Direct coupling methods for the analysis of open & closed boundaries containing incompressible fluid and the applications on offshore structures

Abstract

In engineering analysis, a variety of analytical and numerical methods have been developed for analysing the hydrodynamic response of marine vehicles or offshore structures under different sea state, including the multipole expansion & integral equation methods for rigid body analysis and hydroelastic analysis involving elastic deformation. Most of these existing procedures require the specification of the boundary conditions over the wetted surface in advance so that functions of the flow are solvable. For rigid body analysis, each degree of motion is explicit while in hydroelastic analysis, the boundary condition is assignable through modal expansions. When additional free fluids exist the problem is a little more complex, but solvable, since the principle modes for analysis is unchanged with sloshing. However, new challenge arise when an incompressible liquid fills the inside of a structure as the modes are significantly changed due to the restriction of internal volume. The boundary conditions are, therefore, unassignable through normal dry hull analysis. In this dissertation, a different approach is developed with associated software developed by the author. The only conditions left that controls the motions in these problems will be continuity of normal velocity at the structural surface and the fundamental strain-stress relationship of the materials. These two conditions cannot provide the results directly. However, they enable us to formulate proper force-movement equations that contain the boundary conditions implicitly. In other words, the hydrodynamic forces and the structural response are directly coupled without separating the model into different orders of components. Direct coupling method itself is not a novel concept. It has been developed for solving simple acoustic and hydroelastic cases. This dissertation focuses on alternative forms, which allow one to solve problems that are more general. The matrix operation technique utilised enables solution of fully closed boundary problems in a boundary element analysis. The study was initially proposed for analysing the Anaconda wave energy convertor (WEC) device, which is a highly flexible rubber tube filled with seawater. The device is submerged under the free surface and generates internal propagating bulges waves through the excitation of external ocean waves. Only 1-dimentional equations were initially proposed to solve the behaviour of the structure. The direct coupling method developed in this dissertation is the first study that treats it as a three-dimensional model. Existing engineering packages and software are not capable to deal with the mathematical problem raised by the closed incompressible fluid domain. Hence, the hydrodynamic codes together with the coupled structural analysis are written and implemented independently. Only few packages related to matrix operation and special mathematical functions are borrowed. The configuration of the model is achieved by command flows under carefully geometric design and the results are exported in raw data to be analysed. Chapter 1 discusses the engineering significance of the proposed novel method and presents a literature review for both existing fluid mechanic methods and Anaconda WEC. The difficulties for solving fully closed problems and the deductions of important formulations are introduced in Chapter 2. Modelling of different problems are presented in Chapter 3. Chapters 4 & 5 provide verifications of developed method and the solutions of general offshore structure and a fully filled closed structure. Chapter 6 provides analysis of Anaconda solved applying the new method. Some unfinished works and discussions are addressed in Chapter 7.

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