Temperature control in multiphase pipelines

Temperature control in multiphase pipelines Content: Hydrate and wax control Basic Methods Case examples: Gullfaks bundles Kristin DEH pipelines

Hydrate control - basic Conditions for hydrate formation: Light components: C1-C4, H2S, CO2, N2 Free water 900 800 Relatively high pressure Relatively low temperature Pressure, bar 700 600 500 400 300 200 100 0 Potenial for hydrate formation Safe operating conditions Assure operation outside hydrate region during normal production and transient operations (as shut-in, start-up) 0 5 10 15 20 25 30 35 Temperature, C

Forskjellige utseende av hydrater

Wax control - basic Voks avsettes på rørveggen når temperaturen ligger under WAT (wax appearance temperature) Voksavsetning over lengre tid vil medføre - Redusert røretverrsnitt - Øket trykktap - Blokkering av røret Metoder for å hindre/fjerne voks - Holde fluidtemperturen over WAT - Injeksjon av voks inhibitor - Pigging

Hydrate (and wax) control methods Fluid displacement Insulation Direct electric heating Methods based on heating Hydraulic metodes Gas sweep Bundles Depressurisation Compression Hydrate control methods Heat tracing MeOH Dense phase Water removal/ processing methods Gas dehydration Water cut reduction Salt X Low concentration inhibitors Glycols Chemical methods Natural inhibition Hydrate region operation Production with no measure Sound engineering practice/design

Prevention of hydrate formation Typical domains for hydrate control methods in flowlines Choice of method will depend on Complexity of fluid Water depth Environment Pipeline length Water rate. Pipeline length 50 km No measures (oil transport lines) Once through LDHI KI/AA Outside NCS Insulated pipeline Depressurization Displacement Compression Gas sweep 100m³/d Continuous injection of chemicals Thermodynamic inhibitors MEG/MeOH Regeneration Water separation Heated pipeline 0.5% Produced water rate and water cut

Eksempel på hydratkontrollmetode - isolering, inhibering og trykkavlastning : Normal produksjons: isolasjon på rør holder temperatur over hydrat likevektstemperaturen Nedstengning av rør: Ved planlagt nedstenging inhiberes røret. Ved uplanlagt nedstengning: Isolasjon holder temperatur over hydrat likevektstemperaturen et gitt tidspunkt ( nedkjølingstid ) Innen nedkjølingstiden må hydrat kontroll metode implementeres (for eksempel trykkavlastning)

Eksempel nedkjølingstid: 5 0-5 -10-15 -20-25 -30-35 0 20000 40000 60000 80000 100000 Difference between hydrate equilibrium temperature and fluid temperature ( C) -40 Distance (m) 0 h 3 h 7 h 11 h 15 h 19 h

Eksempel hydrauliske metode Trykkavlastning Trykk Nedkjølingstid B * * A No-touch time * C T omgivelse Temperatur

Example Active heating Bundle: Warm fluid circulated generates heat Direct electrical heating: Electric current through pipeline wall generates heat. Eksempel Gullfaks Bundle

Oppvarming Ulik bruk av oppvarmings systemet: Periodisk oppvarming Hindrer hydrat dannelse og stivning under nedstengning Eliminerer behov for trykkavlastning Kan redusere behov for inhibering Fjerne/smelte voks avsettinger Smelte hydrat plugger? ikke kvalifisert. Kontinuerlig oppvarming Hindre voks oppbygging Redusere behov for kjemikalie Eksempel Gullfaks Rørbundt Forlenge levetid Redusere viskositet for tunge oljer

Case Gullfaks South phase 1 bundle

Fluid challenges for Gullfaks South oils High wax content (11-14 weight %). High ability for forming stable emulsions. Pour point temperature between 10-15 C for dead oils (gelling during shut-downs). The yield stress estimated very high for stabilised oil at 5 C WAT 30 C. HET 20 C A bundle system chosen for hydrate and wax control

Bundle functional requirements Insulation + hot water circulation should ensure: Fluid arrival temperature > live oil WAT (30 C) in normal production (no hot water circulation) Maintain fluid temperature > HET (20 C) for 6 hours during a shutdown (no hot water circulation) Heat the pipeline from ambient to 30 C within 48 hours (hot water circulation) Achieve 60 C in the whole pipeline for the purpose of melting deposited wax (production and hot water circulation) Thereby: Avoid continuous injection of wax and hydrate inhibitor Very limited need for depressurisation and pigging

The bundle configuration Free space within sleeve pipe is filled with nitrogen gas pressurised to about 13 bara.

Thermal performance of the bundle The bundle has bad thermal performance. This was identified during the flushing of heat-up system (prior to production start-up) and during production measurements. Therefore, temporary operational procedures based on continuous chemical injection had to be introduced.

Heat transfer mechanisms in bundle system A full scale test performed by the bundle supplier showed that the U- value increased proportionally with nitrogen gas pressure within the sleeve pipe. U-value (W/m²K) 14 2 0 0 Pressure (bara) 30 The heat loss is dominated by free convection through the porous insulation material. v Δp / μ p ΔT h / μ h T 2 T 1 p 0

Improvement measures Goal: To establish a fluid control philosophy which does not require continuous chemical injection. Main improvement required: The heat-up system must be able to heat the fluid in the production line above the HET (20 C) during shut-ins. Temperature ( C) 40 35 30 25 20 15 20 m³/h 30 m³/h 35 m³/h 40 m³/h 10 0 2000 4000 6000 8000 10000 Distance from Stafjord template (m)

Field test 25 20 Base case No flow improver: Flowrate Temperature 15 Flowrate [m3/h] and temperature [C] 10 5 0 0 10 20 30 40 50 60 time [hours] 35 30 25 20 15 Flow improver: Flow improver test Flowrate Temperature 10 5 0 0 10 20 30 40 50 60 70 80 time [hours]

Conclusions The poor thermal performance of the GFS1 bundle is explained by free convection through the insulation layer. The increased free convection is caused by the elevated nitrogen gas pressure surrounding the insulation. The thermal performance of the GFS1 bundle has been improved by increasing the capacity of the heating system (adding surfactant flow improver to the heating medium and installing a new heating medium pump). This has provided new hydrate and wax control strategies that does not require continuous injection of chemicals. Lesson learned for future bundle design!

Kristin 6.8 km 6.0 km Facts Water depth 320 m Reservoir P 900 bar Reservoir T 170 C GOR 800 Sm 3 /Sm 3 Number of wells 12 6.1 km 6.0 km 6.7 km 6.7 km Production capacity gas 18.3 MSm 3 /d Condensate 20 000 Sm 3 /d

Field schematic P N 10 flowline 10 flowline 12 - Oil export Fibre optical cable To Åsgard C To Midgard N Umbilical: -2 MEG injection -Controls 3.5 Scale squeeze line: Kristin A 16 Gas export loop Åsgard Transport S 4 x 10 flowlines R

Kristin s operasjons vindu Det høye trykket og den høye temperaturen i Kristin reservoaret (911 bar, 167 C) gjør det utfordrende å operere innenfor design trykk og design temperatur til produksjons systemet.

Temperatur kontroll Reservoar temperatur 167 C Maksimum strømmende brønnhodte temperatur: 157 C Design temperaturer Well & X-mas tree 162 C Flowline 155 C Flexible riser 132 C Isolering til U=8 W/m²K gir naturlig kjøling fra 155 C til 132 C

Hydrat kontroll metode for Kristin Ulike hydrat kontrol metoder er vurdert for Kristin. Direkte elektrisk oppvarming (DEH) ble valgt ut fra robusthet, LCC analyser og HMS perspektiv. Hydrat kontroll under normal produksjon Under normal produksjon sørger den moderate isolasjonen (U=8 W/m²K) for nok kjøling av fluidet til at temperatur ved riser base er under 132 C. Den moderate isolasjonen gir forholdsvis liten nedkjølings tid etter nedstengnig-> 4-7 timer før hydrate likevekts temperaturen nås. Hydrat kontroll for en planlagt nedstengning Systemet blir inhibert med MEG før nedstengning og under oppstart Hydrat kontroll for uplanlagt nedstengning Fortrenge endesoner med MEG (riser, X-mas tree and flowline spool) Direkte elektrisk oppvarming (DEH) systemet brukes til å holde temperatur på fluid i rørledning over HET (23 C) DEH system er på under nedstengning og under oppstart til høye ankomst temperaturer er nådd.

DEH systemet Sørger for at fluid temperatur holdes over 25 C under nedstengninger Er i stand til å varme opp fluid i flowline fra omgivelses temperatur (4 C) til 23 C innen 24 timer. Electrisk strøm gjennom rørvegg genererer varme. Effektivitet 60-70 % (kraft tap til sjø og i kabel). Installert kraft: 1.3 MW @ 1400 Amp for hver flowline

DEH beskyttelses struktur Kabel må beskyttes Electric cable mot tråling. Kabel må beskyttes mot høye temperaturer: PEX material I kabel degraderes ved temperaturer høyere enn 90 C Maksimum operasjons temperatur for kabel er 113 C Flowline OLGA FEMtherm simulation Protection structure Seawater Steady state operation of DEH system

Thermiske belastninger på DEH kabel Nedgravd rør og posisjon av kabel under rør gir spesielt høy temperatur på kabel. Flowline Production temperature shutdown Cable temperature DEH turned on Survey utført i etterkant for å identifisere posisjon på kabel. DEH må ikke settes på før etter 6-7 timer nedstengning. Et online verktøy skal gi råd til bruk av DEH. 0 6 12 18 hours

Oppstart Kristin Oppstart 3Q 2005. Temperatur og trykk beregninger fra design stemmer innenfor 10 %. (Litt kaldere enn antatt i design). Bruk av DEH og temperatur målinger logges, levetids beregninger utføres med jevne mellomrom.