A [subsea pipeline] often needs to be [coated]. The reasons for this may be:

[Corrosion protection]
[Thermal insulation]
[Stability]
Wear & [Impact protection]

Corrosion protection often consists of an air-tight protective layer around the [pipeline] exterior, and also on the inside if the pipeline is to carry corrosive fluids. The exterior protective layer is often [fusion bonded epoxy] or [asphalt], while interior protective layers often consists of [plastic liner]s. Asphalt if often more popular as exterior coating on larger pipelines, as an epoxy layer is quite thin and easily damaged. When manufacturing large diameter pipelines, the manufacturer will also accept larger [surface defect]s, which can be hard to cover with a thin layer of epoxy. A typical asphalt layer is around 6 mm - but this depends very much on the type of pipeline and it's environment.

Fusion bonded epoxy is very often used as a [primer] for other types of plastics.

Pipeline stability is equally important. If the pipeline is too light it can shift on the seabed due to wave action and currents. This is especially important for pipelines carrying gas or for pipelines close to the surface (where waves have more impact). There have been some pipelines that have ignored this, and started floating from the combined effect of waves and the [buoyancy] of the gas they carried (the waves washed away the rock that was dumped on top of it).

To increase the pipeline weight you can either:

increase the [wall thickness] of the pipe (expensive!),
cover it with [rock dumping|rock] (pricey),
[trenching|bury the pipeline] (not that cheap),
add on extra weights (inconvenient), or
apply a layer of concrete on the outside of the pipeline (dirt cheap!).

If you choose to coat the pipeline in concrete, the concrete has to be held in place by [rebar]s placed around the pipeline. The reason for this is that if you get a small damage in the concrete the damage won't propagate and clear off all the coating (concrete can take lots of [compressive force], but no [tensile stress|tensile] - that's the rebar's job.

The amount of extra weight needed on the pipeline varies, but a typical amount for a 30" gas pipeline in the [north sea] is 5-10 mm of asphalt covered by 20-40 mm of [concrete coating], adding on approx. 150 kg/m to the pipeline's weight.

Thermal insulation is often very important for pipelines - especially those who carry raw well-stream. This is because the oil contains a high percentage of waxes. At high temperature, the wax will be [in solution] with the oil - but if the temperature drops, the wax will solidify on the inside of the pipeline. This can cause the pipeline to block, and is not acceptable.

Since this is a subsea pipeline, there will be a significant temperature loss to the [outside environment] - oil directly from the well is often very hot (~100 deg C), while the surrounding sea is usually only 4 deg C.

Insulation of the pipeline is often done by either coating the pipeline with a plastic ([polypropylene] is very popular), burying the pipeline into the seabed, or both.

Most pipelines also need some form of wear and impact protection . Pipelines are laid in areas where there is a danger of impacts with other objects, be it [dropped objects|objects dropped] from the surface, or moving along the seabed. In the vicinity of [platform]s there can be [container]s accidentally dropped over the side, while in clear water [the local fishermen] often believe that there is more fish around the pipeline, and therefore bring their [huge industrial trawls] up against the pipeline. If the pipeline has subsea valves they need special protective cages to ward against this.

To avoid damaging the pipeline, it is often either buried into the seabed, buried under an pile of rock, or coated with concrete. Burying a pipeline is a legal requirement in Norway if they are under 16". Pipelines over 16" are believed to be stable enough to withstand most damage.

As a few final comments, all pipeline coating has to fulfil a few key points:

[Ease of application]: Today most pipelines are coated in 12 m [line pipe|segments] in the [fabrication yard], so that during pipeline lay one only has to coat the welded joints (a [field joint coating]). Since it would not be economically viable to wait for hours to coat every pipeline segment, a key property is speed of application.
Good adhesion to pipeline steel: If the coating does not adhere to the steel, it can shedded off - or even worse: remain in place and not do it's job. This is especially true for the corrosion coating, where water ingress between the coating and pipeline will actually speed up the [corrosion process].
Low price: Really goes without saying, especially for the longer pipelines.
Flexibility: During pipeline lay the line is bent, often so that the steel yields. The coating needs to be able to handle this as well, so that it doesn't disintegrate when going over the [stringer|side].
Resistant to [biodegradation] and environmental forces.: A pipeline lifetime can be up to 50-60 years, and therefore needs to be able to withstand both the marine growth and the forces occuring on the seabed. It needs to be able to survive shifting seabeds, and collisions with other objects.
[Non-toxic]: Probably not that true 30 or 40 years ago when [tar and pitch] was popular as coating, but today a lot of focus falls on both the workers handling the pipeline, and the environment it is placed in.

Pipeline coating (thing)