Hydrocarbon Transport Options
Unlike steel, traditional polypipe is highly resistant to corrosion. This, combined with its low cost and high availability, has even made traditional polypipe a popular liner material for aging steel pipelines. It is relatively inexpensive, flexible, lightweight and far easier to install. Readily available in large coils, traditional polypipe also minimizes the number of joints and fittings required in any given installation. However, the Achilles heel of traditional polypipe is a vulnerability to chemical permeation.
TRADITIONAL POLYPIPE’s Permeation Problems
At the risk of oversimplifying complex chemistry, permeation is exactly what it sounds like - the tendency of one substance to move through another. In the case of polyethylene, gas and liquid hydrocarbon compounds will eventually be driven into and through the plastic pipe wall by the pressures and temperatures inside. This process expands and thus weakens the pipe, making it more susceptible to permeation and leading to early-life failure.
Since liners are not designed for pressure, the space between the pipe and the liner must be minimized; when the gas volume in the annulus expands, the subsequent pressure can otherwise cause the liner to collapse. This particular problem requires that the annulus be frequently vented, and because traditional polypipe comes in long spools or sections and tie-ins aren’t always an option, repairs often mean replacing the entire line.
To prevent permeation, you have to consider the density of the polymer. In the case of short-chain or aromatic hydrocarbons, the molecular density is very low, which means the hydrocarbons can more easily pass through the plastic wall. Long-chain polyamides, often referred to as nylons, are much denser and chemically stable than traditional polypipe. As a result of their chemical resistance, long-chain polyamides have for decades been successfully used in offshore flexible flow lines and risers.
In 2015, polyamide-6 (PA6) and polyamide-12 (PA12) products were developed specifically as liners for gathering pipelines, serving as a barrier between the flowing hydrocarbons and the host pipe.
Composite pipe by Baker Hughes features a unique design that includes an inner nylon liner for ultra-low permeation. For this liner, there are actually three different ultra-low permeation options: a nylon PA6 liner, which provides a strong chemical barrier for a limited range of applications at a lower price point; a nylon PA12 liner, which provides a stronger chemical barrier and is the only approved nylon for standalone gas piping systems; and a polyphenylene sulfide (PPS) liner, which provides the strongest chemical barrier. These options are resistant to H2S, CO2, bacteria and harsh chemicals; sustain no paraffin buildup; are fully piggable and suitable to hot oil. Featuring the lightest weight and longest lengths among spoolable pipe solutions, Thermoflex pipe can be installed with less manpower in less time.
Finally, with any of these inner liner options, no pressure de-rating is necessary, even at higher temperatures.
 (H. F. Mark. Encyclopedia of Polymers Science and Technology – 3rd Ed. Vol 12. John Miley & Sons Inc. 2004)