There are many ways in which fuel and emissions can be reduced in ship operation. One of them is by optimising the trim and draft of the vessel to achieve minimum fuel consumption. However, getting accurate data needed for trim optimisation is not easy, while being essential for its success. This is where CFD comes in. Read about how it is used by one of the largest shipping companies here.
Trim Optimisation using CFD
The main purpose of the method used to acquire trim optimisation data, is to relate propulsion power to draft, speed and static trim of the vessel. The method offers draft optimisation as a byproduct since fuel consumption can also be compared for various drafts in ballast condition. The differences between individual trims can be small from the hydrodynamic point of view, which is why a precise method is necessary to produce reliable data. Model tests present the most reliable way of acquiring data, while CFD offers similar accuracy, for a fraction of the cost.
Trim optimisation data is acquired by performing self-propulsion simulations at different sailing conditions. The sailing conditions are selected based on the operational profile of the vessel, to minimise the cost of the study. The sets of sailing conditions are further refined during the study itself, as the calculated data sheds light on powering trends, showing where to focus more to achieve higher savings.
Trim Optimisation for Tankers and Bulkers
Even though they generally have smaller installed power compared to faster ships such as container vessels and car carriers, tankers and bulkers can also benefit from trim and draft optimisation. The large difference in draft between the laden and ballast condition creates an opportunityfor optimising trim. Savings are usually larger in ballast condition, since there is more room to change the trim of the vessel, but also since draft can be changed as well. Draft optimisation can be a very effective way of reducing power requirements for these vessels.
As an example, below is a table showing savings we calculated for an Aframax tanker owned by Minerva Marine. Total yearly savings amount to 44.2 tons per vessel. With seven sister vessels, the total yearly savings are are ound 136 000 USD with current HFO prices. This translates into an ROI of only two months. Read the full article here.
Container Vessels and Car Carriers
Since they sail at higher speeds, container vessels and car carriers have larger installed power. This means that savings of fuel and emissions get larger in absolute terms, helping to justify the investment in trim optimisation using CFD. Adding on top of that the fact that these vessels often sail in intermediate drafts, explains most trim optimisation studies in the past have been conducted for these types of vessels. On the other hand, adjusting trim on these vessels demands more effort in planning cargo loading distribution.
In the image at the bottom of the page a trim optimisation map is shown for a car carrier. In extreme cases, certain trims achieve more than 20% of savings. This should be interpreted with a grain of salt however, since the same condition might be subject to operational limitations. Nonetheless it gives an indication of the potential of trim optimisation. You can read the case study of car carrier trim optimsation here.
In most cases the savings achieved using trim and draft optimisation fall somewhere between 0.5% and 3%. Savings depend heavily on the type of the vessel, but also on the operational constraints limiting the change of trim and draft. For ships that often sail in partially loaded condition such as container vessels and car carriers, the savings can be significantly higher. Ships that are especially well poised to benefit are the ones that often sail off-design speed and draft. This can be a consequence of applying slow steaming, a retrofit or similar
Limitations of Trim Optimisation Application
Operational restraints limit using trim optimisation for some loading conditions. Structural integrity of the hull, propeller submergence, maneuverability, and other factors need to be considered when changing the trim of the vessel. These limitations need to be observed before the trim optimisation is conducted. The trims, drafts and speeds considered in the study need to be selected so that their application in real ship operation is maximised, to optimise the costs of the study.
Optimising Trim in Waves
The simulations used for trim optimisation are typically performed in calm water conditions. It is generally accepted that otpimisation trends calculated in calm water will be reflected in ocean waves conditions. It is however useful to check whether that is true for a few conditions where the savings exist, to verify that the savings will also be present in realistic predominant sea conditions which the ship encounters. Have a look at a study done for an Aframax vessel on this link, and a video showing the comparison.