Maritime transport of hydrogen is a critical component of the emerging global hydrogen economy, enabling the movement of large volumes between production and demand centers. However, leakage risks during shipping—whether in the form of liquefied hydrogen (LH2), ammonia, or liquid organic hydrogen carriers (LOHCs)—pose environmental and safety challenges. Addressing these risks requires an understanding of failure scenarios, their consequences, and the development of robust international regulations to minimize emissions and hazards.

Liquefied hydrogen is transported at cryogenic temperatures (-253°C), presenting unique challenges. Boil-off gas is inevitable due to heat ingress, even with advanced insulation. If not managed properly, this can lead to pressure buildup or direct hydrogen release into the atmosphere. In the event of a tanker accident, catastrophic failure could result in rapid vaporization and dispersion of hydrogen, which is highly flammable at concentrations between 4% and 75% in air. The buoyancy of hydrogen means it disperses quickly, but ignition risks remain significant in confined spaces or near ignition sources.

Ammonia, a common hydrogen carrier, has different risks. While it does not combust as readily as hydrogen, it is toxic and can cause severe health effects at concentrations as low as 25 ppm. Leakage from storage tanks or pipelines due to corrosion or accidents could lead to hazardous vapor clouds. Ammonia also contributes to nitrogen pollution if released into marine environments, with potential impacts on aquatic ecosystems.

LOHCs, such as toluene-methylcyclohexane systems, are less volatile but pose risks related to organic compound leakage. Spills could contaminate marine environments, and incomplete dehydrogenation during unloading may release trace hydrogen. The flammability of some LOHCs adds another layer of risk, though their lower volatility compared to LH2 reduces the likelihood of rapid dispersion.

Three primary leakage scenarios must be considered:
1. **Boil-off gas release** – During normal operations, boil-off from LH2 tanks must be reliquefied, burned, or vented. Uncontrolled venting contributes to hydrogen emissions, which indirectly affect climate due to hydrogen's role in prolonging atmospheric methane lifetime.
2. **Tanker collisions or structural failures** – A breach in containment could lead to large-scale release. For LH2, this would cause rapid vaporization and potential fire hazards. Ammonia leaks would require emergency response to prevent toxic exposure.
3. **Loading/unloading failures** – Transfer operations are high-risk phases where leaks may occur due to equipment malfunction or human error.

Quantitative studies indicate that hydrogen losses during maritime transport can range from 0.1% to 5% per day for LH2, depending on insulation quality and voyage duration. Ammonia leakage rates are typically lower, around 0.01% to 0.1%, but the toxicity risk outweighs the lower emission volume. LOHCs exhibit minimal hydrogen loss under stable conditions but may face risks during dehydrogenation.

To mitigate these risks, international regulations should focus on:
- **Enhanced containment standards** – Mandating double-walled tanks for LH2 with vacuum insulation, corrosion-resistant materials for ammonia, and leak-proof systems for LOHCs.
- **Boil-off management** – Requiring onboard reliquefaction or combustion systems to prevent direct venting of hydrogen.
- **Emergency response protocols** – Establishing clear procedures for collision, leakage, and fire scenarios, including crew training and coordination with coastal authorities.
- **Emissions monitoring** – Implementing real-time sensors to detect leaks and quantify losses, with reporting requirements for international shipping bodies.
- **Route risk assessments** – Avoiding high-traffic or ecologically sensitive areas for hydrogen carrier transport when possible.

The International Maritime Organization (IMO) should expand its existing codes, such as the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code), to include hydrogen-specific provisions. These could set maximum allowable leakage rates, mandate vapor recovery systems, and define penalties for non-compliance.

Regional cooperation is also essential. Coastal nations must harmonize emergency response plans, particularly for ammonia spills, where cross-border vapor drift could occur. Ports handling hydrogen carriers should invest in specialized firefighting equipment, given hydrogen's invisible flame and ammonia's toxicity.

Technological solutions can further reduce risks. Advanced composite materials for tanks, automated leak detection systems, and improved insulation can minimize boil-off. For ammonia, catalytic converters could break down accidental releases into harmless nitrogen and water. Research into more stable LOHCs with lower environmental persistence should also be prioritized.

The climate impact of hydrogen leakage must not be overlooked. While hydrogen is not a direct greenhouse gas, its release increases tropospheric hydroxyl radicals, which reduce methane's atmospheric lifetime but also contribute to stratospheric water vapor formation. Quantifying these indirect effects is critical for lifecycle assessments of hydrogen supply chains.

In conclusion, maritime hydrogen transport is feasible but requires stringent regulations to address leakage risks. A combination of technological improvements, operational safeguards, and international policy coordination will be necessary to ensure safe and sustainable shipping. The IMO, alongside national regulators, must act swiftly to establish standards that prevent accidents, minimize emissions, and protect both human health and the environment. Without such measures, the hydrogen economy's expansion could face significant public and environmental opposition due to unresolved transport risks.

Future work should focus on real-world pilot projects to validate leakage mitigation technologies and refine regulatory frameworks. Collaboration between industry, academia, and governments will be key to developing a robust and safe global hydrogen shipping network.