Risks Associated with the Installation of Hull Fixed LH2 Tanks Onboard Ships

The opportunities offered by green hydrogen as energy carrier/fuel for ships and industrial port areas are getting of great interest for the environmental and climate-related challenges of decarbonizing the maritime sector [1]. It is known that, among its various storing options, liquid hydrogen (LH2) might offer the most suitable solution [2], due to its higher density (70.9 kg/m3 ) compared to compressed hydrogen (cH2), for large ships zero-emission navigation on medium/long range voyages [3]. However, LH2 applications are still limited due to supply difficulties, even though hydrogen demand is expected to rise with new zero emissions technology development [4]. Clean hydrogen, produced with renewable energy sources will play a key role in reaching the targets of the European Green Deal by 2050 [5]. In fact, almost 80% of the current global hydrogen production derives from steam methane reforming [6], the adoption of carbon capture and storages (CCS) solutions might make blue hydrogen suitable for meeting the market demand while green hydrogen production and distribution scale up. The storage of LH2 presents its unique technological challenges due to the cryogenic temperature of 20 K, which requires super-insulating solutions and boil-off gas (BOG) management systems [7]. LH2 storage tanks and systems onboard ships present peculiar safety issues, which are not yet explicitly dealt with in the existing maritime regulatory framework (e.g. IGF Code). The adoption of LH2 technologies for ships applications is fraught with challenges, including the need for robust risk assessments and safety protocols [3]. In fact, due to hydrogen wide range of flammability and detonability in air, 4 to 75 vol.% and 18 to 59 vol.% respectively [8], leakage scenarios, both of liquid and gaseous phases are to be carefully considered and analyzed [9]. Effectively addressing these challenges is essential in order to unlock the full potential of hydrogen on a large scale, as an energy carrier and as a technology enabler also. World-wide transportation and distribution of LH2 will inevitably be made possible by ships [10]. The Suiso Frontier is a promising start as the world’s first liquid hydrogen carrier ship, despite its modest capacity of 1250 m3 . Large LH2 carrier ship designs (160000 m3 ) with individual tank capacities of 40000 m3 are currently being developed [11]. At first, solutions engineered for liquified natural gas (LNG) technology were exploited with adequate adjustments for LH2 characteristics. Materials designed and certified for cryogenic applications require specific considerations on mechanical properties at low temperatures (e.g. ductile-to-brittle transition, hydrogen embrittlement) [12]. Moreover, the small size of hydrogen molecules makes it extremely difficult to prevent their slipping between gaskets and flanged couplings [13]. Newly developed materials need to be tested, and equipment failure rates are to be estimated due to the absence of consolidated records. It is common practice, for ordinary equipment, referring to widely recognized reliability databases (e.g. HSE, OREDA), natives of the offshore oil & gas sector. In order to allow the development of new technologies and industrial solutions, Classification Societies are adopting a risk-based certification (RBC), a goal-based approach, consistent with MSC.1/Circ.1394, releasing approval in principle (AiP), an initial process which guarantees that a project is being developed in compliance with the state of the art or latest available standard, when dedicated regulations issued by the International Maritime Organization (IMO) are missing or incomplete. All the innovative project involving LH2 need to receive an AiP, as in the International Code of Safety for Ship Using Gases or Other Low-flashpoint Fuels (IGF Code), which was originally developed in order to allow the use LNG as a fuel for ships other than LNG carriers, there is no explicit mention of alternative fuels (i.e. ammonia and methanol) or hydrogen. Therefore, adopting LH2 as fuel onboard ships requires an alternative design approval process. Developing a tool to facilitate the evaluation procedure of safety assessments and quantitatively estimate the risk associated with a proposed design solution for LH2 ship systems is of great interest, both for stake holders and Classification Societies.