Yoo Sang Choo

Localization in underwater sensor networks: survey and challenges

In underwater sensor networks (UWSNs), determining the location of every sensor is important and the process of estimating the location of each node in a sensor network is known as localization. While various localization algorithms have been proposed for terrestrial sensor networks, there are relatively few localization schemes for UWSNs. The characteristics of underwater sensor networks are fundamentally different from that of terrestrial networks. Underwater acoustic channels are characterized by harsh physical layer environments with stringent bandwidth limitations. The variable speed of sound and the long propagation delays under water pose a unique set of challenges for localization in UWSN. This paper explores the different localization algorithms that are relevant to underwater sensor networks, and the challenges in meeting the requirements posed by emerging applications for such networks, e.g. offshore engineering.

Robust adaptive boundary control of a flexible marine riser with vessel dynamics

In this paper, boundary control of a flexible marine riser with vessel dynamics is developed to reduce the risers vibrations. To provide an accurate and concise representation of the risers dynamic behavior, the distributed parameter model of the flexible marine riser with vessel dynamics is described by a partial differential equation (PDE) coupled with ordinary differential equations (ODEs) involving functions of space and time. Boundary control is developed at the top boundary of the riser based on Lyapunovs direct method to regulate the risers vibrations. With the proposed boundary control, uniform boundedness is achieved. The boundary control is implementable with actual instrumentation since all the signals in the control can be measured by sensors or calculated by a backwards difference algorithm. Simulations are provided to illustrate the effectiveness of the proposed control.

Robust Adaptive Position Mooring Control for Marine Vessels

In this paper, robust adaptive control with dynamic control allocation is proposed for the positioning of marine vessels equipped with a thruster assisted mooring system, in the presence of parametric uncertainties, unknown disturbances and input nonlinearities. Using neural network approximation and variable structure based techniques in combination with backstepping and Lyapunov synthesis, the positioning control is developed to handle the uncertainties, input saturation and dead-zone characteristics of the mooring lines and thrusters. Full state feedback with all states measurable and output feedback using high gain observer to estimate unmeasurable states are considered. Dynamic control allocation is presented for actuation of the position mooring system. Under the proposed robust adaptive control, semi-global uniform boundedness of the closed-loop signals are guaranteed. Numerical simulations are carried out to show the effectiveness of the proposed control.

Boundary Control of a Coupled Nonlinear Flexible Marine Riser

In this paper, boundary control for a coupled nonlinear flexible marine riser with two actuators in transverse and longitudinal directions is developed to reduce the risers vibrations. The dynamic behavior of the flexible riser is represented by a distributed parameter system with partial differential equations (PDEs) and the control is applied at the top boundary of the riser based on Lyapunovs direct method to regulate the risers vibrations. With the proposed boundary control, uniform boundedness under ocean current disturbance, and exponential stability under free vibration condition can be achieved. The proposed control is independent of system parameters, which ensures the robustness of the system to variations in parameters. Numerical simulations for demonstrating the effectiveness of the proposed control are presented.

Neural network tracking control of ocean surface vessels with input saturation

In this paper, robust adaptive tracking control is proposed for ocean surface vessels based on neural network. In the tracking control design, parametric uncertainties, unknown disturbances and input saturation are explicitly considered. Using neural network (NN) approximation and backstepping control techniques, full state feedback control and output feedback control are investigated to tackle system uncertainties and control input saturation. An auxiliary design system is presented to analyze the effect of input saturation, and states of auxiliary design system are utilized to develop the tracking control. Under the both of developed tracking control approaches, semi-global uniform boundedness of all closed-loop signals are guaranteed via Lyapunov analysis. Simulation studies are given to illustrate the effectiveness of the proposed tracking control.

Advanced analysis for the assessment of steel frames in fire

Abstract This paper describes the methodology of an advanced analysis technique for studying the large-displacement inelastic behaviour of building frames subjected to localised fire. The main feature of the proposed analysis is its use of one element per member to model each structural component and thereby obtain a realistic representation of material and geometrical nonlinear behaviour of the overall framework. Both ISO fire and natural fires can be simulated by the analysis model. The transient heat transfer is calculated by using a refined finite element mesh, whereas the structural responses are calculated with a nonlinear finite element technique based on an elasto-plastic beam–column formulation. Numerical studies are carried out on portal frames and multi-storey frames exposed to natural fires. The computed results are compared and contrasted with those from the conventional approach based on ISO standard fire, so that the advantage of using advanced analysis for direct assessment of the performance of steel structures in fire can be highlighted. Effects of fire load and ventilation resulting in localised fires in building frames are studied. The design implication of fire spreading in the storey of a building in which cooling and heating are taking place simultaneously is discussed.

Dynamics and control of mechanical systems in offshore engineering

Preliminaries.- Dynamic Load Positioning.- Coupled Nonlinear Flexible Marine Riser.- Flexible Marine Riser with Vessel Dynamics.- Riser System with a Torque Actuator.- Marine Installation System.- Riser Installation System.- Mooring System.

Static strength of thick-walled CHS X-joints—Part I. New approach in strength definition

This paper presents a new approach in the definition of joint strength for thick-walled circular hollow section (CHS) X-joint subjected to brace axial loading. This is based on the plastic load approach for limit analysis of pressure vessel components proposed by Gerdeen. A consistent coefficient λ associated with the plastic load approach has been determined, and the corresponding joint strength for X-joint under brace axial loads is found to correlate closely with the results obtained using Lu’s deformation limit. The strength prediction for thick-walled CHS X-joint using the present approach is found to be in good agreement with design recommendations by ISO and CIDECT for joints with chord radius-to-thickness ratio γ>7. For X-joints with γ<7, significant difference in results is observed especially with the ISO recommendation for joints with brace–chord diameter ratio β≥0.7.

Characteristics of photopolymeric material used in rapid prototypes Part II. Mechanical properties at post-cured state

Abstract The curability and mechanical properties of an acrylic-based photopolymer (namely De Solite SCR-300) used in building rapid prototypes have been studied. A differential scanning calorimeter (DSC) was used to monitor the degree of curing of the resin after laser scanning. It was observed that the curability of the resin in the stereolithography process was a function of layer pitch and laser exposure density. Tensile tests were conducted on multi-layered specimens generated under different layer pitch and laser exposure density. It was found that the elastic modulus, ultimate tensile strength and elongation to fracture of the specimens were also functions of laser exposure density and layer pitch. Morphological tests using a scanning electron microscope (SEM) showed changes in the cross-sectional profile of a multi-layer model with laser exposure density and layer pitch. By increasing the laser exposure density and decreasing the layer pitch, the curing percentage of the resin can be increased, leading to a prototype built with better mechanical strength.

Control of Coupled Vessel, Crane, Cable, and Payload Dynamics for Subsea Installation Operations

In this paper, the coupled dynamics and control of the vessel, crane, flexible cable and payload under environmental disturbances with attached thrusters for subsea installation operations is investigated. For the practical system with physical constraints, we employ Barrier Lyapunov Function in the design of positioning control for the flexible crane-cable-payload subsystem to ensure that the constraints are not violated. Uniform stability of the flexible subsystem is shown and asymptotic positioning of the boundaries is achieved. Next, we tackle the scenario where nonuniformity of the cable, uncertainties and environmental disturbances are considered. Boundary controls are formulated using the nonlinear partial differential equation of the cable. Numerical simulations are provided to illustrate the performance of the proposed controls.