1. The principles of Long range ultrasonic testing (LRUT):

- Long Range Ultrasonic Testing : the technology operates by placement of an array of probes placed in a belt, circumferentially wrapped around the pipe being inspected. 

Picture 1: A belt of probes warpped around the pipe.

- The system then generates ultrasonic guided waves longitudinally down the length of pipe, distance covered by the ultrasonic waves is dependent on a number of factors related to pipe coating, branch connections, fittings and above ground or buried locations. 

- The system can inspect both directions from each inspection location and therefore can maximize production.

Picture 2: set up of Long range ultrasonic testing and signal of discontinuities.

- The  LRUT systems excites low-frequency ultrasonic waves that propagate in the direction of the pipe, from both sides of the probe collar. Various factors influence the maximum inspection distance: the pipe configuration and environment, the type of fluid inside the structure, and the type of coating. The propagated torsional waves detect general variations in the total cross-sectional area of the pipe, as well as changes in the material properties.

Picture 3: The principles of Long range ultrasonic testing

2. Advantages and Disadvantages of Long range ultrasonic Testing

- Advantages:

• Rapidly provides comprehensive condition information on large areas of structure.
• Provide a screening tool to quickly identify any discontinuity along the pipe.
• 100 % of volume is inspected.
• Detects internal and external metal loss.
• Requires minimal preparation, Insulation removal, scaffolding, excavation,  coating removal, etc.
• Inspects inaccessible areas remotely.
• Minimum Detectable Defect Size 2 % of pipe wall cross section.
• Reduces inspection cost and time.
• Inspection Range: 100m or greater depending on pipe conditions.

- Disadvantages:

• Dead zone is large.
• Pipe conditions and geometry influence performance
• GWT is a screening technique. This method does not provide a direct measurement of wall thickness or exact dimension.
• Need of experienced and skilled operator. 
• Very difficult to detect defects under supports.