There is an increasing trend towards using numerical wave simulations for the design of offshore structures, especially for the stochastic prediction of nonlinear wave loads like those related to air-gap and wave impact. Unlike experimental facilities, where the complex nonlinear physics of wave propagation is simply enforced by the laws of nature, numerical wave tanks (NWTs) rely on assumptions and simplifications to solve the propagation equations in a reasonable amount of time. It is therefore important to verify the quality of the waves generated by NWTs in terms of realistic physical properties. As part of the effort to develop reliable numerical wave modeling practices in the framework of the “Reproducible Offshore CFD JIP”, qualification criteria are formulated for the wave solutions generated from either potential-flow based or CFD-based codes. The criteria have been developed based on experiences from physical wave tank tests and theoretical/numerical studies. They are being evaluated using results from several numerical models and available benchmark data. This paper presents the proposed qualification criteria and on-going evaluation efforts by comparing results from different codes.
The paper presents results of steady simulations based on Reynolds-averaged Navier-Stockes (RANS) equations of the loads due to wind and current on a semi-submersible oil platform. Flow angles range from 0 to 360 • . The loads due to current and wind are treated separately. The simulations of current are performed in a uniform flow, whereas in the simulations of wind, the atmospheric boundary layer is taken into account. The computational mesh is locally refined to include several levels of detail. Viscous boundary layers are not modeled. The computed results are compared to wind tunnel measurements.
The evacuation of personnel from floating production, storage and offloading (FPSO) vessels in case of accidental scenarios is generally ensured by free-fall lifeboats launched from skids. Safe lifeboat launches occur for a specific range of pitch angles at water entry: flat water impacts may cause dangerous acconal criteria for safely launching the lifeboats. The importance of the wind velocity and direction with respect to the lifeboats is investigated. The study mainly focuses on the acceleration loads experienced by the passengers during water-entry and it is based on numerical simulations and model tests for the validation of the simulation tools and models. Introduction Free-fall lifeboats launched from skids constitute the primary means for evacuating FPSOs in sea states that do not permit helicopter landing, i.e. typically when the significant wave height Hs is larger than 5m. These lifeboats are generally installed at the bow of the FPSO at ca. 20m above the calm water level. The lifeboat launch is then influenced by environmental parameters such as the significant wave height Hs, the spectral peak period Tp and the 1-hour mean wind velocity Uw. In addition to environmental parameters, other factors like the heading strategy of the FPSO with respect to waves and wind, the loading conditions of the FPSO and the lifeboat, possible damage scenarios for the FPSO including loss of thrust or additional trim and list are also of significance. The present paper is concerned with the establishment of operational criteria for safe launch of free-fall lifeboats from a turret-moored FPSO in the Norwegian Sea. The final goal is to estimate the probability of successful launch as a function of a selected set of the parameters listed above. So far, the main failure mode considered in this study has been possible human injury during water impact due to large accelerations. An example of quantity one aims to estimate is the conditional probability of being injured when seating in the aft seat row of a fully loaded lifeboat, if Hs is between 8 and 10 meters and the mean wind velocity Uw is larger than 20m/s. Provided a weather forecast, the obtained probabilistic model can then be used to decide if the lifeboats can be used in this sea state or if some seating restrictions have to be applied.
Hydrodynamic force coefficients are important parameters in the design and assessment of marine risers. The hydrodynamic coefficients are widely used for assessing marine riser responses due to floater motion excitation and vortex-induced vibrations (VIV). Traditionally, the hydrodynamic coefficients have been obtained from physical model tests on short rigid riser sections. Recently, the offshore industry has started to use Computational Fluid Dynamics (CFD) analysis for predicting the hydrodynamic coefficients, due to the recent advancement of CFD software and high-performance computing capabilities. However, a reliable CFD modeling practice is required for CFD analysis to be a more widely accepted prediction tool in the industry. A joint industry effort has been initiated for developing and verifying a reliable CFD modeling practice through a working group of the Reproducible Offshore CFD JIP. Within the working group, a CFD modeling practice document was written based on existing practices already validated with model test data and verified by blind validations with three CFD practitioners. The first year work focused on a bare riser with circular cross-section and has been published in OMAE 2021. This paper presents the working group’s second-year verification activities for a staggered buoyancy module and a straked riser. The verification work covers three numerical test problems: 1) stationary riser in steady current, 2) riser under forced-oscillation in calm water, 3) riser under forced-oscillation in steady current. In the stationary riser simulation, drag coefficient and lift coefficient from verifiers are compared. In the forced-oscillation simulation in calm water, the fully-submerged riser section oscillates with a sinusoidal motion, and damping and added-mass coefficients are compared. In the forced-oscillation simulation in steady current, where the riser oscillates in either inline or perpendicular direction to the steady current, lift coefficient and added mass coefficient are compared. By following the modeling practice, the CFD predictions are consistent with each other and close to the model test data for the majority of the test cases.
Hydrodynamic force coefficients are important parameters in design and assessment of marine risers. The hydrodynamic coefficients are widely used for assessing marine riser responses due to floater motion excitation and vortex-induced vibrations (VIV). Traditionally, the hydrodynamic coefficients have been obtained from physical model tests on short rigid riser sections. Recently, the offshore industry has started to use computational fluid dynamics (CFD) analysis for predicting the hydrodynamic coefficients due to the recent advancement of CFD software and high-performance computing capabilities, but a reliable CFD modeling practice is requested for CFD analysis to be a more widely accepted prediction tool in the industry. A joint industry effort has been made for developing and verifying the reliable CFD modeling practice through a working group of the Reproducible Offshore CFD JIP. In the working group, a CFD modeling practice document was written based on existing practices already validated for model test data, and verified by blind validations with three CFD practitioners. The first year works are focused on the bare riser with circular cross-section, and the second year work will be extended to the other riser sections such as staggered buoyancy module and straked riser. This paper presents the working group’s first-year verification activities for a bare riser with circular cross-section. The verification works covers three test problems: 1) stationary simulation in steady current, 2) forced-oscillation in calm water, 3) forced-oscillation in steady current. In the stationary simulation, mean drag coefficient, standard deviation of lift coefficient, and Strouhal numbers are compared. In the forced-oscillation simulation in calm water, the fully-submerged riser section oscillates with a sinusoidal motion, and damping and added mass coefficients are compared. In the forced-oscillation simulation in current, the riser section oscillates in cross-flow direction to the steady current, and lift coefficient and added mass coefficient are compared. By following the modeling practice, the CFD predictions are consistent with each other and close to the model test data for a majority of test cases.
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