Page 143 - Handbook Of Multiphase Flow Assurance

Hydrate of natural gas 139

removal. During depressurization, gas flow carried partially dissociated hydrate slush to the
vent line where slush deposited inside the 2-in. transition nipple and at the end of the vent
stack. The hydrate deposited on the vent stack end got sheared off from the vent pipe wall by
gas flow and formed a projectile which flew in the air for 4 s to an estimated height of 20 m.
Approximately half of the nipple cross section area, initially clear, was found to be filled with
hydrate.
May 2003 US onshore
Operator was cycling a well on which a plunger lift had been installed. The pressure in the
well decreased rapidly, and operator shut in the well. As the operator walked away from the
well, the plunger lift assembly on top of the wellhead blew off the wellhead lubricator.
Plunger in the well or hydrate in the well was driven up the well bore by well pressure and
impacted the plunger assembly with enough force to dislodge it from the wellhead. Hydrate
plug in tubing was considered as the cause. There were no injuries and no environmental
damage.

Commissioning/dewatering of pipelines to avoid hydrates
During commissioning of the newly installed pipelines, they are hydro tested to check
if the pipes can hold water before they can hold hydrocarbons. Displacing water from the
pipeline after the hydrotest is usually accomplished with scrapers, and drying the remaining
water with nitrogen. Remaining water can also be inhibited with glycol.
Several multiphase flow considerations are present:

• multiphase flow in dewatering lines
• nitrogen requirements
• scraping efficiency
• tools and methods to verify the line is dry
A water sweeping scraper can have a very tight seal against the pipe wall. However, water
will remain in the pipe wall roughness. Also some water will bypass the scraper cups. For
long distance pipes the amount of water trapped in pipe wall roughness can be substantial.
For example, assuming a perfect scraper seal against pipe wall and a 45-μm roughness typi-
cal for a carbon steel, in an 18-in. diameter pipe wall roughness there is 33 liters of water per
kilometer or 0.33 barrels per mile.
Gas dew point analyzer is used to check nitrogen arriving from the dried pipeline for mois-
ture content in order to detect the absence of water in the pipeline.
Alternatively, a water detector scraper may be used to check for the presence of water
holdup in a pipeline as shown in Fig. 5.26.
Water detection scraper after deployment in an 18 × 20 inch multidiameter oil export pipe-
line indicated some water presence. The detector at the scraper front as shown in Fig. 5.27
is a pre-weighed PVC fitting with inserted plastic tie-strap for mounting on the scraper and
filled with molten sugar (caramelized sugar candy). Sugar dissolves in proportion to the time
of contact with water which is measured by weight on arrival. While soluble in water, sugar
does not dissolve in oil or refined products such as gasoline. Identical detector was fitted on
the scraper back, which indicated little to no contact with water.