The eight production and two water injection Steel Catenary Risers (SCRs) connecting the Bonga Floating ProductionStorage Offloading Facility (FPSO) with the seabed at 1000m depth each contain about 100m of CRA clad pipe at the TouchDown Point (TDP), Figure 1. The girth welds joining theseclad pipes are of an austenitic nature with a coarse grainstructure when compared to the ferritic grain structure of girthwelds in carbon steel. This coarse structure complicatesinspection of the welds by conventional Automatic UltrasoundAUT (including Time of Flight Diffraction TOFD) especiallyin view of the tight acceptance criteria for these TDP weldsthat are subject to fatigue loading conditions.
Figure 1(available in full paper)
The Bonga flowlines and risers contract was executed by Acergy on behalf of Shell Nigeria Exploration and Production Company Limited (operator) and its co-venturers Esso Exploration and Production Company Nigeria Limited, Nigeria Agip Exploration Limited and Elf Petroleum Nigeria Limited. The licence holder for the Bonga Field is the Nigerian National Petroleum Corporation (NNPC). The authors gratefully acknowledge the permission from thevarious organisations to publish this paper.
In parallel to the On-shore double and quadruple joint production welding, a new automatic ultrasonic inspection method has been developed for austenitic pipeline girth welds. This method is using dual crystal focused angle beam compression - and creep wave probes to cover the entire SCR CRA weld volume, including the clad layer. In addition, the TOFD technique has been used to increase the overall inspection integrity and to detect and size root concavity. This approach has been applied to Off-shore welding as well.
Throughout the progressive qualification process, on-shore as well as off-shore, it has been verified that the CRA inspection technique developed by Shell Global Solutions International and Rontgen Technische dienst (RTD) adequately detects and qualitatively sizes the weld imperfections that also have been observed in destructive, microscopic, (destructive) evaluation.
The Primary & Coincidence inspection method has been demonstrated to be very versatile in comparison with conventional shear wave tandem techniques. The technique is able to cover austenitic welds between :
overlay clad components
CRA clad pipe joints
CRA clad pipe joint and overlay clad component.
Besides the 10 SCR's with CRA clad pipe (approximately 550welds on-shore in 1G, and 66 Off-shore in 2G), the presented CRA inspection technique was used on more than 1000 offshore CRA welds on the water injection flowline system.
The Bonga field is in production since November 25th 2005 Dutch patent registrated under nr: 1024726, awarded in May9th 2005 and filed for PCT application Nov 8th 2004 under nr: PCT/NL2004/000874)