export scr riser analysis report_kim young tae
TRANSCRIPT
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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
pressure (psig) 3250
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density (lb./ft3) 12.5
hydrotest conditions
pressure (psig) 4875
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
Free stiffness Infinity stiffness
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)
Free stiffness Infinity stiffness
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)
Free stiffness Infinity stiffness
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
Strake10 Strake30 Strake50 Strake70 Strake90
0.00
0.50
1.00
1.50
2.00
2.50
8000 8500 9000 9500 10000
RMS Stress Near TDP
Strake10 Strake30 Strake50 Strake70 Strake90
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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
CoverageStrake30 Strake50 Strake70 Strake90
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
CoverageStrake30 Strake50 Strake70 Strake90
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
CoverageStrake30 Strake50 Strake70 Strake90
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
Stake20Strake60EddyDNVE
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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"
Stake20Strake60EddyDNVE
Stake20Strake60EddyDNVC
0
2000
4000
6000
8000
10000
0.00E+001.00E-012.00E-013.00E-014.00E-01
RMS Stress
DNV "E"& DNV "C"
Stake20Strake60EddyDNVE
Stake20Strake60EddyDNVC
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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.