Coating of Subsea Pipelines Against Corosion
Subsea pipelines which carry offshore crude oil and natural gas must be protected both externally against the seawater and damage and internally against acids emitted from the high temperature hydrocarbons and condensates.
The pipes are normally treated against corrosion ashore at a specialist coating facility, the treatment varying with the type and temperature of the oil or gas passing through it, insulation required, seabed location and terrain. A current method of anticorrosion coating follows below;
The pipe is shot blasted externally and internally to remove any loose particles or rust and to give the primer a good key.
Nowadays the pipe is treated internally using a spraying device which moves along the inside of the pipe, efficiently applying a two-part polyurethane primer. This is followed by another coat of primer and once dry a single coat of polyurethane applied in the same manner rendering the internal surface acid and heat resistant.
The outer surface is treated similarly with two coats of primer and a slightly modified polyurethane topcoat of anti-impact design, then a coat of concrete or aluminum spray applied. A further coat of polyurethane can be applied on top of this if deemed necessary.
For extra insulation several layers of this type of protection can be applied, with extra care taken to repair any damage to the pipe coatings especially in areas where underwater pipe welding has been used to join the pipeline lengths. It is imperative that these areas are repaired, as corrosion will attack the weakest point, therefore it is recommended that a specialist contractor experienced in this field is employed to carry out the work.
Once the pipeline is complete cathode protection must be applied.