Thermoplastic composite pipes
|Publication Date:||1 September 2019|
This standard provides the design philosophy and requirements to specifications of loads and global analysis valid for TCPs. The standard applies to all newbuilt TCP systems and may be applied to the modification, operation and upgrading of existing pipelines and risers.
Description of thermoplastic composite pipe
TCP are flexible bonded structures and the composites are fibre reinforced laminates with a thermoplastic matrix. The pipes are similar to composite pipes made of fibre reinforced thermoset composites. However, the thermoplastic material is more flexible allowing the thermoplastic pipes to be used in applications where higher bending strains are needed. This standard will therefore be largely based on technology and knowledge related to composite thermoset pipes or risers and unbonded metal reinforced flexible pipes or risers. The novelties will be the combination of requirements, and some aspects being unique for bonded flexible thermoplastic pipes and thermoplastic materials in general.
Qualification of thermoplastic composite pipes for offshore use
The standard provides recommendations and information including technical provisions and acceptance criteria pertaining to TCPs in offshore use. The standard addresses three main areas:
- design of TCPs for designated offshore use
- testing of performance characteristics for TCP´s materials and pipes
- analysis of TCP as part of an integrated system.
In addition, at a high level:
- production and delivery of TCPs.
The standard provides recommendations for the design, testing and analysis of TCPs for use in:
- dynamic riser systems in accordance with DNVGL-ST-F201
- subsea pipelines in accordance with DNVGL-ST-F101.
In both cases this standard describes methods to meet the acceptance criteria for:
- Composite materials in accordance with DNVGL-ST-C501.
- Dedicated partial load and resistance factors are specified herein for limit states relevant to TCPs.
The standard provides extensive acceptance criteria versus requirements for basic material properties. The need for extensive material testing is due to the nature of components made from composite materials.
Transfer of responsibility at delivery
On delivery, the responsibility for the pipe is transferred from the supplier to the recipient. Safe and reliable service depends on the proper use and maintenance of the TCP as well as timely decommissioning should its condition deteriorate unacceptably. In support of this, the standard includes production and operational (inspection and repair) requirements, evolving from the design, and assumptions made for the design, that should be observed in service.
However, this standard does not specify complete operational requirements and manuals for users and operators to ensure that the TCPs remain fit for service when in use. Such specifications depend on the system that the TCP is part of and would need to be developed on a case-by-case basis. Any such specifications shall ensure that the TCP is used within its design envelope.
Range of applications for thermoplastic composite pipe
The applications addressed in this standard are static and dynamic pipes used, for example, as:
- flow lines
- choke and kill lines
- expansion spools
- well access lines
- chemical injection lines
- downlines for pipeline pre-commissioning
- intervention lines.
No restrictions are put on fluids or environments, as long as the component's compatibility with the fluids and environment can be demonstrated. The fluids inside the TCPs addressed in this standard are typically liquids and gases such as hydrocarbons, water, air, nitrogen or injection chemicals.
The global loads acting on the TCP will be different for each application and are not covered in this standard. They need to be assessed based on system analysis. Some guidance and requirements are given in [3.6].
General build up - components
The TCP assembly comprises a pipe body and end fittings, see Table 1-1.
The pipe body is made up of three main components: an inner thermoplastic liner, a thermoplastic laminate (TCP laminate), and an outer thermoplastic cover. The inner thermoplastic liner serves mainly as a barrier towards the inner fluid. The TCP laminate is the load bearing component. The outer thermoplastic cover is a protective layer. The inner liner, laminate and cover are all bonded or fused to the TCP laminate. A schematic of a typical cross section is shown in Figure 1-1. The interface between the liner and TCP laminate, and interface between the TCP laminate and cover are also defined as pipe components. This is because their properties and behaviour are important to the function of the TCP. Additional outer layers may be added as special-purpose layers, e.g. as local wear and tear protection or fire protection.
Typically the inner liner is an extruded thermoplastic pipe. The TCP laminate is a thermoplastic matrix reinforced by fibres, typically made by tape winding or filament winding. The cover is usually extruded over the laminate.
Sometimes the TCP does not have a liner or cover. In these cases the TCP laminate shall additionally fulfil all the requirements of the omitted liner or cover.
The end fittings (or end terminations) are attached to the ends of the TCP. A generic end fitting is shown in Figure 1-2. This standard addresses the composite-to-metal interface and aspects of the end fitting that relate to TCP performance, such as load transfer and tightness, see also [1.2.9].
Upper and lower end fittings may be designed differently due to different loading conditions.
All the components are summarized in Table 1-1.
Ancillary equipment may be added to the TCP, such as an outer special purpose layer, bend stiffeners/restricto
Interactions between such equipment and the TCP shall be considered.
This standard is applicable to all thermoplastic matrix materials.
Thermoset and ceramic matrix systems are not covered by this standard. These materials may still be used in the end fittings. Their performance shall be documented according to DNVGL-ST-C501.
Thermoplastic composite pipe fibres
The main load-bearing fibres shall form a continuous reinforcement. Short fibres may be used as fillers.
The fibre material shall not undergo a phase change while the TCP is in use.
The fibres shall always be kept below their melting temperature Tm during the full life cycle of the TCP, such as during production and operational phases.
Some splicing of fibres is acceptable, as described in [7.8.14] and [13.2].
Temperatures during any phase shall never cross the glass transition temperature. Exceptions may be made if TCP properties are measured above and below Tg of the fibres, see [A.2].
This standard covers the following fibre types:
Laminates with a mixture of different fibres (hybrids) may be used. Other fibres may be considered if the requirements of [1.2.8] are fulfilled. This standard does not cover steel and metal fibres.
End fitting - composite metal interface
End fittings are generally made of metals. This standard covers the composite metal interface and interactions of the metal end fitting with the pipe section. It covers aspects of the metal that relate to TCP performance, such as load transfer and tightness.
The design of the metal parts of the end fittings themselves and of flanges, etc. is not covered by this standard. Metal components shall be designed in accordance with relevant standards, see [8.3].
Any taper in the metal end fitting to ensure smooth transition of forces into the pipe shall be considered in the design.
If the TCP has extra layers or additional thickness at or near the end fitting, the component shall be designed and analysed as a new section of the TCP. Any tapers in the TCP laminate shall also be considered in the design.
Any sealing mechanisms (O rings etc.) shall be described in the design. Their particular function is not covered by this standard.
Possible metal corrosion shall be addressed in the design. Metal corrosion is not addressed in this standard.
This standard refers extensively to:
- DNVGL-ST-C501 Composite components
- DNVGL-ST-F201 Dynamic risers
- DNVGL-ST-F101 Subsea pipelines.
The requirements of these standards shall be fulfilled for the aspects they are referred to.
Relationship of this standard to other standards
No standard exists for unbonded composite pipes, but API17J partially covers this subject by referring to DNVGL-ST-C501 (dotted line).
The standard is linked to other DNV GL standards, as shown in Figure 1-4.
If requirements in the above documents also apply to TCPs, they are referred to or replicated in this document for convenience. References are not given for each individual case.
API 15 S is a specification for spoolable reinforced plastic line pipe which has some similarities with TCPs. However, API 15 S is not shown in Figure 1-3 and Figure 1-4, because it is only applicable to flow-lines and does not cover offshore applications. Internal pressure and reeling are the only load cases considered.
Structure of this standard
This standard is divided into five main parts:
- The first part describes the standard itself, its introduction, philosophy and scope.
- The second part specifies the design input.
- The third part describes the required material properties and failure criteria.
- The fourth part describes the design process, covering analysis, design criteria, prototype tests and safety factors.
- The fifth part covers activities after the design is finished, such as inspection, quality assurance and documentation.
A link between the main parts and the actual sections of the standard is given in Table 1-2.