Understanding the challenge of manufacturing deepwater line pipe

Deepwater line pipe is usually small-diameter, thick-walled pipe which, by definition, is a pipe with a high thickness-to-diameter (t/D) ratio.

For pipe manufacturers who deploy the UOE process (where the material is formed into a “U” shape and then an “O” shape before being expanded to the final dimensions), the creation of line pipe with a high t/D ratio rather than necessarily the manufacture of specific deepwater line pipe represents the greatest challenge. As the limit state for the design of such line pipe is not guaranteed to be the collapse point, the manufacturer has to produce a high t/D line pipe that can meet the most demanding design limit state requirements, with any specific collapse element coming through extra testing.

For low t/D line pipe, the predominant collapse mechanism is elastic and controlled primarily by the dimensional performance of the line pipe. For high t/D line pipe, one of the most frequently dominant collapse mechanisms that influences the design is plastic collapse, and it is the mechanical strength and shape of the line pipe that control this.

FIGURE 1. Modeling and validation are vital tools for understanding the UOE process for the manufacture of high t/D line pipe for deepwater applications, with this trial demonstrating the consistency of the manufacturing process for X70-grade material.

Deepwater line pipe key factors

A key factor for the manufacture of deepwater line pipe is the consistency of both the dimensional and mechanical properties around the circumference and along the full length of the pipe.

The pipe manufacturing route can significantly influence a number of factors in the DNV OS-F101 collapse equations, which when optimized can yield improvements in the collapse resistance of UOE line pipe. Understanding the design requirements and the UOE manufacturing process is essential to manufacturing a product that exhibits all of the desired qualities. If the intrinsic collapse resistance of the pipe can be increased, there are benefits of cost reduction, increased safety margins, and facilitation of laying at greater depths and speeds.

A model solution

Due to the stringent requirements for deep water, it is essential to be able to manufacture high t/D line pipe to the necessary quality while doing so in a commercially viable production environment.

Each step of the forming process must be fully understood from a theoretical and operational perspective. Finite element (FE) modeling and validation of the individual processes within the UOE manufacturing route allow the dies for each step of the process to be designed at the optimum specification. This is essential to give good shape to the final pipe at a production rate that maintains the commercial viability.

The UOE forming processes for high t/D line pipe involve large amounts of pressure, and the possibility of damaging equipment is high if the process is not controlled. The use of modeling that has been validated on previous projects allows the design of the pressing dies to be optimized. This optimization allows the minimum level of pressing force necessary to be used, thereby reducing both the risk of damage to the equipment and to the mill’s ability to complete an order on time and in full to the necessary standard of quality.

Each of the individual forming processes has an impact on the others in the whole UOE process, and an understanding of these interactions is necessary to establish a process that is optimized to deliver quality line pipe. For example, a good crimp profile for shape control out of the O-press, which allows consistent weld preps and reduces the peaking on the final product, is essential since this not only affects the final product quality in terms of shape (peaking cannot be fully removed by the expander) but can have a large impact on the throughput of the mill by allowing the welding to be set up for optimum running.

FE modeling has allowed the design of dies that not only give the correct shape and profile but, due to the bespoke nature of the process, reduce the amount of wear. This gives better shape control for a longer period of time while reducing the costs associated with lower quality and throughputs.

Modeling and validation are essential tools for understanding the UOE process with respect to the manufacturing of high t/D pipe. Strain reversals on the material during the forming process in the UOE mills are unavoidable and necessitate the deration of the pipe manufactured through this process due to the Bauschinger effect. This fabrication factor for the pipes is a calculated value. These fabrication factors should be used as a design tool only and not as a quality control tool. Quality control for the product should be determined by whether the actual test results meet the calculated tensile and compressive minimum yield strength as set out in the specification.

The way forward?

Natural progression within the industry has been to develop higher t/D ratio products with a steel grade strength of X65, and the next logical step is to increase the specified grade to X70 to give more mechanical resistance to collapse.

The manufacture of high t/D line pipe involves the application of large forces during each stage of the UOE forming process, and an increase in strength of the starting plate material will require larger forces to be applied during the forming stages against an equivalently sized X65 material. To prove the UOE process for the manufacture of a high t/D line pipe, it is essential that the manufacturer understands the dimensional and mechanical properties along the entire length of the line pipe manufactured by this route since the in-service pipeline will be subjected to large external hydrostatic pressure along its entire length during its operational lifetime.

To show this phenomenon and to try to understand what the variation is at various points around the ring, a trial was carried out to quantify the differences, both around the circumference and along the full length of an X70-grade line pipe. The trial also demonstrated the consistency of the UOE manufacturing process.

Test success

The data from the manufactured line pipe for the mechanical properties showed a normally distributed X70 tensile and compressive yield strength at Rt 0.5, which comfortably exceeded the 0.85 fabrication factor associated with the UOE process. The testing regime showed the properties were consistent regardless of the position along the pipe from where the tests were taken. However, there was some variation seen for tests taken around the circumference of the ring due to the different stresses of the individual processing stages (Figure 1), with the most conservative location of 180° to the weld being the location where the standard production testing was taken. The results of the tests carried out at the 180° position all passed the requirements and showed that if tested at this location, it was appropriate to assume that this was the minimum performance for the entire pipe.

FIGURE 2. For high t/D line pipe, getting as close to a perfect circle as possible is best when it comes to collapse resistance. In this example, the ovality level for this X70-grade line pipe is less than 0.5% of the OD, positively impacting the fit of line pipe for laying operations in deep water.

Practical application testing of the collapse resistance should be carried out in a way that ensures a valid result while being a test with a turnaround time that allows sensible release timings to be maintained.

There are three main tests for collapse with varying degrees of conservatism and viability for production release testing. The first and most “service-realistic” option is the full-scale pipe collapse test. It is impractical for this method to be used for production testing due to cost and logistics. Validation of the UOE manufacturing method and benefits associated with the low-temperature heat treatment of the coating process have been reported. This work has shown the relationship between the different methods and how there are varying degrees of conservatism associated with each.

The preferred production release test is small-scale compression testing, which is the most conservative of the three options. The small-scale test pieces can be taken as part of the routine release tests, and results can be generated within a sensible timescale. The test is carried out as per ASTM International compression testing standard ASTM-E9, with the sampling of the material taken in a way to ensure that all of the grain size variation in the material is sampled equally.

The other subtlety that requires consideration is the strain rate of the test. The current experience and calibrations for mills are based on the ASTM-E9 strain rate, and this should be maintained for any tests carried out. The other test, the small-scale ring collapse test, is between the full-scale and compression tests in terms of conservatism. This not only allows the mechanical collapse strength to be tested but also the influence of any imperfections in the shape of the pipe or ring. This is a mid-point between the other two tests and gives realism due to the fact that a ring of the material is tested. It also can be carried out at a sensible rate for testing on an extended basis (e.g. one in 10 heats).

Dimensional data

The second important factor for high t/D line pipe is the dimensional performance along its length. A major factor in the collapse limit state equation is the ovality of the manufactured line pipe, with a perfect circle giving the best collapse resistance. The dimensional properties based on the outer diameter (OD) ovality of the trial pipes can be seen in Figure 2.

The performance of the UOE process is excellent, with an ovality level of less than 0.5% of the OD. This level of ovality will have a positive impact on the fit of the line pipe for laying operations and also on the collapse resistance of the operational pipeline. All other dimensional measurements taken during the trial showed the excellent performance of the UOE process against the applicable standards.

To be able to produce line pipe suitable for deepwater applications, it is essential that a UOE pipe mill can manufacture high t/D pipe with consistent mechanical and dimensional properties along the entire length of the pipe. To do this, a UOE mill can use modeling and validation along with comprehensive data analysis to ensure that the line-pipe products being manufactured meet all of the necessary property specifications for deep water.

References available.