The maritime sector plays an important role in energy transition because offshore vessels are used for building offshore renewables but also have high emissions. The European Union (EU) has determined that from 2025 the maritime sector must pay per ton of CO2 emission. Heerema Marine Contractors (HMC) is an essential player in the maritime sector and plans to have reduced its emissions by 50% by 2025. This thesis aims to build a model in which different emission-abating options can be compared on financial and technical aspects to determine the best roadmap to achieve sustainability. To do this properly, literature research is done on emissions-abating fuels and technologies/processes applicable to offshore crane vessels. Five sub-questions are answered to find an answer to the main research question: What will be the future energy configuration of the Heerema fleet and the most technical and financial feasible roadmap to meet the sustainability goals look like? • What are an offshore crane vessel's main operational energy-consuming modes? • Which fuel switches and blends are technically and financially feasible, and what are the key characteristics of these fuels? • What are the key characteristics of the ship-based emission-abating technologies like batteries and carbon capture and storage? • What are the current vessel-specific levelized cost of Energy (LCOE) and levelized cost of carbon abatement (LCoCA) of the fuel switches and technologies, and how are they expected to develop over time? • Which emission-abating future scenarios are technically and financially feasible, and how will these sustainable roadmaps look? With the evaluation of the answers to these sub-questions, the importance of using hydrotreated vegetable oil (HVO) is shown until e-fuels are available. From 2026 e-fuels are assumed to be wide available for usage on board the three vessels, the Sleipnir, Aegir and Thialf the vessels within the scope. The financial evaluation of the e-fuels is in favour of ammonia. The technical evaluation is based on the limited available storage volume favour for methanol. These findings result in a roadmap using batteries on board the vessels together with HVO as fuel and when available installing designated ammonia or methanol tanks, and using ammonia or methanol as a 100% fuel blend in the internal combustion engines of the three vessels.