OC6 Phase Ia: CFD Simulations of the Free-Decay Motion of the DeepCwind Semisubmersible

Currently, the design of floating offshore wind systems is primarily based on mid-fidelity models with empirical drag forces. The tuning of the model coefficients requires data from either experiments or high-fidelity simulations. As part of the OC6 (Offshore Code Comparison Collaboration, Continued, with Correlation, and unCertainty (OC6) is a project under the International Energy Agency Wind Task 30 framework) project, the present investigation explores the latter option. A verification and validation study of computational fluid dynamics (CFD) models of the DeepCwind semisubmersible undergoing free-decay motion is performed. Several institutions provided CFD results for validation against the OC6 experimental campaign. The objective is to evaluate whether the CFD setups of the participants can provide valid estimates of the hydrodynamic damping coefficients needed by mid-fidelity models. The linear and quadratic damping coefficients and the equivalent damping ratio are chosen as metrics for validation. Large numerical uncertainties are estimated for the linear and quadratic damping coefficients; however, the equivalent damping ratios are more consistently predicted with lower uncertainty. Some difference is observed between the experimental and CFD surge-decay motion, which is caused by mechanical damping not considered in the simulations that likely originated from the mooring setup, including a Coulomb-friction-type force. Overall, the simulations and the experiment show reasonable agreement, thus demonstrating the feasibility of using CFD simulations to tune mid-fidelity models.

Numerical Modelling of a Floating Wind Turbine Semi-Submersible Platform

A detailed study is undertaken of the computational modelling of a sub-platform for floating offshore wind using the software Star-CCM+ with the application of the RANS approach. First, a mathematical introduction to the governing equations is carried out. Then, the computational grid is defined, and the grid-independence of the solution is verified. A time-dependent study is performed with the selected time-step. Finally, two examples of 3D decay tests in heave of the sub-platform without and with moorings are presented, accompanied by a damping factor study, with the aim of providing a better understanding of the hydrodynamic damping of the platform. Throughout the process, three degrees of freedom (DoFs) are locked due to the limitations imposed by the use of a symmetry plane; this implementation allowed us to reduce the computational cost of each simulation by 50%. Therefore, three DoFs (heave, surge and pitch) are considered. The coupling study, adding a mooring system in the decay tests and the regular wave tests, shows good agreement between the experimental and computational results. The first half-period of the simulations presents a greater discrepancy due to the fact that the damping of the platform is lower in the computational simulation. However, this does not imply that the hydrodynamic damping is underestimated but may be directly related to the lock of various DoFs associated with the hydrodynamic damping.

Subsea Tree Fatigue Mitigation Solutions For Shallow Water Drilling

The offshore drilling industry is advancing technologies to extend deep water drilling technologies and attain feasibility of operations at deeper depths and higher pressures. However, shallow water operations themselves pose a certain unique set of challenges that need to be addressed with customized and innovative solutions. While shallow water poses certain benefits and conveniences to the operations, like ease of retrieval and better access to wells, there are significant challenges in terms of operational down time caused by limited operability and poor drilling riser and subsea hardware fatigue performance. Shallow water operations do not have the advantage of deep water drilling where the motions and loads imparted to the subsea blowout preventer (BOP) are relatively decoupled and damped out by hydrodynamic damping from the significant length of the water column. Thus, the vessel motions and wave hydrodynamic loads imparted on the riser are transferred to the wellhead and subsea hardware. In this paper the fatigue challenges encountered for drilling wells in 530 ft water depth from a sixth generation moored semi-submersible rig are explored. The fatigue loading is critical for the subsea tree connector which is characterized by a high stress amplification factor (SAF). Multiple riser space-out solutions were evaluated including fairings, helically-grooved buoyancy, joints with rope, and modifications to the telescopic joint each of which will be presented in the paper along with combination of different damping parameters to optimize the fatigue performance. The paper will present the subsea tree connector fatigue performance for different riser space-out options and make recommendations for future operations with similar conditions. Other challenges encountered in fatigue evaluation will be discussed. This will highlight the current assumptions and unknowns in data that can form the subject of evaluation for a future joint industry study.

Bayesian Experimental Design of Cyber-Physical Tests for Hydrodynamic Model Calibration

An application of cyber-physical testing to the empirical estimation of difference-frequency quadratic transfer functions is presented. As an alternative to today’s procedure based on hydrodynamic tests with broad-banded or realistic (e.g., JONSWAP) wave spectra, tests in bichromatic waves are considered. The laboratory setup is the one developed by Sauder & Tahchiev (2020) that enables magnifying the sensitivity of the floater response to the low-frequency wave loading by adjusting the stiffness and damping parameters of a virtual soft mooring system. Bayesian experimental design is proposed to optimize the selection of the control variables (frequencies in the bichromatic wave and properties of the virtual mooring system) for a batch of cyber-physical tests. The experimental design algorithm is based on the recent work of Huan & Marzouk (2013). In a virtual yet realistic case study using an uncertain parametric quadratic transfer function, we demonstrate how the uncertainty of its describing parameters and other calibration parameters (low-frequency added mass and hydrodynamic damping) can be reduced. Results indicate that the proposed procedure has the potential for reducing experimental cost for calibration of hydrodynamic models.