Numerical Modelling and Global Response Assessment of Floating Docks towards Efficient, Safer and Autonomous Docking Operations

Abstract

Floating docks are known for their construction efficiency and operational flexibility compared to traditional graving docks. They play an important role in shipyards by serving as essential platforms for vessel construction, maintenance, and repair. Docking a vessel relies on precise ballasting and de-ballasting operations for achieving the desired floating position of the floating dock. Traditionally, these tasks are manually performed by skilled dock masters who regulate ballast valves and pumps. The entire vessel-docking operation takes hours, and the motions of the floating dock and vessel are slowly and steadily. However, the floating dock and vessel are still facing safety challenges during operations. According to the reported accidents occurring in floating dock operations, malfunctions of the ballast water system, overloading and improper ballast control are the main threats to the stability and structural integrity of the floating docks. To address these concerns and enhance operational safety, a thorough response assessment of vessel-docking operations is important. This thesis focuses on developing an in-house code to facilitate a comprehensive global response assessment of a full-scale floating dock, aiming to enhance overall operational safety and efficiency. The in-house code is developed under a quasi-static assumption and enables dynamic, stability and global structural response assessments of various types of floating dock operations. Multiple numerical tools are incorporated into this code. Various loads applied to the floating dock and vessel are determined using the numerical tools: a hydrostatic force model, a hydrodynamic force model, a mooring force model, and a contact force model. Within the load calculations, the dock-vessel coupling loads are highlighted, including contact loads between the docking blocks and the docked vessel and the loads attributed to the mooring ropes between the dock and vessel. A six-degree-of-freedom (6-DOF) model is developed to determine the motions of the dock and vessel based on the obtained loads. In the 6-DOF model, the dock and vessel are represented as rigid bodies with six degrees of freedom. The ballast piping network of the floating dock is modelled in a hydraulic model for the ballast water system. The flow rates into or out of all ballast tanks are computed for updating the ballast water volumes due to ballast water adjustment. Furthermore, a modified proportional controller (P-controller) is introduced to achieve automatic ballast water control, regulating opening angles of ballast tank valves to minimize roll and pitch motions during vessel-docking operations. The developed numerical tools are verified against theoretical models and various commercial software. They are also validated through experimental tests on a model-scale floating dock. The motions and loads obtained from the dynamic analysis of floating dock operations are important inputs for stability and structural response assessments. The curves of metacentric height and righting arm are obtained using dock motions, hydrostatic loads, and the coordinates of the dock’s centre of gravity (CoG) and centre of buoyancy (CoB). For structural response assessment, a bending model is proposed to evaluate the global bending deformation of the floating dock based on the applied loads obtained from the dynamic analysis. This deformation is also fed back to the dynamic analysis to update the applied loads and motions of the floating dock and vessel. The proposed numerical tools have practical applications in the floating dock’s design, maintenance and operations. The dynamic processes of gravitational ballasting for the maintenance of a floating dock are investigated. Effective tank valve status arrangements are designed for lifting different parts of the floating dock out of water for inspection. Simulations of ballasting and de-ballasting operations for a single floating dock demonstrate the reliable performance of the automatic ballast control algorithm in minimizing the roll and pitch of the dock. Moreover, the control performance of the proposed automatic ballast control algorithm is examined during de-ballasting operations with a malfunctioning pump. The importance of using mitigation measures and smart ballast control strategy is highlighted. Finally, the vessel-docking operations considering the automatic ballast control and global deflection of the dock are studied. The dock’s motions and deflections are computed using two methods: one-way and two-way couplings between the dock’s deformation and motions. The maximum roll and pitch angles obtained using the two methods are close to each other, maintained below 0.13deg and 0.04deg, respectively, which indicating a robust control performance of the proposed automatic ballast control algorithm.