Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 01 , 2025

Abstract

Reinforced thermoplastic pipelines (RTP) are emerging as a serious alternative to steel pipelines for gaseous hydrogen transport, particularly where hydrogen-induced embrittlement and corrosion of metallic systems are a concern. This work synthesizes and critically evaluates a recently proposed design methodology for high-density polyethylene (HDPE)–glass fiber reinforced plastic (GFRP) RTP, originally developed in a 2023 Master’s thesis by Le Thi Doan. The pipe concept consists of an inner SDR11 PE100 liner, one or more glass/epoxy reinforcement layers, and an outer PE100 sleeve. Material properties are obtained from tensile tests on HDPE and woven E-glass/epoxy laminates using ASTM D638 and ASTM D3039 procedures. These data feed into an orthotropic cylinder model and a finite-element (FE) framework in ANSYS, validated against closed-form elasticity solutions for thick-walled composite cylinders.

A parametric study explores fiber thicknesses of 1.1, 2.2 and 3.3 mm and winding orientations of 0°/90°, ±45° and ±30° under internal pressures from 2 to 10 MPa. Results are presented in terms of utilization ratio (UR), defined as maximum principal stress divided by laminate strength, with a conservative design limit of UR ≤ 77% (safety factor ≈1.3). The analysis confirms that the GFRP wrap carries essentially all hoop load, while the HDPE liner remains in a low-stress, highly compliant state. For 1.1 mm GFRP, 0°/90° and ±45° lay-ups are suitable up to approximately 8 MPa; thicker wraps (2.2–3.3 mm) extend safe operating pressure into the 8–23 MPa range. Orientations of ±30° are consistently less efficient.

Beyond reproducing these findings, the present paper discusses the implications for hydrogen pipeline design, highlights limitations of purely pressure-based assessments, and outlines required steps toward qualification, including permeation, cyclic loading, multi-axial stress states and manufacturing defects. The outcome is a coherent design envelope for HDPE/GFRP RTP and a roadmap for turning this numerical framework into a robust engineering basis for hydrogen infrastructure.

Keywords

Reinforced thermoplastic pipeline; hydrogen transport; HDPE/GFRP composite pipe; winding angle optimisation; finite-element analysis; utilization ratio; non-metallic hydrogen pipeline

1. Introduction

Hydrogen is at the centre of many national low-carbon strategies. It can be produced from renewable sources, stored, transported and then converted back into energy with water vapour as the primary combustion product. However, the infrastructure inherited from natural-gas networks is not directly compatible with pure hydrogen service. Conventional steels can suffer hydrogen-induced cracking, accelerated fatigue, and reduced fracture toughness when exposed to high-pressure hydrogen, leading to shorter service life and higher inspection demands.

Composite and polymeric pipelines have therefore attracted attention as an alternative linepipe technology. Reinforced thermoplastic pipelines (RTP) combine a thermoplastic liner, typically high-density polyethylene (HDPE), with one or more layers of fibre reinforced plastics (FRP) and an outer protective sheath. Such non-metallic pipelines offer inherent corrosion resistance, lower weight, smoother internal surfaces and easier spool-based installation compared with welded steel.

The key engineering question is whether RTP can be designed to withstand hydrogen pressures in the range of several megapascals, while accounting for structural integrity, permeation, and long-term reliability. A recent thesis by Le Thi Doan (2023) addressed this question by characterising HDPE and glass/epoxy materials, validating an orthotropic cylinder model through FE analysis, and proposing design recommendations for HDPE/GFRP RTP under internal pressures up to 10 MPa.

The present paper revisits and consolidates that work in a structured format suitable for use as a reference document in the hydrogen pipeline community. It distils the experimental data, modelling strategy and parametric results into a coherent design framework, while also discussing limitations and future qualification needs. The focus is on RTP made from PE100 HDPE liners and woven E-glass/epoxy wraps, but the methodology can be generalised to other non-metallic pipeline configurations.

2. Literature Review

2.1 Hydrogen Transport and Steel Pipeline Challenges

Hydrogen offers clear environmental benefits over fossil fuels by eliminating direct CO₂ emissions at the point of use and reducing nitrogen oxides when combustion is optimised. Its low volumetric energy density, however, requires either high-pressure compression, liquefaction, or carriers such as ammonia. For gaseous transport in pipelines, steel remains the incumbent material, but hydrogen embrittlement is a persistent risk. Atomic hydrogen can diffuse into the steel lattice, promote decohesion, and trigger intergranular cracking, which may reduce service life by 20–50% depending on pressure, microstructure and loading history. Mitigation strategies include internal coatings, inhibitors, and pipe-in-pipe concepts, but these add complexity and cost.

2.2 Non-Metallic Composite Pipelines

Reinforced thermoplastic pipelines and other composite pipe concepts have been developed for oil, gas, and water applications. They typically use thermoplastic liners such as PE, PA11 or PA12 for fluid containment and corrosion resistance, coupled with fibre reinforced plastic (FRP) layers—glass, aramid or carbon fibres in an epoxy or thermoplastic matrix—for structural capacity. External sheaths protect against abrasion and ultraviolet exposure.

Design rules for RTP often derive from netting analysis and thin-shell theory, treating the composite wrap as a winding of helical fibres that carry hoop and axial loads. Optimal winding angles for purely internal pressure loading are around ±55°, where fibres carry hoop stress efficiently while still contributing to axial strength. Once additional axial loads, bending moments or pressure-induced tension are present, optimum angles shift toward ±45–50°.

2.3 Fibre Types and Winding Angles

Aramid fibres provide high tensile strength and reduced density, carbon fibres supply stiffness and low thermal expansion, while E-glass remains the most widely used reinforcement for economic reasons. E-glass fibres combined with epoxy typically exhibit tensile strengths up to several gigapascals and moduli in the 20–30 GPa range.

Winding angle plays a decisive role. Experimental burst tests on composite cylinders have shown that ±55° windings under pure internal pressure can achieve high burst pressures, but at the expense of axial stiffness. Other studies on RTP risers and spools report that adding axial plies (0°) or hybrid lay-ups improves resistance to bending and external loads. These findings motivate systematic parametric studies where thickness and angle are varied within the same material system.

2.4 Hydrogen-Specific Considerations for RTP

For hydrogen service, non-metallic pipelines address embrittlement but introduce new questions. Hydrogen permeation through PE liners must be controlled to prevent leakage or pressure build-up in the annular spaces. Barrier layers such as aluminium or polymeric laminates have been used in commercial hydrogen RTP products. Temperature, pressure cycling and long-term diffusion must be examined.

Structural design and permeation are therefore coupled but separable problems. The work by Le Thi Doan focuses on structural capacity under internal pressure using HDPE and GFRP, with permeation, temperature variation and cyclic loading identified as future work. The present paper follows that scope: structural design is treated in detail, while hydrogen permeation and barriers are discussed qualitatively.

3. Methodology

3.1 Pipe Concept and Materials

The pipeline concept studied consists of an inner HDPE pipe, one or more glass/epoxy reinforcement layers and an outer HDPE pipe. The thermoplastic material is a PE100 grade selected from commercially available SDR-rated pressure pipes in accordance with AS/NZS 4130. The final configuration uses an SDR11 DN80 pipe (internal diameter ≈72.5 mm, wall thickness ≈8.1 mm) as the inner liner and an SDR11 DN100 pipe (internal diameter ≈93.3 mm, wall thickness ≈10.4 mm) as the outer sleeve.

The reinforcement comprises woven E-glass/epoxy laminates. Three prepreg products were used: two with 0°/90° yarn orientations and one with ±45° orientation, each supplied as an eight-ply laminate with total thickness of approximately 1.1 mm. The epoxy binder is Kinetix R240. By stacking these sheets, reinforcement thicknesses of 1.1, 2.2 and 3.3 mm can be achieved.

3.2 Mechanical Characterisation

HDPE coupons were machined from PE100 pipes and tested in tension according to ASTM D638-10 (Type IV geometry). Tests were performed at a displacement rate of 50 mm/min. True stress–strain curves were obtained, and elastic moduli were determined using the ASTM toe-compensation procedure. Across 15 specimens, the average Young’s modulus was approximately 333 MPa, with yield strength in the range 21–23 MPa. These values reflect the thin-wall coupon geometry and are lower than bulk HDPE moduli typically reported for compression tests, but they are internally consistent and suitable for the FE model.

Glass/epoxy laminates were tested according to ASTM D3039. Rectangular coupons 250 mm × 20 mm × 1 mm with bonded tabs were loaded in tension. Twelve specimens from the 0°/90° laminate and five from the ±45° laminate were tested. From the 0°/90° tests, an average modulus of 27.1 GPa and ultimate tensile strength of roughly 364 MPa were measured in the fibre direction. From the ±45° tests, an in-plane shear modulus of about 3.9 GPa and a shear strength of ≈61 MPa were extracted. The measured Poisson’s ratio was approximately 0.149, in line with published ranges for E-glass/epoxy.

These material parameters form the basis of the orthotropic material model for the GFRP wrap in the analytical and numerical analyses.

3.3 Analytical Cylinder Model

The composite pipeline is idealised as a long, thick-walled orthotropic cylinder subjected to internal pressure. Classical elasticity in cylindrical coordinates is applied, using an orthotropic compliance or stiffness matrix in the hoop–radial–axial coordinate system. Radial, circumferential (hoop) and axial stresses and displacements are obtained as functions of radius.

The model assumes axisymmetry, no external pressure, and no bending. The HDPE liners and GFRP reinforcement are treated as concentric layers with individual material properties. For validation purposes, an auxiliary configuration—a single-layer orthotropic cylinder with known properties and geometry—is analysed both analytically and numerically.

3.4 Finite-Element Modelling and Validation

Finite-element models are created in ANSYS Workbench, using the ACP (Composite PrepPost) module to define the laminate stacking sequence and fibre orientations, and the Static Structural module to apply loads and boundary conditions. The inner liner, GFRP wrap and outer liner are represented using shell elements (SHELL181) with appropriate through-thickness integration points.

Internal pressure is applied to the inner HDPE surface, and end constraints replicate a long pipe with restrained axial movement at the ends. A mesh convergence study is performed to ensure stability of stress and displacement predictions. A mesh size producing approximately 9950 elements (around 50 × 200 in hoop and axial directions) yields converged maximum principal stresses.

Validation takes place in two steps. First, the hypothetical orthotropic cylinder is analysed analytically and via ANSYS; discrepancies in radial displacement and hoop stress are within about 2%, and axial stress differences remain below 6%. Second, the multi-layer RTP configuration is assessed qualitatively to ensure stress gradients and deformations behave as expected when internal pressure increases.

3.5 Parametric Study and Design Metric

The validated FE model is used to evaluate various reinforcement configurations. Parameters include:

• GFRP thickness: 1.1, 2.2, 3.3 mm
• Winding orientations: 0°/90°, ±45°, ±30°
• Internal pressure: 2, 4, 6, 8, 10 MPa

The principal design metric is the utilisation ratio (UR), defined as

where σ_{max, principal} is the maximum principal stress in the most highly stressed GFRP ply, and σ_allowable is taken as the measured laminate strength in the relevant loading mode. A conservative limit of UR ≤ 77% is adopted, corresponding to a safety factor of approximately 1.3. This limit defines the maximum allowable operating pressure for each configuration.

4. Results

4.1 Material Property Summary

The tensile tests confirm a sharp contrast in stiffness between the HDPE liners and the glass/epoxy reinforcement. HDPE exhibits a modulus around 0.33 GPa and a relatively low yield strength near 22 MPa, indicating significant compliance and capacity for strain without brittle failure. By contrast, the GFRP laminate has a modulus approximately two orders of magnitude higher (27.1 GPa) and an ultimate tensile strength in the mid-hundreds of megapascals.

The ±45° laminate tests yield an in-plane shear modulus of about 3.9 GPa and shear strength near 61 MPa, which are consistent with expectations for woven E-glass/epoxy. Overall, the HDPE behaves as a deformable liner and sheath, while the GFRP acts as the primary load-bearing component.

4.2 FE Model Validation

The comparison between analytical and numerical results for the reference orthotropic cylinder shows excellent agreement. Radial displacements and hoop stresses differ by less than approximately 2%, and axial stresses by less than about 6%. These levels of discrepancy are acceptable for structural design, particularly since material property scatter and manufacturing tolerances will typically exceed such numerical differences. This validation step gives confidence that the ANSYS model accurately captures orthotropic pressure response, including the effect of laminate stacking and thickness.

4.3 Stress Distribution in RTP

Under internal pressure, the HDPE liner experiences low stress compared with the GFRP. Typical simulations show inner liner stresses of the order of 0.4 MPa even when GFRP layers carry stresses exceeding 130 MPa. The GFRP wrap takes nearly all the hoop load, while the outer HDPE sleeve serves mainly as environmental protection and a secondary mechanical barrier.

Stress through the GFRP thickness is relatively uniform for the considered laminate structures and pressure ranges. For a 1.1 mm thick reinforcement layer with 0°/90° orientation, the first and last plies show almost identical principal stresses, reflecting the axisymmetric nature of the loading and the stiffness dominance of the composite over the liner.

4.4 Effect of Winding Angle at 1.1 mm Thickness

For the thinnest reinforcement (1.1 mm), the utilisation ratio grows approximately linearly with pressure. In the 0°/90° case, principal stresses climb from roughly 71 MPa at 2 MPa pressure (UR ≈ 20%) to about 357 MPa at 10 MPa (UR ≈ 98%). The ±45° configuration shows slightly lower stresses for the same pressure, leading to UR values around 19% at 2 MPa and 94% at 10 MPa.

The ±30° orientation is less favourable. At 6 MPa, UR is already above 80%; at 8 MPa, UR exceeds 100%, indicating that failure would be expected before reaching this pressure. Interpolation indicates that ±30° windings with 1.1 mm thickness can safely sustain only around 5.6 MPa at UR = 77%.

By contrast, both 0°/90° and ±45° orientations reach UR ≈ 77% at approximately 8 MPa. In this thickness range, the two configurations perform comparably, with ±45° offering slightly lower peak stress.

4.5 Effect of Increased Reinforcement Thickness

Doubling and tripling the GFRP thickness substantially reduces the utilisation ratio at a given pressure. For 2.2 mm of GFRP, the UR at 10 MPa drops to around 50% for 0°/90° and 48% for ±45°. Extrapolating the linear trend of UR with pressure suggests that these configurations could safely handle around 14 MPa before reaching the UR = 77% limit.

For 3.3 mm thickness, UR at 10 MPa falls further to about 32–33% for the 0°/90° and ±45° lay-ups. The corresponding safe pressure at UR = 77% is approximately 23 MPa. The ±30° configuration remains less efficient across all thicknesses, with safe pressures of about 11 MPa (2.2 mm) and 17 MPa (3.3 mm).

4.6 Consolidated Design Envelope

The results can be summarised as an operational envelope:

• 1.1 mm GFRP:
  • 0°/90° or ±45°: safe to ≈8 MPa
  • ±30°: safe to ≈5.6 MPa
• 2.2 mm GFRP:
  • 0°/90° or ±45°: safe to ≈14 MPa
  • ±30°: safe to ≈11 MPa
• 3.3 mm GFRP:
  • 0°/90° or ±45°: safe to ≈23 MPa
  • ±30°: safe to ≈17 MPa

Within the studied space, 0°/90° and ±45° lay-ups are almost indistinguishable in terms of pressure capacity, while ±30° is consistently inferior. These figures provide clear, thickness-based guidance for structural design of HDPE/GFRP RTP under pure internal pressure.

5. Discussion

5.1 Structural Role of HDPE and GFRP

The pronounced stiffness difference between HDPE and GFRP means that the composite wrap is the structural backbone of the pipeline, while the HDPE serves as a liner and protective cover. In practice, this implies that changes in HDPE grade or liner thickness will have limited influence on burst pressure as long as the liner does not collapse or wrinkle under vacuum or bending. The designer’s principal lever for pressure capacity is therefore the GFRP thickness and fibre architecture.

This behaviour mirrors observations from other composite pipeline systems, where the thermoplastic liner is sized primarily for handling, permeation and local buckling resistance, while the composite layer is sized for internal pressure, external pressure and bending moments.

5.2 Winding Angle Selection

The parametric results confirm familiar trends: angles closer to the hoop direction (0°/90° in the chosen notation) and ±45° combinations provide efficient hoop load carrying, while more shallow angles such as ±30° introduce unnecessary axial stiffness without proportional benefits in circumferential strength.

In pure internal pressure loading without axial tension, classical netting analysis would recommend angles near ±55°. The present work considers a woven laminate and a specific test-based allowable strength, which shifts the optimal range somewhat. The similarity between 0°/90° and ±45° performance suggests that, within the limits of this configuration and test data, both angles are structurally acceptable.

From a practical viewpoint, ±45° windings can be easier to implement in filament-wound or tape-laid pipes than strict 0°/90° combinations, and they offer better torsional stiffness and impact distribution. Therefore, ±45° lay-ups may be preferred where manufacturing methods and installation conditions benefit from more balanced in-plane properties.

5.3 Design Envelope and Safety Margin

Using utilisation ratio as a design metric provides an intuitive way to connect numerical results with safety factors. A limit of UR = 77% corresponds to a typical design factor of 1.3 on strength under static loading. This is reasonable for early design studies, but several points merit attention:

• The HDPE and GFRP strengths are based on short-term tensile tests at ambient conditions. Long-term creep, environmental ageing, and thermal cycling will reduce allowable stresses.
• Hydrogen service may result in temperature changes, local hot spots or cooling due to gas expansion, all of which can affect material response.
• Fatigue from pressure cycling is not captured by a simple static UR criterion.

Consequently, the design envelope described in Section 4.6 should be treated as an upper bound for static pressure capacity. Engineers responsible for real hydrogen networks will need to apply more conservative strength reductions when considering lifecycle loads and environmental exposure.

5.4 Model Assumptions and Limitations

The analysis relies on several simplifying assumptions:

• Perfect bonding between HDPE and GFRP layers. In practice, interface quality depends on surface preparation, cure cycle, and manufacturing defects. Debonding could shift load paths and alter stress fields.
• Shell elements represent the composite layer, which may not fully resolve through-thickness stress gradients near discontinuities, bends or fittings.
• Only internal pressure is considered. External pressure, bending due to soil movement, temperature gradients and installation loads can all influence local strain states.
• Material behaviour is treated as linear elastic up to the allowable stress, ignoring matrix cracking, fibre-matrix debonding and nonlinear HDPE response beyond yield.

These simplifications are acceptable for a first-order design framework, but they must be supplemented with more detailed analyses and testing before code-level design rules can be issued.

5.5 Relation to Hydrogen-Specific Challenges

While the structural capacity of the HDPE/GFRP system appears adequate for pressures up to approximately 23 MPa with 3.3 mm reinforcement, hydrogen service brings additional concerns:

• Permeation through HDPE: without barrier layers, hydrogen can diffuse through the liner and accumulate in the interface or outer sheath. This may cause blistering or require venting strategies.
• Temperature effects: compression and decompression, as well as environmental exposure, can shift operating temperatures away from the conditions used in material tests.
• Cryogenic excursions: some hydrogen applications may include cryogenic liquefaction, which demands different materials and design rules.

The present structural results are valuable because they isolate the mechanical dimension of the problem. They form one layer of evidence that must later be combined with permeation data, barrier design, and long-term durability testing to support deployment of RTP in hydrogen networks.

5.6 Path Toward Qualification and Standardisation

To transform this design framework into a basis for industrial practice, several steps are advisable:

1. Burst and cyclic tests on full-scale HDPE/GFRP RTP segments built according to the recommended thicknesses and orientations, under hydrogen and inert gases, to verify the predicted pressure capacities and failure modes.
2. Multi-axial loading studies, including combined internal pressure, bending and external pressure, to refine the choice of winding angle and laminate architecture.
3. Hydrogen permeation and barrier evaluations, possibly introducing metallic or polymeric barrier layers and assessing their impact on mechanical behaviour.
4. Long-term ageing programmes, including creep rupture tests, pressure cycling and environmental exposure, to derive time-dependent design factors.

Experience from existing RTP products in hydrocarbons and water distribution provides a strong foundation. The numerical work discussed here, supported by targeted testing, can help bridge the gap between conventional RTP design and the specific demands of hydrogen transport.

6. Conclusion

A structured design framework for HDPE/GFRP reinforced thermoplastic pipelines under internal hydrogen pressure has been presented, drawing on experimental characterisation and validated finite-element analysis. The main conclusions are:

1. Material hierarchy: PE100 HDPE liners and sheaths are highly compliant compared with woven E-glass/epoxy reinforcement. Under pressure, the GFRP wrap carries almost all hoop load; the HDPE primarily functions as a corrosion-resistant liner and environmental shield.
2. Validated modelling approach: An orthotropic cylinder model implemented in ANSYS, validated against analytical solutions, can predict stresses and displacements with discrepancies below about 6%. This offers a reliable platform for parametric design studies.
3. Thickness and orientation effects: For a 1.1 mm GFRP reinforcement, 0°/90° and ±45° lay-ups safely sustain internal pressures up to roughly 8 MPa at a utilisation ratio of 77%. Increasing thickness to 2.2 mm extends safe pressure to about 14 MPa, while 3.3 mm supports approximately 23 MPa. Orientations of ±30° are consistently less efficient.
4. Design envelope clarity: The utilisation-based design envelope expressed in terms of thickness, angle and pressure gives immediate guidance for sizing HDPE/GFRP RTP in preliminary hydrogen pipeline layouts. Engineers can match required operating pressures to minimum reinforcement thicknesses and fibre angles.
5. Remaining gaps: The present framework addresses static internal pressure in idealised straight pipes. It does not yet incorporate multi-axial loads, fatigue, hydrogen permeation, temperature variation or manufacturing defects, all of which need to be considered for full qualification.

In combination with permeation and durability studies, the methodology outlined here can support the development of robust, non-metallic pipelines for hydrogen transport. It offers a technically grounded starting point for future design codes, recommended practices and industrial standards in the hydrogen economy.

References

1. Le Thi Doan, T. (2023). Design of Reinforced Thermoplastic Pipelines (RTP) for Transport of Hydrogen. Master of Philosophy Thesis, Griffith University, Australia.
2. Bai, Y., Bai, Q. (2014). Subsea Pipeline Design, Analysis, and Installation. Gulf Professional Publishing.
3. Hermkens, R. J. M., et al. (2018). “Hydrogen Permeation in Polyethylene Pipes.” International Journal of Hydrogen Energy, 43(4), 1966–1975.
4. Qiao, X., et al. (2018). “Mechanical Behaviour of Thermoplastic Composite Pipes Under Combined Internal Pressure and Tension.” Ocean Engineering, 151, 92–102.
5. Sebaey, T. A. (2019). “Burst Pressure of Glass Fiber Reinforced Polymer Pipes Under Internal Pressure.” Composite Structures, 210, 196–205.
6. Fang, H., et al. (2022). “Failure Modes of Composite Risers with Glass Fiber Reinforced Plastic Layers.” Composite Structures, 279, 114732.
7. DNV. (2022). Non-Metallic Composite Pipes for Transportation of Hydrogen. DNV Report and Guideline.
8. Pipelife. (2023). “Hydrogen Transportation Using Reinforced Thermoplastic Pipe Solutions.” Technical Brochure.
9. AS/NZS 4130:2018. Polyethylene (PE) Pipes for Pressure Applications. Standards Australia.
10. ASTM D638-10. Standard Test Method for Tensile Properties of Plastics. ASTM International.
11. ASTM D3039/D3039M-17. Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. ASTM International.