materials feature | ETPs & composites
composites). Long-term material strength is also determined by small-scale testing. Once the character- istic properties are determined they can be used in the analysis phase. It should be noted that the results of the material
tests – and the scatter we observe in them – affect the choice of the safety factors.
Design criteria Once material properties have been determined, the next factor is design criteria. A number of these are described in the RP. There are three separate categories of short-term design criteria. The first deals with the fibre-reinforced laminate, and addresses fibre-dominated ply failure, matrix cracking, delamination and laminate failure. The second factor addresses the complete TCP, and includes fluid tightness, fracture of the non-reinforced polymer layers, plastic deformation (such as yielding of isotropic materials), maximum deformation and debonding. The third category addresses degradation, and this includes degradation by chemicals, through swelling or shrinking, leaching of additives, rapid gas decompres- sion, UV exposure, and thermal softening and hardening. For long-term design criteria, we must address two main phenomena: cyclic fatigue; and stress rupture. For cyclic fatigue, we should look at the various and peculiar failure mechanisms of composites, including fibre-dominated failure, matrix cracking, laminate failure and thermal fatigue. In stress rupture, there are a number of failure mechanisms, including through- thickness delamination and debonding, and non-rein- forced polymer failure of liner and cover.
Being safe Finally, we need to choose the correct safety factors. A number of these must be included in the analysis: for example, we have safety factors to account for uncer- tainties in both the load and the material properties. There are also safety factors to account for uncertainties in the design criteria of each of the failure mechanisms. Safety factors are calibrated new based on a
statistical approach, similar to that used for steel risers. Load and resistance safety factors can be combined and applied as one factor. There are three safety classes in the RP: low,
medium and high. This is chosen depending on the criti- cality of the component. At this stage – now that safety factors, design criteria
and material properties are determined – we can move to the next phase, which is to start the design and analysis of the actual TCP.
Confirmation testing To do that, we use a numerical approach like the finite element method. An analytical approach can also be used. But the question is: how reliable is the finite element model that we use for the design? To address this, the RP requires a number of confirmation tests for representative pipes. There are three types of test to perform here:
l static tests, including axial compression, nternal pressure, hoop strength and crushing ring;
l long-term tests for cyclic fatigue (axial compression, axial tension and internal pressure); and,
l long-term tests for stress rupture (axial compression and internal pressure). It should be noted that these confirmations can also
be used to determine the model factor accounting for the deviation between tests and analysis predictions. Once this is complete, and we see that finite element
results are close to those obtained fro the test, we can move to the final stage of the qualification – full-scale prototype testing. Limited full-scale testing is used to verify the analysis and the actual pipe plus end fitting. This generates confidence in the design calculations, allowing for calculating other conditions and load cases that have not been tested. Thermoplastic composites offer huge potential to the oil and gas industry, as pipes made from these materials are less brittle and more spoolable than their thermoset equivalents. Putting this recommended practice in place will help to ensure full confidence in this type of pipe. ❙
www.dnvgl.com
Confirmation tests are always done to ensure that the upper level is properly described by the lower level
16 PIPE & PROFILE EXTRUSION | January/February 2017 l Ramin Moslemian is a senior engineer at DNV GL
www.pipeandprofile.com
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