Hardness prediction models can simplify design and welding needs for deepwater pipelines.

The definition of "deep water" has changed dramatically over the past 7 years. Pipeline technologies have made substantial improvements, such as those found in drilling and exploration techniques. Development of steel grades higher than X70 and X80, as well as heavy-wall seamless line pipe (with wall thickness up to 40 mm), have allowed oil and gas companies to cross the deepwater frontier of 4,922 ft (1,500 m). One good example of a project in this field is the Total Canyon Express pipeline in the Gulf of Mexico, which holds the deepwater record at 7,200 ft (2,200 m) below sea level. A second example is BP's Thunder Horse, also in the Gulf of Mexico, which is facing the high temperature/high pressure (HT/HP) challenge with an innovative design of 12-in. outside diameter (OD) x 40 mm wall, grade X70, seamless pipeline.

Pipe development

Greater water depths dictate conditions unheard of only a few years ago, creating a need for new breeds of pipe. Deepwater pipeline projects demand a very technologically advanced line pipe. Generally, this type of line pipe is designed for HT/HP applications and corrosive environments. Because the pipe is being laid thousands of feet below the surface, it must withstand both internal and external pressures. In addition, as laying methods become more complex, additional plastic characteristics and collapse resistance are often required to ensure good performance during the laying process. Also, the ability to weld with ease during installation is another relevant aspect of deepwater line pipe design because the time required to perform this task has an important economic impact.

From the technical point of view, the most critical issues of deepwater line pipe and risers are:

• Collapse resistance above 10,000 psi
• Stringent dimensional tolerances
• Low variation in actual mechanical properties
• Sour service resistance (NACE)

Collapse resistance above 10,000 psi is critical because the variation in terms of yield strength and wall thickness (WT) should be very low to ensure a high-collapse-resistant pipe. As long as exploration of the fields moves to water depths deeper than 3,281 ft (1,000 m), the WT of the line pipe or risers will increase. As a result, achievement of high strength (X70 steel grade and above) on wall thicknesses ranging from 30 mm up to 45 mm becomes a challenge.

Applying stringent dimensional tolerances aid in expediting the welding process in order to avoid mismatching during welding. This stringent tolerance refers to the internal diameter tolerance (i.e. +/-1.5 mm or more stringent), which is usually demanded by the end user. Also, risers are usually exposed to fatigue loads, so the dimensional tolerances at the pipe ends becomes a critical issue.

Low variation in actual mechanical properties is essential due to the fact that pipes will be welded through a qualified welding procedure. A low variation in actual yield strength (i.e actual yield strength should not exceed specified maximum yield strength + 100 mega pascals) will contribute to having consistent mechanical properties in the girth weld. During the laying process, the line pipe or risers are exposed to different load regimes. If pipes are going to be reel laid, the material will be exposed to deformation cycles, the lower the yield strength variations, the easier it is to guarantee against any buckling during the laying process.

Sour service resistance is key because, if line pipes or risers are exposed to a sour environment, clean steel is mandatory. Residual elements should be kept to the minimum (i.e. sulfur less than 0.003% by weight, phosphorus less than 0.015% by weight). Furthermore, a good heat treatment process will produce a good final homogeneous microstructure through the wall thickness and will ensure better corrosion resistance through the wall.

The hardness value across the wall thickness is a function of the steel chemical composition and the heat treatment process during the line pipe manufacturing. However, as the pipes are welded at the end, hardness becomes an issue in the girth welds. Furthermore, final hardness will be a function mainly of the welding process.

Deepwater pipelines demanding high strength and heavy wall thickness are real challenges during both pipe manufacturing and welding. Accordingly, the best way to approach these projects is to work jointly with experienced line pipe manufacturers that can bring their knowledge of materials' behavior and metallurgical expertise into the search for the right solution.

Prediction models

Tenaris, in conjunction with the Centro Sviluppo Materiali, has developed two models to predict, with a high level of accuracy, the mechanical properties of the base material (yield strength, ultimate tensile strength and hardness), and the final hardness in the girth weld, given a chemical composition.
The first model (pending license) can determine mechanical properties and the steel microstructure of the pipe after heat treatment. It is a static model based on the microstructural changes that take place during cooling in the tempering tub at the heat treatment process. As a result of this model, continuous cooling transformation (CCT) graphs were created.

CCT graphs for different types of steel are the base for conducting research about the effect of chemical composition, cooling speeds and austenitic grain size on the tempered microstructure. The model is based on a neuronal network process which analyzes a CCT graph database. Using the calibration ratios of the tub in each mill, the model estimates the cooling speeds and the temperature distribution throughout wall thickness. This data and the given chemical composition result in the percentage of phases (i.e. percentage of bainite, martensite, ferrite) derived from the tempering process. Additionally, the model has a microstructural evolution model attached, which estimates the mechanical properties based on the tempering temperature and time of the current phases.

Using this model reduces time and the costs associated with heat trials, avoiding to some extent, the need for completing rolling, heat treatment and testing processes when designing deepwater line pipes.
The second model, already licensed as Tenweld 2002, is a tool that predicts the hardness value and microstructural constituents in the heat affected zone (HAZ) of the welded joints, based on a given chemical composition and a welding method. It is specialized on low-carbon steels (0.05-0.18 maximum carbon percentage); on steels microalloyed with vanadium; vanadium and niobium; and vanadium, niobium and titanium; and on wall thickness ranging from 6 mm to 50 mm.

The model allows for adjusting the chemical composition design for tube manufacturing from the beginning, taking into account steel performance during the welding process. Hardness predictions can also be used as a function of only the chemical design in order to establish the welding procedure to be adopted in fabrication. This information represents a great deal of time and cost savings in the development of the right product and welding procedure, ensuring a good level of accuracy without incurring expenditures for unnecessary experimental trials.

As oil and gas companies advance into deeper waters, the need for new technologies that are economically viable will demand joint work between users and suppliers to leverage the accumulated experience on both ends. Previous work and experience should bring success in the future, if they are used to enhance technologies, coming up with more cost-effective techniques and processes.