materials feature | ETPs & composites
Alternative approach: full scale testing
There’s also an alternative approach based on full scale testing. Here, a number of full scale tests are typically required, depending on the target performance.
Within the full-scale tests, long-term integrity of the TCPs should be
verified – and we should account for environmental effects such as temperature and degradation. This approach is only valid for the tested condition. So, if the TCP is supposed to be used in another condition – or loading scenario – then additional testing will be needed. Qualification using this approach is useful if a simple, well-known
performance is required. However, it is not practical for complex fluid loadings or where you have different types of loading acting on the pipe body.
Need for qualification TCPs are a relatively new technology, and for any new technology there are always concerns about safety – and having a framework for qualification. To address this, a DNV GL joint industry project to develop rules for safety assurance and qualifications of TCP was launched in 2015. Its 18 partners included polymer manufacturers, TCP manufacturers, service companies and operators. This recommended practice (RP) for qualification of
TCPs – called DNVGL-RP-F119 – is based on structural reliability analysis. The full RP is available here. It addresses applications for static and dynamic pipes that are used in applications such as risers, jumpers, flowlines and intervention lines. The new RP looks at the TCP’s structural integrity. Dynamic loads acting on the TCP – such as in a riser – are expected to be determined using specially designed dynamic analysis software – such as DNV’s Helica – which can account for the anisotropy of the TCP. The new RP is connected to three main documents:
l DNV-OS -C501 – a standard that mainly addresses thermoset composites;
l DNV-OS-F201 – used for dynamic risers; and, l DNV-OS-F101 – which is used for submarine pipeline systems.
The principles and concepts in these standards have all been used to develop the new RP. The most extensive part, which covers ‘Develop- ment’, describes a design basis for TCPs – encompass- ing its entire performance envelope. The section also includes information on: materials (including material qualification and characterisation); failure mechanisms (such as matrix cracking and delamination); and design criteria for both the pipe body and end fittings. It also
14 PIPE & PROFILE EXTRUSION | January/February 2017
includes a section on qualifying TCP using full-scale testing – as an alternative to the design calculation approach developed in the RP. The ‘Testing’ chapter describes the full requirements of prototype testing. There is also a chapter on safety factors, which describes how these should be determined and used.
Pyramid approach The main approach for qualification is based on a test pyramid. At the bottom, there is small scale testing. This includes short- and long-term material testing, and is quite extensive. Next, there is medium-scale testing – which has a narrower scope. And finally, there’s full-scale testing – which is quite limited in com- parison with small-scale testing. The intention is to reduce the scope for doing lots of full-scale testing – which is expensive and difficult to perform. The basic structure of the qualification in this RP
starts with the short- and long-term materials testing, which is relatively extensive. When we do materials testing, environmental effects like temperature should be considered – as well as degradation due to internal media.
https://www.dnvgl.com/oilgas/download/dnvgl-rp-f119-thermoplastic-composite-pipes.html
The basic properties in this phase are determined based on statistical evaluation and statistical safety factors. Once we have the material properties, the relevant failure modes and mechanisms should be determined. After that, safety factors are determined. Then, we can start with analysis and design of the TCP which is done with numerical methods such as finite element modelling. Once this is done, and the TCP is designed and analysed, the finite element model must be verified using some medium-scale tests on the pipe body. Finally, we enter the last qualification phase that includes the full-scale testing, including end fitting. In this phase, the full survival test is used to verify the predicted performance of the end fitting and pipe body – and especially the long-term performance. Once it has gone through this full process, then a
TCP is qualified. Any failed verification means that the TCP is moved one step back in order to understand the failure – and repeat the tests until we fulfil all the required testing.
Material characterisation Short- and long-term characterisation of material properties can be demanding and may become expensive. However, it will pay off later with flexibility: once we have the material properties characterised, we can use it to design TCPs of various sizes, wall thick- nesses and dimensions. The properties are statistically determined using an approach similar to DNV-OS-C501 (for thermoset
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