Business Name Date

Flow Assurance & Operability

Hydrates

Business Name Date 1

Hydrates

• Hydrates are crystalline material that forms when light

hydrocarbon molecules mix with water at appropriate

pressure and temperature conditions.

• The following conditions are required to form

hydrates – Free water (water in liquid form)

– Small molecules like: methane, ethane, propane, n-

butane, carbon dioxide

– Sufficiently high pressure: typically above 10-20bar at

ambient temperature

– Sufficiently low temperatures: typically below 20 - 25 °C.

Business Name Date 2

Hydrates

• A massive and uncontrolled formation of hydrates can

result in restriction in the flowline that can eventually

develop into a full blockage; resulting in stop in

production.

• Hydrate prevention is a key flow assurance focus area.

Business Name Date 3

Hydrates – common strategy

• Operate outside the thermodynamic hydrate formation

envelope in all operational scenarios.

• Hydrate control philosophy is field specific and selection

of hydrate control strategy is based on theoretical

estimated hydrate equilibrium conditions.

0

50

100

150

200

250

300

0 5 10 15 20 25

Temperature [°C]

Pre

ssu

re [

kg

f/cm

²]

Wellfluid with gaslift

Wellfluid

T = 4°C

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Hydrate strategy

• All different operational modes needs to be covered

by a hydrate control strategy

– Normal production

– Planned shutdown

– Unplanned shutdown

– Restart after the different shutdown scenarios

– Remediation

Business Name Date 5

Hydrate prevention

• The following conditions are required to form hydrates – Free water (water in liquid form) – Small molecules like: methane, ethane, propane, n-butane,

carbon dioxide – Sufficiently high pressure – Sufficiently low temperatures

• In order to prevent hydrates from being formed, one

have to eliminate at least one of the conditions required for their formations

• Hydrate prevention methods can be categorized as follows: – Removal of water – Chemical injection – Hydraulic methods – Heat control methods

Business Name Date 6

Hydrate prevention – removal of water

• Water can be removed by

– Well completion; avoid producing from where water is

dominating in the reservoir

– Choking back on well with high water production

– Subsea processing

– For gas export lines the gas is dried so that no free water

will be present during operation

– For oil systems seperators are needed to seperate out the

water phase.

• Topside

• Subsea

Business Name Date 7

Hydrate prevention – chemical injection

• Chemical injection is commonly used for prevention of

hydrates forming

– Continuously or for short periods during specific operations

such as start up and shutdown.

• Two main classes of hydrate inhibitors exist

– Thermodynamic inhibitors are chemical that lower the

hydrate equilibrium temperature. Commonly used

chemicals are MeOH and MEG.

– Low-concentration inhibitors

• Kinetic inhibitors delay the formation of hydrates by a certain

length of time

• Anti-agglomerants allow hydrates to form as transportable

slurry thereby preventing plugging.

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Hydrate prevention – chemical injection

Business Name Date 9

Hydrate prevention – hydraulic methods

• Fluid displacement

– Fluid displacement means replacing production fluid with a

non hydrate forming fluid during or prior to a planned

shutdown.

• Compression method

– For gas systems compressing the system prior to restart

will result in a temperature increase which places the fluid

outside the hydrate forming domain.

• Depressurization

– Depressurization is a widely used strategy for avoiding

hydrates formation for a planned and unplanned

shutdown.

Business Name Date 10

Hydrate prevention – heat control

• Thermal insulation

– Limiting the temperature loss of subsea equipment and

shorter flowlines

– Buying the operator more time reducing how quickly the

system cools down after a planned or unplanned shutdown

• Cool down time = No touch time + Implementation time

• The NTT is the time required after shutdown where the fluid is

allowed to stay untouched before the start of implementing

any hydrate control methods.

• The IMT is the time to implement hydrate control methods.

– Design of thermal insulation is a key activity in detailed

design

• Focus is on components transporting heat to the ambient.

These are typically valves, support structures, instrumentation.

• Focus is on dead legs in the piping design

Business Name Date 11

Hydrate prevention – heat control

Business Name Date 12

Hydrate prevention – heat control

• Active heat control methods

– Hydrates can be prevented by adding heat to the

production fluid. Several options exist:

• Heat tracing adds heat to specific components

• Pipeline bundling consist of a carries pipe with one or

several internal oil production and/or gas injection lines

together with lines for circulation of a heating medium.

• Direct Electric Heating may be used for long pipelines during

shutdown to maintain the flowline temperature above the

hydrate forming temperature.

Business Name Date 13

Hydrate prevention - design

• The best way of avoiding the formation of forming is to

properly design the subsea production equipment. Key

points in a design are:

– Eliminate low points (or high points depending on fluid)

– Eliminate dead legs

– Optimal location of instruments

– Optimal location of chemical injection points

– Liquid drainage to remove liquid from the actual pipe

section

Business Name Date 14

Hydrate remediation

• Hydrate remediation methods may be organized as follows: – Chemical injection

• Thermodynamic hydrate inhibitors are used to melt hydrate plugs. The chemical needs to be able to reach the plug so the number of and the locations of injection points are of great importance.

– Heating

• By heating the system the temperature will move out of the hydrate region and plugs may melt. Heating to remove hydrates presents a high risk as large amounts of gas is released when melting hydrates causing a large pressure buildup. 1 m3 of hydrates may contain typically 0.8 m3 of water and 150 Sm3 of gas.

– Depressurization

• Depressurization is commonly used for removing hydrate plugs. Care has to be taken though. Reducing the pressure on one side of the plug only may cause the plug to travel like a projectile though the piping driven by the high pressure on the other side of the plug. The plug may then cause great damage to the piping.

– Mechanical methods

• Hydrated may be removed by mechanical means using pigging, hydrate tractors or by replacing the piping.