Robust adaptive preview control design for autonomous carrier landing of F/A-18 aircraft

Purpose The purpose of this paper is to design an innovative autonomous carrier landing system (ACLS) using novel robust adaptive preview control (RAPC) method, which can assure safe and successful autonomous carrier landing under the influence of airwake disturbance and irregular deck motion. To design a deck motion predictor based on an unscented Kalman filter (UKF), which predicts the touchdown point, very precisely. Design/methodology/approach An ACLS is comprising a UKF based deck motion predictor, a previewable glide path module and a control system. The previewable information is augmented with the system and then latitude and longitudinal controllers are designed based on the preview control scheme, in which the robust adaptive feedback and feedforward gain’s laws are obtained through Lyapunov stability theorem and linear matrix inequality approach, guarantying the closed-loop system’s asymptotic stability. Findings The autonomous carrier landing problem is solved by proposing robust ACLS, which is validated through numerical simulation in presence of sea disturbance and time-varying external disturbances. Practical implications The ACLS is designed considering the practical aspects of the application, presenting superior performance with extended robustness. Originality/value The novel RAPC, relative motion-based guidance system and deck motion compensation mechanism are developed and presented, never been implemented for autonomous carrier landing operations.

Dynamic Analysis on Critical Responses of Pipeline and Cable During Pipeline End Termination Installation

In order to figure out the dynamic characteristics of the pipeline and cable during pipeline end termination (PLET) installation based on S-laying, numerical simulation is carried out based on a practical operation project performed at Liwan oil and gas fields in the South China Sea. Four scenarios are selected from the PLET installation process in sequence for simulation. Critical responses of the pipeline and the cable in different scenarios of the operation are analyzed in this paper with a coupled model using RIFLEX module of SIMA software. Both the pipeline and the cable are modeled by the finite element method, and the pipelaying vessel is controlled by a dynamic positioning system. The simulation results are validated by the commonly used OrcaFlex software. The critical responses analyzed include static configuration, time-domain variation of axial tension at the top of the cable and bending moment variation near the touchdown point (TDP) of the pipeline. Furthermore, the time-domain variation of the tension at the top of the cable under different wave and current directions are also compared and analyzed, in order to study the effect of sea environment on the pipeline and cable during PLET installation operation. The results show that the responses of pipeline and cable vary in different operation scenarios, and the sea environment has remarkable effect on the pipeline and cable. The study in this paper is of value to the design of PLET installation based on pipelaying and can help predict the response of pipeline and cable during the operation.

An experimental study on vortex-induced motion responses of a moored semi-submersible with and without riser

This article presents an experimental study on the “vortex-induced motion” responses of a moored “semi-submersible without catenary riser” and “semi-submersible with catenary riser.” An eight-point catenary mooring system is adopted for the study in which eight rollers are installed on the pontoons for guiding the mooring lines and the semi-submersible design is of two rectangular pontoon and two circular columns on each of the pontoons. To replicate the real-world scene, the experiments are conducted in wave-cum-current flume with five directions of current (i.e. 0°, 30°, 45°, 60°, and 90°) to study the vortex-induced motion under the surge and sway degrees of freedom at high dimensionless reduced velocities (i.e. 6< Urs<50.86). Along with the surge and sway degrees of motion, the roll and pitch degrees of motion are investigated too with their paths for semi-submersible (i.e. amplitude traces). A “stainless steel flexible braided hose” with an aspect ratio of 157 till the touchdown point is used as a catenary riser. Results are presented with the definitions of degrees of freedom with respect to the axes defined at the center of gravity of semi-submersible and show that above the reduced velocity of 25 for the sway and surge degrees of freedom, the vortex-induced motion responses show a complex variation without any monotonicity. Furthermore, it is observed that (1) in traces there are no eight-shaped trajectories; (2) for both the semi-submersible without catenary riser and semi-submersible with catenary riser, the higher responses occur along the diagonals; (3) there are couplings of the pitch with surge, and roll with sway dofs; (4) the catenary riser reduces the vortex-induced motion responses for most of the current directions; and (5) galloping instabilities may be occurring and higher amplitudes may be related to them. Finally, it is shown that the vortex-induced motion response of semi-submersible reduces the vortex-induced vibration response of riser and that the motions in pitch and roll are likely to become critical and they demand a detailed analysis to be addressed in future to improve the operational range of semi-submersible and catenary riser in deep water depths under harsh marine environment of high current velocities.

Finite Element Analysis of Seafloor-SCR Interaction in Touchdown Zone

The touch down zone (TDZ) of steel catenary riser (SCR) is subject to fatigue damage. As a significant fatigue indicator, the variation of the stress range largely depends on the instantaneous changing bending moment. In this paper, a non-linear interaction model between seafloor and SCR is developed to consider the formation of trench at TDZ and its effect on the variation of bending moment. The seafloor soil is modeled by the nonlinear springs, whose stiffness is calculated according to nonlinear load-deflection relationship, such that the effect of cyclic loading can be simulated. The modeling of trench evolution and bending moment variation near the touchdown point (TDP) under cyclic loading are further conducted using the finite element software ABAQUS. The results show that the trench shaping is related to the uplift displacement of SCR and its number of cycles. The dependence of the bending moment on the displacement cycles near TDP is also investigated.

An Experimental Study of Mooring Line Damping and Snap Load in Shallow Water

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water, where snap loading may occur more frequently. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used, and the tension force at the fairlead was measured by tension gages. Moreover, the line geometry, touchdown point speed, and mooring line velocity were derived from image processing techniques. Surge motions at fairlead were driven from a programmable wavemaker. Regular surge motions with different frequencies and pretensions were tested in this system in order to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, the mooring line keeps a typical catenary shape, and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed, and it cyclically goes through the stage of semitaut and fully taut. We successfully reproduced a snap event in the laboratory scale, and the resulting mooring line damping can considerably increase in this manner. Two criteria for snap event were examined, and both of them were verified by the experiments.

Pilot Performance Using Head-up and Head-down Synthetic-Vision Displays for SA CAT I Approaches

Thirteen two-person crews flew 39 approaches each to runway 34R at Seattle-Tacoma airport in a 737-800 level D flight simulator using both head-up and head-down synthetic-vision displays. The within-subject design manipulated display type, runway-lighting configuration, ambient illumination, and two combinations of decision height and runway visual range associated with Special Authorization Category I and II approaches. Data collected/analyzed included training trials, instrument landing system tracking, touchdown point and vertical velocity at touchdown, and pilot opinions on the adequacy of the display formats for the specific task. Results indicated that there was a small but reliable difference in touchdown point between day and night approaches (longer in day) and that distance from centerline was also slightly affected by display type. Mean vertical velocity at touchdown was also affected slightly. Data indicated that there appeared to be no operationally significant differences as a function of display type or other variables examined.

On the dynamics of a thin viscous film spreading between a permeable horizontal plate and an elastic sheet

The two-dimensional dynamics of a thin film of viscous fluid spreading between a permeable horizontal plate and an overlying thin elastic sheet is explored. We use a lubrication model to describe the balance between the elastic stress, the hydrostatic pressure gradient and the viscous resistance of the flow, as fluid spreads laterally from a source and simultaneously drains through the plate. A family of asymptotic solutions are described in which the flow is dominated by either the hydrostatic pressure gradient or the elastic stress associated with the deformation of the sheet. In these solutions, although the deformation of the sheet above the porous plate arises from the fluid flow below the sheet, the fluid typically separates from the sheet a short distance upstream of the full extent of the draining zone, with the region of flow being driven purely by the hydrostatic pressure gradient. As a result, an air gap develops below the sheet up to the point where it touches back down onto the plate. With a very light or stiff elastic sheet, this touchdown point may extend far beyond the fluid draining zone, but otherwise it is similar to the extent of the draining zone.