Natural gas fields occur not only on land. There are also offshore deposits: oil and gas are occasionally encountered in the subsurface covered with water.
Shore and shelf
Geologists survey both onshore and offshore areas. If a field is close to the shore, in a coastal area, then directional wells are drilled from the shore towards the sea. Fields that are farther away from the shore belong to the shelf area. Underwater continental edge having the same structure as the shore is called “shelf” and its boundary is the crest – a steep difference in depth. Floating platforms and rigs are used for such fields, or simply high piles from which they drill if the depth is shallow.
Three main types of floating rigs – special platforms – are used for hydrocarbons production at offshore fields: gravity-based, semi-submersible, and self-elevating.
For shallow depths
Self-elevating platforms are floating pontoons; the rig is installed in the middle, and support columns are placed in the corners. At the drilling area, columns are lowered to the bottom and sank into the ground, while the platform is elevated above water. Such platforms can be huge with premises for workers and crews, helipad and on-site power plant. They are used at shallow depths and their stability depends on the seabed quality.
At greater depths
Semi-submersible platforms are used at great depths. Platforms are not elevated above the water but float over the drilling site held by heavy anchors.
Gravity-based drilling platforms are most stable as they have a thick concrete base supported by the seabed. Drill strings, storage tanks, and pipelines are integrated with this base, while the rig is located above the base. Dozens and even hundreds of workers can live on such platforms.
Gas produced on the platform is transported for treatment either on special tank ships or via subsea gas pipelines (as, for instance, in the Sakhalin-2 project)
Offshore production in Russia
Since the world’s largest shelf with many deposits belongs to Russia, the development of the offshore production is very promising
for the oil and gas industry. Sakhalin Energy started to drill first Russian offshore wells for gas production at the Lunskoye field offshore Sakhalin in 2007. Production of gas at the Lunskaya-A platform started in 2009. Today, the Sakhalin-2 project is one of the major projects implemented by Gazprom. Two of three gravity-based platforms installed on the shelf of Sakhalin are the heaviest offshore structures in the entire history of the international oil and gas industry.
Furthermore, Gazprom is implementing the Sakhalin III project in the Sea of Okhotsk and preparing the Shtokman field in the Barents Sea and the Prirazlomnoye oil field in the Pechora Sea for development. Geological exploration is carried out in the offshore areas of the Ob and Taz Bays.
Gazprom also works offshore Kazakhstan, Vietnam, India, and Venezuela.
How is the subsea gas production system arranged
Currently, there are more than 130 offshore fields worldwide where offshore techniques for hydrocarbons production are used.
In Russia, Gazprom will erect the first production system offshore Sakhalin as part of the Kirinskoye field pre-development. It is also projected to apply submerged technologies in the project for the Shtokman gas and condensate field development.
A subsea production system with several wells features a spider configuration with a manifold as its body.
The manifold is a part of oil and gas equipment representing several pipelines usually fixed to one base, resisting high pressure and coupled according to a specific design. The manifold gathers hydrocarbons produced from several wells. The surface equipment running a well is called a Christmas tree or X-tree. Several X-trees may be integrated and fixed by one template (seabed plate) like eggs in a basket. A subsea production system also includes control systems.
In terms of sophistication subsea systems may vary from an individual well to several wells drilled in a template or gathered near a manifold. The output may be delivered from wells either to a floating production vessel for extra treatment or immediately to the coast if it is near.
Hydrophones for dynamic positioning of a vessel
Vessel has diving equipment