Page 118 - Handbook Of Multiphase Flow Assurance

114 5. Flow restrictions and blockages in operations

stability pressure at an operating temperature, and supercooling or the difference between
the operating temperature and hydrate stability temperature at an operating pressure.
The higher the overpressurization, the sooner a KHI can fail. Similarly the higher the su-
percooling, the sooner a KHI can fail. Empirical evidence (Talley, 2000) suggests that over-
pressurization is more important to reducing KHI effectiveness time than supercooling.
The first of the two sub-categories, the KHI have no limitation on the amount of water
they can protect from hydrate. Operators use that property to deploy KHI in the areas of
high water production such as late life deepwater oil wells, gas and gas condensate produc-
tion with limited residence time of water in the pipeline. KHIs are also deployed in regions
where environmental regulations preclude the use of more toxic thermodynamic inhibitors or
anti-agglomerant chemicals because typically KHI chemicals are non-toxic.
KHI chemicals are usually short polymers or oligomers with carbonyl groups present in
the side chains. Some of the early KHI chemical active ingredients like PVP were same as
used in shampoos and other non-toxic products. Some details and examples of chemicals are
presented further.
AA chemicals are similar in their chemical structure to corrosion inhibitors, which can
be quite toxic and corrosive themselves, depending on concentration. An example of an AA
chemical is a quaternary ammonium salt.
AA chemicals work by allowing the hydrates to form, but control the solid surface to keep
the solids finely dispersed in a carrier fluid. This makes AA chemicals have no limitation on
the time of protection, similar to thermodynamic hydrate inhibitors like methanol but AA do
have a limit on the amount of water they can protect from a hydrate blockage. As the formed
hydrate solids need to be dispersed into a carrier fluid such as live liquid hydrocarbons pro-
duced from a well in order to be transported from the flowline where they form to the pro-
cessing facility, there is a limitation for AA chemicals on the water cut which can be protected.
Typical water cut limit for AA chemical use is between 40 and 50 vol%. Water expands when
it transforms into gas hydrate similar to as when it freezes into ice. Actually water expands
more when it forms a hydrate. Ice takes approximately 1.11 volumes when 1 volume of water
freezes. Hydrate takes between 1.2 and 1.26 volumes, as calculated by the hydrate crystallo-
graphic unit cells size for different crystal structures when 1 volume of water forms a gas hy-
drate. The difference in expansion ratios between ice and hydrate is explained by the volume
of gas trapped inside the hydrate crystal lattice. The expanded volume of solids can be dis-
persed in a liquid hydrocarbon and remain fluid up to 50–60% solids volume which dictates
the AA chemical applicability limit of the water cut. There are specialty formulations which
increase the limit to 60–70% water cut but these systems are rare. In one instance a formu-
lation was developed by Yale University team led by Prof. Firoozabadi which allows an AA
chemical to be effective up to 100% water cut by forming a water-in-oil-in-water dual reverse
emulsion. The formed hydrate slurry remains fluid but is understandably very viscous as
reported by oil majors who tested this method (Walsh, 2014). The water cut limit for normal
operating conditions is verified in a laboratory for a specific oil which will be produced. This
step saves operator cost as each crude may contain different amounts of naturally-occurring
hydrate dispersant chemicals which may reduce the required AA chemical concentration.
Some crudes known as self-inhibited possess a property to naturally disperse forming
hydrate particles. This is usually associated with acidic crudes. Studies of the properties of
naturally-inhibited crudes indicated that crudes with high TAN number reduced hydrate