Steel Lazy Wave Risers - 2H Offshore
Transcript of Steel Lazy Wave Risers - 2H Offshore
Steel Lazy Wave Risers A Step Change in Riser Technology for the NWS
Tze King Lim, Dhyan Deka, Elizabeth Tellier, Hugh Howells
AOG 2018, Perth
15th March 2018
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Agenda
Lazy wave risers – an enabling technology for the North West Shelf (NWS)
Design drivers
Configuration development
Riser strength and fatigue
Installation feasibility
Reducing conservatism through measurements (STREAM JIP)
Summary
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Riser Design in the NWS
The Australian NWS is a challenging design environment:
Directional swell seastates
Tropical cyclones
10,000 year design criteria
Calcareous soils
Lack of existing subsea architecture
Large diameter gas lines
Long service lives
Harsh chemical environment
Temperature and pressure ranges
Flexible risers are limited by size, fluid pressure and long service requirements Learn more at www.2hoffshore.com
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Why SLWRs for NWS
Steel catenary risers (SCRs) could be a viable solution:
Simple concept and long track record
SCRs are subject to fatigue issues, especially with FPSOs or semisubs
Steel lazy wave risers (SLWRs) are a feasible solution:
Robust configuration
Allows flexibility (vessel type and top loads)
Field proven on BC10, Blind Faith, Julia. Liza & Mad Dog2 will have SLWRs
Overall cost of hull, mooring and riser can be lowered
Hang-off region
Simple catenary
Some operators have developed feasible SLWR configurations for NWS 1000m+ fields Learn more at www.2hoffshore.com
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Lazy Wave vs. SCR
Pros: Reduces top tension
Decouples (partially) vessel and TDZ movements
Improves strength performance 10-30%
Fatigue performance improves 3-10x
Permits tieback to vessels with poor motion characteristics e.g. semi-sub or FPSO
More riser configuration and heading arrangements allowed (for lateral and vertical clearance)
Cons:
Installation (and hardware) costs
Increased design complexity Learn more at www.2hoffshore.com
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SLWR Suitability for North West Shelf
Swell seastates, cyclones
SLWR reduces TDZ compression and fatigue allowing large diameters, long design lives (40+ years)
Allows larger motion vessels (e.g. semisubs, FPSOs)
All risers need not be orthogonal to dominant NE swell direction
Internal fluids
SLWR better suited than flexibles for sour service, high pressure and large OD
Calcareous soil
Reduces interaction at TDZ with calcareous soil which has high vertical and horizontal stiffness
Hence reduced fatigue loading Learn more at www.2hoffshore.com
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Steel Lazy Wave Risers (SLWRs)
Tapered stress joint or flex joint
SCR pipe with strakes (as required)
Buoyancy modules
Touch down zone (TDZ) Learn more at www.2hoffshore.com
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SLWR Design Challenges Riser fatigue in the hang-off and TDZ
Slugging induced fatigue
VIV of buoyancy modules
Motion induced vibration (MIV, sometimes called heave induced VIV) is more pronounced than SCRs
Sagbend may touch seabed during hydrotest
High flex joint rotations in extreme cyclones
Installation of empty riser with buoyancy modules
Stability/fatigue in wet parked condition
Loss of buoyancy
High pull in loads Learn more at www.2hoffshore.com
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Preliminary Configuration
Sufficient length to avoid TDZ overstress
Minimise height to minimise buoyancy & associated costs
Avoid seabed contact with heaviest fluid & extreme near offset
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SLWR Configuration Development
Configuration is highly dependent on:
Internal fluids
Vessel motions
Fatigue life improvement
Increased buoyancy
Increase in overall suspended length
Hang-off angle
Typically, what is good for fatigue is bad for installation
Configuration development requires iteration
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High Arch Strength Response
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FPSO Feasible
Better stress response than SCR
Highest stresses distributed between hang-off, sag and hog bends, and TDZ
High stress fluctuation at critical locations
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Low Arch Strength Response
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Critical location at start of buoyancy
High stress driven by:
Sag bend heave motion
Less damping
Increased likelihood of local buckling
Can be optimised by increasing arch height
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Typical Wave Fatigue Response
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Fatigue Life Slick Section Buoyancy Section
Critical locations
TDP
Hang-off point
Better fatigue life than SCR
Relatively low fatigue life occurs at hog and sag bends with high curvature fluctuations
Fatigue life increased by longer suspended length - by increasing hang-off angle or buoyancy coverage
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Arch Height Effect on Fatigue
Fatigue Life (years)
Low Arch = 670
Mid Arch = 750
High Arch = 1,500
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Low Arch Mid Arch High Arch Buoyancy 1742 ft Buoyancy 2261 ft Buoyancy 2722 ft
Sagbend
Archbend
Sag Bend Height: 2400ft
Arch Bend Heights:
Low Arch 2600ft
Mid Arch 3100ft
High Arch 3600ft
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Sagbend Effect on Fatigue
Fatigue Life (years)
Low Sagbend = 2,700
High Sagbend = 2,100
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Lazy Wave Sagbend 2400ft Buoyancy 10inch Thickness
Lazy Wave Sagbend 600ft Buoyancy 10inch Thickness
Buoyancy 2722 ft
Buoyancy 2189 ft
High Sagbend
Low Sagbend
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Lazy Wave Riser VIV
SLWRs in the NWS are expected to be fully covered with VIV suppression due to the possibility of strong cyclonic currents.
Buoyancy section is typically not fitted with suppression. Lock-in of buoyancy module
and gaps can occur.
Lazy wave riser VIV is still an active research topic. Previous studies include – Shell tests at MARINTEK (OTC 23672), CFD studies by Chevron (OMAE 24522).
CFD based flow visualization (Constantinides and Zhang, OMAE-24522)
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Lazy Wave Riser VIV
Buoyancy modules and bare pipe modes compete
Preference is to have buoyancy modules vibrate (low modes)
Buoyancy module L:D ratio and module to gap length ratio critical in determining VIV response
Bounding approach suggested while using empirical programs such as SHEAR7
2 peak power spectrum (Jhingran et. al., OTC-23672)
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SLWR Motion Induced Vibrations
MIV of SLWRs can contribute significantly to fatigue, particularly in cyclonic seastates
MIV is not yet well characterised
Deepstar® SLWR test campaign (OMAE 54970) demonstrated MIV in the out of plane direction at up to 4 times applied frequency
MIV can be modeled using a global model and VIV software and updated hydrodynamic coefficients based on tests/field data
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SLWR MIV Modeling
Extract riser velocities at discrete timesteps Relative “current” profiles for VIV assessment
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Optimisation for Clearance
Configuration envelope due to fluid variation
Configuration envelope optimised to allow crossing without interference:
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SLWR Installation
Major challenge - high strain during installation of buoyancy modules
Installation aids include clump weights, retrofit buoyancy modules
Installation and wet-parked conditions should be assessed for strength and fatigue
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SCR/SLWR Monitoring Programs
While laboratory tests are a good indicator of riser behavior, they are not a substitute for real field behavior
A number of catenary risers have been monitored in the past 2 decades
The ongoing STREAM (STeel Riser Enhanced Analytics using Measurements) JIP is focused on leveraging field measurements to improve typical design standards
TLP SCR
• 14 inch OD
• 3,000 ft WD
• 20% straked
• 20% faired
TLP SCR
• 8 inch OD
• 4,000 ft WD
• 40% faired
Mini-TLP SCR
• 12 inch OD
• 3,000 ft WD
• 15% straked
Spar SCR
• 9 inch OD
• 4,000 ft WD
• Fully straked
Spar SLWR
• 7 inch OD
• 5,000 ft WD
• 50% straked
• 30% faired
STREAM JIP Datasets
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Learnings from Monitoring (STREAM JIP)
Measurements indicate wave response damps down the riser to a greater extent than predicted by current industry standard modelling methods
Fatigue near TDP is over-predicted by factor of ~3
Inline and higher harmonic VIV are infrequently observed but can cause significant fatigue
Motion induced vibrations occur during storm seastates
3 more datasets to be assessed – including 1 SLWR with semi
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Conclusions
Steel lazy wave risers are suitable for the NWS
Field proven extension to SCRs
Enables larger diameters and pressures than flexibles
Better fatigue and strength response
Calibration of analysis parameters by monitoring is reducing conservatisms
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Questions?
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Learn more at www.2hoffshore.com