Gravity platforms, also known as Gravity Based Structures (GBS), offer interesting potential for the development of nearshore liquefied natural gas (LNG) projects. After studying the possibility of integrating a liquefaction plant with such a structure, Pascal Collet and Aroquiaradj Gervais from the LNG product line explain the whys and wherefores of this solution. The concept draws on expertise that we have long been developing in this area.
When did the oil and gas industry start using concrete gravity platforms?
Pascal Collet: In the 1970s, the oil industry was confronted with the need to operate in increasingly challenging environments, and began to investigate the use of concrete. As a material, concrete offers many advantages: construction can take place locally and on floating docks, and maintenance requirements are low. France was in the vanguard of this trend, with structures such as the Hibernia platform. And so the idea of floating concrete structures began to gain traction. In 1996, we became the first to commission a concrete barge carrying oil and gas processing facilities, offshore Congo: the N’Kossa barge, which is still in operation today. In recent years, the growth of the LNG industry has given rise to FLNG* vessels, enormous plants dedicated to exploiting offshore resources.
Aroquiaradj Gervais: Meanwhile, the architecture of onshore LNG facilities remained quite conservative: a pretreating unit installed on the gas field is connected by pipeline to the liquefaction plant along the coast, adjacent to a storage facility and a loading dock for LNG carriers. In addition to its huge footprint, this layout requires the construction of a breakwater to ensure calm seas for the tankers. To remain competitive despite the collapse in natural gas prices, we needed a disruptive change in this architecture.
How did you come to the idea of building a liquefaction plant on a GBS?
P.C.: On the one hand, we can now boast fifty years of experience with concrete gravity structures. We also have a product to store (LNG) for which concrete happens to be the ideal material. We have an industry that sometimes has trouble finding a site for its plant, with less and less time to build it and significant cost constraints. On the other hand, we have a concept that is a perfect match for the specifications: built and assembled in the dry dock of a shipyard, then floated out to the designated location, the GBS can serve as breakwater, storage facility, host platform for the liquefaction plant, and loading dock.
What advantages does this solution afford?
P. C.: Structurally, the GBS is extremely robust and requires little maintenance. Building it does not require sophisticated technical know-how. Moreover, if the reservoir is in a remote location, the yard can be set up in another country with better conditions of access and more abundant materials.
A.G.: Above all, it is an agile concept with architecture that can be tailored to the context of each project. Studying potential sites nearshore rather than confining our search to onshore sites on stable ground opens up many more prospective locations. We compared the costs for one of our current projects. Compared to an onshore liquefaction plant, installation on a GBS would save 20% on the upstream pipeline and the LNG plant. There is still more room for optimization, but the concept is certainly viable and deserves to be considered for our projects.
What kinds of optimization?
A. G.: Optimizations mainly relate to integrating the liquefaction plant with the GBS and to the yard, which accounts for 40% of the cost of the platform. We have drawn inspiration in particular from a polder-building technique in which small GBS caissons are precast right on site by deploying floating docks, and then assembled, at a cost well below what it costs to build a yard ($30 million versus $450 million respectively). However, this option creates other constraints relating to topsides installation, so we need to tweak it further before we can consolidate our options.
P. C.: The GBS option for LNG creates an opportunity for civil engineering and oil & gas companies to brainstorm together to challenge the projects and propose better solutions. New opportunities will take shape as we build bridges between these two fields of expertise.
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