Choosing the right cargo containment systems

Ever since liquefied natural gas (LNG) carriers first came into use in the maritime world, cargo containment systems (CCS) have preoccupied equipment manufacturers, technology providers, shipyards and designers.

These specialized, insulated tanks contain LNG, keeping it cold and facilitating gas transport and delivery from one facility or vessel to another. To ensure this is done securely and efficiently, CCS are held to high standards of safety and regulatory compliance. New technologies must be carefully developed, assessed and certified.

Despite this, cargo containment technology is flourishing, with new designs being created, certified and brought to market. As of today, marine stakeholders can choose among four types of CCS, each with its own benefits, challenges and applications.

The foundational regulation behind CCS

All CCS are subject to the International Code of the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, i.e., the IGC Code. Most recently updated in 2016, the Code contains specific regulations dedicated to CCS. They address a range of safety and structural aspects, including materials selection and testing, prevention of leaks and venting, and pressure and temperature control.

The IGC Code also covers elements such as materials approval and class, making it the definitive reference document for all classification societies and flag administrations. For technology providers and equipment manufacturers, IGC Code compliance is always the baseline when developing and approving a new CCS design.

The ABCs of independent tanks

The mostly commonly used CCS for small and mid-sized LNG carriers are independent tanks, which are built separately from the ship itself. Independent tanks come in three varieties.

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  Type A tanks are prismatic tanks of the atmospheric type, containing flat panels and external insulation. Originally developed for liquefied petroleum gas (LPG) carriers, Type A tanks have since also been approved for use onboard LNG carriers.
  
  The key concern when using a Type A tank for LNG is temperature: LPG can be kept at -50°C during transportation, while LNG requires a minimum temperature of -160°C. Type A tanks must therefore install a complete secondary barrier made of appropriate insulation material that goes all around the tank.
  
   

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  Type B tanks are prismatic or spherical tanks used primarily for mid-sized and large LNG carriers. Designed around a  “leak before failure” principle, Type B tanks are constructed to limit cracks and escaped LNG.
  
  Nonetheless, Type B tanks feature a partial secondary barrier in the lower part of the cargo hold to recover leaks, should they occur. Prismatic Type B tanks are similar to Type A tanks in shape and maximum pressure, and provide better volume optimization than their spherical counterparts.
  
   

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  Type C tanks are insulated cylindrical, bi-lobe or tri-lobe shaped tanks that can be fully or partially pressurized, depending on the liquefied gas to be stored. Type C tanks are typically found onboard small and mid-sized LPG carriers, as well as small-scale LNG carriers.
  
  Type C tanks do not require a secondary barrier, thanks to the limited risk of leakage or structural failure. Type C tanks can store LNG at a higher pressure than Type A or Type B tanks, though space optimization in these tanks is lower.

Membrane tanks: the built-in solution

In contrast with independent tanks, membrane tanks are built as part of the vessel. Designed with two thin membranes that act as barriers, and supported by layers of insulation, membrane tanks keep design pressure low. The thick inter-barrier and insulated spaces enable these tanks to maintain low temperatures and keep LNG at a minimum of around -160°C.

Membrane tanks were originally developed for LNG, and their shape provides strong volume optimization, making them the preferred technology for today’s standard large LNG carriers. Membrane tanks have also begun to be installed onboard bunkering ships, including MOL’s Gas Agility, the world’s largest LNG bunkering vessel.

The next evolution of CCS

As the boom in LNG use for transportation continues, more and more industry players are looking to build the next generation of CCS. To do this, solutions providers, designers and manufacturers will need to work closely with classification societies, ensuring that their technologies are safe and compliant with regulations.

Bureau Veritas has a long history as a partner of choice for stakeholders throughout the LNG value chain. In addition to providing class approval for the world’s largest and most innovative LNG carriers, our experts have worked closely with GTT on innovative projects like the CMA CGM Jacques Saadé.

Our extensive knowledge of CCS for LNG carriers and LNG bunkering ships has helped Bureau Veritas class small, mid-sized, and large-scale vessels around the world. Looking forward, this will enable us to continue providing insight and technical excellence to new players entering the dynamic, fast-evolving LNG market.

 

Photo Credit: GTT