Plastic lining has been applied as a means of providing cost effective corrosion protection to pipelines, and it is widely used for water injection pipelines both onshore and offshore.
Previous Part 2 captured the Plastic Lining Geographical, Economic, Social Factors.
4.0 Liner Technology
The technology of liner insertion within a conventional steel pipeline is long established. There are a number of proprietary techniques for doing this which were mainly developed for the refurbishment of old gas and water pipelines onshore and there are thousands of miles of lining installed using these techniques.
The fundamental difficulty for plastic liners in the offshore Oil and Gas production context is however that plastics are permeable and small amounts of gas can be trapped in the micro-annulus between the liner and steel, and this gas can drive liner collapse if the pipeline is depressurized.
The collapse pressures are low (typically 4 or 5 bar, or 40m of water) relative to operating pressures and some form of venting is required.
There are various venting techniques but the two vent technologies which offer promise are:
- A grooved liner
- A perforated liner
4.1 Grooved Liner
The grooved liner has grooves cut in its outer surface so that permeating gasses collect and flow to some convenient vent point. It is essential that the pressure at the vent point does not exceed the liner collapse pressure at that point and the liner must be sufficiently thick to allow a workable groove to be cut that will not collapse with creep over time.
4.2 Perforated Liner
The perforated liner has a series of small holes drilled through it. Any substance behind the liner is therefore free to flow back into the flow path when the pipeline is depressurized. The holes are sized such that corrosion is self-limiting and it has been demonstrated that corrosion can be controlled in this way.
4.3 Integrity Management of a Lined Pipe
Because the liner is thick, tough and abrasion resistant it is unlikely to be damaged. However, obtaining assurance of the pipeline’s condition will pose novel inspection challenges.
There are two areas of interest: the condition of the liner, and the condition of the steel pressure-containing envelope. Once liner technology becomes established and there is confidence that they work reliably, the focus may move from the concern for both liner and steel, to assuring the condition of the liner only, but this will take some time.
Initially it will be desirable to inspect both liner and steel. The condition of the liner can be inspected using existing pigging technology including caliper and vibration measurement. Its condition can then be used to infer the likely condition of the steel.
Obtaining assurance directly that the steel has not deteriorated in some unexpected way is more difficult because the liner inhibits inspection. Novel techniques are likely to be required.
See Part 4 for Plastic Lining Future Prospects.