Page 212 - Pipeline Pigging Technology

Ethylene pipeline cleaning

interface length of contaminated ethylene. To expedite the process,
decommissioning was done in three stages with three nitrogen injection
points (see Fig.l). Nitrogen injection would begin at the south end of the
pipeline; as the interface passed the next injection site, the previous section
was shut in, depressured, and prepared for capital work. Due to the amount
of nitrogen involved in decommissioning, it was necessary to use three
nitrogen service companies, each with one injection point.

Capital works

In order to clean and inspect the entire 180-km line in a 28-day period, the
pipeline had to be separated into four sections. The section lengths were set
at 75km, 51km, 35km, and 19km, based primarily on the amount of polymer
expected in each section. The deposition problem was considered to be more
severe at the north end of the line, which is furthest from the plants, than at
the south end, so the section lengths decreased proportionally. Each section
had its own launch and receive traps, as well as facilities to separate the
polymer from the nitrogen. Four simultaneous pigging operations proceeded
on a 24-hour-a-day basis.
For capital works, Novacorp was retained to design, procure, fabricate and
install all additional pig trap sites complete with polymer-separation systems.
The receive sites had separation facilities to remove any debris from the
nitrogen stream as it was vented to the atmosphere. These consisted of a
separator/knock-out drum, pressure let-down valve and final filtration bags
(see Fig.2).

Cleaning and inspection

Cleaning commenced immediately upon completion of the capital works
for a section. All cleaning and inspection tools were propelled by nitrogen,
with their speed governed by a control valve at the receive sites. The
proposed schedule of cleaning and inspection runs is shown in Fig.3; this
selection of pigs was designed to progressively remove the polymer debris
from the pipe wall and successfully carry it out to the separator and filter bags.
The cleaning programme assumed the majority of polymer would be
removed during the 1400-kPa (200-psO runs when the separator was in
service. The separator would then be by-passed for all inspection runs,when
pressures were 3500kPa (500psf).
The four sections were totally independent for cleaning. Each had dedi-
cated resources with operations proceeding 24 hours a day.

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