For the calculation of the Energy Efficiency Existing Ship Index (EEXI), it is necessary to determine the reference speed (Vref) at a certain draft of the vessel.
For many existing vessels subject to the EEXI, it is unlikely that compliant model tests at the required draft exist or that a scale model still exists. Therefore, the EEXI calculation guideline offers the option to calculate the reference speed using an approximate formula for the ship type and installed power. With an included margin factor of 5%, this approximated reference speed will be conservative.
The EEXI rating can be improved by determining the Vref from model tests. DNV GL can arrange, support and verify these model tests. However, building a new model to scale and conducting towing tank tests only for the purpose of evaluating the effect of improving the Vref is unreasonably expensive and time consuming, especially for conducting EEXI screenings of entire fleets.
With Computational Fluid Dynamics (CFD), an alternative method exists which is faster, cheaper and – with DNV GL’s state-of-the-art standardized and validated procedure – provides results at the same level of reliability to model tests.
The EEXI improvement can be evaluated quickly and efficiently for an existing fleet of vessels. The impact of additional energy efficiency technologies on the Vref can also be assessed accurately.
With the CFD approach, additional model tests are only necessary for the EEXI Technical File if no compliant reference model tests are available and if CFD is still not accepted as an alternative method when the EEXI Technical File is to be issued.