Roelf Luppes

Extreme Wave Impact on Offshore Platforms and Coastal Constructions

Hydrodynamic wave loading on structures plays an important role in areas such as coastal protection, harbor design and offshore constructions (FPSO’s, mooring), and there is a need for its prediction up to a detailed level (max./min. pressures, duration of pressure peaks, shear stresses, etc.). In close cooperation with industry, long-year joint-industry projects are carried out to develop a numerical simulation method: the CFD method ComFLOW. The two major application areas are the prediction of extreme wave forces on offshore platforms and offloading vessels, and the prediction of impact forces on coastal protection structures. The paper will present a short overview of the method, some recent results and future plans.Copyright

Free-surface flow simulations for moored and floating offshore platforms

During the development of the ComFLOW simulation method many challenges have to be tackled concerning the flow modelling and the numerical solution algorithm. Examples hereof are wave propagation, absorbing boundary conditions, fluid–solid body interaction, turbulence modeling and numerical efficiency. Some of these challenges will be discussed in the paper, in particular the design of absorbing boundary conditions and the numerical coupling for fluid–solid body interaction. As a demonstration of the progress made, a number of simulation results for engineering applications from the offshore industry will be presented: a wave-making oscillating buoy, a free-fall life boat dropping into wavy water, and wave impact against a semi-submersible offshore platform. For those applications, MARIN has carried out several validation experiments.

Generating and Absorbing Boundary Conditions for Free-Surface Flow Simulations in Offshore Applications

Numerical simulations of wave phenomena necessarily have to be carried out in a limited computational domain. This implies that incoming waves should be prescribed properly, and the outgoing waves should leave the domain without causing reflections. In this paper we will present an enhanced type of such generating and absorbing boundary conditions (GABC). The new approach is applied in studies of extreme hydrodynamic wave impact on rigid and floating structures in offshore and coastal engineering, for which the VOF-based CFD simulation tool ComFLOW has been developed.Copyright

Adaptive grid refinement for free-surface flow simulations in offshore applications

In many (wave) impact problems the area of interest does not change in time and is readily pointed out by hand, allowing for a one-time design of an efficient computational grid. However, for a large number of other applications, e.g. involving violent free-surface motion or moving objects, a reasonable efficiency gain can only be obtained by means of time-adaptive refinement of the grid. In previous studies a fixed, block-based Cartesian local grid refinement method was developed and implemented in the CFD simulation tool ComFLOW [1], a VOF-based Navier-Stokes solver on Cartesian grids with cut-cell discretization of the geometry. Special attention was paid to the interface discretization in cut-cells as well as the fluid displacement algorithm across refinement boundaries. The method was successfully applied to a range of offshore applications, including for example wave-impact on a semi-submersible (figure 1)and sloshing in a moonpool. In the present paper we present the first results of our attempts to extend the method to support adaptive refinement.Copyright

Turbulence Modeling for Free-Surface Flow Simulations in Offshore Applications

To study extreme hydrodynamic wave impact in offshore and coastal engineering, the VOF-based CFD simulation tool {ComFLOW} is being developed. Recently, much attention has been paid to turbulence modeling, local grid refinement, wave propagation and absorbing boundary conditions. Here we will focus on the design of the turbulence model, which should be suitable for the coare grids as used in industrial applications. Thereto a blend of a QR-model and a regularization model has been designed, in combination with a dedicated wall model. The QR-model belongs to a class of modern eddy-viscosity models, where the amount of turbulent eddy viscosity is kept minimal. The performance of the model will be demonstrated with several applications relevant to the offshore industry. For validation, experiments have been carried out at MARIN.