Katrin Ellermann

The Motion of Floating Systems: Nonlinear Dynamics in Periodic and Random Waves

Floating systems, such as ships, barges, or semisubmersibles, show a dynamical behavior, which is determined by their internal structure and the operating conditions caused by external forces e.g., due to waves and wind. Due to the complexity of the system, which commonly includes coupling of multiple components or nonlinear restoring forces, the response can exhibit inherently nonlinear characteristics. In this paper different models of floating systems are considered. For the idealized case of purely harmonic forcing they all show nonlinear behavior such as subharmonic motion or different steady-state responses at constant operating conditions. The introduction of random disturbances leads to deviations from the idealized case, which may change the overall response significantly. Advantages and limitations of the different mathematical models and the applied numerical techniques are discussed.

Nonlinear Dynamics of Offshore Systems in Random Seas

This paper addresses the analysis of nonlinear stochastic dynamical systems. As example, the motion of moored floating structures is considered. Firstly, different steps of a gradually refined mechanical model are described, including the combination of linear and nonlinear components of a floating system. Secondly, techniques which allow for an approximation of the probability density function are briefly introduced and applied to the mechanical models developed before. Advantages and disadvantages of the techniques and limitations with respect to shape, the possible number of degrees of freedom or environmental conditions are discussed.

Research in offshore transfer cargo operations; challenges, developments, and new frontiers

Currently, offshore operations are considered activities with high impact on the economy, which standsin direct relation to the products of great importance and value for diverse economic sectors. Thus, itbecomes necessary to implement new technologies that make the manipulation of these products fasterand easier. In this work, the authors introduce the problem in offshore cargo transfer operations. Thisproblem involves different kinds of areas: logistics, dynamics, and control are some of them. The authorspresent an approach for the last two. In the dynamic problem is presented a study on the dynamics of asuspended load connected to a crane via a mechanism with two prismatic degrees of freedom. The studiesshow the complex large-amplitude motion of the load given the visibly nonlinear behavior of the ship.Therefore, the development of a fuzzy controller was necessary to decrease oscillations and position theload in one definitive point of interest. The work presents the test results, demonstrating that this typeof manipulator in combination with an effective control strategy allows for the reduction of oscillationsin offshore activities.

Analysis of offshore systems in random waves

This paper addresses the analysis of the motion of floating offshore systems in waves. The focus lies on different approaches for the determination of probability density functions. The analysis of the dynamical behavior of systems in ocean waves is an important part in offshore engineering. These structures involve components with very distinct nonlinear characteristics which affect the dynamics. The systematic analysis of the nonlinear dynamics of floating structures is generally facilitated by additional assumptions, such as waves modeled by periodic functions or linearizations of parts of the equations of motion. These assumptions make conclusions about the first passage time or upcrossing probabilities difficult or even impossible. This paper addresses different approaches for the description of random forces acting on floating structures and for the analysis of the resulting motion.

Statistical Investigation of the Nonlinear Behavior of Structures Operating Offshore

Due to the increasing amount of ship and offshore operations taking place in the ocean environment, the amount of reports concerning damage, loss of cargo or even loss of human lives increases, too. The reasons for these incidents have been a subject of intensive research for many years. Recently, the aim of some research concentrated on extreme wave phenomena such as ‘rogue waves’ or ‘the three sisters’. These phenomena are a severe danger, but they do not play a decisive role for most of the incidents happening at sea. More often, the reason for endangerment of structures in the environment is a fluid-structure-interaction that leads to unstable dynamical system behaviour. Including nonlinear effects in the calculations, through statistical investigation of structures in waves, it can be shown that critical behaviour of ships and structures can occur without the absolute necessity of heavy sea conditions or the occurrence of dangerous sea phenomena. The reason that many of these incidents do not lead to severe accidents lies in the experience of the master and his crew and their ability to change system parameters in a way that prevents the catastrophe. The aim of this paper is to show the current state of development of a program that uses a Monte-Carlo-Simulation technique based on a common panel-method for the creation of added masses and added damping with hydrodynamic coupling to predict the behaviour of the structure in several wave conditions.Copyright