repair designed, installation proce-
dures must provide the permanency
required of the repair. This means the
composite repair, when completed,
must be able to compete with traditional repair alternatives in terms of
safety, economics, control of installa-
tion variables and effectiveness.
To be reliable and predictable, a
composite should be manufactured
under controlled conditions. Manu-
facturing material in a facility allows
accurate control of the ratio of glass
to resin under conditions that can be
monitored. Within a facility, the unidirectional glass strands can be carefully positioned, pre-tensioned and
aligned to maximize strength, and
the composite can be compressed,
dried, heat-treated, cured, and inspected be-
fore being shipped as a completed unit to the
repair location. This approach allows design
variables to be controlled by the manufacturer,
producing repair units that are consistent and
documented.
Regardless of the manufacturing process
and its location, a quality composite requires
that the reinforcement be completely saturated
with the resin. The composite also must be
compacted to squeeze out air bubbles and excess resin and be fully cured before it is suitable
for carrying loads. In a factory environment,
quality control procedures can deliver these
properties more consistently in the finished
product than can be achieved in field applications. Without this predictable performance,
long-term durability would be questionable.
The composite laminate layers produced by
Clock Spring are nominally 0.065-in. ( 1. 65 mm)
thick and have a glass fiber content ranging from
60% to 70% by weight (45% to 55% by volume).
The resulting material exhibits linear elastic
behavior up to the point of failure in tension,
typically 1.5% to 2% strain. Elastic modulus values
are 5 by 106 psi (0.34 by 7. 3 bar) in the fiber
direction and 1. 4 by 106 psi (0.09 by 7. 3 bar) in
the transverse direction, with tensile strength in
the range of 75-100 ksi (517-690 MPa).
It is important to understand the structure
and application of a composite material to
gauge its suitability for a particular repair.
Composites can fail in three areas: the fibers,
shear/laminate bonds, and interface failures
between the resin and the fiber. In addition,
the composite repair is also susceptible to deg-
radation overtime due to moisture absorption,
temperature fluctuations, and fatigue. Uniaxial
fibers oriented and protected by “size” that
is specially designed and heated to promote
bonding of the fiber, size, and resin prevent
degradation, delivering a much stronger and
longer-lasting composite. These fabrication
risks are best managed in a controlled manu-
facturing environment. Wet wraps applied in
the field contend with many variables that can
be eliminated in a properly managed facility.
Composite repair
Composite repairs reinforce the damaged
section of the substrate by wrapping the defect
area with a composite sleeve that shares the
hoop load, reducing the stresses in the pipe
wall. The hoop load must be efficiently transferred to the composite for the appropriate
load sharing to be achieved. The only way to
accomplish this is to fill external defects with
a high-compressive strength material that pro-
tects the thinned ligament from further yield.
This can be designed into the repair system
by using unidirectional rovings or woven cloth
as the reinforcing element. Unidirectional
rovings provide more strength than woven
cloth for the same ratio of glass to resin and
are less prone to damage caused by cyclical
loading. Unidirectional E-glass embedded in
a polyester resin provides the best method for
high-pressure reinforcement. This part of the
repair is crucial because it re-establishes the
strength of the pipe, delivering performance
that exceeds the “as new” condition. Repair
techniques that do not use filler
are unsuitable for critical high-
pressure repairs.
Two application techniques are
available for the repair of both low-and high-pressure systems: full-
cure and wet wrap. In the full-cure process,
the composite sleeve is completely cured in
the manufacturing facility and installed using
filler and adhesive in the field. The compos-
ite sleeve is coiled slightly smaller than the
outside diameter of the pipe and cut to the
length required by the repair. This method
allows the glass direction and composition to
be fully controlled. Wet-wrap requires that the
glass cloth be wetted with resin in the field and
applied to the pipe in the wet condition. The
amount or length of the wrap is variable and
determined in the field. Curing takes place as
part of the installation process.
A typical repair consists of locating and
cleaning the damaged area. Filling the defect
and other voids under the repair with a high
compressive strength filler material to transfer the loads from the pipe to the externally
applied composite sleeve. Using adhesive to
secure the composite to the pipe, and keeping
the repair under tension while it cures to force
excess adhesive and filler out the edge of the
unit ensures a fully filled, tight fit. Installation
takes approximately 30 minutes, and the repair
cures in about two hours.
Building a better composite
Creating a composite that can withstand harsh environments and high pressures requires engineering that factors
site conditions into the formula of the composite. The Clock
Spring repair sleeve is a fiberglass composite that
has a memory, created by a manufacturing method
that constrains its cylindrical shape over several
concentric layers, similar to the winding spring of
a clock or watch. The glass fibers in the composite
wrap are continuous and are aligned circumferentially to maximize the composite strength. When the
sleeve is installed on a pipeline with the proper adhesive, the resulting structure provides circumferential
reinforcement of the defect and reduces the hoop
stress of the steel pipe under the wrap.
This three-part system comprises a unique unidirectional composite structure of glass fibers and polymer base, a patented two-part adhesive system, and a proprietary high-compressive strength load transferring compound.
Clock Spring
Composite
Reinforcement
Filled
Corrosion
Defect
While relatively new to the offshore
arena, fiberglass composite repair
sleeves have a long track record in
other industries.
To be reliable and predictable, a
composite should be manufactured
under controlled conditions that
allows accurate control of the ratio
of glass to resin under conditions that can be monitored. Here,
unidirectional glass strands are
carefully positioned, pre-tensioned
and aligned to maximize strength.
