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Innovative design work with hoops of carbon fibre reinforced polymer (CFRP) plates by consultant Mouchel has saved 70 extra days of shutdown on a urea prill tower for the Qatar Fertiliser Company (QAFCO).
The 65 metre high slip-form concrete structure has been strengthened with 3,500 metres of bonded-CFRP plates in just 20 days instead of the 90 days needed to carry out the job with steel or concrete.
Constructed 30 years ago, the tower is a critical part of the Qatari company’s process stream and produces 400,000 tonnes of urea fertiliser prills per year. When spalling faults and cracks appeared in the 380mm thick wall, Mouchel structural repair experts were brought in to assess the problem and specify remedial measures that would minimise business disruption.
Detailed inspection followed by finite element analyses — under dead, live, wind and thermal loading — indicated that the 31°C gradient between the heated process and outside temperature was creating hoop stress problems. The answer was to reinforce the tower exterior with bonded steel jacketing, cast concrete rings or CFRP plate hoops.
Explained Mouchel’s group head of advanced engineering, Dr Sam Luke: “We recommended CFRP plates because the work could be completed within a 25-day planned shutdown period. The steel or concrete options would take at least 90 days and the tower’s foundations would have to be strengthened to support the extra weight.”
Local firm Appollo won the main contract, with highly specialised CFRP plate bonding work going to sub-contractor Balvac of the UK after a four-way tendering process with European structural repair companies. Work was carried out under the control of QAFCO’s lead civil engineer Helge Hamnevoll.
With scaffolding and shrouds in place, the work of bonding 1.8mm thick by 160mm wide CFRP plates, supplied by MBT, around the 36 metre circumference tower for every 500mm of its height, was carried out away from direct sunlight.
The shrouds kept the structure’s surface at about 6°C below the 41°C ambient, giving suitable temperature and humidity levels for effective bonding with an epoxy adhesive specially formulated for use in the region.
As the tower is not perfectly circular, and has a three-metre square concrete access tower running up its exterior, the lengths of CFRP plate were firmly anchored in recesses cut into either side of the obstruction, instead of forming a complete ring. Three 11-metre long strips of CFRP with lap-plates at the joints were used for each ‘hoop’.
The CFRP also had to be supported for up to 12 hours in intimate contact with the layer of epoxy adhesive on the concrete substrate, using backing plates screwed to the tower structure. After 24 hours the bond was acoustically tested and any significant voids were injected with low viscosity resin.
Said Mouchel project manager Steve Godman: “With the shutdown period for the tower set at a critical 25 days maximum, the four-man team from Balvac with local support from Appollo, had to complete over four ‘hoops’ each day. After a three-day learning curve, the crew was able to bond efficiently seven hoops a day. The project was therefore completed five days early, allowing QAFCO to recoup significant amounts of its lost production.”
The tower exterior was then painted to give additional protection and enhance its durability. Remedial work was also carried out to the internal face of the tower, where broken concrete was removed and replaced for the top ten metres of the structure. Further internal concrete repairs will be carried out at the next annual maintenance shutdown.
Reported QAFCO’s Helge Hamnevoll: “We were concerned about the integrity of the CFRP technique when it was initially recommended by Mouchel. But the success of similar work on silos and chimneys in Sweden and Russia, together with the cost and time implications of the alternatives, helped us to make the right decision.”
CFRP is an outstanding structural material. It is one-fifth the weight of high strength steel and at least four times as strong — a combination that makes handling easier and reduces the installation costs. It also has superior resistance to fatigue, creep and fire in comparison to steel. Moreover, maintenance costs are minimal as the material does not corrode.
The use of bonded-CFRP plates for strengthening concrete structures has been thoroughly investigated and developed by Mouchel for use on numerous bridges, buildings and other structures.
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