Investigation of ship anti-roll stabilizer systems

In this study, general design features of roll stabilizer systems which are used to reduce ship roll motion are investigated. In order to demonstrate the application of roll stabilizer technology a typical 50 m motoryacht is selected as case study and the the design features of the active fin stabilizer to give the necessary performance were predicted. The roll motion can cause severe problems in ship operations including damage to equipment or cargo, personnel or passenger injury, reduced efficiency of crew and passenger discomfort. In warships the weapon and sensor systems are adversely affected by the roll motion. Problems occur in aircraft / helicopter landings and departures and the ability of the warship as a weapon system is reduced. Many systems have been developed since the 1870s to reduce the problem of the roll motions with motor ships. The most well-known of these systems can be classified as follows: • Bilge Keel, • Fixed weight transfer • Anti-Roll tanks • Gyroscopic anti-roll system • Fin stabilizer systems Three different cross-sectional types are generally used in active fin stabilizers: • All movable fin • Fin with flap • Fin with trailing edge wedge In the first part of this thesis, anti-roll stabilizer systems which are developed in order to reduce the negative effect of the rolling motion on the marine vehicles are examined comparatively. The oldest and the simplest of the roll stabilizer systems are the bilge which are easy to manufature and simple to operate. However, bilge keels have limited effectiveness in reducing roll motions and need to be supported by other more sophisticated roll stabilizer systems. The most common roll stabilizer systems are; passive or active fin stabilizers, tank stabilizers, gyro stabilizers and rudder controlled stabilizers. Different types of vessels have different priorities depending on the type and purpose of use of the stabilizer system. The crucial element in passenger ships is that passengers are transported in a safe manner and that they are able to travel comfortably despite the adverse conditions of the sea. Modern cruise vessels are considered as floating luxury hotels and promise the passengers a comfortable voyage with minimum discomfort due to the roll motion. An important element in warship design is that weapon and sensor systems are not affected by environmental conditions and the warship may fulfill its mission in all kinds of weather conditions with maximum efficiency. In a luxurious mega yacht it is desirable that the guests are least affected by the roll motion due to adverse conditions of the sea. Because of their relatively smaller size, mega yachts may easily get affected by the waves which would result in dicomfort and even sickness of the owner and the guests. In order to prevent the negative effects of the roll motion useful solutions have been produced for many years. The predecessors of those who deal with this problem are William Froude (1874) [1] with free surface tanks and Frahm (1910) [3] with U-type Passive Tanks. The bilge keels were designed and used even before the 20th century. The use of fin systems began in the 1930s. The development of new roll stabilizer systems has shown that each system has their advantages and drawbacks. The stabilizer manufacturers in Europe and America continue to work on the design and production of more efficient stabilizers in their R & D laboratories and take patents for the new generation systems to be presented to the market. Among the passive systems, the most common equipment used in the past after the bilge keel is free surface tanks and U-Tube tanks. U-Tube tanks are more attractive because their free surfaces are reduced. Nowadays, active roll tanks are more preferred because they are more efficient and controlled than passive systems. However, the active tank stabilizers are more complex equipment and therefore have high costs. The choice of pumps on the system is very important in terms of efficiency of the active roll tanks and strong pumps are required to complete the transfer on time. Tank systems are used mainly in offshore support ships as specified at the beginning of the section, in cargo ships and fishing vessels according to need. In order to reduce the volume loss, the tank systems in the empty spaces separating the compartments of the container ships between the hatches of the container ships, the tanks under the hatch of the cargo ships and the unused areas in the warships may be preferred according to the relevant requirements. The rudder-controlled antiroll system, due to the advantages of not requiring additional equipment weight and not occupying extra volume within the vessel, has been done and has been carried out to improve this system, but the interest in the system has been reduced as the system did not work at low speeds and in zerospeed condition. It has been reported that the active fin stabilizers can be damaged by striking a ship as an equipment outside the ship, and that there is a danger that it may be able to tear the ship and sink the ship. Foldable type of active fin stabilizer systems have been developed in order to avoid such a situation. Thus, when not needed, the fin is folded into the storage box inside the vessel and does not create additional resistance to the vessel. Fin stabilizer profile should not be out of the water if the vessel is inclined, should be in the protected area in the interior of the bilge round form and placed in the vessel so as not to disrupt the flow of water to the propeller. Gyro-stabilizers are more successful in preventing roll movement than when sailing on a ship or when sailing at low speed. Since there are no mechanisms in the outer parts of the ship, there is no risk of damage to the stabilizers. Due to the fact that all the equipment is in the boat, it is possible to cruise in shallow waters with gyro stabilizer boats. Fin stabilizers have lower lifetime maintenance costs compared to gyro stabilizers. Gyro-stabilizers should generally be removed from the ship for maintenance activities such as bearing replacement. In addition, fin stabilizers have less weight and volume than gyro stabilizers. Fin stabilizers have a better navigational control performance, especially during head waves and cross waves where other stabilizer systems are forced and does not work. It is not possible to wait for suitable flywheel turn speed, such as gyro stabilizers, for 45-60 minutes, and improves roll movement as soon as the system is activated. Years of experience and practice have demonstrated that fin stabilizers are the most efficient equipment for reducing roll motion, particularly at higher speeds. Currently, there are many companies that manufacture fin stabilizers equipped with the latest technology for a wide range of vessels, including yacht industry, cruise ships, war ships and commercial freight ships. Although the fin stabilizers are costly to manufacture and operate, they are the most preferred stabilizer systems today. In order to demonstrate the application of roll stabilizers in ship design process, a 50 meter motoryacht is selected as case study. In order to reduce the roll motion to acceptable levels a fixed fin section based on NACA family was selected and the performance and cavitation properties of the fin stabilizer was estimated by using an empirical design procedure. In order to calculate the lift and drag forces generated by the fin stabilizer a commercial CFD software (star-ccm+) was used. The the lift and drag forces and coefficients of the fin profile were computed for a cruising speed of 12 knots and angles of attack of 0, 5, 10, 15, 20, 25 and 30 degrees. The results of the CFD analyses indicated that the maximum lifting power is achieved at angle of attack of 20 degrees and the fin stabilizers should be placed on the bilge turn at midship section with 20 degrees angle.