HYDRODYNAMIC INTERACTION OF SIDE-BY-SIDE FLOATING BODIES

Ph.DDOCTOR OF PHILOSOPH

Stability of floating offshore structures

Stability of floating offshore structures at intact and damaged conditions is currently being addressed by adherence to prescriptive regulations, on the basis of qualities of the righting moment curves. However, as experience shows, a prescriptive set of regulations is not adequate, especially when dealing with larger structures or complicated configurations. Following relevant developments in the shipping industry and the recent Macondo Prospect (Deepwater Horizon) incident, it is considered appropriate to propose a probabilistic framework for the stability of offshore structures. This framework will take into consideration different damage cases, loading conditions and damage extents, and will account for their probability of occurrence and potential consequences. The framework would be developed on the basis of parametric use of first-principles tools and integrated analysis of dynamic behaviour of the said floating structure in a wind-wave environment and would make use of accident data. The considerations above are directly related to risk-based design. Existing regulatory requirements are discussed, and key areas for future development are proposed

Development of recommendations for digital testing of MASS navigation safety prior to sea trials

For Maritime Autonomous Surface Ships (MASS), a key area that has seen active development is in the use of autonomous capabilities for vessel navigation and control. This can range from a simple use case of waypoint navigation, which takes into account bathymetry and navigation markers, to complex collision avoidance scenarios where the autonomous systems are required to detect, evaluate and execute evasive manoeuvres based on time and spatially varying dynamic behaviour of other vessels. In Singapore, it has been identified that there is a need to carry out accurate digital testing of MASS navigation safety before sea trials. This is where a vessel developer is required to demonstrate that the MASS is able to carry out the sea trials safely, and to stress test high risk scenarios that may not be practicably tested in the sea trials. A study has been carried out to develop recommendations for the digital testing, which takes into consideration the need for accurate representation of the actual MASS being built, as well as the verification of the autonomous navigation algorithm’s capabilities to safely control the vessel in real-world scenarios. Based on the study, a three-stage framework is proposed. Firstly, the accuracy of the digital model in representing the dynamic responses of the actual vessel is verified and any discrepancy with benchmark data is to be quantified. Secondly, tests are carried out to ascertain that the autonomous navigation algorithm is able to control (virtually) the dynamically-accurate vessel from one point to another, taking into account the real-world environmental loads. Lastly, the ability of the autonomous navigation algorithm in carrying out collision detection and avoidance is verified. As part of the study, a review of the current state-of-art and engagement with the industry has been carried out. These details are described in this paper

Adaptive discretisation and dual-rate time-stepping of mooring cable dynamics

Catenary mooring lines experience touchdown and liftoff from the seabed as the applied fairlead tension varies with time. The numerical modelling of touchdown/liftoff effects requires a finer discretisation at the touchdown point to avoid the production of spurious line tension fluctuations. The disparity in element sizes within and outside the local refinement zone gives rise to a stiff dynamical system. As the touchdown point changes with time, the locally refined zone has to shift in tandem to limit the spatial extent of the refined domain. This work introduces an approach for applying adaptive discretisation to a numerical mooring cable model with a non-uniform mesh, and dual-rate time integration for the resultant stiff dynamic system.EDB (Economic Devt. Board, S’pore)Published versio

Historical data is useful for navigation planning : data driven route generation for autonomous ship

This work presents a method for automated generation of navigation plan for autonomous or robotic surface vessel. Historical Automatic Identification System (AIS) data is of significant value to this problem. The method joins AIS locations of a same vessel at different time and locations in a region into a route. Next, it automatically computes navigation plans using nearest neighbour based path retrieval relying on two representations, Ship Feature and Navigation Feature. Before starting service, existing AIS records in the form of ship properties and corresponding route are preprocessed and stored in the form of Ship and Navigation Feature. During online retrieval, given input constraints in vector form, nearest neighbour of this query vector in the same space is found and corresponding path of the neighbour is returned as recommended path. Analysis was done in four and two dimensional spaces for Ship and Navigation Feature respectively. Application of the method is demonstrated in two regions of Australian, covering Bass Strait and Great Australian Bight.Economic Development Board (EDB)Nanyang Technological UniversityThis project was carried out under funding from Economic Development Board of Singapore, Lloyd’s Register and Nanyang Technological University through Industrial Postgraduate Programm