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Structured Training and Advanced Research in Marine Active Structures

Periodic Reporting for period 1 - STARMAS (Structured Training and Advanced Research in Marine Active Structures)

Reporting period: 2016-05-01 to 2018-04-30

Over recent years, ever more globalised trade connections and the developments in terms of world trade, energy markets, climate change concerns or security threats have stressed the importance of seaborne transport for the prosperity of Europe and its citizens. With over 80% of world trade being carried by sea, maritime transport remains the backbone of international trade. For the EU, the world’s most important exporter and the second biggest importer, shipping and related services are essential in helping European companies to compete globally.

Whereas all angular degrees of freedom (roll, pitch and yaw) are important for sea-keeping characteristics of any ship, roll motion is known to be the critical one. This is because ship roll is typically lightly damped and the restoring moment of the ship is small in the cross-plane in comparison with the other planes. As a result, excessive roll can occur under unfavourable or extreme sea conditions. This can lead to reduced effectiveness of the crew, damaged or lost cargo, limited operability of the on-board equipment, or even to catastrophic sea accidents which include capsizing of the ship and the loss of human lives.

Roll reduction devices include keels, fin stabilizers, rudders, gyro stabilizer, azimuthing propellers, and anti-roll tanks (ARTs). Among these devices anti-roll tanks have raised a considerable attention since they do not cause highly concentrated loads as, for example, gyro stabilisers, they are effective at low forward speeds.

In the traditional design of passive anti-roll tanks (ART), the energy associated with the tank fluid motion is dissipated. The research carried out within this project explores the possibility that this energy is instead harvested.

On the other hand, U-tube ARTs can be made active. With the active approach, the water can be transported from one side reservoir to the other by a pump located in the connecting conduit between the two reservoirs, according to a controller command. This could potentially enable generating a more effective anti-roll moment than that produced by passive ARTs.

Results of research carried out under the STARMAS project suggest that the maximisation of the ART harvested power corresponds to the minimisation of the mean kinetic energy of the ship roll. This is of particular technological importance since the ART tuning that minimises the roll motion also maximises the recuperated energy. Therefore prospective efficient ART energy harvesters would not be conflicting with efficient roll control.

Regarding the active ART technology, it was found that a reduction of the ART natural frequency results in a reduction of power injected into the system by the pump. The same roll control performance can be achieved with a lightly damped, low natural frequency ART as with the heavily damped ART having its natural frequency similar to that of the natural frequency of the ship. The former design however requires much less power to drive the pump.
With respect to the energy harvesting, roll control and survival probability of ships rolling at rough seas, a special attention has been paid to the determination of the wave-generated input power and how this power is distributed between a portion absorbed by the tank fluid motion and another portion dissipated by the ship roll. As part of the work carried out, it has been analytically determined how the natural frequency and the damping ratio of a U-tube ART should be tuned to maximise the power harvested by the tank turbine assuming a perfectly flat spectrum of the moment representing the excitation of the ship by waves. It has been found that such tuning also minimises the average kinetic energy of the ship roll, thus reducing the average roll velocity squared.

With respect to the ship roll control using active ARTs, the stability, performance and power flow of an active U-tube ART has been studied. A feedback control scheme has been considered in which a pump, located in the connecting channel of the ART, generates a pressure difference proportional to the ship roll rate via a constant gain. An equivalent mechanical model has been developed in order to provide a clear and physically transparent insight into the problem to wider community without a particular expertise in hydrodynamics. The stability analysis indicated that the ART tuned in a classical way does not perform well, and suffers from severe stability limitations. By reducing the ART natural frequency below the ship roll natural frequency, the feedback loop stability margins significantly improve. This also results in improved active control performance. In other words, the feedback loop implementing the same gain becomes more efficient in reducing the ship roll. The analysis of the power flow indicated that with a reduction of the ART natural frequency it is possible to mitigate unnecessary large power being injected into the system by the pump, only to be subsequently dissipated by the ART water damping.

The technical and scientific results of the STARMAS project have so far been disseminated through 6 conference and 5 published journal articles. The results of the work include numerical and analytical tools and guidelines. The results will be exploited for efficient design of anti-roll tanks for energy harvesting as well as recommendations for design of active anti-roll tanks with emphasis on the energy efficiency of such installations.
The work carried out advanced the state of the art since the power flow in both passive and active ART has been thoroughly examined and a clear guidelines for designing effective passive ART energy harvesters and active ARTs with low-power consumption have been provided.

According to the EU policy document: “Commission Communication - Strategic goals and recommendations for the EU’s maritime transport policy until 2018”, a major challenge is how to come up with new ship designs and equipment to improve safety and environmental performance. Targeted RTD initiatives should lead to new forms of design, advanced structures, materials, clean propulsion and energy-efficient solutions. For example, in the maritime transport sector there is wide scope for improving energy efficiency in ships, reducing their environmental impact, minimising the risks of accidents and providing better quality of life at sea.

As a result of the STARMAS action we are one step closer to achieving the strategic goals and recommendations. Advanced marine structures with improved safety, environmental performance and quality of life at sea have been investigated. Innovative active control concepts for improved sea-keeping and increased probability of survival at rough seas have proposed, and novel technologies for wave energy recuperation have been suggested.
Active anti-roll tank scheme
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Ship_and_tank_hydro_structure
Anti-roll tank CFD
Equivalent mechanical system
Summary image for publication
Diagram performance active vs passive