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A Coupling Strategy for Modelling Dynamics of Moored Floating Structures

Tristan de Lataillade,'•[1] Aggelos Diniakoponlos, [2] Chris Kees[3] and Lars Johanning[4]

Introduction

Assessing the performance of a multiphysics system such as moored floating structures subject to environmental loads is a challenging task, which can be performed by means of physical or numerical modelling. The latter can be categorised depending on the inclusion of: nonlinear effects (linear or nonlinear models), dynamic effects (static/quasi-static or dynamic models), and the interaction between multiple physical processes (coupled or uncoupled models). Until recently, relatively simple numerical tools using linear, uncoupled and static/quasi-static models were proven to be relatively reliable and computationally efficient for simulating floating structures that are not expected to significantly respond to environmental conditions, e.g., moored vessels in a port or floating LNG platforms, as the slow or limited motion of the structure makes the use of these tools appropriate. The recent growth of business sectors such as marine renewables has nevertheless been a game changer for the design of floating and moving structures. In order to harvest energy, offshore renewable energy devices are designed to be deployed in sites that exhibit a high energy potential, which usually translates to a hostile environment with harsh conditions. For example, the construction and operation of a floating offshore wind platform and associated components such as mooring cables and turbine must be designed to withstand severe environmental loads that may cause failure due to extreme loading or even fatigue. Other devices, such as some wave energy converters (e.g., attenuators, point absorbers) are designed to actually resonate with the dominant wave frequency of the deployment site, making the use of coupled nonlinear approaches vital for developing successful devices. The performance of these smaller scale devices cannot be reliably assessed using linear, static/quasi-static and uncoupled tools as these either require tuning parameters on a case-by-case basis to perform adequately, or in some cases, are simply inappropriate for assessing the performance of the structure.

Nonlinear processes in Fluid-Structure Interaction (FSI) applications for floating bodies typically include, but are not limited to: viscous effects, vortex shedding, wave overtopping, wave-wave interactions, and mooring damping. Many of these physical processes are interdependent, i.e., their presence or magnitude affects the other effects, hence the need of a viable coupling approach when they are to be modelled numerically. Prototype testing and large scale physical models, are technically able to reproduce real-world conditions in an exact manner, but are generally considered prohibitively expensive for early development, due to their high cost of initial production and testing, as well as any subsequent iterations in their design. These methods are instead commonly used in later design stages, before deploying devices for commercial use. In early design stages, engineers have the option of using small scale physical testing or coupled numerical models. The former option can indeed offer useful insights in physical processes, but small scale experiments typically suffer from scaling effects that make it difficult for accurate quantitative predictions. High-fidelity coupled models therefore offer a strong alternative to simulate highly nonlinear FSI processes numerically. While promising, they still present several shortcomings (e.g., numerical stability issues, time-consuming simulations) that current and future research is anticipated to address. When compared to simpler models, high-fidelity models such as Computational Fluid Dynamics (CFD) typically require access to significant computational power such as High Performance Computing (HPC) clusters. While this can be prohibitive in certain cases, it is becoming lesser due to recent and continuing growth of available computational power at a reduced cost.

The objective of this chapter is to propose a viable coupling strategy employing a combination of two-phase flow CFD model with a fully dynamic multibody and mooring dynamics model, suitable for high-fidelity simulation of floating moored structures. A key difference between the models mentioned above and the one developed here is that the coupling between moorings and structure is strong, and that fluid velocities from the CFD solver are retrieved to calculate hydrodynamic pressures along the mooring line. The CFD and solid dynamics solver are based the Proteus® (https: //proteustoolkit. org) and Chrono® (https://projectchrono.org/) open-source projects. The implementation herein is based on these particular tools, but it is also our ambition to provide a blueprint for interested researchers and engineers aiming to develop similar coupling strategies based on different tools. A decision has therefore been made to make the description of the coupling strategy as generic as possible. It is worth noting that this chapter follows on the work performed in de Lataillade (2019) where a more detailed account of the particulars of the strategy implementation is presented.

This chapter therefore describes the complete development process, implementation, and testing of a high-fidelity numerical framework for FSI of moored floating structures. The main numerical techniques and tools selected for this purpose are:

  • • CFD using Finite Element Method (FEM) for fluid flow, free surface tracking, and any auxiliary models solving Partial Differential Equations (PDEs) on the fluid mesh;
  • • Arbitrary Lagrangian-Eulerian (ALE) formulation for the motion of mesh-conforming solids within the fluid mesh, and mesh deformation through the equations of linear elastostatics;
  • • Discrete Element Method (DEM) coupled with capable Multibody Dynamics (MBD) solver for rigid structures, including collision detection features; [5]

• FEM method for mooring dynamics using beam theory with gradient deficient Absolute Nodal Coordinate Formulation (ANCF) elements.

Following this introduction, the present chapter contains 5 additional main sections. To start with the governing equations of each uncoupled model used in this work are presented in Section 2. Then, an overview of the different fluid-structure coupling schemes is given in Section 3. This is followed in Section 4 by an overview of the coupling strategy employed here for the different coupled interfaces: fluid-structure, fluid-mooring, and mooring-structure. The coupled framework in then validated with several simulations in Section 5. Finally, discussion, conclusions and future work are laid out in Section 6.

  • [1] 2 Research Engineer at US Army Corps of Engineers ERDC, Vicksburg MS, US and EngD Researcher at the University ofEdinburgh, Eduiburgli, UK.
  • [2] Principal Engineer at HR Wallingford, Wallingford, UK.
  • [3] Research Hydraulic Engineer at US Anny Coips of Engineers ERDC, Vicksburg MS, US (now Associate Professor atLouisiana State University, Baton Rouge LA, US).
  • [4] Professor of Ocean Technology at University of Exeter, Exeter, UK. Emails: This email address is being protected from spam bots, you need Javascript enabled to view it ; cekees:@gmail.com; L This email address is being protected from spam bots, you need Javascript enabled to view it * Corresponding author: This email address is being protected from spam bots, you need Javascript enabled to view it
  • [5] Statics/quasi-statics model for estimating tensions of catenary mooring lines at equilibrium:
 
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