Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering
OMAE2007
June 10-15, 2007, San Diego, California, USA
OMAE2007-29049
WEIGHT OPTIMIZED SCR – ENABLING TECHNOLOGY FOR TURRET MOORED
FPSO DEVELOPMENTS
Elizabeth Foyt
Senior Engineer
2H Offshore Inc
Cary Griffin,
Senior Engineer
2H Offshore Inc
H. Howard Wang
ExxonMobil Development Company
1
Mike Campbell,
Manager - Engineering
Projects, 2H Offshore Inc
Wan C. Kan
ExxonMobil Development Company
Copyright © 2007 by ASME
INTRODUCTION
Steel catenary risers (SCRs) have been used successfully
with floaters such as TLPs, SPARs and Semis. Recently SCR
applications have been extended to FPSOs. However, the SCRs
have been attached at mid-ship of the FPSOs with milder
motion. SCRs in conjunction with a turret moored FPSO in
deepwater environments present significant design challenges.
The large vertical motions at the FPSO turret induce severe
riser response including compression and potential buckling at
the TDP area and result in difficulty meeting minimum fatigue
life requirements. Special efforts are needed to develop an
optimized, feasible SCR configuration to be used in
conjunction with a turret moored FPSO for West African
environment.
The feasibility of a conventional SCR on turret moored
FPSO is assessed initially and is confirmed to be difficult to
achieve the desired strength and fatigue requirements. Based on
previous industry work [1], an optimization study expands on
the idea of using varying weight, in the form of different
density coatings, along the length of the riser in order to
improve SCR strength and fatigue performance. The study
demonstrates that the SCR with weighted sections, which
improve SCR response at critical area, is a potential alternative
solution for the application of SCRs on turret moored FPSO.
Further study to understand why the mechanism of the
increased weight at the TDP area improves riser performance is
carried out, which is key to addressing the issues involved with
the feasibility of this type of application. The study also
determines the key parameter for SCR peak response, which
included static shape, dynamic motions and velocities. This
2
understanding would help provide further insight for future
optimization design work.
DESIGN DATA AND CRITERIA
A production SCR mounted to a turret moored FPSO in a
water depth of 1500m off West Africa is evaluated. The key
properties are as follow:
• 10.75in O.D., 1.0in (25.4mm) wall thickness;
• X65 steel;
• Insulated with 3.5in thick, 768kg/m³ syntactic foam.
Typical West African environmental data is used in the
analysis. The concept of the proposed SCR configuration with
weighted sections is shown in Figure 1.
1600
1400
Height Above Seabed (m)
ABSTRACT
Steel catenary risers (SCRs) used in conjunction with a
turret moored FPSO in deepwater environments present
significant design challenges. The large vertical motions at the
FPSO turret induce severe riser response. This results in
difficulty meeting strength and fatigue design criteria at the
Touch Down Point (TDP) and at the riser hang off location. It
is typically considered challenging to achieve feasibility for a
conventional SCR application on a turret moored FPSO.
Previous industry work for an SCR application used with
other floating hosts has demonstrated that SCR strength and
fatigue response can be improved using heavy and light
coatings strategically placed along the riser [1]. An
optimization study is performed, based on previous industry
work, which demonstrates that a weight optimized
configuration can enable the application of an SCR on a turret
moored FPSO. The effect of adding different coatings along the
length of the SCR is discussed. The position, length, and
density of the coating type are varied to determine an optimum
configuration for both strength and fatigue response. This paper
will also discuss observations which may help explain why
weighted sections can improve SCR response at the critical
area.
1200
Light - Low density coating (3.5in, 670kg/m^3)
Normal - As is (3.5in, 768.9kg/m^3)
Heavy - High density coating (3.5in, 2800kg/m^3)
1000
800
600
400
200
0
0
200
400
600
800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
Horizontal Distance from Vessel Attachment Point (m)
Light
Normal Riser
Heavy Riser
Figure 1 – SCR Configuration with Different Density
Coatings
The strength analysis is performed based on a 12-hour 100
year extreme swell condition. Extreme strength response is
considered to be acceptable if, along the entire riser, the
maximum von Mises stress does not exceed 80% of the yield
stress. It is also desirable to maintain positive effective tension
in SCR at all load cases. Three FPSO draft conditions are
considered including ballast, intermediate and fully loaded.
The target field life is 15 years so that the design fatigue
life needs to be 150 years or more with a safety factor of ≥10.
Fatigue analysis is conducted for 41 condensed swell bins at 3
dominant swell directions. Fatigue damage is calculated using
rainflow counting approach to reduce conservatism. Vessel
orientation is 15° from the direction of the swell or local sea
(165° vessel heading).
FEASIBLITY OF A CONVENTIONAL SCR
An initial attempt was needed to see if a conventional
SCR, without weight optimization, could be made feasible for
the turret moored FPSO application.
Preliminary strength analysis results showed compression
and unacceptable stresses with a wall thickness of 25.4mm as
given in Table 1. It was found that the hang off angle has to be
increased to 15 deg and the wall thickness to 31.8mm to
Copyright © 2007 by ASME
Riser
Description
Draft
Condition
Min
Effective
Tension
(kN)
Stress
Utilization at
TDP (%)
MINIMUM EFFECTIVE TENSION
100 Yr Swell with Associated Local Sea, Failed Mooring
165deg VH 15deg WH, Near Offset
2000
Minimum Effective Tension (kN)
marginally satisfy the strength. But the effective tension
condition is still not satisfied for the ballast draft condition.
The maximum stress utilization with a 31.8mm wall
thickness at the TDP is given in Table 1 and shown in Figure 2
for three different draft conditions. For the ballast draft
condition the maximum compression at the TDP is 146kN
resulting in a maximum stress utilization of 102%.
Wall Thickness = 31.8mm
Top Angle = 15 Deg
Attachment Heave Range ~ 10m
Cd = 1.1
1500
1000
500
0
-500
0
500
1000
1500
2000
2500
Distance from Vessel (m)
25.4 mm WT,
10 deg Top
Angle
Ballast
31.8 mm WT,
15 deg Top
Angle
Fully
Loaded
Intermediate
Ballast
Ballast
116
Intermediate
Fully Loaded
-99.5
Figure 3 – Minimum Tension at TDP
85
1.7
94
102
-18.6
-146
First order fatigue analysis is carried out using time
domain random sea analysis. The minimum fatigue life along
the length of the riser is shown in Figure 4 for three different
SN curves and given in Table 2.
Table 1 – Strength Results for Conventional SCR at TDP
S-N
Curve
B
C
D
VON MISES STRESS/YIELD STRESS
100 Yr Swell with Associated Local Sea, Failed Mooring
Zero Offset, Ballast Draft
0.9
0.7
Wall Thickness = 31.8mm
Top Angle = 15 Deg
Attachment Heave Range ~ 10m
Cd = 1.1
0.6
0.5
Table 2 – Fatigue Life at TDP for Conventional SCR
0.4
0.3
0.2
0.1
0
1500
1600
Base Case - 180deg VH, 15deg WH
165deg VH, 15deg WH
1700
1800
1900
Distance from Vessel (m)
180deg VH, 35deg WH
Allowable
2000
2100
180deg VH, -15deg WH
Figure 2 – Von Mises Stress at TDP
The minimum effective tension is shown in Figure 3 for
the different drafts considered. The cause of the high stress for
the ballast draft condition is due to large compressive tension in
the TDP region of the SCR, as a result of the large heave
motion at the hangoff location in the order of about 10m.
The fatigue life at the TDP area due to all swell bins is
below the target fatigue life of 150 years when considering the
use of the best weld “C” fatigue curve. The fatigue
performance may be improved due to variations in vessel drafts
during operating. This spreading effect may increase fatigue
life by a factor of about 2.5 at the TDP area. However, the
fatigue performance is still considered to be very marginal even
if the best weld fatigue curve is used.
UNFACTORED FATIGUE LIFE
41 Condensed Swell Sea States, SCF=1.1
1.0E+06
1.0E+05
Fatigue Life (Yrs)
Von Mises Stress/ Yield Stress
0.8
Min Fatigue Life at TDP
(Yrs)
227
62
15
1.0E+04
1.0E+03
1.0E+02
1.0E+01
1.0E+00
0
500
1000
1500
2000
2500
Distance from Vessel (m)
Swell - B
Swell - C2
Swell - D
Figure 4 – Fatigue Life of SCR for Different Curves
In summary, feasibility of a conventional SCR with turret
moored FPSO is questionable. Hence, as a means of improving
3
Copyright © 2007 by ASME
OPTIMIZATION OF SCR CONFIGURATIONS
Previous studies for SCR applications on other floating
hosts indicate that a heavy section in the sag-bend region and
light section along the touch-down region improves response
[1]. To improve the response of the SCR on a turret moored
FPSO in both strength and fatigue performance, an
optimization study on SCR configurations with weighted
sections is carried out. The general arrangement used as the
base case for this study is shown in Figure 5. An initial weight
optimized SCR configuration is adopted based on the use of
heavy 2800kg/m³ and light 650kg/m³ density coatings along the
length to vary the weight of the riser at key locations.
PRODUCTION SCR - WEIGHT OPTIMISATION CONFIGURATION DEFINITION
1600
Height Above Seabed (m)
1400
•
•
Removing the lightweight coating at the TDP;
Reducing overall weight of the heavy section whilst
still meeting design criteria.
While the strength performance is acceptable as the
coating density is decreased to 1500kg/m³, effective tension
approaches zero at the touch down zone, as shown in Figure 6.
A final configuration is chosen which uses an 1850kg/m³ heavy
coating along 425m of the sag-bend region and no light weight
coating.
PRODUCTION RISER - MINIMUM EFFECTIVE TENSION
100 Yr Swell, Ballast Draft Condition
2500
Minimum Effective Tension (kN)
the SCR response, an optimized SCR configuration is needed
to obtain a feasible solution.
2000
1500
1000
500
1200
Normal
Coating
0
1000
0
500
1000
1500
2000
2500
Distance From Vessel (m)
800
Heavy
Coating
600
Light
Coating
1850kg/m³ Heavy Coating Only (Conf S)
1500kg/m³ Heavy Coating Only (Conf R)
2800kg/m³ Heavy and Light Coatings (Conf I)
Normal
Coating
400
Figure 6 – Effective Tension along Riser Length
200
0
0
200
400
600
800
1000
1200
1400
1600
Horizontal Distance From Riser Attachment Point (m)
Figure 5 – Layout of Weight Optimized Sections
To obtain an acceptable configuration, a total of 9
configurations which vary the light and heavy section lengths
and thicknesses are evaluated for strength and fatigue response.
Of these, a configuration is found which satisfies both fatigue
and strength criteria. A comparison between the weight
optimized configuration and the conventional SCR is given in
Table 3.
Case
Conventional
Riser
Weight optimized
Max von Mises
Stress/Yield
Hangoff
TDP
Mini Fatigue Life
DoE C/2 (yrs)
Hangoff
TDP
0.50
0.82
372
31
0.57
0.48
318
264
Table 3 – Weight Optimized Configuration with Heavy and
Light Sections
Analysis of the multiple weight optimized configuration
indicates that a section of heavy coating closer to the
touchdown zone improves the strength and fatigue performance
of the SCR.
Following the development of a feasible weight optimized
configuration, further optimization is conducted by:
4
DESIGN OF SCR WEIGHTED SECTIONS
High density coating provides an acceptable solution from
a structural standpoint. Other weight options are desirable to
allow flexibility in cost and installation conditions. Hence,
alternative means of achieving increased weight are evaluated,
including:
• Increasing the J-lay collar diameter to meet weight
per-joint weight requirements
• A pipe-in-pipe configuration where an external steel
sleeve is added to the riser
• Increasing the wall thickness of the riser to meet
weight requirements
• A clump weight solution where a weight is added
during installation to each joint in the heavy section
The above solutions would all allow for proper insulation
of the riser. However, due to the amount of extra weight needed
per riser joint, increasing the weight by thickening the wall or
adding a pipe sleeve are not feasible; the required wall
thickness pipe could not be welded. Increasing the J-lay collar
thickness would also not be feasible due the 60in diameter
associated with a 2ft long collar. A qualitative comparison of
these options is listed in table 4.
Copyright © 2007 by ASME
Solution
Pros
Cons
High Density
Insulation
Even distribution
of weight
Limited proven
supplier
Increased Wall
Thickness
Even distribution
of weight
Required
thickness not
feasible
Steel Pipe Sleeve
J-Lay Collar
Modification
Clump Weight
Even distribution
of weight, retains
insulation
properties
Collar already
integrated, will not
slip or fall off
Cheap, deployable
through the
moonpool, can be
located to avoid
high SCF at welds
Sleeve too thick,
pipe ovality
mismatches
High cost, locally
high stresses over
welds
Further
development
required to
prevent sliding,
locally high
stresses
Table 4 – Pros and Cons of Weight Optimization Options
The most promising solution appears to be a clump weight
design which could be installed on an installation vessel, and
away from welds, as shown in Figure 7. It allows for the riser
to be fully insulated and potentially reduces procurement and
installation costs. Clump weights have been successfully
installed on other SCR application in the Gulf of Mexico in
order to improve riser response during extreme events [2].
deck with existing equipment and narrow enough to fit through
the J-lay tower. Only one clump weight is required per joint
over the heavy section.
Length
(m)
OD
(m)
1.000
1.261
ID
(m)
0.457
Weigh
t
(lb)
5520
Number
Require
d
34
Table 5 – Clump Weight Parameters
Strength and fatigue analysis is performed on a clump
weight solution with a weight distribution equal to the riser
with only heavy coatings and equivalent density of 2250kg/m³.
Strength response is found to be acceptable. However, the
fatigue performance is significantly reduced. The best welds
with “C” class fatigue performance are required in the TDP
area to satisfy the fatigue design requirements, as shown in
Table 4. This change is probably due to the additional drag
imparted from the clump weights.
The locally higher drag and higher weight caused by the
clump weights also decreases fatigue life along the clump
weight region, as seen in Figure 8. However, because the
relative dynamic motions are significantly lower and the fatigue
performance is higher, the decrease in life in this region is
acceptable.
Unfactored Fatigue Life (yrs)
TDP
Hang Off
(C Class)
(C/2 Class)
455
819
190
595
Sensitivity
Heavy Coating
Clump Weights
Table 4 – Effect on Fatigue Life due to Clump Weights
CLUMP WEIGHT VS. HEAVY COATING FATIGUE LIFE
C Fatigue Curve, TDP Fatigue Spreading
1000000
Fatigue Life (years)
100000
10000
1000
100
10
1
1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000
Distance From Vessel (m)
Clump Weights
Heavy Coating
Figure 8 – Fatigue Response of Clump Weight Riser
Figure 7 – 2H Design Clump Weight
As shown in Table 5, the weight and OD of the clump
weight are such that it is light enough to be manipulated on
5
SCR PEAK RESPONSE ASSESSMENT
The typical parameter used in design to predict peak
response of an SCR is the maximum wave height. However,
Copyright © 2007 by ASME
other parameters also drive the SCR response. A study was
performed to understand the correlation between the peak
response and the following parameters:
• Maximum and minimum wave height
• Maximum upward porch velocity
• Maximum downward porch velocity
• Maximum upward porch acceleration
• Maximum downward porch acceleration
• Hang off degree of freedom 1 through 6
Environmental loading conditions consist of 12 one hour
random sea analyses. Each random sea analysis uses a 100 year
swell conditions and associated current and a different random
seed. Extreme motions and SCR response are plotted against
each other in order to identify the correlation between peak
SCR response and the various vessel motion characteristics.
The relationship between maximum downward porch
velocity and minimum effective tension at the TDP for the SCR
is shown in Figure 9.
The correlation between significant wave height and minimum
TDP effective tension, which is a commonly used parameter to
determine response, is shown in Figure 10. The results show
that there is only moderate and considerably scattered
correlation between wave height and peak response.
STUDY OF THE WEIGHTED SCR CONFIGURATION
As discussed above adding distributed weight to the SCR
above the sag-bend region improves the SCR strength and
fatigue response. It is desirable to understand the reason why
the added weight improves the response of the SCR, which
may facilitate better SCR design. The mechanism behind the
improvements in response is investigated by performing a
detailed assessment of the SCR strength response.
The SCR response for three SCR configurations is
assessed including evaluation of: static shape, tensions and
bending moments, dynamic motions, and velocities. The results
are evaluated in a number of ways including time traces,
envelopes and snapshots. The SCR configurations including
normal, heavy coating, and clump weight are shown in Figure
11.
MAX DOWNWARD PORCH VELOCITY against MIN EFFECTIVE TENSION
700
TDP Minimum Effective Tension
(kN)
600
ExxonMobil - Turret Moored SCR Feasibility Study - Phase IV
500
PRODUCTION RISERS CONFIGURATION DEFINITION
Nominal Position - Top Angle = 15deg
400
Compression
1600
1600
1400
1400
1200
1200
300
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Max Downward Porch Velocity (m/s)
SEED01
SEED02
SEED03
SEED04
SEED05
SEED06
SEED07
SEED08
SEED09
SEED10
SEED11
SEED12
Figure 9 – Maximum Downward Porch Velocity vs. TDP
Minimum Effective Tension
The results show that maximum downward porch velocity is a
reliable indicator to capture the occurrence of minimum
effective tension at the TDP. The results show that compression
is expected to occur in the SCR for maximum downward porch
velocities greater than 2.5m/s.
1000
Normal
Riser
800
Heavy
Riser
600
Lump
Weight
Height above Seabed (m)
100
Height above Seabed (m)
200
1000
800
600
400
400
200
200
0
0
0
-200
-400
-600
-800
-1000
-1200
-1400
Horizontal Distance from Hang-Off (m)
0
-200
-400
-600
-800
-1000 -1200 -1400
Horizontal Distance from Hang-Off (m)
Figure 11 – SCR Configurations
Static Response
SCR static response shows that the addition of weight
increases tension but reduces TDP bending moments and
curvatures. The reduction in static TDP curvature for the heavy
coating and clump weight configurations is shown in Figure
12. The SCR static curvature at the TDP is reduced by a factor
of 0.8 with the introduction of the additional weight (See
Figure 16). As expected, the single lump mass causes a locally
high increase in curvature that can be reduced with the use of
smaller distributed lump masses.
MAX DOWNWARD PORCH VELOCITY against MIN EFFECTIVE TENSION
70
TDP Minimum Effective Tension (kN)
60
50
40
30
20
10
0
0.
0.
1.
1.
2.
2.
3.
3.
4.
4.
5.
Max Wave Height (m)
SEED01
SEED02
SEED03
SEED04
SEED05
SEED06
SEED07
SEED08
SEED09
SEED10
SEED11
SEED12
Figure 10 - Maximum Wave Height vs. TDP Minimum
Effective Tension
6
Copyright © 2007 by ASME
STATIC RESULTANT CURVATURE
DYNAMIC RISER SHAPE ENVELOPE
No Environmental Loading
45
3.0
40
35
Elevation above Seabed (m)
Curvature (1000/m)
2.5
2.0
1.5
1.0
30
25
20
15
10
0.5
5
0
1800
0.0
1700
1750
1800
1850
1900
1950
1850
2000
1900
1950
2000
2050
2100
Distance from Vessel (m)
Distance From Vessel (m)
Bare SCR
Heavy Coating
Clump Weight
Figure 12 – Static Curvature
MOTION RANGE ALONG SCR LENGTH
Middle of the Heavy Coating Section
Lump Weight Position
Vertical Motion Range (m)
12
TDP
TDP
vterm =
Where:
•
•
•
•
8
6
4
2
0
2
Bare SCR
3
Location
Along SCR4 Length
Location Along SCR Lenght
Heavy Coating
Clump Weight
SCR Velocity Response
The SCR velocity response is assessed to help
understand if it is a factor in the changing motion
characteristics. The terminal velocity and actual velocity along
the riser length is calculated. Terminal velocity of a free falling
body is the speed at which the surrounding fluid drag force
matches the pull of gravity, resulting in a constant fall rate. It is
defined as follow:
10
1
Bare SCR
Figure 14 - Dynamic Riser Shape Envelope in TDP Region
Dynamic Motions
Riser dynamic motion envelopes and snapshots of the
structure are investigated in order to determine how the motion
response of the various SCR configurations differs. The vertical
motion range extracted from 6 positions along the length of
each SCR is shown in Figure 13.
14
Heavy Coating
5
6
•
2W
C d Aρ fluid
vterm
- terminal velocity of the free falling body
W
- weight of the body
Cd
- drag coefficient
A
cross
sectional
projected
area
(perpendicular to flow direction)
ρfluid
- density of the surrounding fluid
Clump Weight
Figure 13 – Vertical Motion Range Along Riser Length
The addition of weight increases the motion response in
the region of the weight by a factor of 1.2, and consequently
reduces the motion response directly above the TDP by a factor
of 0.6. The reduced motion response in the TDP region of the
weight optimized SCRs results in lower velocities and
consequently the dynamic response is improved as shown in
Figure 14. Therefore, reduced loads and stresses occur in the
TDP region.
7
Terminal and actual velocity along the length of the SCR
for both configurations are shown in Figure 15 and Figure 16.
The actual vertical velocity response in the region of the added
weight of the SCR configuration with the heavy coating is
approximately 20% greater than the bare SCR, as shown in
Figure 15. Below the distributed weight, the velocity reduces to
50% of the value for the normal configuration. For the non
weight optimized SCR, without the additional weight, the
velocity is transferred to the TDP region. Hence, locally high
velocities, that exceed the terminal velocity, occur directly
above the TDP, where compression occurs.
Copyright © 2007 by ASME
ACTUAL VERTICAL VELOCITY
VERTICAL VELOCITY - ACTUAL AND TERMINAL
Heavy Coating Configuration
4.0
4.5
4.0
2.0
3.5
Vertical Velocity (m/s)
Vertical Velocity (m/s)
3.0
1.0
0.0
-1.0
Compression
-2.0
2.5
2.0
1.5
1.0
-3.0
0.5
-4.0
0
400
800
1200
1600
2000
2400
0.0
Distance from Vessel (m)
Bare SCR
1000
1400
1600
1800
2000
2200
2400
Distance from Vessel (m)
Terminal Velocity
Actual Velocity
Figure 17 - Terminal and Actual Velocities Heavy Coating
Configuration
Effect of Drag Coefficients on SCR Response
The effective tension at the TDP is lower for higher
drag coefficients and compression occurs when the actual
downward vertical velocity exceeds the terminal velocity by a
certain amount. For the non-optimized SCR, compression
occurs for drag coefficients of 0.8, 1.1 and 1.4, as shown
Figure 18.
VERTICAL VELOCITY - ACTUAL AND TERMINAL
Bare SCR Configuration
4.0
3.5
Compression
3.0
1200
Heavy Coating
Figure 15 - Actual Peak Vertical Velocity Envelope along
Riser Length
Vertical Velocity (m/s)
3.0
2.5
2.0
MINIMUM EFFECTIVE TENSION
1.5
Bare Pipe Drag Coefficient Sensitivity
Bare SCR Configuration
1.0
2500
0.0
1000
1200
1400
1600
1800
2000
2200
2400
Distance from Vessel (m)
Terminal Velocity
Actual Velocity
Figure 16 - Terminal and Actual Velocities of Bare SCR
Configuration
The addition of weight locally increases the terminal
velocity, allowing the riser to ‘fall’ faster and hence further
during cyclic downward vertical motion. For the heavy coating
configuration, the actual maximum downward velocity at the
TDP only slightly exceeds its terminal velocity, as shown in
Figure 17.
Thus, one possible explanation for TDP compression
is that if the hang-off forces the TDP region to move downward
faster than its terminal velocity, then the upward drag force in
this region tends to inflect the riser and the catenary’s shape is
locally deformed as a consequence of high compression.
8
Minimum Effective Tension (kN)
0.5
2000
1500
1000
500
0
-500
0
400
800
1200
1600
2000
2400
Distance From Vessel (m)
Cd=0.5
Cd=0.7
Cd=0.8
Cd=1.1
Cd=1.4
Figure 18 – Minimum Effective Tension for Different Drag
Coefficients – Bare SCR Configuration
Compression does not occur for the case with a drag
coefficient of 0.5 because the actual velocity is only slightly
higher than the terminal velocity as shown in Figure 19. For the
weight optimized configuration the minimum effective tension
is achieved for a drag coefficient of 1.4 and, as shown in Figure
19, compression does not occur even though the actual velocity
slightly exceeds the terminal velocity as discussed above.
Further work is required to better understand the physics of
these observations.
Copyright © 2007 by ASME
•
VERTICAL VELOCITY - ACTUAL AND TERMINAL
Bare Pipe Drag Coefficient Sensitivity
Bare SCR Configuration
4.0
•
Vertical Velocity (m/s)
3.5
3.0
2.5
2.0
1.5
1.0
•
0.5
0.0
0
400
800
1200
1600
2000
2400
Distance from Vessel (m)
Terminal - Cd = 0.5
Terminal - Cd = 0.8
Terminal - Cd = 1.1
Terminal - Cd = 1.4
Actual - Cd=0.5
Actual - Cd=0.8
Actual - Cd=1.1
Actual - Cd=1.4
Figure 19 - Terminal and Actual Velocities of Bare SCR
Configuration for Different Drag Coefficients
MINIMUM EFFECTIVE TENSION
Bare Pipe Drag Coefficient Sensitivity
Heavy Coating Configuration
•
The clump weight option, which may provide
flexibility from installation and cost perspective,
gives acceptable fatigue and strength response.
The study shows that there is a strong correlation
between maximum downward porch velocity and
peak SCR response. The maximum downward
porch velocity is a reliable indicator for the
occurrence time of the minimum effective tension
at the TDP.
The addition of a clump weight or heavy coating
appears to result in a reduced static curvature and
dynamic motion response in the TDP area of the
weight optimized SCRs.
TDP compression appears to be related to max
downward velocity that significantly exceed the
terminal velocity. Weight optimized SCRs may
enhance performance, in part, by changing the
terminal velocity profiles of the rises.
Minimum Effective Tension (kN)
2500
REFERENCES
2000
[1]. D. Karunakaran, Subsea 7, & T.S. Meiling, S.
Kristoffersen, & K.M. Lund, Statoil – “Weight Optimized
SCRs for Deepwater Harsh Environments”, OTC 2005.
1500
1000
500
0
0
500
1000
1500
2000
2500
Distance From Vessel (m)
Cd=0.5
Cd=0.7
Cd=0.8
Cd=1.1
Cd=1.4
[2]. M Vanderbossche & J. Brooks BP America, C. Masson %
J. Fang INTEC Engineering – “Design and Installation of the
Mardi Gras Large Diameter , Deepwater Steel Catenary
Risers”, IOPF 2006.
Figure 20 - Minimum Effective Tension for Different Drag
Coefficients – Heavy Coating SCR Configuration
CONCLUSIONS
The design challenges of utilizing an SCR on a turret
moored FPSO are addressed in this paper. An optimum
weighted SCR configuration is presented as an alternative
solution to these challenges. This configuration takes
installation flexibility and cost into account. In addition, the
physics behind the reasons for improvement in response have
also been discussed. The following conclusions can be drawn
from the study:
• With the implementation of optimized weighted
sections, SCR strength and fatigue response is
improved sufficiently to be within acceptable
limits provided high fatigue performance welds
be used.
• Static response shows that the addition of weight
increases tension but reduces TDP bending
moments and curvatures. Stress and fatigue
performance at the TDP is improved as weight
above the TDP increases.
9
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