Engineered composite wraps are becoming the solution of choice for dealing with corrosion issues in the North Sea on an increasing range of offshore pipework and structures. Elaine Maslin reports.
Caisson repair using Technowrap Splashzone. Photos from ICR.
The trouble with metal and water is that they don’t get along. The same goes for metal and corrosive fluids. There’s a lot of corrosion on oil and gas facilities in the North Sea, many of which were installed decades ago when it wasn’t expected they would still be operating today. As a result, there’s an increasing amount of composite repair wraps used to temporarily, and even permanently, repair damaged areas, from service lines to caissons and structural members.
With the long-term integrity of engineered composite repairs somewhat unknown, the UK’s Health and Safety Executive (HSE) launched a shared research project.
Wrap it up
Composite repairs started to make their presence felt in the early 2000s, and indeed, in 2001, the HSE produced guidelines on use of composites for repairs. However, it took until 2015 for an ISO standard to be drawn up and there’s no validated inspection method for checking bonds. Composite repairs were initially used as a temporary solution on service lines through to the next shutdown. The use has also increased dramatically (in the words of the HSE) as well as the types of repair and the the length of time for which they’re expected to be used. In some cases, where a facility is near the end of its life, repairs will be expected to last until decommissioning.
Epoxy, used to bond the materials, has also been developed for use in or under water, which means engineered composite repairs are now also being used on caissons, applied by trained divers, which otherwise might need to be replaced, as well as structural members, decks, blast walls, bulk heads and more.
These repairs have proved very good to date, says David Johnson, of HSE, but there are failures. More knowledge is needed about their long-term integrity, given the “steady proliferation” in their use, the length of time that they’re being used, as well as their use on more safety critical applications.
“Engineered composite repairs play an important role for aging assets,” he told the Topsides UK conference in Aberdeen earlier this year, including for structural repairs, such as helidecks or gas turbine struts, and I-beams. “We are [also] starting to see people wanting to use these repairs for 5-10 years,” he says. “We have started to see some failures, generally short-term. Are there any long-term modes we are not aware of at the moment? What is our understanding of repair lifetime?”
Engineered composite can have 2-20-year design lives, says Gareth Urukalo, composite repair senior technical engineer, at Walker Technical, an ICR Integrity company. Initially, many were designated as temporary repairs, but because the HSE wanted a design life so that they and operators could better assess repairs, or mark points at which repairs needed assessing, specific design lives were developed.
A 48in discharge caisson with large number of defects over a vertical length of 23m.
“They wanted to stop repairs sitting on pipework forever,” says Simon Frost, composite repair technical director, Walker. This would mean a plan could be put in place to manage the repairs, which are often between 5-20mm thick, including revalidating or replacing them at the end of their life.
The advantage of an engineered composite repair is the ability to repair complex geometries and essentially manufacturing a part in situ, instead of using clamps or other mechanical means, or welding – or evening replacing an entire pipe spool. There are different variants of engineered composite repairs, which comprise various layers of glass fiber or carbon fabric impregnated with a type of resin, typically epoxy, and applied directly to a prepared substrate. Depending on what the repair needs to do – i.e. withstand internal pressure or bending moments – glass fiber or carbon fiber is used and in some cases, both. Glass fiber is useful for areas of corroded substrate because it has more adhesive properties, where carbon fiber has more strength, Urukalo says.
Urukalo says that Walker uses epoxy because of its adhesive properties and chemical resistance to most fluids seen offshore. The firm uses various resins for different applications, depending whether they will get wet, the temperature of the environment or application in which it is being used, or if going to be subject to impacts, in which case rubber material might be used.
Johnson adds that there’s a variation in duty holder approach to use of composite repairs, with some using them on hydrocarbon lines, and others not. One example, composites were used to repair a 30in gas export header, which was deemed impractical to replace. Some 18 repairs were done over 130m of the pipeline. The project, described as a “Rolls Royce job” by Johnson, was carried out in close cooperation with the HSE.
For the HSE, such repairs should only be done as a temporary measure and only as a permanent solution if a replacement to what it is repairing cannot be sourced. HSE says that safety critical elements should be replaced like for like.
Recently, Walker Technical applied its engineered composite wrap technology to I-beams on a stair tower offshore, as well as to areas of caisson underwater, which first required a live leak seal before the wrap could be applied.
There is a standard for such repairs, ISO 24817, which covers the entire repair system, i.e. the composite laminate, surface preparation, and filler.
But, Johnson says, the quality of installation depends on the competency and proficiency of the person doing it. “Human performance is key. There are clear ramifications when it comes down to the competency of the people doing the job. You can qualify this in a lab, but you have to replicate that out on plant.”
There are two main failure modes, interfacial delamination, usually caused by inadequate surface preparation, and media coming through thickness, which is rarer, it implies a clamp has not been isolated properly and affected the cure of the resin. Failures usually occur quickly (minutes, days or weeks), following the repair, and tend to be a benign leak before being a break.
Another issue is corrosion protection continuity, i.e. where surface has been prepared but then not re-protected where it hasn’t be recovered by the repair, leaving bare carbon steel open to corrode.
Following a repair, Johnson says that steps should be taken to monitor it, i.e. taking photos to create a baseline for inspection and updating P&IDs (piping and instrumentation diagram/drawing), as well as ongoing integrity management work. This should then feed in to decommissioning work, i.e. understanding when the asset life is ending.
“Part of the problem is what happens to the pipework itself,” Frost says. While the repair might not fail, “we have to make an assumption about the wall thickness or the size of the hole [once it is covered up].” I.e. it’s not necessarily the life of the composite, it’s the life left of the pipe underneath.
Inspection regimes – how and when to inspect – should also be considered. Visual inspection is still the most commonly used inspection mode and is used to identify continuity of protection, dry fibers, or exposed or damaged fibers, or the repair starting to life, which could be the bond failing.
But, new techniques are also being used to inspect composite engineered repairs, including pulsed eddy current, radiography, and sensors embedded between the repair and substrate, which can be used to give thickness measurements, and dynamic response spectroscopy, and microwaves. Walker has been working with Sonomatic in this area, to monitor the steel underneath repairs.
However, none have been fully validated as a method for validating the bond, Johnson says. A full assessment of existing techniques is set to be part of the shared research project. One of its five work packages will cover inspection techniques and will result in a report detailing the strengths, limitations and resolution of currently available inspection techniques along with an overview of operational experience/perspective to date. There will also be a report setting out the results of tests to failure of specimen trials.
The shared research project is due to run for 24 months and will include gathering and then testing actual repairs, which have been brought back from installations. Those involved in the project include North Sea operators Total, Nexen, ConocoPhillips, TAQA, and Shell, as well as nuclear firm Sellafield, gas pipeline operator the National Grid, and EDF, from the power industry, highlighting the interest from other sectors in this type of repair work.
The five work packages are: quality assurance and integrity management; inspection and the criticality of defects; in-service performance; fire performance; and a repair installer proficiency scheme.
Following a planned inspection program, it was identified that the C8 18in Caisson on Shell’s Lomond platform in the North Sea had sustained external corrosion, resulting in a through-wall defect and a 4in spool requiring replacement as it was no longer functioning as per its design.
To avoid a dropped object situation, Shell worked with ICR’s composite engineering team on an engineered composite repair solution to re-instate the caisson with a 15-year design life. The work required one Technowrap rope access technician working in the splashzone to apply a 14-layer repair, overwrapping two metal plates with the ca. length of 2.20m. ICR’s Technowrap LT (low temp) resin system and structural strengthening cloth was used to reinstate the structural integrity of the caisson and provide pressure retention to any through wall defects.
ICR’s carbon fiber repair system, Technowrap SRS, was used to accommodate the huge wave loading. During surface preparation, the caisson holed in two areas, 75mm from the end of the bottom landing zone. ICR and Shell reviewed the issue and agreed to increase the repair size in line with the engineered design calculations.
Shell plates were installed and profiled prior to the 14-layer wrap installation being applied. The repair was completed with a design life of 15 years.
Delays were experienced due to adverse weather conditions and the surface preparation grit blasting was slow due to the paint thickness. A long tarpaulin sleeve was also required to divert water coming from the 4in line and isolate the repair area.
The work avoided a potential dropped object and was more cost effective than replacing the caisson, avoiding shutdown.