2h offshore - deep water riser engineering · 2020. 5. 21. · 2h offshore - deep water riser...
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Effects of High Temperature on the Design of Deepwater Risers
March 2003
2H Offshore Engineering
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Outline of Talk
Overview of Riser SystemsWhat is “High Temperature”?High Temperature (HT) DevelopmentsIssues Relating to SteelEffect on Insulation MaterialsProblems with H2S, corrosion and fatigue issuesBuoyancy IssuesPipe-In-Pipe SystemsUse of Flexible Jumpers for COR™ & SLOR™Summary of HT Design IssuesAlternative Options for Dealing with HP/HT
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Riser Systems - Configurations
Free Standing RisersSingle line
Bundled
Catenary RisersFlexible
Steel
Top Tensioned
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Riser Systems - Pipe Options
Flexible
Steel Pipe Non Insulated
Steel Pipe Insulated
Steel Pipe in Pipe
Bundled Steel Pipes
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What is High Temperature?
Typical production temperatures 40-80 deg C
High temperature 80 deg C - 100 deg C
Very high temperature 100 deg C +
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Current 2H HT Project Involvement
High
Very High
Very High
Production Pressure
TFEVery HighMoho Bilondo, Congo,600m
bpVery HighThunder Horse, GoM, 1800m
ChevronTexacoVery HighTahiti, GoM800m
OperatorProduction Temperature
Field
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Steel - Effect of High Temperature
Reduced yield strength at high temperatureAPI derating for T > 120°C (250°F)
150°C = 4.5% strength reductionDNV derating for T > 50°C (120 °F)
150 °C = 30MPa strength reduction (5.5% on X80)
Example:steel manufactured to X70 Normal operating stress checks performed against 65ksi to account for steel derating at HTStress checks for shut-down conditions checked against 70ksi yield strength since no HT
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Insulation Materials – Effect of HT
Why would risers for HT wells need thermal insulation?Cool down times during shut-in must be long enough to prevent hydrate formation
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Insulation - RequirementsMeet thermal requirements
Steady state conditions (U-value) and specific heat capacityMaintain temp. above critical value during shut-in (12-24hrs)
Long-term hydrothermal stabilityMaintain material properties in the long-term (20years)Water depth dictates compressive strength and water absorption rates
DensityIncreases with increase water depth and temperatureIncrease in density increases thermal conductivity (k)Desire to keep riser weight and k-value low Increased tension, drag loading, fatigue
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Insulation - Requirements
Corrosion protection & adhesionFBE typically used – mechanically bonds insulation to pipeAt high temperatures (>110 deg C) ordinary epoxy not suitable and high temperature epoxy requiredResist cathodic disbondment if insulation coating is damaged
Dynamic serviceResistant to cracking under fatigue loadingResist large strains during storm loading and installation (S-lay, J-lay and reeling)
Impact strengthResistant to crushing and mechanical strength during storage and laying with tensioners and over stinger rollers
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Insulation – Material Selection
3 major types available for deep and ultra deepwater riser application
Epoxy based syntactic foamsPolyurethane based syntactic foamsMulti-layer polypropylene systems
Selection influenced by:Design temperatureManufacturability of resulting insulation thickness based on required U-value and cool down time (lower k-value gives lower insulation thickness)Cost
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Insulation - Epoxy Syntactic Foams
Composite - fiberglass macro spheres in an epoxy binderCast directly onto pipeLow density (600 kg/m3 at 1200m)High thermal efficiency (k = 0.09 W/m.K at 1200m depth)Low costLess flexible than PU or PP (cracking concern in high fatigue areas)Limited to 100 deg C serviceUsed on 1000m GoM Shell King (1999) with 6 miles of C-THERM for 6” OD flowlines (80 deg C design temp.)Limited riser track record
C-Therm with macrospheres
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Insulation - PU Syntactic Foams
Polyurethane matrix loaded with gas filled glass micro spheresApplied by standard molding techniquesHigher density compared with epoxy syntactic foams (800 kg/m3 at 1200m)Moderate thermal efficiency (k = 0.165 W/m.K at 1200m depth)Medium costLimited to 90 deg C serviceUsed on 1600m GoM BP King (2001) with 59km of GSPU for 12” OD flowlines (83 deg C design temp.)Limited riser track record
Glass Syntactic Polyurethane(GSPU)
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Insulation - Multi-layer Polypropylene (PP)
3-layer corrosion barrier plus PP foam insulation and solid PP shieldApplied by side or cross extrusion processHigher density (800 kg/m3 at 1200m)Moderate thermal efficiency (k = 0.13 to 0.22 W/m.K)Medium / highest costHigh temp service up to 140 deg C Used on:
1500m GoM BP Nile (2000) with Thermotitefor 6” OD SCR (90 deg C design temp.)Asgard flowlines (140 deg C design temp.)
Will be used for:GoM BP Thunder Horse production SCRs
5-layer Thermotite® System
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Insulation - Field Joints System Quality
Specialist joints made in factory for main riser lengths but same quality also needed for field jointsIf high quality field joint are not achieved:
Water ingressCorrosion - pitting- fatigueReduced insulation – cold spotConvection – cold spot
Minimise number of field joints since costly due to
Speed of assemblyProblems associated with working on the vessel
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Insulation - Material Qualification
Little long-term data available for high temp. serviceTesting required for successful applicationSmall scale hydrothermal ageing tests under pressure, accelerated using an elevated temperature (least expensive)Large scale pipe simulated service tests to determine thermal performance and long-term degradation (expensive)Mechanical tests (tensile, shear, bend, adhesion, impact and fatigue)Cathodic disbondment
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H2S – Effect on Fatigue Performance
Recent tests at 60deg C indicate significant reduction in fatigue performance if H2S present (Factor of 20 on life)Effects of HT are currently unknown
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Corrosion – Effects at High Temperatures
Rate of corrosion increases with increase in temp. due to reduced efficiency of cathodic protection systemTherefore require more anode mass and/or reduced spacing of anodesIntegrity of coating barriers also an issue at HTTherefore corrosion allowances must increase – thicker walled pipesImpact on cost and weight
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Corrosion – Effect on Stresses and Fatigue
Calculation of stresses in riser need to take account of potential wall thickness loss due to corrosion
Corrosion allowances may lead to thick-walled pipes and wall thickness correction factor must be applied when assessing fatigue performance (DnV 2001 > 25mm)
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Fatigue Life Enhancement
Low fatigue lifes due to high corrosion levels etc. may be improved by:
Improve weld quality (double side weld)Reduce stresses using upset (thickened end) pipe Overlay of critical welds with corrosion resistance alloy (CRA)Reduce stresses using external sleeves
Cost Increase
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Buoyancy - Typical Properties
Temperature toleranceWater ingress
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Buoyancy/Insulation - Hot / Wet Issues
Accelerated water permeationLoss of buoyancyLoss of insulation
SolutionUse of pure syntactics - no spheres- (heavy)Use of resistant materials – Amines (costly)Low permeability coatings polyethylene – (damage)Barrier insulation material temp. gradient – (complex)Bonding direct to pipe to prevent ‘hot/wet’ – (not practical with bundles)
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Buoyancy/Insulation - Conventional Bundle
Hot/Wet interfaceComplex buoyancy profile
Difficulty castingNumber of piecesAssembly procedure
Poor heat sharing
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Buoyancy/Insulation - Internal Bundle
Buoyancy Modules
Carrier Pipe
Production Flowlines
Encircling Water
Production Fluid
Gel Medium
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Buoyancy/Insulation - Internal Bundle Riser
Lower buoyancy contact temperature Better heat sharing between prod. linesAbility to circulate cooler waterAbility to circulate hot waterSimpler buoyancy shapesLower buoyancy material spec.Similar thermal expansion
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PIP, TTR and COR - HT Issues
For pipe-in-pipe (PIP), top tensioned risers (TTR) and concentric offset risers (COR)Different pipes are at different temperatures and the relative thermal expansion of pipes must be considered:
Inner pipe expansionBucklingCentraliser design and spacingPreloadingCrushing of insulation Cold spots
Friction
Centraliser Spacing
Tension
Compression
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Flexible Jumpers - Free Standing Risers
Difficult to design CORs /SLORs for HT due to problems with flexible jumpers
Internal pressure sheathEnd fittingsStiffener design
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Flexible Jumpers – Internal Pressure Sheath
Material Temp. Range (°C)
Water Cut (%) Comments
HDPE -50 to +60 0 - 100 High tensile and impact resistance at low temp and low pressure
XLPE -50 to +90 0 – 100 Upper temperature limit reduces if pressure >2000psi
-20 to +100 0
-20 to +90 0 – 5 PA-11
-20 to +65 5 - 100
Weak resistance to high water cut at HT
PVDF -20 to +130 0 – 100 Appropriate for HP/HT applications
Maximum operating temperature is highly dependent on the design life of the pipe
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Flexible Jumpers – Internal Pressure Sheath
Example using PA-11:Design life of 30-years – operating temperature 55°CDesign life of 10-years - operating temperature 70°CDesign life of 1-year – operating temperature 100°C
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Flexible Jumpers – End Fitting Design
When PVDF used as internal pressure sheath:End-fitting design (crimping/sealing mechanism) is criticalPotential for plasticizer loss is high – reduced seal efficiencyPVDF has higher thermal expansion coefficient – cyclic expansion/contraction – gradual pull-out of sheath from end fittingSignificant development by manufacturers mean these issues are being addressed and continual improvements are made
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Flexible Jumpers – Stiffener Design
Stiffener at connection jumper/manifold is made from structural polyurethane (PU)
PU susceptible to aging at relatively low temp. (e.g. 50 ºC) therefore need to accurately determine stiffener internal wall temperatureTemperatures can be reduced by using water circulation around stiffener or active cooling
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Flex Joint – Issues at HT
Design IssuesAging of rubber at HTRapid decompression at HP
SolutionCRA Bellows (pressure balanced)Prevents direct contact of rubber with hydrocarbonsProduces temperature gradient across bellowsReduces rubber temperature
ImpactCostAdditional stiffnessFatigue of bellows
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Summary of Riser Design Issues at HT
HT wells result in additional complexity for riser design
Material performance – derating of steel yield strengthProblems with buoyancy / insulation material at HTPotential issues with H2S (if present)Accelerated corrosion ratesKnock on effects on riser wall thicknesses and fatigueAdditional considerations for pipe-in-pipe systemsIssues for flexible pipesIssue for key components such as flex-joint design
These all results in significant increase in riser cost due to higher spec. materials, increase weight etc.
Attention should be turned to finding alternative ways of dealing with HP/HT problem………
Attention should be turned to finding alternative Attention should be turned to finding alternative ways of dealing with HP/HT problem………ways of dealing with HP/HT problem………
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Alternative Approach for HP/HT Risers
High TemperatureCooling loops or heat exchanges on seabedPotential problems with hydrates and wax in exchanger during shut down – can be solved with chemicalsTherefore high temperature issues with insulation/buoyancy, steel etc. are no longer a problemThis approach is yet to be implemented
High PressureHigh Integrity Pressure Protection System (HIPPS)Complex control and additional subsea valving/choking to reduce pressure in riser systemIssues relating to reliability and risk must be addressedAlready implemented on a number of systems
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Alternative Approach for HP/HT Risers
Use both cooling loops and HIPPS to eliminate problems with HP/HT risers. Benefits include:
Reduce weight of riserReduce vessel payload / buoyancy requirementsReduce insulation/buoyancy material issuesFaster installationReduced hydrodynamic drag Improved dynamic responseLower quality welding may be acceptable
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