Peter R. Wellens

CFD Simulations of a Semi-Submersible With Absorbing Boundary Conditions

The CFD tool COMFLOW is suitable for simulations of two-phase flows in offshore applications. COMFLOW solves the Navier-Stokes equations in both water and (compressibl e) air. The water surface is advected through a Volume-of-Flui d method, with a height-function approach for improved accur acy. By employing Absorbing Boundary Conditions (ABC), boundaries can be located relatively close to an object, wit hout influencing outgoing waves or generating numerical reflections that affect the waves inside the flow domain. Tradition ally, boundaries are located far from the obstacle to avoid reflect ions; even when numerical damping zones are used. Hence, with the ABC approach less grid points are required for the same accuracy, which reduces the computing time considerably. Simulations of a semi-submersible model are compared to measurements. The overall agreement is reasonably good, fo r a wide range of wave conditions. The ABC performs well; numeri cal reflections are almost absent. Moreover, computing time s reduce with a factor four compared to damping zone techniques.

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

Towards a General-Purpose Open Boundary Condition for Wave Simulations

For the design of FPSO’s in harsh environments an accurate assessment of the ability of the platform to survive in extreme sea conditions is of prime importance. Next to scaled model tests on the FPSO in waves also CFD capabilities are at the disposal of the designer. However even with the fastest computers available it is still a challenge to use CFD in the design stage because of the large computational resources they require. In that respect to use a small computational domain will improve the turn around time of the computations, however at the expense of various numerical artifacts, like reflection on artificial boundaries in the computational domain. In order to mitigate the reflection properties new absorbing boundary conditions have been developed. The work in this paper is constructed on the previous study about the generating and absorbing boundary condition (GABC) in the ComFLOW project. We present a method to apply the GABC on all the boundaries in a three dimensional domain. The implementation of the GABC in ComFLOW is explained in detail.Copyright

Application of an Absorbing Boundary Condition in a Wave-Structure Interaction Problem

For the design of offshore structures, an accurate assessment of the ability of the structure to survive in extreme sea conditions is of prime importance. Next to scaled model tests on the structure in waves, also CFD capabilities are at the disposal of the designer. However even with the fastest computers available, it is still a challenge to use CFD in the design stage because of the large computational resources they require. In this study we focus our attention on the implementation of an absorbing boundary condition (ABC) in a wave-structure interaction problem. Unlike the traditional approach where the boundaries are located far from the object to avoid reflection, we gradually locate them closer while at the same time observing the influence of the absorbing boundary condition on the solution. Numerical calculations are performed using the CFD simulation tool ComFLOW which is a volume-of-fluid (VOF) based NavierStokes solver. Comparisons with experimental results are also provided and the performance of the ABC is discussed.

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