Metacentric height

About: Metacentric height is a research topic. Over the lifetime, 219 publications have been published within this topic receiving 1505 citations.

Deterministic analysis of extreme roll motions and subsequent evaluation of capsizing risk

Abstract: Encountering a freak wave remains one of the most horrible visions for a ship master. Until now the mechanisms which underlie large rolling and subsequent capsizing due to a rogue wave are only partly disclosed. In the framework of the German research project ROLL-S which is funded by the German Federal Ministry of Research and Education (BMBF) nine partners are cooperating on the investigation of large rolling and capsizing mechanisms covering fields like numerical analysis and simulation, validation, and evaluation of capsizing risk. The project part presented here deals with the validation of numerical tools for the analysis of large rolling and capsizing. The main goal is the implementation of a sophisticated capsizing test procedure. For providing useful data for the analysis of the capsizing process as well as for the validation of numerical methods, exact correlation of wave excitation and ship rolling is indispensable. All wave trains are tailored for the specific purpose of each capsizing test and generated in the model tank. Unusual wave trains like regular waves followed by a freak wave and special wave groups within a defined random sea as well as realistic wave scenarios have been generated. The parameters of the model seas are systematically varied to investigate the ship model response with regard to metacentric height, model velocity, and course angle for different ship types. The wave elevation at the position of the ship model in time and space is calculated (and controlled by registrations during model tests) in order to relate wave excitation to the resulting roll motion.

On the Occurrence of Mathieu Instabilities of Vertical Cylinders

Abstract: Large offshore platforms with vertical circular cylindrical hull shapes have been designed and employed in recent times. Classical spar platforms and mono-column concepts are just two examples of designs with such simple configuration, supposed to perform limited motion responses in waves. Reports on investigations on the occurrence of parametric resonance of spar platforms have been published recently in which the relevance of Mathieu amplifications have been assessed making use of different mathematical models. However, some uncertainties still remain on the influence of crucial design parameters as, for instance, metacentric height, draft/diameter ratio, associated damping and mooring system. In an attempt to clarify some of these aspects, in this paper the dynamic stability of a vertical cylinder in regular waves is investigated theoretically and experimentally. A coupled non-linear mathematical model is employed to model and simulate the coupled heave, roll and pitch motions. Theoretical aspects related to the development of resonant motions are discussed. In addition to the numerical and theoretical investigations, an extensive series of experiments with a model of a typical mono-column have been recently conducted at LabOceano. The findings of these investigations are compared and summarized.Copyright © 2008 by ASME

Pitch motion problem induced by dynamic positioning system for new sandglass-type floating body

Abstract: In this paper, a new concept of sandglass-type floating body, which can overcome the performance limitations of traditional ship-type and cylindrical FDPSO (floating drilling production storage and offloading), is proposed as research subject. In the general design of dynamic positioning systems, it is adequate to deal with a three-degree of freedom problem in the horizontal plane. However, the new floating body with small water-plane area and low metacentric height may cause an unintentional coupling phenomenon between the pitch and surge motions by the thruster system. Therefore, first by numerical boundary element method (BEM) based on wave potential theory and experiments, the pitch motion characteristic of the sandglass-type model is studied and meanwhile the numerical method of this paper is validated. Furthermore, two dynamics models with and without consideration for the additional pitch moment by the thruster systems are introduced and numerically simulated, which can show the problem of pitch motion induced by the positioning thrusters. Then by the mass-spring-damper systems with two-degree of freedom, the influence of additional pitch moment by the thrusters on the pitch and surge motion performances is theoretically analyzed. Finally, based on the essential reason of pitch motion problem, a control law considering pitch inertia effect has been used and proven to be effective to decrease the pitch motion response.

System and method for monitoring stability of a vessel

Abstract: An automated stability system which is accurate but simple enough to be implemented on small vessels such as fishing boats is provided. It provides this by integrating the measurements of a digital magnetometer, digital accelerometer, and digital gyroscope which are used to calculate the natural roll period of the vessel which in turn permits calculation of the GM (metacentric height). GPS may also be provided to provide for time and velocity correction.

Parametric roll mitigation using rudder control

Abstract: Severe roll angles can be developed by parametric excitation in relatively moderate weather without any apparent pre-warning for the crew onboard. In this study the prospect of using rudder control to mitigate parametric roll was investigated using multi-degree of freedom simulations. A typical modern Pure Car and Truck Carrier was considered and modelled by coupling a roll model with a planar motion manoeuvring model. The combined model was calibrated using in-service, full-scale trials and model tests. Irregular variations of the metacentric height were applied to simulate recorded, full-scale events of parametric roll that have occurred with the considered design. These simulations with rudder roll control showed promising results and demonstrate that the approach could be very efficient for mitigation of parametric roll.