D Ranmuthugala

Reviewing seafarer assessment methods to determine the need for authentic assessment

The Standards of Training, Certification and Watchkeeping (STCW) Convention set global, minimum standards of competence for seafarers. Maritime Education and Training institutes (METs) are responsible for ensuring assessment processes not only assure attainment of STCW outcomes but also produce competent graduates that meet the expectations of core stakeholders such as regulators and employers. A review of literature in this area, however, suggests that some current assessment methods employed by METs are largely failing on both accounts. This paper argues that STCW falls short in its ability to provide appropriate standards and looks at the need for authentic assessment in seafarer training. A brief review of authentic assessment presented herein highlights characteristics that may improve the shortcomings of current assessment methods and STCW. The paper proposes authentic assessment as a way to elevate the collection of evidence of a seafarers competence using methods that promote student engagement and transfer of competence in different contextual scenarios.

Development and Control of a Low-Cost, Three-Thruster, Remotely Operated Underwater Vehicle

This paper presents the development of a low cost Remotely Operated Vehicle (ROV) which consists of open source hardware and has three thrusters. First, the hardware of the vehicle, including the actuators, sensors, and control structure, is described. Second, to study the relationship between the thrust forces and the performance of the ROV, a mathematical model of the vehicle in the form of a kinematic and kinetic model is established. Next, a hybrid control algorithm consisting of two components, namely model-based and PID algorithms, is proposed for surge speed, depth, and heading control. The effectiveness of the hybrid control algorithm is then verified by the ROV mathematical model-based simulations. Finally, free running tests for depth control are conducted to verify the robustness and reliability of the control structure and proposed algorithms.

Autonomous underwater vehicle motion response: a nonacoustic tool for blue water navigation

Autonomous underwater vehicles (AUVs) use secondary velocity over ground measurements to aid the Inertial Navigation System (INS) to avoid unbounded drift in the point-to-point navigation solution. When operating in deep open ocean (i.e., in blue water—beyond the frequency-specific instrument range), the velocity mea- surements are either based on water column velocities or completely unavailable. In such scenarios, the velocity-relative-to-water measurements from an acoustic Doppler current profiler (ADCP) are often used for INS aiding. ADCPs have a blank- ing distance (typically ranging between 0.5 and 5 m) in proximity to the device in which the flow velocity data are undetectable. Hence, water velocities used to aid the INS solution can be significantly different from that near the vehicle and are subjected to significant noise. Previously, the authors introduced a nonacoustic method to cal- culate the water velocity components of a turbulent water column within the ADCP dead zone using the AUV motion response (referred to as the WVAM method). The current study analyzes the feasibility of incorporating the WVAM method within the INS by investigating the accuracy of it at different turbulence levels of the water column. Findings of this work demonstrate that the threshold limits of the method can be improved in the nonlinear ranges (i.e., at low and high levels of energy); however, by estimating a more accurate representation of vehicle hydrodynamic coefficients, this method has proven robust in a range of tidally induced flow con- ditions. The WVAM method, in its current state, offers significant potential to make a key contribution to blue water navigation when integrated within the vehicle’s INS.

Wetdeck slamming loads on a developed catamaran hullform – experimental investigation

ABSTRACT Catamaran wetdeck slamming has been experimentally investigated using a servo hydraulic slam testing system. A series of controlled-speed water impacts was undertaken on a rigid catamaran bow section with two interchangeable centrebows. Entry into the body of water was at two fixed trim angles: 0° and 5°. The vertical velocity was varied from 3 to 5 m/s in 0.5 m/s increments. This study presents a new dataset of pressure distributions and slam forces on the arched wetdeck structure of catamaran vessels. The relationships between the peak force magnitudes, relative impact angle and vertical velocity are observed, with a small reduction in slam force for an amended centrebow. Limited pressure measurements along the archway were not found to be representative of wetdeck slamming loads.

Predictor-based model reference adaptive control of an unmanned underwater vehicle

Unmanned Underwater Vehicles (UUVs) are being deployed in advanced applications that require precise manoeuvring close to complex underwater structures such as oilrigs and subsea installations or moving objects such as ships and submarines. The effect of vehicles hydrodynamic parameter variations is significant in such scenarios and in extreme conditions the UUV may experience loss of control. In addition, external disturbances and actuator failures degrade the performance of the UUV. Adaptive control has been identified as a promising solution that can improve the performance in such situations. However, adaptive control is not widely used in UUVs mainly due to the trade-off between fast learning and smooth control signals. The latter can be guaranteed at low learning rates but require additional input to improve learning. The Predictor Model Reference Adaptive Control (PMRAC) is one such method that uses a prediction error to improve learning. In this paper, the performance of PMRAC in UUV applications is investigated and compared to standard Model Reference Adaptive Control (MRAC) at low learning rates under normal operational conditions, partial actuator failure, and under the influence of external disturbances. Simulation results show that PMRAC significantly reduces the tracking error compared to MRAC. In addition, PMRAC is less affected and recovers quickly from actuator failure and external disturbances, while generating smooth control signals with less oscillation compared to MRAC.

Computational fluid dynamics re-mesh method to generating hydrodynamic models for maneuvering simulation of two submerged bodies in relative motion

An Autonomous Underwater Vehicle (AUV) operating closer to a larger vessel experiences significant hydrodynamic forces requiring an adaptive control mechanism to maintain acceptable trajectory. It is therefore important that the designer understands the hydrodynamic characteristics of the vehicle in this scenario in order to develop appropriate control algorithms to deal with its dynamic behaviour. This requires developing simulations of the vehicles behaviour close to the larger vessel, the control algorithms, and the dynamic interface between the two. This paper presents a method to generate a complete hydrodynamic model of underwater vehicles using the Computational Fluid Dynamics (CFD) code ANSYS CFX, which can then be interfaced with the vehicles control algorithms within a simulation environment. The essential aspect of the method is the re-mesh approach, where the mesh deforms locally around the bodies using an Arbitrary Lagrangian-Eulerian form of the governing fluid equations and re-meshes when the deformation significantly compromises the quality of the mesh. This overcomes the motion limitations imposed by a pure deforming mesh approach. Preliminary work to validate the method is based on two smooth spheres moving relative to each other. It shows that this method is able to adequately simulate the fluid behaviour around the bodies. The paper also describes the future work focused on a 6 degrees-of-freedom (6-DOF) AUV modelled in CFD to obtain its hydrodynamic behaviour to be interfaced to the control system within MATLAB.

Threshold Capability Development in Intensive Mode Business Units.

Purpose – The purpose of this paper is to explore: student perceptions of threshold concepts and capabilities in postgraduate business education, and the potential impacts of intensive modes of teaching on student understanding of threshold concepts and development of threshold capabilities. Design/methodology/approach – The student experience of learning was studied in two business units: strategic management, and accounting. The method involved two phases. In the first, students and unit coordinators identified and justified potential threshold concepts and capabilities. In the second, themes were rationalized. Findings – Significantly more so in intensive mode, the opportunity to ask questions was reported by student participants to support their development of the nominated threshold capabilities. This and other factors reported by students to support their learning in intensive mode are consistent with supporting students to traverse the liminal space within the limited time available in intensive mo...

Command governor adaptive control for an unmanned underwater vehicle

This paper presents the design and simulation of a command governor based adaptive controller (CGAC) for a remotely operated underwater vehicle. The command governor modification is applied for the first time to an underwater vehicle simulation of the actual vehicle for improved transient performance and disturbance rejection. The vehicle dynamics are assumed to be decoupled thus allowing for the design of separate heading and depth controllers. The results show that in contrast to standard adaptive controllers, command governor based adaptive controllers are able to produce better transient performance as well as improve the overall response even at low learning rates. Furthermore, simulation results verify the disturbance rejection capability of the command governor based adaptive controller, which is necessary to effectively and safely operate an unmanned underwater vehicle in real environments.

Design, modelling and simulation of a remotely operated vehicle – Part 1

Continuing the previously published study [4], this paper focuses on hardware and Virtual Reality (VR) model development of a three-thruster Remotely Operated Vehicle (ROV). The paper included setting up an on-board electronic system with the associated suite of sensors and the required communication protocol. This system utilises a master-slave structure, which consists of an onshore station computer and an on-board open source microcontroller. To improve the controllability of the driving system, a VR model of the ROV is designed to reflect the altitude and attitude of the physical vehicle. By using the feedback signals from the sensors, the VR model operates in a similar manner to the actual vehicle. Hence, it provides the operator with the capability to monitor the ROV operation within a virtual environment and enables the operator to control the ROV based on the visual inputs and feedback. Finally, real time simulations are presented to validate the interaction between the ROV operator and the VR model. To provide realistic operational conditions, the effects of sensor noise and water current disturbances are included to the simulation programme. The results show that the performance of the VR ROV is stable even with these disturbances.

Influence of restricted water on the time domain interaction forces and moment on a berthed ship due to a passing ship

Abstract An investigation was conducted into the effect that different berthed ship bow and stern blockage arrangements has on the interaction forces and moment experienced by a berthed ship due to a passing ship. Physical scale model experiments were used to quantify the magnitude and form of the interaction forces and moments for five different blockage arrangements. The interaction force and moment traces from the experiments were compared to traces predicted using existing empirical formulae based on open water scenarios. Additionally, two methods were used to adjust idealised curves using the experimental results to better represent the form and magnitude of the interaction forces and moments. To demonstrate the effect that the changes in form and magnitude of the interaction forces and moments has on the predicted berthed ship motions the interaction forces and moment from the physical scale model experiments, empirical predictions and the adjusted idealised curves were extrapolated to represent a full scale ship and used as input to a commercially available time domain numerical simulation code to predict the berthed ship motions. The bow and stern blockage was shown to have a significant effect on both the form and magnitude of the interaction forces and moments, and hence, the predicted berthed ship motions. The empirically predicted interaction forces and moments correlated poorly with the experimental results.