Corrosion in the marine environment is a process constantly acting on offshore structures, due to immersion in salt water and constant exposure to oxygen through wind and waves. Corrosion weakens structures and can lead to buckling or complete failure of supporting members and attached cross-bracing.
Routine maintenance will help preserve steel structures, but over years, the constant exposure can lead to exterior pitting and penetration. In addition, caissons for fire support, which are open internally to move seawater, and drainage pipes are subject to hidden damage. If corrosion weakens pipes carrying crude or natural gas in the processing system, there is the potential for burst pipe and fire.
Replacing the weakened components is one option, albeit an expensive one. Another option is to clean and reinforce the affected supports with exterior protection: welding additional metal to cover and seal the corroded area, or applying a reinforcing clamp for added structural strength. Both increase dead weight on the structure and can change load dynamics. Clamps often must be custom built, have limited length, and may not be strong enough to hold wave loads.
Replacement or clamping strategies work well on simple cylindrical members, but become problematic where complex joins bring two or more support members together. They can also be problematic for process piping with complex bends, and clamps are not adaptable for flanges or other protrusions.
Another option is to use an easily applied composite material that can be shaped to the compromised complex structure. Composites combine two or more materials to form a new material with the strengths of both. Fiber-reinforced polymers combine the strength of fibers (glass, carbon etc.) with the mold-ability and corrosion resistance of polymer resins. It is best to use glass fiber where internal pressure retention is important and carbon fiber where structural loads are critical.
Composite materials can be used in a wide variety of environments due to their service range. Workers can apply them in temperatures from +10°C to +50°C. Composites are stable at temperatures from –50°C to +150°C and can be built to contain pressures up to 120 bar. They do not require heavy lifting equipment to install, are applied wet, and cure in place. They are also ease to use for splash zone repairs.
Composite material can restore the structural integrity of caissons and other large diameter pipes affected by mechanical damage or wall thinning due to external and internal corrosion. Using finite element design to identify high stress areas, a compositebased repair can be created to resist bending, shear, and axial stresses from static loads (i.e. jackets and process equipment), internal pressures, and bending loads generated by wind and wave.
A repair using composite materials can be designed to meet all international engineering standards for static and induced loads, can be used on all geometries, can be worked cold (no hot-work permits needed) on surfaces prepared to ST2 standard, can be applied by individuals working from ropes or from scaffolding, and is both durable and cost effective.
Case study
A major offshore platform in the southern North Sea had severe problems on the fire pump return caisson with multiple, corrosion-related penetrations. The operator attempted to repair the problem with multiple clamps, but was not able to completely restore integrity. Leakage was still evident and new leaks were appearing.
The operator decided to use a composite repair. After removing the clamps, over 400 holes were found in the caisson, ranging from 2-100mm, over a 15m section of pipe. The affected area was cleaned to ST2 standard and the caisson holes were plugged. Kevlar-reinforced epoxy putty was spread over the 15m section, which was then sealed by wrapping with fiber sheeting. The epoxy permeated the fiber sheath and was allowed to cure. Next, an additional 3mm layer of epoxy putty was applied, and then ten layers of a colored composite laminate were applied and compressed around the pipe to form a lasting seal. The repair was designed for a service life of 20 years and took six days to complete. A test of the repair flowed 8500 gal/min through the caisson without a leak, proving the effectiveness of the operator’s choice.
Maintaining integrity of offshore structures is a challenge for operators. Fiber-reinforced composites are a durable way to address the problem and extend the working life of load-bearing structures and essential piping. OE
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