Corrosion can lead to damage, structural failure, lost production, and in extreme conditions, environmental incidents. To date, the common repair option has been limited to removing damaged sections of pipe and replacing with new spool pieces, which introduces a range of safety risks associated with heavy lifting and welding. These activities also require a weather window for safe execution, and production must be shut in while the repair is being carried out, which can escalate costs.
Composites were developed specifically to contend with corrosion. In general terms, a composite combines two or more materials, a high-strength reinforcement in fibrous form, incorporated into and bonded by a matrix, usually a thermosetting polymer. The most common strength component is glass.
Most glass fibers consist of E-glass, a term that derived from the words “electrical grade glass.” This super-cooled mixture of metallic oxides is brittle and transparent but has very high tensile strength, 500 ksi (3,400 MPa). Glass is produced in a furnace at about 2,192°F (1,200oC) and spun into fibers of approximately 10 microns in diameter by allowing it to drain under its own weight through many heated bushings.
When engineers are designing a composite, they focus on three characteristics:
• Fiber type: glass, carbon, or aramid (strong, heat-resistant synthetic fibers frequently used in aerospace and military applications)
• Fiber form (typically roving, tow, mat, or woven fabric)
• Fiber orientation or architecture (Reinforcement can be oriented in any direction the designer desires. The most common structural elements are designed with greater strength in the direction subjected to the greatest load.
CONSIDERING THE OPTIONS
It is important to distinguish among the composite repair offerings on the market because not all composites are equal. The distinctions can be critical specific to the particular repair applications, so it is important to understand how composite repairs differ.
Any composite repair being considered for an offshore application should be:
• Non-intrusive, limited disruption to normal production
• Suitable for in-service applications
• Permanent, restores the serviceability to the pipe beyond its design life
• Cost effective
• Fully predictable and verifiable by modeling and/or definitive equations found in current design standards
• Able to eliminate all installation variables
• Eective in all locations and environments
• Eective for all pipe grades and sizes
• Designed to meet or exceed current code requirements.
• Formulated and constructed to eliminate field design and field engineering
• Able to conservatively restore the pipe to its original strength
• Rigorously tested and subjected to peer review
• Field proven.
A composite that will work in an offshore environment must be designed carefully to ensure the mechanical properties can provide the necessary strength to restore the line to the appropriate level of operation, typically to its original design standard. With the proper repair designed, installation procedures 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 installation variables and effectiveness.
To be reliable and predictable, a composite should be manufactured under controlled conditions. Manufacturing 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 before 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.