Guided Waves inspection of pipelines: experiences of Italian Institute of Welding
May 30, 2008
The article shows the experiences of the Italian Institute of Welding in inspection of above ground pipelines and road crossings using advanced diagnostic techniques, above all Guided Waves technology.
Since very low frequencies are applied, the indication sensitivity is limited to large areas of (corrosion) wall loss. Welds cause reflection signals at regular distance, providing reference for sensitivity settings. Internal features in the weld, such as weld root erosion, may be discriminated in the reflection signal by advanced signal processing techniques. In a similar way, guided wave inspection could discriminate between corroded and unaffected pipes at support locations. The full potential of the technique could become more evident when it is applied widely.
The lines external and internal surface condition did not reveal any corrosion.
The guided wave inspection, in this case, allowed a distance of 140 m with single scanning.
The echographic situation (Fig. 4) shows all single welds in the line without reduction in the attenuation factor (around 0,1 dB/m) due to the absence of corrosion.
- visual examination to highlight the construction anomalies and the status of painted surfaces;
- wall thickness measurement of each component to censure the different schedule of piping.
As no significant localized anomalies were present single echo with significant amplitude were absent.
The single echoes evident on the screen were due to welds, piping supports and change in direction of piping. Automatic ultrasonic pulse echo “corrosion mapping” (T-SCAN) was conducted to allow measurement of wall thickness.
The following figures show the difference between a section of the same line showing slight corrosion and extreme corrosion.
In the first case (Fig. 11) a single scan permitted the inspection of about 30 m of pipe with attenuation factor about 1,5 dB/m.
In the second case (Fig. 12) apart from the high attenuation signal (3,2 dB/m) caused by generalized corrosion, the inspection length was reduced to about 15 m.
The examination revealed a localized crater form of corrosion reducing the wall thickness in the proximity of 1,5 mm comparing to a nominal wall thickness of 6,35 mm.
These lines are exposed to splash zone of sea waves results in external localized corrosion due to the marine ambient where surface protection is insufficient. These areas are also difficult for access for surface protection and maintenance. The guided wave inspection is an ideal method in these cases, where even pig intelligent cannot be launched.
For this reason Italian Institute of Welding has conducted a lot of inspections revealing situation of serious damages.
This inspection revealed the presence of local crater, heavy oxidation and corrosion on supports and around the circumferential welds (Fig. 14).
That's why guided waves resolution is good enough for generalized corrosion but can be lack a small localized corrosion on a big pipe.
In these cases IIS suggests a spot inspection with automated UT technique (TSCAN) for corrosion mapping.
Results could be analyzed using a statistical evaluation, finally performing a remaining life assessment.
The scope of inspection was to detect external corrosion in bottom part of pipes caused by localized presence of tufe.
- 22 areas with external corrosion, 3 -5 mm in depth;
- 6 areas with external corrosion, 5 -6 mm in depth;
- 2 holes.
Inspection has been conducted only using guided waves, giving the following qualitative result:
- on 3 lines, light internal corrosion;
- on 4 lines, medium internal corrosion;
- on 1 line, severe internal corrosion.
As shown in figure 18, three areas of corrosion (one severe and two medium) have been detected at 1.2 m from the entrance.
- geometrical features don't allow the correct installation of the ring (guide waves can pass only one bend);
- high attenuation (caused by ground, protection or corrosion) doesn't permit the entire coverage of inspection.
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Contact
Francesco Bresciani (Istituto Italiano della Saldatura)
16141 Genoa, Italy
Phone:
+390108341-406
Fax:
+390108367780