Page 126 - Handbook Of Multiphase Flow Assurance

122 5. Flow restrictions and blockages in operations

Wet insulation is exposed to ambient elements. Wet insulation has to be more rigid and
thus is more thermally conductive and less efficient compared to dry insulation. Wet insu-
lation may be used for multiphase pipeline tiebacks up to 20–30 miles. Wet insulation also is
limited by water depth to which it can be deployed depending on type of the material. Wet
insulation absorbs moisture from ambient elements with time, and its efficiency may be ex-
pected to decrease by 3–5%.
Dry insulation is contained in the annulus between two concentric pipes for a pipe-in-pipe
configuration. Having two pipes nearly doubles the cost but provides more effective insula-
tion which can be deployed to greater seawater depths. Dry insulation may be effective to
tieback lengths of 30–40 miles.
For tieback lengths >40 miles actively heated insulated pipes are used. Cost of active heat-
ing adds the power generation equipment and platform, cables, subsea transformers, instal-
lation costs. Actively heated pipes are used seldom. To-date <30 subsea projects are known to
have actively-heated pipes.
Chemical inhibition is used as often as insulation. While oil-dominated systems (GOR
<6000 scf/stb) rely mainly on insulation and use chemicals such as methanol or LDHI
for short-term operations such as well startup or a planned shutdown, gas-dominated
systems rely on the continuous injection of glycols, KHI or seldom methanol in the unin-
sulated multiphase pipelines. The choice of glycols is dictated by economics. While glycols
can be economically recovered from produced water at the onshore or topsides facility
and re-used, the technology for economic recovery of LDHI or methanol chemicals is still
in development. Over 99% of gas production through multiphase pipelines uses glycols.
One example of methanol use with a recovery plant is the Malampaya gas condensate
field. Glycol regeneration plants product contains 70–80 wt% glycol and balance water.
The dilution needs to be accounted for in the design of the plant capacity to meet the field
glycol demand.
Local supply of an inexpensive hydrate inhibitor such as ethanol and its easy and reliable
delivery to the field can be deciding factors in selecting the hydrate prevention methods.
However, safety of the application of a method should be the main parameter for the selec-
tion of a hydrate prevention method.

Environmental impacts of hydrate remediation
Various chemicals may be used in hydrate remediation. However, it is expected that all
chemical will be captured and not released to the environment.
• Glycols are less toxic and non-flammable.
• Ethanol is less toxic unless it is denatured but is flammable.
• Methanol is very toxic, flammable and will damage the environment if released.
• Low-dosage inhibitors vary in their toxicity. Polymers composing the KHI chemicals
active ingredients are usually non-toxic or have low toxicity. Chemicals constituting AA
usually have high toxicity.
Each field abides by the regional environmental regulations. The use of chemicals for hy-
drate control has to be approved by the appropriate authority if this chemical can be released
to the environment.