Mechanical Ultrasonic Inspection of Offshore Platform Structures

 

Radiographic testing (RT) is commonly used for inspecting welded sections on offshore drilling platforms. This technique, however, is subject to radiation regulatory requirements, which impose restrictions on other production activities and logistics. This month's column shows how one company dealt with these issues by using mechanized ultrasonics (UT), and completing the inspection in a timely fashion. G.P. Singh Associate Contributing Editor

 

Figures 1-3
Figures 4-6
Figures 7-8

Editor's Note: As referred to by the authors in this article, mechanized ultrasonic testing involves the use of a continuously moving scanning machine to test welded seams in large-scale operations.

Background
Major offshore drilling projects are being carried out around the world. Drilling platforms used
in petroleum exploration are subjected to extensive inspection during their construction to ensure the integrity of components. The main load bearing components of such a platform are the legs and piles. These are usually tubular sections welded together, with the lower sections having larger diameters and thicker walls than the upper sections. These tubular sections are constructed from plates rolled into cylindrical shapes, with a submerged arc weld forming the long-seam. The section thus formed can then be joined to other sections by submerged arc welding of circumferential seams. See Figures 1 and 2.

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For both the manufacturer and the customer this was their first experience in effective application of mechanized UT...

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Traditionally, both of these seams (longitudinal and circumferential) have been tested using RT. Results of RT (using Co-60) for thicknesses up to 100 mm (4 in.) of steel are doubtful at best, but not many facilities have linear accelerators, which can produce better quality X-ray images. Under these circumstances, and using Co-60, the standard 2-2T sensitivity is fuzzy at best. At normal production rates about 1 or 2 long seams could be radiographed in an eight or twelve hour shift.

A large manufacturing company was awarded the contract to build two offshore drilling structures in an 18-month period. This would be a significant burden on their radiography department, but more important were the logistics of timing and handling. Radiation regulatory requirements require that certain areas be roped off while performing RT. This meant that the very large sections of legs and piles would need to be lifted away to a dedicated area, or RT would need to be carried out in the shop during the night shift, when the access restrictions imposed by roping areas off would have the least effect on production activities. To address this potential bottle-neck, the manufacturer requested that the customer consider mechanized ultrasonic testing (UT) as an option to RT. The customer agreed to consider the option, provided that the technique was proven to be as reliable as RT.

This paper describes the techniques adopted and the experiences gathered, as UT eventually supplanted RT and allowed the project to be completed in a timely fashion.

 

Code Requirements
Nondestructive testing (NDT) on the project was regulated by the dictates of the American Petroleum Institute's Recommended Practice for Offshore Structural Fabrication and Guidelines for Qualification of Technicians, known as API RP2X, as interpreted by the customer (API 1996). In order for UT to be applied the procedures would need to meet the requirements of API RP2X.

This API Recommended Practice document requires inspection of 100 percent of all longitudinal seams and 10 percent of circumferential weld seams. Although the API RP2X document gives extensive descriptions of general advantages and limitations of ultrasonics, its main coverage is devoted to technical recommendations for manual applications of ultrasonic testing and methods for qualifying ultrasonic technicians. This concern for technique and
operator ability is based primarily on the traditional application of UT to the very difficult T, K, and Y joint configurations. Application of UT to the simple symmetric butt weld configurations used for this project would place less demand on operator ability to plot indications than in the complex T, K, and Y geometries. As with most such documents, however, API RP2X has not given special consideration to the specific aspects of mechanized UT. To address the absence of specific guidance in the API RP2X document, both the manufacturer and the customer developed a qualification program to allow the introduction of mechanized UT. For both parties involved this was the first experience in effective application of mechanized UT, and therefore considerable debate and cooperation was required for this new venture.

This became a multistage program, requiring:

• developing a procedure and techniques to inspect the range of butt welds proposed for inspection
• training operators on the use of the equipment and application of the techniques
• construction of a mock-up of a weld with typical anomalies
• qualification of the equipment and technique on the mock-up
• moving to the field and performing trials of the equipment, techniques, and operators to qualify the entire concept, including operators
• correlating ultrasonic and radiographic results to establish a confidence in the system with decreasing dependence on radiography. This would mean moving from 100 percent radiography and 100 percent ultrasonic testing to zero percent radiography and 100 percent ultrasonic testing, with the percentage of radiography being reduced in several stages.

Every effort was made by both parties to observe the intent of the API Recommended Practice document, while recognizing that strict adherence to the document was not possible due to its lack of information on mechanized UT information. As a result of the extensive efforts to ensure that the results of RT were the minimum expectations of the system, a significant advance in technology was possible.

To ensure maximum integrity, the manufacturer agreed to use the Level A acceptance criteria in Appendix D of API RP2X. This is based on workmanship quality and is not related to the component's fitness-for-purpose. These criteria would more closely relate to the radiography acceptance length criteria. These acceptance criteria, however, base evaluation response on a 1.5 mm (0.06 in.) diameter side drilled hole at the depth of concern. This applies to a 25 mm (1 in.) thick section as much as to as a 105 mm (4 in.) thick section. Effectively a 25 mm (1 in.) thick section is evaluated to a hole six percent of wall thickness, whereas a 105 mm (4 in.) section is evaluated to a hole 1.5 percent of wall thickness.

This is in contrast to radiography, where the image quality indicator (IQI) varies with the thickness,while trying to maintain a similar sensitivity with respect to wall thickness. In a 25 mm (1 in.) section requiring a #25 penetrameter (ASME source side) the essential hole is the 2T and is 1.3 mm (0.05 in.) in diameter. This represents about five percent of the wall thickness. For a 64 mm (2.5 in.) wall the source side IQI is the #40. The #40 2T hole is 2 mm (0.08 in.) in diameter, or about three percent of the wall thickness.

Technique
The UT technique developed was a combination of rastered pulse-echo inspection coupled with time of flight diffraction (TOFD). This ensured that the entire weld volume was inspected with the pulse-echo probes, paying special attention to the weld bevel fusion faces, with TOFD providing extra coverage for the volumetric anomalies and off-angle planar anomalies not optimally orientated for the pulse-echo probes.

Figure 3 shows a typical set-up. This uses two pair of 45° probes for the full skip, a pair of 60° probes for the half skip, and a TOFD pair set for nominal 65° refraction in compression mode. The data for the TOFD pair are collected with the probes placed symmetrically on either side of the weld centerline. For the pulse-echo probes a raster scan is used in which the probes are moved about 10 mm (0.4 in.) perpendicular to the weld centerline after each scan, with five such scans being made to cover the inner and outer third of the fusion faces.

In addition to the full volume being ultrasonically tested, an additional NDT was carried out on both the inside and outside surfaces, using magnetic particle inspection. This ensured that fine surface anomalies that might be masked in the unground weld cap could be detected. As well, a manual UT inspection using 0° compression mode was used to ensure no that laminations were present in the plate where the mechanized inspection was carried out.

 

System Integration
R/D Tech designed the full integration of automatic UT inspection system for Daewoo Heavy Industries Co. Ltd. The procedure, designed by Material Research Institute, requires that three to five linear scans, depending on the wall thickness, are made with a different offset from the weld centerline. Figure 4 illustrates the diagram of system integration. The components of the system are:

• a magnetic strip is located at a constant offset from the weld centerline, the scanner follows the magnetic strip and moves along this axis with its magnetic wheels (see Figure 5); an arm on the scanner positions the probes at a correct offset from the weld centerline
• the UT acquisition system synchronizes the pulses to the probes with the position encoders from the scanner. The encoded ultrasonic signals will permit generation of A-scan, B-scan, C-scan, and volumetric presentation of the information
• the system is completed by a motor drive unit which delivers the power to the scanner, a pump for local immersion UT coupling, and an optical disk for data files archive.

After the operators had become familiar with the equipment, a 4 m (13 ft) longitudinal seam on a 70 mm (2.75 in.) thick section could be inspected and evaluated within one hour. This was a significant improvement over RT.

 

Qualification and Trials
Calibration blocks using side drilled holes and slots were made by the manufacturer and used to develop and optimize the techniques. Mock-ups were made by the manufacturer and used to qualify the techniques and operators.

The mock-ups were long-seam welds made with welding anomalies imbedded at various depths and locations. The techniques were confirmed and qualified on the mock-ups. Figure 6 shows a sketch of the mock-up weld used for qualification with the positions of the imbedded anomalies. With the exception of the porosity, "anomalies" were made using ceramic inserts approximately 4 ´ 45 mm (0.15 ´ 1.75 in.). Figure 7 illustrates the processed data from the pulse-echo probes. Customized software allowed the data collected to be presented in a typical "top/side/end view" format. Figure 8 illustrates the A-scan and B-scan representation of the TOFD channel.

 

Results
After the technique had proven it could detect the anomalies in the mock-up, there was a transition from RT to mechanized UT. This involved inspection of actual production welds using both methods. During this transitional period only, all significant indications found initially by RT were also detected subsequently by mechanized UT. As the transition progressed, and less RT was being performed, RT was carried out where indications had been located initially by mechanized UT. All indications located by mechanized UT were confirmed by manual UT but in many cases they could not even be detected by RT. This did not negate the validity of either NDT method, but merely reflects differences in the ability of the methods to detect different types of anomalies.

 

Findings and Conclusions
Laboratory techniques, which were verified on customer mock-ups, were used to phase out the use of RT for this application. Further requirements to satisfy the customer's concerns involved site trials. Therefore, several hundred welds tests by mechanized UT were compared with tests performed by RT. It was seen that all significant anomalies found using RT were also detected using mechanized UT.

It was concluded that mechanized UT was an effective replacement for radiography in this instance and offered a comparable confidence level in probability of detection of all significant anomalies.

 

Acknowledgments
The authors would like to thank Chevron Oil for their help in adapting this new technology and for the image of the Lomba Project Platform used in Figures 1 and 2. We would also like to thank R/D Tech and Daewoo Heavy Industries for the images of equipment and the report results.

 

References
API 1996: Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Technicians, 3rd edition, November 1996, American Petroleum Institute, Washington DC.

 

* Materials Research Institute, 368 Lexington Rd., Waterloo, Ontario, Canada N2K 2K2; (519) 886-5071; fax (519) 886-8363; e-mail ginzelea@wat.hookup.net.

+ R/D Tech, Inc., 4495, Wilfrid-Hamel Blvd, Québec City, Québec, Canada G1P 2J7; (418) 872-1155; fax (418) 872-5431; e-mail glegault@rd-tech.com.

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