Long Range Ultrasonic Testing of Offshore Pipelines
Prawin K Sharan ANDT Engineer Sievert India Pvt Ltd

Long-Range ultrasonic Testing (LRUT) Technology was useful for integrity assessment of the oil and petrochemical industries for a method to detect corrosion in pipelines and in pipework. The aim of the LRUT inspection is to test long lengths of pipe rapidly from a single test location with 100 percent coverage of the pipe wall and to identify areas of corrosion or erosion for further evaluation using other NDT techniques. The technique is now widely accepted and used to inspect pipes in inaccessible areas such as tubing, risers, offshore topside pipework, jetty lines, Spiral welded pipe and refinery pipework.

An important driver that impacts the performance of a production plant, like an offshore oil and gas structure, is the high utilisation of the equipment. This can be achieved effective-efficient inspection maintenance service. While meeting the high performance goals, offshore structure must also satisfy the structural health monitoring. The maintenance of technical integrity comprises of all activities that investigate the extent of decline in performance of equipment and systems take cognizance of the degradation processes and seek to prevent further degradation or, if the level or rate is unacceptable, repair or replace the degraded component (maintenance).

Inspection is one of the many dedicated activities within offshore management that contribute to controlling and minimising offshore risks. The role of inspection is to check/confirm whether degradation is occurring, to measure the progress of that degradation, and to help ensure that integrity can be maintained.

The degradation of a component can take place externally and internally. The rate at which the degradation takes place at the external or internal surfaces depends upon the combination of the following parameters: Material of construction, product services (for internal degradation), Environment surrounding the part (for external degradation), Protective measures and Operating conditions.

Localised damage does not interfere with the load bearing capacity of the equipment wall. Probability of failure refers to a leak at wall penetration. Uniform damage is such a large area that affects the load bearing capacity of the equipment wall. Probability of failure refers to the state when the wall ligament cannot accommodate the loading as calculated using structural reliability analyses.

LONG RANGE ULTRASONIC TESTING
Guided waves can propagate a long distance and are excellent for screening and circumferential defect location analysis. Ultrasonic Guided waves propagates along the length of the structure like plates, rods, tubes, Pipelines etc confined by the boundaries of the structure. Long-range guided wave inspection technique is an effective method for rapidly screening a pipe for any service induced defects[1]. Lamb waves are propagated in plates or pipes (made of composites or metals) only a few wavelengths thick. A Lamb wave consists of a complex vibration that occurs throughout the thickness of the material. The propagation characteristics of Lamb waves depend on the density, elastic properties, and structure of the material as well as the thickness of the test piece and the frequency. The wave is guided by the geometric boundaries of the medium; the geometry has a strong influence on the behavior of the guided wave[2-6].

The long range, guided wave ultrasonic technique was developed for the rapid pipe line inspection, to detect internal and external metal loss and also it can find other pipe line defects such as any mechanical damage. The oil and gas industries now use it for detection of corrosion and other metal loss defects and it has gained acceptance as a valid means of assessing the condition of pipes and pipelines where conventional ultrasound test is difficult, expensive and it takes a lot of time. During long range ultrasonic test a series of transducer in form of three or more rings is used to transmit and receive of Lamb ultrasonic wave. Lamb Wave propagation properties in pipe lines are extremely complicated. Lamb wave have several mode of propagation in two modes of wave, torsional, longitudinal and flexural modes, so that related to pipe line diameter and thickness, selecting proper frequency and wave mode with the least dispersion is very important during the practical exam (Figure 1).

The state-of-the-art of guided waves utilisation defect sizing is limited. The focusing technique developed by Li and Rose[7] is used to increase the detection potential and to achieve a sizing possibility. Guided waves were performed to detect and study defect circumferential location, circumferential length and depth.

Guided waves can propagate in cylindrical type of structure with three types of wave modes; those are Longitudinal, Torsional, and Flexural modes. The selection of the wave mode will be dependent on the DISPERSION CURVE. A major complication for guided wave systems as distinct from conventional ultrasonic inspections is the dispersive nature of guided waves; that is to say, the velocity of most guided waves varies with their frequency.

How Long Range Ultrasonic Testing Helps – Offshore Pipework
Due to limited accessibility and time constraint, on-line and remote inspection is most preferable for offshore pipework. LRUT is being capable of performing online/offline and for a long diagnostic range with single test location. It can be suggested that Guided Waves is a suitable technique for inspection of the pipelines over very long distances, and can be efficiently used instead of conventional ultrasonic methods which are based on point by point inspection. It offers wide inspection capabilities with minimum surface clearance requirements.

EQUIPMENT
The Guided Wave long range ultrasonic testing is a process of rapidly surveying pipelines from a single test location by generating low frequency ultrasonic guided waves through the material boundary.

Test Equipment - The Plant Integrity Limited Teletest® MK4 FOCUS system (Figure 2). Multi-Channel transmitters and receivers for controlling the wave propagation direction and the system have Frequency Range from 20 kHz to 100 kHz. Multi-frequency data analysis is used for finding different size of defects. Focusing Technique will give support to defect confirmation and finding the location on the pipeline on the clock wise.

Inspection was carried out using Teletest Focus+ diagnostic system developed by PI, UK and based on piezoelectric transducer technology. Tool (collar) installation is possible for the monitoring the pipelines. The features of the system are given in Table 1. Piezoelectric transducer uses the direct and inverse piezoelectric effect that occurs in materials. It is possible to induce ultrasonic wave propagation along the pipeline as the longitudinal and torsional wave modes. The presence of defects due to corrosion and pipe features along the pipeline generates a reflected wave which will be detected by the same piezoelectric transducer tool that were used in the transmission using the inverse piezoelectric effect.

CASE STUDY - PIPELINES INSPECTION & RESULTS
Inspection of Smaller dia. using Piezoelectric Transducer System The generation of the torsional wave mode is possible for better sensitivity and long diagnostic range. The data acquisition and signal generation part comprises by using Teletest Focus+ system. The output of the inspection will be in terms of cross sectional area (CSA) % reflection and corresponding distance as represented in the scan in Figure 3 (A & B).

The greater the S/N ratio the easier it is to identify and interpret signals from small change of cross-sectional area (CSA) of the pipeline.

The signals are represented in distance against amplitude scale. The data analysis and the reporting is done using DAC plot. Since their velocity is influenced by wall thickness, guided waves exhibit their most important characteristic for NDT that of being sensitive to changes in wall thickness .

CONCLUSION
Piezoelectric transducer on long range pipelines in-service systems and also their ability for a long range ultrasonic inspection could be used to monitor the offshore pipe network & jetty lines for any damages due to degradation mechanisms.

ACKNOWLEDGEMENT
The authors express their sincere gratitude to Sievert India Pvt Ltd (A Bureau Veritas Company) for permitting to carry out the investigation and also permitting to publish the results.

References
  1. Kwun, H. And Kim, S. Y., G. M. Kight, "The magnetostrictive sensor technology for long range guided wave testing and monitoring of structure", Mater. Eval. 61:80-84, 2003.
  2. M. Redwood, Mechanical Wave Guides, the Propagation of Acoustic and Ultrasonic Waves in Fluids and Solids with Boundries, Pergamon, New York (1960).
  3. J.D.ACHENBACH, WAVE PROPAGATION IN Elastic Solids, Elsevier, New York (1975).
  4. H.Kwun, K.A.Bartels "Magnetostrictive Sensor Technology and it’s Applications" Ultrasonic 36 (1998) 171-178
  5. Hegeon Kwun ,Sang –Young Kim ,and Glenn M.Light “Long Range Guided Wave Inspection of Structures Using the Magnetostrictive Sensor” (2001 )
  6. "REVIEW PAPER ON APPLICATION OF MAGNETOSTERCIVE SENSOR TECHOLOGY” Glenn M. Light, Ph.D., Hegeon Kwan, Ph.D., Sang Y. Kim, Ph.D., Albert Parvin, SWRI
  7. Li, J., and Rose, J.L., 2001, "Excitation and Propagation of Non -axisymmetric Guided Waves Hollow Cylinder," JASA, 109, pp. 457-464