReferencesConor Galvin, R. H. (2007). Independence Trail-Steel Catenray Riser Design and Materials. OTC.

(1996).

DEEPSTAR IIA Project: "Steel Catenary Riser Performance On A Floating Production System.

DNV-RP-C203. (2012). Fatigue Design of Offshore Steel Structures. DET NORSKE VERITAS.

INTEC. (2006). SCR Integrity Study. MMS.

Introduction to VIV and SHEAR7. AMOG.

RISER DESIGN BASIS & METHODOLOGY.

Vandiver, K. (2012). SHEAR7 USER GUIDE Version 4.7. AMOG.