API TR 17TR2:2003 pdf download

API TR 17TR2:2003 pdf download.The Ageing of PA-11 in Flexible Pipes.
For evaluation of manufactured or aged pipes, the initial molecular weight or inherent viscosity should be that measured on the manuticturing run of the actual pipe in question. according to procedures in Appendix D.
Operational data must be provided, in as much detail as possible. For example, at the design stage, a typical data set might include: temperature: water cut with time: CO2 levels:
pressure; predicted downtime; crude acidity; hot oil regime:
etc. It is also necessary to include estimated cumulative exposure to production chemicals, e.g., methanol, acid etc. Temperature gradients across the pressure sheath thickness should not he considered.
Given the sensitivity of the service life models to temperature, inclusion of the temperature profile of the development over field life in any analysis can improve the reliability and accuracy of the predictions derived.
As a project moves into the operational phase, the extent and accuracy of this data should increase and PA- I I service life verification can form part of the overall flexible pipe integrity assurance process.
3.3 INITIAL ACCEPTANCE CRITERIA
The initial acceptance criterion for PA- I I pressure sheathes is based on the corrected inherent viscosity of the most highly degraded material on the inside bore of the sheath reaching a certain defined level.
It is acknowledged that in practise there may be a gradient of corrected inherent viscosity across the wall thickness of the pressure sheath. Therefore, depending upon the filure mode, consideration of only the most highly degraded material is a conservative approach.
Once the initial acceptance criterion is reached, the pressure sheath has aged significantly, as illustrated in Figure 1. In order to continue using the pipe, an analysis of potential failure modes should be performed. This will aim to evaluate the margin of safety and address risk and criticality, in a remnant life assessment, considering recovery and re-use if applicable.
The initial acceptance criteria is based on a failure criterion derived and validated from both the examination of pipe retrieved from service, detailed in Appendix F, and from laboratory mechanical property, fatigue and crack growth testing, detailed in Appendix G.
The failure criterion for a PA-I I pressure sheath is a corrected inherent viscosity of 1 .05 dl/g. Therefore, the initial acceptance criterion is set at a corrected inherent viscosity of
1.2 dug. This initial acceptance criterion typically includes a
time based safety factor of 1.6 or more over failure, e.g., 1.64
atpH=4, 1.75atpH=5,and2atpH=6.
It is recognised that this initial acceptance criterion is not based on a large statistical base. However, from the field failure results indicate it is conservative, and it is consistent with laboratory experience.
It should be noted that there is potentially a good case for reducing the initial acceptance criterion to 1. 1 dug in case of static applications. There are several reasons for this:
a. Stress levels—In static applications, a pressure sheath is typically only subjected to thermal stresses which are lower than 3 MPa, e.g., in the case of cool down from 75°C to 20°C. In a dynamic application, stresses as a result of vessel motion, typically range from 6 to 26 MPa. ignoring thermal stresses.
b. Number of eve/es—A static pipe is typically subjected to 1000 to 5000 temperature and pressure cycles over its lifetime, whereas a dynamic pipe will see a much larger number of stress cycles, typically 108.
c. Low strain rate—In static applications, strains rates are typically very low. There is a growing body of evidence to suggest that under such circumstances the ductile I brittle transition occurs at temperatures below those encountered in typical field conditions.
d. Field experience—As shown in Appendix F, all tlie failures of static lines have involved PA- 11 material with corrected inherent viscosity lower than 0.95 dlJg.
This case for setting a reduced initial acceptance criterion for static pipe is currently undergoing further refinement.
3.4 AGEING CURVES
The prediction curves in Figure 2 are derived from the initial acceptance criterion of CIV = 1.2 dl/g.
The mathematical relations for these curves are tabulated in Table 2.
The API RP 17B relations have been taken from close examination of Figure 21 in the Third Edition3. The “unsaturated” RP 17B curve is still to be employed in dry service, as defined in 2.2.
The “Batched methanol treatment” relation can he used to give a conservative indication of the effect of the batched methanol on a PA-I I pressure sheath, by considering cumulative exposure time and temperature for methanol batch treatments.