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Antonis Chatzigeorgiou PHD at UNIWA

Antonis Chatzigeorgiou is a sname member and at 2014 he graduated from TEI Athens, today’s University Of West Attica. At the same year, he started working at Bureau Veritas. At 2015, he started his Master’s Degree in NTUA on Computational Mechanics and his bachelor’s thesis published, titled ‘’ Code development for the computational analysis of crack propagation in structures ‘’.

(https://www.fracturae.com/index.php/fis/article/view/2767).

Antoni started officially his PHD last November, and its title is: “Optimum design of Stiffened Panels under compressive loading using Non-Linear Analysis”.  In practice, CSRs describe how the ultimate strength of a stiffened panels can be calculated by a numerical method. It is also stated that one can calculate the above maximum strength, using non-linear finite elements, but there is no instruction on how such a calculation can be performed. Therefore, the main occupation of the doctorate is the creation of a methodology sufficiently scientifically proven, based on which one can calculate the ultimate strength of stiffened panel which is subject to compressive loads, using non- linear finite elements.

Last but no least, he has given a few lectures in courses of Mr. Theodoulidis, in the course of "Longitudinal Endurance of a Ship", as well as in the course "Application of Finite Elements in Shipbuilding.

Giannis Chalaris Bio – Founder of SNAME-UNIWA

Giannis is proud member of SNAME and the Founder of UNIWA SNAME Student Section. He graduated as a Naval Architect and Marine engineer from the University of West Attica in 2020 and currently continues his postgraduate studies (MSc) in Technical Ship Management in the University of Strathclyde. Simultaneously with his studies he was working in Bureau Veritas for two years in Plan Approval department and Bureau Veritas Solutions department as a naval architect. Last but not least, Giannis is a water polo athlete with honors as he was selected for the junior men’s National team in the past.

SNAME activity

Giannis is one of the most active students of SNAME and a great example to emulate for the younger students. He was the Founder of our Student Section in 2016 and the Chairperson until 2018. However, he still helps by giving advices to the younger members of the section. During his undergraduate years, Giannis was highly involved in every event of the SNAME Greek Section by volunteering and representing the University. Furthermore, he had the chance to travel in the United States of America and socializing with other members around the world for three years in a row, in SNAME Maritime Conventions. As a result of his hard work and passion, he received the prestigious SNAME Undergraduate Scholarship in 2018, for helping him with his studies.

Gravitational type launching

The launching of a ship is a major operation where the vessel is transferred from dry land, where it is being built, to the water. An operation that must be calculated correctly from all aspects or else the results of the launching may cause serious damage to the ship. The launch of a vessel nowadays, for the most part, takes place in docks. An important feature of the launching is that the vessel is not completed yet. Therefore, the ship’s weights must be revaluated to establish the displacement and the center of gravity position. These hydrostatic quantities are critical for the ship’s launching.

Longitudinal oiled slideway launching

In the more general case, the ship is launched down inclined ways and one end, usually the stern enters the water first, because it is more buoyant than the stem. This launching requires a wide-open water area that can fit the length of the ship and allows it to move for several meters away from the spot of the launching. If the area is restricted and the water resistance is not enough to stop the progress of the ship into the water, drag chains are used. They are arranged to stop the ship before it hits the far bank. Another method to stop the ship is by building wooden barriers over the propellers. This way the water resistance is increasing and the ship’s movement is impeded.

The ship is being supported outside the water to remain upright with the following

mechanism. Shores are used as columns placed at the ship’s keel, stem and stern to ensure that the ship remains upright and prevent any movement. Usually building blocks placed at the ship’s keel, are used to support the keel and most of the ship’s weight. The height of these blocks, must be adequate to provide space under the ship to enable the outer bottom and its fittings to be worked on, to facilitate the insertion of the launching cradle.

In order to have a successful launching, it is needed to plan the mechanism that will guide the ship safely into the water, the building slip. The ship is rested on this mechanism. The building slip’s floor has a slope to ensure that the ship slides down to enter the water. On top of the floor a solid structure of blocks, the groundways, is pinned on the ground on either side of the building blocks. The grounding ways must have a slope that will enable the ship to overcome the initial sliding. In the later stages of the launch the slope must be adequate to overcome the resistance of the grease and the water resistance. The top of the groundways is being greased. Above the groundways are instated the sliding ways, on which

there are rested cradles that are built to support and guide the ship on its way down the slipway. The pressure exerted on the grease depends on the area that the sliding ways sit in.

A relatively small area for the load of the ship, may cause the grease to be squeezed out from between the sliding ways and the grounding ways. Launch cradles are the transmitters of the ship’s weight loads, between the hull and the sliding ways. The cradles nearest the stem and the stern are the fore and after poppets.

Before the launching starts the building blocks and the shores are removed. This causes the weight to transmit to the cradles therefore to the slipping way which will slide on the grounding ways due to the effects of the grease, as soon as the trigger that keeps the ship stationary is released. The ship follows the curve of the ways. As the stern enters the water the buoyancy is increasing and a moment tending to lift the stern is created. When the moment of buoyancy about the fore poppet exceeds that of the weight, the stern lifts. If the slipway is long enough the vessel finally floats off. If the ways are not long enough for this the fore foot will drop off the end. The water’s depth at the end of the ways must be enough to allow this to happen without the fore foot striking the bottom.

The calculations of the naval architects in this operation are very critical and are related with the hydrostatic quantities of the ship especially the weight and center of gravity position. An assessment must be made of the weight and centre of gravity position at the time of launch. As the ship enters the water the waterline at various distances down the ways can be noted on the profile. From the Bonjean curves the immersed sectional areas can be read off and the buoyancy and its longitudinal centre are calculated. The ship will continue until the moment of the weight about the fore poppet equals to the moment of buoyancy about the same position. That is when the stern is lifted. At that point the force on the fore poppet is very large and stability can be critical. The load they then carry may be about 20 per cent of the total weight. The maximum force on the fore poppet will be the difference between the ship’s weight and the buoyancy of the immersed portion of the ship at the moment the ship’s stern is lifted. The ship becomes fully waterborne when the buoyancy equals the weight. The moment of buoyancy about the after end of the ways must be greater than the moment of weight, the least distance between these two moments about the after end of ways, gives the least moment against tipping about the end of ways.

The data are usually presented as a series of curves called the launching curves. This set of curves will enable the user to predict the behavior of the launch in a safe condition.

By Konstantinos Tsourekas