The liquefaction vessel is based on a standard Moss type LNG carrier  of 155 – 170,000 m3 capacity and equipped with Dual Fuel Diesel Electric (DFDE) propulsion.  The Moss containment system eliminates risk of damage from sloshing in slack tanks during filling.  There is no documented evidence of sloshing damage to the tanks in a Moss type LNG containment system although some early designs required reinforcement of the pump tower foundations.

A standard LNG carrier with DFDE power plant will have 30 – 40 MW of installed power.  This is increased by 72 MW of installed power by adding 4 x 18 MW MAN V18 DF 51/60 engines in a new mid-ship section engine room.  The power required to drive a 2 MTPA liquefaction plant is approximately 70 MW, depending on gas composition and cooling water temperature.  The four mid-ship engines provide primary liquefaction power while the existing ships power plant is electrically interconnected to provide sparing and redundancy.

The 30 m long mid-ship “sandwich” section is spliced in between the Nos. 2 and 3 cargo tanks and houses:

• 4 x 18 MW DFDE engine gensets
• disconnectable turret, turret trunk & auxiliaries
• switchgear including transformers, variable frequency drives & magnetic bearing controls
• seawater/freshwater heat exchangers & cooling water pumps

By inserting the sandwich piece at mid-ship, any increased hull stresses resulting from increasing  the waterline length are primarily in the new section and can be accommodated in the design.  The turret contains a single10 in. NB, ANSI 900 gas swivel to deliver pre-cooled gas to the liquefaction vessel from the host platform.  The turret contains an additional swivel for a deep seawater intake hose to provide cold seawater for process cooling.  At a depth of 1500 ft., typical seawater temperatures will be 48 – 50 F.

The turret is designed for quick connect/disconnect (< 4 hours).  Maximum seastate for turret connection is Hs≤4 m and for disconnect Hs ≤5.5 m.  Once the turret is connected, the liquefaction vessel can remain on station in any weather condition subject to design of the buoy mooring system, although it would not be planned to remain on station for extreme weather such as tropical cyclonic storms or severe winter storms at high latitudes.

The sandwich piece provides a footprint for the liquefaction module.  The liquefaction module contains 2 x 1 MTPA liquefaction trains arranged port and starboard.  Each train consists of:

• 1 x Cold Box containing 5 x Braised Aluminum Heat Exchanger (BAHE) cores
• 1 x “Warm Loop” and 1 x “Cold Loop” Turbo Expander/Compressors
• 3 x Integral Sealed N2 compressors (ISC)
• Printed Circuit Heat Exchanger (PCHE) inter-stage coolers
• 3 x Labyrinth seal Boil-off Gas (BOG) compressors (shared between 2 trains)
• 1 x Liquid Expander

ISC compressors are directly driven by a high speed electric motor which is housed within the compressor body.  The motor and impellers are installed on a common shaft supported on magnetic bearings and is cooled by a slipstream of the gas being compressed.  The entire assembly is hermetically sealed within the compressor body with no external shaft seals.

Benefits of the ISC include:

• extremely compact footprint
• no shaft seals or seal gas system
• no lube oil system
• no gearbox
• high reliability with minimal planned maintenance

ISC’s were originally developed for remote, unmanned pipeline booster stations.  In recent years, they have been selected for seabed compression.  The use of ISC’s and the elimination of nearly all hydrocarbon inventory in the liquefaction system enables the liquefaction module to be extremely compact and lightweight.  Overall dimensions of a 2 MTPA liquefaction module are approximately 16 m long x 17 m high x 45 m wide.

In contrast to most FLNG vessels, because the liquefaction vessel described here is simply a modified ship, it can remain classed, flagged and crewed as a ship and periodically dry dock for emergency repair and maintenance.  This operating philosophy is expected to significantly reduce capex and opex compared to a liquefaction vessel which must be designed to remain on station indefinitely and which much be crewed as a production unit.

For liquefaction capacities up to 1.5 MTPA, the liquefaction vessel can be based on conversion of an existing 138,000 m3 Moss steamship.

The ability to reliably transfer LNG from the liquefaction vessel to an LNG tanker is crucial to the successful operation of any offshore FLNG installation.   Most FLNG concepts have been constrained to utilize some form of “Side-by-Side” offloading where a standard LNG ship is maneuvered into position with tugs and berthed alongside the FLNG vessel, typically against Yokohama fenders.

Side by Side loading entails several operational limitations and risks including:

• Seastate limitations < 2m Hs
• Limits on tug effectiveness with increasing Hs
• Relative motions between vessels
• Risk of collision when berthing/un-berthing
• Exceeding safe tensions in mooring lines & appurtenances

To avoid risk of collision, common practice in deepwater offshore upstream operations is to avoid having vessels approach within the 500 m exclusion zone of a production facility unless the vessel is dynamically positioned and rated to DP2 capability.  Standard LNG ships do not have DP2 capability.

Some form of “Tandem Offloading” is usually proposed to overcome the limitations of “Side by Side” loading.  Tandem Offloading requires use of a DP shuttle tanker or tug assist and the increased distance between vessels can result in higher rates of Boil Off Gas (BOG) compared to Side by Side loading.

In the LoneStar FLNG concept, a special form of tandem loading offloading is utilized incorporating the following features:

• DP2 LNG Shuttle Tanker
• Floating Cryogenic Hoses
• “Bow to Bow” offloading orientation in close proximinty

The liquefaction vessel weathervanes around the midship turret using thruster assist – similar to turret moored drill ships from the 1970’s and 80’s.  It is normally oriented stern into the wind.  This keeps the living quarters upwind of the liquefaction plant and offloading operation.

The DP2 shuttle tanker is equipped with a DFDE propulsion system.  Three retractable azimuthing thrusters are added, two at the stern and one at the bow, along with an upgraded bow tunnel thruster.  This provides a DP envelope capable of over 40 kts wind longitudinally or 20 kts on the beam, more than adequate for offloading in moderate metocean conditions.

Time domain simulations of the DP system in a metocean environment of 20 kts wind and 2.4 m Hs show that it is capable of holding the bow of the ship within a watch circle of approximately 5 m diameter.

During offloading the two vessels are oriented bow to bow, with a separation distance of about 50 m, approximately equal to the ships beam.   The relatively small overlap between the two bows along the longitudinal axis reduces the risk of collision in spite of the small separation between vessels.  If the shuttle tanker loses station keeping, it will drift aft, away from the liquefaction vessel.

The liquefaction vessel is equipped with reels for 3 x 16 in floating cryogenic hoses and the DP shuttle is equipped with a retractable bow loading manifold.   Two hoses are for liquid transfer and one for vapor return.

Longer hose lengths require higher pumping power which imparts heat energy into the LNG stream.  The relatively short hose length in the bow to bow configuration reduces pump power during offloading, reducing boil off gas rates.  If all three hoses were used for liquid transfer, at 6500 m3/hr total transfer rate the BOG rate is 6 tonnes/hr, which could be handled by a re-liquefaction system on the shuttle tanker rather than using a vapor return line.

Dual N2 Expansion liquefaction (DNX) was selected for liquefaction because of the following advantages:

• compact/lightweight
• intrinsically safe
• convenient refrigerant makeup
• simple operability/restart/maintenance
• proven track record on LNG carriers

The primary concerns or drawbacks typically cited against DNX are:

• process efficiency/fuel consumption
• lack of large scale plants

Liquefaction Process Optimization

Through a series of process improvements, it was found that unit power for N2 expansion could be reduced about 6% to 425 KW-hr/Tonne LNG by increasing feed gas pressure and N2 discharge pressure from ca. 900 psi to 1300 psi.

Unit power is reduced by 20% from the Base Case by further lowering the interstage cooling temperature on the N2 compressors from 100 F to 60 F. In practice, this would be achieved by a deep seawater intake hose – seawater at 1500 ft. depth is 47 – 50 F, with little sensitivity to surface temperature.

A further reduction of overall unit power to 325 KW-hr/Tonne LNG, (28% less than the Base Case) is achieved by pre-cooling the feed gas from 80 F to -20 F. The pre-cooling is achieved by heat integration with the hydrocarbon dew pointing system/export booster compression on the host facility and/or use of a CO2 refrigeration system.  The power requirement for pre-cooling is included in the total liquefaction power.

DNX Optimization:Total Unit Power(KW-hr/Tonne)

The unit power cited above includes the power on the host facility for both feed gas booster compression and pre-cooling on the host plus the liquefaction power on ship.  Focusing on the latter, the power required to drive the liquefaction plant on board the ship is reduce by 34% compared to the Base Case, from 64,500 HP to 42,500 HP for each 1 MTPA train.

The ship power is mainly for N2 compression, but also includes BOG compression and a power “credit” from the liquid expander.

DNX Optimizaton:  Ship & Host Power (HP)

Unit power in the Optimized Case for DNX is still higher than typical values for Single Mixed Refrigerant or Dual Mixed Refrigerant processes. However, the higher thermal efficiency of the DFDE power plant, 44% net of switch gear losses vs. 38% for an aero-derivative gas turbine, compensates this difference in unit power. Total fuel consumption (host facility + ship) for liquefaction power is reduced from 7% of feed gas in the Base Case to 5.2% in the Optimized Case.

Thus, the Optimized DNX liquefaction process with DFDE power achieves near parity in fuel efficiency compared to mixed refrigerant processes, but retains the benefits of safety, compactness, lightweight and operational flexibility which are intrinsic to N2 liquefaction processes.

Background and Experience with N2 Expansion Liquefaction

N2 expansion liquefaction has been used in dozens of LNG peakshaver plants. However, the refrigeration duty is significantly less than that required for a baseload LNG facility. There are over 50 LNG vapor re-liquefaction plants in service on board LNG carriers based on N2 liquefaction, but these are typically small scale, single expander processes with much higher unit power than DNX systems.

Dual N2 expansion is the standard process used in the air separation industry to fractionate liquid oxygen (LOX) from the atmosphere with scores of air separation plants using this process all around the world for decades; a large percentage of these plants have LOX capacity of 1 MTPA equivalent or greater.

Large scale N2 cycle cooling for LNG was successfully accomplished for the first time on the AP-X® LNG Process trains in Qatar, for which the subcooling refrigeration loop utilizes a N2 process that has equivalent refrigeration duty to liquefy 1 to 2 MTPA of natural gas if it were a stand-alone liquefier.

The two FLNG vessels built for Petronas will produce 1.2 MTPA and 1.5 MTPA respectively utilizing a triple N2 expansion process provided by Air Products AP-NTM, finally confirming full acceptance of N2 expansion liquefaction processes for application on offshore FLNG vessels.