WHY CHOOSE AN UNDERWATER PIPELINE TO CONNECT A MARITIME TERMINAL TO THE MAINLAND RATHER THAN AN OVERHEAD INFRASTRUCTURE?

Underwater solutions with less environmental impact

Jetties and quays require a large footprint and extensive infrastructure, with a visual impact that is often substantial and alters the coastal landscape. In contrast, terminals with underwater pipelines, installed several kilometers from the coast, have no impact on the landscape. The environmental impact can be further reduced through advanced techniques such as directional drilling and microtunnelling, which allow the pipeline to be installed in a tunnel under the seabed, minimizing disruption to marine habitats.

Example of a traditional jetty

The choice of subsea pipelines: reduced CAPEX, shorter lead times

The choice of infrastructure type depends mainly on the water depth available at the site intended to accommodate LNG carriers or FSRUs.

When the bathymetry allows ships to approach very close to the coast (less than 200 metres) without the need for dredging, quay and jetty systems are generally preferred.

Conversely, when the seabed slopes gently and the waters remain shallow for a long distance from the coastline (more than 1,000 metres), terminals equipped with submarine pipelines and ship anchoring systems on the seabed are logically the most popular.

Between these two configurations, both types of infrastructure can coexist.

At the Wilhelmshaven gas terminal, a jetty complements the 1.7 km long gas import pipeline installed by Geocean 14 metres below sea level to connect the Excelerate Excelsior FSRU to the German network.

For a long time, jetties were the preferred option, as the installation of subsea pipelines required specialised pipe-laying vessels, which were few in number and whose availability was uncertain. Jetties, requiring less complex and often locally available marine construction resources, were therefore a more accessible solution.

Over the past fifteen years or so, the emergence of innovative techniques has made it possible to install submarine pipelines several kilometres long without the need for a pipe-laying vessel, thereby significantly improving the economic competitiveness of this infrastructure compared to jetties. These techniques include pipe pulling (manufacturing the pipeline on land and pulling it into the sea) and the use of modular barges, which are much lighter than pipe-laying vessels.

Ship mooring systems can also be simplified: either through the traditional use of anchors, which are completely invisible and reusable, or, in the case of a channel where the footprint is limited, through the use of mooring dolphins.

However, CAPEX assessments now clearly favour pipeline solutions and underwater mooring systems in this zone between 200 and 1,000 metres from the coast.

In terms of OPEX, the two types of infrastructure have relatively similar costs, both for mooring support vessels and for infrastructure maintenance. However, the complete dismantling of the infrastructure – the cost of which is often overlooked at the time of investment – proves to be more advantageous for the subsea solution.

Another advantage of subsea systems is significantly reduced construction times. Lighter, requiring fewer engineering hours, based on available components, with little metal prefabrication and simplified authorisation procedures, some projects are completed in 12 months, an exceptional turnaround time in this sector that meets the needs of investors.

Pipelines installed by Geocean in shallow waters