Jeremy Beckman; Editor, Europe
Soaring oil prices have turned even antiquated platforms in the North Sea into assets. Rusting equipment, however, remains a liability, and in extreme cases it is threatening tail-end production due to high replacement costs.
As an alternative approach, some operators are looking to repair their damaged items by applying of carbon fiber or epoxy resin composites. The Furmanite and DML Composites alliance in the UK have pooled their expertise in this field since 2001 and have worked for all the North Sea’s major operators, they say. Their track record ranges from pipework and pipelines up to 60-in. diameter, to caissons, storage tanks, valve bodies, and flanges. Where possible, repairs are conducted with the equipment still live to avoid interruptions to production. The current operating limit for implementing this approach is 200° C and 209 barg.
DML Composites, part of the Devonport Royal Dockyard in southwest England, is a center of technical expertise with over 30 years’ experience in composites engineering. Early applications were largely naval, but in recent years the company has branched into offshore and civil engineering repair. Today, its majority owner is KBR. Other stakeholders are Weir Group and Balfour Beatty.
Furmanite, in Kendal, northwest England, is a long-established maintenance specialist. Its services include on-line pressure leak sealing, pipeline intervention, valve repairs, and on-site machining, among others.
The alliance came about, according to Furmanite’s managing director, Tony Nicholls, “because our customers were seeking lasting repairs of five to 25 years, rather than a quick fix.”
DML, for its part, needed better marketing and greater installation capability, says composites technical manager Paul Hill. Aside from the North Sea, Furmanite also offered a strong track record offshore West Africa and in the Far East.
Working in tandem, the two parties have adapted their techniques to complex geometrical shapes encountered on process modules and pipework such as Y-pieces, elbows, and tees, and to substrates ranging from carbon or stainless steel to duplex steels, cunifer, and GRP. All solutions are validated by in-house tests, including FEA modeling, at DML’s facility in Plymouth, as stipulated by the material design codes.
“Our database there covers a large number of combinations,” says Hill. “But there are times when damage to an item is so severe that even we have to say no.”
Composite solutions are typically devised to repair corrosion and fatigue damage, to protect and strengthen structures that might be subjected to blast loading, or to increase a structure or module’s load-bearing capacity. Carbon fiber composites can be tailored to endure over the field’s life span or to a specified design life with minimal maintenance requirements. Where external corrosion has occurred, the partners say, the composites will prevent further damage. Internal corrosion can be addressed by engineering a solution that allows for total loss of the substrate without threatening the integrity of the subsequent repair.
Following thorough surface preparation - considered critical to achieving a durable bond - a glass fiber tie coat is applied to provide a repeatable interface with the steel and to insulate the carbon fiber from the substrate. Epoxy resin-impregnated carbon fiber plies are then applied in layers until the pre-determined thickness has been reached. This is followed ultimately by a sacrificial peel-ply to take away excess resin and provide a suitable finish for paint or another type of coating. The materials’ flexibility allows application in confined or cluttered areas requiring restricted radial clearance and in situations where only rope access is possible.
Designs are based on data from validation trials relating to strength, stiffness, fracture toughness, bond strength, and the impact of temperature on these properties. Substrates featuring copper-nickel, mild, or stainless steels can differ widely in performance. The original surface preparation method can also impact the performance of the repair. Other factors affecting calculations include the size and damage of the defect and the load-bearing capability of the substrate.
Ninian tank fix
The alliance has worked on numerous installations in the North Sea. Each solution is unique. On CNR’s Ninian North platform in the UK North Sea, a 3.5-m-diameter vessel used to store cement for downhole grouting had experienced internal erosion damage. The suspected cause was incorrect positioning of a fluffer nozzle near the tank’s base, leading to the cement - in concert with air from an internal manifold outlet - abrading the vessel. The result was wall thinning over a 2-cm diameter area, with a 5-mm-diameter through-wall defect below a circumferential weld.
Cost and logistical issues ruled out replacement or welding, which would have entailed complex post-weld heat treatment to reduce stresses and avoid embrittlement in the welds. Instead, the Furmanite/DML alliance was called in to rehabilitate the tank. The company had engineered previous solutions for this platform, having also worked with previous operator Kerr-McGee.
The first step was to clean and prepare the tank surface to the specified roughness, via abrasive grit blasting. An E-glass layer was next applied as a tie coat, followed by six layers of epoxy resin-impregnated carbon fiber. The resultant 6-mm-thick repair covered the damaged area with an overlap in line with design calculations. Finally, a sacrificial peel-ply layer was employed to absorb excess resin and to leave a surface ready for future painting or coating.
A similar situation arose on the platform’s K87 clear water caisson, which was undergoing wall loss due to internal root erosion of circumferential welds. Again, shutdown or replacement was out of the question due to the anticipated disruption to production. According to the alliance, high axial stiffness and strength are critical to a successful caisson repair because of the high level of external loading induced by wave actions, which can lead to bending in the caisson.
In this case, the repair was designed to take into account buckling, fatigue, and transverse shear that could be induced by a 100-year return wave period current and wave loading. The repair section extended nearly 20 m. Nine-millimeter thick carbon fiber repairs were used for the lower 3 m, with 5-mm repairs for the remainder. The partners calculate that their solution cost 5% of the £2 million CNR would have had to pay for a new caisson. The resultant repair should allow for total loss of the caisson steelwork between the affected upper and lower welds while still retaining full structural and pressure integrity through 2020.
Recently, the alliance has also designed repairs incorporating composites to a closed drain vessel experiencing corrosion on the Scott platform (operated at the time by EnCana); to cunifer piping suffering wall thinning and pinhole leaks on Total’s Alwyn North B platform; and to a corroding high-pressure/high-temperature carbon steel gas compression line (124° C, 80 bar) on Amerada Hess’AH001 floating production semisubmersible.
“In the latter case, we had to ensure that our geometry was applied carefully to take into account the strength of the carbon fibers,” Hill says.
Recent research and development undertaken by the alliance has focused on engineering and implementing new types of resins and curing processes. Last year, it introduced what it says is the first resin suitable for application in wet or humid offshore environments. This was the result of an eight-year study and testing program. Most conventional resin systems need a dry surface on application to realize their full bond strength. Also, water-activated systems continue to absorb water after application, resulting in degradation of their mechanical properties. In serious cases, this can lead to failure of the bond.
Persistently wet weather offshore can hold up remedial work for days. And in some particularly humid regions, daytime condensation has necessitated leaving repairs until nighttime. Splash zones can also be no-go zones for composite repairs. Previous attempts to achieve an adhesive composite bond on a wet surface have involved initially displacing the water - but this has demanded a resin of high viscosity, rendering it impractical for application.
The challenge was to find a practical, low-viscosity resin for this task. The new moisture-tolerant system is said to meet this challenge by displacing the water chemically rather than physically. It also removes the problems and high costs associated with composite repairs in wet or humid environments, allowing permanent repairs to be undertaken in conditions where not previously possible. Permanent repairs can be effected at pressures of up to 100 bar and temperatures up to 70° C, provided there is no through-wall defect.
Other innovations are a resin capable of working at temperatures to 200° C, a development triggered by a need to strengthen structures around a gas turbine exhaust. The alliance has also introduced a portable post-cure system for permanent repairs of pipes above 40° C. Traditionally, this task has been logistically difficult due to the cumbersome nature of most modified pipe re-heating equipment.
