Pipelines & Flow Assurance - Subsea UK littler... · Flow Assurance Wax Study on the Ravn Field Production System Wood Group Kenny: Hooman Haghighi, Jamie Littler, Fujiang Zhu, Temitope

Pipelines & Flow Assurance

Jamie Littler

Technical Disciplines and Assurance Manager

Shell UK

Flow Assurance Wax Study on the Ravn Field

Production System

Wood Group Kenny: Hooman Haghighi, Jamie Littler, Fujiang Zhu,

Temitope Solanke

Wintershall: Leif H. Blidegn, Amir Mofidi

4 - www.woodgroup.com

Agenda

• Introduction to the Risks of Wax Deposition

• Fluid Tuning

• Ravn Field Overview

• Results of In-field and Export Lines

• Conclusions and Recommendations

Wax deposition risksWax is long-chain simple hydrocarbons.

• Long Chain n-paraffin

• Branched chain paraffin

• Cyclo-paraffin (Naphthene)

Wax deposition can cause:• Reduction in flow area

• Change in wall friction

• Blockage of the pipeline

These pictures are from public domain.

Wax managementMaintain the system temperature above the wax appearance and/or fluid pour point.

• Insulation

• Displacement with stabilized crude, diesel or condensate

• Active heating of the pipeline

Physical removal of wax

• Periodic scraping of the wax layer via pigging operations.

• Heating

Chemical treatment

Blideng et al. (2011), Running-in a new Platform, 22nd

International Oil Field Chemistry Symposium

Wax deposition (Molecular Diffusion)

• Molecular Diffusion is the dominant wax deposition mechanism

• Radial diffusion of dissolved wax molecules in the oil

• Concentration gradient between dissolved wax in the turbulent core

and the wax in solution at the pipe wall

• Dissolved wax diffuses towards the wall where it precipitates

Turbulent Core

• There are a few lab techniques available for wax measurements:

• Viscosimetery

• Cold finger

• Differential Scanning Calorimetry (DSC)

• Cross Polarization Microscopy

• Filter Plugging

• Fourier Transform Infrared Spectroscopy (FTIR)

Wax Appearance Temperature (WAT):

The temperature below which the paraffin's start to precipitate as wax crystals is defined as crude cloud point or WAT.

Pour Point:

The temperature at which oil sample movement stops is defined as the crude oil pour point.

Wax testing

These pictures are from public domain.

Wax properties

Lab Data

WAT [oC] 27.5

WDT [oC] 55

Wax Paraffinic content [wt%] NOTE1 2.625

Cold Finger Test

Note 1: C17+

Static Cold Finger Set-up

• Hayduk Minhas correlation was used to calculate the diffusion Coefficient (9.78E-08 cm2/s)

• Wax Inhibitor from lab test was shown to reduce the deposition rate by 40-80% (40% has been assumed as a conservative approach in this study)

Viscosity tuning

Shear Rate Calculation-Catcher

Shear rate= 10 S-1

Temp= 15 C

μ= 22 cp

ID= 0.1016 m 4"

ρ= 834.9702 kg/m3

QLT= 1950 bpd

0.003588 m3/s

u= 0.442594 m/s

Velocity

Re= 1706.661 Re<2300 Laminar

Re>4000 Turbulent

Laminar

τ= 1.0 N/m2

s= 0.045805 S-1 Turbulent

s= 34.84989 S-1 Laminar

note: use the lab data with shear rate =10 S-1

• A shear-thinning behaviour of the fluid has been observed at low temperature.

• The shear rate has been identified to represent the actual flowing condition (for each flow rate) and viscosity has been tuned based on the selected shear rate.

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Dyn

amic

Vis

cosi

ty m

Pa.

s [c

P]

Temperature °C

Steady State Dynamic Viscosity Profiles For Ravn Oil

1 s-1

10 s-1

100 s-1

non-Newtonian Behaviour at Low temperature

Fluid modelling Effect of pressure & light end component on WAT

Note: The dynamic changes in the fluid composition (e.g. Gas Oil Ratio) in the pipeline and the effect on WAT has been considered in thermo-hydraulic simulation. However the model has not taken into account the composition change due to wax drop-out (conservative).

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Pre

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ra]

Temperature / Wax Appearance Temperature [oC]

Wintershall Ravn Field

Fluid Modelling

in-Field Line

Wax Appearance Temperature (WAT) as a Function of GOR and Pressure

Bubble Line (Case 1+GL - GOR = 1900 Scf/bbl)

WAT (Case 1+GL - GOR = 1900 Scf/bbl)

Bubble Line (Case 2 - GOR = 533 Scf/bbl)

WAT (Case 2 - GOR = 533 Scf/bbl)

Bubble Line (Case 3+GL - GOR = 986Scf/bbl)

WAT (Case 3+GL - GOR = 986 Scf/bbl)

Ravn system schematic

Ravn

F3-FB Platform

4” Oil Export Line (119 km)

72 m 5°C

0.34 m/s

- 10°C

54 m/s

66 m

API° 38.2

Viscosity at 60°F (cP) 24.1

WAT (°C) 27.5

N-Paraffin Content (wt%) 2.625

Pour Point (°C) -51

Fluid Properties

Export T = 55°C

Ambient Conditions

FWHT= 60°C

8” Infield Line (18 km)

A6A Platform

Risk of wax deposition (in-field line, early life)

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per

atu

re [ C

]

Length [km]

Wintershall Ravan Field

Wax Deposition Simulation

In-field Line

Temperature and Wax Depostion Profiles (Case 1+ GL)

Fluid T

WAT

T Ambient

Wintershall Ravn Field

Wax deposition thickness (in-field, early life)

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Wa

x T

hic

kn

ess

[mm

]

Length [km]

Wintershall Ravn Field

Wax Deposition Simulation

In-field Line

Wax Depostion Profiles (Case 1+ GL)

30 days

20 days

10 days

Risk of Wax Depostion at the Topsides

Risk of Wax Depostion at the Subsea

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tal

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m3]

Time [days]

Wintershall Ravn Field

Wax Deposition Simulation

In-field Line - No Inhibitor

Volume of Wax Deposition

Case 1

Case 2

Case 3

Case 4

Total wax deposition (without inhibitor)

Total growth rate of wax (in-field pipeline) is <0.4 m3/d without inhibitor

Oil Gas Water

[m3/d] [m

3/d] [m

3/d] [m

3/d]

1 310 29448 0 + GL

2 620 58896 0 0

3 620 58896 0 50000

4 369 39979 52 100000

Case

Production RatesGas Lift

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x D

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Time [days]

Wintershall Ravn Field

Wax Deposition Simulation

In-field Line - With Inhibitor

Volume of Wax Deposition

Case 1

Case 2

Case 3

Case 4

Total wax deposition (with inhibitor)

Total growth rate of wax (in-field pipeline) is <0.3 m3/d with inhibitor

Oil Gas Water

[m3/d] [m3/d] [m3/d] [m3/d]

1 310 29448 0 + GL

2 620 58896 0 0

3 620 58896 0 50000

4 369 39979 52 100000

Case

Production RatesGas Lift

Summary of the results (in-field line)

• WAT is lower at higher pressure for the live fluid• The effect of pressure is more pronounced for the fluids with a

higher GOR (i.e. Gas Oil Ratio)• As soon as the fluid reaches ambient temperature, no wax

deposition would occur (No heat flux to drive the wax deposition –cold slurry flow).

• The first location for wax to deposit depends on the flow rates, GOR, phase fractions, etc.

• After 30 days of operation <4mm and <13mm of (max) wax thickness can be expected at seabed and topside conditions respectively without inhibitor.

• The recommended frequency of pigging operation is every month (based on maximum 4mm of wax deposition in the system) without inhibitor and every 45 days with inhibitor injection (40% efficiency).

Risk of wax deposition (Export Line)

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Tem

per

atu

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Distance (km)

Wintershall Ravn Field Wax Deposition Simulation - Export Line

Temperature Profile and Risk of Wax Deposition

Ambient T

WAT

1950bpd Fluid T

2500bpd Fluid T

3500bpd Fluid T

4500bpd Fluid T

Risk of Wax Depostion at the Subsea

Risk of Wax Depostion at the Topsides

Wax deposition thickness (Export line)

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Wax T

hic

kn

ess

[mm

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Length [km]

Wintershall Ravn Field

Wax Deposition Simulation

Export Line

Wax Depostion Profiles (2500 bpd)

5 days

10 days

15 days

20 days

30 days

Risk of Wax Depostion at the Topsides

Risk of Wax Depostion at the Subsea

Pressure drop vs. max wax thickness (Export Line)

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rop

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Max Wax Thickness [mm]

Wintershall Ravn Field

Wax Deposition Simulation

Export Line

Pressure Drop vs. Max. Deposition Thickness

1950 bpd

2500 bpd

3500 bpd

4500 bpd

Self insulation on wax deposition (Export Line)

Results are for the topsides (i.e. the highest deposition thickness and rates)

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ax T

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Max D

eposi

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Rate

[m

m/d

ay]

Time [days]

Wintershall Ravn Field

Wax Deposition Simulation

In-Field Line

Effect of Wax Thickness on Depostion Rates

Case 1 - Deposition Rate Case 2 - Deposition Rate Case 3 - Deposition Rate Case 4 - Deposition Rate

Case 1 - Max Wax Thickness Case 2 - Max Wax Thickness Case 3 - Max Wax Thickness Case 4 - Max Wax Thickness

Summary of the results (Export line)

• Higher flow rate leads to longer section of the export line subject to wax deposition risk.

• Self-insulation effect was observed (Lower rate of deposition by time).

• The maximum wax thicknesses identified for the 4 cases are comparable, however the total wax deposited is more at higher flow rates.

• After 22 days and 31 days of operation <4mm of (max) wax thickness can be expected at seabed condition without and with inhibitor (40% efficiency), respectively.

• Pigging of 4” >100 km export line is challenging and is currently under further evaluation.

• Alternative wax mitigation strategy like wax dispersant, gas condensates has been considered.

Other Flow Assurance challenges• Slugging in the in-field line at the early life and during the start-up

and turn-down operations has been observed. The following mitigation methods has been considered: o Increased back pressure (for start-up and turn-down operations)o Gas lift injection (if required)

Special thanks to:

Questions?