Hyunchul Jang

An Experimental and Computational Development of a Benchmark Solution for the Validation of Numerical Wave Tanks

A joint effort between MARINTEK, DNV GL and Technip has been made to develop a set of benchmark solutions for the validation of numerical wave tanks. High-fidelity measurements of the wave field around a vertical circular cylinder from a scaled model test have been selected as the benchmark cases. A reference numerical wave tank based on a commercial CFD software and the Euler-Overlay Method as far-field closure has been validated by grid convergence test and comparison with the model test data. The validated numerical solution can be used as a benchmark solution to validate other numerical wave tanks.Copyright

Guidelines for CFD Simulations of Spar VIM

Vortex-Induced Motion (VIM), which occurs as a consequence of exposure to strong current such as Loop Current eddies in the Gulf of Mexico, is one of the critical factors in the design of the mooring and riser systems for deepwater offshore structures such as Spars and multi-column Deep Draft Floaters (DDFs). The VIM response can have a significant impact on the fatigue life of mooring and riser components. In particular, Steel Catenary Risers (SCRs) suspended from the floater can be sensitive to VIM-induced fatigue at their mudline touchdown points.Industry currently relies on scaled model testing to determine VIM for design. However, scaled model tests are limited in their ability to represent VIM for the full scale structure since they are generally not able to represent the full scale Reynolds number and also cannot fully represent waves effects, nonlinear mooring system behavior or sheared and unsteady currents. The use of Computational Fluid Dynamics (CFD) to simulate VIM can more realistically represent the full scale Reynolds number, waves effects, mooring system, and ocean currents than scaled physical model tests.This paper describes a set of VIM CFD simulations for a Spar hard tank with appurtenances and their comparison against a high quality scaled model test. The test data showed considerable sensitivity to heading angle relative to the incident flow as well as to reduced velocity. The simulated VIM-induced sway motion was compared against the model test data for different reduced velocities (Vm) and Spar headings. Agreement between CFD and model test VIM-induced sway motion was within 9% over the full range of Vm and headings. Use of the Improved Delayed Detached Eddy Simulation (IDDES, Shur et al 2008) turbulence model gives the best agreement with the model test measurements. Guidelines are provided for meshing and time step/solver setting selection.Copyright

Investigation on the VIM Mitigation of the HVS Semisubmersible

The mitigation of Vortex Induced Motion (VIM) of the HVS (Heave and VIM Suppressed) semisubmersible is investigated through extensive comparisons between CFD analysis and VIM model test results. It is shown that the lower VIM response of the HVS semisubmersible results from the break in coherence of vortex shedding along the length of column due to the column step. The present CFD application was carried out on the basis of in-house best practices for VIM analysis of multi-column floaters. The analysis results show excellent comparison with the model test results. The present findings and methodology can be applied to optimize semisubmersible hull designs for suppressed VIM response.Copyright

Benchmark of CFD Modeling of TLP Free Motion in Extreme Wave Event

In this study, a numerical wave tank was set up to simulate the free motion of a Tension Leg Platform (TLP) in extreme wave event. For better computational efficiency, a nonlinear potential flow solver is coupled with a CFD software, with the former to simulate the far-field wave domain and the latter to simulate the near-field wave domain and platform motion. In order to benchmark against model test, a five-minute time window of interest was selected from the extreme sea state in model test. The incoming irregular wave was firstly reconstructed from the measured wave time history using the nonlinear potential flow solver and then applied as input to CFD simulations for two different headings to the platform. Static offset tests and free decay tests were simulated in CFD initially to confirm that the platform and tendon properties were properly modeled. The 6-DOF platform motions were then obtained from the CFD simulations and the time histories of motion, air gap, and tendon tension were compared with model test measurements. Good agreements were achieved except for the initial transient period and low-frequency motions. In particular, the air gap or relative wave elevation compared well for all the locations around the platform. The high frequency response in tendon tension and the different tension characteristics of weather side tendons and leeside tendons were also well captured.Copyright

Technical and Economic Readiness Review of CFD-Based Numerical Wave Basin for Offshore Floater Design

As Computational Fluid Dynamics (CFD) and High Performance Computing (HPC) technologies matured in many other industries, the offshore industry has begun to recognize CFD-based Numerical Wave Basin (NWB) as a design tool to evaluate offshore floater design more efficiently and with less uncertainty than the conventional ways relying on empirical methods. The recent NWB technology development has focused on the customization of CFD software for offshore design practices and validation of the developed analysis tools/procedures against physical model tests. Development has now extended to simulation of fully coupled hull-mooring-riser systems. Technology readiness of the NWB for field application is demonstrated for two benchmark problems: Vortex-induced motion of a multi-column floater Global performance of a multi-column floater in extreme wave environment The results indicates that the CFD-based numerical wave basin, although still computationally expensive, is technically ready to be a complementary tool to physical wave basin for offshore platform global performance design.