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

Learn more at www.2hoffshore.com

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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


2H Offshore - deep water riser engineering · 2020. 5. 21. · 2H Offshore - deep water riser...

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Transcript of 2H Offshore - deep water riser engineering · 2020. 5. 21. · 2H Offshore - deep water riser...