LNG Book

Liquefaction process – Cascade Cycle

  • Separate refrigerant cycles with propane, ethylene and methane
  • High efficiency (~0.3 kWh/kg LNG)
  • Optimized technologies are evolved and proposed
  • Large equipment count
  • Requires large plot area forn refrigerant storage and management
  • Increased risk due to flammable liquid inventory
  • Not recommended for offshore environments

Liquefaction process – Mixed Refrigerant (MR) Cycle

  • Uses a single multi-component refrigerant comprising typically nitrogen, methane, ethylene, propane and butane
  • The MR evaporates over a wide range of temperatures and thereby follows the natural gas condensing curve closely
  • High efficiency (~0.3 kWh/kg LNG) Up to 50% less equipment items than the cascade process
  • Several modifications are developed and proposed
  • Sensitive to change in feed composition
  • Requires extensive plot space for refrigeration generation, storage and management
  • Flammable refrigerants

Liquefaction process – Expander Cycle

  • Based on the classic Reverse Brayton / Claude Cycle
  • Several options:
  • Single expander cycle
  • Double expander cycle
  • Open expander loop (methane as refrigerant)
  • Closed expander loop (nitrogen as refrigerant)
  • Efficiencies down to 0.4 kWh/kg LNG
  • A pure refrigerant (typically nitrogen) is deep-cooled by expansion to condense the natural gas to LNG in the cold box

Topside design challenges

  • Limitation in available efficient drivers. Available driver with high efficiency and offshore references is approx. 50MW. Larger LNG trains have lower efficient drivers.
  • Cold design temperatures in combination with large pipe sizes give significant pipe stress issues and layout implications.
  • Open LNG drain and possible leakages on ship deck in process areas.
  • High gas flows give very high flares compared to other FPSOs.
  • Large acid gas removal columns.
  • Potential environmental challenges related to handling of large sour gas rates.

Field specific and pre-treatment systems

Field specific and pre-treatment systems

  • Field specific and pre-treatment systems are conventional and not new to the offshore environment.
  • Energy optimization is required to integrate the heat- and energy demanding systems in the overall topside.
  • Optimize and include the pre-treatment and field specific systems in the fuel gas balance.
  • Tall columns with internals must be carefully designed in order to minimize flow maldistribution.
  • Avoiding stabilization issues of the condensate or recycle of middle components like propane and butane through the process system.

LNG FPSO topside weight

  • Topside weights are closely linked to the equivalent investment cost.
  • Topsides costs are in the range of 50 to 70% of the investment cost for a ship sized solution with nitrogen expander cycles.
  • Proposed sophisticated mixed refrigerant solutions on barges indicates 50% to 100% more topside weights per mtpa LNG produced.

Efficiency

  • The efficiency factor “kWh/kg” is commonly used for efficiency comparison.
  • The factor is often understood as the energy required at the main compressor shafts per kg LNG produced.
  • Most onshore mixed refrigerant plants have high process efficiencies ‘kWh/kg’, but low thermodynamic efficiency due to their driver selections.
  • The more correct approach is to compare the percent feed shrinkage for operation of the FPSO for given field data.
  • The result is typically 1-3% points lower feed shrinkage for the most efficient proposed technologies compared to the others.
  • Maximum LNG production for a topside is not limited by the process selection as long as the upstream system is no bottleneck for the extra 1-3 % points feed shrinkage.
  • A higher feed shrinkage will lower the lifetime of the field equivalent to the difference in the feed shrinkage.
  • For a 2mtpa plant and a ten years production scenario, the lifetime of the field will be increased by approx 3 months by selecting the most efficient technology.
  • The cost of fuel is the value of feed gas at the end of the field life time
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adrian

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