The thickest 18in UOE pipe manufactured to date was required for Petrobras' Tupi field. Corus' Simon Slater, Richard Freeman, Martin Connelly and Shuwen Wen describe the manufacturing steps taken to meet the technical linepipe requirements of the project.
With such a large volume - 118km of 457mm x 31.75mm grade X65 - of sour linepipe required for Petrobras' Tupi field in the Santos Basin offshore Brazil in water depths exceeding 2200m, Corus sourced plate from two suppliers. To meet linepipe specifications, Corus used its database and knowledge of major European plate manufacturers to procure the highest quality sour plate for the foundation of the final product.
The chemical composition supplied by each mill differed slightly and was based on the material requirements and the plate production route employed at each mill, specifically the capabilities of the rolling mill-stand and accelerated cooling.
For optimum sour service performance pipe, the foundations are built during steel making, when it is critical to control the amount of phosphorous, sulphur and carbon in the steel. The project had tight controls on the phosphorous and sulphur content: below 0.015% maximum phosphorous and 0.0015% maximum sulphur.
Secondary steel making and plate rolling also influence the sour performance and ultimately the final microstructure. The first mill produced a predominantly acicular ferrite structure with some bainite colonies, the second an even proportion of acicular ferrite and bainite; both are ideal structures for sour service performance.
Using a dual plate supply impacts on the control of the mechanical properties and pipe manufacture. To some extent, the mill settings can be optimized for incoming plate characteristics; however, they cannot be treated as a truly dynamic variable if production rates and delivery demands are to be met. Any variability in plate strength characteristics may be displayed as variation in shape from the O-press and expander. This means that for heavy wall, small diameter projects, the forming line must have the capability to accommodate variation in plate forming characteristics in line with a load capability to press the high strength plates.
As part of the pre-production planning, a development program was undertaken to optimize pipe shape, weld form and minimize the loads in key pipe forming processes.
Tooling
Ensuring the combination of correct size and shape over a long production run places huge demands on a pipe mill. The most problematic shape irregularities that manifest at the O-press are shoulders at the 2 o'clock and 10 o'clock positions and peaking near the weld area.
Although the UOE process - which is carried out by pressing a steel plate into a U-shape and then an O-shape, then expanding the formed pipe circumferentially - includes a shape correction step, the expander, it is not enough to rely solely on this capability to ensure shape and size, especially at high wall thickness. At this high thickness/diameter (t/D), severe flats and peaking would be very difficult to remove even with exceptionally high expander loads. As such, Corus carried out extensive modeling of high t/D pipe forming.
This resulted in a structured tooling management system fundamental to the success of this project.
Welding
Welding heavy wall linepipe presents a number of challenges. The parameters of the single pass (inner diameter and outer diameter) submerged arc welding (SAW) process are specifically designed to ensure weld integrity, reduce the occurrence of defects and meet the production rates required in the mill. The consumables are chosen to ensure a stable and robust weld with the required mechanical properties.
The welding process was qualified at the start of production using the plate material with the highest pipeline current matter, theoretically representing the most difficult material to weld. This required significant planning between the plate and pipe mill, ensuring the first batches of plates delivered to the pipe mill contained the highest pipeline current matter. The pipeline current matter distributions from each plate mill showed exceptional control below the specified maximum of 0.21 dimensionless.
As with all offshore linepipe projects, the weld integrity and properties were monitored through production with extensive testing, including Charpy impact toughness and hardness. Although not required during production by the project specification, weld Charpy testing at -30°C was performed on some of the project pipe, as this is recognized as the current temperature limit for single pass SAW linepipe at this thickness. The data showed excellent properties achieved on the project with average values all above 100J.
Properties, performance
A number of fundamental steps ensured the required sour service performance was not lost through the pipe forming process. A strain management approach ensured the best sour linepipe possible at the t/D ratio. Corus has established a nominal strain limit of 0.035, equating to the pipe t/D ratio, for full sour service certification to the NACE TM0284 standard, or the National Association of Corrosion Engineers standard that certifies the evaluation of pipeline and pressure vessel steels for resistance to hydrogen-induced cracking, although with increasing demand for sour service products this is currently being extended. 'Micro-damage', in the form of small cracks and voids around precipitates/ hard inclusions, will increase the number of available sites for molecular hydrogen formation throughout the microstructure; therefore, the total strain during forming, including pressing and expanding that causes micro-damage should be limited where possible.
A t/D ratio of 0.0695 for this product was clearly challenging; however hydrogen induced cracking testing was performed throughout production, on 26 body and 13 weld samples, all of which passed with no cracks.
Such a large quantity of pipe makes offshore lay rate a paramount consideration. If optimum lay rates are to be realized it is important that the product remains consistent throughout manufacture and that the pipe mill presents a uniform size and shape to the laybarge. The project team requested a range of 2.8mmID and an ovality controlled against a maximum of 3.4mm. Corus met the tight control: every pipe was supplied with ovality <2mm. To confirm the ability to maintain shape control out of the O-press, the expansion ratio recorded across the project was an average of 1% with no individual pipes having an expansion ratio greater then 1.2%.
Conclusion
Through the application of targeted research and development into mill tooling and operation, a regime has been developed to optimize pipe production performance. This has allowed the design of bespoke tooling for large volume offshore pipelines, raising the bar in terms of finished pipe properties.
It has also increased the understanding of the formability of sour service steels and the limits of their application in thick walled pipeline, which can be used globally on future demanding projects. OE
About the Authors
Martin Connelly is technical manager at Corus, which he joined in 1993 after graduating with a first class honors degree in metallurgy & engineering material from Strathclyde University. He previously worked in a number of technical, quality and operational roles.
Richard Freeman is business development manager at Corus Tubes, which he joined in 2004 as development manager. He is responsible for sales into new markets and leads the product development function of the organization. Freeman graduated from the University of Leicester with a first class honors degree in mechanical engineering.
Simon Slater is manager of technical development and mill support for Corus Tubes Energy Business, based at the Corus pipe mills in Hartlepool. He is responsible for technical support to both the UOE and 3 roll bending mills and strategic development projects such as deepwater, clad and high strength sour linepipe.
Shuwen Wen is a principal scientist at Corus Research Development & Technology's Swindon, UK technology centre. He has over 20 years of experience in numerical modeling of materials processing, including metal forming, casting, welding and galvanizing. For the last 10 years he has been involved with the through process FE modeling of UOE linepipe manufacturing at Corus 42 pipe mill in Hartlepool, with specific emphasis on mill capabilities, process optimization as well as plate-to-pipe properties characterization for deepwater pipelines.