Towards automatic trim optimisation
As advances in control systems and computational power offer the prospect of constant automatic trim optimisation, Kyma marine engineer and University of Cantabria postgraduate student Carlos Gonzalez explores how the process could help operators further reduce fuel costs.
Shipping companies go to great lengths to operate their ships in the most efficient way. To be competitive, low operation costs are essential. And with fuel one of the biggest costs, any improvement in fuel consumption can have a major impact on operational expense. Among many fuel-saving measures, one of the most fundamental methods is optimising a vessel’s trim to any load condition.
Research from various parties has found that by sailing under optimal trim conditions, vessels can reduce fuel consumption by between 2-5%, with a corresponding reduction in atmospheric emissions. Many classification societies and vessel monitoring system providers already offer trim optimisation services – examples include Eniram’s Dynamic Trimming Assistant, Force Technology’s Sea Trim, Kyma's KSP Trim Module and DNV GL’s ECO Assistant.
Major shipping companies are also investigating ways to predict and sail with optimum trim. Mitsui OSK Lines (MOL) is just one example – the company reports that it is engaged in joint research towards developing a system that will calculate optimum trim from only small amounts of vessel data.
What is trim?
Trim is defined as the difference between the draft aft and the draft forward. Optimal trim is the trimming condition at which a vessel, at constant displacement, can travel at the desired speed using the least power possible. Alternatively, it can be defined as the trim condition that permit the ships reach the maximum speed keeping constant the shaft power. To find that condition, crew must position weight onboard in the correct way.
Any modification to trimming condition directly affects the resistance acting on a ship, by modifying the area of the hull under water. This a major influence on the residual coefficient. commonly known as “wave resistance”, and has big impact on the ship’s advance.
When the vessel is trimmed at bow – the normal situation at laden condition - the reduction on the residual resistance coefficient is very significant. This is because the optimal trim will allow the bulbous bow to work properly. On other hand, it must be taken into account that at ballast condition, the normal best trim is astern, to avoid problems with propellers such as cavitation.
As trimming affects ship resistance, it influences hull efficiency due to the effect of resistance on thrust deduction and wake deduction. The increase of wake fraction for bow trims can be up to 20% and the decrease for stern trims can be up to 10%. The differences in wake fraction can therefore change the power demand by up to 5%, according to a study by Force Technology.
There are two different trim conditions to be considered. Static trim is the condition obtained at port, when the ship is berthed. In this condition, trim is easy to measure directly, as the difference between draft forward and draft astern. There is no influence by ship motions, sea current or hull deflections.
Under dynamic trim, the conditions shift due to external factors that affect to the ship stability. Hydrodynamic forces (squat, propeller thrust and manoeuvring rudder angles, for example) and the impact of weather conditions have a very big influence on the ship.
Predicting optimal trim
There are two methods for predicting optimal trim. One is based on very complex computational calculations and sensors readings, while other systems use the data recorded onboard to obtain the optimal trim empirically.
Computational Fluid Dynamics (CFD), an alternative to model tests in water, is the fastest developing area in marine fluid dynamics, using Raymonds Averaged Napier Stokes (RANS) equations. Based on the first principle of mass and momentum conservation, CFD-RANS methods provide versatile and increasingly fast solutions. This method is dedicated to the prediction of the steady turbulent flow around ship hulls, allowing the calculation of the best trim condition that minimises the frictional resistance on the ship.
There are several companies offering software with this mathematical method as the base of their systems. In addition, the software can interface with sensors (giving measurement of the dynamic-static vessel trim) showing excellent results in order to ensure the best trimming, and offering a real-time tool for officers to keep the best trimming during sailing.
The empirical method is based on real trials, data recording and empirical evaluation of the results obtained during the trials. The method consist in running several trials at same mean draft condition (cargo condition), keeping power constant and varying the trim condition from maximum astern to maximum ahead condition. For each trim condition the speed reached (at constant power) is recorded, and the results are plotted and by mean of an iteration method. It is then calculated which trim condition allows the vessel to reach the maximum speed for the power and mean draft set level. As with CFD-RANS, there are companies that offer software to help the crew to get the optimum trim based upon automatically recorded data.
Using modern advanced software and sensors technologies, ships could be fitted with systems that allow the predicting of optimal trim conditions easily. Such systems will help the crew to sail in the best trimming condition without any extra work.
The trimming on ships is done by means of the ballast tanks. By displacing the water from one tanks to others, it is possible to vary the trim. As the fields of IT, software and electric engineering are advancing rapidly, organisations are beginning to explore the possibility of combining CFD-RANS based trim prediction with controls on ballast water systems. The result, if it is achieved, would be a single system that ensures the ship sails under optimal trim condition, all the time and automatically.
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