due to the room for mobility that the fibres inadvertently
have in such an architecture.
Stiff uniaxial composites are stiffer than cloths
because the fibres are pre-tensioned as they are wound
around a cylindrical mandrel, meaning that there is no
room for the fibres to stretch.
Independent test measures
Independent testing of the two major types of
composites (stiff and wet) was completed by a major
pipeline operator. Apparent tensile strength and modulus
of elasticity were the two main mechanical properties
analysed. The wet wrap showed an average tensile
strength of 28.38 ksi and an average modulus of 1.34 x
10
6
psi. Stiff sleeves showed an average tensile strength
of 34.6 ksi and an average modulus of 4.47 x 10
6
psi.
Note the much lower modulus for the wet wrap. It
should also be noted that these values are a bit lower
than those previously mentioned due to the nature of
the independent testing. Another important result of
the testing showed that after
exposure to water, the wet-
lay demonstrated a significant
reduction in strength whereas
the stiff sleeve was unaffected.
This finding is most likely due
to woven cloth’s ability to
allow moisture to wick into the
composite.
Summary
If the operating pressure of a
given pipe is low enough, there
are advantages of the wet cloth
over stiff composites. The main
advantage of a wet-lay is that
they can be wrapped around
geometries other than straight
sections of pipe, such as bends,
tees, and nozzles. They are
also not restricted to a certain
diameter of pipe, whereas the
stiff composites are prefabricated
to fit a specific size. Wet lays
can be incredibly effective
for corrosion mitigation, rock
shielding, and abrasion resistance.
They provide many advantages
when used as a coating. However,
they must be carefully examined
when used as a repair on high
pressure pipelines.
References
1. GRI –95/0072 “Engineering Properties
of Clock Spring® For Repair of Defects
in Transmission Pipelines”, Northwestern
University/ Nicor Technologies.
2. GRI-98/0032 “Field Validation of
Composite Repair of Gas Transmission
Pipelines”-Final Report-Gas Technology
Institute, 1700 South Mount Prospect
Road, DesPlaines, IL 60018-1804.
3. Internal Clock Spring Report, Bell
Evaluation Laboratories, Houston, Texas
4. MATEER, M. W., and J. Chang, (January
1 2005), Performance of Pipeline
Composite Repair Sleeves, NACE
International.
5. POWERS, B. D., Composite Repairs- They
are not all the same!, Houston, Texas.
