Making Shipping Greener Part III: EEXI Calculations with Fine Marine

23 May 2024 • 5 minute read

CFD simulation of a rudder propeller interaction

The maritime industry is undergoing transformative changes to mitigate environmental impact and promote sustainability. One crucial development in this transition is the Energy Efficiency Existing Ship Index (EEXI), established by the International Maritime Organization (IMO) to reduce greenhouse gas emissions from existing ships. With advancements in technology, computational fluid dynamics (CFD) simulations have emerged as a valuable tool for assessing the energy efficiency of maritime vessels. These simulations provide a cost-effective and flexible alternative to traditional towing tank tests.

In the third part of this blog series, we examine how Fidelity Fine Marine full-scale CFD simulations are useful for EEXI calculations to optimize vessel performance, contributing substantively to the maritime industry's pursuit of sustainability and reduced environmental impact.

What is EEXI?

EEXI is a technical measure developed to assess the energy efficiency of existing ships in relation to their greenhouse gas emissions. It calculates emissions by expressing the grams of CO2 produced per ton-mile, thereby allowing a standardized measure of a ship's environmental impact. EEXI is analogous to the Energy Efficiency Design Index (EEDI), which evaluates new ships' design efficiency. Effective January 1, 2023, the EEXI regulation will apply to all vessels above 400 gross tons under MARPOL Annex VI, as stipulated by the International Maritime Organization (IMO).

Relevance of EEXI to CFD Simulation for Ship Design

The EEXI regulatory committee officially accepts numerical calculations as a valid alternative to towing tank tests. CFD offers significant advantages, including cost-effectiveness and flexibility. Towing tank tests involve substantial costs, logistical complexities, and scheduling constraints.

By leveraging CFD, the industry eliminates the need to wait for testing slots or run multiple iterations within physical facilities. Additionally, CFD simulations can be conducted at full scale, which is crucial. Full-scale simulations mitigate the inaccuracies introduced by scale effects in model tests, often leading to overestimating the benefits of energy-saving measures. This is a favorable stride forward in the precision and reliability of energy efficiency evaluations for maritime vessels.

How Does Fine Marine Contribute to EEXI Calculations?

Fine Marine is crucial in precisely determining EEXI calculations through comprehensive assessments and simulations. Engineers provide essential factors such as specific fuel consumption and ship capacity; the correction or conversion factors, either for the ship or fuel-related components, are provided by IMO guidelines. Fine Marine helps determine the main engine EEXI contribution based on reference velocity and resistance through self-propulsion analysis and speed-power curve.

An actuator disk can be used to make an initial estimate but still quite accurate for global trends, followed by a final design assessment of the local flow and particular appendages performances with a sliding grid propeller, rudders, and others. Fine Marine allows for the consideration of multiple points on a speed-power curve within a single project. It's worth noting that the same approach can be applied to the EEDI for new ships.

Full-scale EEXI Computations with Fine Marine

There are two primary methods for performing EEXI calculations with Fine Marine:

Method 1: Actuator Disk Approach

The first method employs the actuator disk approach based on momentum theory. This technique can be enhanced with open water data of a propeller, known as the enriched actuator disk method. This method offers accurate predictions at a fraction of the cost associated with fully modeled propellers. The computational effort for actuator disk simulations is comparable to resistance-type computations and usually takes a few hours, contingent on case specifics and resource availability.

When open water data for the propeller is unavailable, the C-Wizard tool can be utilized to set up the computations using the propeller's characteristics. This enables the creation of a performance curve that can be directly applied to self-propulsion analysis.

Steps in actuator disk approach using Fine Marine

Running the propeller in isolation facilitates smooth fluid flow without free surface interaction, leading to quick convergence and minimal computational expense in terms of cell count. This enriched actuator disc method can then be applied to self-propulsion analyses, with a safe computation time that may extend to a few days, depending on the complexity of the case.

Method 2: Full Propeller Modeling

There are scenarios where full propeller modeling becomes indispensable. For instance, EEXI and EEDI calculations submitted to class societies typically mandate full propeller modeling according to ITTC procedures. This requirement is unlikely to change in the foreseeable future. Additionally, many energy-saving devices operate by altering local flow conditions, creating scenarios where flow stratification occurs due to rotational effects. This necessitates full propeller modeling to accurately capture the induced velocities and assess the benefits of such energy-saving devices, especially when mounted close to the propeller, either upstream or downstream.

Standard approach and Cadence approach for full propeller modeling

Accurately representing marine propulsion physics demands the use of sliding grid modeling to simulate the propeller's rotation. Initially, a rotating frame method is employed to set up the calculation, followed by fully resolving the propeller's rotation. Fine Marine utilizes a self-propulsion dynamic library that integrates propeller thrust and vessel drag balance within a single computation, obviating the need for interpolation across multiple operating speeds. This approach ensures accurate and relatively quick simulations, crucial for optimizing ship designs and operational parameters to meet strict regulatory standards and improve environmental performance.

The enactment of the EEXI underscores the maritime industry's commitment to reducing its environmental footprint by enhancing the energy efficiency of existing ships. Fine Marine plays an integral role in this mission, providing accurate and reliable methods for EEXI calculations through advanced CFD simulations. By offering accurate solutions for both the actuator disk approach and full propeller modeling, Fine Marine ensures that maritime vessels meet stringent regulatory standards while optimizing performance.

Read the previous parts of this blog series:

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