Hydrate Plugs – Still a Major

Flow Assurance Challenge

By Xiaoyun Li, Statoil R&D

Flow Assurance Lecture at NTNU, 16th April 2012

2012-04-10

Contents

• Introduction

• Plug detection/localization

• Hydrate plug remediation methods

• Efficiency

• Safety

• Some field examples

2012-04-10 2

Hydrates may have very different appearances

2012-04-10 3

Conditions for forming hydrates

Free water

Small/light gas

molecules

High pressure

Low temperature

Smørbukk

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30

Temperatur (°C)

Try

kk (

bara

)

Hydrates

Temperature (°C)

Pre

ssu

re (

bar)

No

hydrates

Hydrate curve

2012-04-10 4

Typical subsea development today

• Very long subsea tie-ins

• Multiple & complicated tie-ins

• Deep water

• Sub-zero ambient temperature

2012-04-10 5

Alternative hydrate control strategies

• Low dosage hydrate inhibitors

• Transport of hydrate slurries

• Transient operation through/in hydrate

domain

• Utilization of hydrate formation kinetics

• ......

2012-04-10 6

Some consequences of the new trends

• May increase hydrate plugging probability

• More difficult/expensive to remove plugs due to

− Larger water depth

− Longer pipelines

− Sub-zero ambient temperatures

− Multiple tie-ins, sharing of risers

− ……

2012-04-10 7

Important issues for plug remediation

• Plug detection/localization

• Melting efficiency

• Safety aspect

Applicability and efficiency of remediation

methods depend on plug location!

2012-04-10 8

Plug detection/localization

• Relatively easy at topside (for non-insulated pipes)

− Temperature gradient (Joule-Thomson cooling)

− Locally no flow (or flow restriction)

− Gamma-ray densitometer

• Limited accessibility for subsea plugs

− Monitoring pressure development across a plug(s)

− Gamma-ray densitometer (mobilization needed)

− More efficient and accurate detection & localization

methods are highly needed!

2012-04-10 9

Hydrate plug remediation methods

Depressurization

External heating

Thermodynamic inhibitors

methanol (MeOH)

mono-ethylene glycol (MEG)

nitrogen

Mechanical methods

coiled tubing / “tractor”

High frequency & low amplitude pressure pulses

2012-04-10 10

Plug melting by depressurization

*

*

*

A

B C

Pre

ssure

Temperature Tmelt Tamb.

Need to remove hydrostatic pressure at deep water

Very low efficiency for insulated pipes

Time consuming for gas (condensate) lines

Need to maintain low pressure to assure melting

Often “only” applicable method

2012-04-10 11

PI PI

•Depressurization from both sides

Most efficient and safer, but often not available

Could be risky for multiple plugs with a large entrapped

gas volume between plugs

• Depressurization from only one side

less efficient & high risk, but often “only” available method

Plug removal by depressurization

2012-04-10 12

Large differential pressure => May cause run-away (loosen) plugs!

High

pressure Low

pressure

2012-04-10 13

Hydrate cannon

2012-04-10 14

Hydrate Cannon experiments

2012-04-10 15

Hydrate projectiles: experiments and modelling

Open pipe outlet case –

Initial DP across plug: 21-1bar

Closed pipe outlet case –

Initial DP across plug: 10-1bar

Damaged coiled tubing by a hydrate

projectile – before plug melting started

High P

end:

80 bar

Low P

end:

40 bar

2012-04-10 17

Plug melting by external heating

• Steaming or heat tracing at top side

• Subsea

• Direct electrical heating (DEH)

• Bundle heating

• ......

• Safety: the most important issue!

2012-04-10 18

Gas content in gas hydrates

1 m³

hydrate 150-170 Sm³

gas

0.8 m³

Water 150 bar

20 x 50 litre gas bottles

+

A lot of gas!!

≈

2012-04-10 19

Avoid heating at the middle of a plug

2012-04-10 20

Heating along the whole plug/pipe

gas transport

Potential danger if too slow gas transport rate!

Avoid P > Pburst_pipe

2012-04-10 21

DEH or bundle heating on plug melting

• Extensive studies have been performed at Statoil

• A comprehensive & rigorous mathematical model developed

• Major conclusions:

• Bundle heating can be applied to plug melting

• Uncritical deployment of DEH for plug melting NOT allowed

• A thorough analysis and detailed procedure must be made for each DEH application on plug melting

• General precautions needed for run-away plugs

2012-04-10 22

Melting by thermodynamic inhibitors

* Pre

ssu

re

Temperature

Tmelt Tamb

Inhibitor & plug must be in

contact

Efficiency depend on plug

properties

Need to remove diluted

inhibitor during melting

Generally not applicable for

hydrate plugs in subsea

flowlines (except in well head

areas & risers)

2012-04-10 23

Insufficient contact between methanol and plug

Water released during initial melting

Water film preventing contact

between methanol and plug

Methanol unable to reach the

plug due to its low density!!!

2012-04-10 24

High glycol viscosity at low temperatures

may prevent efficient melting

2012-04-10 25

Melting Porous versus Compact Plugs

Small contact area

gives low melting

efficiency

Inhibitor

A compact

plug

2012-04-10 26

Hydrate Plug Removal Vehicle (HP-RV)

•Tractor/pig technology

•Topside/subsea launching

•Up to 15 km reach

•Pass 90°, 5D bends

•Pull force up to 30 ton

Turn the plug into crystal dispersion

high pressure jetting

Hydrate melting & restart aspects

Hydrate crystal melting with inhibitor Permanent inhibition effect

Direct access to the plug

through the flowline

Plug crushing

2012-04-10 27

Some field examples

2012-04-10

Icing – downstream plug (J-T cooling)

Tamb. = 7-8°C!

2012-04-10 29

Plug seen through a thermo-camera – during depressurization

2012-04-10 30

Hydrate formation downstream choke valves

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

25.12.03

00

14.01.04

00

03.02.04

00

23.02.04

00

14.03.04

00

03.04.04

00

23.04.04

00

13.05.04

00

02.06.04

00

Tem

pera

ture

(C

)

AVG_FLOWLINE_TEMP AVG_TS_DSC_TEMP

Time

2012-04-10 31

Plugging also around local high points

-80

-60

-40

-20

0

20

40

0 5000 10000 15000

Distance from Wellhead (m)

Dis

tan

ce

fro

m S

ea L

ev

el (m

)

Often plugged location

Tommeliten field tests

2012-04-10 32

Hydrate removal by inhibitor injection Increased pressure due to

hydrate deposition

MEG injection removed hydrate

deposits & pressure normalized

2012-04-10 33

3D g-ray measurements showing even

hydrate depositions at the pipe wall

2012-04-10 34

ID_pipe = 350 mm

g-ray measurements showing hydrate

deposts before & after dynamic pigging

2012-04-10 35

Gas condensate field: lots of gas need to be removed

Low melting efficiency!

Depressurization from 1 side – field test

Plug

2012-04-10 36

No contact between inhibitor and plug

9.0 °C

15.5 °C

10

12

14

sp1

sp2

sp3

sp1: Temperature 10.0 °C

sp2: Temperature 13.1 °C

sp3: Temperature 9.4 °C

Sp1: Hydrate Sp2: Gas

Sp3: MeOH

2012-04-10 37

Melting by steaming from one end

7.8 °C

27.8 °C

10

15

20

25

sp1

sp2

sp1: Temperature 8.8 °C

sp2: Temperature 20.0 °C

Hydrate equil. curve in pipe 20L0004 - ÅSG B

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12 14 16 18 20 22 24

Temperature (°C)

Pre

ssu

re (

bara

) Hydratlikevekts temperatur

Hydrate equil. curve in pipe 20L0004 - ÅSG B

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12 14 16 18 20 22 24

Temperature (°C)

Pre

ssu

re (

bara

) Hydratlikevekts temperatur

Hydrate curve

Melting zone temp.:

20°C = HET

2012-04-10 38

Summary • Hydrate plugging still a major FA challenge

• Avoid hydrate plugs by better process/pipe design and

procedure control! And more knowledge!

• Novel techniques for subsea hydrate plug localization

and remediation are highly needed!

• Better hydrate remediation methods allow less

conservative hydrate control measures – less chemical

usage & improved field economy!

Need to deal with hydrate plugs as long as there are

gas & oil production!

2012-04-10 39

Presentation title

Presenters name

Presenters title

E-mail address ……@statoil.com

Tel: +4700000000

www.statoil.com

Classification: Internal 2012-04-10 4

0

Thank you!

comments/questions?