Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : April 08 , 2026
Abstract
Unbonded reinforced thermoplastic pipe (RTP) is widely used in oil and gas transportation due to its flexibility, corrosion resistance, and spoolable installation capability. However, field observations have shown that RTP may exhibit non-intuitive deformation under internal pressure, including unexpected axial strain response and pressure-induced torsional rotation.
This study examines the mechanical behavior of a long-length RTP system reinforced with non-impregnated aramid cords, focusing on the role of cord-slack and layer imbalance in governing deformation. Experimental pressure testing is combined with analytical modeling to explain the transition between different deformation regimes and the reversal of torsional direction under increasing pressure.
Results demonstrate that the mechanical response of RTP is not solely determined by nominal fiber angle or material stiffness, but by the activation sequence of reinforcement layers, which is controlled by manufacturing-induced slack differences. A multi-layer plane-strain model provides improved prediction accuracy compared to conventional thin-wall approaches.
The findings highlight that in spoolable composite pipe systems, manufacturing geometry directly influences structural behavior, with important implications for offshore installation, system alignment, and long-term integrity.
Keywords
Reinforced thermoplastic pipe (RTP), cord-slack, torsional behavior, composite pipe mechanics, aramid reinforcement, pressure deformation, spoolable pipelines, oil and gas transportation
1. Introduction
In composite pipeline systems, strength is only part of the story. For unbonded RTP, how the pipe deforms under load is often more relevant than its ultimate capacity.
Field engineers have reported cases where RTP lines:
- rotate under internal pressure
- show non-linear axial strain response
- behave differently from predictions based on classical laminate theory
These behaviors are often misinterpreted as defects or instability. In reality, they are inherent to the construction of spoolable RTP systems.
Unlike bonded composite pipes, unbonded RTP relies on helically wrapped reinforcement layers that are not fully constrained by a rigid matrix. This enables flexibility, but also introduces mechanical effects that are not captured by simplified models.
This paper focuses on one critical factor—cord-slack—and demonstrates how it governs deformation, torsion, and load transfer in RTP under pressure.
2. Literature Review
Early RTP studies focused on qualification and pressure capacity, treating the pipe as a thin-walled structure with balanced fiber reinforcement. These models assumed:
- immediate load transfer to reinforcement
- uniform stress distribution
- negligible through-thickness variation
Subsequent research identified limitations in this approach, particularly for unbonded systems. Observations showed:
- delayed engagement of reinforcement layers
- non-linear strain response
- coupling between hoop, axial, and torsional deformation
More recent work emphasized:
- the role of fiber orientation in pressure resistance
- manufacturing effects such as fiber waviness and layer misalignment
- the importance of multi-layer modeling
However, the influence of cord-slack and differential layer activation has remained underexplored, despite its clear impact on observed pipe behavior.
3. Methodology
3.1 Pipe Structure
The RTP system analyzed consists of:
- Inner HDPE liner (fluid barrier)
- Two helical layers of twisted aramid cords
- Outer HDPE protective layer
The reinforcement cords are non-impregnated, meaning there is no rigid bonding between fibers and matrix. This construction enables flexibility but allows relative movement during loading.
3.2 Definition of Cord-Slack
Cord-slack refers to the excess length of reinforcement fibers relative to the pipe geometry, resulting in:
- initial slack state where fibers do not carry load
- required “take-up strain” before engagement
Slack originates from:
- cord construction strain
- manufacturing shrinkage
- geometric differences between layers
3.3 Experimental Testing
Hydrostatic pressure tests were conducted on pipe samples, measuring:
- tangential (hoop) strain
- axial strain
- torsional rotation
The pipe was allowed to deform freely without axial restraint.
3.4 Modeling Approach
Two modeling strategies were used:
- Improved plane-stress model
- Includes slack and geometric non-linearity
- Assumes thin-wall behavior
- Multi-layer plane-strain model
- Treats pipe as a thick-walled cylinder
- Accounts for radial stress variation
- Incorporates slack terms in force equilibrium
4. Results
4.1 Three-Stage Deformation Behavior
The pressure response can be divided into three regimes:
Stage 1: Low pressure
- Reinforcement layers remain slack
- Pipe behaves like pure HDPE
- High compliance, minimal torsion
Stage 2: First layer activation
- Inner reinforcement layer becomes engaged
- Load transfer begins
- Torsional rotation develops
Stage 3: Second layer activation
- Outer layer engages
- Torsional balance shifts
- Rotation direction reverses
4.2 Torsional Behavior
The observed torsion is driven by:
- opposite torsional contributions from inner and outer layers
- unequal slack between layers
- difference in activation pressure
This explains the experimentally observed rotation reversal at critical pressure levels.
4.3 Model Validation
- Plane-stress model captures general trends but lacks accuracy
- Multi-layer model shows strong agreement with experimental data
- Axial strain discrepancies attributed to viscoelastic behavior of HDPE
5. Discussion
5.1 Manufacturing Influence on Structural Behavior
The study demonstrates that RTP behavior is strongly influenced by manufacturing:
- small variations in winding angle
- differences in layer geometry
- shrinkage during production
These factors create layer asymmetry, which directly affects deformation.
5.2 Implications for Offshore Systems
In offshore applications, torsional behavior has practical consequences:
- pipe rotation during pressurization
- stress concentration at fittings
- alignment challenges in tie-ins
Design and installation procedures must account for these effects.
5.3 Limitations of Simplified Models
Traditional thin-wall models are insufficient because:
- stress distribution is not uniform
- reinforcement engagement is not simultaneous
- torsion is coupled with pressure loading
Accurate prediction requires multi-layer analysis with slack consideration.
5.4 Design Optimization Opportunities
Understanding slack behavior allows:
- tuning of fiber angles
- balancing torsional moments
- reducing axial strain
This opens the possibility of designing RTP systems with controlled deformation characteristics.
6. Conclusion
This study shows that the mechanical behavior of unbonded RTP cannot be fully understood using conventional composite pipe assumptions.
Key conclusions:
- Cord-slack governs the activation sequence of reinforcement layers
- Layer imbalance leads to pressure-induced torsion
- Multi-layer modeling is required for accurate prediction
- Manufacturing geometry is a critical design parameter
The broader implication is:
In spoolable composite pipelines, structural performance is shaped not only by materials and design, but by how the reinforcement is introduced and engaged during loading.
For engineers working with RTP systems, this means that manufacturing details must be treated as part of structural mechanics, not as secondary considerations.
References
- Kruijer, M., Warnet, L., Akkerman, R. (2005). Analysis of the mechanical properties of a reinforced thermoplastic pipe. Composites Part A.
- API RP 15S – Qualification of Spoolable Reinforced Plastic Line Pipe
- ASTM D2992 – Hydrostatic Testing of Plastic Pipe
- CSA Z662 – Oil and Gas Pipeline Systems
- Industry studies on RTP deformation and reinforcement behavior