Membrane tank application in a Gravity Based Structure
In a context where gas resources are localised far from consumption zones, frequently in remote areas, Oil & Gas companies must have a broad range of solutions available adapted to various conditions and localisations. Recently, GTT has significantly invested in the development of Gravity Based Structures (GBS). Concrete offshore structures have been successfully employed in the petroleum industry as production or storage units for crude oil since the 1970’s. However, the use of the concrete offshore structures for LNG storage was considered much later. As of today, only one of this kind has been built and is in operation in the Adriatic. This Adriatic GBS will soon no longer be the only LNG GBS project as the new Arctic LNG 2 project is on the way.
Last year, GTT signed a contract with SAREN B.V. (Russian joint venture of Renaissance Heavy Industries Russia and Saipem) for the detailed design and construction assistance of the storage tanks for three GBS dedicated to the Arctic LNG 2 project.
GBS structures can be made of concrete or steel caissons equipped with GST® or Mark III membrane containment tanks. GST® membrane’s full integrity system is similar to GTT Mark III technology. About 800 storage tanks are on board the Mark III vessels, in severe operational conditions compared to land storage tanks. GST® system had been proven with 37 tanks in service since 1972. With almost 50 years operations and such large industrial references, GST® and Mark III membrane systems are proven worldwide technologies.
The GBS concept consists of prefabricating reinforced concrete or a steel structure with caissons equipped with liquefied gas containment tanks in floating docks or in dry-docks and then towing the finished facilities to the installation location, where they are ballasted to rest on the seafloor. GBS can be installed in a harbour or in nearshore shallow area with very limited requirements for any additional infrastructure. At the same time, this concept guarantees a high level of safety, reducing installation costs while limiting the environmental and visual impact.
While membrane containment systems for offshore and on-shore applications are very similar, regulatory frameworks are not the same. This implies major differences, for example, for the secondary barrier definition (full secondary barrier for offshore structures as per IGC code and corner thermal protection system up to 5 m as per EN14620 for on-shore tanks). With the lack of the normative base for GBS, which regulations should be applied? For structural and liquid tightness functions, it shall be demonstrated that the liquid is contained by the secondary barrier and the concrete container in order to avoid any chain reaction effect. The combination of the insulation system with the barrier protection shall be designed to contain the liquid and to protect the concrete slab and walls from the leakage of LNG.
Another innovation is the use of the sump pits for pumps, dedicated to perform more efficient pumping of LNG at the bottom of the tank by minimizing the residual liquid level. For the first time in the LNG industry, the sump pit in the concrete structure will be applied on Arctic LNG 2 project. This is possible thanks to the specific design which can withstand the operational and accidental loads and also ensure a flat bearing surface for the primary membrane sheet. Pumps inside pump columns will be installed in the sumps thanks to the pump column base support. The main functions of the pump column base support are to withstand forces issued from tubular structure and to limit tubular structure moment deflections and stresses by a guiding system at tubular structure lower part.
GBS is also suitable for different liquefied gas applications as LNG, ethane, LPG… It is appropriated for storage capacities from 5k m3 to 200k m3 or more (several caissons can be combined to increase storage capacity). Thus it allows the mooring of all types of vessels (small LNG tankers, container ships, ferries…). It can contribute to the LNG supply chain as liquefaction or regasification plants, peak shaving storages, satellite stations and also for inland distribution. It can also be used for power generation (industry companies, captive power…) and bunkering (LNG as fuel). GBSs can be located in remote areas such as islands and remote coastal areas, furnishing power to isolated populations and feeding their industrial requirements.
The engineering works for Arctic LNG 2 project are still on-going, with tangible perspectives of using GTT’s membrane containment system in GBS. With the future successful implementation of GBS in the Gydan Peninsular in Russian Arctic, the development of other potential GBS projects in the world will be boosted.
