Taking pipeline inspection and testing to new depths
Although the pipes used in various underwater 'oil rig' structures are manufactured to high standards, they may not be able to meet life-cycle requirements without failure. Research indicates that 'start-of-life' welding defects may go undetected at the manufactured non-destructive testing (NDT) and inspection stage. To further complicate matters, conventional NDT techniques cannot be applied once components have been installed in their subsea environment. This raises concerns for in-service NDT and inspection of subsea risers, steel catenary risers (SCRs) and subsea flow lines. Undetected fatigue, corrosion and cracking could lead to premature failure and bring about major environmental pollution. The 'Development of a guided long range ultrasonic inspection system for the examination of offshore subsea risers, steel catenary risers (SCRs) and flowlines' (Risertest) project aimed to realise a large-scale monitoring solution detecting time-dependent degradation in subsea pipelines and risers. The EU-funded project proposed that guided wave ultrasonic testing developments could overcome the limitations of existing technology and greatly increase its applicability and productivity. System requirements were evaluated, and testing showed it was possible to transmit ultrasonic guided waves in steel at external pressures of up to 150 bar. This is equivalent to a water depth of 1,500 m. Developed prototype transducers were shown to work well at all pressures, and in an additional test, the transducer worked well after a 200 bar pressure test. This is equivalent to water depth of 2,000 m, the project's target operational depth. Risertest partners carried out a series of experiments on bare as well as coated pipe to determine the performance of guided waves in detecting metal loss defects such as corrosion. Results revealed that such an approach is feasible for pipes with coatings, with a maximum inspection length of up to 12 m to 15 m. Risertest also tested the performance of long-range ultrasonic testing (LRUT) for detecting growing fatigue cracks, using a permanently mounted sensor to withstand stresses of the resonance rig. The permanently mounted transducer system has the potential to monitor pipelines and risers over long periods for time-dependent degradation such as fatigue cracking. Field trials demonstrated the applicability of the system, with results being almost the same as those achieved during in-air tests. With oil companies turning to specialist outsourcing for all inspection and maintenance requirements, advances in this area promise to drive business opportunities for EU inspection and maintenance small to medium sized enterprises (SMEs).
