Turbomachinery for offshore lng

Turbomachinery Fundamentals

Turbomachinery for offshore LNG
 

 

Liquefaction of natural gas to LNG in floating module is the only available option for remote offshore gas reservoirs. Aero-derivative gas turbine driven centrifugal compressors -- because of compact, light-weight, simple and reliable design -- are the best and optimum selection for offshore LNG units. Presented conceptsand details in this article allow substantial reduction in total cost of ownership, better production flexibility and reliability of offshore refrigeration centrifugal compressor trains.

The LM2500G4 gas turbine is a 34 MW aero-derivative two-shaft machine with efficiency above 42%. Two-stage power-turbine provides shaft normal speed around 6100 rpm (operating range 3600 rpm to 6400 rpm).Variable geometry air compressor inlet guide vane and variable stator vane technologies are used to improve starting as well as reasonable efficiencies in part load operation. LM2500 gas turbines are usually offered in single base-plate configuration including gas turbine, enclosure and auxiliaries, total dimension around 6.5x3.6x4 m and weight 40 t.

LM6000 is multi-spool machine that exhibits alarge speed range. It is an around 42% efficiency gas turbine offered more than 44 MW simple-cycle power. Six stage turbine offers 3600 rpm speed (speed range capability at around 55-105% of nominal speed). It is usually offered in single base-plate configuration.

Trent 60 is a large and modern aero-derivative gas turbine available today for industrial use. It can deliver up to 64 MW power (42%efficiency). This machine offers great flexibility.

The LMS100 is a large aero-derivative three-shaft gas turbine. It can produce more than 100 MW with around 45% efficiency. The LMS100’s aero-derivative core and power turbine allows operating flexibility which is very rare in any other gas turbines of 100 MW size. It uses annular combustion chamber and advanced inter-cooling technology toincrease power and efficiency. It has three turbine sections (HP, MP and LP). Power turbine can be optimized for different power turbine speeds (3,000rpm, 3,600rpm, etc). The modular design, part-load efficiency, operation characteristics and ability to change-out the main modules around 24-48 hours, provide increased flexibility, availability, reliability and easy maintenance.

 

LNG PROCESSESFOR OFFSHORE

Perhaps the process contributing the most tonnage of LNG worldwide is APCI LNG. This process includes two main refrigerant cycles: 1- A propane pre-cooled cycle. 2- Mixed refrigerant (MR) cycle. The pre-cooling stage of the incoming stream is in a separate propane chiller and can cool the process gas down to around -40°C. The MR in this process usually consists of nitrogen,methane, ethane and propane, used in the liquefaction and sub-cooling cycle. In the MR cycle, the partially liquefied refrigerant is separated into vapor and liquid streams that are used to liquefy and sub-cool the process stream to a typical temperature below -150°C. The overall vaporized MR stream from the bottom of the cryogenic heat exchanger is recovered and compressed by the MR compressor toaround 48 Bara. It is cooled and partially liquefied first by water or air, then by the propane refrigerant, and recycled to the exchanger. The Shell Double Mixed Refrigerant process (DMR) is close to the APCI system. Rather than propane, the pre-cooling is accomplished by a refrigerant consisting mainly of ethane and propane. Another key difference is that the pre-cooling is carried out in heatexchangers rather than kettles. These two processes (APCI and DMR) are suitable processes for large offshore floating LNG units.

The PRICO LNG process is a single MR LNG process. The refrigerant usually consists of nitrogen, methane, ethane, propane, butane and pentane. A single driver compressor system compresses the refrigerant (using a single multi-casing compression train). The cooling and...