Abstract
This study evaluates the impact of fabrication parameters on the mechanical and physical properties of filament-wound risers exposed to varying seawater conditions. Using a Box-Behnken design, three key process parameters including winding tension, winding speed, and curing time were selected for multiple sets of repeated experiments. The composite risers were immersed in seawater at 5°C, room temperature (RT), and 70°C for 4 months. After aging, their compressive performance was tested to investigate how the manufacturing parameters affected mechanical behavior, and micro-morphology was used to examine changes in resistance and damage. The results showed that the average peak compressive force (PCF) changed very little at 5°C and room temperature. But at 70°C, it dropped sharply by 23.8%. Specimens gained 1.8% mass at 5°C and RT due to moisture uptake but lost 8.8% at 70°C because metal-liner corrosion exceeded hygroscopic gain. Process optimization showed that a winding speed of 4 m/min and tension of 45 N consistently improved anti-aging performance across all conditions.
Highlights
• High-temperature aging greatly accelerates structural damage. • Aging temperature affects the failure mode of composites. • Aging reduces energy absorption and weakens process regulation.
Introduction
As offshore oil and gas exploration progresses, there is a growing demand for offshore pipes with improved performance. Composite offshore watertight pipes are key structural components that are widely used in offshore oil and gas exploration and production.1,2 Compared with traditional metal materials, composites have superior specific strength and corrosion resistance,3–5 and are widely used in marine and aerospace applications.6–9 However, it is worth noting that under hot and humid operating conditions such as marine environments, the composites may be subjected to erosion failure,10–15 which can lead to degradation of the riser performance. The mechanical properties of marine risers are inextricably tied to their service life. Hence, conducting systematic research into these properties is not only necessary but also of profound significance for developing marine risers with far superior performance.
Currently, some studies on the mechanical properties and failure mechanisms of composite risers have also been conducted.16,17 For example, Ragheb et al. 18 conducted a strength-based reliability assessment of full-size carbon/epoxy composite risers. They determined the limiting response by combining a static global catenary model with classical lamination theory and verified its performance using the finite element method. Özbek et al. 19 discussed the crushing behavior of glass/carbon intra-layer hybrid filament wound composite tubes under quasi-static lateral and axial compressive loads. The experimental results demonstrated that interlayer delamination is the main damage mode of composite tubes under lateral compression. Xu et al. 20 investigated the effects of crushing speed, temperature treatment and winding angle on the load carrying capacity of aramid/carbon FRPS hybrid tubes. The results showed that the energy absorption capacity could be improved by adjusting the temperature treatment. Liu et al. 21 analyzed the voiding and compression properties of CNT-reinforced fiber-wound risers under axial compression loading. They found that the damage mechanism of composite risers is mainly characterized by initial resin splitting at the end of the structure, crack growth in the intertwined region and local buckling at the end of the structure. As well as localized asymmetric buckling with increasing winding angle. Lisbôa et al. 22 developed an FE-based damage-evolution framework, updated through experimental force-displacement data, showing that different winding ratios lead to distinct stiffness responses and failure mechanisms. Their results demonstrate that models ignoring the actual winding pattern cannot accurately reproduce damage progression under radial compression. Almeida et al. 23 proposed a sequential FEMU framework that couples genetic algorithms with gradient-based optimisation to identify creep parameters for cylinders under radial compression, achieving close agreement with experimental results across different winding angles and hygrothermal conditions. Their work demonstrates the effectiveness of optimisation-based FEMU routines for characterising time-dependent behaviour in filament-wound structures.
However, in actual working conditions, marine risers are subjected to seawater all the time. Therefore, in order to better explore the characteristics of composite risers in the actual environment. It is necessary to further study the seawater aging of composite risers. In recent years, many scholars have conducted studies related to seawater-aged composites. At present, the research on seawater aging of composites mainly focuses on two aspects: (1) the effects of material type and manufacturing processes on the aging properties of materials; (2) the effects of seawater environments on material properties. For example, Antunes et al. 24 evaluated the effect of exposure to seawater at 80°C for different periods of time on glass fiber/epoxy composite cylinders with different degrees of cure. It was concluded that the mechanical properties of fully cured cylinders were slightly higher than those of partially cured cylinders. Aging in seawater for 7 days had a positive effect on both circumferential tensile strength and stiffness, while there was no significant effect on radial compression properties. Orkan et al. 25 characterized the impact of PTFE particles as reinforcement in epoxy resin matrix on the erosion behavior of solid particles. Ultimately, it was found that PTFE particle reinforcement increased the erosion rate, and this negative effect persisted in proportion to filler loading. Eslava-Hernandez et al. 26 explored the implications of salt water environment on the physical and tribological properties of laminated composites reinforced with aramid fibers when subjected to erosive wear by sand particles. The results indicated that the aged material absorbed more impact energy compared to the unaged material. Mamalis et al. 27 found that the combination of moisture absorption and interfacial adhesion due to seawater aging reduced the fracture resistance of the composites. Demircan 28 aged glass fiber reinforced epoxy nanocomposites with 1% and 2% Al2O3 addition to artificial seawater at 70°C for 3 months. The results demonstrated that the incorporation of 1% nano-Al2O3 effectively reduces the water absorption rate and thus protects the mechanical, thermal and structural properties in the hydrothermal environment.
The examples given above are studies in terms of material types and manufacturing processes. Many scholars have also studied the effects of different seawater aging environments. For example, Zhou et al. 29 conducted accelerated aging tests on glass fiber-reinforced polypropylene (GFRPP) rods immersed in distilled water (DW), alkaline solution (AS), and seawater and sea-sand concrete (SWSSC) solutions at 21°C, 40°C, and 60°C for 120 days. The most pronounced degradation was observed at 60°C, where the residual compressive strength retention of specimens exposed to DW, AS, and SWSSC solutions decreased by 25.8%, 30.0%, and 33.0%, respectively, relative to initial values. Gualberto et al. 30 immersed steel/GFRP (glass fiber reinforced polymer) bonded joints in distilled and saline water at three different temperatures (40°C, room temperature, and 4°C) with prior exposure to UV radiation and without exposure to UV radiation. Results demonstrated that immersion temperature significantly influenced water absorption kinetics in both adherend and adhesive constituents, consequently altering post-cure stiffness properties. Golewski et al. 31 examined the seawater aging behavior of carbon fiber-reinforced polymer (CFRP) composites under elevated temperatures (250°C and 300°C) with exposure durations of 60, 65, 75, and 90 minutes. Both temperature parameters and exposure times were found to significantly influence thermal decomposition kinetics and resultant mechanical property degradation. Separately, Oğuz et al. 32 evaluated moisture absorption effects on the tensile properties of aramid/glass fiber-reinforced epoxy hybrid composites. The results indicated that hydrothermal aging significantly reduced the tensile strength of the composites, and delamination increased with aging and temperature.
A comprehensive review of the literature reveals extensive documentation on both the degradation of mechanical properties in seawater-aged composites and the failure mechanisms of composite tubular structures under operational loads. However, it is evident that prevailing research on seawater aging of composites predominantly focuses on isolated factors such as temperature or manufacturing parameters. Significantly less attention has been directed toward elucidating the compressive failure mechanisms of filament-wound risers with a metal liner following seawater aging. Furthermore, there exists a critical knowledge gap regarding the synergistic effects of fiber winding process parameters on the compressive performance of aged composite risers. To address this research gap, this study presents an experimental investigation specifically examining seawater aging effects in carbon nanotube (CNT)-reinforced composite risers manufactured with varying fiber winding parameters. The work is described as follows: In Section 2, the experimental procedures are described, including materials, experimental design, fabrication process, and sample preparation. Section 3 presents a comprehensive analysis of the interdependent relationships governing fiber winding process parameters, seawater aging conditions, and their collective influence on the mechanical performance and failure mechanisms of composite risers. Finally, conclusions are given to summarize our work.
Materials and experiments
Materials
In this experiment, carbon nanotube-reinforced T700/epoxy composite risers were prepared using the fiber winding technique. Carbon fiber, epoxy resin, and nanoparticles were used in the manufacturing process of the composite risers. T700/SC-12K-50 C carbon fiber, purchased from Toray, Japan (provided by the Xi’an Institute of Aerospace Composites), was selected for the fabrication. Multi-walled carbon nanotubes (model XFQ041) were supplied by Nanjing Xianfeng Nanomaterials Technology Co. The volume content of nanoparticles in the composite riser was set at 1%.
Experimental design
Experimental variables and levels.
Experimental design and test results.
Preparation process and sample preparation
Composite risers were produced by a fiber winding process, following a validated five-axis lay-up sequence of [90°/±15°/±55°/±45°/90°]. The process started with T700 carbon fibers passing through an epoxy resin bath containing uniformly dispersed carbon nanotubes. Subsequently, fibers were wound onto a metallic mandrel (outer diameter: 30 mm, wall thickness: 1 mm, length: 400 mm, surface-treated by grit blasting) following programmed helical and hoop winding trajectories defined in G-code instructions. Fiber volume fraction was regulated through precise calibration of the doctor blade-resin bath roller gap, with operational settings determined by certified process technicians. Throughout deposition, winding tension and traverse speed were maintained within the ranges specified in Table 1 to ensure optimal fiber consolidation and complete mandrel coverage.
Following winding completion, the composite structures underwent non-pressurized thermal curing in a convection oven stabilized at 150°C. Three distinct cure durations like 3, 6, and 9 hours were implemented according to experimental design requirements. All specimens with approximately 52% fiber volume fraction were subsequently passively cooled at ambient conditions to room temperature. Final specimen preparation involved precision sectioning of wound risers into 6 mm-wide test coupons using computer numerical control lathe machining.
Experimental characterization
Hydrothermal aging
Subjected to testing, with each batch divided into three groups consisting of 14 specimens per group. To examine the effect of seawater temperature, the specimens were aged in separate glass vessels containing natural seawater collected from the Bohai Sea (5–10 m depth near Huludao, China). The seawater was replaced monthly during the 4-month aging period and maintained at a salinity of 29.0–31.18‰ and a pH of 8.1 ± 0.1. The aging temperatures were controlled at 70°C, room temperature, and 5°C.
During the experiment, the sample was immersed directly in the seawater beaker without any treatment. To maintain the temperature at a constant 70°C, the sample container was placed in a thermostatic water bath. The thermostatic water bath model is LC-WB-2 from LICHEN, with a temperature control range of 5 to 100°C. For temperatures of 5°C, samples are kept in a household refrigerator with a constant internal temperature of 5°C. RT samples are kept in a natural indoor environment where the temperature changes throughout the day and season.
The samples were weighed before immersion in seawater at different temperatures. After the experiment, they were removed, cleaned with an ultrasonic cleaner, dried in an electric blast oven, and weighed again. Radial compression tests were then conducted on both the dried aged specimens and the unaged specimens.
Radial compressive experiments
The radial compression test was carried out on the TTM-100 universal testing machine with a load capacity of 100 kN. During the experiment, the composite riser specimen was placed horizontally on the support platform of the testing machine. It was compressed in the radial direction at a constant rate (5 mm/min) until the compression displacement reached 20 mm. All compression tests were conducted under identical conditions and divided into four stages (5 mm, 10 mm, 15 mm, and 20 mm) based on the displacement of the loading head. Meanwhile, data is recorded in real time through the M230 C testing machine control system. Experimental data collection occurred in segments based on predetermined displacement intervals. The compression loading, however, proceeded continuously throughout the entire process. This is to avoid affecting the coherence of the material deformation behavior due to intermediate stops. Figure 1 shows a schematic of the aging device and testing machine. Aging device and radial compression test. (a): hydrothermal aging; (b): radial compression test; (c): experimental procedure; (d): post-test specimen.
By analyzing these load-displacement curves, the crashworthiness of the composite riser can be evaluated. Crashworthiness analysis helps to characterize the crushing performance of the structure at different stages. Therefore, five important indicators are used to examine the performance of composite risers under different process conditions. During the elastic phase, load-carrying capacity is expressed through peak crushing force (PCF). The PCF refers to the first significant peak load on the load-displacement curve. As displacement progresses, the decreasing load fluctuates due to evolving damage modes beneath the descending load head. This stage employs crashworthiness parameters, including total energy absorption, mean crushing force, specific energy absorption, and crushing force efficiency. These metrics collectively illustrate the structure’s energy absorption capacity. Finally, a sharp rise in compression load indicates the third stage, marking the termination of the crushing process.
If the load is integrated in the direction of displacement, the total compression work is calculated as the total energy absorption of the composite riser. The total energy absorption can be obtained as follows,
The mean crushing force (MCF) can be obtained by calculating the ratio of total energy absorption to total displacement in compression. This index reflects the energy absorption capacity of the specimen per unit compression distance. Dividing the total energy absorption by the specimen’s mass yields the specific energy absorption (SEA). This metric evaluates the material’s energy absorption efficiency per unit mass. The calculation of these two key parameters is expressed as:
In addition, the uniformity of load variation during compression can be expressed by the crushing force efficiency as.
Crashworthiness parameters for unaged specimens.
Crashworthiness parameters of 5°C aged specimens.
Impact resistance parameters for RT aged specimens.
Crashworthiness parameters of 70°C aged specimens.
Results and discussion
Effects of moisture and heat aging on mass and morphology
Prior to the aging experiments, each riser specimen had a mass of 5.7 g. After 4 months, the masses were 5.8 g at 5°C, 5.8 g at room temperature, and 5.2 g at 70°C. It is worth noting that the mass of the riser increased by 1.8% at both 5°C and room temperature. Conversely, at 70°C, the mass of the riser not only did not increase, instead, it decreased by 8.8%. This may seem confusing because the composite and adhesive components of the risers increase in mass upon immersion. However, the risers also contain a metal component that degrades in the presence of moisture. The mass loss due to corrosion of the metal in degradation is greater than the mass gain due to adhesive and composite uptake at elevated temperatures of 70°C. The phenomenon is also observed in the metal/GFRP joints, with the joints degrading when immersed in seawater. The reduction in the mass loss was caused by corrosion in metal adherends. 33 This loss is greater than the amount of water absorption that occurs in composites.
End-face microscopic morphology of risers aged at 5°C, RT, and 70°C was examined using a DSX1000 super depth-of-field microscope. The resulting end-face morphology appears in Figure 2. Observing all the specimens, we found that the corrosion aging of the risers was similar. Corrosion was observed in the metal lining of all risers, with obvious rust visible. Pore expansion occurred in the composite material layer, with moisture penetrating along the initial micropores and generating hydraulic pressure, thus enlarging the pore size. Humid heat conditions caused the resin network to break, resulting in microcracks in the composite layer. Local interlayer delamination was also observed in the 70°C aged samples. This was mainly due to the local pull-out caused by the resin-rich areas being more prone to moisture absorption and expansion. To further reveal the microscopic damage mechanism, scanning electron microscopy (SEM) was performed on the 70°C aged samples (Figure 2). SEM images showed resin peeling on the sample surface, microcracks, and localized fiber exposure and interfacial debonding. The end face morphology and surface micrograph of the aged riser.
In summary, the aging temperature has a deteriorating effect on the performance of risers with different process parameters. Medium and low temperatures (5°C, RT) mainly cause slight moisture absorption and interfacial damage, which have a limited effect on the performance. High temperature (70°C), on the other hand, is a key factor in accelerating metal corrosion, resin degradation and interfacial debonding.
Load-displacement curve
Analysis of unaged specimens
The radial compressive load-displacement curves of the unaged risers under different process conditions are shown in Figure 3. Stiffness measurements across all 14 specimen groups exhibit relatively consistent values. This finding indicates that process parameters exert a minimal effect on the initial stiffness of the riser specimen, largely due to the metal liner functioning as the dominant load-bearing component during this stage. However, as the load approaches the initial peak, differences in the fiber-wound layers begin to appear. This leads to a dispersion characteristic of the curves in the subsequent linear section. In addition, the history curves show smaller fluctuation peaks before reaching the peak load. This indicates that the compression process is often accompanied by a transition from initial crack initiation to crack extension with interlayer delamination. In the nonlinear phase, there is a sudden drop in the loading process. Failure occurs mainly due to debonding of the winding layer from the metal liner, as well as fracture of the winding layer or fiber breakage. Load-displacement curves for unaged risers.
From Figure 3(a), it can be seen that the load in the elastic section of the curve (0-0.6 mm displacement interval) shows a rapid load rise, reflecting the elastic phase response of the material. Comparing the different processes (specimens 1 to 4), the slope of the curve for specimen 3 (winding tension 65N) has a stronger re-bearing capacity after yielding. This finding suggests that high tension is likely to enhance the fiber-matrix interfacial bonding strength. It is noteworthy that specimen 4 exhibits optimal stiffness performance, while specimens 1 and 3 have low initial peak loads. This is mainly due to the sudden delamination damage.
For specimens 5 to 8 (Figure 3(b)), the stiffnesses of specimen 5 and specimen 7 are similar, while the stiffnesses of specimen 6 and specimen 8 are close. It indicates that lower winding tension has a positive effect on stiffness. The linear segments of the curves for specimens 5 and 8 (curing time 6 h) are smoother and the load transfer is more stable. A short and long time (3 h, e.g., Specimen 3) results in weak post-curve yield load maintenance due to insufficient curing reaction. However, too long curing (9 h) may also lead to the accumulation of internal stresses, limiting the growth of the peak load of the curve.
For specimens 9 to 14 (Figure 3(c) and (d)), a comparison of specimens 9, 11, 13, and 14 reveals that specimen 13 exhibits the optimal stiffness, with an initial peak load of 0.514 kN. This indicates that both the low winding speed and the long curing time are favorable for riser stiffness. Also, the linear compression process was accompanied by the appearance of cracks and a slight brittle sound. This is basically consistent with the compression phenomena of other specimens.
Comparative analysis of aging specimens
Figure 4 show the radial compressive load-displacement curves of the aged risers at 5°C, room temperature, and 70°C, respectively. Figure 4(a), (e), (i), and 3(a), indicate that the stiffness of the aging specimens at 5°C and RT shows any insignificant change. The maximum initial load peak distributions are all close to 0.5 kN. However, specimens aged at 70°C exhibited significantly reduced stiffness. Their peak initial loads decreased by 2% (Specimen 1), 20% (Specimen 2), 18% (Specimen 3), and 31% (Specimen 4). Specimens 5 to 14 displayed roughly similar load variations, demonstrating an unchanged composite riser damage mode. This consistency further indicates minimal moisture-heat aging impact on the fundamental failure mechanism. From the overall comparative analysis of Figures 3 and 4, it is noted that the stiffness of the specimen pieces decreased after the humid-heat aging experiments. This is mainly caused by the physical and chemical bonding degradation of the fiber-reinforced composites, as well as the corrosion of the metal liner. Load-displacement curves for 5°C, RT, and 70°C aged risers.
Failure analysis
Failure of composite materials
Figure 5 shows the compression process of the composite wound risers. Observing the compression process of all types of specimens, it is found that all types of risers show similar deformation patterns. That is, the metal liner buckled significantly, resulting in synchronized concavity of the upper and lower sections of the riser. At the same time, the composite layer flares out to both sides and gradually collapses into a flat ellipse with the increase of displacement. Compression process of filament-wound risers.
Notably, the process is accompanied by typical progressive damage evolution, including initial crack initiation, crack propagation, interlayer delamination, and fiber fracture. In addition, the rebound phenomenon also occurs in the risers when the compressive loads are removed. This is mainly due to the rebound effect caused by the rebalancing of internal stresses due to changes in boundary conditions. However, rebound deflection is limited. This is largely attributed to the plastic deformation in the nonlinear region experienced during compression. The restricted rebound behavior indicates an irreversible deformation phase dominated by plastic deformation during compression.
Figure 5 shows photographs of the compressed end faces (A and B) of the riser specimens at two positions, highlighting the damage patterns developed during compression. Figure 6 presents a synthesized view of the end face damage state of all specimens at the end of compression. When the load is in the linear elastic range, the compression process is often accompanied by a crisp sound. The load curve shows slight fluctuations or small peaks, indicating that cracks have been generated at this stage, and the cracks are mainly manifested as interlaminar cracks. This is a key factor in inducing subsequent interlaminar delamination. As the displacement increases, the load reaches an initial peak and then begins to drop significantly or fluctuate violently. At this time, the main failure mode of the composite riser changes to interlayer delamination. Delamination was mainly concentrated between ±15° and 90°, ±15° and ±55°, and ±55° and ±45°. The cross-sectional damage profiles in Figure 6 clearly show that the adjacent layer interfaces at ±15° are more prone to delamination. Microscopic damage of all specimens after compression. (a): unaged specimen; (b): 5°C aged specimen; (c): RT aged specimen; (d): 70°C aged specimen.
As the load continues to increase, the compressive behavior of the filament-wound riser enters the nonlinear region. Some sharp drops are evident on the load-displacement curve. This is mainly related to the occurrence of various failures. As evidenced by Figure 5, it can be seen that debonding occurred between the 90° inner layer and the metal liner due to resin fragmentation and fiber breakage. The stresses in the damaged layer became more severe after the riser was further deformed and expanded. Rapid expansion of fractures in the transverse direction (fractures within a single layer) and in the longitudinal direction (fractures in orthogonally anisotropic layers). This leads to the fracture of individual layers and delamination of orthogonally anisotropic layers. In addition, it was observed that orthogonal anisotropic layers at ±15° and ±45° were more likely to fracture. This is mainly due to the weak off-axis stiffness caused by the winding angle. The 90° winding, on the other hand, is relatively less likely to fracture. Side by side, the fracture location of the windings along the layer thickness direction is more likely to occur at ±45° and ±55°than at ±90° (as labeled by the red line in Figure 6). It is also found that all specimens as a whole showed the above trend of compression damage.
For the unaged specimens, the damage is dominated by fiber breakage and interface debonding. Under compressive loading, fiber breakage occurs due to stress concentration, and local cracking occurs in the winding layer fracture. However, the interlayer delamination is relatively small, mainly the interface debonding. This type of damage originates from the material’s own mechanical response, with no additional degradation from seawater erosion.
After aging with seawater at 5°C, the damage condition adds new metal liner rust and microcracks sprouting. Slow seawater penetration caused localized rust spots to form on the surface of the metal liner. Cracks appear between the layers of the composite material due to moisture absorption, and the delamination between layers increases.
Under seawater aging conditions at 70°C, the samples exhibited severe metal corrosion and extensive interlayer debonding. The high temperature accelerated resin degradation and metal lining corrosion, further exacerbating interface separation. Concurrently, the number of cracks within the winding layer increased significantly, and through-cracks were generated. The outer layer material also showed widespread and severe crumbling.
From Figure 6, it can also be noted that the final deformation and damage trends are similar for all specimens. However, the compression evolution of the specimens during the compression process is not consistent, mainly in the severity of the final damage. These phenomena indicate that process parameters and seawater aging have an important influence on the compressive capacity of the riser in the nonlinear region.
Interface failure between composite and metal
Interfacial failure is a central aspect of compression damage in filament-wound risers. It mainly involves three forms of fiber/matrix interface debonding, interlayer delamination and interface separation between the metal liner and composite layer. Based on microscopic observations and mechanical responses, the unaged and aged specimens at 5°C show only limited corrosion. Their failure is mainly characterized by localized debonding and minor delamination. At room temperature and especially 70°C, seawater erosion intensifies, and high temperature accelerates resin degradation, metal corrosion, and moisture penetration. As a result, resin constraint is lost in larger regions, forming unbonded fiber bundles and severe interfacial delamination and separation. The influence of the winding angle shows a consistent trend, with the interface between the ±15°and ±45° layers being the most vulnerable to failure. In contrast, failure in the 90° layer is more strongly affected by metal corrosion and interlayer stress.
The micrograph of the metal surface between the metal liner and the composite layer after compression is shown in Figure 7. Comparing the unaged specimens, it is clear that the metal in this area is essentially uncorroded. Especially at 5°C, there is basically no corrosion. This indicates that the composite layer can effectively isolate the effect caused by seawater on the metal liner. However, the corrosion of the metal at the interface was enhanced by seawater at room temperature and 70°C. This may be due to the increase in temperature, leading to the intrusion of seawater at the interface which accelerates the erosion. Micrograph of the metal surface between the metal liner and the composite layer.
Residual resin and fibers are present on the separated metal surfaces. This indicates that interlayer resin debonding (adhesive layer separation) and fiber exposure have occurred. No separation of the metal liner from the composite layer was observed at the interface. This phenomenon typically indicates a mixed failure mode of interfacial adhesion failure and resin cohesive failure, rather than pure interfacial debonding. This suggests that sandblasting enhanced the interfacial bond between the metal and the composite material. The fiber residue at the metal is the most after interfacial separation at 70°C. This indicates that high temperature promotes resin hydrolysis with, thus making fiber/matrix debonding more severe. This is direct evidence that seawater aging weakens the interfacial bond strength.
Crashworthiness
Analysis of indicator ranges for unaged specimens.
Analysis of indicator ranges for 5°C aged specimens.
Analysis of indicator ranges for RT aged specimens.
Analysis of indicator ranges for 70°C aged specimens.
Crushing force efficiency
The peak crushing force is a key index for evaluating the structural safety and compressive capacity. Figure 8 shows the peak crushing force of four groups of specimens under different process parameters. Observing Figure 8(a), the average peak crushing force of composite risers is higher when the winding speed and curing time are at level 1 and level 3, respectively. The average peak crushing force of the composite riser is lower at level 2. This may be due to the inhomogeneity and incomplete curing of the interlayer resins under Level 2 process conditions. The PCF is relatively large when both winding speed and curing time are at maximum and minimum levels. Effect of individual variables on peak crushing force. (a): unaged specimen; (b): 5°C aged specimen; (c): RT aged specimen; (d): 70°C aged specimen.
In addition, the winding tension has an opposite effect on the average peak crushing force compared to the curing time and winding speed. Composite risers have the largest PCF at a winding tension of 45N. This is because when the winding tension is insufficient, the fibers are prone to relaxation, folding, or misalignment during the winding process. This results in lower fiber volume fraction and higher porosity. High winding tension may increase the residual stresses and form microscopic cracks. It may also lead to localized microdamage (e.g., fiber breakage), which in turn reduces the PCF. A comparison of the range of fluctuations revealed that the relative sensitivity of the PCF of the unaged specimens had the greatest effect on the winding tension. Curing time was next in importance and finally winding speed.
An analysis of Figure 8(b)–(d) in comparison with the unaged group (Figure 8(a)) reveals that the PCF values of the 5°C aged group are generally close to those of the unaged group, with the former even exhibiting the highest overall value (0.516 kN). This may be due to the fact that the low temperature temporarily suppressed the plastic deformation of the resin so that the rigidity of the material was preserved. The performance of the specimens in the room temperature and 70°C groups showed a significant decrease. This is due to a variety of factors such as accelerated chemical reaction rate, increased penetration of water molecules, destruction of interfacial bonding and deterioration of fiber properties. The PCF decreases significantly when aged at room temperature compared to unaged. Process parameters can still affect the PCF, but not as much as when it is not aged. In particular, the mean PCF value after aging at 70°C decreased by 23.8% compared with the unaged group. At this point, it is almost impossible to regulate the performance of the process parameters, proving that the high-temperature seawater environment is the main cause of the riser performance deterioration.
Figure 9 shows the mean crushing force distribution of the composite riser. This index can directly reflect the energy absorption capacity of the structure. It is found that both the non-aged group (Figure 9(a)) and the aged group (Figure 9(b)–(d)) exhibit the highest MCF at a Level 1 winding speed. This indicates that before and after aging, the effect of winding speed on energy absorption capacity is greater than other variables. It indicates that winding speed has an increasing effect on the load-carrying capacity of each compression process. In contrast, winding tension and curing time can change the MCF, but the changes are relatively small. This may be due to the fact that the quality of composite risers is mainly determined by the winding speed. In addition, it is observed that composite risers with 4 mm/min winding speed have higher MCF. However, the PCF is not the best at the same winding speed. This suggests that the differences between the risers at the first failure were primarily due to defects induced during the winding and curing stages. Whereas the continued resistance at later stages may reflect more a combination of defects and fiber density. Overall, the winding speed has a greater impact on energy absorption both before and after aging. Effect of individual variables on mean crushing force. (a): unaged specimen; (b): 5°C aged specimen; (c): RT aged specimen; (d): 70°C aged specimen.
Figure 10 depicts the crushing force efficiency of the composite riser for different process parameters. Analyzing Figure 10, for winding speed, the CFE first decreases, reaches its lowest point and then increases. It indicates that the composite risers fabricated at 12 m/min exhibit poor energy absorption. This is mainly due to the relatively small PCF at level 2 of winding speed. The winding tension is similar, again falling and then rising. In the 70°C aging group (Figure 10(d)), the law is disturbed because defects such as fiber damage and matrix degradation within the material become dominant in energy absorption. We found that the CFE of the unaged risers was generally higher. And 137.94% of the maximum value occurs at the level 1 curing time of the unaged specimens. The variation of crushing force efficiency at curing time varies in different groups of specimens, but the variation is smooth in general. Overall, the composite risers fabricated with 6 h curing time have better overall crushing efficiency. Effect of individual variables on crushing force efficiency. (a): unaged specimen; (b): 5°C aged specimen; (c) RT aged specimen; (d): 70°C aged specimen.
Comparative analysis reveals that all aging conditions (5°C, RT, 70°C) significantly influence CFE. Furthermore, the aging process causes the effect of each process parameter on the CFE to change. It indicates that the winding process parameters and curing time have different effects on the energy absorption capacity of the composite risers under different aging conditions. In addition, it can be observed that the CFE is basically greater than 100%. This indicates that the composite risers under the selected process parameters have good compressive strength in the nonlinear phase. The phenomenon is influenced by both process parameters and aging conditions. These factors are mainly characterized by significant fluctuations in the CFE values.
Energy absorption capacity
Figure 11 displays the total absorbed energy of the specimen under compressive loading. From Figure 11(a), the winding speed shows a strong effect, and the EA decreases with the increase of winding speed. Curing time impacts EA non-monotonically, namely, initial reduction followed by an increase. Similarly, EA peaks then declines with escalating winding tension. The effect of process parameters on EA is consistent with the trend of MCF. As is consistent with the actual situation, and the average EA of each sample is around 10 J. This indicates that the EA is not sensitive to the three variables, and the EA is primarily determined by the structural parameters of winding angle and sequence. Effect of individual variables on energy absorption. (a): unaged specimen; (b): 5°C aged specimen; (c): RT aged specimen; (d): 70°C aged specimen.
Analyzing Figure 11(b)–(d), the EA showed a decrease after seawater aging. Among them, the decreases in the 5°C and room temperature aging groups are small, while the decrease in the 70°C aging group is large. All three groups maintained small EA fluctuation ranges (<10 J). Similarly, it is found that the effect of winding speed and winding tension on EA before and after aging is constant. The influence of curing time on EA exhibits distinct patterns under varying aging conditions, which suggests that curing time and aging environment have a coupling effect on EA. Such behavior likely stems from seawater-induced resin degradation and interfacial debonding, where aging degradation overrides curing-related crosslinking optimization. It is found that seawater aging has a greater effect on the EA values, while the difference in the fluctuation is not obvious.
Composite riser specific energy absorption exhibited variability due to specimen mass differences. This variability, however, did not alter alignment with the fundamental EA trend. The unaged group (Figure 12(a)) presents a larger SEA at level 1 of winding speed and level 2 of winding tension, which are 2.0252 J/g and 1.9393 J/g. And SEA continued to decrease with the rise of winding speed. This is because excessive velocity exacerbates layup misalignment. These defects then reduce the energy absorption efficiency. SEA demonstrates an initial increase followed by a decrease as winding tension rises. It can be seen that the moderate tension enhances the energy absorption capacity of the riser. In the seawater aging group (Figure 12(b)–(d)), the law still holds, except that aging depresses the SEA values overall. The SEA of the specimens before and after aging had a small range of variation with curing time. And the trend of the effect on SEA is similar to that of EA. This indicates that the effect of curing time on SEA is weak and is influenced by the aging environment. Comparing the range of fluctuations in specific energy absorption, it is evident that the parameter that most affects this metric is winding speed. This is followed by winding tension and curing time, as shown in Figure 12. Effect of individual variables on specific energy absorption. (a): unaged specimen; (b): 5°C aged specimen; (c): RT aged specimen; (d): 70°C aged specimen.
Analysis of moisture and heat aging
Effect of temperature on compressive performance
To investigate the effect of aging temperature on the performance of composite risers individually, specimens with the same process parameters (45N, 12 m/min, 6h, corresponding to the 11th specimen) were selected. The performance differences of the risers were compared under the conditions of no aging, 5°C seawater aging, RT seawater aging and 70°C seawater aging.
The macroscopic performance shows that the core crashworthiness index of the riser shows a gradual decreasing trend as the aging temperature increases. The PCF of the unaged specimen is 0.464 kN, which fluctuates slightly after aging at 5°C and RT (0.456 kN and 0.424 kN, respectively). However, the PCF plunged to 0.308 kN after aging at 70°C, a decrease of 33.6%. The change of SEA was more intuitive, which was 1.855 J/g in the unaged state. Following aging at 5°C and room temperature, specific energy absorption declined to 1.770 J/g and 1.737 J/g, respectively. A further reduction to 1.579 J/g occurred under 70°C, representing a 15% decrease relative to the unaged state (Tables 3 to 6). Aged specimens at 70°C exhibit significantly reduced initial stiffness in load-displacement curves. Their linear segment slopes decrease relative to unaged specimens, while post-peak load declines become steeper. This reflects the weakening of the material load-bearing stability with increasing temperature (Figures 3 and 4).
Microscopic observations show that high-temperature hydrolysis causes resin fracture and promotes interlayer delamination in the composite cross-section (Figure 13(a)). Radial compression testing further induces fiber fracture and crack propagation. These damages exacerbate debonding at the fiber/matrix interface (Figure 13(b)). Microscopic image of the sample after compression test at 70°C.
Effect of process parameters on seawater aging
Under different aging conditions (5°C, RT, and 70°C seawater), the three process parameters were further analyzed to regulate specimen performance. Process combinations with superior anti-aging properties were thereby identified. Across all temperatures, winding tension at level 2 provided optimal aging resistance. The SEA reached 1.8106 J/g at 5°C with minimal PCF fluctuations, 1.8068 J/g at RT, and 1.6019 J/g at 70°C. This improvement was attributed to tension-induced fiber compaction, which reduces moisture penetration and pore expansion. Notably, winding speed at level 1 consistently enhanced aging resistance, yielding the highest SEA values: 1.8321 J/g at 5°C, 1.8287 J/g at RT, and 1.6181 J/g at 70°C. The lower speed decreased initial microporosity, shortened the moisture penetration path, and reduced interfacial debonding, while also improving fiber–resin infiltration. This effect is beneficial in resisting resin hydrolysis and fiber/matrix debonding at both low and high temperatures. For curing time, the optimal values varied with conditions: 9 h at 5°C, 3 h at RT, and 6 h at 70°C, corresponding to SEAs of 1.7995 J/g, 1.7729 J/g, and 1.5920 J/g, respectively. Thus, curing time should be selected based on the riser’s actual service environment. Overall, integrated performance across the three conditions identified optimal parameters: winding speed of 4 m/min (Level 1) and winding tension of 45 N (Level 2). Condition-specific curing times were determined as: 3 h (Level 1, RT), 6 h (Level 2, 70°C), and 9 h (Level 3, 5°C).
Conclusion
This study presents a comprehensive experimental work on seawater aging of carbon nanotube reinforced filament-wound risers with different processes at different temperatures. It is found that mass changes due to seawater aging reflect the degree of structural deterioration. At 70°C, the metal lining corroded more than the composite material absorbed moisture, resulting in a mass loss of 8.8%. This indicates that structural damage is more significant at high temperatures. Therefore, seawater aging temperature is a crucial factor affecting riser performance. As aging progresses, the failure mode of the riser shows progressive deterioration. Aging at 5°C has a limited effect, causing corrosion, microcracking, and interlayer delamination. At room temperature, aging leads to increased delamination and interfacial debonding, slightly reducing performance. Aging at 70°C accelerates metal corrosion and resin degradation, resulting in significant performance loss, manifested as metal-composite separation, crack propagation, aggravated interface debonding, and extensive outer layer fragmentation. In addition, seawater aging reduces the energy absorption capacity of the composite risers, and the effectiveness of process parameter regulation diminishes with increasing aging temperature. Process optimization showed that a winding speed of 4 m/min and a tension of 45 N optimized anti-aging performance under all aging conditions. The curing time, however, should be determined based on the specific application environment.
Footnotes
Acknowledgements
This study was supported by the Fund of Provincial Key Laboratory of High-end Deepsea Machinery Equipment (No. SYH2024006), and Natural Science Basic Research Program of Shaanxi Province (Program No.2024JC-YBQN-0405). In addition, we would like to thank all the colleagues and anonymous reviewers who helped to improve the paper.
CRediT authorship contribution statement
Yuan Jiang: Writing – original draft, Investigation; Chao Kang: Review & editing, Supervision; Ni Zhuang: Analysis; Zan Liu: Review & editing; Yuan Hu: Review & editing; Jian Zhang: Review.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the Fund of Provincial Key Laboratory of High-end Deepsea Machinery Equipment (No. SYH2024006), and Natural Science Basic Research Program of Shaanxi Province (Program No.2024JC-YBQN-0405).
Data Availability Statement
Data will be made available on request.