A Numerical Study on Berthing Problem between Two Floating Bodies in Waves

Abstract

This thesis considers the berthing problem between two floating bodies in waves. A numerical method is developed for solving the berthing problem in the time domain based on the potential flow. The wave-induced motion and force responses during the berthing process are investigated by the developed method. Berthing involves a moving vessel closely approaching a fixed or moored offshore structure from a far distance. The theoretical formulation for the berthing problem is suggested with respect to the earth-fixed coordinate system because the relative positions of the two vessels continuously change. The linearized boundary value problems are derived by applying perturbation series expansion method. Acceleration potential method is introduced to evaluate the hydrodynamic forces that act on the bodies during berthing. The finite element method is used to solve the Laplace equation in the fluid domain with appropriate boundary conditions. The weak formulation of the governing equation is obtained by introducing the test functions and applying integration by parts. The fluid domain is discretized using a finite number of elements, and the linearized free-surface boundary conditions are integrated in time by applying 4th-order Adams-Bashforth-Moulton method. To satisfy the radiation conditions numerically, damping zone technique is utilized. A new re-mesh algorithm is developed for efficient updating of the mesh considering the horizontal movement of the vessel. The concept of local and global meshing is employed in the algorithm, and the re-mesh process is replaced by a simple connection operation. The developed numerical method is used to investigate the berthing problem, with particular consideration of two benchmark berthing problems. The first is berthing between two rectangular barges, which are assumed to comprise a transportation barge and an installation barge. Under various regular wave conditions, a series of berthing simulations are conducted by the developed numerical method, with focus on the hydrodynamic forces that act on the barges during the berthing process. The effects of the wave frequency and heading on the hydrodynamic force are discussed in this thesis. To validate present numerical results, model tests were conducted in the Ocean Engineering Bain of KRISO. The berthing tests were performed using two barge models under the head and beam sea conditions. Fairly good agreement is observed between the numerical simulation and experimental results. The effects of the barge size and berthing path are also briefly discussed based on the numerical results. The second berthing problem considered in this study is that between a floating production, storage, and offloading (FPSO) unit and a shuttle tanker. The focus in this case is on the wave-induced motion response during the berthing process. The characteristics of the wave-induced motion response in a head sea are examined with various wave frequencies and berthing speeds. The proposed numerical method is also used to investigate the berthing process in quartering and beam seas, with particular interest in the sheltering effect and the strong modulation of the motion response. This thesis further discusses the berthing problem in irregular seas as well as the effects of the wave period, height, and random seed on the motion response.