TCP Design Feature #2: Minimum Bend Radius

6 June 2018

We often receive feedback from our clients that highlight the unique benefits and impact that TCP makes on their operation and subsea infrastructure. However, since TCP is different from steel and flexible pipe, our clients also have questions and try to understand this promising product much better. In a series of articles, we highlight and explain the various features of TCP. This second article covers the Minimum Bend Radius (MBR) of our Thermoplastic Composite Pipes.

Minimum Bend Radius and composite fibre angle
The TCP has a simple, fully bonded, solid wall. It consists of an inner liner, a strong melt-fused composite layer and a protective coating. This solid wall makes TCP a strong pipe rather than a hose. Due to this nature, deformations will always cause internal stresses. Bending the pipe will for instance result in tensile stress in the top of the pipe, and compression in the bottom of the pipe, see Figure 1. Since stresses grow with increasing deformation, the maximum bending strain is reached when a certain stress reaches its allowable value. At this maximum bending strain, we have reached the smallest bending radius, or Minimum Bending Radius (MBR). It is important to realize that the bending strain is the strain in axial direction of the pipe and not the strain in fibre direction. The strain in fibre direction will be different and will depend on the orientation of the fibres, i.e. the orientation of the composite layers. This explains why the pipe design influences the Minimum Bend Radius.  

Figure 1 shows two typical layups for our products: the so-called “high-tension” layup and “high-flexibility” fibre layup. The high-tension layup has fibres more or less oriented in the axial direction of the TCP in combination with fibres oriented in hoop direction. This layup is mainly used for risers and downlines, where a high tensile strength is needed. The high-flexibility layup uses a pressure balanced fibre orientation, i.e. a fibre orientation of alternating +55 and -55 degree angles with respect to the axial direction. This layup is used for applications with lower tensile strength requirements, such as flowlines and jumper spools, but with higher focus on flexibility and smallest possible Minimum Bend Radius.

Figure 1 (A) Relation between bending radius and bending strain. (B) Typical lay-up for TCP Flowlines and Jumper spools and (C) TCP Risers

MBR of TCP Flowlines and Jumper Spools
For flowlines and jumper spools the fibre orientation is such that bending strain will not result in significant strain in fibre direction. Therefore, the polymer of the composite material determines the allowable strain and not the fibres. 

Thanks to the high ductility of our thermoplastic polymers, the axial strain is not really a limiting factor for the MBR. The polymers we use (HDPE, PP, PA12 and PVDF) have a very high failure strain and in our designs we therefore allow a strain of 3.5% or more, even after long term storage on a reel. As an example, we recently measured one of our products that had been stored for two years at its storage MBR. The total ovality was still within our manufacturing tolerance and in very good alignment with the calculated ovality.

For thick-walled pipes, the most relevant failure mechanism for polymer failure is delamination caused by through-thickness tensile stress at large bending strains. However, such delaminations will not occur below the limit of 3.5% bending strain. Hence, for our TCP Flowlines and Jumper spools we allow a bending strain of at least 3.5%, which means that the MBR is roughly 14 times the outer diameter of the TCP.

Contrary to TCP Risers, the bending stiffness of TCP Flowlines mostly depends on the stiffness of the matrix material and not on the fibre stiffness. The relatively low stiffness of our thermoplastic polymers gives a low bending stiffness of our TCP Flowlines, which results in reduced loads on subsea connectors.

MBR of TCP Risers
For risers the fibres are more or less oriented in axial direction. Consequently, bending strain will cause higher strains in fibre direction and fibre failure will be the relevant failure mechanism to be considered in the design. Fibre failure can be either fibre failure in tension or in compression. As discussed in our article on collapse strength, the fibre tensile strength is approximately twice the compressive strength, making fibre failure in compression the critical failure mechanism for bending. 

So far, we did not discuss bending stiffness of TCP risers, but it is quite intuitive that a stiffer pipe will give higher stresses at a given bending radius. And higher stress at given strength and safety factors will result in a larger MBR. So, what defines the bending stiffness? Obviously, it is in the first place the composite layup. The more the fibres are oriented in axial direction, the higher the bending stiffness of the TCP will be. Secondly, it is the stiffness of the uni-directional composite layer itself, i.e. the stiffness of the fibre and the fibre volume fraction. As one can imagine, an important aspect of riser design is finding the optimal balance between performance (in terms of pressure rating and tension capacity) and spoolability. 

For our carbon based TCP Risers, this has led to a riser design with more than sufficient tension capacity even for ultra-deep water applications, while maintaining a MBR, small enough to ensure efficient installation. As a rule of thumb, we allow a bending strain of approximately 1.5%, which means that the MBR is roughly 35 times the outer diameter of the TCP.
In practice, this means that the 6” 10ksi Carbon PVDF riser we are currently developing has a minimum bend radius of 7.5m (24.6 ft) allowing cost effective installation. In addition, we can manufacture such risers in single long lengths of up to 3 kilometres.

Concluding remarks
In this article we have summarized the most relevant design aspects to define the MBR of our products. It is worth noticing that Strohm's design methodology is very structured and follows DNVGL-RP-F119, which means that all potential failure mechanisms are checked to prove that they are not critical, covering not only the as-installed situation but also the installation and transport case. As stressed in our earlier article on collapse, the properties of thermoplastic composites (such as compressive strength and delamination strength) are not a constant, but depend on time, temperature and fluid exposure. An evaluation of storage durations and conditions is therefore essential in defining the storage MBR of a TCP. 

In this series of articles, we will cover all relevant TCP Design Features.
Next time: Fluids & Gases for which TCP can be used. Contact us for more information on TCP and its benefits.