April 2021, Vol. 248, No. 4

Features

Offshore Pipelines Remain Vital Energy Links for Many Regions

By Nicholas Newman, Contributing Editor 

Sometimes overlooked in discussions of pipelines are those located offshore pipelines, which are increasingly constrained by various factors, but still essential for energy transportation in many regions. 

Offshore pipelines are comprising metal or plastic tubes, are laid directly on the seabed or inside a prepared trench. They typically transport crude oil, petroleum, natural gas, and gas liquids, but in future may carry hydrogen.  

This article looks at some types of pipeline, their chief characteristics, and obstacles in the path of new constructions. Here we have a look at three main types of offshore pipeline. These are: intra-field and export pipelines which are ubiquitous and the less common transmission pipelines: 

Intra-field (or infield) pipelines, include flowlines and feeder lines, and connect wellheads, manifolds and platforms within an oil or gas field development. They are designed to transport liquids, but a designated small number will carry processed water from platforms to service injection wells which drive down water to boost production.  

Export pipelines are designed to transport the extracted oil or gas from the field’s platform to the nearest coast. Generally, longer but often less than 100 miles in length, they are to be found in the Gulf of Mexico, the North Sea where export pipelines from the Brent-Fortis field carry crude to northern Scotland, in the Arabian Gulf and in S.E. Asia.   

Where the export pipeline carries a combination of crude oil and natural gas, it is often called a multiphase pipeline. If the pipeline carries solely oil or natural gas, then it is known as a single-phase pipeline. 

Transmission  

Typically, longer in length and with higher capacity, transmission pipelines carry crude oil, petroleum, or natural gas under water from one country’s coast to another. A prime example is Nordstream, which leaves Russia’s Baltic Coast near St. Petersburg and emerges at the German Baltic Coast.  

Long established is the pipeline connecting Tunisia with Southern Italy, while more recent is the Trans Adriatic Pipeline connecting Albania with Southern Italy. Currently being considered is a proposed East Mediterranean transmission pipeline to connect gas fields around Cyprus, Israel, and Egypt with mainland southern Greece.  

In the future, offshore transmission pipelines could carry hydrogen. Already, there is discussion for hydrogen produced in the United Kingdom being transported across the North Sea to mainland Europe. 

Steel pipes lined with anti-corrosion materials and coatings are made to resist the corrosive effects of oil and gas and are therefore the most common.  Pipes vary in diameter from as little as 3 inches to as much as 76 inches, while wall thickness can range from as little as 0.39 to 3 inches, according to the prevailing temperature and pressure. For example, high yield steel pipeline walls are designed to withstand between 50,000 to 70,000 psi. 

Inside walls, especially those of petroleum pipes, are typically coated with epoxy, polyurethane or polyethylene and are also often cement-lined. External coatings against corrosion include those based on bitumastic or epoxy, often augmented by cathodic protection together with sacrificial anodes.  

Further protection can be gained from installing Portland cement jackets. To prevent unacceptable external leaks into the pipeline, internal pressures up to 1,500 psi are often applied alongside mechanical joint.  

Well-protected pipelines, including the offshore part of the Trans-Adriatic pipeline and the 404-mile (650-km) Nyhamna-Sleipner stretch of the Langeled offshore North Sea pipeline. 

Construction Offshore 

Laying pipe on the seafloor is not without its challenges, more so when the water is deep. However, there are three major ways in which sub-sea pipes are laid: the S-lay, J-lay and tow-in installation. 

When performing an S-lay pipe installation, the pipeline section is eased off the stern of the vessel as the boat moves forward. The pipe curves downward from the stern through the water until it reaches the “touchdown point,” or its destination on the seafloor. As more pipe is welded in the line and eased off the boat, the pipe forms the shape of an “S” in the water. Hence its name! 

A J-lay pipeline installation puts less stress on the pipeline by inserting the pipeline in an almost vertical position. Here, pipe is lifted, via a tall tower on the boat and inserted into the sea. Unlike the double curvature obtained in the S-lay, the pipe only curves once taking on the shape of a “J” under the water. 

The Tow-in installation is just what it sounds like:  Here, the pipe is suspended in the water via buoyancy modules and one or two tugboats tow the pipe into place. Once on location, the buoyancy modules are removed or flooded with water and the pipe floats to the seafloor. 

Multiple Obstacles  

The route of a sub-sea pipeline is governed by natural features including volcanoes, historic wrecks, dumped munitions as well as artificial constraints such as the location of marine reserves. Man-made barriers include existing pipelines, marine activities, and of equal importance politics.   

A case in point is Turkey which objects to the route of the proposed 1,200-mile natural gas EastMed pipeline to link gas fields in Cypriot, Israeli and Egyptian waters with mainland Greece and Italy. 

In general, seabed conditions are of paramount importance. Seabed unevenness in the form of ridges may leave a section of the pipeline unsupported and subject to undue stress and vibrations. Another consideration is whether the seabed is sufficiently strong to support the weight of a full pipeline. 

If the soil is too soft, the pipeline could sink, and if it is too hard, it could necessitate expensive trenching. Ideally, the seabed soil should be just soft enough to allow the pipeline to settle.  

In many parts of the world, such as the Gulf of Mexico, the North Sea, Arabian Gulf and South China seas, the seabed is already crowded with offshore pipelines. This necessitates a bridging structure where the new pipeline crosses another. 

There many other obstacles, including large commercial fishing vessels, merchant ships, tankers and naval vessels plying the seas around the world. For example, fishing vessels may drag large, often kilometers length trawl nets, along the seabed causing disturbance to marine life as well as the condition of the seabed.  

While trailing ship’s anchors can damage pipelines in their wake. For example, five crude oil and natural gas pipelines belonging to Enbridge in the Straits of Mackinac in the Great Lakes area are vulnerable to damage from trailing anchors from freighters.  

This is not to forget military and naval zones, where activities including the impact of bombing or gunning practice take place, as well as the presence of specialized anti-submarine detection equipment.  

The golden age of offshore fossil fuel pipeline construction may be coming to an end owing to the suppression of demand for oil and gas initiated by the Paris Accord and increasingly reinforced by environmental and climate concerns.  

However, there is a golden future for pipelines carrying hydrogen from producers in the Middle East, principally Saudi Arabia as well as elsewhere, especially Australia, North America and Europe.