Analysis of Hull Shape Impact on Energy Consumption in an Electric Port Tugboat
Department of Ship Automation, Gdynia Maritime University, Morska St. 83, 81-225 Gdynia, Poland
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Energies 2022, 15(1), 339; https://doi.org/10.3390/en15010339
Received: 26 November 2021 / Revised: 24 December 2021 / Accepted: 31 December 2021 / Published: 4 January 2022
(This article belongs to the Special Issue Modeling, Control and Optimization of Electric Vehicles and Ship Electric Power Systems)
Abstract
The trend to replace internal combustion engines with electric zero-emission drives, visible in the automotive industry, also exists in the shipbuilding industry. In contrary to land vehicles, the requirements for the electric propulsion system of tugs are much greater, which combined with the limited space and energy on board, makes any amount of energy valuable. Strategic changes in the policy of many countries, such as the “Fit for 55” package, introduce plans to significantly reduce CO2 emissions, which favors the development of alternative drives and their introduction to new areas of operation. This article presents that it is possible to reduce the amount of energy an electric tug spends for movement by applying the Particle Swarm Optimization method to modify the shape of its hull. A statistical analysis of public data was performed to determine the speed profiles of actual port tugs. The Van Oortmerssen method was used to determine the hull resistances of the proposed tug and the impact of the hull shape modification sets on reducing these resistances. Based on the six obtained speed profiles, it was determined that one of the tested variants of modifications made it possible to reduce energy consumption on average by 2.12%, to even 3.87% for one of the profiles, and that some modifications increase energy consumption by even 6.59%.
1. Introduction
Port tugs are small, special-purpose ships (width approximately 10–12 m, length approximately 24–32 m). Their task is to assist larger vessels during maneuvers, when precise and reliable control over the position of the assisted vessel is essential.
Examples of situations where the assistance of tugs is necessary include entering and leaving the port, berthing and unberthing from the quay, changing the berth in the port or turning the ship around in place. Port tugs are also at the disposal of ship captains in situations where a ship equipped with devices enabling independent execution of the above-mentioned maneuvers is, for some reason, unable to perform them independently, or there is a risk that it will not be able to execute them safely. Such situations may include, for example, too strong a wind force or an extremely unfavorable wind direction; partial or total failures of the ship’s equipment; specific requirements of the cargo carried by the ship (e.g., increased possibility of explosion or fire); and finally, when the captain has any doubts regarding the planned maneuver, e.g., has insufficient experience.
Contrary to other types of tugs, such as seagoing or river tugs, port tugs are designed for short-term operations, usually not lasting more than a few hours, and having a range of activity limited to the harbor basin and its immediate vicinity.
A characteristic feature of the tug is its high power to hull size ratio. In order to perform maneuvers, a tug needs to apply large tractive force, which requires a powerful propulsion system. In order to generate the required force, the tug uses at least one propeller, but in the vast majority of cases, it has at least two propellers or thrusters.
Until recently, the only practical source of mechanical energy capable of moving a tug’s propellers was the diesel engine. Recent advances in the technology of electric drive and constantly developed battery energy storage allow for wider replacement of the classic diesel drive with a drive based on electric motors, whether in the form of a hybrid propulsion [1,2,3] or fully electric drive [4,5,6,7].
In hybrid systems, a combination of diesel engines and electric motors is used, allowing them to work in such a way that they complement each other’s capabilities. The advantage of the electric drive is high efficiency in a wide load range, while its disadvantage is operating time, limited by the size of energy storage. In turn, the diesel engine can operate for a long time, due to the much higher volumetric and mass energy density of diesel fuel. Unfortunately, when contrasted with an electric drive, the efficiency of diesel engines is low, especially with light loads. The use of electric motors during low-load operation allows for extending the downtime of diesel engines, which are only started in periods when high propulsive power is required [8].
Fully electric drive allows infrastructure associated with diesel engines and the engines themselves to be eliminated from the deck of the ship. In an all-electric ship, the following components are redundant: fuel tanks, lubricating oil tanks, massive cooling installations and mechanical transmission systems such as reduction gears and drive shafts. In their place, it becomes possible to install an electric energy storage, which for maintenance purposes requires less space around it. This allows the use of smaller passages than in a conventional engine room and improves the mass distribution by placing the energy store at the lowest possible point of the hull.
The integration of the electric drive may also facilitate the use of the tug in an autonomous, unmanned mode, which may speed up the response time and allow the anti-collision algorithms to work effectively [9,10].
Global trends force the development of clean, eco technologies that do not rely on energy obtained from conventional fossil fuels. Taking into account economic aspects, it is beneficial to exploit such energy sources, which is not necessarily favorable to the natural environment [11]. When designing modern means of transport, which electric tugs certainly are, it is possible to select their operational parameters so that the energy spent for operation is used as efficiently as possible. It is an important way of reducing the Total Cost of Ownership (TCO).
One of the tools used when designing a new vessel are the EEDI—Energy Efficiency Design Index, and the EEOI—Energy Efficiency Operational Indicator. The obligation to use the EEDI applies only to vessels with a tonnage greater than 400 GT, i.e., only the largest tugboats [12].
Optimizing the configuration of the ship’s propulsion system is an issue widely discussed in the literature due to the multitude of possible implementation options. Paper [1] presents an impact analysis of converting two tugs from conventional propulsion to hybrid propulsion with a serial structure. Several variants of the implementation of the energy storage have been presented, including a variant with a removable electrical energy store. Depending on the variant, a reduction in fuel consumption of between 6% and in excess of 50% was achieved, together with a reduction of NOx emissions of up to 91%.
The effect of the type of current in the electricity distribution system (Alternating Current or Direct Current) on energy consumption was analyzed in [13]. It has been shown that it is possible to reduce fuel consumption by 5–15% by using a DC system compared to a traditional distribution system using AC power.
Optimization of the PMS system operation using evolutionary algorithms in a hybrid drive system with a parallel structure was the subject of research in [14]. The effect was a reduction of energy consumption in a hybrid river barge by an average of 4.5%. In [15], genetic algorithms were used for multi-variant optimization of the hybrid drive system on an AHTS unit. As a result, two configuration variants were obtained, one that reduces GHG emissions and significantly reduces initial capital expenditure, while the other reduces both the GHG emissions and fuel consumption.
Optimization of the components of the drive system is justified only in the case of designing new vessels or a thorough refit of existing ones. Paper [16] presents how the selection of hybrid drive components using the bi-level nested optimization method can enable a long-term reduction of operating costs by more than 35%, while allowing the reduction of engine size by more than 20% and engine operation time by up to 80%. In turn, the authors of [17] conducted an analysis in economic terms concerning various configurations of the propulsion system in the process of designing a tug. The result of their work was the selection of a favorable configuration of three medium-power engines, which minimizes the risk of failure capable of disabling the vessel, and limits its CO2 emissions and fuel consumption.
When starting the process of designing the structure of a new tug, its load characteristics should be considered, which will then allow determination of the total energy demand during typical tasks performed during the service.
The classic tug propulsion uses stern propellers, often in conjunction with devices that improve their efficiency and low speed towing, such as the Kort Nozzle. The invention of improved kinds of propulsion, such as azimuth thrusters, allowed for two important changes. A free rotation of each thruster in the horizontal plane by 360° became possible, so that the force exerted by its propeller could be directed in any direction. Secondly, it became possible to move some propellers from the stern to other places, e.g., to the midship or to the bow. This allowed the distance between the thrusters to be increased, which made it possible to exert higher torsional moments on the hull of the tug, contributing to increased maneuverability of the vessel.
Currently, both tugs with a classic propulsion system (Figure 1, left) and those with improved maneuverability are used. A typical group of such tugs are the azimuth stern drive (ASD) class tugs, using a pair of azimuth thrusters located at the stern (Figure 1, middle). These tugboats are more maneuverable than tugs with classic propulsion. Even greater maneuverability is offered by tugs with azimuth thrusters located not only at the stern, such as the Rotor Tug manufactured by Damen Shipyards. This type of tug has three azimuth thrusters, two of which are located near the bow and one is located at the stern (Figure 1, right).