Sebastian Küchler

Real-time estimation of a ship's attitude

During subsea lifting operations in harsh sea conditions, the involved crane system is subjected to extensive dynamic forces due to vertical vessel motion. Thus, active heave compensation systems can be used to compensate for vertical vessel motion and to reduce forces acting on the crane structure. Furthermore, such systems allow an exact positioning of the load on the seabed. However, active heave compensation systems always require knowledge about the vertical position of the crane depending on the ships heave, roll, and pitch motion. Hence, an attitude estimation method for ships during subsea lifting operations is proposed. To estimate the roll and pitch motion of a vessel with high accuracy, rotation rate sensors are fused with accelerometers using an Extended Kalman Filter. Since an exact knowledge of the yaw motion is not required to determine the cranes vertical motion, the yaw angle is stabilized around zero with an additional virtual sensor signal. The attitude estimation algorithm is evaluated with simulation and measurement results from an experimental setup.

Heave Motion Estimation of a Vessel Using Acceleration Measurements

Abstract Ocean waves continuously disturb every vessel resulting in horizontal and vertical motions. Especially the vertical motion has a significant effect on certain marine applications like subsea lifting operations. Subsea lifts are required for underwater installations on the seabed. Since such operations are normally performed from vessels using offshore cranes, the vertical vessel motion results in excessive dynamic loads acting on the crane structure. Furthermore, an accurate positioning of the load on the seabed is nearly impossible during harsh sea conditions. To avoid these problems active heave compensation systems can be used. These systems actively compensate for the vessels vertical motion and therefore reduce the dynamic loads acting on the crane structure and enable precise positioning of the load. However, active heave compensation systems always require the knowledge about a vessels heave motion. This article presents an observer based method to estimate the heave motion of a vessel from accelerometer signals without requiring any vessel specific parameters. The observer model is formulated by a sum of periodic components that approximate the heave motion of a vessel. The parameters of these components are identified online. The identified model is used with an extended Kalman filter to estimate the heave motion with high accuracy. The proposed method is evaluated with simulation and measurement results from an experimental setup.

Nonlinear control of an active heave compensation system with time-delay

Crane systems used during offshore installations in harsh sea conditions must satisfy rigorous requirements in terms of safety and efficiency. The vertical vessel motion which is due to ocean waves excites dynamic forces which have an extensive effect on the overall crane structure and its lifetime. Furthermore, the operator gets handicapped during fine positioning of the payload. Thus, compensation systems for the vessel motion during offshore operations become more and more important in the near future. This paper presents such an active compensation system. The proposed system controls directly the hydraulic driven winch of an offshore crane to compensate the vertical vessel motion and to achieve trajectory tracking of the payload in an earth fixed coordinate frame. The control algorithm is divided into two parts. The first part is used to decouple the loads motion from the one of the vessel. The second part is designed for trajectory tracking and stabilization. It takes the time-delay between the winch and the payload, which is due to very long rope lengths used in deep water lifting operations, into account. The active heave compensation approach is evaluated with simulation results.

New22Ne(α,n)25Mg-resonances at very low energies relevant for the astrophysical s-process

The22Ne(α,n)25Mgreaction is thought to be one of the main neutron sources for the astrophysical s-process. Thus cross section data for this reaction have been obtained from near the threshold (Eα≈570keV) up toEα=2100 keV using the 4 MV DYNAMITRON accelerator at Stuttgart, the windowless gastarget system RHINOCEROS and a 4π neutron detector. Two new resonances have been observed atEα=623 ± 6 and 838 ± 6 kev, which dominate the reaction rate at T9<0.3.Possible background reactions are discussed.

Modeling and Boundary Control of a Hanging Cable Immersed in Water

A nonlinear distributed parameter system model governing the motion of a cable with an attached payload immersed in water is derived. The payload is subject to a drag force due to a constant water stream velocity. Such a system is found, for example, in deep sea oil exploration, where a crane mounted on a ship is used for construction and thus positioning of underwater parts of an offshore drilling platform. The equations of motion are linearized, resulting in two coupled, one-dimensional wave equations with spatially varying coefficients and dynamic boundary conditions of second order in time. The wave equations model the normal and tangential displacements of cable elements, respectively. A two degree of freedom controller is designed for this system with a Dirichlet input at the boundary opposite to the payload. A feedforward controller is designed by inverting the system using a Taylor-series, which is then truncated. The coupling is ignored for the feedback design, allowing for a separate design for each direction of motion. Transformations are introduced, in order to transform the system into a cascade of a partial differential equation (PDE) and an ordinary differential equation (ODE), and PDE backstepping is applied. Closed-loop stability is proven. This is supported by simulation results for different cable lengths and payload masses. These simulations also illustrate the performance of the feedforward controller.

Vibration damping for a hydraulic driven luffing cylinder at a boom crane using feedforward control

Oscillations in the cylinder force of a boom crane effect the high-cycle fatigue of the cylinder and hence decrease the life-time of the whole crane structure resulting in higher maintenance. Thus it is desired to reduce the stress cycles in the cylinder force due to unwanted dynamics in the hydraulic driven luffing cylinder of the boom crane. To suppress the oscillations in the cylinder force due to the hydraulic eigendynamics and excited through motions of the cranes boom two different control approaches are proposed and compared in the paper. It is required that the controller only consists of feedforward terms, since feedback controllers depend on sensor signals which must fulfill certain security requirements in industrial applications resulting in higher costs. The control approaches are a Finite Impulse Response (FIR) filter suppressing one nominal frequency and a flatness based feedforward controller inverting the system dynamics. Both control approaches are evaluated and compared with simulation and measurement results obtained at a real harbor mobile crane of Liebherr.

Beobachtergestützte Prognose der Vertikalbewegung eines Schiffes

Zusammenfassung Im Bereich der Meerestechnik ist die Kompensation von vertikalen Schiffsbewegungen eine allgemein bekannte Regelungsaufgabe. Ein Ansatz für solche Kompensationssysteme ist die Verwendung von Trajektorienfolgereglern. Zur Online-Planung der benötigten Trajektorien ist eine Kurzzeitprognose der vertikalen Schiffsbewegung von Vorteil, da hiermit Antriebsbeschränkungen schon vorab in der Planung berücksichtigt werden können. Dieser Beitrag zeigt eine beobachtergestützte Methode für eine Kurzzeitprognose auf. Abstract Compensation of vertical vessel motion is a common control task in the field of ocean engineering. Such compensation systems can be realized with trajectory tracking controllers. To plan the trajectories online, a short-time prediction of the vertical vessel motion is advantageously, since actuator constraints can be taken into account more efficiently. This article proposes an observer-based method for a short-time prediction.