water or pond water. In both onshore and offshore scenarios,
it is also possible for recycled produced water or aquifer
water to be used; use of potable water and drill water has
also been commonplace. It is possible for any of these
water sources to cause corrosion due to several different
mechanisms, including:
)
)
Oxygen induced corrosion: most of the water types
used for hydrotesting are fully saturated (up to and
beyond 8 mg/L) with oxygen. This can cause flash rusting,
especially around welds where the heat affected zones
and weldment material are particular susceptible.
)
)
Microbially induced corrosion (MIC): this is a major
threat to pipelines and storage tanks. While this may be
limited during a hydrotest because of a lack of nutrients
(if fresh water is used), any biofilm left behind after a
hydrotest can further proliferate after the equipment
is commissioned, resulting in severe localised pitting
corrosion. Under-deposit corrosion can also occur as a
result of MIC and biofilm build up. MIC is a well known
phenomenon observed with poorly treated, or untreated,
hydrotest water. Once sessile colonies of bacteria have
been established, it is very difficult to control them.
One example of how aggressive the MIC can be is from
the North Sea, in systems such as deoxygenation vessels
from a MINOX tower. In one example the tower was
hydrotested and, 18 months later after installation on the
platform, was brought into service. The time delay was
due to several challenges in commissioning the seawater
injection system. A hole the size of a US 25 cents was
found at the base of the vessel and in 18 months, MIC had
occurred in the carbon steel of the tower and corroded
through 18 mm of steel that was itself glass lined.
)
)
Chloride induced corrosion (CIC): depending on the type
of water used for the hydrotest, CIC can be induced as a
result of increased electrolytes in the hydrotesting water.
The severity of the corrosion from the above three
mechanisms is reliant upon the quality of water and source of
water used for the hydrotest.
Single functionality hydrotest chemicals have long been
available and generally include oxygen scavengers, such as
ammonium bisulfite or sodium bisulfite; biocides including
glutaraldehyde and tetrakis (hydroxymethyl) phosphonium
sulfate (THPS). Corrosion inhibitors typically include
proprietary quaternary ammonium compounds. Use of dyes
is common in hydrotesting for leak detection, especially in
subsea applications where ROVs equipped with black light can
determine a leak from ppm levels of fluorescent dyes. The
most common dye used is fluorescein dye (Acid Yellow 73),
due to its excellent detection at very low concentrations
under black light. This dye is, however, environmentally
persistent and not favoured by many legislations worldwide.
Combination hydrotest chemicals are available in the
market place in a one-drum cocktail that comprises multi-
functionality. The challenge is that many of the single
functionality products are not compatible with one-another,
for example: it is well known that bisulfite inhibitors are not
typically compatible with biocides such as glutaraldehyde
and THPS. The most common types of current combination
products comprise dimethyl quaternary ammonium
compounds with ammonium bisulfite. This can prove to be a
further challenge as the quaternary ammonium compounds
themselves can be environmentally persistent and toxic. This is
a challenge because once the hydrotest is complete, the test
fluid needs to be disposed of. It is very typical to empty the
water back to its source; the sea, a lake or a river. Depending
on where in the world these operations are performed,
and given that the water has been chemically treated, it is
necessary that the residual level of chemical meets regulatory
limit requirements. By their very nature, the typical chemicals
used in hydrotesting can be environmentally hazardous, such
as biocides and corrosion inhibitors. In relatively high doses,
biocides are highly toxic and, therefore, have the potential to
damage the ecosystem into which they may be discharged.
Multi-functional products
The current work developed a novel and patented hydrotest
combination chemical formulation system including an oxygen
scavenger, biocide, microbially induced corrosion inhibitor,
general corrosion inhibitor, localised (pitting) corrosion
inhibitor and a visual leak detection. The applicability of this
product includes pressure testing operations for upstream and
downstream oil and gas operations, pipeline commissioning
Figure 1.
Oxygen scavenging performance in seawater and
freshwater reduces the oxygen content from saturated to
below 10 ppb.
Figure 2.
LPR corrosion testing shows that the corrosion rate is
reduced to below 0.01 mm/yr in under one hour.
82
World Pipelines
/
JUNE 2015
