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Pipeline Technology Journal 2-2015

Latest developments and news from the pipeline industry

RESEARCH / DEVELOPMENT / TECHNOLOGY As indicated in previous section and for optimisation purposes, the fatigue component has been carried out for two fatigue regimes, “Normal” and “Reduced” in the assessments. It was found that the ECA for the Oil Export Pipeline is dominated by the limiting peak loading for installation and practically unaffected by fatigue from either installation condition or operational schedule. However, the ECA for the Gas Export Pipeline was mostly driven by the operational fatigue. The consequences of these observations are briefly presented in the following section. It was also found that the good workmanship levels for flaw accept- ance criteria was conservative for the Oil Export Pipeline, if compared with ECA based criteria. However, this was not the case for relatively deep flaws in the Gas Export Pipeline girth welds: Flaws with heights in excess of 1.5mm approximately could not be sustained if 25mm long, as dictated by workmanship. Shorter flaws were set up as AUT limits for offshore production. ECA SENSITIVITY AND CONCLUSIONS The green lines to the left in each of the graphs presented in the previous section capture the allowable fabrication flaws during con- struction for each case analysed. In that way, it is ensured that the welding flaw does not extend to any unacceptable levels by the end of the pipeline design life. The results correspond to theoretical limits for surface breaking flaws and materials with qualified tensile prop- erties and fracture toughness. Such results are converted to AUT cri- teria and presented in Figure 8 and Figure 9 presented in this section. It can be seen from the step functions presented in the graphs that the benefit from an alternative, reduced operational schedule for the Oil Export Pipeline is small, see Figure 8. However, the benefit from a reduced operational schedule for the Gas Export Pipeline is sig- nificant, as demonstrated in the graph of Figure 9 by comparing the green and purple lines. As can be also seen from the graphs, a small percentage of reject- ed flaws would have been accepted during construction and a repair would have not been called if the fatigue design of the pipelines was based on a less restrictive, more realistic operation regime. This is especially true for the Gas Export pipeline. FINAL REMARKS The performance of an ECA for thin wall pipelines is still paramount for thin wall pipelines, particularly for shallow flaws for which it is ben- eficial against good workmanship criteria. The performance of an ECA must include fatigue loading, since var- iable loads at different stages of design life have an effect on flaw acceptance limits. For the purpose of concluding this paper, Saipem Ltd’s production graphs on board Castoro Sei are presented below, Figure 10. As can be seen, excellent production rates were achieved during con- struction of the pipelines and the rejected flaws were a small percent- age of production welds. This was indeed the case for flaw accept- ance criteria which were in full compliance with DONG E&P fatigue requirements for the operation regime. 0 50 100 150 200 250 300 350 400 0 1 2 3 4 5 6 7 8 FlawLength[mm] Flaw Height [mm] 12" Oil Export Pipeline AUT Flaw Acceptance Criteria ECA Criteria (Reduced Fatigue) ECA Criteria (Full Fatigue) Good Workmanship [DNV OS-F101] Rejected Defects Offshore 0 50 100 150 200 250 0 1 2 3 4 5 6 7 8 FlawLength[mm] Flaw Height [mm] 12" Gas Export Pipeline AUT Flaw Acceptance Criteria ECA Criteria (Reduced Fatigue) ECA Criteria (Full Fatigue) Good Workmanship [DNV OS-F101] Rejected Defects Offshore Figure 8: Oil Export Pipeline – AUT criteria and Rejected Flaws Figure 9: Gas Export Pipeline – AUT criteria and Rejected Flaws References 1. DNV OS-F101 “Submarine Pipeline Systems”, Det Norske Veritas, 2010. 2. BSI BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures”, Incorporating Amendment No. 1, 2005. PIPELINE TECHNOLOGY JOURNAL 43 012345678 012345678

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