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Ageing Effects, Continuous Quality Control and Repair of Fibre-reinforced Thermoplastic Materials

Anastasios TOULITSIS 1, Richard FREEMANTLE 2, Valentina MULTANE 1, Vineet JHA 3,

Jonathan LEYLAND3, Morris ROSEMAN1

1 Element Materials Technology, Wilbury Way, Hitchin, SG4 0TW, UK

2 Wavelength NDT, The Paddock, Main Street, Elton, Derbyshire, DE4 2BU, UK 3 GE Oil & Gas, Wellstream House, Wincomblee Road, Walker Riverside, Newcastle upon Tyne, NE6 3PF, UK

Abstract This work, which is divided into two stages, investigates the ageing effects of porosity on carbon reinforced thermoplastic composites under various environments. Initially, carbon thermoplastic specimens were exposed to three different environments (dry, acid and wet). Following exposure, they underwent mechanical testing to determine any changes in behaviour compared to un-aged material. The second part of this work focused not only on the development of an ultrasonic inspection prototype to continuously assess the quality of the composite pipe as it is manufactured, but also the development of a tool and a methodology to repair the material following detection of the defect. Keywords: Ultrasonic Testing (UT), Ageing, Quality Control, Repair, Fibre-reinforced Thermoplastic Materials

1. Introduction In terms of their resistance to chemicals, thermoplastic materials are well known in the oil and gas industry for their advantages over both steel and thermosetting plastics. Moreover, their flexibility can create new designs and manufacturing opportunities for pipelines, as well as reparability, which could assist in reducing both the cost of the equipment and the impact on the environment. A thermoplastic is a polymer that softens on approach of its glass transition temperature and can be reconsolidated without significant changes in mechanical properties. Consequently, the thermal consolidation (or reconsolidation) demands careful control of both the pressure and temperature, as any change during this procedure could affect the quality of the material by introducing voids or delaminations. Focusing on the pipe industry where long lengths are manufactured, a region containing defects can lead to large amounts of material needing to be scrapped, as removing a pipe section and grafting in a new section is not an economic and viable solution. For that reason it is essential to detect and address any problems during the production run of a pipe that can affect its structural performance and service life. 2. Experimental Approach A study to assess any ageing effects on carbon reinforced thermoplastic composites has been performed. The aim of that study was to experimentally determine the effects of laminate damage on the mechanical performance of the material through exposure to oil and gas environments. For the first part of this work, material was supplied in plates having different porosity levels. Specimens were cut in the appropriate dimensions for obtaining three point bending data following full immersion tests in three different environments, the first being dry only gas and the rest being wet. There were three exposure periods and mechanical tests took place after 1, 3 and 6 months of exposure. The results were compared to un-aged material. For the second part of this work, a thermoplastic carbon fibre reinforced pipe having PA-12 both as matrix and protective liner was supplied. There were parts on the pipe with different consolidation levels to represent any possible defects during its manufacture.

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2.1. Specimens Preparation The material was carbon reinforced Polyamide 12 (PA-12) with a proprietary quasi-isotropic fibre layup. During manufacture of the plates, the porosity was controlled to simulate incorrect processing parameters, to understand how they affect the performance of the material in combination with ageing. Consolidation pressures were selected to be 0.04%, 4%, 10% and 100% of nominal. Double through transmission ultrasonic C-scan amplitude inspection of these samples showed even distribution of porosity over the area of the plates (Figure 1), and a difference in attenuation was recorded which correlated with consolidation (26.0 dB attenuation for 0.04% consolidation, 12.0 dB attenuation for 4% consolidation, 4.4 dB attenuation for 10% consolidation and 3.7 dB attenuation for 100% consolidation).

Figure 1: Comparison of Transmission for Plates Pressed at 0.04%, 4%, 10% and 100%

of nominal consolidation pressures

2.2. Exposure Environment Three different environments were chosen to assess the degree of ageing study; two environments used a pressurised gas mixture with a dry and wet environment as follows: Environment 1 was dry and contained a mixture of 70% CO2 and 30% CH4. Environment 2 utilising the same gas mixture but with NORSOK Oil (70% Heptane, 20% Cyclo-Hexane, 10% Toluene) and water added to make the environment wet and acidic. Spacers were loaded into the vessel to prevent immersion of the specimens into the liquid. Following closure of the vessel, it was filled with the test gas mixture and the environment was boosted to the test pressure of 414 bar. Heating was applied to 65oC, using an electrically controlled band heater. The pressure and temperature were maintained for the specified test duration, and recorded with a pressure transducer (calibrated using a dead-weight tester) and thermocouple inserted into the vessel close to the samples (Figure 2). These were linked to a computer running dedicated data acquisition software. In order to reduce the risk of decompression damage at the end of each exposure period, the pressure vessels were allowed to cool to ambient temperature. The remaining pressure was then released at a slow rate (less than 5 bar/minute) and the samples left undisturbed until the following day. Gas release from the vessel was undertaken via automated depressurizing equipment. For environment 3, the specimens were fully immersed in NORSOK Oil at ambient pressure and, following the closure of the vessel, the vessel was put into pre-heated oven at 65oC. In all cases the specimens were loaded vertically into the vessels and exposure periods of 1, 3 and 6 months were adopted followed by mechanical testing of the samples after exposure.

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Figure 2: Pressure Vessels for Environments 1 and 2

2.3. Testing

All flexural tests were performed on a screw-driven universal testing machine (Zwick Z50) equipped with a 5kN load cell. The flexural tests were conducted following the procedure within ASTM D7264 [1]. Displacement was applied at a rate of 1 mm/min. All results have been normalised using the average modulus and strength values of the unaged material wit 10% of nominal consolidation pressure in order to obtain a better comparison and understanding of the level of material degradation due to exposure. 3. Experimental Results The force increased linearly with displacement up to the break point, following which the equivalent stress and strain were calculated according to ASTM D7264. Tests began with un-aged specimens in order to understand the properties of the un-aged material. A comparison between the different porosity levels is presented in Figure 3.

Figure 3: Comparison of the results for different consolidation pressures

By increasing the consolidation pressure (resulting in lower porosity), there is an increase in both the maximum strength and the flexural modulus. As to be expected, the increase is more significant between pressures of 0.04% to 4% rather than 4% to 10%, as on the first occasion the difference in relative consolidation pressure is greater. All the specimens were exposed to three different environments, followed by a comparative study of the flexural properties. As already mentioned, there is an increase in the modulus with the higher consolidation pressure, because the layers are better consolidated (i.e., they behave like a unit connected together) making the specimen stiffer. As shown in Figure 4, there are no significant ageing effects on the flexural modulus for Environment 1 (the dry environment). However, there are signs of ageing for Environment 2 (the acid environment), as there is a drop of more than 10% after 6 months of exposure for all the different porosity levels.

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Figure 4: Comparison Study for Flexural Modulus

The maximum stress shows a gradual drop for all environments following exposure (Figure 5). This can be explained as the maximum flexural stress occurs in the mid span at the outer surface, which is in direct contact with the environment and has been affected more by the environments. Particularly for environment three (wet environment), there is a large drop after 1 month of exposure. The reason for this is because in this environment specimens were immersed in NORSOK oil, which results in absorbing moisture and destroying bonds until saturation point. After that, the material cannot absorb more, so the drop in the maximum stress comes from the chemical reaction from the already-absorbed liquid, a process which is much slower than absorption.

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Figure 5: Comparison study for Flexural Stress

These results show that there are minimal changes in the mechanical properties (for example that could be detected by NDT) due to porosity during environmental changes. However the NDT results confirm that the effect of porosity must be carefully considered as consolidation pressures below 10% of nominal would seriously compromise the ability of ultrasound to detect defects such as delaminations and voids. 4. Inspection Module The second stage of this work began with the design of an ultrasonic module to continually inspect the composite pipe during manufacture. The final design allows for real-time inspection during production of the pipe and reduces scrap and testing costs for product development, and enables concurrent, precise determination of the damaged area, avoiding wasting material. Moreover, it connects the transferring signal with the quality of the pipe, enabling the set of threshold and acceptance criteria of the final product. Finally, it adds value to the final product and increases the reliability for lifetime service of the manufactured products. The module consists of three parts, starting with the probes, which undertake the inspection. Then there is the case inside which the probes are located, and finally the conveyor which allows the pipe to move forward and backwards. A single probe (Figure 6) consists of the probe and the nylon delay line, which is used for transferring the signals between the composite pipe and the probe, and both are attached to a carriage system. In this way, the system is allowed to move on a linear guide to facilitate smooth movement. Moreover, there are two compression springs, which are used for forcing the system into contact with the composite pipe; the bearing between the nylon block and the carriage takes care of any misalignment.

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Figure 6: NDT System - Single Probe Design

The module uses six of the probes as described above, located circumferentially in the inner side of the case (Figure 7) with a water manifold for water irrigation, which was used for enabling the coupling of the signals between the probes and the pipe. Finally, there is the conveyor, which aligns the NDT system and the pipe and allows the pipe to move forward and backwards (Figure 8).

Figure 7: NDT System - Case Design

Figure 8: NDT System – Conveyor

The final assembly of the prototype is presented below in Figure 9, with all probes in service, using water for the coupling of the signals between the probes and the pipe.

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Figure 9: NDT System - Final Assembly

In conclusion, results of a scan are presented below in Figure 10, verifying the coupling of the system. Moreover, A and B scans through the thickness of the pipe were obtained, showing that the inner side of the pipe reflects properly. The consistency of the signals and scans proved that both the methodology and the prototype were able to monitor the quality of the pipe and detect any manufacturing defects. On the basis of these results a commercial pre-production scanner was developed by GE Oil and Gas to make further inspections of longer lengths of pipe product. A photograph of this system is shown in Figure 11.

Figure 10: Repeat A-scan and B-scan testing of the same area of pipe

Figure 11: GE’s prototype NDT system

5. Repair Module A repair module incorporating infra-red heaters and heat shrink tape to repair the affected area through the application of heat and pressure was designed and trialled. This enables concentric mounting of a maximum of eight heaters with adjustable positions in order to

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accommodate pipes up to 8-inches in diameter. In addition, the temperature was controlled and monitored during the procedure. During the repair, the pipe is rotated to ensure even heat distribution, and polyimide heat shrink tape was used as a method of applying uniform pressure over the whole surface of the pipe. For demonstration purposes only four heaters of 500 watts were used, positioned as close to the surface as possible without disturbing the rotation of the pipe (Figure 12). Moreover, LabView was used to control the heaters and monitor the temperature of the pipe.

Figure 12: Repair Module

Following the repair, visual inspection showed changes of the appearance of the outer surface of the pipe (Figure 13). The surface became smoother, without the undulations which were present prior to repair. Further investigation took place using phased array ultrasound inspections to observe the quality of the repaired pipe as shown in Figures 14 a and b. Scans prior to the repair displayed strong reflections from surface plies, no reflection from carbon/liner interface, nor from the far side of the liner, indicating poor consolidation quality of the material. Scans after repair, showed consistent reflection from carbon/liner interface and very consistent reflection from the far side of the liner suggesting the majority of pipe in the repair area has been properly consolidated.

Figure 13: Surface Appearance Prior (Left) and After (Right) to Repair

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Figure 14a: NDT Inspection prior to repair

Figure 15b: NDT Inspection after repair

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6. Conclusions Thermoplastic reinforced specimens of three different materials were supplied and have been exposed to three different environments (dry, acid and wet). Following exposure they underwent mechanical testing to determine any changes in the behaviour in comparison to un-aged material. Mechanical tests were conducted before ageing and after 1, 3 and 6 months of exposure to show any changes in the performance of the materials. A comparison between the different porosity levels has shown an increase in flexural modulus with the increase of consolidation pressure. After the end of the exposures, there is a small drop in the flexural modulus only in environment 2 following 6 months of exposure, while no noticeable changes were observed in the other environments. However, flexural stress shows a downward trend, which indicates that the outer surface has been affected by ageing mechanisms. Prototype inspection and repair modules were designed, manufactured and tested demonstrating the feasibility of the approaches that were employed. The ultrasound inspection system managed to obtain coupling on both the full length of the pipe and captured the areas with defects. Following that, the repair module managed to reconsolidate areas exhibiting excess porosity, using elevated temperatures and a polyimide heat shrink tape for applying uniform pressure over the surface of the pipe. References 1. ASTM D7264-07, Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials. ASTM International, West Conshohocken, PA, 2007.

Acknowledgements Anastasios Toulitsis performed this research work while placed at Element Materials Technology Hitchin Ltd as Marie-Curie researcher. His research and training activities were funded by the European Union's Seventh Framework Programme managed by the Research Executive Agency (http://ec.europa.eu/research/rea) and he participated in a Marie Curie Action (GlaCERCo GA 264526). As the UK’s innovation agency, one of the main roles of the Technology Strategy Board is to achieve business and economic growth for the UK. One way the organisation supports this is through funding innovative Collaborative Research and Development (CR&D) projects, one of which is the above work under the project Continuous Quality Control for Composites for the Next Generation Flexible Pipe (CQCC). Collaborative research and development (R&D) encourages businesses and researchers to work together on innovative projects in strategically important areas of science, engineering and technology, from which successful new products, processes and services can emerge, contributing to business and economic growth. Find out more about the CR&D programme here: https://www.innovateuk.org/-/collaborative-r-d