Stern slamming on an offshore heavy lift vessel

Abstract

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.