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Development of a Guided Long Range Ultrasonic Inspection System for the examination of offshore subsea Risers, Steel Catenary Risers (SCRs) and Flowlines

Final Report Summary - RISERTEST (Development of a Guided Long Range Ultrasonic Inspection System for the examination of offshore subsea Risers, Steel Catenary Risers (SCRs) and Flowlines)

The RISERTEST project aimed to implement a large-scale monitoring solution for detection of time dependent degradation -corrosion and fatigue cracking- in sub-sea pipelines and risers using ultrasonic guided waves.

The main tasks in the project were:
-to specify the requirements for the test system;
- to develop sensors and the associated electronics to meet those requirements;
- to enhance the techniques and procedures for performing long range ultrasonic testing (LRTUT) for improved performance;
- to demonstrate the capabilities of the laboratory;
- to demonstrate the feasibility of performing such tests sub-sea by means of underwater demonstration of a prototype system and tests under high pressure to simulate depths down to 2 000 metres.

The requirements for the system were evaluated for three different cases: fixed risers, flow lines and steel catenary risers. For the final demonstrations of the system 12 in (323 mm) diameter pipe with 27 mm wall and 3 mm polyprolylene coating was chosen to be representative of pipe used for deep water risers. After proper testing it was concluded that:
- it is possible to transmit ultrasonic guided waves in steel at external pressures of up to 150 bar (equivalent to a water depth of 1500 m).
- the prototype transducers worked satisfactorily at all pressures, although they did show signs of degradation towards the end of the test.
- in the limited additional test, the transducer worked satisfactorily after a pressure test at 200 bar, equivalent to the 2000 m water depth which is the target operational depth for this project.

A series of experiments was carried out on both bare and coated pipe to determine the performance of guided waves for the detection of metal loss defects, such as corrosion. The results have shown that the inspection for pipes with coatings in feasible. The maximum inspection length can be up to about 12 - 15 m.

The performance of LRUT for the detection of growing fatigue cracks was also tested. A permanently mounted sensor to withstand the stresses of the resonance rig was used. The results showed that whilst there is some variation between the successive tests, the differences between the readings, including the initial baseline, are 0.4 dB for the first peak, 3.4 dB for the second and 2.2 dB for the third. The permanently mounted transducer system offers potential for monitoring of pipelines and risers over long periods for time dependent degradation such as fatigue cracking.

Field trials were also carried out to demonstrate the applicability of the system. The results were virtually identical to the in-air tests.

The project coordinator, TWI, hosts a website for the RISERTEST consortium at: http://www.resertest.eu.com.

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