1.1. Introduction
An experiment is of paramount importance for the design and operation of naval and offshore structures. Especially, due to the impact of survivability of these structures on human life and safety. Two types of experiments are possible: on real objects and on reduced models. The experiments carried out on the real objects are the most valuable but often impossible to be implemented due to their complexity, costs or risks. Therefore, the physical model tests being carried out in hydromechanics laboratories, occupy a privileged position when predicting the properties of objects such as ships, oil rigs or wind turbines.
Fundamentally, the model tests carried out at model scale, allow to predict the properties of the naval and offshore, full scale objects, to improve the human safety and survivability of constructions.
Moreover, from a scientific and research point of view, a particularly important feature of physical model tests is the possibility of verification of the developed physical theories.
The naval and offshore objects, in their environmental conditions, are mostly affected by the wind and waves. The influence of waves is usually of far higher importance, than the influence of the wind, due to the amount of the wave energy as compared to the amount of the wind energy, resulting from the difference in water and air density. This influence can result in many undesirable phenomena, related to the objects, i.a.: flooding the deck, broaching of the propeller, dynamic load of the hull and equipment and transported goods, manoeuvrability deterioration and resistance increase. Mentioned phenomena can deteriorate facility’s economic performance and be unbearable for people located on these objects or even be destructive for the object and people.
Consequently, physical modelling of waves is an essential part of the process focus on Modelling of Environmental Conditions (MEC), specific to the working area of the naval or offshore object. The MEC process is carried out in hydromechanics laboratories worldwide, during model tests, that are called seakeeping tests which are performed in a reduced scale on the models of naval and offshore objects subjected to simulated marine conditions influence, i.a.: towed or free running ships (Fig. 1.1), anchored structures like oil rigs or bottom-mounted structures like wind turbines.
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Fig. 1.1. The example of seakeeping tests in a hydromechanics laboratory: free running model of a ship
For the needs of the seakeeping model tests, the waves are generated in a wave tanks equipped with wave makers. The movements of the wave maker actuators, generate waves in the wave tank. Two basic types of waves can be generated: regular and irregular. The regular wave is induced by monoharmonic movement of the wave maker actuator. The irregular wave is induced by multiharmonic movement of the wave maker actuator and is desired to contain the expected harmonics of Energy Spectral Density (ESD) to reflect the real environmental conditions in a model scale. The generated irregular waves are consistent with the States of Sea (SS) [2]
desired for seakeeping model tests scenarios.
Measurements carried out during the seakeeping model tests allow an experimental determination of full scale object properties, in order to improve the human and construction safety and sustainable development of naval architecture and offshore sectors.
1.2. Thesis and Objectives
The scientific objective of this doctoral dissertation is to solve the complex problem related to the generation of the waves on the water surface in a wave tank with a required accuracy, for the needs of the seakeeping model tests to improve survivability of naval and offshore constructions and therefore human safety.
Waves on the water surface in wave tank are generated as a result of the oscillatory movements of wave maker actuator. Unfortunately, there is no direct relationship between the ESD of the generated wave and ESD of movements of the wave maker actuator. This is due to hydromechanical phenomena, which complexity causes that hydromechanical models are not sufficiently general and robust. Finally, in order to obtain ESD of generated waves, that reflects the real environmental conditions with the required accuracy, it was necessary to manually and iteratively apply corrections to the input signal of the wave maker.
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Moreover, the widely used technique of wave profile measurement was based on the phenomenon of variable resistance or capacitance between two electrodes immersed into the water, while the resistance or capacitance depends on the depth of immersion. This common solution is a nuisance for the user, due to settlement that result in necessity of cleaning the electrodes and adjusting the amplifier before each use. In addition the probes immersed into the water were the source and receiver of hydromechanical disturbances, related to the measured waves.
Each of the hitherto way of generating and measuring the wave was a time-consuming, high-cost and non-automatic solution based on iteratively cycles of: preparation, generation, measurement, analysis and correction.
In the scope of this doctoral dissertation newer types of regulation of plant has been considered and the target one has been developed. The fuzzy-logic controller and the adaptation module have been implemented to a high performance embedded system with an intuitive computer application.
A new method and an ultra-sound device for a wave profile measurement have been invented for the needs of the developed system. The method and the device have been applied for a Polish patent [44] and, subsequently, for an European patent [45]. The method and device are based on contactless ultrasonic measurements and thus, it is non-invasive and maintenance-free.
The entire solution has been worked out and implemented in the hydromechanics laboratory in the CTO S.A. Maritime Advanced Research Centre. However, both the adaptive system and the ultra-sound device are a ready-made products that can be broadly implemented to others hydromechanics laboratories.
The implementation of the objective of the dissertation, solved the significant scientific and technical problem of MEC in hydromechanics laboratory during the model tests, carried out for needs of naval and offshore industries.
The novelty and the main contribution of the doctoral dissertation are:
development of a new complete control system of the wave maker for a real towing tank;
development of the fuzzy-logic controller to control the velocity and the position of the wave maker flap;
development of the non-invasive, contactless, and maintenance-free ultra-sound system for measurement of the wave profile;
development of the robust and sufficiently general method of adaptive wave control based on the wave spectrum-feedback;
consideration of modern and advanced electric drive of the wave maker.
The thesis of the doctoral dissertation is formulated as follows:
“It is possible to control the energy spectral density of the generated irregular waves, in an automatic manner with omission of complex and often inapplicable and not robust hydromechanical models, with use of the adaptive controller.”
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Whereby “the pragmatic attitude that an adaptive controller is a controller with adjustable parameters and a mechanism for adjusting the parameters” [K. J. Åström and B. Wittenmark, 2013, p. 1], was taken.
The obtained results validated this assumption and brought a number of additional benefits described along the dissertation.