Mathias Paschen

Incremental Gain Scheduling and Sensitivity-Based Control for Underactuated Ships

Abstract Maneuvering of ships in difficult situations is one of the main tasks in maritime and inland navigation. To increase the operational capacity in water transport as well as to improve its reliability and safety, new control strategies are required. Besides the optimization of parameters of existing control approaches, it is necessary to develop new techniques which are applicable to robust velocity control and path following. For that purpose, novel techniques for incremental gain scheduling in nonlinear control systems as well as for sensitivity-based feedforward control are summarized in this paper. Numerical simulation results are presented to highlight the applicability of the presented approaches in a basic control structure for path and trajectory following with and without underlying velocity control. Possible application scenarios of the presented techniques include the automatized navigation of ships, the avoidance of collisions by evading oncoming vessels, the assistance of captains during both steering in critical route segments and piloting of ships into ports, as well as offline path planning.

Assessment of Transient Sloshing Due to Encounter of an LNG Carrier With a Steep Wave

This study investigates a transient sloshing flow in a rectangular tank by comparing model tests and unsteady Reynolds-Averaged Navier-Stokes CFD simulation. The sloshing flow is excited by a normally periodic surge motion, which is then subjected to a transient akin to an LNG carrier encountering a particularly steep wave. It is found that the impact pressure recorded in the transient motion is several times greater than that observed at resonance. High speed video data and CFD simulation is used to analyse the sloshing flow. The severity is explained by the absence of entrained air bubbles prior to impact and the second mode response one half period before impact.

Fluid-Structure Interactions in Pelagic Trawls and Probable Consequences for the Selectivity of the Fishing Gear

To study the phenomenon of fluid-net-structure interaction of a trawl the authors investigated effects of hydro-elasticity of stiff net panels (grids) by means of special experiments in a wind tunnel. The analyses were especially focused on how the grids affect the current. One of the targets was to observe the correlation between the actual distribution of velocity distant and close to the grid and the respective hydrodynamic load distribution namely under consideration of prevailing geometrical and kinematical parameters. Another aim of the tests was to get an idea of the fluid-dynamic interactions between neighbouring mesh bars which is implicitly expressed in the well known nomographic diagram by Koritzky (1973). And last but not least the third target was to analyse the impact of the grid on the fluid parameters in correlation with further investigations regarding the understanding of the phenomenon of selectivity of fishing gears. In context of these objectives the hydrodynamic loads as well as the current distributions close to and far from the grid surface were measured by a six-component balance, by hot-wire anemometers as well as by a 2d-PIV-system. The tests were focused on small angles of attack. To control the motion of fluid two end plates and a base plate were used. The consequences of a fluid blockage at the end of a grid were analysed. Results are presented and discussed.

Sloshing in LNG Tanks: Assessment of High and Low Pressures

In the assessment of 2D numerical simulations of liquid sloshing in partially filled membrane-type cargo tanks of LNG Carriers high pressure impacts and also particularly low pressures have been identified. One high pressure impact at low filling levels and one sloshing impact leading to particularly low pressures at high filling levels are studied in greater detail. The results of these investigations are presented and conclusions are drawn with respect to the underlying physics of the two phenomena.

Investigation of a Cup Anemometer in Oblique Flow for Measurement of Wind Velocity in Offshore Installations in the Field of Aquaculture

In Germany the combination of environmentally friendly changed wind energy with aquaculture is seen as an opportunity for multiple use of natural marine resources. Loads and stresses of “open ocean” offshore installations depend on wind velocity. For measurements of wind velocity within the field of wind energy conversion the only cost effective device is the cup anemometer. It has a linear relation between number of revolutions and velocity. Questionable is the measuring result if the wind vector is not within the ‘measuring plane’. Results of CFD calculations of the flow around an anemometer cup are given. With the force coefficients (lift and drag coefficients) an anemometer in inclined flow is calculated. It will be shown that the measured velocity within the measuring plane is higher than the cosine component of velocity.© 2007 ASME

Fluid Structure Interactions at Towed Fishing Gears

From the point of view of mechanics, trawls are considered as extremely flexible and partly extensible rope and net structures which are exposed to flow. Form and loads of such gears mainly depend on the corresponding velocity of inflow and also on the so-called rigging elements that are required for the horizontal and vertical spreading of the fishing gear. At the same time the fishing gear is acting on the surrounding fluid. These reactions can on the one hand lead to unsteady states in the fishing gear. On the other hand changes of pressure and velocity can be detected by the fish and can possibly influence the selectivity of the fishing gear. This lecture is focused on the presentation of special numerical and experimental methods both for calculating large net systems and for analysing the reactions of the structure to the fluid.© 2004 ASME

Prediction of the initial movement of objects on the sea floor

The presence of a huge amount unexploded ordnance devices (UXO) on the sea floor is one of the major problems during installation and maintenance of offshore wind farms and other offshore structures. To identify all targets, time and cost consuming campaigns are necessary. The clearance of UXO is often limited by time due to the idea that UXO can migrate on the sea floor. Therefore the UXO-measure has to be revisited in some cases before installation or repair. Thus it might be possible, that additional UXO-measures are necessary before starting any operations touching the seabed. Insofar a better understanding of the migration of objects on the sea floor may help to improve currently used methods of time-dependent clearance by more knowledge-based decisions. The aim of this study is to investigate the requirements of initial movement of objects on the sea floor. This has been done by the analysis of literature, previous investigation to scour and burial of such objects, wind-tunnel experiments as well as experiments in a water channel. Additionally, numerical simulations allowed comparisons, combinations and a generalization of experimental results. A new mathematical model developed and validated allows a prediction of the incident fluid velocity that is necessary for an inertial motion of defined cylindrical and spherical objects. This model allows a reliable prediction of the initial migration of objects on a sandy sea floor. It is based on physical parameters, which depend on predictable and measurable events like currents, tides and indirectly on the weather conditions.

Hydrodynamic Loads on Twisted Ropes and Hawsers

The loads on both a model of a smooth cable and a helical strake (also known as scrouton spiral) were measured in a wind tunnel. The coefficients of drag, lift and transverse force were ascertained for an angle of attack between 0 and 360 degrees. Results show that the drag coefficient increases by the spiral in each case whereas the lift coefficient changes only insignificantly. A defined transverse force is caused by the spiral. To find an explanation of this force by the circulation a simple mathematical model is given. Finally the behaviour of a twisted rope in water calculated by a known code is predicted in case of steady current.Copyright

Study of Compression Soft Porous Foam Materials

The paper presents the findings of the research into neoprene-like soft foamed materials during compression in water. The specific features of the internal structure of such materials lead to complex deformations. This is related to the specific features of the internal structure of materials that contain a large amount of inert air. The paper also presents the findings of structural studies which explained the relationship between the elastic properties of materials and the strength of polymer bonds forming internal air cavities. When foamed the polymer sections are destroyed under compression, it results in the loss of enclosed volume of air voids. This changes the total volume and thickness of the material, which defines some physical and thermal properties of products made of such material. Hydrostatic pressure environments have their own specific features. Rheological properties of soft polymers in a hydrostatic pressure environment give rise to a composite effect of compression deformation. The study of recent developments in the research into polymer deformations in a hydrostatic pressure environment shows that it is difficult to record the stages of reversible and nonreversible compression deformation in near-real experimental simulation of diving operations. The paper presents the developments and findings of experimental design studies for a product (wetsuit) made of foamed materials that were conducted in a hydrostatic pressure environment in an enclosed volume using special Drucktank equipment by the Marine Engineering Department of the University of Rostock

Investigations on the Fluid-Structure Interactions of Fishing Nets

Modern nets consist of meshes made of threads or twines with spirals or helical strakes. Fluid-structure interactions have been investigated in Rostock for a long time applying different theoretical models. Because of great net flexibility there is a need of calculation methods which consider the main physical qualities. This is done by the approximation of wake of threads by results from circular cylinders and influence of circulation, which is known from measurements of transverse force. Results of measurements with two models with and without spirals are given for comparison.Copyright