Floating liquefied natural gas (FLNG) project solutions are located at the offshore reservoir to monetise stranded offshore hydrocarbon reserves. Natural gas is drawn from a single well or multiple wells and brought to the topsides of the FLNG vessel through subsea completion systems, manifolds, risers or turret systems.
On board the FLNG, the raw feed gas, which might also carry condensate, water and other hydrate formation inhibitors, is first sent through an inlet facility for separating these elements. The gas is then sent to a gas pretreatment and dehydration unit to make it liquefiable before it enters the cryogenic portions of the facility.
FLNG solutions are also being considered for exporting nearshore gas reserves or gas from an existing pipeline network by locating the LNG production facility near the shore. The natural gas, in this case, is typically subject to pipeline quality specifications with prescribed limits on variation.
Most of the condensate or heavy hydrocarbons are generally removed upstream and sold as natural gas liquid (NGL) products prior to putting the gas in the distribution network. Because of this, the pipeline gas would not require an elaborate gas inlet facility but still needs to be treated to remove impurities and moisture prior to sending it to liquefaction.
Data on the anticipated feed gas is key for setting the design parameters for a successful FLNG project. However, there is no guarantee that feed gas quality during operation will be the same as that considered in the design of the facility.
There may be some certainty in the feed gas drawn from a pipeline network on shore, but for natural gas coming directly from the aggregation of multiple producing wells, the quality of the feed gas is likely to vary over time, changing pressure, temperature and composition.
It is therefore imperative that FLNG developers consider several eventualities in their design basis. Developers must select technology and equipment with enough flexibility to operate safely at or near the required performance level without interruptions and shutdowns due to changes in the feed gas conditions over time.
In addition to the flexibility to adopt varying gas quality, other parameters affect the efficiency of the liquefaction process on an FLNG, such as the temperature of seawater and of ambient air, and the pressure of the feed gas.
Typically the heat of refrigeration is rejected to the ocean, and thermodynamic efficiencies are reduced when the heat sink is at a warmer temperature. The same can be said of the air temperature for air-cooled facilities.
FLNG facilities are typically powered by gas turbines, whose output depends on the combustion air temperature, with warmer conditions resulting in less throughput. This relationship varies by turbine model, but is readily available from the manufacturers.
Liquefaction of natural gas is more efficient at a higher feed-gas pressure, since the overall thermal efficiency increases when the phase change from gas to liquid occurs at a corresponding warmer temperature.
An FLNG must be able to operate efficiently over the entire temperature range for its assigned location and to handle anticipated variations in pressure as gas fields mature from seasonal variations in pipeline gas demand or for other reasons.
Developers must consider the annual production rate from a facility in their offtake planning and the possible complications. For example, a liquefaction facility in the southern hemisphere may have lower production in summer, a season that coincides winter in the northern hemisphere and the peak demand of its customers there.
For a successful FLNG application, the liquefaction technology should be proven, reliable, lightweight, space-efficient and simple to operate and have the flexibility to operate under varying feed gas and other conditions. Technology like the PRICO single-mixed refrigerant (SMR) process is ideal for FLNG applications.
The refrigerant composition is easily tuned during operation to match the cooling curve for the gas being liquefied. This is accomplished in the distributed control system by varying the residence time of various refrigerant streams in the loop.
Additionally, the integrated heavies recovery system is designed conservatively for the anticipated feed gas. Experience shows that this portion of the plant will experience the greatest variability in flows, and appropriate design margins are incorporated in key equipment items such as the tower internals and heat exchangers used to process the NGL and condensate streams.
The PRICO design uses a simple closed-loop refrigeration cycle in which the refrigerant is compressed, partially condensed, cooled, expanded and then heated as it supplies refrigeration and flows back to the compressor. The refrigerant is a mixture of nitrogen, methane, ethylene, propane and isopentane.
The design basis of the FLNG should be comprehensive and include possible conditions during normal operation and end-of-the-run conditions for a depleted feed gas source.
The SMR process has been used successfully in many applications with varying feed gas compositions and pressures.
In 2005, Praxair and Black & Veatch commissioned the first LNG production facility in Brazil. Feed to the Praxair facility is supplied from various resources and there is a wide composition range and a 450-1,000 lbs per square inch gauge (psig) pressure range, or 3,200-7,000 kPa, depending on the feed source.
The process was designed to process any of the feed streams without plant shutdowns, changes to the refrigerant charge in the system, or severe intervention.
Turndown flexibility may be a desired attribute for various reasons, such as a phased development of gas production or seasonal variability in supply or offtake agreements. Multiple train arrangements allow discrete production increments to be turned on and off.
Experience shows that PRICO is very flexible to turndown with efficient operation throughout the entire speed range of a gas turbine. Further turndown can be achieved by opening the compressor antisurge valves, although this recycle will reduce overall process efficiency.
Operational flexibility is critical for a successful FLNG project. Flexibility challenges can be resolved with proper selection of the liquefaction technology and appropriate design of key systems within the facility.
PRICO SMR technology has the intrinsic ability to adjust operations on the fly, to cope with anticipated and unexpected changes and to provide a robust process solution for FLNG.
Javid H Talib is vice-president and Shawn Hoffart vice-president LNG technology, oil and gas at Black & Veatch