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Topsides/storage arrangement
The hull of the company's FLNG design has a 106-m (348-ft) diameter and a double bottom, supporting a three-deck topsides: uppermost is the process deck, with an area of 13,000 sq m (139,931 sq ft), with a utility deck underneath, and below that the main deck. Total topsides weight is 31,500 metric tons (34,723 tons), of which 17,000 metric tons (18,739 tons) is accounted for by the gas processing and liquefaction facilities. There is storage capacity for 180-210 cu m (6,357-7,416 cu ft) of LNG in six main tanks, 20,000 cu m (706,293 cu ft) of liquid petroleum gas (LPG) in a central tank (which could also be used for LNG if LPG storage is not required); and 250,000 bbl of condensate in six smaller tanks – condensate and LPG are extracted during the pre-processing process.
The liquefaction process makes use of KANFA Aragon's patented technology which is based on the well-proven dual nitrogen expander cycle optimized for offshore application and reportedly using up to 15% less energy than similar dual expander cycles. The FLNG unit is equipped with two LNG trains, each sized to produce 1.2 MMt/yr of LNG, making a total capacity of 2.4 MMt/yr based on a gas feed of 333 MMcf/d.
The process requires a large intake of cooling water – typically 18,000 cu m/hr (635,664 cu ft/hr) for 2.4 MMt/yr. In areas with high sea water temperatures, process efficiency can be significantly improved, Sevan adds, by taking colder water from deep down through free-hanging vertical hoses. Again, as the unit is stationary, there is no interference between the hoses and the mooring or risers to prevent the deep deployment of these hoses.
There is flexibility with respect to hull fabrication, which can be performed in a drydock, on a skidway, slipway or submersible barge. The self-supporting prismatic type B SPB tanks included in the design are prefabricated, offering a choice of location, while the membrane tanks are fabricated in situ. The topsides are modularized and fabrication can therefore take place at a variety of locations.
Hull/topsides integration and topsides hook-up will normally take place in the hull fabrication yard, allowing a fully commissioned unit to be delivered and on-field hook-up reduced to a minimum. Transportation to the field location can either involve a dry tow on the back of a heavy-lift vessel, in the case of a longer distance, or a wet tow, with a full crew on board if desired.
Major claims the cost of the Sevan FLNG unit would be reasonably competitive, at just under $1 billion/MMt per year. And there are further advantages in terms of offloading of LNG to tankers. For the standard ship-shape unit, the normal method is to have the tanker moored alongside, meaning that the two vessels are in contact. In such circumstances loading operations are limited to relatively low sea states. Use of hinged loading arms for LNG transfer also means there are restrictions on the relative motions that can be permitted.
Various methods, in contrast, are feasible with the Sevan FLNG unit, including side-by-side offloading to a conventional LNG carrier alongside the platform, or offset side-by-side, with the carrier stationed a small distance from the platform.
These would allow loading operations in sea states up to 2.5 and 3.5 m (8.2 and 11.5 ft) significant wave heights respectively. Operations in up to 4.5 m (14.7 ft) sea states would be possible through tandem offloading to carriers adapted for bow loading.
Last year, in recognition of the need for loading in higher sea states, Sevan acquired the LNG loading technology that employs the HiLoad docking system, establishing the company's HiLoad LNG for this purpose. The HiLoad unit, which is equipped with four thrusters and a DP-2 dynamic positioning system, carries the loading hose to the tanker, locking onto its side and holding it in position while loading takes place. The fixed connection means there are no relative motions between the HiLoad unit and the tanker, allowing operations to take place in sea states in excess of 4.5 m. It also means that standard LNG carriers can be used, without any modifications.
LNG is transferred through a cryogenic floating hose. Trelleborg has developed a 20-in., 20-bar (290-psi) hose for this purpose, which is currently undergoing qualification to the EN1474-II standard.
Operating costs for Sevan floaters of all types are comparatively low, the company adds, mainly because there is no turret requiring regular maintenance. Experience from the North Sea floaters indicates a need for an overall crew of about 30, a savings of roughly five compared with a conventional FPSO of equivalent capacity. The marine crew only works the day shift.
