Repair & Strengthening of Pressure Pipes for
oil, gas and water industries with Carbon FRP

(Patent Pending)
by Professor Mo Ehsani, PhD, PE, SE

FlexiTube® is a patent-pending economical techniques developed by Prof. Mo Ehsani for repair and strengthening of pressure pipes used in water, oil and gas industries. These pipes may have diameters ranging between 6-48 inches (150-1200 mm) that would be impossible or very costly to repair with current techniques. This repair and strengthening technique is particularly suitable for high pressure oil, gas and water mains that may have lost their original strength due to corrosion of steel or defective welds, etc.

At least once a month, there is a major news story about a gas pipeline failure, explosion, death, etc. The most severe recent failure in the U.S. was the San Bruno pipeline and the news of how PG&E is dealing with those problems still lingers in the media. More recently there have been other news story about a federal lawsuit settlement caused by a gas line explosion due to corroded welds and the challenges that the U.S. Government is facing because of a US-built pipeline in Afghanistan heading towards catastrophic failure.

With billions of dollars spent on repairs and fines and considering the aging pipeline infrastructure there is clearly a need to address this issue and develop cost-effective methods to strength and extend the life of these pipelines. While our previous award-winning products such as SuperLaminate™ are being widely used to repair short segments of gas pipelines, FlexiTube® carbon FRP system described here is a new product that allows repair and strengthening of miles of deteriorated pipes economically.

What is FlexiTube®?
FlexiTube® is a radially expandable braided tube made of high-strength, high- modulus carbon fibers as shown here. The tube has a thickness of less than 0.05 inches (1.2 mm). Unlike all other FRP tubes used in repair of pipes that have a fixed diameter, the unique design and construction of FlexiTube® allows it to change its diameter by approximately ± 30% of the base diameter. For example, a 12-inch (300 mm) diameter FlexiTube® can fit pipes with diameters ranging from approximately 8 to 16 inches (200 to 40 mm). This means unlike other repair techniques, FlexiTube® will change its diameter around bends and elbows, allowing it to remain in full contact with the host pipe at all points.

How is FlexiTube® Installed?
First the interior surface of the pipe is cleaned. Next, FlexiTube® is placed inside a long plastic bag and epoxy resin is added to saturate the FlexiTube®. The plastic bag is pulled inside the pipe and it is laid along the length of the pipe requiring repair. A cylindrical installation device similar to a Pipeline Intervention Gadget (PIG) consisting of spring-loaded heating surfaces is inserted inside the FlexiTube®. As the PIG is slowly pulled inside the FlexiTube®, the heated spring-loaded plates push the FlexiTube® outward against the surface of the pipe. The epoxy is designed to cure in about 3 minutes once it is heated. The number and size of the heated plates and the speed of travel of the PIG will be selected to ensure that the FlexiTube® is in contact with the heated plates for the required 3 minutes. If necessary, the procedure can be repeated and another FlexiTube® installed.

The FlexiTube® can be designed to resist the entire internal pressure of the pipe with virtually no reliance on the host pipe. Alternatively, the system can be designed such that the internal pressure of the pipe is resisted in a shared manner by both the FlexiTube® liner and the host pipe. It is possible to spray a final coating inside the pipe to achieve an impervious liner of desired smoothness.

What Kind of Pipe Can be Fixed with FlexiTube®?
The method presented here can be used to strengthen any pipe including cast iron and steel pipes for larger throughput capacity. This technique works equally well for corroded pipes as well as undamaged pipes. In both instances, the installation of FlexiTube™ will strengthen the pipe so that the pipe can be operated safely at a higher pressure that would allow for a larger volume of materials to be transported through the pipe. More importantly, it will allow the pipe to operate safely at the current or original design pressure levels.

Many water mains made with steel or concrete that operate at relatively high pressures can also benefit from this technology. Long segments (e.g. several miles) of such pipes can be strengthened for additional capacity by internal lining of the pipe with FlexiTube®.

To print a pdf copy of an article that describes some of the features of this system in more detail, please click here.