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Crack detection tool set for initial U.S. trials.

British Gas Inspection Services has opened its first overseas pipeline inspection facility with a new 14,000-sq-ft building in Houston, Texas. The facility will initially house a range of smaller-diameter pigs, from 6 to 14 in., for inspections throughout the U.S. BGIS also has access to pigs up to 48 in.

The opening of the facility was quickly followed by the arrival of BGIS' new Crack Detection Tool for its first U.S. commercial trials. The U.S. market is expected to be the largest in the world for the tool, which detects longitudinally oriented stress corrosion cracks and fatigue cracks that cannot he found by magnetic inspections.

Trials are to be completed by late 1994, when the tool will be available for commercial use.

The new vehicle transmits ultrasonic signals pulsed in a circumferential direction around the pipe wall. Any defect located longitudinally and radially will provide an ultrasonic reflector, which is collected and recorded by the vehicle.

The transducers are mounted in couplant-filled wheels, so the tool can work in natural gas as well as liquids pipelines, without the need for a liquid slug.

Crack Detection

Of all forms of planar defect that can occur in a pipeline, those oriented radially and longitudinally have the greatest structural significance. laboratory work carried out by British Gas has reproduced the type of cracking experienced in a wide range of pipe steels.

Susceptibility to cracking has been found to be non-uniform. Although British Gas has not suffered an operational failure due to cracking, the inherent variability of the phenomenon and the consequences of failure prompted the decision to develop a pig-based system to detect and size longitudinal cracks.

A wide range of inspection techniques was studied during the first research phase, with particular reference to their suitability for pipeline applications. A method that uses elastic waves at ultrasonic frequencies was selected as the basis of development.

Pulses of elastic waves, produced by the transducer probes, propagate around the pipe circumference and are reflected by any defect oriented longitudinally.

Performance

Ideally, any inspection technique used for detection of cracking should have at least the same detection as a proof- pressure test.

The nature of SCC propagation and growth rates proved difficult to specify, but British Gas decided to aim for the detection of cracks deeper than 20% of the pipe wall thickness. The length and depth of all cracks over this parameter had to be measured to assess the structural significance of the defects. The system must had to be able to discriminate between signals from defects and benign metallurgical anomalies.

The tool had be locate all defects accurately and be able to work in a gas pipeline without the need for introduced liquid couplant.

Vehicle Design

The 36-in. elastic-wave vehicle consists of two modules with the same design considerations as the magnetic-flux leakage systems.

In addition to the electronic design requirements for high- volume data handling and recording, a challenging aspect of the development was production of the wheel-mounted transducer system. High-frequency elastic waves will not propagate through gas, the essential requirement is for some means of transmitting the energy into the pipe wall without excessive attenuation.

The critical elements are the transducer and a fluid-filled wheel with a rim and tire of carefully selected and tested materials. The tire must be soft for wave transmission yet robust to survive the impact of weld beads, valves and bore changes. The wheel design must maintain accurate alignment and provide acoustic efficiency while coping with non- circular pipelines, avoid gas ingress, provide temperature compensation and eliminate unwanted echoes.

Each wheeled transducer has a limited range, so to achieve full circumferential coverage the 36-in. vehicle carries 32 wheels. Elastic waves are directed in both directions to allow a comparison of echoes from both sides of a reflector.

An important aspect of the system design is the synchronization of beam pulsing with vehicle speed. A constant pitch is essential for consistent characterization of a defect, so the scanning pulse rate must vary with speed. The elastic wave tool maintains a constant scanning pitch at speeds to 5 mph.
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Title Annotation:pipeline inspection
Publication:Pipeline & Gas Journal
Date:Dec 1, 1993
Words:684
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