Ship owners and operators are facing numerous difficulties as a result of the International Maritime Organization's Energy Efficiency Existing Ship Index (EEXI), which is already having a considerable influence on the industry. Put simply, the EEXI is the result of applying the Energy Efficiency Design Index (EEDI) to ships that are already in service as opposed to ones that are being built. Since the majority of departing ships do not have an EEDI value, the EEXI must be determined. Ideally, this should be less than a specific threshold. EEXI calculation is challenging since it must be based on the available data for each vessel. This data is only partly available for older vessels. Most vessels will first choose to modify their power rating in order to comply with EEXI regulations, but investing in energy efficiency technologies (EETs), for which the power savings must be measured and confirmed, appears to be better option. On the other hand, EEXI compliance is a technical requirement needing a single certification. The Carbon Intensity Indicator (CII), an operational indicator that requires annual improvements in operational efficiency, is more difficult. The carbon intensity of a ship, or the amount of carbon emissions produced by one unit of transport work, or one nominal tonne of cargo transported over a nautical mile, is created on a downward trajectory by the CII. Every ship is given a "energy efficiency" rating (ranging from A to E) by the CII. Every year, vessels in categories D and E must show progress in order to advance to category C. A mandatory review of the Ship Energy Efficiency Management Plan (SEEMP) and the creation of a corrective action plan are required for ships that have three consecutive years in category D or one year in category E. The goal is to achieve the Required Annual Operational CII. In this master thesis, we focus on Steam Turbine LNG Carriers calculation of the EEXI, CII and energy flows among the machinery items of this type of vessel. More specifically: • Chapter 1 is a brief description of the Liquified Natural Gas (LNG), its properties and the LNG Carrier ship types, • Chapter 2 presents the basic energy flows of the machinery items of a Steam Turbine LNG Carrier on Normal Sea Going Condition and at Port Condition utilizing Sankey diagrams. • Chapter 3 analyzes the calculating process of the Required and the Attained EEXI for the Steam Turbine LNG ships. • Chapter 4 shows the process to determine the Required and the Attained CII of a vessel. • Chapter 5 presents the results of 2 similar ships (Steam Turbine LNG Carriers) in terms of EEXI and CII. • Chapter 6 is the final section of this thesis incorporating the final results and conclusion regarding the EEXI, CII and energy flow identification of this specific type of vessel which examined.