January 2021, Vol. 248, No. 1
Features
Choosing, Using Composite Repair Systems
By Casey Whalen, Global Engineering Manager, CSNRI
With so many repair options in the oil and gas industry, choosing the right repair with the best materials can be difficult. There are plenty of manufacturers that would have operators believe their products are adequately suited to handle any repair because of their one-size-fits-all construction.
Realistically, these products can provide a good repair solution only in the right conditions. Even then, those conditions don’t necessarily make the repair material the best option. There are many variables when considering a composite repair, including operating pressure and temperature, defect type and severity, pipe geometry and external or internal chemical presence.
Considering all these variables, it is almost impossible for a single, ready-made composite to work well in every situation.
“Predesigned” and ready-made repair products should only be used to address very specific situations, which should be clearly and plainly understood. Manufacturers of a truly engineered composite repair product should be able to explain how the engineering of said product will address the specific problem being considered.
Finding a product that is engineered to the exact specifics of a given scenario will not only offer the optimum design required to provide a sufficient repair but can also reduce costs. Additional conditions, such as bending or combined loading, must be addressed through the repair as well. All these conditions have a much higher likelihood to be taken care of if the repair is custom-designed vs. assuming an off-the-shelf product has predetermined these loads.
Some manufacturers may claim the low number of layers that come standard with their composites are a key benefit; however, layer count is not necessarily as important as total repair thickness and how the system addresses certain stress loads, such as bending.
If a composite decision is made solely based on its constant repair thickness (layer count) alone, the results could be costly. Having more material (constant repair thickness) than necessary will mean purchasers are paying for material they don’t need. If the operator decides to err on the side of perceived expedience, depending on the product type and kit, the repair could consist of too thin a composite, resulting in a repair that could potentially fail. This scenario would mean a second repair is imminent, which adds costs that could have been avoided with an initial proper solution.
For example, a manufacturer might offer a standard, fixed-layer composite for any given pipe size as a stocked repair product. If, in fact, the circumstances of a repair require more layers than provided to address the hoop stress, the only option is to use two fixed-layer composites, which creates layers of unnecessarily similar composite, but that still may not address bending or axial loads.
Can your manufacturer explain the importance of Poisson’s ratio with regard to how the composite product will address your repair? If a manufacturer cannot demonstrate knowledge surrounding this crucial point of engineering, the company may not be able to provide an adequate and purposefully engineered repair.
Stress on a composite repair due to an increase in the internal pressures of the pipe will cause the composite repair itself to shrink axially. This could lead to an issue in adhesion if it is not properly considered.
Even if you know to choose a specifically engineered repair vs. a predesigned one, however, close attention also should be paid to the product’s fabric orientation when considering a composite repair. A product listing may reference uni-, bi-, tri- or quad-directional fabrics. These terms aren’t just sales speak or marketing terms, but rather are important structural characteristics that can ensure the right composite is selected for a repair.
Unidirectional
Unidirectional fiber-reinforced composite fabrics are designed just as they sound; fibers are oriented and aligned in a single direction, making these fabrics extremely strong in that direction. Depending on the orientation of the composite when applied to a pipe, the composite will inherently neglect either the hoop or axial stress state of the pipe, making the repair extremely weak to any loads not parallel to the fiber construction.
A unidirectional fabric can result in a repair that is unable to handle bending, thermal loading or multiaxial load conditions that are associated with pipeline dent and wrinkle bend repairs. The abovementioned limitations and weaknesses do apply to precured systems as well because the fiber orientation neglects axial stresses.
Bidirectional
Bidirectional fabrics are usually created in a 0°/90° woven configuration, which can address the stress load bearing ratio of 2:1 to adequately support both the hoop and axial stresses if designed accordingly. The amount of support is determined by the ratio of fibers aligned in each direction. Typically, hoop stress will be predominately supported, but the fabric can be tailored to provide certain percentages of support without needing to sequence layers. Additionally, the thin composition of bidirectional fabrics makes them relatively flexible and easily formed to complex shapes while either wet or dry.
This composition is not without its faults. When exposed to torsional or shear forces along the 45° line, bidirectional fabrics are weak. Also, because of the space required to include fabric for supporting axial stresses, there is a reduced modulus and strength—generally making bidirectional fabrics slightly weaker than unidirectional fabrics with regard to only pure hoop stress.
The woven composition of bidirectional fabric increases unused space between fibers, decreasing the overall strength compared to fabric in which the fibers are stacked and stitched together.
Tri-Directional
Tri-directional fabrics venture away from a woven design. They are typically engineered in a stacked fashion, with a prevalent strength in the hoop direction, normally aligned along the 0°, +45° and -45° lines of the fabric. This configuration provides a much greater distribution of load-bearing capabilities.
Because the fabric is stacked and stitched together, this sequence also minimizes space between fibers, making a tri-directional fabric great for leak repairs. Although slightly thicker and slightly less pliable than a bidirectional fabric, its tight fiber structure makes it the best option for addressing multidirectional stress loads.
If no fibers are placed directly along the 90° line, axial stress loads are distributed to all other non-perpendicular fibers. This composition gives tri-directional fabrics a reduced modulus and strength compared to unidirectional fabrics along any single direction; however, it provides a higher overall strength in all directions due to the total number of fibers.
Quad-Directional
Quad-directional fabrics are stacked and stitched composites that include fibers typically placed along the 0°, +45°, -45° and 90° directions, making this composite relatively strong in every direction. Acting mostly isotropic (like metal), this composition is ideal for leak repairs due to its strength and sequencing. Because it is thick and stiff in all directions, however, it is an extremely difficult composite to apply to complex shapes.
While quad-directional fabrics have the highest average overall strength and modulus in stress, they have the lowest strength and modulus in any single direction. These fabrics are typically overkill when it comes to more simplistic repairs, which can add unnecessary costs.
Random Orientation
Randomly oriented fabrics are not as common as the previously mentioned compositions but are yet another available repair product. These fabrics contain no directional preference, making them isotropic and very stable in the planar direction. They can also be easily formed around complex shapes. However, due to lack of direction, this construction provides minimal strength – only about 30% or less of what can be achieved by unidirectional fabrics.
Because of this, randomly oriented fabrics are typically used in low-stress applications where they can be utilized mostly as a preventative measure. Randomly oriented fabrics may be sequenced with unidirectional materials oriented in various lines of action to make up for the lack of support in other directions.
By choosing the correct fabric orientation, design and manufacturer, it is possible to ensure repairs will be an adequate match for the project.
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