Page 169 - Handbook Of Multiphase Flow Assurance

Paraffin wax 165

FIG. 5.34 A wireline scraper used to scrape wax in a deepwater dry tree well.

through a lubricator. This method in onshore wells encounters operational issues such as
wireline tearing or cutter sticking, which are resolved by a well service crew.
In deepwater subsea production only dry tree wells are conducive to wireline cutting of
wax. A cutting tool on a string as shown in Fig. 5.34 is lowered from topsides into the tubing
and mechanically removes the wax deposit. In a subsea tree, access to the well is significantly
more complicated and requires the use of a workover rig, which makes wax removal by cut-
ting in subsea tree wells uneconomic.
• Scraping
Scraping is the most common way to remove wax deposits from gathering flowlines and
from export pipelines.
Wax scraping requires a careful balance of forces acting on a scraper, in order to not break
the scraper or not to make it stuck on the wax deposit in a pipeline.
Typical forces acting on a scraper include:
‐ Force of scraper friction on pipe wall, which depends on scraper size relative to the pipe size.
‐ Force required to cut the wax from pipe wall, which is a function of wax content, which
in turn is a function of time. The diffusion of normal paraffin molecules into the porous
wax deposit causes the deposit to harden with time.
‐ Force required to propel the wax cuttings in front of the scraper, which depends on
scraper efficiency in removing wax and on the oil bypass of the scraper from back to
front, which depends on scraper size relative to the pipe size.
The sum of these forces should not exceed the value recommended by the manufacturer,
typically 30–40 psi, otherwise the scraper may get deformed and stuck in a pipeline.
Scraping typically removes up to 50% of a wax deposit per pass. Data on the wax removal
efficiency by scraping were measured by Wang et al. (2001, 2008) for cup and disk scraper
types.