Lifetime testing | technical paper
trichlorobenzene solution. GPC is a fractionation method aimed at gaining information concerning the molecular weight distribution within a sample of polymeric material. Analyis was performed on exposed and unexposed
samples of both the foam and fi eld joint material. For comparative purposes, the molecular weight (Mw) and the number average molecular weight (Mn) were determined, from which the polydispersity index (PDI) was determined as Mw/Mn. [Mw is the average weight of polymer chains in the sample weighted according to weight fractions, Mn is the average weight of polymer chains in the sample weighted according to number fractions, PDI is a measurement of spread in the molecular weight distribution within a sample.] Signifi cant chemical change, such as cleaving of
polymer chains, would be expected to change the values of Mw and Mn and to narrow the PDI. The results are shown below in Table 1 and Figure 2.
Samples from the exposed section were taken from the hot region of the fi eld joint during operation. The values of Mn and Mw for exposed and unexposed samples are identical within the uncertainty of the measurement and, as can be seen in Figure 2, the GPC curves for exposed and unexposed samples lie on top of each other for both the line pipe and the fi eld joint materials. Therefore, at the molecular level there is no evidence of signifi cant change.
MFI test results Melt Flow Index (MFI) measurements were carried out according to ISO 1133; 230˚C, 2.16 kg load. MFI is a technique that is used as a QC tool to determine whether a material has the same melt behavior under a given set of conditions. As the rheological properties of polypropylene
materials are dependent on the molecular weight and the molecular weight distribution at a given tempera- ture and under a given loading, this simple tool has been used to indicate change. Although not as exhaus- tive in terms of data as the GPC analysis performed above, the test is simple and cheap to perform and the
Figure 2: Comparison of GPC curves for exposed and unexposed PP samples
results are generally more available to the reader. Increase in MFI is assumed to be indicative of reduction in average molecular chain length and thus degrada- tion.
MFI results are shown in Table 2. As with the GPC
analysis, there is little evidence of signifi cant change in the MFI of the materials pre- and post exposure. While a slight increase in MFI can be seen between the exposed and unexposed foam, the signifi cance of this relative to the accuracy of the measurement is questionable.
Oxygen induction testing Oxygen Induction Time (OIT) measurements were carried out according to ISO 11357-6; 210˚C, O2. OIT refl ects the amount of residual anti-oxidant additive present in a material. For many materials the presence of such additives is essential to prevent oxidative cleavage of polymer chains and the corresponding reduction in mechanical properties. OIT is often measured following material application to ensure that suitable processing conditions have been used. The question of change in OIT in the subsea environ- ment is often asked and, until this point, little material has been retrieved for analysis. In the Tordis case, however, samples were available for analysis post application and post exposure. The results in Table 3
Table 4: Three-point fl exural tests for exposed and typical raw materials Exposed Typical
FLEXURAL MODULUS FLEXURAL STRENGTH
MPa MPa
MPa MPa
FLEXURAL STRAIN AT FLEXURAL STRENGTH % 6,9 FLEXURAL STRESS AT 3,5% STRAIN FLEXURAL STRESS AT BREAK FLEXURAL STRAIN AT BREAK
% 7,9
23,1 27,1
1000* 26* 6,6 21* - -
Exposed Typical raw
foam raw material fi eld joint material 1010,8 27,5
754,8 21,3 6,8
17,6 21,5 7,7
800 21,5 6,5 18 - -
May 2013 | PIPELINE COATING 23
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