Oil and gas production handbook An introduction to oil and gas production

particular importance is the high integrity pressure protection system 
(HIPPS). This is a highly reliable system that is needed to maintain 
protection against overpressures, and manage shutdowns. The strategy 
used by normal safety system to isolate and depressure, or simply fail to 
safe condition, often cannot be applied to pipelines due to the large volumes 
of product in the pipelines. Simply opening or closing a valve would 
potentially cause overpressure conditions or overload safety devices such as 
flares HIPPS monitors and executes these responses in a specific 
sequence. 
5.5 LNG 
LNG is a gas transport product. The gas, which is primarily methane (CH
4
), 
is converted to liquid form for ease of storage or transport, as its volume is 
about 1/600th the volume of natural gas in the gaseous state. It is produced 
close to the production facilities in an LNG liquefaction plant, stored, 
transported in cryogenic tanks on an LNG carrier, and delivered to an LNG 
regasification terminal for storage and delivery to a pipeline system. 
 

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71 
 
LNG carriers are used when the transport distance does not justify the cost 
of a pipeline. The main drawback is the cost of the liquefaction, calculated as 
how much of the total energy content of the gas is used for liquefaction. 
About 6% of energy content is used to produce LNG in a large modern plant, 
due to overall thermal efficiency. More than 10% could be consumed with 
smaller, gas turbine-driven trains. This compares to losses of about 0.9% per 
1,000 km of transport distance for large pipeline systems. 
Melkøya LNG Plant with LNG Carrier Arctic Princess Photo: Statoil 
 
The LNG feedstock comes from a gas plant as outlined above. It must 
satisfy sales gas specifications. Ethane, propane and butane all have 
freezing points of less than -180 °C and can be part of the LNG, but the 
concentration of methane is generally above 90%. Some NGLs are also 
needed as refrigerant for the cryogenic process. 
5.5.1 LNG liquefaction 
LNG processes are generally patented by large engineering, oil and gas 
companies, but are generally based on a one- two- or three-stage cooling 
process with pure or mixed refrigerants. The three main process types of 
LNG process with some examples of process licensors are: 
 
• Cascade 
cycle: 
o
 Separate 
refrigerant 
cycles with propane, ethylene and 
methane (ConocoPhillips)  
•  Mixed refrigerant cycle: 
o
  Single mixed refrigerant (SMR) (PRICO) 
o
  Single mixed refrigerant (LIMUM
®
) (Linde) 
o
  Propane pre-cooled mixed refrigerant: C3MR (sometimes 
referred to as APCI: Air Products & Chemicals, Inc.) 
o
  Shell dual-mixed process (DMR) (Shell) 

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72 
 
o
  Dual mixed refrigerant (Liquefin Axens) 
o
  Mixed fluid cascade process (MFCP) (Statoil/Linde) 
• Expander 
cycle 
o
 Kryopak 
EXP
®
 process 
 
Each process has different characteristics in scalability, investment cost and 
energy efficiency. For smaller installations, e.g., to handle stranded gas or 
isolated small gas fields, a single cycle process is preferable due to its low 
CAPEX (and possibly lower weight for floating LNG), even if energy 
efficiency is significantly lower than the best cascade or DMR processes, 
which cost more but also allow the largest trains typically, 7.8 million tons per 
annum and lowest energy consumed per energy unit LNG produced.  
 
Most processes use a mixed refrigerant (MR) design. The reason is that the 
gas has a heat load to temperature (Q/T)  curve that, if closely matched by 
the refrigerant, will improve stability, throughput and efficiency (see the figure 
below). The curve tends to show three distinct regions, matching the pre-
cooling, liquefaction and sub-coiling stages. The refrigerant gas composition 
will vary based on the individual design, as will the power requirement of 
each stage, and is often a patented, location-specific combination of one or 
two main components and several smaller, together with careful selection of 
the compressed pressure and expanded pressure of the refrigerant, to 
match the LNG gas stream.