Floating Storage Regasification Units made easy

Floating Storage Regasification Units (FSRUs) are offshore facilities that are used for the storage and regasification of liquefied natural gas (LNG) to be transported through pipelines to onshore markets. These vessels play a crucial role in enabling countries or regions without access to natural gas pipelines to import LNG. Below, I’ll provide a detailed explanation of the design, construction, operations, and maintenance of FSRUs

1. Design and Construction

Hull Design and Materials

The hull design of an FSRU is similar to that of a conventional LNG carrier, but with modifications to accommodate the regasification and storage processes.

  • Hull Shape: Typically, FSRUs have a hull design that is optimized for stability, durability, and the unique requirements of floating regasification. The hull must ensure that the unit can withstand harsh ocean conditions, including high winds, waves, and temperature changes.
  • Materials: The hull is made from high-strength steel, designed to withstand the extreme environmental conditions at sea. The material used is usually a carbon steel alloy that is resistant to corrosion, with coatings applied to prevent damage from seawater. The choice of materials is critical because the FSRU operates in an aggressive marine environment where corrosion, impacts, and mechanical stress are common.
  • Size and Capacity: FSRUs are often large vessels, with storage capacities ranging from 125,000 to 265,000 cubic meters of LNG. The hull is designed to house both LNG storage tanks and the regasification units. The size also enables FSRUs to operate as standalone regasification stations that can connect to local pipelines.

Storage Tank Design and Materials

The storage tanks on an FSRU are crucial for holding the liquefied natural gas before it is regasified and sent to shore.

  • Tank Types: There are two main types of tanks used on FSRUs:
    1. Membrane Tanks: These tanks use thin, flexible membranes that are bonded to the internal surface of the tank. The membrane contains the LNG and keeps it at very low temperatures, typically between -160°C to -164°C. The design allows for a high degree of space efficiency.
    2. Moss Type Tanks: These are spherical tanks made of reinforced concrete with a layer of insulation. Moss-type tanks are designed to minimize the risk of leakage in the event of a rupture and offer robust protection against harsh marine environments.
  • Materials: The materials used in LNG storage tanks must provide excellent thermal insulation to keep the LNG at cryogenic temperatures. Stainless steel, reinforced concrete (in the case of Moss-type tanks), and specially formulated insulating materials are used. These materials must also resist corrosion and be able to handle the dynamic pressures that arise during the transfer and regasification processes.

2. Operations and Maintenance

Regasification Processes

Regasification is the process of converting LNG back into natural gas to be transported through pipelines. FSRUs are equipped with regasification systems that operate under several potential technologies:

  • Open Rack Vaporizer (ORV): The ORV system uses seawater to heat the LNG and convert it into gas. It consists of heat exchangers where LNG is pumped through pipes submerged in seawater. The heat from the seawater causes the LNG to vaporize and revert to natural gas.
  • Closed-Loop Vaporizer: In this system, LNG is heated using a closed-loop system in which a heat transfer fluid (such as glycol or seawater) is used to transfer heat to the LNG to convert it into gas.
  • Ambient Air Vaporizer (AAV): Some FSRUs use air vaporization, where ambient air is drawn over a set of heat exchangers to warm the LNG. This method is more environmentally friendly as it doesn’t use seawater.
  • Pressure Letdown Vaporization: This process uses the reduction of pressure to allow the LNG to expand, which causes it to vaporize. This is typically used as a supplemental vaporization method.

Once the LNG is regasified, the gas is either pumped into local pipelines or stored for further use. FSRUs can also offer flexibility in terms of storage and regasification capacity, with regasification rates typically ranging from 500 million to 1 billion cubic feet per day.

Safety and Emergency Response

Safety is a key concern for FSRUs due to the highly flammable nature of LNG and the complex equipment involved in its storage and regasification. Several measures and systems are implemented to ensure safe operations and to address any emergencies that might arise.

  • Safety Systems:
    1. Leak Detection: Advanced leak detection systems, such as gas detectors and thermal imaging, are used to quickly identify leaks or abnormal situations. These systems help in monitoring LNG storage tanks, pipelines, and regasification units.
    2. Fire Suppression Systems: FSRUs are equipped with advanced fire suppression systems, including water spray systems, foam systems, and fire extinguishers to quickly control any fires or explosions. Special attention is given to the prevention of fire in the cargo hold and during the LNG transfer process.
    3. Emergency Shutdown Systems (ESD): These systems are designed to stop operations automatically in case of a detected emergency, ensuring that regasification, storage, or transfer is halted if necessary to mitigate risks. ESD can be manually triggered as well by the crew.
  • Emergency Response Plans:
    1. Evacuation Procedures: In case of an emergency, evacuation procedures are implemented. Crews are trained to abandon ship if required, and escape routes and life-saving appliances (e.g., life rafts) are always available.
    2. Collision Avoidance: FSRUs are often positioned in offshore locations away from shipping lanes to avoid collision risks. However, if a collision does occur, FSRUs are designed with reinforced hulls to mitigate potential damage. Additionally, tugboats may assist in preventing drift during emergency situations.
    3. Contingency for LNG Spills: LNG, while non-toxic, poses risks in the form of fire and explosion. For LNG spills, FSRUs are designed to minimize the impact on the environment and human health, with containment systems for spills and rapid dispersal mechanisms for vaporized gas.
  • Maintenance and Inspections:
    1. Regular Inspections: Routine inspections are essential for ensuring the structural integrity of the FSRU, particularly the hull, tanks, and piping systems. These inspections include non-destructive testing (NDT) techniques like ultrasonic, radiographic, and visual testing.
    2. Tank Maintenance: The insulation of LNG tanks is regularly checked for wear and tear, especially in areas exposed to seawater or extreme temperatures.
    3. Regasification Equipment Maintenance: The mechanical and electrical components used in the regasification process must be maintained regularly to ensure efficiency and prevent operational failures.

Conclusion

FSRUs are sophisticated vessels designed to safely store and regasify LNG, enabling energy access in regions without pipeline infrastructure. The hull and storage tank designs are specifically chosen to ensure safety, durability, and space efficiency, while the regasification systems are built to operate efficiently in a variety of environmental conditions. Rigorous safety and emergency response protocols are implemented to protect the crew, the environment, and the infrastructure in case of any issues. Regular maintenance ensures long-term operational reliability.

You cannot copy content of this page