New tool merges marine engineering and hydrodynamics in the towing tank
Hardware in the Loop (HIL) in the towing tank opens up a new direction of combined marine engineering and hydrodynamic research. HIL open water tests in fact allow to study the performance of any propulsion system in extreme environments.
A wide range of options is being considered to reduce the climate impact of marine diesels. These options vary greatly in terms of impact on ship design and supply chains. Yet, they have one thing in common: their potentially profound effect on the ship’s capability to deal with the dynamic load as caused by wind, waves, and manoeuvres. With this in mind, the Delft University of Technology has developed a new tool, merging marine engineering and hydrodynamics in a single experiment.
Over the past two centuries, cargo capacity and energy efficiency have been key drivers for maritime technological development. Ship propulsion, in particular, has been profoundly impacted by these developments. Until well into the 19th century, sea transport was mostly powered by wind. By the beginning of the 20th century, wind had largely given way to steam. Still, this new technology left much room for improvement, and fuel consumption became a point of interest. Eventually, steam was superseded by more efficient and compact diesel engines, which today power the vast majority of merchant ships.
Safety of future ships
Marine diesel engines are known for their favourable fuel economy: overall efficiencies of the largest two-stroke engines exceed 50 per cent. However, with growing concerns regarding greenhouse gas emissions and their effect on the climate, diesel engines are under increased scrutiny. Limits to installed engine power, different fuel compositions and new technologies such as fuel cells are all being investigated and they all impact the ship’s capability to deal with the dynamic load as caused by wind, waves, and manoeuvres.
This has raised concerns regarding the safety of future ships. The Energy Efficiency Design Index (EEDI), for example, was met with criticism when it was introduced by the International Maritime Organization (IMO). The EEDI aims to abate maritime emissions of CO2 by limiting installed engine power in newbuilds. However, there are concerns that the EEDI reduces power to unsafe levels, particularly in adverse weather. Similarly, there is much uncertainty regarding the dynamic capabilities of novel propulsion technologies.
Solid oxide fuel cells, for example, are considered for ship propulsion for reasons of efficiency and tolerance towards contaminated fuels. However, they are also known for their sluggish dynamic response to load variations. This, too, may limit performance in highly dynamic environments such as rough seas.
Combining simulated and hardware components
Before such technologies can be safely integrated in ship power and propulsion systems, their performance in marine environments must be investigated in detail. With this in mind, the department of Maritime and Transport Technology of Delft University of Technology (TU Delft) has developed a new tool, merging marine engineering and hydrodynamics in a single experiment. By introducing Hardware in the Loop (HIL) functionality into an open water test setup, the interaction between waves, propellers and machinery can be emulated at model scale whilst ensuring correctly scaled dynamic behaviour.
An open water test setup is a specialised tool to measure the static performance of scale model propellers in open water. The setup comprises an electric motor, a propeller and a sensor to measure propeller torque and thrust. HIL experiments, on the other hand, combine simulated and hardware components in a single real-time experiment, and are already common in other industries such as automotive.
In the HIL open water test developed by TU Delft, engine room machinery is simulated on a dedicated simulation computer, while the propeller and its environment are physically present; a schematic drawing of such a setup is shown in the figure below. On the simulation side, specialised processor boards were used, while a state-of-the-art open water setup was designed and manufactured by MARIN.
Schematic drawing of the HIL open water test, demonstrated by TU Delft.
By combining machinery simulations with complex hydrodynamics around the propeller, HIL open water tests allow to study the performance of a wide range of propulsion configurations in realistic conditions. These conditions range from regular wave fields to highly complex phenomena that are only partly understood, such as propeller ventilation events. As such, questions related to the safety, effectiveness and environmental friendliness of future propulsion systems can be answered on a scientific level.
However, there are some issues with HIL open water tests that have remained unmentioned so far. As with other experiments in the towing tank, there are viscous scale effects on propeller torque. Flow regimes are different at full scale and model scale, resulting in relatively high propeller torque at model scale, while thrust is relatively low. In addition, the drive system in the test setup is fundamentally different from the ship’s propulsion system.
With the electric drive, a dynamic system is introduced, which is not present at full scale. As such, it is a potential source of unwanted, dynamic behaviour. As another consequence, the HIL drive and ship propulsion system have different geometries. This implies that the moment of inertia and friction are different too, further distorting shaft dynamics.
These issues were analysed in detail, resulting in the insights and solutions necessary to conduct successful and correctly scaled HIL open water tests. Viscous scale effects, to begin with, mostly impact static propeller performance. Effects on dynamic performance, which is most relevant during the considered HIL tests, are found to be sufficiently small to be dismissed.
To tackle the remaining sources of unwanted dynamics, mathematical descriptions of the HIL setup were developed, taking into account the dynamic behaviour of the electric propulsion system, step sizes of the simulation models and sample frequencies of the sensors. This allowed to formulate tuning guidance for the electric propulsion system.
The mathematical analysis also resulted in a method to modify the inertia of the propulsion shaft, without making any physical changes to the system. This inertia correction, which could be described as a virtual flywheel, presents the setup with a high degree of flexibility.
Simulation and interface computers, motor drives and signal amplifiers required for the HIL open water tests, mounted on a towing tank carriage at TU Delft.
These solutions were put in practice in actual HIL open water tests, yielding promising results. To produce a proof of concept, experiments were conducted with a simulated diesel-mechanical propulsion system and a propeller provided by MARIN. Although these results could not be compared with full scale measurements, the close correspondence between experiments and simulations proves that HIL open water experiments can indeed accurately emulate the interaction between engine, propeller and waves.
New experiments become available
With this, a wide range of new experiments becomes available, shedding new light on the complex, dynamic interaction between load and drive. For example, HIL open water tests allow to study the performance of any type of propulsion system in extreme environments. However, this is only one example among many.
Topics such as advanced propulsion control strategies based on predicted incoming waves and oblique propeller inflow related to wind assisted propulsion can all benefit from HIL, while more fundamental research topics such as the entrained inertia between the propeller blades can also be studied. Moreover, the developed HIL functionality can be extended to free-sailing models, further increasing the potential of model scale experiments.
To demonstrate the added value of HIL, open water experiments were conducted with a ventilating propeller. In these experiments, the propeller was brought close to the surface and moved through different types of wave fields, causing the blades to pierce the surface and draw air in wave troughs. Measurements from two of these experiments are shown in the figure below.
Propeller thrust measured while the propeller moved through a wave train, drawing air in the deepest wave troughs. Both HIL experiments and traditional, constant speed experiments were conducted in these conditions. As can be seen, the interaction between the (simulated) diesel-mechanical propulsion system and the ventilating propeller results in a complex, dynamic breakdown of propeller thrust, which cannot be reproduced by traditional open water experiments. This complex interaction will be demonstrated in more detail in the doctoral dissertation by Lode Huijgens, co-author of this article.
As can be seen, the resulting interaction between rapidly changing propeller load and the propulsion system has a considerable effect on propeller thrust. Such interactions have a large, yet underexposed impact on ship manoeuvrability and seakeeping capabilities, as well as on machinery load and wear.
In summary, HIL in the towing tank opens up a new direction of combined marine engineering and hydrodynamic research. With this new tool in its inventory, TU Delft is ready, together with its partners in research and industry, to continue with groundbreaking research on safe and sustainable ship propulsion systems in the years to come.
As a closing note, the authors would like to extend their thanks to Jan de Boer and his team of specialists at MARIN, who managed to deliver a world-class piece of scientific equipment despite all complex requirements. For the experiments conducted at TU Delft, the efforts of Cornel Thill, Peter Poot and the other team members of the towing tank were invaluable.
Picture (top): The Hardware in the Loop (HIL) open water setup as it moves through the towing tank.
This article was published in SWZ|Maritime’s November 2020 issue. It was written by: