Hexagon Purus has received an Approval in Principle for its 350L Type 4 composite high-pressure hydrogen cylinder with 381 bar working pressure issued by DNV, one of the world’s leading classification and certification bodies. Although this tank design has been certified and in service for many years in automotives and cargo transport applications using compressed hydrogen gas, an additional assessment considering maritime specifications was deemed necessary for fuel system approval in professional ships.The Approval in Principle process is an early phase verification level for new design concepts or for existing designs in new applications. This included individual assessment of design documentation of the Hexagon Purus’s designed and manufactured composite cylinder, including an evaluation of the suitability for fuel tank system of compressed hydrogen gas in maritime applications.Generic common governmental regulatory and approval rules are not completed for compressed hydrogen gas tanks for maritime fuel using carbon-fibre composite materials. Therefore, the Approval in Principle project was requested to carefully investigate and review relevant tests and calculations carried out for the auditors and reviewers’ satisfaction. Based on the AiP Hexagon Purus will continue the efforts targeting general type approval certificates from DNV which will improve futural delivery process. Similar assessments will be carried out for other selected maritime fuel tanks design in due time.This AiP represents Phase 1. In Phase 2, DNV, will review the safety of handling hydrogen as a fuel and the safety of hydrogen gas installation and systems in maritime applications.