Page 154 - Handbook Of Multiphase Flow Assurance

150 5. Flow restrictions and blockages in operations

Diamondoids are translucent crystals and are may be observed in natural gas production.
Diamondoids may also form gas hydrates. A study by Lederhos et al. (1992) showed that
12 8
adamantane can fit in the largest 5 6 cavity of structure H hydrate. As structure H hydrate is
mainly observed in natural environments and not in production systems.
A study on diamondoid molecules is published by Mansoori (2007) provides more de-
tailed properties of diamondoids.

Ice

Ice formation in multiphase flow assurance is relevant not only to Arctic and permafrost
areas, but also to regular production where Joule-Thomson cooling can cause ice solids for-
mation at chokes and at other flow restrictions.
The presence of solids such as ice alters the erosional consideration of flow design and the
solids-free formulas may no longer apply.
Ice can also form in stagnant parts of the system which are thought to be flowing. This may
occur in parallel or looped line configuration where flow rate is not monitored in each leg of the
flow network. Ice, like hydrate, expands upon freezing into a solid. Ice expands by around 9%
relative to the original volume of water. In comparison, hydrate expands even more by approx-
imately 26% to make room for the guest molecules. Similar to the automobile engine blocks,
which can get cracked by ice if water is used as engine coolant, ice can rupture the pipeline.
Ice can be managed by heat tracing the line to keep pipe walls from reaching the freezing
temperature. Ice may also be managed by the addition of salt or antifreeze chemicals in static
systems. However, the chemical route can be prohibitively costly or difficult to implement in
flowing systems.
Unlike hydrate solids, ice cannot be melted by pressure reduction. If an ice blockage
formed in the pipe, it either needs to be warmed up or chemically dissolved, or mechanically
jetted out with brine or milled.
Hydrate however, can be converted to ice when ambient temperature is below freezing
(e.g. 0 °C) and pressure is below hydrate stability. Caution should be taken while developing
a hydrate blockage dissociation plan in regions with cold ambient temperatures to avoid con-
verting a hydrate plug into an ice plug because an ice plug may remain stable as long as the
environment remains below 0 °C.

Liquid holdup

Water in gas and oil lines

Water is heavier than hydrocarbons and may accumulate in the gathering lines or in the
export lines. Accumulation of water increases hydraulic resistance to flow, increases the po-
tential for pipe wall corrosion and increases the possibility of hydrate formation.
Systems are normally designed to minimize the accumulation of liquid while increasing the
pipeline throughput by minimizing the pressure drop through a pipeline. When hydrocarbon