The concept is the offshore application and marinisation of the NicheLNG process, a proprietary dual turbo-expander based LNG liquefaction scheme developed by ABB Lummus Global.
The equipment layout is similar to a typical FPSO, however, the concept for this new purpose-built FPSO allows for processing facilities onboard which incorporate both the gas feed pre-treatment (removal of CO2, mercury etc., dehydration of the gas and LPG extraction) and LNG liquefaction.
LNG and LPG have different compositions thus requiring storage at different temperatures. LNG must be stored at extremely low temperatures (-162 degrees C) while LPG can be stored at a much higher or warmer temperature (-40 degrees C).
Described by ABB as primarily a floating liquefaction facility for LNG production, storage and export, its process facilities are compact enough to fit onto the deck of a 312-metre long Niche LNG FPSO, yet is capable of delivering 1.5 million tonnes per year combined output from its three LNG production trains, according to ABS.
The design storage capacity is in the 200,000m3 range, with six tanks total - four for LNG storage and two for LPG storage. The design features a flat deck making it easier to arrange and accommodate the topside process modules.
The vessel will utilise a storage concept such as Ishikawajima Harima Heavy Industries, Co., Ltd's. (IHI) Self-supporting, Prismatic-Shape, IMO Type-B tank system or SPB.
ABS has previously classed the only LNG carriers to use this containment system and also classed the first LPG FSO newbuild, the Escravos, and the first LPG FPSO newbuild, the Sanha, both operating offshore Angola.
According to ABS’ project manager Phil Rynn, the uniqueness of the proprietary liquefaction process from ABB on the vessel required close review by ABS. The ‘dual turbo-expander cycle' to chill the gases to a liquid form for transport involves cooling and condensing the methane rich stream available from the gas pre-treatment step.
Gas is liquefied in two independent cycles, using methane as refrigerant for the first stage, and then moving to nitrogen, with both cycles adopting turbo-expanders. These refrigerants are always in gas phase, which simplifies the equipment layout compared with traditional LNG refrigeration systems.
“Since there is a complete LNG process plant onboard which also incorporates LPG extraction facilities, there are more equipment concerns with regard to spacing and general arrangements,” says Rynn. During preliminary design review of all the drawings, Rynn says special attention was given to the process facilities, general arrangements, cryogenic transfer and facilities for loading and offloading the LNG and LPG products, as well as in-service inspection and security issues for the design.
“Industry has successfully dealt with oil and gas processing facilities on Floating Production Storage Units for a number of years. However, the liquefaction and cryogenic storage of LNG and LPG offshore is a whole new arena,” said ABS vice president of Energy Development William J. Sember.
“This transition requires a comprehensive approach toward the integration of marine and typically shore-based gas processing facilities in the design of a floating concept,” he adds. “A key issue for a floating concept like this is designing for the relative motions likely to occur during loading/offloading operations.”
Transfer of LNG at subzero temperatures through a loading hose or arm presents industry with a technological challenge. The society said it is working with industry to evaluate appropriate technologies to optimise reliability and flexibility for the LNG transfer in an offshore environment.
The ABB Niche LNG FPSO design has incorporated a number of measures to minimise the effects of the motions experienced on a floating platform. The vessel will be held on station via an external turret mooring system at the bow and offloading will most likely be conducted tandem-style from the stern although side-by-side offloading is a possibility and is being explored.
The power generation demands for this concept call for an onboard power plant with the capability of providing 40 megawatts of generation. Three main gas turbines will drive the main compressors. Together the three trains can liquefy 225 million cubic feet per day of gas.
AIP from ABS included design review of the general arrangements, topsides and system integration, process flow diagrams (or PFDs), heat and mass flow diagrams (or MFDs), safety shutdown and firefighting system philosophy, as well as review of the hull structure, cargo tanks, refrigeration and re-liquefaction plants in accordance with the International Gas Code (IGC).
