Wind-wave misalignment significantly affects the dynamic responses of floating offshore wind turbines (FOWTs). A fully coupled aero-hydro-mooring numerical model was established in OrcaFlex for a FOWT with a three-line shared-anchor mooring system. The numerical analysis evaluated three representative sea states (LC1, LC2, and LC3) alongside multiple wind-wave heading combinations. The six-degree-of-freedom (6-DOF) motion responses of the floating platform were systematically investigated using motion trajectories, kernel density estimation (KDE), violin plots, and probabilistic statistical analyses. The results indicated that: (1) the motion responses of the floating platform were jointly governed by the sea state and wind-wave heading. The sea state primarily determined the response intensity and variability, whereas the wind-wave heading governed the motion pattern. Increasing wind-wave misalignment angle shifted platform motion from unidirectional response to multidirectionally coupled motions. This shift led to increasingly dispersed probability distributions. (2) Different motion degrees of freedom exhibited distinct environmental sensitivities. Roll and pitch were primarily controlled by the wind direction, whereas relative wind-wave heading dictated yaw sensitivity. Increasing sea-state severity significantly amplified motion responses without altering their directional response characteristics. (3) Variations in platform motions altered shared-anchor loading characteristics. Higher wind-wave misalignment shifted shared-anchor loads from unidirectional to multidirectional distributions. Consequently, mooring lines underwent continuous load redistribution. These findings clarified the coupled effects of wind-wave misalignment on floating platform motions and shared-anchor loading. This work provides theoretical guidance for designing and assessing shared mooring systems in FOWTs.
Model Predictive Control (MPC) is applied to track control of unmanned surface vessels. To adapt to dynamic changes in speed and water depth, a water depth and speed dependent predictive model, with coefficients identified from free running tests (e.g., acceleration and zigzag trials), is developed. The nonlinear least square method is used for parameter estimation of the model. Furthermore, a novel self-tuning method adapts MPC’s essential parameter, prediction horizon, to diverse operating conditions. Validation via manoeuvring simulations of the full-scale benchmark KVLCC2 ship covers under keel clearances (UKC) from 20 to 100 % ship’s draft (from shallow water to medium deep water) and vessel speeds of 7-15 knots, spanning 25 distinct scenarios. The designed controller achieves high-accuracy path following across all conditions, demonstrating exceptional adaptability to parameter variations.
To balance NOX emissions and performance in ammonia-fueled marine engines, this study investigates exhaust gas recirculation (EGR) effects on combustion, performance, and emissions, and proposes a full-load EGR strategy. Based on a validated CFD model of a methanol/diesel dual-fuel engine and an ammonia spray model, an ammonia/diesel dual-fuel engine model was developed. Simulations at four typical loads were conducted with various EGR rates. Results show that higher EGR slows ammonia combustion, prolongs combustion duration, intensifies after-burning, and increases ammonia emissions. Heat release rate is load-dependent, with peaks regulated by EGR. Engine performance varies with load: the gross indicated mean effective pressure (IMEPg) increases with EGR at low-to-medium loads, but first rises then decreases at high loads. EGR effectively reduces NOX emissions at all loads, with maximum reduction ranging from 71.5 % to 90.2 %. Fuel NOX shows higher sensitivity to EGR than thermal NOX. However, EGR also increases ammonia and N2O emissions—marginal at low loads but significant at medium-to-high loads, with a distinct surge threshold. Recommended EGR rates are 50 %, 40 %, 30 %, and 10 % at 25 %, 50 %, 75 %, and 100 % loads, respectively. This strategy enables compliance with IMO Tier III NOX regulations, achieves low ammonia and N2O emissions, and improves IMEPg.
Numerical studies on parametric roll have mainly focused on head and following seas, whereas parametric roll in oblique head seas remains less fully understood due to the coupled effects of roll, sway, yaw, and the propulsion-steering system. In this study, a CFD-based numerical framework is developed for predicting parametric roll of a self-propelled KCS ship in oblique head seas, considering the coupled interactions among the hull, rudder, and propeller. The effects of model speed and wave heading on parametric-roll responses are investigated, and two mitigation strategies are examined, namely passive roll reduction using a bilge keel and active avoidance through speed control. The results show that parametric roll in oblique head seas is highly sensitive to model speed and wave heading. The roll response becomes pronounced when the encounter condition falls within the parametric-roll-sensitive range, whereas it is significantly weakened when the ship moves away from this range. Wave headings closer to head seas correspond to a more critical parametric-roll region. The bilge keel suppresses roll growth by increasing roll damping and reducing the roll moment, while speed control mitigates the nonlinear response by adjusting the encounter frequency. The present study demonstrates that CFD can be used not only to identify parametric-roll occurrence, but also to support the development of passive mitigation and active operational guidance for self-propelled ships in oblique head seas.
This study investigates parametric rolling of a container vessel in regular head waves using a hierarchical modelling framework combining a nonlinear time domain motion solver, a blended Ikeda-based roll damping module, and high-fidelity CFD simulations employing single-mesh and overset mesh strategies. The selected case corresponds to the classical resonance regime for first order parametric instability. All numerical approaches consistently reproduce the fundamental resonance mechanism, including the characteristic frequency relationship between pitch and roll and the exponential growth phase. A key result is the close agreement in instability onset time among the reduced order model and both CFD configurations, demonstrating robust capture of the excitation/restoring interaction governing parametric resonance. Experimental measurements exhibit an earlier visible onset, attributed to unavoidable perturbations and slight irregularities in the basin wave record, highlighting the sensitivity of parametric instability to initial disturbance levels. Differences in saturation amplitude are primarily associated with viscous dissipation modelling and nonlinear free surface phenomena, including green-water events observed experimentally but intentionally excluded from the present CFD setup. The results confirm that accurate representation of the resonance mechanism is achievable across modelling levels, while amplitude prediction remains sensitive to nonlinear damping and deck interaction effects. The developed framework provides a validated and computationally efficient basis for further investigations of damping thresholds, irregular wave excitation, green-water modelling, and coupled propulsion–roll interaction.
