Guided Waves inspection of pipelines: experiences of Italian Institute of Welding

1. INTRODUCTION

In the recent years, the technological progress has given rise to a significant improvement of the consolidated NDE techniques and to a noticeable development of new ones. Between them, particular attention has to be given to Long Range Guided Waves inspection which allows a fast and global screening avoiding service interruption. These systems are very interesting every time it's necessary to guarantee the total integrity of the pipelines using a cheap and extensive inspection able to detect external and internal corrosion.

The result of test consists in localization of thinned areas whose effective thickness, showed in qualitative way, and nature shall be quantified using conventional or automatic ultrasound techniques, better if showed as corrosion map.

Guided waves inspection usually doesn't involve any particular operating difficulty but the interpretation of result, that must be conducted on field , needs the evaluation of complex echoes carried out by expert and skilled personnel.

Following the job experiences in these years, this paper describes emerging problems and adopted solutions in order to apply to these diagnostic technique in different operating conditions such as the inspection of marine terminal pipeline, interconnecting piping, road crossings, oil pipeline etc.

2. EQUIPMENT DESCRIPTION

Guided ultrasonic technique involves transmitting ultrasonic Lamb waves along the pipe length. Using this method several hundred feet of pipe can be inspected from a single location. The system has the ability to transmit waves from a remote single location of the pipe and inspect difficult areas, such as road crossings and insulated pipes. The technique is specially sensitive for detection of corrosion damage in pipes.

This equipment allows a rapid screening of the all pipe; screening tools for fast assessment of large parts of installations seem to have a growing inspection potential. Instead of spot checks, plant Users demand complete 100% inspection coverage of their installations; where conventional ultrasonic techniques, based on bulk wave propagation, have a limited range up to one meter, Lamb waves have the potential of propagating over much longer distances. In a confined geometry, such as a pipe, guided waves build up and can travel over tens of meters.

The equipment is composed by an ultrasonic unit that clamps onto the surface of the pipe to be examined and sends a torsional and longitudinal wave along the pipeline. This technique allows to detect locations where a minimum of 5% cross section area has been eroded or corroded away.

It is an useful tool for searching corrosion of pipe surfaces at pipe supports, underneath insulation, soil to air interfaces, underground piping, road crossings and underwater piping, inspecting diameters from 2” to 48” inches. As a screening tool, this technique provides on line information of long lengths of pipework. Guided waves travel across straight stretches of pipes, bends, supports, T-joints, etc but cannot pass across flange joints and end pieces.

It also gives qualitative information about defects, as only an UT measurement allows to define how much the thickness reduction in the indicated area has been.
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.

3. FIELD EXPERIENCE

In the following some significant IIS field experiences using guided wave technique are reported.

3.1 A new Interconnecting pipeline inside a tank farm (Fig. 3)

Details:

• 12" Diameter - Fluid
• Never been on service
• Surface protection - Painted

The inspection was carried out to ensure the internal conditions of piping by a rapid 100% examination due to the reason that this line was never on service after the construction.

Results:
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.

Analyzing the echographic pattern it is also possible to distinguish, besides the echoes from the circumferential welds also the ones from piping supports without supporting pads. (Fig. 5).

The guided wave inspection was supported by the following inspection:

• 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.

Taking into account the good condition of the line ultrasonic inspection has not been performed. 

3.2 Interconnecting pipeline inside a tank farm (Fig. 6)

Details

• Diameter 36"
• Fluid: crude oil
• Surface protection: painted

This line has been in service for last 40 years constantly with petroleum products to tank farm.

Results: Visual inspection did not reveal any important anomaly. But the guided wave inspection revealed a generalized internal corrosion.

Analyzing the echographic pattern (Fig. 7), it shows a significant reduction in the echo amplitude between two circumferential welds due to the generalized corrosion phenomena.

The attenuation factor, in this part, is about 1,6 dB/m comparing to a value of about 0,6 dB/m measured at the same line without any corrosion.
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.

For this technique, direct twin crystal probes with 4 MHz were fixed on a scanner which moves along the piping and transmits the information to a central processing unit (Fig. 8). The central processing unit elaborates and produces a C-scan map (plan view) of the examined area and a B-scan view (cross section transversal and longitudinal)).

This procedure verified the presence of internal corrosion at the bottom side of pipe (Fig. 9) detecting a minimum value of 3.3 mm, comparing to original thickness of about 10 mm.

3.3 Fuel gas system inside a petrochemical plant (fig. 10)

Details:

• Diameter: from 4" to 14"
• Fluid: fuel gas
• Surface protection: painted

These lines transport aggressive medium and are subjected to internal corrosion where acids are accumulated. Aim of this job was to detect these area allowing a total replacement of all damaged sketches.

Results
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.

Manual ultrasonic examination was conducted using twin crystal normal probes 4 MHz with pulse echo technique to verify the above indications.
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.

3.4 Marine terminal pipelines (Fig. 13)

Details:
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.

The following is an example of inspection:

• Diameter: 8"
• Fluid: GPL
• Surface protection: epoxy painting

Results:
This inspection revealed the presence of local crater, heavy oxidation and corrosion on supports and around the circumferential welds (Fig. 14).

Figure 15 shows the craters where surface protection was lacking.

3.5 Big diameter Lines

Over 24"-26" lines have to be inspected using guided waves in combination with other spot techniques.
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.

3.6 Over sand pipeline (Fig. 16)

Details:

• Diameter: 20"
• Fluid: crude oil
• Surface protection: none

It's a pipeline of 130 km length placed on desert sand, and sometimes partly covered by it. Line has been composed with helicoidally welded pipes, thickness 9.52 mm.
The scope of inspection was to detect external corrosion in bottom part of pipes caused by localized presence of tufe.

At the end of job (guided wave inspection and verification by visual testing after excavation) have been found:

• 22 areas with external corrosion, 3 -5 mm in depth;
• 6 areas with external corrosion, 5 -6 mm in depth;
• 2 holes.

3.7 River crossing (Fig. 17)

Details:

• Diameter: from 3" to 16" (34 lines)
• Fluid: various
• Surface protection: painted

Aim of job was to find external /internal corrosion without installing a complete and expensive scaffolding.

Result:
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.

3.8 Line covered by concrete

Details:

• Diameter: 16"
• Fluid: fuel

Taking into account high attenuation caused by concrete directly in contact with pipe, guided waves system allowed the inspection of few meter.

Result:
As shown in figure 18, three areas of corrosion (one severe and two medium) have been detected at 1.2 m from the entrance.

3.9 Road crossing (Fig. 19)

Inspection of road crossing is one of peculiarity of guided waves.

Road crossing having casing pipe are easy to inspect otherwise difficulties may be araised on the following situations:

• 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.

In the first case it's possible to perform an excavation (Fig. 20), while in the second case the only possibility is to use special probes which lightly increase inspectionable length.

4. CONCLUSION

Guided waves inspection is a fast and economical methodology which permits a complete screening of pipeline without interrupting the process conditions. Italian Institute of Welding, first in Italy and one of pioneer in the world, today has a consolidated experience to demonstrate advantages and limitation and also to point out risks involved in the incorrect interpretation of the results. Examples shown in this paper, in fact, demonstrate the necessity of a dedicated inspection plan for any singular situation, taking into account the damage mechanisms, which is composed by guided waves and other supplementary NDT examination. Paper also shows the complexity of results evaluation that needs a skilled and competent inspector to avoid the misinterpretation of test results.