As facilities and structures mature, engineers are looking for ways to safely sustain the assets' lives and usefulness as cost-effectively as possible. Composites may be the answer.

Recent work undertaken by FD Alliance (FDA - the alliance between Furmanite and DML Composites) on caisson rehabilitation, demonstrates the ever-widening circumstances in which relatively new composites technology can be successfully applied to provide permanent solutions at a fraction of the cost of replacement. A further development in the use of composites technology could prove of significant interest and high value to operators of mature assets in the North Sea and around the world. The use of highly engineered, custom composites solutions to restore permanently full structural strength to damaged or weakened caissons, enables operators to avoid extremely costly replacement - this idea was put to the test recently on CNR's Ninian North platform in the North Sea.

While composites are now increasingly accepted as a highly cost-effective route for permanent pipeline repairs, use of the technology in more demanding stressfields such as caissons has not been widely seen to date - perhaps because the engineering and design skills required are more challenging than those involved in pipeline repairs, and are not something all composites providers can offer. Yet composites are ideally suited to such applications, capable of being designed to cater to the complex loading requirements, and to allow for total loss of the original caisson steelwork - effectively providing a permanent new structure, but one that's created while platform operation continues.

Paul Smith, FDA business manager, pointed out, "Perhaps because caissons don't involve the same high pressures seen elsewhere on the platforms, they don't always seem to be subject to the same strict maintenance routine, and as assets have aged, often outliving their design life, quite a number are now seeing corrosion or erosion damage. Given that the cost of a single caisson replacement is estimated at some £2 million (US $3 million), and that most existing structures in the North Sea are believed to have at least one or two caissons needing repair, the value of such a solution to operators speaks for itself."
In the case of CNR's clean water caisson K87 on Ninian North, this was found to be suffering from wall loss during a routine inspection, due to internal root erosion of circumferential welds. Clearly a solution was needed, but shutdown and replacement were to be avoided. Mark Ferguson, CNR facilities superintendent, explained, "A costly unplanned shutdown to replace the caisson was not really an option, due to the monetary outlay and disruption to production, not to mention the logistics involved to replace the caisson. Fortunately we have been working with FD Alliance on a number of other rehabilitation projects on our assets so were fully aware of the advantages and capabilities of composites to provide an alternative solution in this case."

Key to a successful caisson repair using composites technology, is the need for high axial strength. "Whereas in the case of most pipe repairs the pressure requirements that need to be taken into consideration are predominantly those within the pipe (so the design pressure is the governing factor), with caissons there is clearly considerable external loading from wave action, causing bending, and the caisson's own weight must be accounted for - so axial and bending strength are key as it's the structural loads that are the most critical," Dr. Paul Hill, FDA technical manager, explained.

This is a factor that sets advanced composites repairs apart from many other composites solutions - the ability to provide multi-axial strength (in other words, to handle both hoop and axial stresses) by using carbon inclusions with multi-directional fibers to withstand maximum operational loads. In the case of the Ninian North caisson, FDA designed the repair against a 100-year return period current and wave loading, taking into account the bending caused by wave loading in terms of static and fatigue strength and buckling, and transverse shear due to wave loading.

Pressure integrity was clearly also a key factor in engineering the repair, which was designed against a pressure of 0.1 bar. Just nine carbon fiber plies [about 0.35-in. (9 mm) thickness in total] were required for the lower 10 ft (3 m) of the repair, and only five plies [about 0.2-in. (5 mm)] for the remainder. Significant modeling and finite element analysis were used to design the repair and ensure it was fit for purpose.
The extent of the repair beyond the defective areas was a further factor considered in the design to achieve maximum strength. In this case, the repair extended along a total of nearly 66 ft (20 m) 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 (at EL+73.8-ft (22.5 m) and EL+78.0-ft (23.8 m) respectively).

"Not only are we able to provide an effective repair, but a permanent solution to the widespread problem of caisson damage, and one that can be applied while operations continue, so we not only avoided the need for an unscheduled shutdown, but removed the need for (and cost of) replacement even during a scheduled shutdown," Smith pointed out.

Hill added, "We can engineer the design for a 25-year lifetime, or more. The net result is a caisson repair that is stronger and stiffer, yet lighter, than a steel replacement would be - and it's corrosion-free. In the case of the Ninian North caisson, the repair has been designed conservatively to allow for total loss of the caisson steelwork between the upper and lower welds that are affected, and still retain full structural and pressure integrity for service until 2020 - in line with CNR's requirements," he said.

The light weight and flexibility of the composite materials, and the fact that no pre-fabrication or hotwork is required, are key factors enabling the caisson strengthening to be undertaken while operations continue.
Installation is undertaken by FDA's own highly trained technicians. "It may look like a simple process to wrap layers of epoxy resin-impregnated material around the caisson, but high-integrity repair requires a high level of expertise and knowledge of the materials and their performance," Hill explained. "Every stage, from the critical surface preparation through to curing, will contribute to the long-term performance of the repair and procedures must be adhered to. The best designs are no good if improperly installed. All our repairs are fully validated, and to do so we need to have full control over every stage."
All FDA composites repairs are undertaken in full conformance with Industry Work Group (IWG) specifications and ASME draft standards. The installation is backed by a 12-month warranty.
Hill added that interest is already being expressed by a number of other operators for similar projects. "It is important that really high quality, high performance technology is used for such work," he stressed. "This is one instance where 'standard solutions' simply can't be applied - the repair has to be skillfully engineered to take into account all the factors, or it simply won't have the lifetime performance that makes this technology so valuable."

The capabilities of advanced composites technology clearly go far beyond the now widely accepted domain of pipe repairs. Caisson rehabilitation is just one example. And in the ongoing challenge, particularly in mature regions such as the North Sea, to extend the life of aging assets as cost-effectively as possible, the potential contribution of composites, providing a permanent alternative to replacement, is increasingly evident and desirable. Already hailed as potentially the most significant development since steel, and with new applications successfully expanding all the time, who knows how far this technology could go in helping operators optimize their assets?

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