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Are the new composites better than steel?

The material is experiencing increasing acceptance in the oil and gas industry due to widespread applications and a new draft standard.

Dr. Paul Hill, FD Alliance, Aberdeen, UK

Composites were once considered solely the domain of high-tech industries such as aerospace and motor racing, but they are quickly becoming accepted in the oil and gas industry worldwide as a cost-effective alternative to pipeline replacement. They offer a permanent replacement or repair solution that can be applied with operations online, avoiding the need for costly shutdown.

The importance of establishing industry confidence is magnified by the apparent simplicity of the technology which, though one of its strengths, has to some extent also made it harder to believe in. This ignores, of course, the high level of engineering that has gone into precise calculations to design the repair, from the number of layers and resin matrix needed, to the surface preparation, overlap onto good pipe, and curing specifications, among others, to ensure that the repair performs as required. The thinking is: If it's as simple as building up layers by wrapping them around a pipe – can it really work, and will it really be as strong as steel or able to contain high pressures? The answer is an unequivocal “yes.” The establishment of rigorous standards will ensure strength and performance.

Recognizing and understanding the advantages, coupled, crucially, with both proven field expertise and the introduction of industry standards, can help to overcome any remaining hesitations over using what is, in effect, still an emerging technology. For an industry that is not renowned for its quickness to embrace new and emerging technologies, ever-widening experience and proven expertise in solving problems is behind the growing acceptance of composites within the oil and gas industry worldwide. This article will also show, through real-world examples, how operators are using composites in the field.

BENEFITS ARE THE DRIVER

Correctly designed and installed, the benefits of the technology are considerable and increasingly valuable.

Strength and weight. Is it as strong as steel? In fact, it's stronger – up to 10 times the strength of steel and twice as stiff, yet less than a quarter of the density, which is a considerable advantage offshore, where weight is money. Indeed, since space is also a key issue offshore, the fact that full structural and pressure integrity can commonly be restored with a repair just 5-mm thick is yet another benefit.

Cost. This is always a key driver. A repair solution that avoids shutdown, or keeps a scheduled shutdown short, means substantial cost-savings, both by reducing or avoiding lost production and by avoiding logistical challenges. Such logistics are often high cost, with cutting out and replacing sections of pipe, typically in areas of severely confined access, requiring cranes and lifting equipment.

Other benefits. There are several other benefits, some of which are specific to the job at hand. The fact that no hotwork is required can enable repairs to be carried out online and in environments where steel replacement would simply not be an option. The ease of handling of composites simplifies working in space-restricted areas and getting materials to the repair site. When it comes to complex geometries, the advantages of composites over steel are again apparent. Composites are not limited by length or geometry as a clamp or weld solution might be and, unlike a steel replacement option, they require no prefabrication, since they're applied by a layering process, wrapping the epoxy-impregnated carbon fiber around the damaged pipe and building to the required thickness for the application. Fiber orientation is essential for the repair to achieve proper strength. The more complex geometries require highly skilled installation to ensure correct fiber orientation, but repair of T pieces, Y pieces, elbows, nozzles, clamps and flanges can all be catered for, as can long lengths.

Finally, advanced carbon-fiber and epoxy-resin composites are both non-corrosive and low maintenance, and unlike other composites currently used, including glass fiber (which degrades slowly under UV light), carbon-fiber repairs have a design life of up to 25 years or more. Equally, where corrosion is internal, although it will continue, the composites repair can be designed to encapsulate entire sections of pipe and allow for total corrosion of the substrate, with no loss of pipeline strength or pressure integrity.

INDUSTRY STANDARDS

Growing confidence in composites is not purely due to proven successes. Crucial to establishing and maintaining credibility in this technology has been the introduction of industry standards. Initially, these were the AEA guidelines and, more recently, the progress of an ASME draft standard – and FD Alliance has made significant contributions to both. Operator confidence has been gained by setting out repair design, testing and installation guidelines, including the importance of key hold and inspection points that allow repairs to be fully validated. Moreover, when warranties are offered on materials and workmanship, such as is the case with the author's company, a further comfort level is offered to operators.

FIELD EXAMPLES

      Shell Expro's Dunlin Alpha platform in the North Sea, avoided three weeks' lost production and significant savings by repairing corroded pipework without shutdown. A Non Destructive Test (NDT) inspection revealed considerable external corrosion over an extensive section of pipework, and over 165 ft of reinforcing was required, including numerous flanges, valves and T-sections, much of it in severely restricted access areas. Advanced carbon-fiber composites technology enabled this to be successfully carried out with no disruption to production, resulting in an overall structural strengthening of the sections of pipeline, without adding significant weight, yet restoring full design pressure-containment capability for the required 10-year lifetime.

In another example, a recent repair to a damaged drill cuttings chute while operation continued on the Apache Forties' Charlie platform in the North Sea, was made possible by the ability to carry out the work lowering the necessary materials on a rope, to install the repair, which was designed for the required 15-year lifetime. This would not be possible with most other repair options.

Given the oil industry's aging assets, common high pressures and temperatures, and offshore environments, the issue of corrosion must surely rank high. Moreover, since composites don't oxidize in the same way as metals, their application to externally corroded pipelines, tanks and vessels will prevent further corrosion, even where defects are through-wall. A repair to a 30-in. n/b metering loop on a Statfjord platform for Statoil, for instance, not only repaired damage caused by external corrosion and returned the loop to its original specification, without costly platform shutdown, but it also prevented progression of corrosion damage.

Pressure and temperature boundaries are constantly being pushed, currently at 209 barg (on a water-injection system), and temperatures from – 26°C to 200°C. An application for Amerada Hess' floating production facility AH001, on a corroded carbon steel gas compression line operating at 124°C and 80 barg, represents one of the high temperature and pressure repairs undertaken by the author's company on a hydrocarbon line.

Being able to permanently repair rather than replace will always be the preferred option on caissons, given that replacement cost of a caisson is estimated at over $3 million. With caissons, the inherent strength and stiffness of composites plays a critical role here since, unlike most pipe structures, the design pressure is not the governing load – structural loads are more onerous. There is considerable external loading from wave action, causing bending, and the self weight of the caisson must be accounted for, so axial and bending strengths are key.

Composites were put to the test recently on CNR's clean water caisson K87 on Ninian North platform in the North Sea, Fig. 1. The caisson was suffering from wall loss due to internal root erosion of circumferential welds. The extent of the repair beyond the defective areas was another consideration in the design to achieve maximum strength. In this case, the repair extended along 65 ft from the top of the Monel sleeve to the underside of the captruss, as well as on to the support, and the 24-in. and 18-in. inlets. In addition to the complex loading requirements already mentioned, the repair had to allow for total loss of the caisson steelwork between the upper and lower welds that were affected, and still retain full structural and pressure integrity for service until 2020. Platform operations continued while repairs were underway.

Fig. 1. Repairs on CNR's clean-water caisson K87 on Ninian North platform in the North Sea.

In other applications, length and shapes such as T's and Y's are not a problem. A 250-ft composites repair (one of the longest to date) to a 60-in. saltwater feed line, fabricated from concrete-lined steel at one of Europe's largest oil refineries, restored full structural and pressure integrity to the damaged line, at some 30% of the cost of replacement.

SUMMARY

While growing recognition of the considerable benefits of composites technology is key to its increasing acceptance by the oil industry, so too is the confidence that is being continually boosted as successful application of the technology becomes more and more widespread. The author's company now has experience in wide-ranging applications from pipework and pipelines to caissons, pressure vessels and storage tanks, in circumstances from external and internal corrosion or damage, to uprating for loads or pressure, through-wall defects (at the time or during the lifetime of the repair), and strengthening of embrittled duplex. Furthermore, these have been carried out on a range of substrates, from carbon steel and mild steel to cast iron, stainless steel, duplex, GRP and cunifer. Work has now been carried out on projects for all the major operators.

With such clear benefits, coupled with expanding experience and proven successes plus the establishment of industry standards and validation, it should come as no surprise that the potential contribution of composites repairs to the oil-industry maintenance, shutdown and asset-life extension is increasingly being recognized. Moreover, with research and development ongoing, breakthroughs continue to be made, and the boundaries of expertise are constantly widened as the capabilities of composites are extended. Who knows how far this technology will go in transforming accepted practices in the oil and gas industry. 

THE AUTHOR
	Dr. Paul Hill has worked with polymeric materials for use in the oil and gas industry for over 15 years. Having completed his PhD at Cambridge, where he investigated methods for predicting the durability of fiber-reinforced polymers in the offshore environment, he then spent seven years with the former British Gas looking at the use of fiber-reinforced polymers within the gas industry. He moved to DML in 1998, where he is technical manager of FD Alliance. He leads the technical work in the composites group, and has overseen the design, manufacture and installation of the first composite primary structure in the North Sea. He has been a key driver behind the introduction of the Industry Workgroup and, more recently, the ASME draft standard.