Characterisation of Manufacturing Defects in Composite Pipes
Academic Institution: Robert Gordon University
Academic Supervisor: Professor James Njuguna
PhD Student: Okolie Obinna
Summary
The research, which is on thermoplastic composite pipes (TCP) offers significant benefits over conventional pipes (e.g., steel-based) such as being lightweight and non-corrosive. The composite pipe is a fully bonded pipe that is manufactured through melt fusion bonding which requires very high temperature and pressure.
However, with these parameters, certain defects can be induced and impede TCP functionality and can cause catastrophic damages in the long run. During manufacturing, the pipe is regularly monitored. When the defect is noticed, the process stops, and appropriate action is taken. However, stopping the process is costly and potentially wasteful; hence it is vital to decrease downtime during manufacturing.
The unique contribution to the knowledge of this research is providing an in-situ non-destructive means of predicting and identifying these defects through smart procedures. The research goals are to be achieved by establishing and characterizing the TCP features and the processing parameters that will foster the development of functional manufacturing prototypes at the laboratory scale. This concept will facilitate reduced repairs, reworks and significant defects of the pipe systems which is timely and hugely beneficial to the industry.
Thermoplastic composite pipes (TCP) are a form of fibre reinforced thermoplastic pipes that have proven benefits such as being lightweight (lower strength and stiffness to density ratio) and non-corrosive. Furthermore, TCP is 3 layer pipe comprising of the liner, reinforce and coated layers which serve specific functions during in-service operations (see Fig 1). However, during manufacturing, certain defects are induced because of certain parameters which eventually affect TCP performance in-service.
Current manufacturing techniques are challenged with on-the-spot detection as the pipe is regularly monitored. When a defect is noticed, the process is halted, and appropriate action is taken. However, stopping the process is costly; hence it is vital to decrease downtime during manufacturing.
Potential solutions are through process optimisation for defect reduction and an in-depth understanding of the effect of parameters that cause defect formation in the pipe. There is an intimate linked to the material features, properties, and performance in-service. The material features are the determinants for the manufacturing technique to be used. For TCP, it is a melt fusion bonding process involving heating and consolidation among other factors such as the consolidation speed and pull force. Thermal behaviour is essential at this phase as it determines the cooling rate, and this study indicates that laser heating is the better heat source in efficiency terms. Defects such as fibre misalignments, voids, and delamination are induced during manufacturing are explored.
The sources of these defects should be thoroughly understood as well as the secondary defects caused by them with the consideration of residual stress impact. The presence of manufacturing defects influences the strength and stiffness, interlaminar shear strength, toughness, and creep performance.
In addition, there is a need to explore the state of the art in defect characterization during manufacturing for TCP. Also, to develop an effective consolidation monitoring strategy for early detection of these defects during manufacturing through the application of suitable sensing technology that is compatible with the TCP. Therefore, the in-situ characterization of the TCP manufacturing process aims to derive high-quality TCP with reduced defects and need for repairs, and increased production rate in safe and eco-friendly conditions while maintaining the current manufacturing process.
Key results/outcomes
From the outcome of the reviews, the key factors to consider during the manufacturing process that influences the formation of manufacturing defects are the material selection, type of heat source, temperature distribution, equipment temperature, layup process, compaction force and layup speed.
The identified defects are fibre misalignments, voids, residual stress and delamination (see Fig 3). While fibre misalignments and voids are mainly formed during the manufacturing and processing phase, residual stress formation occurs from the mismatch in material properties which then creates delamination during manufacturing.
These defects have proven to influence the holistic mechanical properties of the TCP. Based on the tests and characterization, the failure mode at operating conditions from applied loads is the matrix cracking from these defects in the reinforced layer plies encourage delamination which results to ply layer separation in the hoop direction (see Fig4). This emphasizes the need to improve the inter and intra-laminar shear strength of the reinforced layer of the TCP during the manufacturing phase.
There is confirmation of the polyethylene functional group with varying amounts. The presence of the amine group can be attributed to siliconization of glass fibre to make it hydrophobic and improve fibre to matrix compatibility. The whole pipe thermally degrades at ~200 0C with full matrix degradation expected at 500 0C (see Fig 5). There is reduction in viscosity due to heating facilitating the rise of thermal expansion contact area and more heat is conducted between the molecules which increases thermal conductivity and diffusivity. However, there is also a contradiction involving thermal conductivity increase with fibre fraction increase and crystallinity. A plausible solution is to conduct this procedure at a broad temperature range.
In terms of damage characterization and defect quantification, X-ray computed tomography (XCT) has been used to provide the internal morphology of the TCP in 3-dimensional formats which have assisted in quantifying internal defects. An attempt of quantifying the defects inherent in pristine TCP samples and the size distribution was made through XCT analysis which finalized that the voids are nanoscaled and that only a few voids are significant. Ultrasonic inspection through Dolphicam2 has been used in examining the damages in the mechanically tested samples (flexural and compression), the compressed samples had more significant failures relative to the flexural samples.
From deriving these parameters, we are now carrying out our in-house TCP fabrication (see Fig 6) which includes utilizing both vacuum bagging and melt fusion consolidation techniques. The next objective will involve robust sensor integration for precise monitoring of the sensitive manufacturing process which can be converted into a prototype. The supervisory team remains extremely supportive and provides essential guidance with this work. The regular meetings held flexibly have enabled the research to progress with the work plan where all the milestones which include the background study, material characterizations and sample fabrication are presently being implemented.
Publications
Okolie, O., Henry Royce student facility grant 2020/2021 and 2021/2022 sessions.Latto, J., Faisal, N., Jamieson, H., Mukherji, A. and Njuguna, J., 2023. Manufacturing defects in thermoplastic composite pipes and their effect on the in-situ performance of thermoplastic composite pipes in oil and gas applications. Applied composite materials, 30(1), pp.231-306.
Awards – Society of Petroleum Engineers Bursary award 2020/2021 session.
Conferences
Conference presentation – Poster presentation at the Nanostruc Conference 2023. International Conference on Fracture, Damage and Structural Health Monitoring 2023.
Contact details
Professor James Njuguna
Professor of Composite Materials, Robert Gordon University
Okolie Obinna
PhD Student, Robert Gordon University