A subsea pipeline (also known as an offshore pipeline), is a length of pipe that is laid on, or below, the seabed. They are used to transport oil or gas across long distances. Whereas pipelines export resources to the shore, a ‘flowline’ (or infield pipeline) is used to connect subsea wellheads, manifolds, and the platform to a development field.

Offshore pipeline construction projects typically require a lot of planning and need to consider many geographical factors, including offshore ecology and environmental loading. Large-scale projects in particular, are typically undertaken by multidisciplinary, international teams.

One of the first major subsea pipeline projects was Operation PLUTO (Pipeline Under the Ocean). It was built between the Isle of Wight and France during World War II and stretched 48 km. The project was an operation by British engineers, oil companies and the British Armed Forces.

Today, the current longest undersea pipeline is the Nord Stream, at 1,224km. It runs across the Baltic Sea from Vyborg, Russia, to the German coast, transporting Russian natural gas to Europe.

The materials used to build offshore gas pipelines

The principal material used in the construction of offshore pipelines is carbon steel, although other steels are used in the industry. The pipes are usually supplied in single joints of 40 feet and are coated with concrete-weight epoxy to protect them against corrosion and enable their installation on the seabed.

In recent times, pipe-in-pipe constructions are often used to provide thermal insulation to the inside of pipelines in deep water. As the name suggests, a pipe-in-pipe construction is a pipe inside another pipe and helps prevent wax and hydrate formation.

The components of a subsea gas pipeline

An offshore gas pipeline system is made up of compressor stations, pipelines, city gate stations and storage facilities. A compressor station is considered the hub of a pipeline system. It supplies energy to ensure that gas flows through the pipeline at the correct flow rate and pressure.

There are typically three types of pipeline in an offshore system: infield, export, and transportation pipelines. 

Infield pipelines carry liquors, which are a mixture of water, oil, and gas from the subsea wells to the process platform.

An export pipeline that transports only oil or only gas from the platform is known as a single-phase pipeline, whereas those that transfer a mixture of the two are known as multi-phase pipelines.

A transmission pipeline carries oil or gas all the way from one shore to another. There are three types of pipe that can be used in this process:

  • A seamless pipe (SMLS) has high flexibility and is suitable for low temperate liquids or gases.
  • A high-frequency welded pipe (HFW) is typically thicker and has pressure welding, making it very robust.
  • Similarly, a submerged arc welded pipe (SAW)has excellent corrosion resistance, so is ideal for offshore pipeline systems.

The construction process of an offshore gas pipeline

Before a subsea pipeline can be laid, the seabed needs to be surveyed for obstacles and to make sure there are no environmental issues with the site. Once the seabed has been cleared of any debris, the pipes are welded, tested for defects and painted with an anti-corrosion coating.

Several approaches can be used to install pipes on the seabed. With a tow-in installation, sections of pipeline are attached to buoys and dragged out to sea by a tugboat. The buoys are then removed and the pipe sinks to the seabed.

With an S-Lay Installation, large ships lower the welded pipes onto the seabed over a boom called a ‘stinger’ that juts out water like a slide, in an s-shape. With a J-Lay installation, the pipes are eased off the ship deck at more of a vertical angle, forming a j-shape.

Each pipe weighs several tonnes, so they don’t need to be fixed to the seabed. They’re laid in trenches and in some locations, they’re secured with sediment from the seabed for extra security.

Once the pipes have been installed on the seabed, part of the pipeline is drawn onshore using a winch. The subsea pipes are then connected to the onshore section and a tie-in is made. With the entire pipeline in place, hydro testing is carried out, where the pipeline is filled with water to check for possible defects. The condition of the pipeline is also carefully monitored using electronic devices for in-pipe inspection.

The future of subsea gas pipelines

According to recent figures reported by Energy Voice, 30% of oil and gas is produced offshore, of which one third is subsea. So, around 10% of overall production comes from subsea pipelines. Subsea infrastructure currently delivers around 5 million barrels of oil and gas per day (bpd), which is set to increase to 35 billion by 2030 if predicted future projects go ahead.

Despite a global effort to move away from fossil fuel production, in the short-term, the global offshore pipeline market is expected to grow from $11.5 million in 2018 to $15.8 billion by 2023.

Due to increasing energy demands in the Asia Pacific region and in Europe, the demand for oil and gas is expected to increase significantly over the next 15 years. Europe in particular, has several large-scale subsea pipeline projects planned, including the Johan Sverdrup Development in Norway, valued at $10 billion, and the Rosebank & Lochnagar Oil & Gas Fields in the UK, valued at $6 billion.

Subsea Engineer jobs in the energy industry

Subsea engineers are responsible for the design and installation of structures below the surface of the sea. They provide technical support and guidance to offshore teams, draw up plans, and manage the installation of pipelines on the seabed, and onshore. Subsea engineers may also manage contractors and project budgets.

Subsea engineers typically have at least a Bachelor’s degree in Engineering, and some have more specialist qualifications in subjects like mechanical, or petroleum engineering.

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Offshore pipelines