The NIC, working with Saudi Aramco Technologies Company (SATC) and Future Pipe Industries (FPI), has begun a feasibility study into innovative joining solutions that will address the common issues currently limiting wider adoption of non-metallic pipes. The main issue with the current joints is the rigorous surface preparation required to ensure a good adhesive bond between the pipe ends - a process that must be carried out in the field, where quality is more difficult to control.
The proposed solution is to replace the adhesive with a thermoplastic welded solution. The main advantage of this approach is that layers of thermoplastic can be applied to the joining surfaces at the manufacturing stage, where strict quality control measures can be relied upon. Subsequent joining in the field would then not require any further preparation as the polymer welding process is much less sensitive to surface condition. The quality of the joints could therefore be guaranteed, significantly reducing the chance of failures in service.
Welding thermoset composites is, however, not straightforward, and depositing the thermoplastic layer on the surface of the thermoset has to be carried out in a way that not only provides sufficient adhesion strength, but is also compatible with 16-inch pipe diameters. The idea of using a thermoplastic joining interlayer has been successfully used to join aerospace thermoset composites by modifying the composite’s surface layers, adding thermoplastic fibres, fillers or particles to the matrix, creating a pseudo-thermoplastic interface. However, this approach is not practical (economical) for joining resin pipes for the O&G industry.
A recent TWI Core Research Programme (CRP) project (Flipo et al., 2019) successfully applied a polymer layer on to the surface of a continuous fibre thermoplastic composite using linear friction welding (LFW); a process originally developed for joining metals. Once coated, these parts were joined to each other using the linear friction welding technique. The resulting joints were found to have strength comparable to an adhesive, without any signs of damage to the underlying composite fibres.
The same approach was adopted by the NIC, after substituting the thermoplastic composite for samples of glass reinforced epoxy (GRE) thermoset laminates provided by FPI, comprising the same fibre/matrix system used in their WaviStrongTM RTR pipes. A series of welding operations was carried out using a variety of surface preparation processes, joint geometries and welding parameters to produce a number of polymer-coated thermoset laminates. These were then welded to each other to produce a series of lap-shear coupons. Similar joint configurations were produced using the same GRE laminates and the epoxy adhesive currently used by FPI in their adhesively bonded joining system.
Mechanical testing carried out on all the joined coupons showed that the welded joints could achieve almost the same strength as when the adhesive was used, demonstrating the potential for a welded joint to replace an adhesive. Considering the practicality of the proposed solution, linear friction welding can readily be adapted to a rotating friction configuration to apply the polymer onto the outside of the pipe ends and to the inside of a pipe coupler. Once in the field, any of the conventional welding processes could be used to complete the joining process.