Answer
To differentiate between a Type A and a Type B LPG carrier, it is important to clarify that under the IMO IGC Code, these designations refer specifically to the cargo containment system (tank design) rather than the hull itself.
While both tank types are independent, self-supporting structures that do not contribute to the ship's hull strength, they differ fundamentally in their engineering design philosophy, secondary barrier requirements, and structural monitoring.
The Core Technical Differences
1. Design Philosophy & Engineering
Type A Tanks: Designed using classical ship structural analysis and standard classification society rules. Because the engineering models do not precisely chart long-term microscopic crack propagation, the design must assume that a major structural failure or leak is a possibility.
Type B Tanks: Designed using highly sophisticated analytical tools, Finite Element Analysis (FEA), and fracture mechanics. They are engineered strictly on the "Leak-Before-Failure" principle. This means you must mathematically prove that if a fatigue crack develops in the tank shell, its growth will be so slow that the crew will detect the vapor leak long before a catastrophic structural failure can occur.
2. Secondary Barrier Requirements
This is the most critical operational and statutory difference for an oral exam defense:
Type A Tanks: A FULL Secondary Barrier is mandatory. Because a sudden, large-scale leak cannot be analytically ruled out, the ship’s hull structure must feature a complete secondary containment system (the inner hull combined with low-temperature insulation) capable of safely containing the entire liquid cargo volume for at least 15 days.
Type B Tanks: Only a PARTIAL Secondary Barrier is required. Because the "Leak-Before-Failure" design guarantees that any leak will start as a minor, slow-growing defect, a massive failure is statistically discounted. The secondary barrier consists only of a Drip Tray directly beneath the tank, paired with splash or spray shields to guide minor leakage safely into the tray without contacting the ship's primary hull girder.
3. Tank Geometry and Shape
Type A Tanks: Almost exclusively Prismatic (box-like with sloped tops and bottoms). This shape is highly efficient because it closely mirrors the internal shape of the cargo hold, maximizing the ship's volumetric carrying capacity.
Type B Tanks: Can be Spherical (such as the iconic Moss-Rosenberg tanks commonly seen on LNG carriers) or Prismatic (known as SPB—Self-supporting Prismatic Type B tanks). Spherical Type B tanks are structurally perfect for uniform stress distribution but leave significant void spaces in the hull.
4. Hold Space Monitoring and Atmosphere
Type A Tanks: The hold space surrounding the primary tank must be completely inerted with Nitrogen when carrying flammable cargoes. The system relies on a heavy-duty inert gas distribution network to maintain a safe environment.
Type B Tanks: While hold spaces are also closely managed, Type B systems place a much higher operational reliance on continuous, ultra-sensitive interbarrier gas detection systems. Because the safety case relies on detecting a microscopic leak early, these sensors are critical statutory instruments.
Summary Comparison Matrix
| Feature | Type A Containment | Type B Containment |
|---|---|---|
| Design Standard | Classical Ship Structural Rules | Advanced Fracture Mechanics & FEA |
| Core Philosophy | Standard Structural Safety Margin | "Leak-Before-Failure" Principle |
| Secondary Barrier | Mandatory FULL Barrier (15-day liquid hold) | PARTIAL Barrier Only (Drip tray & spray shields) |
| Tank Geometry | Strictly Prismatic | Spherical (Moss) or Prismatic (SPB) |
| MARVS (Vapor Pressure) | Low (typically < 0.3 bar) | Low (< 0.3 bar for prismatic, slightly higher for spherical) |
| Typical Application | Standard Fully Refrigerated VLGCs | Specialized modern VLGCs and LNG carriers |
| Cost & Complexity | Lower initial fabrication cost; higher steel weight. | Exceptionally high fabrication and engineering costs. |
Practical Exam Insight: Why Choose One Over the Other?
If an MMD surveyor asks “Why don't all VLGCs just use Type B tanks if they require less of a secondary barrier?”, the answer comes down to economics and construction complexity.
Building a Type B tank requires extremely high-precision welding, expensive materials (like specialized aluminum alloys or high-nickel steels), and extensive non-destructive testing (NDT) to validate the fracture mechanics. For standard LPG trading (Propane at -42°C and Ammonia at -33°C), the traditional Type A prismatic tank built with fine-grained carbon-manganese steel provides the most cost-effective balance of maximum cargo volume and structural safety, despite needing a full secondary barrier.
This detailed diagram serves as a perfect visual reference for an engineering presentation on gas carrier design or a management-level technical oral exam.