Guided waves are structure-borne ultrasonic waves that propagate along a structure confined and guided by its geometric boundaries. As shown in Figure 1, guided waves propagate along the pipe axis and are reflected from any local cross-sectional area change such as crack or corrosion defect. GWT is a technique for finding defect location and estimating the defect size using the arrival time and the amplitude of ultrasonic signal, respectively. The operating frequency of GWT is usually low (5 to 250 kHz) compared to ordinary ultrasonic testing. The low frequency operation helps to generate non-dispersive ultrasonic guided wave and to reduce the attenuation for long-range pipeline inspection.
- Fast and precise qualitative inspection technique, with the capability to inspect up to 1500 m per day;
- In-service inspection with a small amount of surface preparation;
- High inspection speed;
- Inspection of areas with limited access;
- Inspection of isolated pipes;
- Inspection independent of fluid type inside the pipe.
The system used for GW examination – MsS System
The system includes a device that, in electromagnetic mode, generates and receives ultrasonic waves which are propagated in the tested structure in order to perform guided wave analysi over long distances.
The entire system consists of a laptop, the MSs 3030R instrument and MsS probes, as shown in picture:
The MsS software installed on the laptop controls the parameters of the MSs 3030R instrument, collects the data through an USB port, analyzes the data and generates an inspection report.
The MsS 3030R instrument generates electric impulses of short duration, modulated amplitude, that are transmitted to the MsS probes, and then detects the voltage induced by the guided waves passing through the probe.
Maximum distance of the inspection
The most difficult question to be answered prior to the Guided Wave inspection is, “What can the maximum distance of the inspection be ?” How far, in the axial direction, can the system perform the examination? The answer depends mainly on the capacity of the system (which is the strength of torsional or longitudinal generated waves and how much can it reduce the generation of asymmetrical waves :e.g. the flexural waves), flaw size, presence and type of insulation, the depth at which the pipe is buried, nature of the soil around the pipe and the degree of compaction.
Guided Wave monitoring
Long-term monitoring advantages compared to inspection are:
- The new data are obtained fast, from the probes already installed;
- Small variations of the structure condition can be observed by comparing the data obtained initially with the periodically collected data;
- The insulation or soil have to be removed only when installing the probe for the first time;
- Allows the examination of “in service” pipelines at high temperatures or of overhead pipes;
- Provides extra time for maintenance and reduces plant downtime.