Raising the bar for onshore pipelines

Carbon steel pipelines are ubiquitous in oil and gas operations, delivering hydrocarbons to refineries and petrochemical plants and transporting refined products to end users. Although carbon steel has become an accepted material for pipelines, it is not the only option available, and in many applications, it is not the best choice.

Pipeline owners have been dealing with the same challenges associated with carbon steel pipe for decades. Originally adopted for oil and gas transportation because of its strength and affordability, carbon steel became the go-to solution for pipeline owners in the sector despite its shortcomings.

First and foremost, carbon steel is susceptible to corrosion. According to data published by the US Department of Transportation Pipeline & Hazardous Materials Safety Administration (PHMSA), internal corrosion accounts for approximately 60% of all pipeline corrosion incidents in transmission and gathering pipelines. Exposure to corrosive chemicals can lead to wall thinning and weakening and eventually to leaks. Carbon steel is particularly vulnerable to hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) when exposed to hydrogen sulfide (H2S). Coatings, cathodic protection, and corrosion-resistant linings can mitigate this issue, but they can be costly and require constant surveillance and maintenance to ensure their continued efficacy. Carbon steel also becomes brittle at low temperatures and suffers rapid cracking propagation failure.

Installation poses a host of additional challenges – some of which are cost related, while others impact asset integrity. Because carbon steel pipe is produced in 40 ft lengths, welding is required to join the pipe. The reliability of the pipeline is determined by the quality of the welding. Inexpert welds and improper designs can result in weaknesses that could lead to uncontrolled releases and failures. To avoid such events, the welding process must follow stringent procedures, and welds must be extensively tested to ensure their quality. This requires skilled welders to perform the work, appropriate non-destructive testing to verify the quality of the welds, and a considerable time investment, all of which impact project economics.

There also are environmental concerns associated with carbon steel. Leaks and spills pose obvious environmental risks, but the carbon-intense process of producing carbon steel is another serious consideration for pipeline owners who are working to reduce the carbon footprint of their operations.

The benefits of RTP

Reinforced thermoplastic pipe (RTP) is gaining ground as an alternative to carbon steel in applications below 3000 psi and temperature below 230°F (110°C). One compelling reason for using RTP is its corrosion resistance. Thanks to its thermoplastic liner, cover and non-metallic reinforcement material, there is no corrosion mechanism. That not only translates into a longer service life with less maintenance, it also means RTP is ideal for sour service because it is not vulnerable to hydrogen-induced cracking.

RTP has better flow characteristics than carbon steel because the internal plastic lining is smoother than steel, providing greater throughput with the same inner diameter. So, less pumping is required to move product through an RTP pipeline, consuming less energy.

There is no wax adherence to PPS or Nylon liners because they are a natural repellent of wax.

RTPs also have much better thermal insulation properties compared to carbon steel pipe. In steel pipe, bore fluid loses heat faster and allows wax crystallisation to occur. On the other hand, RTPs keep the bore fluid above wax crystallisation temperature over a longer distance. Thus, less pigging and cleaning will be required for removing wax.

In addition, RTP does not require corrosion inhibitors, which can weaken the protective iron oxide layer on the steel, and in the case of inhibitors that contain hydrogen sulfide scavengers, can promote hydrogen absorption into the steel and lead to hydrogen embrittlement. RTP pipe is lighter than carbon steel, which impacts installation time and cost. A narrower right of way is required to install RTP pipe, and reduced trenching is required for pipe burial. Also, given that RTP pipe is flexible, it can be installed with long sweeps/bends lowering energy loss during transmission compared to the 45° and 90° elbows typical on steels lines.

Improvement by design

With the implementation of a new, proprietary manufacturing process, it is now possible to produce spoolable RTP pipe up to 8 in.

The process begins with melting polymer pellets and feeding the liquified polymer through an extrusion head to form the liner, which is pulled to size through a vacuum tank. The material used for the inner lining of the pipe depends on the performance characteristics required for end use:

• HDPE delivers four times the erosion resistant of regular steel pipe.
• Nylon provides gas permeating resistance and deters paraffin deposition.
• PPS delivers the greatest resistance to permeation and performs well in corrosive environments.

A tiebond (aka adhesive) layer is next, followed by an HDPE or PERT layer for high-temperature strength. A coextrusion process forms these three layers simultaneously into a single pipe, minimising the possibility of delamination between layers.

The pipe is then fed through a controlled cooling chamber to ensure it is solidified before reinforcement. Tapes are wrapped helically around liner at a 55° lay angle. The reinforcement materials used are:

• Aramid fibre, which delivers the best tensile strength-to-weight ratio.
• Glass fibre tape, which allows for sour service to pressures up to 1500 psi and provides optimal weight-to-pressure resistance.
• Steel wire tape, which offers pressure resistance up to 3000 psi.

The final step in the process is…

To access the full version of this article and get a free trial subscription to World Pipelines, sign up here!

Read the article online at: https://www.worldpipelines.com/special-reports/27032024/raising-the-bar-for-onshore-pipelines/