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Agenda
Introduction
Riser Concept Feasibility Study
Basis of Design
SLWR Configuration
Extreme Storm Analysis
First Order (Wave) Fatigue Analysis
Riser Pre-lay Assessment
SLWR and Flexibles Riser Costing Comparison
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Introduction
Steel Catenary Risers often favoured for deepwater due to simplicity, versatility, reliability and cost
In deepwater Norwegian and UK North Seas, flexibles are more common due to harsh environment
In comparison, flexible risers can be limited by cost, pressure rating and integrity issues
Steel Lazy Wave Risers (SLWR) may be an alternative to flexibles but cost and technical performance needs evaluating
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Design Basis
Water Depth
The water depth at the proposed FPSO location is approximately 850m (2789ft)
Riser Pipe Specification
Flex Joint
A typical linear rotational stiffness is assumed for the Flex Joint
Parameter Value
Outer Diameter, mm (inch) 609.6 (24) Wall Thickness, mm (inch) 31.75 (1.25)
Corrosion Allowance, mm (inch) 3.0 (0.12) Material Grade API 5L X65
SMYS, MPa (ksi) 448 (65) SMUTS, MPa (ksi) 535 (77.6)
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Design Basis
Internal Fluid Densities and Pressure
Vortex Induced Vibration (VIV) Suppression
The analysis considers the top 250m of the riser to be covered with VIV suppression strakes
Vessel: Sevan Circular FPSO
Internal Fluid Internal Fluid
Density kg/m3 (ppg)
Pressure Bar (PSI)
Gas 140 (1.17) 135 (1,958) Seawater 1025 (8.55) 162 (2,350)
Production 150 (1.25) 200 (2,900) Slug 576 (4.81) 200 (2,900)
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Waves and Current Data Extreme Wave Data
Long Term Wave Data
Return Period (years) 1 10 100
Hs (m) 11.6 13.9 16.2 Tz (central) (s) 10.7 11.8 12.7 Tp (central) (s) 15.0 16.5 17.8
Hmax (3-hr) (m) 21.5 25.8 29.8 Tass (central) (s) 13.7 15.1 16.2
SWELL
Hs (m) 0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 10-11 11-12 12-13 13-14 14-15 15-16 16-17 17-18 18-19 19-20 20-21 21-22
0-1 0.05 1.33 6.67 8.67 8.00 6.47 4.40 2.93 1.67 0.93 0.45 0.21 0.09 0.04 0.02 0.01 0.00 0.00 - - - -
1-2 - - 0.13 2.07 4.27 5.67 5.47 4.80 3.47 2.33 1.40 0.80 0.44 0.23 0.11 0.05 0.03 0.01 0.01 0.00 0.00 -
2-3 - - - 0.08 0.62 1.53 2.07 2.27 2.07 1.60 1.13 0.73 0.45 0.25 0.13 0.07 0.03 0.02 0.01 0.00 0.00 0.00
3-4 - - - 0.00 0.05 0.30 0.66 0.93 1.00 0.93 0.73 0.51 0.33 0.19 0.11 0.06 0.03 0.01 0.01 0.00 0.00 -
4-5 - - - - 0.00 0.05 0.17 0.35 0.45 0.48 0.42 0.32 0.21 0.13 0.07 0.04 0.02 0.01 0.01 0.00 0.00 -
5-6 - - - - - 0.01 0.04 0.11 0.19 0.24 0.23 0.19 0.13 0.08 0.05 0.03 0.01 0.01 0.00 0.00 0.00 -
6-7 - - - - - 0.00 0.01 0.04 0.08 0.11 0.12 0.11 0.08 0.05 0.03 0.02 0.01 0.00 0.00 0.00 - -
7-8 - - - - - - 0.00 0.01 0.03 0.05 0.06 0.06 0.05 0.03 0.02 0.01 0.01 0.00 0.00 - - -
8-9 - - - - - - - 0.00 0.01 0.02 0.03 0.03 0.03 0.02 0.01 0.01 0.00 0.00 0.00 - - -
9-10 - - - - - - - 0.00 0.00 0.01 0.02 0.02 0.02 0.01 0.01 0.00 0.00 0.00 - - - -
10-11 - - - - - - - - 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 - - - -
11-12 - - - - - - - - - 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 - - - - -
12-13 - - - - - - - - - 0.00 0.00 0.00 0.00 0.00 0.00 0.00 - - - - - -
13-14 - - - - - - - - - - 0.00 0.00 0.00 0.00 0.00 - - - - - - -
14-15 - - - - - - - - - - - 0.00 0.00 0.00 - - - - - - - -
Tp (s)
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Hang-off Configuration
FPSO
MWL
15.70m Draft
29m
Flexible joint hang-off angle of 18deg relative to
FPSO vertical axis
Flexible joint hang-off location at base bottom edge of the main hull of
the FPSO
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Design Codes and Standards
Design Stage Codes & Standards Check
Pipe and Material Selection
API Spec 5L Material selection
Wall Thickness Sizing API-RP-2RD Burst, collapse, buckle propagation, load capacity, and reelability (strain)
Strength API-RP-2RD Serviceability, ultimate, and accidental
limit states
Fatigue DNV-GL-RP-0005
{DNV-GL-RP-C203} API-RP-2RD
Fatigue limit state, 1/Σ(damage)>design life
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Analysis Load Case Matrix
Riser Application
Riser OD (in)
Load Case Internal
Fluid Pressure
(bar)
Internal Fluid Density (kg/m3)
Mooring Line
Offset Environmental
Loading
API Allowable vM/Yield
Export 24
Operating Gas 135 140 Intact 6% 1 Year 0.67 Extreme Gas 135 140 Intact 10% 100 Year 0.8 Survival Gas 135 140 Failed 12% 100 Year 1
Installation Empty 0 0 Intact No Offset 1 Year 0.8 Hydrotest Sea Water 162 1025 Intact 3% 1 Year 0.9
Wave Fatigue
Gas 135 140 Intact No offset Long Term Wave
Loading n/a
Production 24
Operating Production
Fluid 200 150 Intact 6% 1 Year 0.67
Extreme Production
Fluid 200 150 Intact 10% 100 Year 0.8
Survival Production
Fluid 200 150 Failed 12% 100 Year 1
Installation Empty 0 0 Intact No Offset 1 Year 0.8 Temporary Slug 200 576 Intact 6% 1 Year 0.8 Hydrotest Sea Water 240 1025 Intact 3% 1 Year 0.9
Wave Fatigue
Production Fluid
200 150 Intact No offset Long Term
Wave Loading n/a
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Effective Tension
Compression of up to 53Te is found in the
sag bend region for gas export riser. Learn more at www.2hoffshore.com
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Extreme – Far Offset
Production riser overstressed by up to 4% at buoyant section.
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First Order Fatigue
Minimum fatigue life below the target life of
200 years in both the TDZ and hang-off region. Learn more at www.2hoffshore.com
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Hang-off Sensitivity Assessment
FPSO
MWL
15.70m Draft
2m
Flexible joint hang-off angle of 18deg relative to
FPSO vertical axis
Flexible joint hang-off location at base bottom edge of the main hull of
the FPSO
Hang-off location moved towards the centre of the hull
Worst load cases from the strength analysis assessed for the new hang-off location
Wave fatigue analysis is repeated
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Production Fluid - Extreme – Far Offset
Acceptable response for all the
dynamic analysis and riser configurations. Learn more at www.2hoffshore.com
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First Order Fatigue - Results
Fatigue life at the hang-off >200 years target
with a DNV C1 Class weld for the top 250m of the riser. Learn more at www.2hoffshore.com
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Riser Pre-lay Assessment
Stress response of the risers if pre-laid
on the seabed is within design limits. Learn more at www.2hoffshore.com
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Capex costs for flexible risers are expected to be similar to SLWR’s costs
Material Cost for Flexible > SLWR due to its complexity
Installation cost for SLWR > Flexible as a result of the offshore welding requirement
Engineering costs for SLWR > Flexibles mainly due to qualifications requirement
SLWR and Flexibles CAPEX Costing Comparison
Item % of the Total Cost
SLWR Flexible
Material cost per riser 18.8% 66.4%
Installation and transportation
43.9% 17.9%
Engineering, commissioning
37.4% 15.7%
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Conclusion
SLWR option is potentially feasible, however some issues such as hang-off fatigue, hydrotest and internal fluid variation need to be addressed
Compression at the sag bend region is expected, it should be confirmed that compression is acceptable
Low fatigue life in the hang-off region is expected;
Better weld quality (DNV C1 Class) in the first 250m of the riser from the hang-off can also be specified to improve fatigue performance
Cost for flexible are expected to be similar to the SLWR - Number of flexibles is result of limiting inner diameter
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Acknowledgements
2H Offshore
Gilles Gardner
Phil Ward
Rohit Shankaran
Norske Shell
Ali Anaturk
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