Summary
Corrosion-resistant alloy (CRA)-clad pipelines have austenitic girth welds.
Such girth welds cannot be inspected by conventional ultrasonic methods and
must to be examined by the nonconventional angle beam compression
wave/creep-wave technique. Because of the dynamic nature of steel catenary
risers (SCR) used in deep waters, very tight acceptance/rejection criteria
apply.
This paper gives background information about the design and installation of
SCRs, the material properties of such a design, and the inspection solution
developed to examine such welds. In addition, the development and qualification
as used in the Bonga project is demonstrated, and conclusions and
recommendations are given.
Introduction
The eight production and two water injection SCRs connecting the Bonga
floating production storage offloading (FPSO) facility with the seabed at
1,000 m depth each contain approximately 100 m of CRA-clad pipe at the
touchdown point (TDP) (Fig. 1.) The girth welds joining these clad pipes are of
an austenitic nature with a coarse-grain structure when compared to the ferrite
grain structure of girth welds in carbon steel. This coarse structure
complicates inspection of the welds by conventional automatic ultrasound
testing (AUT) [including time-of-flight diffraction (TOFD)], especially in view
of the tight acceptance criteria for those TDP welds that are subject to
fatigue-loading conditions.
The Bonga flowlines and risers contract was executed by Acergy on behalf of
Shell Nigeria Exploration and Production Company (Snepco) (operator) and its
coventurers Esso Exploration and Production Company Nigeria, Nigeria Agip
Exploration, and Elf Petroleum Nigeria. The license holder for the Bonga field
is the Nigerian National Petroleum Corporation (NNPC).
In parallel to the onshore double and quadruple joint production welding, a
new automatic ultrasonic inspection method has been developed for austenitic
pipeline girth welds.
This method uses 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 also
has been applied to offshore welding as well.
Throughout the progressive qualification process, onshore as well as
offshore, it has been verified that the CRA inspection technique developed by
Shell Global Solutions International and Röntgen Technische Dienst (RTD)
adequately detects and qualitatively sizes the weld imperfections that also
have been observed in, microscopic (destructive) evaluation.
The primary and 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 joints and overlay-clad components
Besides the 10 SCRs with CRA-clad pipe [approximately 550 welds onshore in
1G (see Fig. 2) and 66 offshore in 2G (see Fig. 3)], the presented CRA
inspection technique was used on more than 1,000 offshore CRA welds on the
water injection flowline system. The Bonga field has been in production since
25 November 2005.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
6 June 2006
- Revised manuscript received:
22 December 2006
- Manuscript approved:
28 February 2007
- Version of record:
20 June 2007
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