FLEXSHIP will facilitate the transition of the waterborne sector towards climate neutrality by delivering a digital green concept for electrification of vessels consisting of a Green Digital Twin (GDT) for designing fit-for-purpose vessel electrical grid architectures and integrating a large battery capacity system into two existing vessel (DEMO 1 & 2) electrical systems, a compact, low-weight, modular and simple, high-efficiency battery system, and a safe integration guide of the system onboard ensuring system interoperability The overall goal of FLEXSHIP is to develop and validate safe and reliable, flexible, modular, and scalable solutions for electrification of the waterborne sector. This includes the reliable design and development of modular battery packs; safe on-board integration including the battery system and its associated electrical distribution grid into the vessel’s existing power grid; optimal design of energy management system (EMS) to maximise the operational flexibility and energy efficiency (both full-electric and hybrid), and smart control for improved lifetime of the battery system and critical power components. The objectives will be achieved by 8 WP and 16 partners within 48 months. In WP1 identification of specification and mapping of requirements will be done. In WP2 the vessel electrical architecture will be designed and optimised by means of the Green Digital Twin. In WP3 the development and optimisation of individual components and sub-systems will be done and the testing of the system at component/sub-system level will consist of hardware-in-the-loop (HiL) and software in the loop (SiL) tests in WP4. The full FLEXSHIP system will be tested in two demonstrations in WP5 with minimum 150nm sailing distance and in WP6 contributing to 300nm by green digital twin and achieving sustainability analysis and business plan. In WP7 the full system will be evaluated in an exploitation strategy. The innovations will be brought from TRL4/5 to TRL7.