Introduction

Growing interest in Wind Assisted Propulsion Systems (WAPS) has led to the need for a systematic and reliable approach to verify in-service performance, assuring shipping stakeholders of their potential and delivered performance as a solution to reduce fuel consumption and emissions.

Anemoi Marine Technologies has developed a robust methodology to verify vessel performance and predict fuel savings not only for Anemoi’s Rotor Sails but for all WAPS. It is driven by several years of research and data gathered from Anemoi installations, inspired by existing approaches from other industries, and has been approved by class society Lloyd’s Register Advisory.

The WAPS verification challenge

The current standards and guidelines landscape for WAPS performance verification is sparse. Existing industry standards such as ISO 19030 can be adapted and used, but were initially designed for other purposes and are not particularly well suited to wind technologies. This can result in protracted test phases due to the requirement to gather a large enough data sample of sufficient quality. Meanwhile recently published procedures and guidelines by the International Towing Tank Conference (ITTC) can be expensive if vessels are taken off hire for a day or more of sea trials, and is hugely dependent on the wind conditions experienced. Even if the wind conditions are good, only a limited envelope is tested and extrapolation is required to predict performance in other conditions and sea states.

Theoretical approaches using Computational Fluid Dynamics (CFD) or wind tunnel tests can provide indicative savings in advance but cannot capture accurately all the real-world effects on the vessel with the same level of confidence. Likewise, direct thrust measurement on board can provide insight into the WAPS behaviour but misses the important effects of the vessel hydrodynamics and propulsion behaviour that affect the actual savings in fuel and emissions.

A practical solution

Anemoi has therefore developed a robust and practical, real-world approach that considers the WAPS devices as a key element of the ship and the ship-wide energy system. The impact of WAPS on ship propulsion performance can be quantified and an existing model calibrated by analysing data collected through ON-OFF testing: switch the WAPS ON, allow the vessel to settle to a new equilibrium then turn it OFF.

The predicted forces from WAPS and their impact on the hydrodynamic force are calibrated with use of the ON-OFF test data. This calibrated model can be used as part of a Performance Prediction Program (PPP), such as the Fuel Saving Assessment Method (FSAM) developed by Anemoi, to predict fuel savings for each voyage with high levels of confidence.

By measuring the ship’s response to ON-OFF tests conducted during the normal operation of the ship, a realistically broad envelope of different wind and wave conditions can be covered without diverting the vessel from its schedule. The analysis approach includes normalising results and deals with variation in other uncontrollable variables such as wind, sea state and currents. This allows for a high proportion of test results to be used, leading to a manageably short period of verification with more relevant results. The method fits the ship model to data points acquired in testing, allowing for predictions of performance to be applied across the entire range of conditions even if tests were not conducted in every condition.

The verified performance model based on the test results accounts for all the real effects on the vessel as well as the performance of the WAPS units. The methodology uses Rotor Sails as an example, but other WAPS types can be treated similarly, subject to considerations such as their power consumption, limits on thrust or wind speed and response to gusts, lulls and changes in wind angle, among others.

ON-OFF testing

Tests are conducted at a range of relative wind angles from ahead to astern on both sides and in a variety of relative wind speeds. For the change in WAPS-generated thrust to be measurable through changes in the vessel’s speed or engine power above their natural variations due to changes in wind and waves, typically, the apparent wind speed should be at least 10 knots (5m/s), ideally 16 knots (8m/s) or more.

For these tests, the main engine rpm and ship heading are kept constant while the Rotor Sails are turned on and off for short periods whilst the vessel speed and main engine power are measured.  This method works well for conventional cargo ships with closed-loop control of the engine rpm. Typically, the percentage change in speed is of the same order as the percentage reduction in engine power, but noting that the resistance increases approximately with the square of vessel speed, this means that in the tests about 75% of the WAPS thrust is used to increase the speed of the ship, while only about 25% is seen as a direct reduction in main engine power.

If the design of the control system allows, there is merit in running some tests at wind angles small enough to give negative thrust, even though this would not be done in normal operation. This allows the angle at which zero thrust is produced to be estimated more accurately by interpolation rather than extrapolation from tests run at higher wind angles.

Through comparison of the changes in Vessel Speed and Main Engine power, the performance benefit of the Rotor Sails can be derived for the particular relative wind conditions at the time of the test. The input power used to spin Rotor Sails are measured and included in the calculation of net power and fuel savings.

Processing vessel data

The processing approach has been divided into two key steps, illustrated in Figure 1.

To quantify performance, data from various sources onboard the vessel needs to be correctly collected, handled and collated. In the pre-processing stage, data is checked for errors, synchronised and corrected before being collated, ready for processing.

After synchronising the raw sensor data from various sources, such as the main engine, speed log and anemometer, it is essential to ensure that the data accurately represents the overall operating conditions of the vessel, rather than merely reflecting localised measurements. For instance, anemometers record wind conditions only at their specific locations, which may not accurately capture the wind conditions upwind of the vessel because the vessel and the WAPS both affect the wind at the anemometers.

Once the data has been pre-processed, it can be processed. Data from discrete ON-OFF tests are used to calculate the net change in forward force from each test so that the performance model can be validated.

Validation and prediction

After processing, the data is verified for the PPP, in this case Anemoi’s FSAM. The net change in forward force on the ship is calculated for comparison with the vessel performance model. The same parameters can then be used to calculate the net fuel savings on any voyage. Additional information needed for that calculation is the SFOC of the main engine and auxiliary generators, and the power consumption of the Rotor Sails.

Conclusion

Anemoi’s robust, practical and widely applicable methodology provides a flexible and relatively straightforward framework to verify WAPS performance. Calibration with real world data gives the method a high level of confidence. The model developed can be used throughout the life of the vessel, with data from the vessel, environment and WAPS operation used to generate accurate fuel savings. This can be done in close to real time or on a voyage-by-voyage basis.

The method developed by Anemoi and approved by Lloyd’s Register Advisory not only allows for WAPS verification in real-world operations, but also enables the creation of advanced predictive tools that can estimate the power and fuel savings of the devices in a wide range of conditions, based on actual data. In this way, Anemoi’s efforts to improve verification deliver value and confidence to operators that their investment in WAPS reflects a meaningful, measurable step to reduce fuel cost and emissions impact.