Offshore pipeline failure statistics have been collected for more than 30 years now and illustrate that the riser predominantly fails as a result of corrosion. The consistent wetting and drying in the splash zone combined with defects in the coating are the usual contributors to the problem. Risers are inspected at some determined frequency and can be done by internal and external methods. Inspecting by either means brings into account caveats and limitation from the technology used as well as human factors. For example, external inspections can be inefficient and inaccurate with some tools missing defects in area of coating disbondment. In addition, internal inspections sometimes create false positives and can miss defects. These inaccurancies in the technologies or the techniques used may miss defects that eventually lead to failure. On the other hand, using corrosion mapping and fitness-for-service (FFS) assessment from the data collected, along with the inherent conservatism of this data from limited measurement accuracy, may result in the premature replacement of risers. A literature search is being conducted to review existing riser inspection methods and identify candidate nondestructive methods for riser inspection. These methods should be capable of detecting and monitoring general corrosion, localized corrosion pitting and stress-corrosion cracking (sulfide or hydrogen induced) as external or internal corrosion damage. Thus far, this search has found that assessing the remaining service life og aging risers is largely dependent on the accuracy of analyzing corrosion damage to the riser surface in the atmospheric, splash (tidal), submerged, and buried environmental zones. The accuracy of each technology was analyzed. The capabilities and limitations of each method/technology used for riser inspection are summerized. The investigation is focused on long- and short-range ultrasonic techniques used for initial screening and corrosion mapping. These techniques can be depolyed to detect a significant reduction in wall thickness using guided and torsional wawes or to map accurately a corrosion damage using single/multiple transducers and phased-array probes in manual or automatic mode. A pulsed eddy-current technique that uses a stepped or pulsed input signal for the detection of corrosion areas under insulation (CUI) is also evaluated. This allows the detection of wall-thinning areas in the riser without removing the outside coating. In addition, it is found that filmless real-time, and digital radiography can be used to find internal and external corrsion defects in an insulated splash zone while the riser remains in service. A survey of nondestructive evaluation (NDE) manufacturing companies. NDE inspection companies, and operating companies was completed to collect information about current instrumentation and inspection/operator's experience for riser inspection. Examples of advanced riser inspection instrumentation and field results are included. The ability of the candidate technologies to be adapted to riser variations, the stage of standardization, and costs are also discussed.
Evaluation of methods for detecting and monitoring of corrosion damage in risers
Entwicklung von Methoden für die Ermittlung und Überwachung von Korrosionsschäden an Steigrohren
2003
12 Seiten, 12 Bilder, 2 Tabellen, 25 Quellen
Aufsatz (Konferenz)
Englisch