The most economical method of manufacturing line pipe is the UOE process, which is performed by pressing a steel plate into a U and then O shape, then expanding the formed pipe circumferentially. This manufacturing route is quicker to market and more cost-effective than seamless alternatives, but has its challenges. Few pipe producers have been able to manufacture UOE pipes at 16 to 20 in. outside diameter, the size required for some deepwater operations.

Petrobras’ deepwater Tupi discovery offshore Brazil required the thickest 18-in. UOE pipe ever manufactured. The line pipe for Tupi, with a wall thickness of up to 31.75 mm for depths of 7,218 ft (2,200 m), is not the deepest globally, but it constitutes a pipe-forming milestone and required pipe-making skills that are at the forefront of current technology.

Design challenges included fatigue and corrosion concerns as well as depth/collapse issues that led to line pipe construction of a previously untried thickness-to-diameter ratio (t/D) of 0.0695.

Tupi discovery

The engineering team initiated a development program aimed at increasing O press die wear resistance while improving strength and toughness. (Images courtesy of Corus Tubes)

The Tupi hydrocarbon discovery lies approximately 155 miles (250 km) south of Rio de Janeiro in Santos Basin Block BM-S-11 and holds an estimated 5 to 8 Bboe. Reserves will be delivered through the Tupi Pilot pipeline, an 18-in. diameter pipeline stretching 140 miles (225 km) to the Mexilhão platform.

Corus Tubes supplied the majority of the pipeline, including the thickest sections, with wall thickness of up to 31.75 mm.

Manufacturing such heavy-wall small-diameter line pipe induces significant forming strain within the pipe, which can be approximated as being the nominal wall thickness divided by the nominal diameter expressed as a ratio, t/D. High forming capacity in terms of the C-press and O-press is required to ensure the pipe is manufactured within the dimensional and property requirements. The process requires detailed attention to tooling design and expert understanding of the feedstock material, forming process, and change in properties from plate to pipe.

Construction challenges

To meet the operator’s requirements, the engineering team used the highest quality sour plate as the foundation for the final product. Using an extensive historical database and knowledge of major European plate manufacturers, engineers defined a plate specification from the technical requirements of the project.

The plate and pipe specification required certification of the pipe to hydrogen induced cracking and sulfide stress corrosion cracking international standards, which are difficult to achieve with high strain, but were met fully by controlling manufacture uniformity and developing the correct base material prior to pipe forming.

The volume of plate material required demanded the use of two suppliers. This presented issues not only in specifying the same high-quality plate through two different routes, but also in understanding how each supply forms into pipe through the mill.

To some extent, 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 can manifest as variation in shape from the O-press and expander. For heavy-wall small-diameter projects, the forming line must be able to accommodate variation in plate forming characteristics allied with the load capability to press the high strength plates.

As part of the pre-production planning, the engineering team undertook a development program to optimize pipe shape and weld form and minimize the loads in key pipe forming processes.

Ensuring the combination of correct size and shape over a long production run presented a significant challenge. Following extensive modeling of high t/D pipe forming, engineers developed a structured tooling management system that was fundamental to the success of this project.

The enhanced process

The UOE method of manufacturing line pipe requires the steel plate to be pressed into a U and then O shape, after which the formed pipe is expanded circumferentially.

The UOE pressing process involves initially crimping the plate into a curve with two pairs of dies operating on the long edges of the plate. Generally, pipe mills have a limited set of dies for this operation. To ensure the integrity of the process, Corus designed specific tooling to ensure the optimum shape was achieved at this stage.

Although the UOE process has the expander as a shape correction step, this should not be relied upon solely to ensure shape and size. Excellent control on shape and formability at the O-press is vital in allowing the expander to correctly form the pipe.

Forming in the O-press was modeled to not only optimize the shape, but to investigate the high pressing force of the nose — where the plate edges meet during forming — of the weld preparation. The engineering team initiated a development program aimed at increasing O press die wear resistance while improving strength and toughness. This was achieved by optimizing the casting process and producing an improved liner microstructure.

Nose deformation happens as the nose comes together during compression. The inside and outside welding process must be robust enough to guarantee a stable consistent weld in view of the nose dimension variation.

The process of welding heavy-wall line pipe presents a number of challenges. The parameters of the single pass (inner and outer diameter) submerged arc welding 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 and to yield the required mechanical properties.

The weld integrity and properties were monitored through production with extensive testing, including Charpy impact toughness and hardness. The data from HAZ (Heat Affected Zone) Charpy impact testing at -22°F (-30°C) demonstrated the excellent properties achieved on the project with average values all above 100J.

For the final forming stage a set of bespoke dies and cone were fabricated to guarantee the final pipe shape and to control the expander pulling force.

Dimensional performance

Accurate pipe dimensions are required to ensure the pipe collapse requirement weld integrity and laying rates are maintained. Pipe production on this project demonstrated excellent control that exceeded project requirements.

A range of 2.8 mm on the inside diameter of the pipe and an ovality controlled against a maximum of 3.4 mm were required for this project. The tight control could only be met through embedded quality control procedures and tooling expertise. Every pipe has been supplied with < 2 mm ovality to confirm the ability to maintain shape control out of the O-press.

The two most revealing shape attributes are peaking and local out of roundness (LOOR), both showing a local variation in shape (peaking in the weld area and LOOR in the pipe body), measured along a gauge length of typically 150 mm to 200 mm. High levels of either indicate a capability gap in the forming process or expansion process.

Corus is commissioning a laser pipe measuring device with the aim of describing the shape in a level of detail previously not possible, while increasing production rates through the finishing stages of process route.

A random selection of pipes from this project was measured using the laser system after expansion. Continuing the analysis and going to the next level of shape definition, the distribution of every individual diameter on each pipe end is measured. This distribution describes what is commonly referred to as the “global ovality,” combining both the ovality and size into one measurement. Pipe manufacturers are increasingly being asked to meet existing acceptance criteria based on every pipe end measurement, and the data provided demonstrate that 97% of the 1,568 pipe diameters measured met the required range of 3.2 mm with an actual range of 4 mm.

The peaking shows a higher mean than those measured manually. However 85% of the pipes measured are still below 1 mm, and all of the results are below the maximum expected of 1.6 mm. The LOOR shows one outlier at 1.4 mm, with 95% of pipes below 0.7 mm. These results combined with the control of ovality and size confirm the outstanding dimensional performance achieved on the Tupi project.

Although presenting a positive step in pipe manufacturing, data from such a laser measurement system must be used with care. It is important to recognize that the existing specifications have been developed based on manual measurements, which can only be expected to have a tolerance of +/- 0.5 mm. If laser type devices are to be implemented in pipe mills, the implications of the increased accuracy will have to be addressed with respect to acceptance criteria in the current standards. However, the increased confidence in pipe shape will allow for optimized utilization in deeper water and enhanced girth weld integrity.

Laser measurement can be used to give increased confidence in pipe shape for girth welding and deepwater collapse resistance. However, a review of current standards and acceptance criteria must be performed to allow sensible implementation.