This study investigates the possibilities of stern slamming and the effects on a heavy lift vessel that is on dynamic positioning during offshore operations. When such a vessel encounters following waves the stern becomes susceptible to high-pressure impacts. These impacts could lead to whipping effects throughout the hull. These possible vibrations can lead to discomfort for the crew and for important mechanical failures of critical DP systems. This poses a risk during the offshore operations and could lead to an abortion of the operation until the slamming stops. This study focuses on identifying how slamming occurs in conditions in which a vessel can perform offshore operations and what parameters have an effect on the pressures generated by the impacts. Also, a relation between the slamming and responses is drawn to find out how certain components could fail due to the whipping effects.

The research consists of two different steps. The first is a two-dimensional incompressible Volume-Of-Fluid model in ComFLOW is used to simulate the hydrodynamic wave impacts on the stern of a heavy lift vessel. Simulations cover two irregular sea states representative of Beaufort scales 4 and 5 and a range of different drafts for the vessel from a minimum draft of 6 meters to the design draft of 8 meters. For this 2D simulation are made to ensure relative quick computation and fundamental results. Because of the 2D simulations and the vessel being stationary, only heave and pitch motions of the vessel are modelled. Also, simulations with a series of different peak wave periods are set up with and without the vessel motions to see the effect of the incoming waves and vessel motions on the impacts on the stern. Mesh refinement and grid convergence analyses are conducted to test the accuracy of the CFD model.
The second approach uses the output of the CFD model to calculate the vibrational responses due to the wave impacts on the stern. The hull of the vessel is idealised as an Euler–Bernoulli beam and decomposed into its first four bending modes. Pressure time histories extracted from the CFD simulations serve as asymmetric loading inputs to two independent vibration response solvers, a Duhamel integral formulation and the Cummins impulse response equation. Both solvers compute time-dependent modal amplitudes and reconstruct spatial velocity fields along the beam to evaluate Root Mean Square velocities at critical locations.

The results from the CFD simulation show that in both irregular sea states significant slamming impacts occur in all loading conditions of the vessel considering the selected sea states. However, the closer the vessel gets to the design draft the lower the average recorded impact pressures are. For a sea state with a Hs of 1.1 meters and a Tp of 4.6 seconds, the average impact pressure reduced from 390 kPa at a draft of 6 meters to 80 kPa for a draft of 8 meters. For a sea state with a Hs of 1.65 meters and a Tp of 5.1 seconds, the average impact pressure reduced from around 4200 kPa at a draft of 6 meters to around 250 kPa for a draft of 8 meters. However, in both sea states impacts with pressures well above the 1000 kPa were recorded on the stern. Also, lower and higher periods seem to increase the average impact pressures, likely due to steeper waves. The vessel motions in all cases reduced the average impact pressures between the 25.7% and 46.1%.
Structural response analyses show close agreement between the Duhamel and Cummins methods, with discrepancies under 2.5% arising from different treatments of memory effects. Predicted RMS velocities at the stern exceed typical comfort thresholds between 4 and 6 mm/s for impact loads around 200 to 300 kPa and can approach equipment safety limits of 18 mm/s even in moderate sea states for the higher observed load of 750 kPa or higher. This indicates that slamming induced vibrations may pose fatigue and operational risks.

This report aims to investigate the dynamic simulation of an IMOCA 60 sailing yacht in big wave condi-tions. These yachts are equipped with hydrofoils, which significantly increases their speed. However, this increase in speed introduces a challenge: when encountering large waves, the yacht can experi-ence a ”crashing” behavior, where rapid acceleration leads to a collision with the wave ahead. These crashes can be severe enough to cause injuries to the crew onboard, making it essential to understand and mitigate these occurrences.
A Dynamic Velocity Prediction Program, DVPP, was developed to explore the yacht’s behavior in waves. This DVPP systematically models all the forces acting on the yacht, allowing them to be solved in the time domain. Particular attention was given to hydrodynamic forces, with a nonlinear Froude-Krylov force calculation to accurately represent the effect of waves on the yacht’s hull. Next to this, a correc-tion has been applied to the diffraction and radiation forces to take the effect of foiling into account. Furthermore, a correction has been applied to the aerodynamic forces to account for the flapping of sails due to changes in apparent wind angle.
To validate the DVPP’s accuracy regarding hydrodynamic and static forces, a heave decay test and RAOs of a Wigley hull were calculated. Based on these results, the DVPP agrees with the refer-ence data, which gives confidence in the DVPP. A qualitative validation was conducted to evaluate the DVPP’s ability to simulate an IMOCA 60 in wave conditions. These simulations demonstrated that the DVPP with the implemented corrections could accurately simulate an IMOCA 60 yacht in waves, as the results corresponded with those from a DVPP developed for an ocean-racing trimaran.
Further investigation was performed on the effect of the foils on the yacht. A parametric study revealed a clear correlation between the yacht’s behavior and sea state: higher sea states lead to more severe crashes. Further investigations into foil chord length and rake angle were also conducted. The analysis showed that a longer chord length tends to result in less influence of waves on the yachts speed, likely due to the increased drag associated with a longer chord, which limits the yacht’s speed.
Additionally, it was found that a lower rake angle leads to more severe slowdowns. This is attributed to the influence of wave orbital motion on the foils; at a lower rake angle, the increased angle of attack generated by the orbital movement increases lift as the wave approaches the stern of the yacht, leading to higher speeds and more significant impacts with the wave ahead. Furthermore, recovery from these crashes is slower with a lower rake angle, as the hydrofoil produces less lift overall. Based on this parametric study, it can be concluded that a larger chord length and a higher rake angle are preferred to minimize accelerations during slowdowns. However, further investigation is needed to understand how the yacht’s overall design influences its behavior in waves.
Lastly, a longer simulation, with challenging environmental conditions, was performed to investigate whether the DVPP could be used to simulate crashes in waves of an IMOCA 60. The results showed several slowdowns where the G force was above the threshold for a crash. This indicates that the DVPP can simulate these extreme events. Upon further analysis, it was concluded that the first part of the slowdown occurs due to the foil submergences, and a second slowdown occurs when the hull enters the water. Based on the results of the parametric study, the recommendations of a larger foil chord length and higher rake angle were applied to the simulation case; with these changes, the slowdowns were much lower, and the occurrence of crashes was reduced.
Furthermore, it is recommended that future research focus on enhancing the accuracy of the DVPP, particularly in the modeling of nonlinear hydrodynamic forces, radiation, and diffraction effects. Since an engineering solution was implemented, incorporating unsteady sail forces into the simulation to account for the effects of sail trimming on the yacht’s performance is also crucial for stable results in big waves. Further research is needed to present a method that is backed by further physics.