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Introduction to Subsea Pipelines
Offshore oil and gas production platforms export their hydrocarbons ashore through subsea pipelines buried beneath the sand and sediment under the seabed.
Before the pipelines are laid they are subjected to a rigorous system of anti-corrosion measures appropriate to the environment in which they are located.
This is an article on anti-corrosion measures currently applied to subsea pipelines in the hostile offshore environment.
We begin with an overview of how the subsea pipelines are installed and go on to examine current anti-corrosion methods.
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Laying of Subsea Hydrocarbon Pipelines
The path along which the pipeline is to be buried is subject to a seabed geotech engineering survey to obtain an image of the undulating ridges and hollows. The sand and sediment is excavated to a depth to suit the type of pipe being laid, with any ridges being removed and hollows backfilled. The pipes are normally laid from a specialist vessel which carries a coil of pre welded, pretreated pipe on a large wheel situated on the deck.
The trench for the pipes may be excavated using equipment operated from a specialized barge or by a Remote Operated Vehicle (ROV) with trenching attachments. It endeavors to follow a straight line from the production platform to the shore-side destination landfall, with several expansion bends incorporated into the pipeline length.
The excavating barge moves ahead of the pipe-laying barge which unreels the pipe, and once they are laid, the trenches are covered over with sand, sediment, and a ballast of stones. The stone ballast gives extra weight to the pipeline, keeping it in position but also acting as dropped object protection against accidental damage. If the pipeline encroaches fishing grounds it is also protected against fouling the trawling devices by a steel ramp over the stone ballast which enables the trawl net mechanism to run over the protrusion without being ensnared.
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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.
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Pipeline Cathodic Protection
Pipeline Cathodic Protection
Exposure to high temperature has been proven detrimental to anode bracelet attachment directly to a steel pipe, therefore another method of cathodic protection has been devised known as cathodic sledge protection.
This consists of a frame onto which numerous zinc anodes are welded. The sledge is transported offshore and lowered to a predetermined location on the seabed, quite close to the pipeline.
Once in the correct location, it is then pinned to the bedrock and connected to the pipeline via flexible copper strips or cables which are welded to the sledge framework and the steel pipe.
For optimum cathodic protection against corrosion, two cathodic sledges are used, being fixed to the seabed in close approximation to each end of the pipeline and connected to the pipe through the copper strips or cables.